Cytoplasmic Membrane and Cytoskeleton

Last updated: May 30, 2026EspañolРусский

Plant cell, like any eukaryotic cell, possesses a complex internal organization in which two interrelated systems play a key role — the cytoplasmic membrane and the cytoskeleton. The former delimits the living contents of the cell from the external environment and regulates the transport of substances; the latter forms a dynamic scaffold that provides shape, movement, and intracellular transport. In plants, these structures have unique features due to the presence of a rigid cell wall and a large central vacuole, and are also closely linked to processes of growth, development, and stress responses.

Cytoplasmic membrane or plasmalemma is a membrane complex consisting of a lipid bilayer and embedded proteins that surrounds the protoplast of a plant cell, separating it from the cell wall and intercellular space (Evert, 2006; Beck, 2010). It performs barrier, transport, receptor, and matrix functions, ensuring selective exchange of substances between the cell and the environment, perception of signals (phytohormones, stress factors), and organization of cell wall synthesis (Mauseth, 2017; Graham et al., 2014). Unlike the plasmalemma of animal cells, the plant membrane lacks cholesterol (replaced by phytosterols) but contains specific glycosphingolipids and linker proteins that tether it to the cellulose microfibrils of the wall (Gronnier et al., 2018; Su et al., 2025).

Cytoskeleton is a dynamic network of protein filaments (microtubules, actin microfilaments, and, putatively, intermediate filaments) that permeates the cytoplasm and provides mechanical support, organelle organization, intracellular transport, movement (cyclosis), and is also involved in cell division and the establishment of cell polarity (Takatsuka et al., 2023; Li et al., 2015). In plants, the cytoskeleton performs unique functions: cortical microtubules guide the movement of cellulose synthase complexes in the plasmalemma, thereby dictating the direction of cell growth, while the preprophase band (a ring of microtubules and actin filaments) predetermines the plane of future division (Evert, 2006; Wu et al., 2025). Plant cells lack centrosomes and cytoplasmic dynein; therefore, microtubule organization and nuclear migration largely depend on actin and calponin‑homology kinesins (Groves et al., 2025; Li et al., 2015). Microfilaments (actin) together with myosins drive the rotational movement of the cytoplasm (cyclosis), which accelerates metabolite transport and gas exchange (Graham et al., 2014; Takatsuka et al., 2023).

Thus, the cytoplasmic membrane and cytoskeleton in plants form an integrated mechanochemical system coupled with the cell wall, a feature that distinguishes the plant cell from the animal cell and is of fundamental importance for its functioning.

1. Functions

The cytoplasmic membrane and the cytoskeleton in plant cells operate as a unified dynamic system, ensuring cell viability, growth, and adaptation to changing conditions. Their functions are closely intertwined, yet for clarity they are treated separately.

Functions of the cytoplasmic membrane (plasmalemma)

The plasmalemma acts as an active molecular filter and signaling center, performing the following key tasks:

Barrier and transport. The primary function of the plasmalemma is to maintain homeostasis by selectively controlling the entry and exit of substances (Evert, 2006; Mauseth, 2017). The membrane exhibits selective permeability (semipermeability): water and small uncharged molecules (O₂, CO₂) diffuse freely, whereas ions (K⁺, Ca²⁺, NO₃^-), sugars, and amino acids are transported via specialized proteins. These include channels (passive transport down the concentration gradient), carriers (passive or secondary active transport), and pumps (active transport consuming ATP). A central role in membrane energetics in plants is played by the proton pump — H^\+-ATPase, which extrudes protons from the cell, creating an electrochemical gradient (Evert, 2006; Su et al., 2025). This gradient drives secondary active transport of all nutrients (sugars, amino acids, ions) via symporters and antiporters. In addition, the plasmalemma contains aquaporins — channels for rapid and regulated water transport, critical for maintaining turgor and cell growth (Evert, 2006; Takatsuka et al., 2023).

Matrix and structural. The plasmalemma serves as a matrix for organizing many cellular processes. The cytoskeleton attaches to its cytoplasmic side via linker proteins, while the cell wall attaches to the external side (Beck, 2010; Gronnier et al., 2018). Particularly important is the role of the plasmalemma in cellulose synthesis: it harbors rosette cellulose synthase complexes (clusters of enzymes) that, moving along guiding microtubules, extrude glucose residues into cellulose microfibrils directly into the cell wall space (Beck, 2010; Wu et al., 2025). Through this property, the plasmalemma dictates the architecture of the cell wall and consequently the shape and growth direction of the cell.

Receptor and signaling. The plasmalemma surface contains diverse receptors that perceive external signals: phytohormones (auxins, brassinosteroids, abscisic acid), peptide signals, pathogen-associated molecules (e.g., flagellin), as well as light and temperature factors (Gronnier et al., 2018; Su et al., 2025). A classic example is the FLS2 receptor complex (flagellin-sensitive), which upon ligand binding activates a MAP-kinase cascade and an immune response. Importantly, these receptors are not uniformly distributed but are concentrated in specialized nanodomains (microdomains enriched in sterols and sphingolipids), which enhances signaling efficiency and specificity (Gronnier et al., 2018; Jaillais et al., 2024).

Involvement in intercellular contacts. Although the main structures of intercellular connections — plasmodesmata — were discussed in detail in the article on the cell wall, it is important to emphasize that the plasmalemma is part of them. In the plasmodesma region, the plasmalemma of one cell becomes continuous with the plasmalemma of the adjacent cell, forming an uninterrupted membranous sleeve around the central desmotubule (Evert, 2006; Brunkard, 2024). This provides not only cytoplasmic continuity (symplast) but also membrane continuity between cells.

Functions of the cytoskeleton (microtubules and actin microfilaments)

In the plant cell, the cytoskeleton serves as the “skeleton”, “muscles”, and “tracks”, and its actions are tightly coordinated (Wu et al., 2025; Li et al., 2015).

Mechanical support and morphogenesis. The network of cortical (plasma membrane‑associated) microtubules acts as “reinforcement” that dictates the orientation of newly deposited cellulose layers in the cell wall (Beck, 2010; Evert, 2006). This determines the shape of the growing cell: if microtubules are oriented transverse to the growth axis, the cell elongates; if oriented longitudinally, expansion is restricted. In specialized cells (e.g., xylem), microtubules form local patterns (e.g., rings or spirals) guiding secondary wall thickenings.

Cytoplasmic streaming (cyclosis) and organelle transport. Cytoplasmic streaming in plant cells results from the action of actin microfilaments and the motor protein myosin (Takatsuka et al., 2023; Graham et al., 2014). Actin bundles traverse the cytoplasm, and myosin attached to the surface of vesicles, chloroplasts, mitochondria, or even the nucleus “walks” along them, moving the cargo. Cyclosis is particularly pronounced in large cells (e.g., in an Elodea leaf), where it facilitates rapid distribution of metabolites and gases.

Organization of cell division. The cytoskeleton is the chief organizer of mitosis and cytokinesis in plants, which, unlike animals, lack centrosomes (Li et al., 2015; Evert, 2006).

Preprophase band (PPB). Even before nuclear envelope breakdown, a ring forms in the cortical cytoplasm beneath the plasmalemma — the preprophase band made of microtubules and actin filaments. It marks the future plane of cell division and the site where the cell plate will subsequently form.
Mitotic spindle. Microtubules form the spindle, which ensures chromosome segregation to the poles. Since centrioles are absent, microtubules are nucleated around the nuclear envelope and organized via specialized proteins (e.g., the γ-tubulin complex).
Phragmoplast and cell plate. In telophase, remnants of the spindle give rise to the phragmoplast — a structure that guides Golgi‑derived vesicles toward the division plane. These vesicles fuse to form the cell plate, which later develops into the middle lamella and the new cell walls of the daughter cells.

Nuclear movement and polarity establishment. The cytoskeleton (especially actin filaments) plays a key role in positioning the nucleus — for example, its migration toward the developing root hair or toward the cell center before division (Groves et al., 2025; Takatsuka et al., 2023). The concerted action of actin and microtubules enables polar transport of membrane proteins, which underlies cell polarity (e.g., the polar localization of the auxin transporter PIN2).

Perception and transmission of mechanical signals. Through the “cytoskeleton – plasmalemma – cell wall” linkage, the cell senses mechanical stimuli (stretch, touch, osmotic pressure) and converts them into biochemical signals (mechanotransduction) (Beck, 2010; Gronnier et al., 2018). This is required for normal growth, stomatal closure, and responses to wounding or pathogen attack.

