Mitochondria of the Plant Cell

This is one of the most important specialised functions of plant mitochondria. Under high light intensity and elevated temperature, the enzyme Rubisco (ribulose‑1,5‑bisphosphate carboxylase/oxygenase) begins to use O₂ instead of CO₂, leading to the formation of toxic phosphoglycolate. During photorespiration, this toxic metabolite is converted to glycolate, which moves from chloroplasts to peroxisomes and then to mitochondria. In mitochondria, glycolate is oxidised to glyoxylate with the formation of glycine. Then two molecules of glycine are converted into serine, releasing CO₂ and ammonia. Thus, mitochondria, chloroplasts and peroxisomes work in a tight metabolic partnership, with mitochondria playing a central role in this process (Graham et al., 2014; Selinski et al., 2024). Although photorespiration is considered energetically wasteful (it reduces net photosynthetic productivity), it serves a protective function, preventing photoinhibition and participating in the regeneration of NAD^\+ (Mauseth, 2017).

In specialised tissues of some plants (e.g., in the inflorescences of skunk cabbage, water lily, and certain magnolia species), mitochondria have the ability to produce heat. This occurs by diverting electron flow from the cytochrome pathway to the alternative oxidase (AOX). This pathway is uncoupled from ATP synthesis, and the energy of substrate oxidation is dissipated as heat. Heat production promotes the evaporation of volatile fragrant compounds that attract pollinating insects and may also protect reproductive organs from frost (Evert, 2006; Chustecki & Johnston, 2024).

In plant cell mitochondria, important metabolic pathways not directly related to ATP production are localised. These include:

Mitochondria are one of the main sources of reactive oxygen species (ROS) in the plant cell, along with chloroplasts and peroxisomes (Serebryakova et al., 2006). Under normal functioning of the respiratory chain, moderate amounts of superoxide and hydrogen peroxide (H₂O₂) are produced, which serve as important signalling molecules. They participate in signal transmission during stress (drought, salinity, pathogens), regulate the expression of defence genes, and even influence the cell cycle (Molina‑Moya et al., 2025; Beck, 2010). When the respiratory chain is disrupted (e.g., by inhibition of complexes I or III), the level of ROS rises sharply, which can lead to oxidative stress and the initiation of programmed cell death (Selinski et al., 2024). Thus, mitochondria act as sensors of the cell’s metabolic state and modulators of defence responses.

In plants, as in animals, mitochondria are involved in programmed cell death processes, for example, during xylem (vessel) differentiation or during the hypersensitive response to pathogens. However, the mechanisms differ from those in animals: in plants, cytochrome c does not appear to play a central role; rather, changes in mitochondrial membrane permeability, accumulation of ROS, and the release of other factors that activate plant‑type caspases are more important (Evert, 2006; Selinski et al., 2024; Chustecki & Johnston, 2024).

Directly linked to mitochondrial function is the phenomenon of cytoplasmic male sterility (CMS). This is caused by mutations in mitochondrial DNA that lead to impaired pollen development (Abe & Numata, 2024; Chustecki & Johnston, 2024). CMS is widely used in the breeding and seed production of hybrid crops (maize, sunflower, onion, carrot) to obtain heterotic hybrids without the labour‑intensive hand emasculation of flowers. Understanding the principles of mitochondrial function and the mechanisms of mitochondrial genetic transfer opens up prospects for editing the mitochondrial genome to increase crop yield and stress tolerance (Abe & Numata, 2024).

In higher plants, mitochondria generally take the form of individual rods, threads or rounded granules (hence their name — “thread‑like granules”). Typically, they are discrete (punctate) organelles, only rarely forming the extended networks characteristic of mitochondria in cultured animal cells (Chustecki & Johnston, 2024; Beck, 2010).

Mitochondrial dimensions vary, but on average they are:

Moreover, within the same cell, mitochondria of different shapes — from spherical to long filamentous — can coexist. This heterogeneity of shape is a consequence of constant division and fusion events (see Section 4). An exception is some specialised cells, e.g., in apical meristems or pollen tube cells, where mitochondria may be highly elongated (Evert, 2006; Chustecki & Johnston, 2024). In algae and lower plants, mitochondrial shape can be even more diverse (e.g., reticulate mitochondria in some green algae) (Graham et al., 2014).

