Plastids

The main function of chloroplasts is the light‑dependent conversion of carbon dioxide and water into organic compounds with the release of oxygen. Photosynthesis includes the light phase (on thylakoid membranes) and the dark phase (Calvin cycle in the stroma). Chloroplasts contain pigments: chlorophylls a and b, as well as carotenoids (lutein, β‑carotene, xanthophylls), which participate in light absorption and photoprotection (Jarvis & López‑Juez, 2013; Evert, 2006). Photosynthesis provides the organic matter that forms the basis of crop yield.

Leucoplasts (amyloplasts, elaioplasts, proteinoplasts) serve as depots for starch, lipids, and storage proteins. Amyloplasts accumulate starch in potato tubers, root crops, and cereal endosperm; they also act as statoliths in the root cap, mediating gravitropism. Elaioplasts synthesize and store oils (rapeseed, sunflower, avocado seeds). Proteinoplasts contain storage proteins (aleurone grains in cereal and legume seeds) (Choi et al., 2021; Sierra et al., 2023; Evert, 2006).

Plastids are hubs for many biosynthetic pathways:

Chromoplasts accumulate carotenoids (lycopene, β‑carotene, xanthophylls), giving fruits and petals yellow, orange, and red hues. This has triple significance: attracting pollinators and seed dispersers, providing a visual ripening marker for harvest, and increasing nutritional value (β‑carotene is provitamin A). Chromoplasts often form from chloroplasts during fruit ripening (tomato, pepper, pumpkin) (Rodriguez‑Concepcion & Lu, 2026; Choi et al., 2021; Sadali et al., 2019).

Plastids synthesize precursors of abscisic acid (ABA) – xanthoxin – as well as cytokinins and some forms of jasmonic acid. In addition, plastids participate in retrograde signaling: under stress (drought, salinity, photodamage) they generate signaling molecules (reactive oxygen species, 3′‑phosphoadenosine‑5′‑phosphate (PAP), β‑cyclocitral, methylerythritol cyclodiphosphate (MEcPP)), which alter the expression of nuclear genes, adapting the cell to new conditions (Jarvis & López‑Juez, 2013; Altamura et al., 2024; Sierra et al., 2023).

Under heavy metals (Cd, Pb, As), salinity, or drought, characteristic ultrastructural changes occur in plastids: thylakoid swelling, increased number and size of plastoglobuli, starch accumulation or conversely its degradation. Plastids generate signals that trigger the expression of antioxidant genes and heat shock genes. Sensory plastids of the epidermis and phloem (SEP) specialize in perceiving pathogens and abiotic stresses, moving toward the nucleus and forming stromules (Altamura et al., 2024; Sierra et al., 2023).

Bright chromoplasts in petals attract pollinators, and in fruits attract seed dispersers. In some plants (e.g., Begonia species under deep shade), chloroplasts convert into iridoplasts – specialized plastids with ordered thylakoid lamellae that enhance light capture efficiency (Sierra et al., 2023). In xerophytes and succulents, plastids participate in water storage and UV protection.

Plastids are closely connected with the endoplasmic reticulum (lipid exchange), mitochondria (photorespiration, malate‑aspartate shuttle), peroxisomes (glycolate pathway), and the vacuole (autophagy of senescing plastids). These interactions occur via membrane contact sites (MCS) and stromules (Renna et al., 2026; Altamura et al., 2024).

For the agronomist, plastid functions are important in the following aspects:

Chloroplasts are the most studied plastid type, carrying out photosynthesis. They contain green pigments – chlorophylls a and b – as well as carotenoids (lutein, β‑carotene, violaxanthin, neoxanthin). Chloroplasts are found in all green parts of plants; they are especially numerous in leaf mesophyll and young stems (Evert, 2006; Wise, 2007, cited in Choi et al., 2021).

Shape, size, and position in the cell. In higher plants, chloroplasts are biconvex lens‑shaped or ellipsoidal, more rarely cup‑shaped or ribbon‑like (in some algae). Typical dimensions: length 5–10 μm, width 2–4 μm, thickness 1–2 μm. A typical mesophyll leaf cell contains 50 to 150 chloroplasts (Evert, 2006). They are located along the cell wall (peripheral position), optimizing light capture and facilitating gas exchange. When illumination changes, chloroplasts can move (phototaxis): under low light they accumulate on walls parallel to the leaf surface; under bright light they orient along walls perpendicular to the incident light to avoid photodamage (Jarvis & López‑Juez, 2013; Evert, 2006).

Chemical composition. Based on dry mass, chloroplasts contain: proteins (50–55%, mainly Calvin cycle enzymes and membrane proteins of photosystems); lipids (25–30%), predominantly galactolipids and phospholipids of thylakoid membranes; pigments (8–12%): chlorophyll a, chlorophyll b, carotenoids; nucleic acids (3–5%): plastome (circular DNA) and RNA; low‑molecular‑weight compounds (ascorbate, glutathione, starch, Mg²⁺, Fe²⁺). Carotenoids play a photoprotective role, quenching reactive oxygen species and contributing to non‑photochemical quenching of excess energy (Evert, 2006; Jarvis & López‑Juez, 2013).

Functions. The main function of chloroplasts is photosynthesis, comprising the light phase (on thylakoid membranes) and the dark phase (Calvin cycle in the stroma). In addition, chloroplasts carry out: fatty acid synthesis, amino acid synthesis, isoprenoid synthesis (carotenoids, tocopherols, phylloquinones), as well as temporary starch deposition. Chloroplasts participate in retrograde signaling, transmitting information about the redox status of the electron transport chain to the nucleus (Jarvis & López‑Juez, 2013; Altamura et al., 2024; Rodriguez‑Concepcion & Lu, 2026).