Thus, the cytoplasmic membrane ensures compartmentalization and selective exchange, while the cytoskeleton provides dynamic organization of intracellular space; together they govern plant growth, development, and adaptation.

2. Architecture and structure

The plasmalemma and the cytoskeleton are not two isolated structures but an integrated system in which the lipid‑protein membrane serves as a platform for dynamic filament assembly. Their architecture reflects both universal principles of eukaryotic cell organization and specialized adaptations related to the presence of a cell wall and the need to maintain turgor pressure.

2.1. General plan of plasmalemma structure: the fluid mosaic model

Diagram of the plasma membrane, showing the phospholipid bilayer, integral and peripheral proteins, and carbohydrate chains — ©Mariana Ruiz commons.wikimedia.org
Public Domain
Detailed diagram of the fluid mosaic model of the plasma membrane
Detailed diagram of the fluid mosaic model of the plasma membrane. Visible are the phospholipid bilayer (heads facing out, tails facing in), embedded channel proteins, transporters, receptors, and carbohydrate chains of the glycocalyx.

According to the classical and repeatedly confirmed fluid mosaic model (Singer & Nicolson, 1972), the cytoplasmic membrane is a two‑dimensional fluid — a lipid bilayer into which protein molecules are embedded (or associated) (Mauseth, 2017; Beck, 2010).

Lipid bilayer is formed by two oriented layers of amphiphilic lipid molecules (phospholipids, glycolipids). Their hydrophilic “heads” face the aqueous environment (cytosol and apoplast/cell wall), while the hydrophobic “tails” (fatty acids) are hidden inside, creating a layer impermeable to charged particles and polar molecules. At room temperature, the lipid bilayer is in a liquid‑crystalline state: lipid molecules diffuse rapidly in the plane of the membrane but rarely flip from one leaflet to the other (“flip‑flop” transitions are rare) (Graham et al., 2014). This fluidity is essential for membrane protein function and membrane traffic.

Membrane proteins are divided into integral (spanning the bilayer entirely or partially immersed in it) and peripheral (attached to integral proteins or lipid heads on the cytoplasmic side) (Evert, 2006; Mauseth, 2017). Integral proteins perform the following functions: - Transporters (channels, carriers, pumps, including the H^\+-ATPase that is key for plants); - Receptors (e.g., FLS2, BRI1, recognizing pathogens or hormones); - Enzymes (e.g., cellulose synthase complexes); - Adhesion proteins that link the plasmalemma to the cell wall (e.g., extensins, formins).

Membrane asymmetry arises because the lipid composition of the outer and inner leaflets differs. The outer leaflet is enriched in glycolipids and phosphatidylcholine, whereas the cytoplasmic leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositols (including phosphatidylinositol 4,5‑bisphosphate, PIP₂) (Gronnier et al., 2018; Jaillais et al., 2024). Moreover, all integral proteins have a strict orientation in the membrane.

Microdomains and nanodomains. Due to interactions among sterols, sphingolipids, and specific proteins, the membrane is not homogeneous. It contains lipid rafts — regions with higher order (less fluid), enriched in sterols and sphingolipids, which serve as platforms for assembling signaling complexes (Gronnier et al., 2018; Jaillais et al., 2024). Current views distinguish nanodomains (20–200 nm in size), within which certain receptors and proteins are concentrated, as well as polar domains (micrometer‑scale) associated with whole‑cell polarization (e.g., the basal or apical domain for the auxin transporter PIN).

2.2. Differences between the plant and animal cytoplasmic membranes

Despite the universal structural plan, the plant plasmalemma has several unique features reflecting the plant lifestyle and the presence of a cell wall (Evert, 2006; Beck, 2010; Gronnier et al., 2018).

Sterol composition. Instead of cholesterol, which is typical of animal membranes, the plant plasmalemma contains phytosterols (sitosterol, stigmasterol, campesterol). They provide the necessary fluidity and permeability and also participate in lipid raft formation and signaling (e.g., binding of phytosterols to brassinosteroid receptors).
Sphingolipids. The outer leaflet of the higher plant plasmalemma is dominated by glycosyl‑inositol‑phosphoceramides (GIPC) — complex sphingolipids with long (up to C₂₄) fatty acid tails. They play a key role in forming membrane domains and also serve as salt‑stress sensors: sodium binds to GIPC, activating a calcium channel and triggering a signaling cascade (Su et al., 2025; Jiang et al., 2019).
Connection to the cell wall. The plant plasmalemma is tightly coupled to the supramembrane matrix of the cell wall via linker proteins (glycoproteins, e.g., arabinogalactan proteins, extensins, and formins). This linkage, on one hand, restricts lateral diffusion of membrane proteins (Martinière et al., 2012, cited in Gronnier et al., 2018) and, on the other hand, allows transmission of mechanical forces from the cytoskeleton to the wall and back (mechanotransduction).
Absence of caveolae. Animal cell membranes often contain specialized invaginations — caveolae — associated with the protein caveolin. Similar structures have not been found in plants. The functions of organizing endocytosis and signaling are performed by other domains, such as clathrin‑enriched regions (clathrin‑mediated endocytosis) and lipid rafts.
Special role of H⁺-ATPase. While animal cells mainly use Na^\+/K⁺-ATPase to create an electrochemical gradient, plants rely on a P‑type H^\+-ATPase. It pumps protons out of the cell, generating a membrane potential (inside negative) and a pH gradient (apoplast more acidic than cytosol). This driving force is used by all secondary active transporters (symporters for sugars, amino acids, nitrates) (Evert, 2006; Su et al., 2025).
Features of endocytosis. Endocytosis does occur in plants, but its mechanisms have several differences. The TPLATE protein complex, not found in animals, plays a key role. Endocytosis is tightly linked to remodeling of the actin cytoskeleton (Li et al., 2015; Gronnier et al., 2018).

2.3. Architecture of the cytoskeleton: microtubules and microfilaments

The plant cytoskeleton consists of two main types of filaments (intermediate filaments remain controversial; their unambiguous homologs have not yet been identified in plants) (Evert, 2006; Beck, 2010).

Microtubules are hollow cylinders ≈ 25 nm in diameter, whose wall is composed of 13 longitudinal protofilaments made of α‑ and β‑tubulin heterodimers. They are polar: the plus‑end grows faster, the minus‑end slower. Dynamic instability (polymerization/depolymerization coupled with GTP hydrolysis) allows rapid microtubule rearrangements (Li et al., 2015; Beck, 2010). Unique features in plants: - Absence of a centrosome as the main microtubule‑organizing center (MTOC). Microtubule nucleation occurs all around the nuclear envelope, at the plasma membrane (via γ‑tubulin complexes anchored by augmin family proteins), and also as branching on existing microtubules (Beck, 2010; Li et al., 2015). - Cortical microtubules — an array of parallel microtubules located directly beneath the plasmalemma. They are generally oriented transverse to the cell growth axis and guide the movement of cellulose synthase complexes, thereby setting the texture of the cell wall (Evert, 2006; Wu et al., 2025). - Preprophase band — a dense ring of microtubules and actin filaments that forms in G₂ phase beneath the plasmalemma at the future division site. It is absent in animals. It sets the plane of cytokinesis and participates in positioning the phragmoplast (Li et al., 2015). - Absence of cytoplasmic dynein. Minus‑end‑directed motors for microtubules in plants are represented only by kinesins‑14 (calponin‑homology kinesins, KCH) (Groves et al., 2025; Li et al., 2015).

Actin microfilaments (F‑actin) are helical polymers of globular G‑actin, ≈ 6–8 nm in diameter. They are also polar: the plus‑end (barbed end) grows faster, the minus‑end (pointed end) slower. Actin dynamics are regulated by ATP and numerous actin‑binding proteins (profilin, cofilin, villin, formins) (Takatsuka et al., 2023; Graham et al., 2014). In plants, actin forms: - Cortical networks beneath the plasmalemma, often associated with microtubules. - Transvacuolar strands — bundles of actin microfilaments that cross the central vacuole and connect the nucleus to the periphery (Takatsuka et al., 2023). - Actin rings in guard cells and in the growth zone of root hairs, enabling polarized growth. - Together with myosin XI, they form the motile apparatus for cyclosis — rapid movement of cytoplasm, organelles, and vesicles.

Interaction between microtubules and actin. In plant cells, these two systems do not work in isolation. They are coordinated via crosslinker proteins (e.g., calponin‑homology kinesins, formins AFH14, MAP190) and signaling pathways (ROP GTPases), which is required for complex morphogenetic processes: stomatal patterning, trichome branching, xylem element formation (Wu et al., 2025; Li et al., 2015).