The number of mitochondria in a plant cell varies widely. It depends on the metabolic activity of the cell, its type and stage of development. Young meristematic cells contain relatively few mitochondria (their number is comparable to the number of dividing nuclei), whereas differentiated cells with high energy demands may contain huge numbers.

(Chustecki & Johnston, 2024; Evert, 2006).

The number of mitochondria can increase rapidly through division when the cell’s energy demands rise (e.g., during the transition from dormancy to seed germination, during fruit ripening).

Unlike animal cells, where mitochondria move along microtubules, in plant cells their movement occurs along actin microfilaments (F‑actin strands). This process is ATP‑dependent and mediated by myosin motor proteins (Evert, 2006; Beck, 2010).

Mitochondria are not uniformly distributed but are located in specific regions of the cell where their function is most needed:

Although plant cell mitochondria usually exist as separate organelles, they constantly and actively divide (binary fission) and can temporarily fuse (fusion) with subsequent separation (a “kiss‑and‑run” phenomenon). This dynamic balance of division and fusion (mitochondrial physiognomy) is critically important for:

Disruption of the balance towards excessive fusion leads to the formation of giant reticulate mitochondria, while excessive division leads to excessive fragmentation. Both states are observed under various stresses (cold, drought, UV radiation) or mutations in genes regulating mitochondrial division (e.g., in the FRIENDLY gene) (Chustecki & Johnston, 2024; Evert, 2006).

Key takeaway for the grower/agriculturist: The localisation of mitochondria in leaf mesophyll cells in close association with chloroplasts and peroxisomes underscores their key role in photorespiration — a process that, in warm and dry climates, significantly reduces crop yields. Understanding the spatial organisation of these organelles is important for developing strategies to improve photosynthetic productivity.

The outer membrane is the outer boundary of the mitochondrion, separating the organelle from the cytosol. Structurally, it is a typical elementary membrane about 6–7 nm thick, which in electron micrographs appears as a three‑layered structure (two dark layers separated by a light gap) (Evert, 2006; Beck, 2010).

Chemical composition and properties. The outer membrane contains approximately 50% lipids and 50% proteins, but its lipid composition differs from that of the inner membrane. It is enriched in phospholipids (phosphatidylcholine, phosphatidylethanolamine) and sterols, which give it a certain fluidity. The most important feature of the outer membrane is the presence of numerous specialised channel‑forming proteins — porins (Evert, 2006; Yakovlev et al., 2008).

Permeability and functions. Due to porins, the outer membrane is permeable to molecules with a molecular mass of up to 5–10 kDa. This means that small molecules such as ions, sugars, amino acids and nucleotides can freely diffuse from the cytosol into the intermembrane space (Evert, 2006; Beck, 2010). Thus, the chemical composition of the intermembrane space with respect to low‑molecular‑weight compounds is practically identical to that of the cytosol.

An important plant‑specific point: unlike in animal cells, the plant mitochondrial outer membrane also contains specific transporters for metabolites involved in photorespiration and other plant‑specific pathways (e.g., for glycine and serine) (Graham et al., 2014; Selinski et al., 2024).

Absence of large membrane invaginations. Unlike the inner membrane, the outer membrane does not form folds or protrusions. It is smooth and closely apposed to the inner membrane but does not fuse with it. Between them remains a slit‑like space — the intermembrane space (more details in Section 3.3) (Mauseth, 2017; Serebryakova et al., 2006).

Interaction with other organelles. The outer membrane serves as a platform for anchoring mitochondria to the cytoskeleton (actin filaments) via specialised receptor proteins. This ensures directed movement of the organelle to sites of active ATP consumption (Evert, 2006; Chustecki & Johnston, 2024). In addition, the import of virtually all mitochondrial proteins synthesised on cytoplasmic ribosomes occurs through the outer membrane. For this purpose, the outer membrane contains the translocase of the outer membrane (TOM complex) — a complex protein structure that recognises mitochondrial signal sequences (pre‑sequences) and facilitates the initial steps of protein transport into the organelle (Evert, 2006; Abe & Numata, 2024).

The intermembrane space (perimitochondrial space) is the region enclosed between the outer and inner mitochondrial membranes. Its width is approximately 6–10 nm (Evert, 2006; Beck, 2010).

