Cell Division

In eukaryotic cells, four sequential phases are traditionally distinguished (Dewitte & Murray 2003; Sablowski & Gutierrez 2022):

In plants, unlike many animal cells, a classical “quiescent” phase (G₀) is absent. Instead, meristematic cells can exit the cycle and differentiate, but upon stimulation (e.g. after wounding) they are able to return to proliferation. In mature parenchyma cells that do not divide, the cell cycle arrests in the pre‑synthetic phase, which functionally corresponds to G₀.

Cyclin‑dependent kinases (CDKs) and their regulatory subunits – cyclins – play a central role in controlling the cell cycle. The activity of the CDK–cyclin complex determines the G₁/S and G₂/M transitions, as well as the progression through mitosis itself (Dewitte & Murray 2003; Liu & Lee 2022).

Several classes of CDK have been discovered in plants, reflecting a more complex regulation compared to yeasts and animals:

Plant cyclins are subdivided into A‑, B‑, and D‑types, as well as the less studied H‑ and C‑types:

To prevent errors in replication and chromosome segregation, the cell cycle contains checkpoints that halt the cycle under unfavourable conditions or upon DNA damage. The best‑studied checkpoints in plants are:

Many differentiated plant cells do not completely stop DNA replication; instead of mitosis, they repeatedly enter S‑phase without subsequent division. This process is called endoreduplication or endomitosis (Dewitte & Murray 2003; Sablowski & Gutierrez 2022). As a result, nuclear ploidy increases (from 2C to 32C or even higher). Endoreduplication is characteristic of:

The switch to endocycles is induced when CDK activity (especially CDKB) declines and when inhibitors of the SMR (SIAMESE‑RELATED) family are expressed. Endoreduplication is often accompanied by an increase in cell size, which is important for fruit growth and storage tissues.

Thus, the plant cell cycle is a multi‑level system in which CDKs and cyclins serve as the executive machinery, while checkpoints and endoreduplication provide ways to adapt to development and stress. Understanding the cycle phases and their regulation is necessary for subsequent analysis of mitosis, meiosis, and the application of this knowledge in agronomy.

Mitosis of somatic cells encompasses the events from the onset of chromosome condensation to the separation of the two daughter nuclei. It usually lasts from 30 minutes to 2–3 hours, whereas interphase (especially G₁ and S) may take from several hours to several days (Dewitte & Murray 2003). Depending on the tissue type and plant species, mitotic activity is restricted to meristems, and only there can all stages of mitosis be observed.

Meiosis (from Greek meiosis – reduction) is a specialised form of nuclear division confined exclusively to cells that give rise to spores (sporocytes, or spore mother cells) in higher plants. Unlike mitosis, meiosis leads to reduction (halving) of chromosome number and to genetic recombination – the reshuffling of hereditary material between homologous chromosomes (Chen et al. 2025; Zou et al. 2024). In plants, meiosis occurs in sporangia: in angiosperms, in anthers (microsporogenesis) and in ovules (megasporogenesis). The products of meiosis are haploid spores, which then give rise to the male and female gametophytes.

Cytokinesis is the final stage of cell division, during which the cytoplasm of the mother cell is physically partitioned and a new cell wall is formed between the daughter protoplasts. Cytokinesis most radically distinguishes plant cells from animal cells. In animals, cytokinesis occurs by furrowing (formation of a contractile actomyosin ring), whereas in plants, due to the presence of a rigid cell wall, this mechanism is impossible. Instead, plants have evolved a unique structure – the phragmoplast – that guides the centrifugal growth of the cell plate (Choi et al. 2025; Li et al. 2015).

Cytokinesis begins in telophase and is tightly coordinated with the drop in CDK activity and spindle depolymerisation. Errors in cytokinesis lead to binucleate cells, polyploidy, or disruption of tissue integrity.

Based on origin and location, primary and secondary meristems are distinguished (Andreeva & Rodman 2002; Mauseth 2017).

In most angiosperms, the shoot apical meristem is differentiated into two histologically distinct layers (Sablowski & Gutierrez 2022; Mauseth 2017):

In many monocots and in dicot roots, a clear tunica–corpus distinction is not evident, but zonation still exists: outer layers give rise to the rhizodermis (root epidermis), and inner layers produce the cortex and central cylinder.

As already noted in section 3.1.1, before entering mitosis, a preprophase band of microtubules (PPB) forms in the cortical region of the cell. The PPB arises in late G₂ and disappears in prometaphase, but leaves a molecular mark – the cortical division zone (Bouchez et al. 2024). It is toward this zone that the growing phragmoplast is guided in telophase, and it is here that the cell plate fuses with the mother wall. Thus, the PPB serves as a spatial landmark that predetermines the position of the new wall. If the PPB does not form (e.g., in ton1 or trm678 mutants), cells can still divide, but the division plane becomes more variable, confirming the role of the PPB in enhancing accuracy (Bouchez et al. 2024).