Thus, the architecture of the plant plasmalemma and cytoskeleton combines conserved elements (lipid bilayer, tubulin, actin) with unique adaptations (phytosterols, GIPC, H^\+-ATPase, absence of centrosome, cortical microtubules), reflecting adaptation to a sessile lifestyle, presence of a rigid envelope, and the need to integrate signals at the whole‑organism level.

3. Structural units

The plasmalemma and cytoskeleton are built from discrete macromolecular components. Understanding their chemical nature and spatial organization is necessary to explain the mechanisms of cell dynamics, signaling, and compartmentation.

3.1. Lipid bilayer

Lipid bilayer is the fundamental structural basis of all biological membranes, including the plasmalemma. It is a double layer of amphiphilic lipids in which the hydrophobic “tails” face inward and the hydrophilic “heads” face the aqueous phase (cytosol and apoplast) (Mauseth, 2017; Beck, 2010).

Chemical composition

The plant plasmalemma contains three main groups of lipids (Evert, 2006; Gronnier et al., 2018):

Phospholipids. They constitute about 40–50% of all membrane lipids. Major classes: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositols (PI), including phosphorylated derivatives (PIP, PIP₂). Phospholipids form the bilayer matrix and participate in signaling (e.g., PIP₂ serves as a precursor of the second messengers IP₃ and DAG) (Gronnier et al., 2018; Su et al., 2025).
Glycolipids. In plants, glycolipids are mainly represented by glycosyl‑inositol‑phosphoceramides (GIPC) — complex sphingolipids containing long‑chain fatty acids (up to C₂₄) and oligosaccharide heads. They are concentrated in the outer leaflet of the plasmalemma and play a key role in forming lipid rafts, and also serve as salt‑stress receptors (bind Na^\+) (Su et al., 2025; Jaillais et al., 2024).
Sterols. Instead of cholesterol, typical of animals, the plant plasmalemma contains phytosterols (sitosterol, stigmasterol, campesterol). They regulate membrane fluidity and mechanical stability, and are required for lipid domain organization and H^\+-ATPase function (Gronnier et al., 2018; Evert, 2006).

The average lipid ratio in the tobacco (Nicotiana tabacum) plasmalemma is approximately 40% phospholipids, 40% glycolipids (mainly GIPC), and 20% sterols (Cacas et al., 2016, cited in Gronnier et al., 2018).

Properties of the lipid bilayer

Fluidity. At physiological temperatures, the lipid bilayer is in a liquid‑crystalline (fluid) state — lipids diffuse rapidly in the lateral plane, but flip‑flop from one monolayer to the other is rare. The degree of fluidity depends on temperature, length and saturation of fatty acid tails, and sterol content (Mauseth, 2017; Beck, 2010).
Asymmetry. The outer and inner monolayers differ in lipid composition. The outer leaflet is enriched in GIPC and phosphatidylcholine, whereas the inner (cytoplasmic) leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositols (including PIP₂). This asymmetry is maintained by active enzymes — flippases and scramblases (Jaillais et al., 2024).
Phase transitions. Upon cooling, the membrane can shift from a fluid to a gel (solid) state — lipids crystallize, reducing permeability and protein mobility. Plants can adapt to cold by increasing fatty acid unsaturation (Su et al., 2025).

Microdomains and nanodomains (lipid rafts)

The lipid bilayer is not homogeneous. It spontaneously forms regions with higher order (higher viscosity) — lipid rafts — enriched in sphingolipids (GIPC) and phytosterols. These domains are depleted in unsaturated phospholipids (Gronnier et al., 2018; Jaillais et al., 2024).

Current classification distinguishes: - Nanodomains (20–200 nm) — dynamic clusters of lipids and proteins that are not visible by light microscopy but are detected by super‑resolution methods (PALM, STED). They harbor signaling complexes (e.g., receptors FLS2, BAK1, and the NADPH oxidase RBOHD). - Polar domains (micrometer scale) — regions associated with cell polarization (e.g., the basal domain for PIN1, the apical domain for PIN2, the Casparian strip in the endodermis, where CASP proteins and GIPC are concentrated) (Jaillais et al., 2024).

Lipid rafts serve as platforms for assembling signaling complexes, endocytosis, and cytoskeleton association. Their disruption with methyl‑β‑cyclodextrin (which binds sterols) impairs signaling and growth (Gronnier et al., 2018).

Biogenesis of the lipid bilayer

Lipids of the plasmalemma are synthesized in the endoplasmic reticulum (ER) and the Golgi apparatus, and then delivered to the growing membrane via vesicular transport (exocytosis) (Evert, 2006; Beck, 2010). New lipids are inserted into the inner leaflet, and then flippases can transfer some lipids (e.g., glycolipids) to the outer leaflet. Asymmetry is established during biogenesis and constantly maintained (Jaillais et al., 2024).

Functions of the lipid bilayer

Barrier function. The bilayer is impermeable to ions and polar molecules, thus ensuring protoplast compartmentation (Mauseth, 2017).
Matrix for membrane proteins. Lipids create a two‑dimensional solvent in which integral proteins maintain their structure and mobility.
Signaling involvement. Phospholipases (PLC, PLD) hydrolyze phosphatidylinositols and phosphatidylcholine, generating second messengers (inositol trisphosphate, diacylglycerol, phosphatidic acid) that activate protein kinases and mobilize Ca²⁺ (Su et al., 2025).
Fluidity regulation and stress adaptation. By altering the ratio of saturated/unsaturated fatty acids and sterol content, the cell maintains optimal membrane fluidity under temperature fluctuations and salinity (Su et al., 2025).
Formation of specialized domains. By assembling certain receptors and transporters in rafts, the bilayer organizes signaling pathways and polar transport (Gronnier et al., 2018; Jaillais et al., 2024).

3.2. Membrane proteins

While the lipid bilayer forms a continuous matrix, proteins provide the specificity of membrane function. They account for 50 to 75% of the plasmalemma mass and perform virtually all of its dynamic functions: transport, reception, catalysis, intercellular adhesion, and cytoskeleton linkage (Evert, 2006; Beck, 2010).

Membrane proteins are polypeptides integrated into the lipid bilayer or associated with its surface. According to their mode of interaction with the membrane, they are divided into three classes (Mauseth, 2017; Gronnier et al., 2018):

Integral (transmembrane) proteins span the bilayer once or several times. Their transmembrane domains consist of hydrophobic amino acids packed into α‑helices or, more rarely, β‑barrels. Examples: H^\+-ATPase, aquaporins (PIP), receptor kinases (FLS2, BRI1), transporters (PIN, AUX1).
Integral monotopic proteins are embedded in the bilayer only partially, anchored in one leaflet (e.g., via acyl chains — myristoylation, palmitoylation) or associated with integral proteins. Example: small ROP GTPases, which are attached to the inner leaflet via lipid modifications.
Peripheral proteins have no direct contact with lipids; they attach to integral proteins or to polar lipid heads via electrostatic interactions. They easily dissociate upon changes in ionic strength or pH. Example: some subunits of the H^\+-ATPase, cytoskeletal proteins (formins that bind the plasmalemma).

Chemical composition and post‑translational modifications

Plant membrane proteins are synthesized on the rough endoplasmic reticulum (RER) and then processed in the Golgi apparatus (glycosylation, lipid modifications, limited proteolysis) (Evert, 2006). The most important modifications are: * N‑glycosylation (attachment of an oligosaccharide to asparagine) — typical of receptor kinases and plasmodesmata proteins (PDLP). It participates in folding and ligand recognition. * Lipid modifications (myristoylation, palmitoylation, prenylation, GPI anchor) target proteins to specific membrane domains. For example, the GPI anchor localizes PDCB proteins to plasmodesmata nanodomains (Zavaliev et al., 2016, cited in Brunkard, 2024). * Phosphorylation (attachment of a phosphate group to serine, threonine, or tyrosine) — a key regulatory mechanism of transporter and receptor kinase activity (e.g., autophosphorylation of BRI1 upon brassinosteroid binding) (Su et al., 2025).