Chemical composition and features. As noted, due to the porins of the outer membrane, this space is freely permeable to low‑molecular‑weight substances (ions, sugars, amino acids, nucleotides) with a molecular mass below 5–10 kDa. Therefore, in terms of low‑molecular‑weight metabolites, the intermembrane space is almost identical to the cytosol (Evert, 2006; Mauseth, 2017).

However, it also contains a set of specialised proteins that are retained in this compartment because the inner membrane is impermeable to them. Among these proteins are:

Functional roles. The main functions of the intermembrane space are related to the creation of the proton gradient and participation in programmed cell death:

Important for the grower/agriculturist: Disruption of the integrity of the outer mitochondrial membrane, for example, under heat or salt stress, leads to uncontrolled release of signalling proteins from the intermembrane space, which can trigger premature cell death and reduce yield. Understanding these mechanisms is necessary for breeding stress‑tolerant varieties (Selinski et al., 2024).

The inner membrane of the mitochondrion is the key structural and functional element of the organelle. It houses the components of the respiratory chain, the electron transport systems and ATP synthase. It is highly selectively permeable and forms numerous folds — cristae — which greatly increase its working surface area (Evert, 2006; Mauseth, 2017).

Chemical composition and properties. The inner membrane consists of about 75–80% protein (including respiratory enzymes, transporters and translocases) and 20–25% lipids. The most important feature of its lipid composition is the high content of cardiolipin (diphosphatidylglycerol) — a unique anionic phospholipid that renders the membrane poorly permeable to protons and is essential for the functioning of respiratory complexes (Evert, 2006; Beck, 2010).

Unlike the outer membrane, the inner membrane is strictly impermeable to most charged molecules and ions (H⁺, OH⁻, K^\+, Na^\+, Cl^⁻, etc.), as well as to large uncharged molecules such as sugars and amino acids. Transport across it occurs only via specialised transporters (carriers) or channels, which ensure selective and often regulated metabolite transfer (Mauseth, 2017; Serebryakova et al., 2006).

Cristae: shape and significance. The inner membrane forms numerous invaginations towards the matrix — the cristae (from Latin crista — crest). Their shape and number vary depending on cell type, metabolic activity and even the systematic affiliation of the organism.

Botanical peculiarity: tubular cristae are considered a more primitive feature, yet they are also characteristic of most modern higher plants (Graham et al., 2014). It is believed that the tubular form provides more efficient packing of respiratory complexes and possibly better adaptation to changing light and temperature conditions (Chustecki & Johnston, 2024).

The surface of the cristae is covered with special protuberances — oxysomes (elementary particles) — mushroom‑shaped structures that contain ATP synthase (the enzyme that synthesises ATP) (Evert, 2006).

Functions of the inner membrane:

Key takeaway: The inner membrane of plant mitochondria is not only an energy centre but also a crucial regulator of metabolism. The alternative pathway of respiration (via AOX), absent in most animals, allows plants to avoid excessive ROS production under stress (drought, salinity, low temperatures) and is one of the targets for breeding for tolerance (Chustecki & Johnston, 2024; Selinski et al., 2024).

The matrix is the internal semi‑fluid content of the mitochondrion, filling the space enclosed by the inner membrane. In terms of chemical composition and physicochemical properties, the matrix is a complex colloidal system resembling the cytosol but with a unique protein and ionic composition (Evert, 2006; Mauseth, 2017).

Chemical composition. The matrix consists of 80–85% water and contains, in dissolved form, hundreds of different enzymes, ions (K⁺, Mg²⁺, Ca²⁺, phosphates), nucleotides (including ADP, ATP) and a number of coenzymes (NAD^\+, FAD). The matrix also contains (Beck, 2010; Evert, 2006; Yakovlev et al., 2008):

Main enzyme systems. A number of fundamental metabolic processes take place in the mitochondrial matrix:

Functions of the matrix:

Agronomic aspect: The efficiency of TCA cycle operation in the matrix directly determines the energy supply of the plant cell. Under stress (e.g., drought or hypoxia), the activity of TCA cycle enzymes slows down, reducing ATP yield and leading to the accumulation of intermediate metabolites that can serve signalling functions (Selinski et al., 2024). Breeding for stress tolerance includes the search for genotypes with more stable activity of matrix enzymes.

