Endoplasmic Reticulum, Golgi Apparatus, and Ribosomes

The Golgi apparatus acts as a sorting station. It receives vesicles from the ER on its cis‑side (facing the nucleus) and, after a series of modifications, dispatches vesicles to the plasma membrane, vacuole or other targets from the trans‑side (Beck, 2010; Serebryakova et al., 2006).

Main functions:

Ribosomes are not part of the endomembrane system in the strict sense, but their work is inextricably linked to it (Graham et al., 2014).

For clarity, let us follow the path of a protein destined for the cell wall (e.g., an expansin):

Thus, the endomembrane system works as an assembly line where each organelle performs a strictly defined stage of processing and delivery of molecules, and ribosomes serve as the “loading device” at the entrance of this assembly line.

In plant cells, the Golgi apparatus (cytological literature often uses the term “dictyosomes”) is a set of individual stacks of membrane cisternae that do not fuse into a single network typical of many animal cells (Evert, 2006; Beck, 2010).

Although ribosomes are not membrane‑bound organelles and formally do not belong to the endomembrane system, their close functional and structural link with the rough endoplasmic reticulum makes their consideration necessary in this section (Graham et al., 2014; Mauseth, 2017). Ribosomes serve as universal “machines” for protein biosynthesis, operating both in the cytosol and on ER membranes.

The chemical composition of the endomembrane system (ER, Golgi apparatus) and ribosomes reflects their specialisation: membrane‑bound organelles are based on a lipid bilayer with integral proteins, whereas ribosomes are ribonucleoprotein complexes. Generalised data for plant cells are given below.

In plant cells, the ER participates in the synthesis of cell wall polysaccharides (via the Golgi apparatus) and in the formation of plasmodesmata (Strasburger et al., 1971; Evert, 2006). In animals, the ER is more specialised for protein export and lipid (cholesterol) synthesis (Beck, 2010). In fungi (e.g., Saccharomyces cerevisiae), the ER is less extensive but contains specific enzymes for ergosterol synthesis (Graham et al., 2014). Prokaryotes lack the ER: they have no internal membranes, except for rare invaginations of the plasma membrane in some photosynthetic bacteria (Mauseth, 2017).

The main difference in plants is the ability of the Golgi apparatus to synthesise matrix polysaccharides (pectins and hemicelluloses), which are not found in animal cells (Mauseth, 2017; Serebryakova et al., 2006). In fungi, the Golgi apparatus also participates in building the cell wall (synthesis of mannans and chitin), but the mechanisms are partly different (Evert, 2006). Prokaryotes lack the Golgi apparatus – all synthesis and secretion processes occur via the plasma membrane or extracellular vesicles (Graham et al., 2014).

Eukaryotic ribosomes (80S) are evolutionarily related to prokaryotic ribosomes (70S) but have additional proteins and rRNAs, reflecting the increased complexity of the translation apparatus (Beck, 2010). Importantly, plant mitochondria and chloroplasts contain 70S ribosomes that are sensitive to chloramphenicol and similar to prokaryotic ones – evidence of the symbiotic origin of these organelles (Graham et al., 2014; Mauseth, 2017).

Prokaryotic cells (bacteria and archaea) do not have an endomembrane system in the eukaryotic sense. Their cytoplasm contains no membrane‑bound organelles (ER, Golgi, nucleus, vacuoles) (Evert, 2006). Exceptions:

Thus, the endomembrane system is a key feature of the eukaryotic cell, enabling compartmentalisation of metabolism. In plants, it is additionally adapted for massive secretion of cell wall polysaccharides and vacuole formation.

These differences are fundamental for understanding plant physiology, stress tolerance, and for the development of herbicides (e.g., some herbicides disrupt lipid synthesis in the plant ER) (Graham et al., 2014).