Constituent elements (substructure). The chloroplast is surrounded by two membranes (outer and inner), with an intermembrane space between them. The internal content is the stroma (a protein‑lipid matrix containing Calvin cycle enzymes, circular DNA, 70S ribosomes, starch grains, and plastoglobuli). Embedded in the stroma is the thylakoid system – flattened membrane sacs. Thylakoids form stacks – grana (10 to 100 thylakoids per granum). Grana are connected by stroma thylakoids (lamellae). Thylakoid membranes contain the pigment‑protein complexes of photosystems I and II, ATP synthase, and the cytochrome b₆f complex (Evert, 2006; Jarvis & López‑Juez, 2013).

It is important to emphasize that the thylakoid membrane is impermeable to protons, which is necessary for generating the electrochemical gradient in the light phase. The stroma contains 70S ribosomes (prokaryotic type), circular DNA (several copies per chloroplast), starch grains (transitory starch deposited during the day), plastoglobuli (lipid droplets containing tocopherols and quinones), and Calvin cycle enzymes (including Rubisco – up to 50% of soluble leaf protein). Chloroplasts divide by binary fission (independently of cell division), and their proteins are partly synthesized in situ (on plastid ribosomes) and partly imported from the cytosol via specialized TIC/TOC complexes (Jarvis & López‑Juez, 2013; Evert, 2006).

Developmental features. Proplastids in meristems differentiate into chloroplasts under the influence of light. In the absence of light, proplastids turn into etioplasts containing a prolamellar body. Upon illumination, etioplasts rapidly (within a few hours) transform into chloroplasts. In many plants, chloroplasts can also arise from leucoplasts (e.g., greening of potato tubers in the light) or from chromoplasts (rare regreening in citrus) (Choi et al., 2021; Evert, 2006; Rodríguez‑Concepción & Stange, 2013, cited in Choi et al., 2021).

Significance for agricultural science. Chloroplasts are the main source of organic matter in agricultural crops. Breeding for increased chloroplast numbers in leaves, higher photosynthetic efficiency, and resistance to photoinhibition are important directions for increasing yield. Understanding protein import mechanisms into chloroplasts and their retrograde signaling is used to create herbicide‑resistant and stress‑tolerant varieties (Altamura et al., 2024; Jarvis & López‑Juez, 2013).

Shape, size, and position in the cell. Chromoplasts are plastids specialized for the synthesis and accumulation of carotenoid pigments, which impart yellow, orange, or red colors to tissues. They exhibit great morphological diversity: shapes may be spherical, ellipsoidal, needle‑like, crescent‑shaped, spindle‑shaped, or even crystal‑like. Sizes range from 3 to 7 μm. In the cell, chromoplasts are usually distributed diffusely, without the strict peripheral positioning typical of chloroplasts. Shape and size depend on plant species, tissue type, and developmental stage: in ripening tomato fruits chromoplasts are rounded, in carrot (taproot) they are elongated, in buttercup petals they are angular (Sitte et al., 1980, cited in Evert, 2006; Sadali et al., 2019; Sierra et al., 2023).

Chemical composition. Chromoplasts are almost devoid of chlorophylls (their content does not exceed 0.1–0.5% of chloroplast levels), so they are incapable of photosynthesis. The main pigments are carotenoids:

Carotenoids accumulate in the stroma either as droplets (in plastoglobuli), or as crystals, or as fibrillar or tubular structures in complex with lipids and specific proteins (fibrillins). Lipid content in chromoplasts can reach 40% of dry mass, ensuring dissolution of hydrophobic carotenoids. Ribosomes and the plastome are retained, but their transcriptional activity is significantly reduced compared to chloroplasts. Proteins are less abundant in chromoplasts because many photosynthetic enzymes are degraded (Camara et al., 1995, cited in Choi et al., 2021; Evert, 2006).

Functions. The main functions of chromoplasts:

For agricultural practice, breeding for intense chromoplast coloration is directly related to the market quality of vegetables and fruits. Tomato varieties with high lycopene content (red) and carrots with high β‑carotene (orange) are created through genetic control of chromoplast differentiation (Giuliano et al., 2008, cited in Choi et al., 2021).

Types of chromoplasts (ultrastructure classification). Chromoplasts are the most heterogeneous category of plastids. Depending on the form of carotenoid‑containing structures, four main types are distinguished (Sitte et al., 1980, cited in Evert, 2006; Sadali et al., 2019):

Globular chromoplasts are considered the most widespread and evolutionarily ancient type (Camara et al., 1995, cited in Evert, 2006).

Development (origin). Chromoplasts can develop by several pathways (Choi et al., 2021; Sadali et al., 2019):

In some species, reverse transition – regreening (chromoplast → chloroplast) – is possible, e.g., in citrus upon blue light treatment, in cucumber and pumpkin. However, in most crops (tomato, pepper) this process is not observed (Choi et al., 2021; Rodriguez‑Concepcion & Lu, 2026).

Constituent elements (substructure). Like chloroplasts, chromoplasts are surrounded by two membranes (outer and inner). The thylakoid system is absent or reduced to single vesicles. The internal space is filled with stroma containing:

Molecular regulators. Key genes controlling chromoplast development:

Significance for agricultural science. Chromoplasts determine the commercial appearance and nutritional value of fruits and root crops. Understanding their biology is necessary for:

Shape, size, and position in the cell. Leucoplasts (from Greek leukós – white) are colorless plastids lacking photosynthetic pigments. They have a rounded, ovoid, amoeboid, or irregular shape; sizes are usually 1–3 μm (in some cases up to 5 μm). In the cell, leucoplasts are most often located near the nucleus or around the central vacuole, as well as in tissues not exposed to light: roots, tubers, seeds, endosperm, storage parenchyma of stems and bulbs. In amyloplasts (a type of leucoplast that stores starch), the shape is determined by the number and arrangement of starch grains: with one large grain they resemble a spindle; with many small grains they remain round (Carde, 1984, cited in Evert, 2006; Choi et al., 2021).