Major classes and functions of membrane proteins

Transport proteins

They ensure selective passage of molecules across the membrane. They are subdivided into (Evert, 2006; Mauseth, 2017): - Channels — transmembrane pores that open under specific conditions (membrane potential, ligand binding, mechanical stretch). They mediate passive transport down the concentration gradient (K⁺, Ca²⁺, Cl^- ion channels, aquaporins). Examples: MCA (mechanosensitive channels), AKT1 (potassium channel). - Carriers — bind the substrate and change conformation, shuttling it across the membrane. They work either passively (uniporters, cotransporters) or secondarily actively (symporters and antiporters using the proton gradient). Examples: AUX1 (auxin transporter), BOR1 (boron transporter), NIP5;1 (water and boron transporter). - Pumps (ATPases) — use ATP hydrolysis energy to actively transport ions against their gradient. The main plant pump is the P‑type H⁺-ATPase (AHA1, AHA2). It generates the proton‑motive force used by all secondary active transporters. V‑type H^\+-ATPase and H⁺-pyrophosphatase are present in the vacuolar membrane (tonoplast), but on the plasmalemma the H^\+-ATPase dominates (Evert, 2006).

Receptor proteins

They recognize extracellular signals (phytohormones, peptides, pathogen‑associated molecules, salts, light) and trigger intracellular cascades. Plants have a large number of such proteins (Gronnier et al., 2018; Su et al., 2025): - Receptor‑like kinases (RLK) — transmembrane proteins with an extracellular recognition domain (LRR, LysM, Lec) and a cytoplasmic kinase domain. Classic examples: FLS2 (flagellin receptor), BRI1 (brassinosteroid receptor), CLV1 (meristem regulator). - Receptor‑like proteins (RLP) — have an extracellular domain but lack a kinase domain; they interact with RLK co‑receptors. Example: Cf‑4 (receptor for Cladosporium fulvum). - Non‑kinase receptors — e.g., CNGC (cyclic nucleotide‑gated channels) that may directly bind Ca²⁺ and be modulated by calmodulin.

Adhesion proteins and cell wall‑linker proteins

They connect the plasmalemma to the supramembrane matrix (cell wall) and to the cytoskeleton. These include (Gronnier et al., 2018; Beck, 2010): - Extensins — hydroxyproline‑rich glycoproteins that form a network in the primary wall and bind to the plasmalemma. - Arabinogalactan proteins (AGP) — anchored in the outer leaflet via GPI; involved in growth‑related signal transduction. - Formins — transmembrane proteins that also bind actin microfilaments and microtubules, providing mechanical linkage between the cytoskeleton, the plasmalemma, and the wall (Wu et al., 2025).

Plasmalemma enzymes

They directly catalyze reactions at the cell–environment interface. The most important example is the cellulose synthase complexes (CSC, rosettes) — large transmembrane complexes assembled from CESA proteins that synthesize cellulose chains from UDP‑glucose and extrude them directly into the apoplast (Beck, 2010; Evert, 2006). Other enzymes: NADPH oxidase RBOHD (produces superoxide anion during stress and immunity) (Su et al., 2025), as well as peptidases and phosphatases.

Proteins involved in endocytosis and exocytosis

They mediate membrane traffic between the plasmalemma and intracellular compartments (Evert, 2006; Li et al., 2015): - Clathrin and adaptor proteins (AP‑2, TPLATE) form coated pits for clathrin‑mediated endocytosis. - Dynamins — large GTPases that participate in vesicle scission. - SNARE proteins (e.g., SYP132) ensure vesicle fusion during exocytosis.

Organization into membrane domains and nanoclusters

Using super‑resolution microscopy (PALM, STED), it has been shown that many membrane proteins are not uniformly distributed but are gathered into nanodomains (20–200 nm in diameter) (Gronnier et al., 2018; Jaillais et al., 2024). For example, the receptor FLS2, co‑receptor BAK1, NADPH oxidase RBOHD, and the protein REM1.3 form stable clusters in resting cells that can enlarge or change mobility upon stimulation (flagellin, abscisic acid). Such nano‑organization provides: * High local concentration of signaling molecules (cooperativity); * Response selectivity (different stimuli activate different sets of nanodomains); * Rapid signal transmission due to restricted diffusion.

Biogenesis and dynamics of membrane proteins

Membrane proteins are synthesized on ribosomes of the rough ER and co‑translationally inserted into its membrane via the Sec61 translocon. From there, they are transported by vesicular traffic through the Golgi apparatus to the plasmalemma (exocytosis). Correct sorting is ensured by signal sequences and post‑translational modifications (Beck, 2010; Evert, 2006).

In the plasmalemma, proteins are not static: * Lateral diffusion is restricted both by lipid rafts and by linkage to the cytoskeleton. Proteins can move in the membrane at speeds from 0.01 to 1 µm²/s (measured by FRAP and SPT) (Gronnier et al., 2018). * Endocytosis and recycling constantly remove proteins from the membrane and return them, allowing rapid changes in plasmalemma composition in response to signals (e.g., PIN2 degradation upon excess auxin) (Li et al., 2015). * Selective proteasomal degradation via ubiquitination and delivery to the vacuole or lysosome provides long‑term regulation.

Functional significance

Collectively, membrane proteins perform the following key tasks: 1. Selective transport (maintaining ion and metabolite homeostasis). 2. Signal perception and transduction (responses to hormones, stresses, pathogens). 3. Cell wall biosynthesis and remodeling (cellulose synthases, matrix‑modifying enzymes). 4. Cell adhesion and mechanotransduction (linkage to cytoskeleton and wall). 5. Membrane traffic (endo‑ and exocytosis, maintenance of plasmalemma composition).

Thus, membrane proteins are the “workhorses” of the plasmalemma, and their dynamic organization into domains, continuous synthesis and turnover underlie the plant cell’s ability to adapt and develop.

3.3. Cytoskeleton: microtubules

Confocal imaging of cortical microtubules — ©Yuliya Krasylenko commons.wikimedia.org CC BY-SA 4.0
Cortical microtubules in hypocotyl cells of <span lang="la" class="biological-name">Arabidopsis thaliana</span> (confocal microscopy)
A network of cortical microtubules beneath the plasma membrane of Arabidopsis hypocotyl cells (labeled with GFP-MBD). The ordered transverse orientation, which determines the direction of cellulose microfibrils and thereby controls the shape and direction of cell growth, is clearly visible.

Microtubules (MTs) are polar, cylindrical polymers of tubulin that in the plant cell serve as “tracks” for transport, stiffening elements, and organizers of morphogenesis. Unlike animal cells, plant microtubules lack centrosomes, and their dynamics are subject to strict spatiotemporal regulation (Evert, 2006; Beck, 2010).

Chemical composition and structure

A microtubule is built from α‑ and β‑tubulin heterodimers. Each dimer binds one molecule of GTP (β‑tubulin) or GDP (α‑tubulin). Dimers polymerize head‑to‑tail to form 13 protofilaments that are twisted into a hollow tube ≈ 25 nm in diameter (Evert, 2006; Li et al., 2015).

Polarity. The microtubule has two ends: a fast‑growing plus‑end (rich in β‑tubulin) and a slow‑growing minus‑end (rich in α‑tubulin). In plants, plus‑ends are often directed toward the cell periphery or toward the plus‑ends of other MTs (Li et al., 2015).
Dynamic instability. Microtubules constantly grow (polymerize) and shrink (depolymerize) with characteristic frequencies. The switch from growth to shrinkage is called “catastrophe”, the reverse switch “rescue”. GTP hydrolysis in β‑tubulin incorporated into the polymer creates mechanical strain that promotes disassembly (Beck, 2010).

Microtubule‑associated proteins (MAPs)

Microtubule function is regulated by a large set of MAPs (Li et al., 2015; Wu et al., 2025): * Stabilizing proteins. MOR1 (Arabidopsis homologue of XMAP215) accelerates polymerization and suppresses catastrophes. CLASP binds to plus‑ends and stabilizes them, especially in the cortex. * Destabilizing proteins. Katanin — an enzyme that severs (cuts) microtubules, generating new free ends for polymerization (Beck, 2010). * Crosslinking proteins. MAP65 crosslinks antiparallel microtubules, forming cross‑bridges necessary for spindle and phragmoplast formation (Li et al., 2015). * Motor proteins (motors). Plants only have kinesins (mainly the Kinesin‑14 family with minus‑end directionality) and lack cytoplasmic dynein. Kinesins participate in organelle and vesicle transport and in microtubule reorganization. Calponin‑homology kinesins (KCH) can bind both microtubules and actin filaments, acting as crosslinkers (Wu et al., 2025; Groves et al., 2025).

Organization of microtubules in the plant cell

Plant cells lack centrosomes. Microtubule nucleation occurs by two pathways (Li et al., 2015; Beck, 2010): . On the nuclear envelope in G₂/prophase of mitosis and in interphase (forming radial arrays). . At the plasmalemma — the γ‑tubulin complex (γ‑TuRC) is anchored to the plasmalemma via Augmin and other proteins, allowing nucleation of cortical microtubules directly in the cortex.