The nucleoid is a region in the matrix where mitochondrial DNA (mtDNA) and its associated proteins are concentrated. Under the electron microscope, the nucleoid appears as an electron‑transparent area with thin fibrillar DNA strands (Evert, 2006; Beck, 2010).

Organisation of the plant mitochondrial genome. Plant mtDNA has a number of unique features that distinguish it from animal and yeast mtDNA:

Functions of the nucleoid:

Key takeaway for the grower/agriculturist: The heterogeneity and recombinational activity of the plant mitochondrial genome create difficulties for its stable inheritance but also provide material for selection. Breeders use CMS genes located in nucleoids for hybrid seed production, which yields a 20–30% increase in yield for many crops.

Mitochondrial granules, also called osmiophilic granules or mitochondrial deposits, are small (20–50 nm in diameter) electron‑dense inclusions visible in the matrix by electron microscopy (Evert, 2006; Beck, 2010).

Chemical nature. Several types of granules exist, differing in chemical composition:

Functions of the granules:

Agronomic significance: Stresses related to iron deficiency in the soil (chlorosis) or excess calcium (salinity) directly affect the formation of mitochondrial granules and, consequently, the efficiency of cellular respiration and yield. Varieties that can efficiently store iron and buffer calcium in mitochondria exhibit higher stress tolerance (Chustecki & Johnston, 2024).

The most obvious difference concerns the shape of the inner membrane folds — the cristae.

A key biochemical difference of plant mitochondria is the presence of a branched respiratory chain. In addition to classical complexes I–IV and ATP synthase (complex V), plants possess:

Physiological significance: the alternative pathways allow plants to:

In animals and fungi, blockage of the main respiratory pathway (e.g., by cyanide) leads to rapid cell death, whereas many plants exhibit cyanide resistance precisely because of AOX (Mauseth, 2017).

The plant mitochondrial genome differs drastically from that of animals and fungi (data summarised from: Evert, 2006; Abe & Numata, 2024; Chustecki & Johnston, 2024; Graham et al., 2014):

This feature has fundamental significance: in plants, full mitochondrial function requires the exchange of gene products between organelles (complementation) — one of the reasons why plant mitochondria actively interact with each other through transient fusion events (Chustecki & Johnston, 2024).

A specific function of plant mitochondria, completely absent in animals and fungi, is participation in photorespiration (the glycolate pathway). Together with chloroplasts and peroxisomes, mitochondria oxidise glycolate and convert glycine to serine, releasing CO₂ (Graham et al., 2014; Selinski et al., 2024). This process is energy‑consuming but protects the photosynthetic apparatus from photoinhibition at high O₂ levels. Photorespiration does not occur in animal or fungal mitochondria because they lack chloroplasts and, consequently, Rubisco.

This difference is related to the overall organisation of the cytoskeleton: in plant cells, microtubules play more of a structural role, while rapid transport is carried out by actin filaments.

In animals, cytochrome c plays a key role in apoptosis: when mitochondria are damaged, cytochrome c is released from the intermembrane space into the cytosol and activates caspases (Evert, 2006; Beck, 2010). In plants, this mechanism does not appear to be the primary one.

Fungal mitochondria, like those of animals, usually have lamellar cristae, lack AOX (with rare exceptions in some phytopathogenic fungi), contain mtDNA of medium size (18–176 kbp), and move along microtubules (Evert, 2006; Graham et al., 2014). In yeasts Saccharomyces cerevisiae, mitochondria often exist as a network, especially under respiratory metabolism. However, unlike animals, fungal mitochondria may contain group I introns and exhibit recombination, albeit to a lesser extent than in plants (Evert, 2006).

Important note: Fungi do not participate in photorespiration (they lack chloroplasts) and generally lack alternative oxidase, although in some phytopathogenic species (e.g., rust fungi) AOX has been described as a factor conferring resistance to fungicides (Selinski et al., 2024).

Table 1. Summary of differences between mitochondria of plants, animals and fungi

Conclusion for agricultural education: Understanding the unique features of plant mitochondria — especially alternative respiration (AOX) and active mtDNA recombination — has direct applied significance. Alternative respiration is a target for improving plant tolerance to abiotic stresses (drought, salinity, extreme temperatures), and cytoplasmic male sterility, which is based on recombination events in mtDNA, is a key tool in modern hybrid breeding (Abe & Numata, 2024; Chustecki & Johnston, 2024).