Chemical composition. Leucoplasts contain practically no pigments (chlorophylls and carotenoids). Their stroma contains:

Starch (up to 50–70% of dry mass in potato tuber amyloplasts) is a mixture of amylose and amylopectin. Lipids in elaioplasts accumulate as triacylglycerols (oils) (Evert, 2006; Sierra et al., 2023).

Functions and varieties. Leucoplasts perform three main functions depending on specialization (Jarvis & López‑Juez, 2013; Choi et al., 2021):

In the same species, different tissues may develop different types of leucoplasts, and in some cases a leucoplast can store two types of substances simultaneously (e.g., starch and protein in maize endosperm) (Evert, 2006; Sadali et al., 2019).

For the agronomist, leucoplasts have direct economic importance:

Constituent elements (substructure). Like all plastids, leucoplasts are surrounded by two membranes (outer and inner). The thylakoid system is completely absent (or represented by single vesicles). The stroma contains:

Interconversions. Leucoplasts have a high potential for transformation:

Leucoplasts are not passive storage depots. They actively participate in regulation of carbohydrate metabolism: in amyloplasts, starch is hydrolyzed to glucose and maltose, which are exported to the cytosol via specific transporters. In storage tissues, leucoplasts form a network with the endoplasmic reticulum and vacuole, coordinating synthesis, storage, and mobilization of reserves (Zeeman et al., 2010, cited in Jarvis & López‑Juez, 2013).

Special cases. In some plants, transitional forms between leucoplasts and other plastids exist:

Significance for agricultural science. Leucoplasts are key organelles for yield accumulation (starch, oils, proteins). Breeding for increased size and number of amyloplasts (potato, cereals), elaioplasts (rapeseed, sunflower), and proteinoplasts (wheat, maize) is a direct route to enhancing productivity. Understanding the mechanisms of leucoplast‑to‑chloroplast conversion is important for preventing tuber greening during storage and for managing root crop quality.

Shape, size, and position in the cell. Proplastids are the smallest and least differentiated of all plastid types. They have a rounded, ovoid, or amoeboid shape, typically 0.5–1.0 μm in diameter (rarely up to 1.5 μm). Proplastids are formed in meristematic tissues – root and shoot apical meristems, cambium, and in the seed embryo. In a dividing cell, proplastids are evenly distributed between daughter cells during mitosis. They can also be found in some differentiated cells that retain the ability to dedifferentiate (e.g., in cortex parenchyma upon wounding) (Jarvis & López‑Juez, 2013; Evert, 2006; Sadali et al., 2019).

Chemical composition. Proplastids are colorless, containing no chlorophyll; carotenoids are present only in traces (usually < 0.1% dry mass). The dry matter consists mainly of:

Energy metabolism of proplastids is maintained by oxidation of sugars imported from the cytosol; they do not photosynthesize nor store significant amounts of substances (Kirk & Tilney‑Bassett, 1978, cited in Evert, 2006; Sadali et al., 2019).

Functions. Proplastids perform a single but fundamental function – biogenesis of all plastid types. Depending on signals (light, tissue‑specific factors, hormones, developmental stage), a proplastid differentiates into:

In addition, proplastids are capable of autonomous division (binary fission), ensuring an increase in plastid number during cell and tissue growth. The rate of proplastid division is highest in meristems and decreases as the cell differentiates. Thus, proplastids are the “mother form” that maintains the plastid genome across cell generations (Jarvis & López‑Juez, 2013; Choi et al., 2021).

For agricultural science, understanding proplastids is important because:

Constituent elements (substructure). Proplastids are surrounded by two membranes (outer and inner), which already contain TOC and TIC translocators (translocon at the outer/inner chloroplast envelope), ready to import nuclear‑encoded proteins. The internal space – stroma – is filled with a fine‑grained matrix. The thylakoid system is completely absent. In the stroma one can distinguish:

It is important to emphasize that proplastids are not passive precursors but actively dividing and metabolizing organelles. They maintain their own genome in a replication‑competent state and are sensitive to light and hormonal signals that switch their differentiation. During aging or stress, can proplastids also form de novo from more differentiated plastids? No, the reverse pathway does not exist: highly specialized chromoplasts and mature leucoplasts do not revert to proplastids. Therefore, maintaining the proplastid population in meristems is critically important for perennial agricultural crops (fruit, berry, grape, forage grasses) (Thomson & Whatley, 1980, cited in Sadali et al., 2019; Sierra et al., 2023).

Division of proplastids. The mechanism of proplastid division resembles bacterial division and involves the protein FtsZ (homolog of bacterial cell division protein). In the proplastid stroma, an FtsZ ring forms, to which additional proteins (ARC6, PARC6, PDV1, PDV2) then assemble on the inner and outer membranes. This ensures accurate organelle division. Proplastid division is coordinated with the cell cycle: in meristems they divide approximately synchronously with the cell, whereas in differentiating tissues their division may continue after the cell has ceased dividing (Jarvis & López‑Juez, 2013; Evert, 2006).

Significance for biotechnology. Proplastids are ideal targets for plastome transformation because they:

On this basis, proplastids are used for the production of recombinant proteins (vaccines, antibodies, enzymes) as well as for biofortification (carotenoid enrichment).