Depending on the cell cycle phase and cell type, several specialized microtubule arrays are distinguished (Evert, 2006; Li et al., 2015): * Cortical microtubules (CMTs). Located beneath the plasmalemma in interphase and during cell growth. They are predominantly oriented transverse to the growth axis. They guide the movement of cellulose synthase complexes in the plasmalemma, thereby dictating the orientation of cellulose microfibrils and consequently cell shape (Beck, 2010; Wu et al., 2025). * Preprophase band (PPB). Forms in G₂ phase as a dense ring of microtubules and actin beneath the plasmalemma at the future division site. It persists until prometaphase. The PPB determines the plane of cytokinesis and positions the phragmoplast (Li et al., 2015). * Mitotic spindle. Forms in prophase/prometaphase from microtubules nucleated on the nuclear envelope (rather than on centrosomes). It ensures attachment to chromosomes via kinetochores and their segregation to the poles. * Phragmoplast. Appears in telophase. It is a double array of antiparallel microtubules (plus‑ends directed toward the division plane). Golgi‑derived vesicles move along them, fuse to form the cell plate. As the plate expands, the phragmoplast moves toward the periphery (Li et al., 2015).

Dynamic processes (polymerization/depolymerization)

Microtubules rapidly reorganize in response to hormonal signals, light, and stress. For example, auxin within minutes can reorient cortical microtubules from transverse to longitudinal, inhibiting cell growth (Li et al., 2015; Su et al., 2025). Reorganizations are mediated by MAP‑kinase cascades and regulation of katanin activity.

Functions of microtubules in the plant cell

Guiding cellulose synthesis (defining cell shape and growth direction) (Evert, 2006).
Formation and positioning of the preprophase band (determining the division plane) (Li et al., 2015).
Formation of the mitotic spindle (chromosome segregation) (Beck, 2010).
Organization of the phragmoplast (directed vesicular transport during cytokinesis) (Li et al., 2015).
Participation in polar transport (interacting with actin, they direct nuclear and some organelle movement) (Groves et al., 2025).
Reception and transduction of mechanical signals (via linkage with the plasmalemma and cell wall) (Gronnier et al., 2018).

3.4. Cytoskeleton: microfilaments (actin)

Microfilaments (F‑actin) are thin (≈ 6–8 nm) polar polymers of globular actin. In plant cells, they form dynamic networks and bundles that drive cytoplasmic streaming (cyclosis), organelle transport, and also participate in shape maintenance and polarized growth (Graham et al., 2014; Takatsuka et al., 2023).

Chemical composition and structure

Actin is one of the most conserved eukaryotic proteins. The monomer (G‑actin) binds ATP or ADP. Polymerization yields a helical filament (F‑actin) in which all subunits are oriented identically, creating polarity: the plus‑end (barbed end) grows faster, the minus‑end (pointed end) slower (Takatsuka et al., 2023).

Plants have several actin isoforms (e.g., ACT2, ACT7, ACT8 in Arabidopsis) that differ in tissue expression and dynamic properties (Graham et al., 2014).

Actin‑binding proteins (ABPs)

The dynamics and spatial organization of F‑actin are regulated by numerous ABPs (Takatsuka et al., 2023; Wu et al., 2025): * Nucleating proteins. Formins (e.g., AFH14) initiate actin polymerization and often bind to membranes (plasmalemma) or microtubules. The ARP2/3 complex creates branched actin networks (important for trichomes and intercellular connections) (Li et al., 2015; Takatsuka et al., 2023). * Depolymerizing proteins. Cofilin accelerates disassembly of old filaments and severs fragments, increasing dynamics. * Crosslinking and bundling proteins. Villin, fimbrin, and tubulin‑binding protein 9 (TMBP9) form actin bundles (actin cables) required for long‑distance transport. * Motor protein — myosin. Myosin XI (the plant analogue of animal myosin V) moves along F‑actin from the minus‑end to the plus‑end, transporting vesicles, organelles (mitochondria, chloroplasts, the nucleus) and driving cyclosis (Groves et al., 2025; Takatsuka et al., 2023).

Organization of actin microfilaments in the cell

Actin in the plant cell occurs in three main forms (Graham et al., 2014; Takatsuka et al., 2023): * Cortical networks. Thin filaments beneath the plasmalemma, often colocalizing with cortical microtubules. They participate in shape maintenance and early endocytosis. * Actin cables (bundles). Thick bundles that traverse the cytoplasm and transvacuolar strands. Along them, myosin XI moves organelles and cytoplasm (cyclosis). * Apical networks (caps). In polarly growing cells (root hairs, pollen tubes), actin forms a fine network in the growth zone (subapical region) that directs exocytosis and maintains Ca²⁺ gradients.

Dynamic processes

Actin dynamics are very rapid. The half‑life of F‑actin can be minutes or even seconds. Regulation occurs via signaling pathways involving ROP GTPases (Wu et al., 2025; Su et al., 2025). For example, abscisic acid triggers rapid depolymerization and reorganization of actin in guard cells, leading to stomatal closure (Takatsuka et al., 2023).

Functions of actin microfilaments

Cytoplasmic streaming (cyclosis). Myosin XI moving along F‑actin generates force that drives the entire cytoplasm, accelerating diffusion of metabolites and gases (Graham et al., 2014).
Organelle and vesicle transport. Myosin XI carries mitochondria, chloroplasts (during light‑induced movements), peroxisomes, and Golgi‑derived vesicles (Takatsuka et al., 2023; Evert, 2006).
Formation of the preprophase band. Actin filaments together with microtubules constitute the PPB and participate in determining the division plane (Li et al., 2015).
Polarized growth. In root hairs and pollen tubes, actin organizes a directed vesicle flow to the growing tip.
Nuclear movement. The nucleus is attached to actin cables and myosin, allowing it to move to the required cell region (e.g., toward a forming root hair or to the center before mitosis) (Groves et al., 2025).
Reorganization upon stress. During pathogen attack, osmotic shock, or heat stress, actin rapidly rearranges, leading to plasmodesmata closure and activation of defense programs (Su et al., 2025).
Interaction with microtubules. Via crosslinker proteins (KCH, formins), actin coordinates its functions with microtubules during division, growth, and signaling (Wu et al., 2025).

Thus, the plant actin cytoskeleton, despite the absence of specialized muscle structures, is a powerful dynamic motor ensuring intracellular traffic and integration of cellular processes.

3.5. Intermediate filaments

The third, least studied component of the eukaryotic cytoskeleton is intermediate filaments (IFs). In animals, they form a mechanically robust network that supports nuclear and cell shape and contributes to resistance to mechanical loads. The question of the existence and functions of intermediate filaments in plants has remained open for decades and is still debated (Evert, 2006; Beck, 2010).

Definition and general characteristics

Intermediate filaments are non‑polar, generally less dynamic (compared to microtubules and actin) filaments about 10 nm in diameter, built from fibrillar proteins (e.g., lamins, keratins, vimentin in animals). They do not bind nucleotides (ATP/GTP), lack motor activity, and primarily serve as a mechanical scaffold (Beck, 2010).

In higher plants, attempts to identify homologs of animal intermediate filament proteins (e.g., using antibodies to keratin or lamins) have long yielded contradictory results, and genomic analysis has not detected genes encoding classical IF proteins (Evert, 2006). However, it has been shown that plant cells contain structures resembling intermediate filaments by electron microscopy criteria (≈ 10 nm diameter, longitudinal striation), especially in the nuclear envelope region (Beck, 2010).

Functional analogue — the plant nuclear lamina (NMCP/CRWN)

In animals, the nuclear lamina (a meshwork of lamins beneath the inner nuclear membrane) imparts shape to the nucleus and participates in chromatin organization. Plant genomes do not contain lamin genes, but a similar role is performed by the NMCP (Nuclear Matrix Constituent Proteins) family in plants, called CRWN (CROWDED NUCLEI) in Arabidopsis (Groves et al., 2025).

Structure. CRWN1–CRWN4 proteins possess a long α‑helical coiled‑coil domain that allows dimerization and formation of filament‑like structures. They are localized at the nuclear periphery, beneath the inner nuclear membrane (Groves et al., 2025).
Properties. Unlike lamins, CRWN proteins are not intermediate filaments by primary structure, but they are functional analogues. They interact with inner nuclear membrane proteins (SUN, PNET2) and with chromatin (Beck, 2010; Groves et al., 2025).
Functions. Mutations in CRWN genes lead to altered nuclear shape (nuclei become more rounded or irregular), reduced nuclear size, and defects in chromatin organization and gene expression. CRWN proteins are required for proper nuclear positioning and perception of mechanical signals (Groves et al., 2025).