Mitochondria, like plastids, possess semi‑autonomy, but their biogenesis requires coordination of two genetic systems — the mitochondrial and the nuclear (Abe & Numata, 2024; Evert, 2006).

Mitochondrial biogenesis includes:

A key rule: all mitochondria arise only from pre‑existing mitochondria (“omnis mitochondrio e mitochondrio”). They are not synthesised de novo from endoplasmic reticulum membranes (although individual lipids and proteins may be supplied via the ER‑Golgi pathway) (Evert, 2006). New mitochondria are formed by division of already existing ones.

An increase in the number of mitochondria in a cell (e.g., during differentiation or when energy demands rise) occurs through binary fission — constriction of the organelle into two approximately equal parts. In plants, this process is mediated by proteins resembling bacterial ones (FtsZ1, FtsZ2, as well as dynamin‑like proteins DRP3A/3B) (Chustecki & Johnston, 2024; Evert, 2006).

In living cells, mitochondria constantly undergo two opposing processes:

The balance between division and fusion determines the morphology of the mitochondrial population:

In plants, unlike animals and yeasts, homologues of the main fusion proteins (mitofusins, OPA1) have not been found. However, a number of regulators have been identified, including the protein FRIENDLY (FMT): its mutation leads to excessive mitochondrial fusion and formation of large clusters, accompanied by reduced organelle motility and developmental defects (Chustecki & Johnston, 2024; Evert, 2006).

Significance of fusion/division:

Mitophagy is a selective form of autophagy aimed at removing damaged, depolarised or old mitochondria. The process involves: recognition of the defective mitochondrion, isolation by a double membrane (phagophore), fusion with the vacuole (in plants) or lysosome (in animals), and subsequent enzymatic breakdown of the contents. The resulting monomers (amino acids, nucleotides, fatty acids) are returned to the cytoplasm for reuse (Chustecki & Johnston, 2024).

In plants, mitophagy is activated during leaf senescence, upon UV light exposure, during hypoxia/reoxygenation, as well as upon mitochondrial damage caused by mutations in mtDNA maintenance genes (Evert, 2006; Chustecki & Johnston, 2024). The protein FRIENDLY, in addition to regulating fusion, participates in mitophagy by helping the phagophore recognise damaged mitochondria (Chustecki & Johnston, 2024).

Agronomic significance: Regulated mitophagy allows crop plants to recover more quickly from stresses (drought, salinity, frost) and prolongs the period of active photosynthesis in leaves, which directly affects yield (Selinski et al., 2024).

Plant mitochondria are not passive particles but actively move around the cell (Chustecki & Johnston, 2024; Evert, 2006). Movement speed varies from 0.5 to 7–10 μm/s depending on cell type and physiological state.

Mechanism: mitochondria “ride” along actin filaments (microfilaments) using motor proteins — myosins (especially the myosin XI family). Microtubules are not directly involved in this movement in plants, but they may set the overall organisation of the guiding tracks (Chustecki & Johnston, 2024).

Physiological significance of movement:

Mitochondria not only receive “instructions” from the nucleus (anterograde signalling) but also send signals back, reporting their functional state, ROS levels and energy status (Selinski et al., 2024; Chustecki & Johnston, 2024).

A key retrograde signalling pathway in plants is the ANAC017‑dependent pathway (Selinski et al., 2024). Upon mitochondrial dysfunction (e.g., upon inhibition of the respiratory chain), the transcription factor ANAC017, anchored to the endoplasmic reticulum membrane, is released, moves to the nucleus, and activates the expression of genes, including the gene for alternative oxidase (AOX) (see Section 3). This allows the cell to adapt to stress without initiating programmed cell death.

Thus, dynamic processes (division, fusion, movement, mitophagy) and molecular signalling are closely intertwined, ensuring the integrity and functional flexibility of the mitochondrial network — a key condition for the survival and productivity of the plant organism.

Key takeaway: Regulating the balance of mitochondrial division/fusion (e.g., via the protein FRIENDLY) and maintaining efficient movement along actin filaments could be targets for biotechnological approaches aimed at increasing plant tolerance to abiotic stresses and improving overall cellular energy efficiency (Chustecki & Johnston, 2024).