Shape, size, and position in the cell. Etioplasts (from Greek etióo – to become pale, to languish) are an intermediate plastid form that arises from proplastids in etiolated seedlings of higher plants grown in complete darkness (or in tissues deprived of light). Etioplasts are round or oval in shape, 1.5–3 μm in size. In cells typical of etiolated grass leaves (wheat, barley, maize) or dicotyledons (pea, bean), etioplasts can occupy up to 20–30% of the cytoplasm volume. Upon illumination, they very rapidly (within 2–6 hours) transform into normal chloroplasts, making them an important model for studying the biogenesis of the photosynthetic apparatus (Boardman, 1968, cited in Jarvis & López‑Juez, 2013; López‑Juez, 2007; Evert, 2006).

Chemical composition. Etioplasts lack chlorophyll but contain protochlorophyllide (a chlorophyll precursor) in a protein‑bound form. Protochlorophyllide gives etioplasts a faint yellow‑orange tint (hence the name “etiolated” – pale). Composition by major components:

Starch is usually absent in etioplasts or occurs in microquantities. The stroma also contains small plastoglobuli.

Functions. Etioplasts do not photosynthesize. Their main function is “waiting for light”. In the dark, they accumulate the enzyme NADPH‑protochlorophyllide oxidoreductase (POR) and its substrate – protochlorophyllide. Upon the first exposure to light, the following occurs:

Thus, etioplasts are a specialized, adaptive form for survival in the dark, allowing the plant to “instantly” (within hours) begin photosynthesis upon emerging into the light. Without such preparation, chlorophyll synthesis in the dark would be impossible due to the lack of light required for protochlorophyllide reduction (Reinbothe et al., 2010, cited in Jarvis & López‑Juez, 2013; Choi et al., 2021).

Constituent elements (substructure). Etioplasts are surrounded by a double membrane. Their stroma contains a unique structure – the prolamellar body (PLB) – a regular three‑dimensional cubic lattice of membrane tubules (tubule diameter ~20 nm), composed mainly of lipids and POR‑protochlorophyllide complexes. Adjacent to the prolamellar body are prolamellar membranes (or proplastid thylakoids) – small flattened membrane vesicles that, upon illumination, turn into primary thylakoids and then into grana (Gunning, 2001, cited in Evert, 2006; Jarvis & López‑Juez, 2013).

The stroma also contains:

Upon illumination, the prolamellar body rapidly (within 1–2 hours) disassembles, and its lipids and proteins are rearranged into thylakoid membranes. This process is one of the most striking models of membrane biogenesis in plant cells (Sundqvist & Dahlin, 1997, cited in Jarvis & López‑Juez, 2013).

Development. Under natural conditions, etioplasts form from proplastids in seed embryos germinating in the dark (under a soil layer). As soon as the seedling reaches the surface and is illuminated, etioplasts rapidly transform into chloroplasts. If the plant remains in the dark for a prolonged period (several weeks), etioplasts may undergo degeneration: the prolamellar body loses order, POR content decreases, and the plastids become unable to green normally (López‑Juez, 2007; Choi et al., 2021).

Regulation. Etioplast formation is controlled by repressors of photomorphogenesis – DET1, COP1 proteins, and PIF transcription factors (phytochrome‑interacting factors). In the dark, these repressors are active, and chloroplast development is blocked at the etioplast stage. Upon illumination, PIFs are degraded and COP1 is inactivated, triggering chloroplast differentiation (Choi et al., 2021; Altamura et al., 2024).

Significance for agricultural science. Although under field conditions fully developed etioplasts are rare (except in seedlings pushing through a thick soil layer or under mulch), understanding the etioplast stage is important for:

Brief summary. Etioplasts are a transient, strictly dark‑adapted form that does not exist in nature under normal illumination, but is extremely useful for experimental plant biology (study of thylakoid biogenesis, role of carotenoids in membrane formation) and for understanding light regulation of plastid development.

Below is a summarizing table that systematizes the key features of different plastid types, their substructure, functions, and examples of tissues. The table is based on information from the cited sources (Jarvis & López‑Juez, 2013; Evert, 2006; Choi et al., 2021; Sierra et al., 2023; Sadali et al., 2019; Rodriguez‑Concepcion & Lu, 2026; Altamura et al., 2024).

Notes on the table:

The best-studied pathway. In leaf cells in the light, the proplastid receives signals (via phytochromes, cryptochromes, retrograde signals from the plastids themselves) that trigger:

The process takes from several hours (in seedlings) to several days (in slow‑growing tissues). This pathway is characteristic of all green organs and forms the basis of photosynthetic activity in plants (Jarvis & López‑Juez, 2013; Evert, 2006; Choi et al., 2021).

In leaves growing in complete darkness, proplastids turn not into chloroplasts but into etioplasts. The leaf meristem is programmed to become a chloroplast, but the absence of light blocks the final stages of chlorophyll biosynthesis. As a result, protochlorophyllide accumulates and a prolamellar body (PLB) forms. Upon illumination, the etioplast rapidly converts into a chloroplast (see 3.6). This pathway is widely used in experimental physiology to study thylakoid biogenesis (López‑Juez, 2007; Choi et al., 2021).

In tissues developing in the dark (roots, tubers, endosperm) or in the light but genetically programmed for storage, proplastids differentiate into leucoplasts (amyloplasts, elaioplasts, proteinoplasts). Signals include low light levels, high sugar concentrations, and tissue‑specific transcription factors. During this process:

In potato tubers, amyloplasts form from proplastids in stolons during tuber initiation; this process is regulated by the phytohormone gibberellin (inhibits) and abscisic acid (activates) (Choi et al., 2021; Evert, 2006; Sierra et al., 2023).