ER membranes are synthesised de novo from lipids and proteins produced by the cell itself (Evert, 2006). The main source of new membrane lipids is the smooth ER, where enzymes (acyltransferases, desaturases) catalyse the formation of phospholipids (phosphatidylcholine, phosphatidylethanolamine) from fatty acids and glycerophosphate (Beck, 2010). Sterols (sitosterol) are synthesised in the smooth ER via the mevalonate pathway. ER proteins (channels, receptors, enzymes) are synthesised on RER membranes and inserted co‑translationally (Graham et al., 2014).

ER membrane growth occurs by insertion of new lipids and proteins into existing cisternae, as well as by fusion with vesicles returning from the Golgi apparatus (retrograde transport) (Evert, 2006). In dividing cells, fragments of the nuclear envelope (which is part of the ER) re‑form into a network of cisternae after mitosis (Serebryakova et al., 2006).

Transport between compartments of the endomembrane system is carried out by specialised vesicles coated with protein complexes that ensure budding and targeting (Evert, 2006).

In plants, vesicles with a coat containing reticulon (RTN) and other proteins involved in the formation of ER tubules have also been found (Evert, 2006).

Movement of vesicles and organelles (dictyosomes, ER) through the plant cytoplasm occurs mainly along actin microfilaments (Beck, 2010). Unlike animal cells, where microtubules and kinesins/dyneins play the major role, in plants the motors are myosins (Graham et al., 2014).

Microtubules also participate in organising the endomembrane system, especially in cells with polar growth (pollen tubes, root hairs), but to a lesser extent (Serebryakova et al., 2006).

Endocytosis in plant cells is less intensive than in animals but plays an important role (Evert, 2006). From the plasma membrane, via clathrin‑coated pits, internalisation of membrane proteins and fluid phase (pinocytosis) occurs. The resulting endosomes fuse with prevacuolar compartments (PVC), which then direct the content either to the vacuole for degradation or back to the plasma membrane (recycling) (Beck, 2010). Endocytosis also participates in retrograde transport of proteins from the Golgi apparatus to the ER (via COPI vesicles) (Graham et al., 2014).

Ribosomes are not static: they can dissociate into subunits, shift from the free pool to the membrane‑bound pool and back (Evert, 2006). Upon activation of secretion (e.g., during differentiation of endosperm cells), the proportion of membrane‑bound ribosomes can increase from 10–20% to 70–80% (Beck, 2010). Ribosomes do not actively move along the cytoskeleton but are passively carried by cytoplasmic streaming (cyclosis) (Mauseth, 2017).

Old or damaged membranes of the endomembrane system are removed by autophagy (Evert, 2006). Fragments of the ER and dictyosomes can be engulfed by a double‑layered membrane (autophagosome) and delivered to the vacuole, where their components are hydrolysed and reused (Serebryakova et al., 2006). This process is particularly active under stress (starvation, injury) and during programmed cell death (e.g., during tracheid differentiation) (Graham et al., 2014).

During plant cell division, the endomembrane system undergoes significant reorganisation (Beck, 2010):

Thus, biogenesis and dynamic processes of the endomembrane system in plants are characterised by high plasticity, active involvement of the actin cytoskeleton, and specialised mechanisms for cell wall formation during division. Understanding these processes is essential for interpreting many physiological phenomena, including growth, differentiation, and stress responses.

The outer membrane of the nuclear envelope is a direct continuation of the rough ER membrane (Evert, 2006; Beck, 2010). The perinuclear space is continuous with the ER lumen, forming a single cavity. Nuclear pores control the export of mRNA and ribosomal subunits into the cytoplasm (Graham et al., 2014). Ribosomal subunits assembled in the nucleolus exit through nuclear pores and, in the cytosol, can either attach to the ER membrane or remain free (Mauseth, 2017). Thus, the nucleus and the ER are functionally integrated into a single “transcription – translation – secretion” system.