In green plant tissues, mitochondria and chloroplasts are in constant metabolic contact. Their interaction is most evident in the processes of photorespiration and the exchange of carbon skeletons (Graham et al., 2014; Selinski et al., 2024).

Photorespiration (glycolate pathway): under high light intensity and elevated temperature, the chloroplast enzyme Rubisco uses O₂ instead of CO₂, leading to the formation of phosphoglycolate. The latter is converted to glycolate, which is exported from the chloroplast to the peroxisome and then to the mitochondrion. In the mitochondrial matrix, glycolate is oxidised to glyoxylate, and then two molecules of glycine are converted to serine, releasing CO₂ and ammonia. This pathway requires close spatial association of mitochondria, chloroplasts and peroxisomes, which are often observed in electron micrographs as a triad (Evert, 2006; Beck, 2010).

Malate and citrate shuttles: mitochondria export malate and citrate into the cytosol, from where they can enter chloroplasts. Malate serves to transfer reducing equivalents (NADH) between compartments, while citrate helps maintain the pool of 2‑oxoglutarate necessary for glutamate synthesis (Evert, 2006; Selinski et al., 2024).

ATP exchange: during the dark period, when photophosphorylation is inactive, mitochondria become the main suppliers of ATP for all cellular processes, including chloroplast metabolism (starch synthesis, lipid synthesis). In the light, conversely, chloroplasts supply mitochondria with excess photosynthetic products (sugars) and oxygen (Chustecki & Johnston, 2024).

Agronomic significance: understanding the relationship between mitochondria and chloroplasts in photorespiration is directly relevant to the problem of yield losses in hot and dry climates. Photorespiration reduces net photosynthetic productivity by 20–50% in C3 plants (wheat, rice, soybean). Breeding to reduce the rate of photorespiration or to increase the efficiency of CO₂ use by mitochondria is one direction for improving yields (Selinski et al., 2024).

Peroxisomes (microbodies) are closely linked to mitochondria both metabolically and often spatially (Evert, 2006; Graham et al., 2014).

Co‑operation in photorespiration: glycolate formed in chloroplasts first enters the peroxisome, where it is oxidised to glyoxylate with the formation of H₂O₂. Glyoxylate is then transaminated to glycine, which is exported to the mitochondrion. In the mitochondrion, glycine is converted to serine with the release of CO₂ and ammonia. Serine returns to the peroxisome, where the metabolic pathway is completed (Evert, 2006).

Detoxification of reactive oxygen species (ROS): catalase, which decomposes H₂O₂ formed during photorespiration, is localised in peroxisomes. However, if peroxisomal catalase is overloaded, H₂O₂ may enter mitochondria and participate in redox signalling (Molina‑Moya et al., 2025; Selinski et al., 2024).

β‑Oxidation of fatty acids: in germinating seeds (of oil crops), glyoxysomes (a type of peroxisome) contain the enzymes of β‑oxidation, converting fatty acids into succinate, which is then used by mitochondria to synthesise ATP via the TCA cycle (Graham et al., 2014; Evert, 2006).

The interaction of mitochondria with the ER (especially with the smooth ER) is multifaceted and is mediated by specialised contact sites — mitochondria‑ER contacts (MERCs) (Chustecki & Johnston, 2024; Neuhaus & Trentmann, 2014).

Lipid and calcium ion exchange: the ER supplies lipids for the assembly of mitochondrial membranes. Mitochondria, in turn, participate in the synthesis of some lipid precursors. Ca²⁺ ions released from the ER during signalling are taken up by mitochondria, affecting their metabolism and potentially serving as a signal to regulate respiration (Neuhaus & Trentmann, 2014; Selinski et al., 2024).

Regulation of mitochondrial division: in animal cells, the ER serves as a site where mitochondrial constriction and division occur. In plants, this role of the ER is less studied, but it has been shown that the protein FRIENDLY and other factors may act in association with ER membranes (Chustecki & Johnston, 2024).

Protein and lipid synthesis: ribosomes on the rough ER synthesise many mitochondrial proteins, which are then transported to mitochondria (Evert, 2006).