In some fruits (watermelon, papaya) and in carrot callus, chromoplasts develop directly from proplastids, bypassing the chloroplast stage. In these cases, carotenoids accumulate without prior formation of the photosynthetic apparatus (Choi et al., 2021; Rodriguez‑Concepcion & Lu, 2026).

The fastest and most thoroughly studied transition. When etiolated seedlings are illuminated:

The process depends on the activity of carotenoid biosynthesis enzymes (carotenes are necessary for PLB formation). In mutants with defects in carotenoid synthesis, etioplasts either do not form or cannot differentiate normally (Choi et al., 2021; Evert, 2006).

Classic examples include ripening of tomato, pepper, citrus fruits, as well as petal senescence and autumn leaves. The process involves:

This transformation is irreversible in most crops (tomato, pepper, carrot). However, in some species (citrus, cucumber, pumpkin) the reverse process – regreening (chromoplast → chloroplast) – is possible under certain conditions (blue light irradiation, removal of the fruit from the plant) (Choi et al., 2021; Rodriguez‑Concepcion & Lu, 2026; Sadali et al., 2019).

Occurs in storage organs when a green organ switches to starch accumulation. The best‑studied example is the potato tuber. In young stolons, cells contain chloroplasts; as the stolon thickens and the tuber initiates, they lose chlorophyll, break down thylakoids, and turn into amyloplasts that accumulate up to 80% of dry mass as starch. Similarly, during legume seed maturation, chloroplasts of the seed coats turn into amyloplasts or proteinoplasts (Jarvis & López‑Juez, 2013; Evert, 2006; Choi et al., 2021).

Possible in tissues that initially developed in the dark or as storage organs but are then exposed to light. Classic examples:

In these cases, starch in leucoplasts is hydrolyzed, thylakoid membranes form anew, and chlorophylls and carotenoids are synthesized. This pathway is reversible: upon return to darkness, chloroplasts may re‑differentiate into leucoplasts (but the process is usually incomplete) (Evert, 2006; Choi et al., 2021).

Occurs in carrot roots (white roots contain amyloplasts, orange ones – chromoplasts), in the cauliflower Orange mutant (leucoplasts of inflorescences turn into chromoplasts with high β‑carotene content), and in the endosperm of transgenic rice and maize upon overexpression of carotenogenesis genes (Choi et al., 2021; Rodriguez‑Concepcion & Lu, 2026).

In the vast majority of plants, chromoplasts are terminally differentiated and do not turn into chloroplasts. Exceptions:

For agricultural practice, it is important that the color of fruits of most crops (tomato, pepper, carrot) is irreversible, so green fruits harvested unripe do not ripen to a bright color after harvest – the chloroplast‑to‑chromoplast transition requires an intact plant and photosynthetically active leaves.

During natural leaf senescence or under stress, chloroplasts turn into gerontoplasts – plastids with a degraded thylakoid system, enlarged plastoglobuli, and accumulation of lipophilic breakdown products. This process is irreversible and is aimed at reutilizing nutrients (nitrogen, phosphorus) from disassembled membranes and proteins. Gerontoplasts cannot revert to chloroplasts (Choi et al., 2021; Altamura et al., 2024).

The ability of plastids to change their type allows the plant to:

Plastid interconversions are not chaotic but strictly regulated processes involving nuclear genes (e.g., the GLK, GATA, RIN, NOR, OR transcription factor families) and plastid signals (redox status, tetrapyrrole accumulation, carotenoids). Understanding these pathways opens opportunities for biotechnological control over ripening timing, fruit quality, and stress tolerance of crop plants (Jarvis & López‑Juez, 2013; Rodriguez‑Concepcion & Lu, 2026; Choi et al., 2021).

Meristematic cells contain proplastids – small (0.5–1.0 μm) organelles with an underdeveloped membrane system. Before cell division, proplastids replicate their DNA and divide by binary fission, after which the daughter proplastids are distributed between daughter cells during mitosis (usually evenly, although cases of asymmetric distribution leading to cytoplasmic hereditary heterogeneity are known). The number of proplastids per cell is maintained with high precision; in Arabidopsis thaliana root meristem there are about 10–20 proplastids per cell, in tobacco up to 50 (Pyke & Leech, 1994, cited in Jarvis & López‑Juez, 2013).

The process of plastid division involves three stages:

As the cell exits the meristem and begins differentiation, proplastids, under the influence of signals (light, hormones, metabolic signals), transform into chloroplasts, leucoplasts, chromoplasts, or etioplasts (for details see sections 2 and 3). This process requires:

Import mechanism: the protein precursor is synthesized on cytoplasmic ribosomes with an N‑terminal signal peptide (transit peptide), binds to TOC receptors, and moves through a channel into the stroma, where the signal peptide is cleaved off by signal peptidase. More than 95% of plastid proteins are imported by this pathway (Jarvis & López‑Juez, 2013; Renna et al., 2026; Evert, 2006).

The level of protein import into plastids is dynamically regulated. A crucial mechanism is ubiquitination and subsequent degradation of TOC complex components by the membrane‑anchored E3 ubiquitin ligase SP1 (Suppressor of ppi1 locus 1). SP1 is localized on the outer plastid membrane and, upon certain signals (e.g., during plastid type switching), marks TOC receptors (Toc159, Toc33) for destruction by the 26S proteasome in the cytosol. This allows switching of import from a “photosynthetic” set of proteins to a “non‑photosynthetic” one and vice versa, which is necessary for plastid interconversions (Jarvis & López‑Juez, 2013; Sadali et al., 2019; Ling et al., 2012, cited in Sadali et al., 2019).