Thus, plants possess a nuclear lamina, but it is built from unique plant proteins (NMCP/CRWN) rather than lamins (Evert, 2006; Groves et al., 2025).

Debate on cytoplasmic intermediate filaments

For the cytoplasm, the presence of classical intermediate filaments (like keratins or vimentin) in plants has not been confirmed. Electron microscopy studies sometimes observe 10 nm filaments in the cytoplasm (especially in phloem cells or suspension cultures), but their protein nature has not been identified (Evert, 2006; Beck, 2010).

Some researchers have reported the isolation of proteins forming 10 nm filaments (e.g., fibroin from lentil or certain glycoproteins), but these data have not gained wide acceptance. Most likely, in plants, the mechanical protection and shape‑maintaining functions of the cytoplasm are performed by actin microfilaments, microtubules, and cell wall elements in conjunction with the plasmalemma (Beck, 2010; Wu et al., 2025).

Intermediate filaments in algae

In some green algae (e.g., Chara, Nitella), filaments with intermediate filament characteristics have been described that participate in forming the cortical layer. However, their molecular nature also remains unclear (Evert, 2006).

Conclusion

At the current state of knowledge, it can be stated: . Plants lack genes encoding typical animal intermediate filament proteins (lamin, keratin, vimentin). . The functions of the nuclear lamina are performed by plant‑specific NMCP/CRWN proteins, which form a filamentous network beneath the inner nuclear membrane (Groves et al., 2025). . In the cytoplasm, no protein that forms classical intermediate filaments has been reliably identified. Presumably, their role in the plant cell is substituted by other cytoskeletal components and the cell wall (Evert, 2006; Beck, 2010).

Thus, in botany courses, when describing the plant cytoskeleton, attention is focused on microtubules and actin microfilaments, while intermediate filaments are treated as a debatable and not fully understood topic.

4. Biogenesis and dynamic processes

The plasmalemma and cytoskeleton are not static structures that become fixed after their formation. Rather, they are constantly renewed, reorganized, and adapted to changing conditions (cell cycle phase, hormonal signals, stresses). Understanding the mechanisms of their biogenesis and dynamics is necessary to explain such fundamental properties of the plant cell as growth, division, polarity, and resistance to damage.

4.1. Biogenesis of the cytoplasmic membrane

The plasmalemma, like all other membranes of the endomembrane system, does not arise de novo but by division of existing membranes and vesicular transport (Evert, 2006; Beck, 2010).

Synthesis of lipids and proteins

Lipids (phospholipids, sterols, sphingolipids) are synthesized in the endoplasmic reticulum (ER). Enzymes for glycolipid synthesis are localized in the Golgi apparatus (Evert, 2006; Gronnier et al., 2018).
Membrane proteins are synthesized on ribosomes of the rough ER (RER). During co‑translational transport, they are inserted into the ER membrane via the Sec61 translocon. Subsequently, the proteins undergo folding, oligomerization, glycosylation, and other post‑translational modifications (Beck, 2010).

Vesicular transport to the plasmalemma

Mature lipids and proteins are delivered to the plasmalemma via the Golgi apparatus by vesicular transport (exocytosis). The flow scheme (Evert, 2006; Beck, 2010):

ER → (transport vesicles) → cis‑Golgi → medial‑Golgi → trans‑Golgi → secretory vesicles → plasmalemma.

In the Golgi apparatus, further glycosylation, sulfation, and synthesis of complex glycolipids and cell wall polysaccharides occur.
Secretory vesicles carrying membrane components bud from the trans‑Golgi and are directed to the plasmalemma. Their transport occurs along actin microfilaments (involving myosin XI) and possibly along microtubules (Takatsuka et al., 2023; Li et al., 2015).
Vesicle fusion with the plasmalemma is mediated by SNARE proteins (e.g., SYP132) and is regulated by signals that determine sites of active growth (polar exocytosis) (Su et al., 2025).

Biogenesis during cell division

During cytokinesis, the formation of the new plasmalemma of daughter cells is closely linked to the formation of the cell plate (Beck, 2010; Li et al., 2015).

In telophase of mitosis, the phragmoplast — an array of antiparallel microtubules, actin filaments, and Golgi‑derived vesicles — assembles at the equatorial plane.
Vesicles containing polysaccharides and membrane proteins fuse with one another, forming a disc‑shaped structure — the cell plate.
The membranes of the fusing vesicles become new portions of the plasmalemma that separate the two daughter cells. The vesicle contents form the middle lamella (the future intercellular matrix).
The phragmoplast and cell plate grow centrifugally until they reach the mother wall. At that point, the new plasmalemma fuses with the plasmalemma of the mother cell.

Insertion of new components into the growing membrane

In interphase cells, especially in elongation zones, the plasmalemma continuously expands via exocytosis. New lipids and proteins are inserted predominantly at specific sites (e.g., the apical zones of root hairs or the growing tips of pollen tubes) (Takatsuka et al., 2023). Old or damaged components are removed via endocytosis and targeted for recycling or degradation (to the vacuole/lysosome).

4.2. Cytoskeleton dynamics: polymerization and depolymerization

The plant cytoskeleton is a highly dynamic system capable of rapid reorganization in response to internal and external signals. The basis of dynamics is the ability of filaments to polymerize from soluble subunits and depolymerize with subsequent reuse of the material (Evert, 2006; Beck, 2010).

Microtubule dynamics

Polymerization of GTP‑bound α/β‑tubulin dimers occurs mainly at the plus‑end, which in plants is often directed toward the cortex or toward the plus‑ends of neighboring microtubules. Polymerization rates can reach 1–10 µm/min (Li et al., 2015).
Depolymerization (disassembly) can be triggered by GTP hydrolysis in dimers incorporated into the polymer, creating mechanical strain that promotes “catastrophes” — sudden switches from growth to shrinkage.
Regulation is carried out by MAPs (MOR1 accelerates polymerization, katanin severs microtubules, CLASP stabilizes plus‑ends) (Li et al., 2015).
Half‑life of cortical microtubules in interphase ranges from a few minutes to tens of minutes. During cell division (spindle formation, phragmoplast), reorganizations occur even faster (Beck, 2010).

Actin microfilament dynamics

Polymerization of G‑actin (bound to ATP) occurs predominantly at the plus‑end (barbed end). Rates can be very high (up to several tens of µm/min).
Depolymerization occurs at the minus‑end (pointed end) and is accelerated by cofilin, which binds to ADP‑actin and severs filaments.
Regulation is carried out by numerous actin‑binding proteins (nucleators — formins and ARP2/3, crosslinkers/bundlers — fimbrin/villin, depolymerases — cofilin) (Takatsuka et al., 2023; Wu et al., 2025).
Half‑life of actin structures can be as short as 10–30 seconds in dynamic networks (cortical actin) and longer in stable bundles (transvacuolar strands).

Impact of stresses on cytoskeleton dynamics

Cold. Induces microtubule depolymerization and actin stabilization (Su et al., 2025). Can lead to growth arrest and disruption of the mitotic spindle.
Heat. Enhances microtubule dynamic instability and actin depolymerization, leading to changes in cell shape and stomatal closure (Su et al., 2025).
Osmotic stress (drought, salinity). Actin rapidly depolymerizes in guard cells, causing stomatal closure. Microtubules change orientation, slowing root growth (Takatsuka et al., 2023; Su et al., 2025).
Mechanical damage and pathogen attack. Actin and microtubules reorganize at the wound/infection site, contributing to plasmodesmata closure and activation of defense signaling cascades (Gronnier et al., 2018; Brunkard, 2024).

4.3. Coordinated dynamics: endocytosis, recycling, and exocytosis

The plasmalemma and cytoskeleton function as a unified system where vesicular transport (endocytosis and exocytosis) is tightly coupled to cytoskeletal rearrangements.

Endocytosis

Endocytosis is the process by which a cell internalizes portions of the plasmalemma to form intracellular vesicles. In plants, it provides: * reutilization of membrane proteins and lipids; * removal of damaged components; * internalization of extracellular ligands (including some phytohormones and flagellin); * maintenance of plasmalemma composition during active growth.

The main pathway is clathrin‑mediated endocytosis (CME) (Gronnier et al., 2018; Li et al., 2015).