The relationship between mitochondria and the nucleus involves two opposing information flows:

Anterograde signalling (nucleus → mitochondria): nuclear genes encode the vast majority of mitochondrial proteins (about 99%), including all TCA cycle enzymes, most respiratory chain components, and factors for mtDNA transcription and replication. Thus, the nucleus determines the number, composition and activity of mitochondria (Evert, 2006; Abe & Numata, 2024).

Retrograde signalling (mitochondria → nucleus): upon mitochondrial dysfunction (e.g., upon inhibition of the respiratory chain or accumulation of ROS), signals are transmitted from the mitochondria to the nucleus, altering the expression of nuclear genes, including those encoding alternative oxidase (AOX) and antioxidant enzymes. This pathway is mediated by transcription factors such as ANAC017, which is released from the ER membrane upon stress (Selinski et al., 2024; Chustecki & Johnston, 2024).

Agronomic significance: the ability of mitochondria to transmit signals about their state to the nucleus allows the plant to rapidly adapt to stresses (heat, drought, salinity) by switching to alternative respiration (AOX) and enhancing antioxidant defence. This property is used in biotechnology to create stress‑tolerant transgenic crops (Abe & Numata, 2024).

The vacuole, especially the central vacuole, interacts with mitochondria indirectly via the cytosol, but direct contacts also exist:

Supply of substrates: the vacuole serves as a reservoir for organic acids (malic, citric), sugars, amino acids and ions. When necessary, these substances are mobilised from the vacuole into the cytosol and enter mitochondria for oxidation (Evert, 2006; Neuhaus & Trentmann, 2014).

Participation in mitophagy (autophagy): damaged mitochondria can be enclosed by a double membrane derived from the ER and then delivered to the vacuole for lysis. Thus, the vacuole functions as a “recycling centre” for worn‑out mitochondria (Chustecki & Johnston, 2024).

Osmotic regulation: the state of the vacuole (turgor) influences the location of the cytoplasm and, consequently, the distribution of mitochondria in the peripheral layer of the cell (Evert, 2006).

Interaction with the Golgi apparatus occurs mainly through vesicular transport.

Delivery of lipids and membrane proteins: vesicles budding from the Golgi can fuse with the outer mitochondrial membrane, supplying lipids for membrane growth and some proteins (although the main pathway for protein import is via the TOM complex, not vesicular) (Evert, 2006; Neuhaus & Trentmann, 2014).

Coordination in secretory cells: in cells that actively secrete mucus, proteins or polysaccharides (e.g., in root cap cells, glandular trichomes), mitochondria often concentrate near dictyosomes, providing them with the energy required for the synthesis and packaging of secretory products (Evert, 2006).

Key takeaway for agricultural education: The interconnections of mitochondria with chloroplasts, peroxisomes and the nucleus underlie such agronomically important processes as photorespiration (yield reduction in C3 plants), tolerance to drought and salinity (alternative oxidase, retrograde signalling), and the use of cytoplasmic male sterility in hybrid breeding. An integrated view of the cell as a system of interacting organelles is essential for developing comprehensive strategies to improve the productivity of agricultural crops (Chustecki & Johnston, 2024; Selinski et al., 2024).

Cytoplasmic male sterility (CMS) is a phenomenon in which a plant is unable to produce functional pollen but retains female fertility. The cause of CMS is mutations in mitochondrial DNA that encode abnormal proteins that disrupt microspore development or the function of the tapetum (the cell layer that nourishes the pollen) (Abe & Numata, 2024; Chustecki & Johnston, 2024).

Agronomic significance of CMS:

Molecular genetic aspects: many CMS‑associated genes have been identified (e.g., orf79 in rice, orf138 in radish, orf355 in maize). They encode chimeric proteins that disrupt the function of ATP synthase or other respiratory chain complexes. Nuclear restorer genes (Rf) suppress the expression of these mitochondrial genes at the post‑transcriptional level (Abe & Numata, 2024).

Current trend: using genome editing technologies such as mitoTALEN (mitochondrial TAL effector nucleases), it has already been possible to “cut out” CMS‑associated genes from the rice mitochondrial genome, restoring fertility (Abe & Numata, 2024; Kazama et al., 2019 — cited in Abe 2024). This opens the way to the targeted creation of new CMS lines.