Plastids are not static organelles. They are capable of movement (taxis) along actin microfilaments with the involvement of motor proteins (myosins). Plastid movement has physiological significance:

Thylakoid membranes of chloroplasts are continuously renewed: photosystems (especially photosystem II) undergo repair after light damage (the D1 protein degrades and is resynthesized every 30–60 minutes). Plastoglobuli serve as reservoirs for lipids, tocopherols, and quinones, and participate in isoprenoid synthesis. Within the stroma, starch turnover is constant: during the day it is synthesized from assimilates, at night it is hydrolyzed to sugars that are exported to the cytosol. In storage‑type amyloplasts, starch accumulates up to a certain limit, but during seed or tuber germination, its mobilization begins (Zeeman et al., 2010, cited in Jarvis & López‑Juez, 2013; Evert, 2006).

Each plastid contains 10–100 copies of a circular DNA molecule (length 120 to 160 kb in higher plants). In young cells (meristems), the number of DNA copies per plastid is low, but during differentiation, especially in chloroplasts, copy number increases by endoreplication (up to 1000–10000 copies per cell in leaf mesophyll). The plastome contains about 110–130 genes encoding: components of photosystems (psa, psb), cytochrome complex (pet), ATP synthase (atp), ribosomal proteins (rpl, rps), tRNAs, and some other proteins (e.g., the large subunit of Rubisco; the small subunit is nuclear‑encoded).

Inheritance of plastids in most flowering plants is maternal (mitotic): plastids of the egg cell are transmitted to the offspring, while plastids of sperm cells usually do not enter the zygote or are degraded. This is important for breeding: traits encoded by the plastome (e.g., resistance to some herbicides) are transmitted only through the maternal line. In conifers and some flowering plants (e.g., Pelargonium), biparental (bilateral) inheritance has been described (Birky, 1995, cited in Jarvis & López‑Juez, 2013; Evert, 2006).

Plastids constantly monitor their status (oxidative stress level, energy charge, tetrapyrrole concentration, integrity of the photosynthetic apparatus) and send signals to the nucleus – this mechanism is called retrograde signaling. Signaling molecules include reactive oxygen species (singlet oxygen, H₂O₂), 3′‑phosphoadenosine‑5′‑phosphate (PAP), β‑cyclocitral, methylerythritol cyclodiphosphate (MEcPP), heme, and others. These signals modulate the expression of nuclear genes, adapting the cell to drought, salinity, high or low light, and pathogens (Jarvis & López‑Juez, 2013; Altamura et al., 2024; Sierra et al., 2023).

A key integrator of retrograde signals is the protein GUN1 (Genomes Uncoupled 1), located in plastids. Mutations in GUN1 disrupt retrograde signaling and render plants unable to properly regulate the expression of nuclear photosynthetic genes (Jarvis & López‑Juez, 2013; Pogson et al., 2015).

During cell aging or programmed cell death, plastids are degraded in two stages:

This process is especially intense during fruit ripening (destruction of chloroplasts during transition to chromoplasts is not complete degradation – the envelope and DNA are retained) and during leaf senescence. Released amino acids and sugars are transported to younger parts of the plant (Hörtensteiner & Kräutler, 2011; Izumi et al., 2017, cited in Altamura et al., 2024; Choi et al., 2021).

Understanding plastid biogenesis is used for:

Animal cells never contain plastids and are incapable of photosynthesis. An exception is some sea slugs (e.g., Elysia chlorotica) that temporarily retain plastids from ingested algae (kleptoplasty), but this is an acquired, not hereditary trait (Rumpho et al., 2008, cited in Evert, 2006). Thus, the presence of plastids is one of the main features distinguishing plants from animals at the cellular level (Yakovlev et al., 2008; Serebryakova et al., 2006).

Fungi have neither plastids nor any homologs of them. Even the most primitive fungi (chytridiomycetes, zygomycetes) lack endosymbiotic cyanobacterial precursors. This is one of the most important differences used in systematics to separate fungi from plants. In some parasitic flowering plants (e.g., Rafflesia, Monotropa), plastids are retained, although they lose photosynthetic function (genome reduction, loss of thylakoids). In fungi, however, plastids are absent even in a reduced form (Yakovlev et al., 2008; Serebryakova et al., 2006; Andreeva & Rodman, 2002).

The most important evolutionary conclusion: plastids are descendants of cyanobacteria, hence their ribosomes, division mechanisms (FtsZ protein), and DNA replication have prokaryotic features. It is on this similarity that the action of some antibiotics (e.g., streptomycin and spectinomycin) is based – they inhibit protein synthesis on plastid ribosomes (70S) without affecting cytoplasmic ribosomes (80S). However, the plant cell as a whole is eukaryotic, and its nuclear genome contains genes inherited from archaea (the informational apparatus) and from alpha‑proteobacteria (mitochondria) (Margulis, 1970; Keeling, 2010; Jarvis & López‑Juez, 2013).

Plastids are present not only in higher plants but also in:

For agricultural science, it is important that in crop plants, plastids are closest to those of green algae and differ from plastids of other eukaryotes in their pigment set (chlorophylls a and b in higher plants and green algae, whereas brown algae have chlorophylls a and c, red algae have phycobiliproteins). This knowledge is important for herbicide treatments (different algal groups may have different sensitivities to inhibitors of plastid processes) (Evert, 2006; Choi et al., 2021).

The presence of plastids is not just a diagnostic trait for distinguishing plants from animals, but also a key reason for differences in metabolism (ability for autotrophy), cellular architecture (need for chlorophylls and thylakoid membranes), and stress responses (e.g., photoinhibition). For the agronomist, understanding these differences is important when developing herbicides (acting selectively on plastids but not on animal cells) and when assessing possibilities for transgenesis (e.g., transfer of plastid genes from plants to animals is impossible under natural conditions) (Yakovlev et al., 2008; Serebryakova et al., 2006; Andreeva & Rodman, 2002; Evert, 2006).