On the cytoplasmic side of the plasmalemma, adaptor proteins (AP‑2) and the plant‑specific TPLATE complex assemble and recruit clathrin.
Clathrin polymerizes, forming a “coat” that bends the membrane and pinches off a vesicle.
Dynamin (a GTPase) tightens the vesicle neck, completing scission.
Endocytosis depends on the actin cytoskeleton (especially at the membrane bending stage). Inhibition of actin suppresses CME (Takatsuka et al., 2023).

There are also clathrin‑independent pathways of endocytosis (via lipid rafts and caveola‑like structures, although classical caveolae are absent in plants). They participate in internalization of certain receptors and lipid material (Gronnier et al., 2018).

Exocytosis and recycling

Exocytosis (vesicle fusion with the plasmalemma) delivers new proteins and lipids and secretes extracellular polysaccharides and proteins. Vesicles can originate both from the trans‑Golgi (de novo exocytosis) and from early endosomes (recycling — return of proteins previously removed from the membrane) (Beck, 2010; Evert, 2006).

Recycling is particularly important for cell polarity: for example, auxin transporters PIN2 continuously cycle between the plasmalemma (apical domain) and endosomes. Actin and microtubules direct vesicles to the correct membrane region (Li et al., 2015; Groves et al., 2025).

Impact of stresses on membrane traffic

Under stress (heat, salt, pathogen), rapid modulation of endo‑ and exocytosis is observed. For example: * Upon pathogen perception via the FLS2 receptor, endocytosis of this receptor is enhanced, possibly as part of an adaptation mechanism (Gronnier et al., 2018). * Under osmotic stress, clathrin‑independent endocytosis of aquaporins PIP2;1 is activated, reducing membrane water permeability (Su et al., 2025). * Cytoskeleton damage (e.g., by herbicides) disrupts endocytosis and polar growth (Li et al., 2015).

Thus, the biogenesis and dynamics of the plasmalemma and cytoskeleton represent a continuous process of synthesis, transport, assembly, disassembly, and recycling. Coordination of these processes is provided by a complex network of signaling pathways, allowing the cell to rapidly adapt to changes in the external environment and internal needs.

5. Interactions with other cell components

The plasmalemma and cytoskeleton are not isolated structures. They form a single integrated network connecting all cell compartments (nucleus, vacuole, endoplasmic reticulum, plastids, mitochondria, cell wall) into a functional whole. This integration ensures coordinated growth, polarity, intracellular transport, and responses to external signals.

5.1. Interaction with the cell wall

Cell wall and plasmalemma are linked both mechanically and biochemically, forming a continuum important for morphogenesis and mechanotransduction (Gronnier et al., 2018; Beck, 2010).

Plasmalemma as a mediator of wall synthesis. The plasmalemma harbors cellulose synthase complexes (CSC). They synthesize cellulose microfibrils and extrude them directly into the apoplast. Cortical microtubules beneath the plasmalemma direct CSC movement, dictating the orientation of cellulose fibers and consequently the direction of cell growth (Evert, 2006; Wu et al., 2025).
Anchor proteins. Transmembrane proteins (extensins, AGPs, formins) and GPI‑anchored proteins link the plasmalemma to matrix components (pectins, hemicelluloses) (Gronnier et al., 2018). This linkage restricts lateral diffusion of membrane proteins and transmits mechanical stresses from the wall to the cytoskeleton.
Plasmodesmata (although not detailed here, mentioned for context). The plasmalemma forms a continuous sleeve of the plasmodesmal channel, connecting with the plasmalemma of the neighboring cell. This provides not only symplastic continuity of the cytoplasm but also membrane continuity (Evert, 2006; Brunkard, 2024). The cytoskeleton, especially actin, participates in regulating plasmodesmata permeability through changes in membrane tension and callose deposition (Takatsuka et al., 2023).

5.2. Interaction with the vacuole (tonoplast and transvacuolar strands)

Central vacuole occupies up to 90% of the volume of a mature plant cell, and its membrane (tonoplast) is closely associated with the cytoskeleton (Takatsuka et al., 2023).

Transvacuolar strands. These are bundles of actin microfilaments that traverse the vacuole, connecting the nucleus (and other organelles) to the peripheral cytoplasm. Along them, myosin XI directs the movement of the nucleus, vesicles, and organelles (Takatsuka et al., 2023; Groves et al., 2025).
Actin on the tonoplast. F‑actin is attached to the tonoplast via protein bridges (e.g., NET4). This linkage regulates vacuole shape: actin depolymerization leads to fusion of small vacuoles into larger ones, while excessive polymerization causes fragmentation (Takatsuka et al., 2023).
Signaling connection. The vacuole (via ion channels of the tonoplast) participates in regulating turgor pressure. Changes in turgor are sensed by the plasmalemma and cytoskeleton, triggering mechanosensitive cascades (Su et al., 2025).

5.3. Interaction with the endoplasmic reticulum (ER)

Endoplasmic reticulum and plasmalemma form specialized contact zones (membrane contacts) important for lipid exchange and signaling (Gronnier et al., 2018; Takatsuka et al., 2023).

ER–plasmalemma contact sites (EPCS). In these zones (enriched in proteins SYT1, MCTP, VAP27), the ER and plasmalemma membranes are brought close together (distance ~10–30 nm) without fusion. Direct transport of lipids and Ca²⁺ occurs across these contacts (Gronnier et al., 2018).
Cytoskeleton as tracks. Actin microfilaments and microtubules direct ER movement and anchor it to the plasmalemma at EPCS. Upon actin or microtubule depolymerization, the ER network collapses (Takatsuka et al., 2023).
Participation in plasmalemma biogenesis. The ER serves as the site of synthesis for most membrane lipids and integral proteins, which are then delivered to the plasmalemma via the Golgi apparatus (Evert, 2006).

5.4. Interaction with the nucleus

Nucleus is physically linked to the cytoskeleton via the LINC complex (Linker of Nucleoskeleton and Cytoskeleton) (Groves et al., 2025).

Structure of the LINC complex. In the outer nuclear membrane are KASH proteins (WIP, WIT, SINE), and in the inner nuclear membrane are SUN proteins. Their interaction in the perinuclear space creates a bridge across the nuclear envelope. KASH proteins bind to actin (via myosin) and/or microtubules (via kinesins), while SUN proteins bind to the nuclear lamina (CRWN) (Groves et al., 2025).

Nuclear movement. Thanks to the LINC complex and motors (myosin XI, kinesins), the nucleus can move along actin cables and microtubules. This is required for: * migration of the nucleus to the division site (preprophase band); * movement of the nucleus into a developing root hair or pollen tube (Groves et al., 2025); * response to mechanical stimuli (mechanotransduction).

Regulation of nuclear shape. The nuclear lamina (CRWN) is linked to SUN proteins, and through KASH proteins to the cytoplasmic cytoskeleton. This allows the cell to control nuclear shape and volume depending on external conditions (Beck, 2010; Groves et al., 2025).

5.5. Interaction with plastids and mitochondria

Mitochondria and plastids (chloroplasts, leucoplasts) move actively through the cytoplasm, using the cytoskeleton as transport tracks (Takatsuka et al., 2023; Evert, 2006).

Transport along actin. Movement of mitochondria and chloroplasts is carried out by myosin XI along actin microfilaments. Speeds can reach several µm/s. This ensures uniform distribution of organelles and their migration to sites with increased energy demand (e.g., toward the plasmalemma during active transport) (Takatsuka et al., 2023).
Attachment to the plasmalemma. In some cell types (e.g., leaf mesophyll), chloroplasts can be anchored at the plasmalemma via actin bridges, optimizing photosynthesis and gas exchange (Evert, 2006; Graham et al., 2014).
Effect on morphology. Actin reorganization under stress (e.g., drought) causes changes in chloroplast shape and positioning, reducing the risk of photooxidative damage (Su et al., 2025).

5.6. Integration into a unified system: symplast, apoplast, and signaling networks

Symplast — continuity of protoplasts. Through plasmodesmata, the protoplasts of neighboring cells are united into a single living system — the symplast. The plasmalemma retains its integrity, lining the plasmodesmal channels (Brunkard, 2024). The cytoskeleton in the plasmodesmata region regulates their permeability (Takatsuka et al., 2023).
Apoplast — extracellular environment. Cell walls and intercellular spaces form the apoplast. The plasmalemma is the main barrier between the symplast and the apoplast.
Signaling networks. The plasmalemma and cytoskeleton serve as platforms for the interplay of signaling pathways (calcium, ROS, hormonal). For example, under salt stress, GIPC at the plasmalemma bind Na⁺, activating Ca²⁺ channels and actin remodeling, leading to changes in gene expression in the nucleus (Su et al., 2025; Jiang et al., 2019).