Abiotic stresses are the main cause of yield losses in world agriculture. Mitochondria are key sensors and effectors of stress responses (Selinski et al., 2024; Chustecki & Johnston, 2024).

Role of alternative oxidase (AOX): under stress (drought, salinity, cold), an excess of reduced coenzymes (NADH, FADH₂) accumulates in mitochondria, leading to the generation of reactive oxygen species (ROS). AOX diverts electrons from ubiquinone to oxygen, bypassing complexes III and IV, thereby reducing ROS production and preventing oxidative stress. Plants with increased AOX expression show better survival under drought and low temperatures (Selinski et al., 2024; Evert, 2006).

Osmotic regulation and proline synthesis: proline — one of the key osmoprotectants that accumulate under salinity and drought — is synthesised in the mitochondrial matrix. Transgenic plants with enhanced proline synthesis in mitochondria are more resistant to water deficit (Chustecki & Johnston, 2024).

Retrograde signalling: mitochondrial dysfunction (e.g., caused by heat shock) activates nuclear defence genes through transcription factors of the ANAC family (especially ANAC017). This includes the expression of AOX, antioxidant enzymes and heat shock proteins (Selinski et al., 2024).

Practical approaches:

Plant mitochondrial DNA has a number of features that make it a convenient object for DNA typing:

Applications:

Many chemical plant protection agents and pathogen toxins act directly on mitochondrial processes.

Inhibitors of the respiratory chain: some herbicides (e.g., dinitroanilines, although they more often act on microtubules) and fungicides (e.g., strobilurins, azoxystrobin) inhibit complex III of the respiratory chain, blocking electron transfer from cytochrome b to cytochrome _c_1. This leads to the cessation of ATP synthesis and death of susceptible species. However, in plants, the presence of AOX can provide resistance to such inhibitors (Selinski et al., 2024; Evert, 2006).

Uncouplers of oxidative phosphorylation: some herbicides (e.g., dinitrophenols) disrupt the proton gradient across the inner membrane, leading to uncontrolled respiration and overheating of tissues. Such substances are toxic to both plants and animals, so their use is restricted (Evert, 2006).

Pathogens and CMS: some fungal pathogens (e.g., Bipolaris maydis — the causal agent of southern corn leaf blight) produce toxins (T‑toxin) that specifically interact with the mitochondrial protein URF13 encoded by the CMS‑associated gene T‑urf13. This led to an epidemic of corn blight in the USA in 1970, when all hybrids were based on a single CMS source (Texas male‑sterile cytoplasm). After this episode, more diverse cytoplasmic sources began to be used (Andreeva & Rodman, 2002; Chustecki & Johnston, 2024).

Modern methods allow targeted modification of the mitochondrial genome and expression of mitochondrial genes.

MitoTALEN and mitoZFN: the creation of artificial nucleases targeted to specific mtDNA sequences. With their help, CMS genes have already been excised in rice (Abe & Numata, 2024) and ATP synthase genes have been edited in Arabidopsis (Arimura et al., 2020 — cited in Abe 2024). This opens the way to “in‑vivo” correction of harmful mutations and the creation of new economically useful mitotypes.

Delivery of nucleic acids and proteins into mitochondria (see Section 6.6 in Abe 2024): using peptide vectors combining a mitochondrial targeting peptide (MTP) and a cell‑penetrating peptide (CPP) allows the introduction into mitochondria of messenger RNA, antisense oligonucleotides or nuclease proteins. This is a promising method for transient modification of mitochondrial expression without transgene integration (Abe & Numata, 2024).

Use of mitochondria as biosensors: by inserting reporter genes (e.g., GFP under the control of a mitochondrial promoter) into mitochondria, one can assess the functional state of the organelles in real time, which is useful for screening stress‑tolerant genotypes (Chustecki & Johnston, 2024).

Prospects: in the coming decades, we can expect the appearance of agricultural crop varieties with an edited mitochondrial genome that combine CMS for hybrid seed production, increased tolerance to abiotic stresses (via AOX), and resistance to specific pathogens. The development of technologies for delivering nucleic acids into mitochondria (peptide vectors, nanoparticles) will allow temporary (non‑transgenic) modification of mitochondrial function in field conditions (Abe & Numata, 2024).