The nucleus and plastids are linked by anterograde (from nucleus to plastids) and retrograde (from plastids to nucleus) signaling.

Anterograde regulation. The nucleus encodes the vast majority of plastid proteins (about 3000 types, including all Calvin cycle enzymes, photosystem components, TIC/TOC import proteins, transcription factors for the plastid genome). Expression of these nuclear genes is controlled by light, phytohormones, and tissue‑specific factors. For example, the GOLDEN2-LIKE (GLK) protein activates transcription of nuclear photosynthesis genes, leading to chloroplast development (Waters et al., 2009, cited in Pogson et al., 2015; Sierra et al., 2023). Photoreceptors (phytochromes, cryptochromes) via cascades of transcription factors (HY5, PIFs) regulate the expression of Photosynthesis‑Associated Nuclear Genes (PhANGs) and sigma factor genes that control plastid genome transcription (Jarvis & López‑Juez, 2013; Altamura et al., 2024).

Retrograde signaling. Plastids transmit information about their functional state to the nucleus: oxidative stress level, photosystem performance, tetrapyrrole accumulation, ATP and metabolite levels. Several signaling pathways are known:

Thus, the nucleus and plastids form a single regulatory network essential for photosynthesis, development, and stress adaptation.

The ER and plastids are closely linked by lipid and protein exchange. Lipids synthesized in plastids (e.g., fatty acids) are exported to the ER for incorporation into membranes or synthesis of extraplastidial lipids (phospholipids, sulfolipids). Conversely, some lipids synthesized in the ER (e.g., phosphatidylcholine) enter plastids. This exchange occurs via membrane contact sites (MCS) – regions where the outer plastid membrane and ER membrane come close (distance 10–30 nm), where lipid transfer proteins (TGD complex, VAP27-ORP2A) operate (Renna et al., 2026; Pan et al., 2024; Wang & Benning, 2012, cited in Renna et al., 2026).

Key contact site proteins:

In addition, a non‑canonical pathway for protein import into plastids via the ER and Golgi apparatus exists (glycosylated proteins, e.g., α‑amylase, carbonic anhydrase CAH1). These proteins are synthesized on the rough ER, pass through the Golgi, and are then delivered to plastids by vesicular transport (Villarejo et al., 2005; Kitajima et al., 2009; Renna et al., 2026).

Plastids are connected to the Golgi apparatus and the trans‑Golgi network (TGN) via vesicular transport. Glycosylated proteins destined for plastids (NPP1, NPP2, NPP6) pass through the Golgi, where they are modified, and then reach the plastid envelope in vesicles (Nanjo et al., 2006; Kaneko et al., 2016, cited in Renna et al., 2026). Furthermore, endosomes (multivesicular bodies) can interact with plastids, possibly participating in membrane recycling and signaling (Spitzer et al., 2015, cited in Renna et al., 2026; Altamura et al., 2024).

Mitochondria and plastids exchange metabolites and energy equivalents. In a leaf during the day, chloroplasts release oxygen and reduce NADP^\+ to NADPH, while mitochondria consume oxygen and generate ATP. Several key points of coupling:

Plastid‑mitochondria contact sites. In Arabidopsis, an MTL complex (mitochondrial transmembrane lipoprotein) has been described, including AtMic60, OEP7, and OMP24 proteins, which ensures transport of galactoglycerolipids between the organelles (Michaud et al., 2016, cited in Renna et al., 2026). Under phosphate starvation, the number of such contacts increases.

Peroxisomes (microbodies) in plants are closely linked to plastids in photorespiration and reactive oxygen species metabolism. In the photorespiratory cycle, glycolate from the chloroplast enters the peroxisome, where it is oxidized to glyoxylate with the formation of H₂O₂. Peroxisomal catalase decomposes H₂O₂. Glyoxylate is then transaminated to glycine, which goes to the mitochondrion. In return, serine exits the mitochondrion, is converted in the peroxisome to hydroxypyruvate and finally to glycerate, which returns to the chloroplast (Huang et al., 1983; Reumann & Weber, 2006, cited in Renna et al., 2026; Evert, 2006).

Peroxisomes and chloroplasts are apposed at a distance of 10–80 nm, and their interaction is mediated by the protein PEX10 (peroxisomal RING protein). In pex10 mutants, contact is disrupted and glycolate metabolism suffers (Schumann et al., 2007, cited in Renna et al., 2026). Optical tweezers have shown that peroxisomes form protrusions (peroxules) that physically connect to chloroplasts, facilitating metabolite transport (Gao et al., 2016, cited in Renna et al., 2026).

The vacuole acts as a depot for metabolites synthesized in plastids or the cytosol. During senescence or stress, the vacuole participates in autophagic degradation of plastids: chloroplasts (or their fragments) are surrounded by an autophagosome membrane and transported to the vacuole, where hydrolases break down their components, releasing amino acids and sugars for reutilization (Ishida et al., 2008, cited in Altamura et al., 2024; Izumi et al., 2017, cited in Jarvis & López‑Juez, 2013). In nectaries, amyloplasts closely adhere to the vacuole and transfer starch grains into it, which are then hydrolyzed to provide energy for nectar secretion (Pacini et al., 2003, cited in Renna et al., 2026).

Anthocyanins and other pigments accumulate in the vacuole, complementing the coloration produced by chromoplasts. The distinction: chromoplasts provide yellow, orange, red hues via carotenoids; anthocyanins in vacuoles give red, blue, purple colors.