The plasma membrane is constantly renewed by fusion with secretory vesicles budding from the trans‑Golgi network (Evert, 2006). The contents of these vesicles – proteins and polysaccharides (hemicelluloses, pectins) – are discharged into the apoplast and participate in the formation of the cell wall (Serebryakova et al., 2006). In growing cells, secretion is particularly active; simultaneously, endocytosis occurs – the retrieval of excess membrane areas back to the Golgi apparatus (recycling) (Beck, 2010). Plasmodesmata (intercellular channels) are functionally linked to the ER: desmotubules – derivatives of the ER – can pass through them, connecting the cytoplasms of adjacent cells (Strasburger et al., 1971).

The vacuole (central vacuole) is part of the endomembrane system: its membrane, the tonoplast, is formed from vesicles budding from the trans‑Golgi and from the ER (Evert, 2006). Through transport vesicles, hydrolytic enzymes (proteases, nucleases) are delivered to the vacuole, providing its lytic function (Graham et al., 2014). Seed storage proteins (e.g., wheat gliadins) are first synthesised on the RER, pass through the Golgi, and are packaged into protein bodies, which then fuse with the vacuole (Mauseth, 2017). Regulation of the ionic and osmotic balance of the vacuole depends on the activity of tonoplast transporters, which are delivered via the Golgi (Neuhaus & Trentmann, 2014).

Mitochondria and peroxisomes are not part of the endomembrane system but closely interact with the ER (Chustecki et al., 2024; Evert, 2006). In plant cells, membrane contact sites exist between the ER and mitochondria, involved in lipid exchange (phosphatidylcholine, phosphatidylethanolamine) and calcium signalling (Beck, 2010; Abe et al., 2024). Active movement of mitochondria along actin filaments is often directed by the cortical ER (Graham et al., 2014). Peroxisomes also interact with the ER, receiving membrane lipids and signals for their biogenesis (Evert, 2006).

Chloroplasts (and other plastids) are not part of the endomembrane system, but they are functionally linked to it, especially during photosynthesis and photorespiration (Serebryakova et al., 2006). During photorespiration, products of the glycolate cycle move from chloroplasts to peroxisomes and then to mitochondria – this metabolic flow requires coordination with membrane systems, but direct membrane fusion does not occur (Mauseth, 2017). Furthermore, chloroplasts receive proteins and lipids synthesised on the ER and processed in the Golgi (via vesicular transport) (Evert, 2006).

Ribosomes do not directly contact mitochondrial or chloroplast membranes, but they synthesise proteins that are subsequently imported into these organelles (Graham et al., 2014). Signal sequences at the N‑terminus direct the protein synthesised on free ribosomes into mitochondria or chloroplasts (Beck, 2010). Some ribosomes attached to the ER are involved in synthesising the membrane and secretory proteins that later end up in the tonoplast or plasma membrane (Mauseth, 2017).

The endomembrane system serves not only as a transport network but also as a platform for the synthesis of signalling molecules. For example, the ER membranes synthesise phosphatidylinositols, which upon hydrolysis yield inositol trisphosphate and diacylglycerol – important secondary messengers in calcium signalling (Evert, 2006). Stress conditions (heat, salt, drought) trigger reorganisation of the ER and Golgi, affecting the secretion of protective proteins (e.g., HSPs) and the formation of osmoprotectants (Graham et al., 2014).

Thus, the endomembrane system permeates the entire cell and interacts with virtually all compartments. These connections – structural, metabolic and signalling – ensure the integrity of the plant cell and its ability to rapidly adapt to changes in the external environment.

Cereal crops (wheat, rice, maize) accumulate storage proteins (gliadins, glutenins, zeins) in the endosperm. These proteins are synthesised on the rough ER and transported via the Golgi apparatus into vacuoles (protein bodies) (Serebryakova et al., 2006; Beck, 2010). Manipulating the expression of genes encoding ER components (e.g., the chaperone BiP) can increase the content and quality of gluten – a critical parameter for the baking industry (Mauseth, 2017). Furthermore, altering the activity of pectin methylesterases (enzymes that modify pectins in the Golgi) affects fruit texture and grain resistance to cracking (Evert, 2006).