Thus, the plasmalemma and cytoskeleton are central elements of cell integration: they connect compartments to each other, coordinate their functions, and ensure a coordinated response to environmental signals.

6. Applied significance

Fundamental knowledge of the structure and function of the plasmalemma and cytoskeleton has direct applications in agronomic practice. Understanding the mechanisms of transport, signaling, growth, and stress tolerance enables the development of new approaches to breeding, chemical crop protection, and biotechnology.

6.1. Herbicides acting on the cytoskeleton and membranes

Many commercially successful herbicides disrupt cytoskeleton dynamics or membrane function, leading to growth arrest and plant death (Li et al., 2015; Beck, 2010).

Tubulin inhibitors (microtubules). A classic example is dinitroanilines (trifluralin, pendimethalin). They bind to α‑/β‑tubulin, inhibiting microtubule polymerization. This blocks the formation of the preprophase band, the mitotic spindle, and cortical microtubules; as a result, cells cannot divide or direct cellulose synthesis. Symptoms include dwarfing, root swelling (“root collar” effect), and seedling death (Li et al., 2015). Other herbicides (e.g., profam) also disrupt microtubule organization.
Cellulose synthesis inhibitors (target the plasmalemma). Herbicides of the dichlobenil and isoxaben groups inhibit cellulose synthase complexes in the plasmalemma. This leads to cell wall weakening, lysis of growing cells, and death of sensitive plants (Beck, 2010; Su et al., 2025).
Actin cytoskeleton inhibitors. Cytochalasin and latrunculin (not used in agriculture as herbicides due to synthetic complexity and animal toxicity) are powerful tools for experimental studies of actin function. However, knowledge of their targets stimulates the search for selective agents that disrupt actin dynamics in plants (Takatsuka et al., 2023).
Membrane permeability disruptors. Contact herbicides such as bipyridyls (paraquat, diquat) generate reactive oxygen species (ROS) at the plasmalemma (via interaction with photosystem I). ROS cause lipid peroxidation, destroying the membrane and leading to rapid tissue desiccation. Diphenyl ethers (acifluorfen, oxyfluorfen) inhibit protoporphyrinogen oxidase, leading to accumulation of photodynamic tetrapyrroles and light‑dependent plasmalemma destruction (Su et al., 2025).

6.2. Improving tolerance to abiotic stresses

Abiotic stresses (drought, salinity, low and high temperatures) damage the plasmalemma and cytoskeleton. Plant tolerance correlates with the ability of these structures to adapt (Su et al., 2025).

Drought and osmotic stress. Drought leads to dehydration, increased apoplastic salt concentration, and deformation of the plasmalemma. Tolerant varieties are able to: * Rapidly synthesize and insert aquaporins (especially PIP2;1) into the plasmalemma to regulate water balance (Su et al., 2025). * Alter lipid composition (increase the proportion of unsaturated fatty acids) and sterol concentration to maintain membrane fluidity (Gronnier et al., 2018; Su et al., 2025). * Remodel the actin cytoskeleton, leading to stomatal closure and reduced transpiration (Takatsuka et al., 2023). * Activate the H^\+-ATPase to acidify the apoplast and maintain membrane potential (Evert, 2006).

Salt stress (NaCl). The plasmalemma contains a salt‑stress sensor — GIPC (glycosyl‑inositol‑phosphoceramides). Binding of Na⁺ to GIPC activates a Ca²⁺ channel, triggering the SOS signaling pathway, which includes transcription of genes that export Na⁺ via the SOS1 (Na^\+/H⁺-antiporter) (Su et al., 2025; Jiang et al., 2019). Breeding for increased SOS1 expression or altered GIPC composition can enhance salt tolerance. Also important is the ability to maintain a high K^\+/Na^\+ ratio, which depends on potassium channels AKT1 and their regulation (Su et al., 2025).

Low temperatures. Tolerant plant varieties (winter crops, frost‑hardy cultivars) are able to undergo cold acclimation: * Alter plasmalemma lipid composition (increase fatty acid unsaturation to preserve fluidity) (Su et al., 2025). * Synthesize antifreeze proteins that interact with the plasmalemma. * Stabilize cortical microtubules and actin, which are required for cell survival during freezing (Li et al., 2015; Takatsuka et al., 2023).

High temperatures. Heat‑shock tolerance correlates with the ability to maintain plasmalemma integrity and prevent its excessive fluidization. Breeding for membrane thermostability and for rapid induction of heat shock proteins (HSPs) that stabilize membrane proteins and the cytoskeleton is an important direction (Su et al., 2025).

6.3. Regulation of growth and development (phytohormones and their analogues)

The plasmalemma and cytoskeleton are targets and mediators of phytohormone action, a fact exploited in agricultural practice (Evert, 2006; Takatsuka et al., 2023).

Auxins stimulate cell elongation by activating the plasmalemma H^\+-ATPase (wall acidification) and remodeling the actin cytoskeleton (Takatsuka et al., 2023). Synthetic auxins (2,4‑D, dicamba) are used as herbicides against dicot weeds, causing unregulated growth and subsequent death.
Brassinosteroids regulate division and elongation via the BRI1 receptor (plasmalemma) and MAP‑kinase cascades, and also influence microtubule organization (Gronnier et al., 2018; Su et al., 2025). Brassinosteroid analogues (epibrassinolide) are used to increase yield and stress tolerance.
Abscisic acid (ABA) induces stomatal closure, which is mediated by actin microfilament remodeling in guard cells (depolymerization and formation of radial cables) and activation of Cl^- and K^\+ channels. Knowledge of these mechanisms allows the development of compounds that regulate transpiration (Su et al., 2025; Takatsuka et al., 2023).

6.4. Biotechnology: transgenic plants and genome editing

Understanding transport and signaling mechanisms enables the construction of transgenic plants with improved traits (Evert, 2006; Su et al., 2025).

H^\+-ATPase (AHA) genes. Overexpression of AHA2 in Arabidopsis and rice increases salt tolerance and nitrate uptake efficiency. This is a promising direction for improving mineral nutrition and stress tolerance in crops.
Aquaporins (PIP). Elevated expression of PIP2;1 in tomato and tobacco increases drought tolerance and recovery rate after water deficit (Su et al., 2025).
Auxin transporters (PIN) and their regulation. Editing PIN genes can alter root system architecture (penetration depth, branching) and consequently drought tolerance and fertilizer uptake efficiency.
Cytoskeleton and fiber quality improvement. In cotton, understanding actin and microtubule dynamics in fiber cells is used to breed for increased length, strength, and fineness of cotton fibers (Li et al., 2015).
Herbicide‑resistant crops. Genetic modification of plasmalemma targets (e.g., introduction of mutant forms of tubulin or EPSP synthase) allows the creation of crops resistant to specific herbicides (e.g., “Roundup Ready” — resistance to glyphosate via modified EPSP synthase, though not directly plasmalemma‑related, the principle is analogous).

6.5. Diagnosis of plant stress status

Measurements of plasmalemma and cytoskeletal parameters can serve as indicators of plant physiological status (Su et al., 2025).

Electrolyte leakage (conductivity measurement). When membranes are damaged, ions (K⁺, Na^\+, Cl^-) leak out of cells, increasing the electrical conductivity of the solution. This rapid test is used to assess frost tolerance, salt tolerance, and herbicide damage.
Plasmalemma thermostability. By testing the ability of tissues to withstand brief heating without membrane integrity loss (electrolyte leakage test), heat‑tolerant forms are selected.
Cytoskeleton state indicators. Although direct methods (immunofluorescence, GFP markers) are not yet routine in breeding, they are used in fundamental research to assess the impact of stressors on microtubule and actin organization (Li et al., 2015; Takatsuka et al., 2023).

6.6. Biological plant protection (use of microorganisms)

Some beneficial rhizobacteria and endophytic fungi can modulate plasmalemma and cytoskeleton functions in host plants (Brunkard, 2024).

Induction of systemic resistance. Rhizobacteria such as Pseudomonas or Bacillus can induce changes in the actin cytoskeleton and closure of plasmodesmata, thereby enhancing resistance to pathogens.
Symbiotic nitrogen‑fixers (rhizobia) and mycorrhizal fungi. During root hair invasion, rhizobia cause local remodeling of actin and microtubules, forming an infection thread. Understanding these processes is necessary to improve symbiosis efficiency (Brunkard, 2024).

Thus, the applied significance of knowledge about the cytoplasmic membrane and cytoskeleton spans key areas of agricultural biotechnology: development of stress‑tolerant and herbicide‑resistant varieties, design of new crop protection agents, and diagnosis of crop status.

References

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