Plastids move through the cytoplasm and are fixed in specific positions due to interactions with actin microfilaments. Plastid movement (chloroplast avoidance/accumulation responses) is mediated by myosins attached to the outer plastid membrane (Wada et al., 2003, cited in Jarvis & López‑Juez, 2013). Microtubules are involved in orienting plastids during cell division (ensuring equal distribution of proplastids between daughter cells) and in forming the prolamellar body in etioplasts (Evert, 2006).

In polarized cells (e.g., in the root cap), amyloplasts (statoliths) sediment to the lower part of the cell under gravity, interacting with actin filaments and transmitting the gravitational signal via pressure changes on membranes. This is a key mechanism of gravitropism (Kiss, 2000, cited in Evert, 2006).

Plastids are connected to neighboring cells via plasmodesmata – cytoplasmic channels that penetrate cell walls. Although plastids themselves do not pass through plasmodesmata (their size is too large), signaling molecules synthesized in plastids (e.g., PAP, β‑cyclocitral, ABA) can move through plasmodesmata. Stromules – tubular protrusions of plastids – can reach plasmodesmal fields and may participate in intercellular communication (Kwok & Hanson, 2004; Hanson & Sattarzadeh, 2011, cited in Renna et al., 2026; Altamura et al., 2024).

Although there are no direct physical contacts between plastids and the cell wall, photosynthesizing plastids supply the wall with sugars (cellulose, hemicelluloses are built from glucose coming from chloroplasts) as well as oxygen for wall oxidative enzymes (peroxidases). In turn, the cell wall determines cell shape, which affects plastid positioning (e.g., the peripheral positioning of chloroplasts in leaves).

Stromules are dynamic stroma‑filled tubular protrusions of plastids, covered by a double membrane and having a diameter of about 0.5 μm. They have been found in all plastid types, but especially often in non‑green plastids and in sensory plastids of the epidermis and phloem. Stromules can:

Stromules do not contain DNA or ribosomes, but soluble proteins (e.g., GFP) as well as macromolecules (up to 550 kDa) can move through them. It is proposed that stromules facilitate retrograde signaling and delivery of metabolites to the nucleus and other organelles (Hanson & Hines, 2018, cited in Altamura et al., 2024).

Understanding the interconnections of plastids with other compartments is important for:

Since plastids in most flowering plants are inherited maternally (through the egg cell cytoplasm), traits encoded by the plastome do not segregate according to Mendelian laws in crosses. This property is used in breeding for:

Plastids (primarily chloroplasts and proplastids) are a promising target for genetic transformation. Plastid transformation (plastome transformation) has a number of advantages over nuclear transformation:

Practical achievements of plastid biotechnology:

Many commercial herbicides act on processes localized in plastids, which explains their selective toxicity to plants (animals lack plastids). Key plastid targets (Jarvis & López‑Juez, 2013; Lichtenthaler, 1999, cited in Evert, 2006; Andreeva & Rodman, 2002):

Understanding the plastid localization of these targets allows the development of next‑generation herbicides with minimal impact on animals. Breeding of herbicide‑resistant crops (e.g., “Roundup Ready” maize and soybean) is achieved by introducing into the plant a mutant plastid (or nuclear) EPSPS gene insensitive to glyphosate (Daniell, 2002, cited in Sadali et al., 2019).

The conversion of chloroplasts into chromoplasts determines the color, taste, and aroma of many fruits (tomato, pepper, citrus, banana, watermelon, pumpkin). Agricultural significance:

The conversion of amyloplasts into chloroplasts in the light (greening) in potato tubers, carrot, and turnip roots reduces market quality and leads to the accumulation of toxic solanines (potato) or chlorophyll that impairs taste. Applied measures:

Some viruses and pathogenic bacteria attack plastids or alter their functions. For example:

Understanding plastids as an arena for pathogen‑plant interaction opens up ways to create resistant varieties, for example, by expressing antimicrobial peptides in plastids or suppressing viral proteins using RNA interference (Daniell et al., 2016, cited in Jarvis & López‑Juez, 2013). In addition, sensory plastids of the epidermis and phloem activate defense programs upon pathogen attack (e.g., the fungus Phytophthora infestans), moving toward the nucleus and forming stromules (Irieda & Takano, 2021, cited in Altamura et al., 2024; Sierra et al., 2023).

Since plastids participate in retrograde signaling (Chan et al., 2016, cited in Altamura et al., 2024), they can be used as biosensors (e.g., genetically encoded fluorescent proteins under the control of plastid promoters to monitor plant status). Enhancing retrograde signals that increase resistance to drought and heat stress is a promising area of biotechnology. Expression in plastids of genes encoding enzymes that destroy reactive oxygen species (superoxide dismutase, ascorbate peroxidase) has already been shown to increase tolerance of tobacco and tomato to oxidative stress (Foyer & Shigeoka, 2011, cited in Jarvis & López‑Juez, 2013). Furthermore, mutations in the MSH1 (MutS HOMOLOG 1) gene, localized in sensory plastids, cause epigenetic changes that can be inherited and increase resistance to abiotic stresses (Virdi et al., 2016; Xu et al., 2011, cited in Altamura et al., 2024; Sierra et al., 2023).

Increasing the content in plastids of essential amino acids, vitamins (A, E, K, C – partially synthesized in plastids), and iron (ferritin‑like proteins) is a task for classical breeding and genetic engineering. Examples:

Plastids serve as a source of valuable compounds used in industry and medicine:

The applied significance of plastids spans all stages of crop production: from breeding and biotechnology to plant protection, storage, and crop processing. Knowledge of plastid biology makes it possible to create crops with desired traits, develop environmentally friendly herbicides, increase the nutritional value of products, and reduce post‑harvest losses. For agricultural specialists, plastids are not only a cytological object but also a powerful tool for increasing the efficiency of agricultural production.