The plant cell wall, formed with the participation of the Golgi apparatus, contains lignin and polysaccharides that hinder digestion by herbivores and the hydrolysis of cellulose for biofuels (Graham et al., 2014). Reducing the synthesis of pectins and hemicelluloses (e.g., by suppressing xyloglucan synthase genes) can increase the accessibility of cellulose to cellulases, which is important for bioethanol production from straw and wood (Mauseth, 2017). In forage crops (maize, alfalfa), decreasing cell wall lignification through targeted modification of ER and Golgi metabolism improves the nutritional value of silage (Evert, 2006).

Some herbicides target key enzymes of the endomembrane system. For example, inhibitors of acetyl‑CoA carboxylase (cycloxydim, fluazifop‑P‑butyl) block fatty acid synthesis in the smooth ER, disrupting membrane and cuticle formation and leading to the death of monocot weeds (Beck, 2010). Inhibitors of sterol synthesis (e.g., triadimefon) suppress squalene epoxidase activity in the ER, causing membrane destabilisation and reducing fungal resistance (Graham et al., 2014). Knowledge of targets within the endomembrane system allows the design of selective herbicides with minimal impact on useful crops (Evert, 2006).

Under drought, salinity, and heat stress, the activity of the ER and Golgi apparatus is reorganised: synthesis of osmoprotectants (proline, trehalose), chaperones, and antioxidant enzymes increases (Beck, 2010; Chustecki et al., 2024). Transgenic plants with enhanced expression of ER protein genes (e.g., BiP) show increased tolerance to water deficit (Serebryakova et al., 2006). Moreover, modifying the synthesis of cell wall polysaccharides in the Golgi can alter wall elasticity and the cell’s ability to retain water (Mauseth, 2017). Managing these processes is a promising direction for breeding for drought tolerance (Graham et al., 2014).

Plant cells are increasingly used for molecular farming – production of recombinant proteins (antibodies, vaccines, enzymes) in vacuoles (Evert, 2006). Signal sequences direct the synthesis of the recombinant protein into the ER and then into the vacuole, where it accumulates without affecting the cytosol (Beck, 2010). The ER also serves as a site for synthesis and accumulation of certain enzymes (e.g., phytase, xylanase) for feed additives (Graham et al., 2014). The use of signal peptides (mitochondrial and vacuolar) allows precise targeting of proteins to the desired compartments, increasing the yield of stable and active products (Abe et al., 2024).

Protein processing in the Golgi apparatus influences the production of effector molecules and defensive compounds (phytoalexins, flavonoids) (Mauseth, 2017). Disruption of sorting of certain hydrolases can lead to premature cell death and enhanced resistance to pathogens (Evert, 2006). In breeding of pea, soybean, and wheat, the glycosylation patterns of enzymes involved in detoxifying fungal toxins are taken into account (Graham et al., 2014). Furthermore, hemicelluloses and pectins synthesised in the Golgi are part of the barriers against phytopathogen penetration (Beck, 2010).

Although mitochondria are not part of the endomembrane system, their functional connection with the ER is important for the energy supply of the secretory pathway. Recent successes in delivering proteins and nucleic acids into plant mitochondria using synthetic peptides (mitochondrial signal sequences) open up possibilities for editing mitochondrial DNA (mitoTALEN, CRISPR) (Abe et al., 2024; Chustecki et al., 2024). This could lead to the creation of lines with cytoplasmic male sterility (CMS) without undesirable side effects, which is critically important for heterosis breeding (Serebryakova et al., 2006).

Thus, knowledge of the endomembrane system and ribosomes of the plant cell underlies many modern agrobiotechnological applications – from breeding for stress tolerance to production of recombinant proteins and management of crop quality. A deep understanding of these processes contributes to the development of “green” biotechnology and addresses food security challenges.