The orientation of the division plane (anticlinal, periclinal, or otherwise with respect to the organ axis) is critically important for tissue formation. Simple rules such as “a cell divides along the smallest surface” (Errera’s rule) operate in the absence of external signals but can be overridden by polar signals (Bouchez et al. 2024). Examples of tissue‑specific orientation:

The positioning of divisions is not entirely autonomous; it depends on signals from neighbouring cells and on mechanical stresses within the tissue. For example, epidermal cells divide synchronously with internal tissues to maintain a continuous surface layer. Tissue injury activates divisions in adjacent cells (callus formation). Mechanical compression or stretching can alter the orientation of cortical microtubules and, consequently, the position of the PPB (Bouchez et al. 2024). Thus, the spatial organisation of division results from an integration of genetic programmes, cell polarity, and mechanical properties of the tissue.

In generative organs, the spatial organisation of divisions is subordinated to the tasks of sporogenesis and gametogenesis:

Understanding where and how cells divide is used in breeding and crop production:

Thus, the spatial organisation of division in plants is the result of evolutionarily fixed meristem distribution, precise orientation of the division plane (with the involvement of the PPB), and perception of local signals. Without this organisation, the formation of functional tissues and organs would be impossible, which underscores its key role in botany and agronomy.

Prokaryotes (bacteria and archaea) divide by binary fission – a simple constriction of the cell after replication of the single circular DNA, without an organised spindle or nuclear envelope. The transition to eukaryotic mitosis required:

It is thought that meiosis evolved from mitosis very early in eukaryotic evolution. A key innovation was homologous recombination – initially, probably as a mechanism for repairing double‑strand DNA breaks, and later as the basis of sexual reproduction. In most modern eukaryotes (including plants), meiosis has retained conserved proteins – Spo11 for initiating breaks, synaptonemal complex proteins, Rad51/Dmc1 recombinases, and crossover components (ZMM) (Chen et al. 2025). However, plants also possess specific proteins, e.g., PAIR1, PRD1–PRD3, which are absent in animals (Chen et al. 2025).

The preprophase band of microtubules (PPB) and the phragmoplast are unique to land plants (and probably their green algal ancestors). They are not found in animals, fungi, or most algae. Their emergence is linked to the colonisation of land by plants and the need for precise positioning of the new cell wall under turgor pressure and in the absence of cell migration (Bouchez et al. 2024; Li et al. 2015). The PPB allows the cell to “remember” the site of future cell‑plate insertion long before its formation, and the phragmoplast guides the centrifugal growth of the plate from the centre toward the periphery. In some green algae (e.g., charophytes – the closest relatives of land plants), primitive forms of cytokinesis resembling a phragmoplast are observed, but a true PPB formed only in bryophytes and vascular plants (Strasburger 1971; Mauseth 2017).

A comparison of the main characteristics of cell division in three major groups of eukaryotes is presented in the table.

Despite differences in spindle organisation and cytokinesis, key proteins involved in these processes are evolutionarily conserved. For example:

Not all plants divide according to a single pattern. In green algae (e.g., Chlamydomonas, Spirogyra), nuclear division can occur as closed mitosis (without nuclear envelope breakdown), and cytokinesis is centripetal septum formation without a phragmoplast (Strasburger 1971). In charophyte algae (genus Chara), a primitive phragmoplast is observed, confirming their relatedness to land plants. In mosses (e.g., Physcomitrella patens), the PPB and phragmoplast are already present, but cells can also divide without them (Bouchez et al. 2024). In ferns and gymnosperms, centrioles are retained in spermatozoids (unlike angiosperms), indicating the gradual loss of centrioles during the evolution of seed plants (Liu & Lee 2022; Mauseth 2017).

Prokaryotic division (binary fission) is fundamentally simpler: no microtubules, no spindle, no nuclear membrane. The main mechanism is polymerisation of the FtsZ protein, which forms a ring (Z‑ring) at the constriction site. No homologues of FtsZ have been found in eukaryotes (although tubulin is likely evolutionarily related to the bacterial FtsZ protein). Thus, mitosis in plants and animals is a later, specialised acquisition associated with multicellularity and increased genome size.

Adaptation of cell division to a rigid wall and sessile lifestyle allowed plants to:

Thus, the evolution of cell division in plants proceeded toward greater complexity of cytoskeletal structures, while retaining a conserved molecular core. Comparison with other eukaryotes shows that differences often concern not so much the proteins themselves, but the ways they are organised in space and time.

As already noted in section 2.2, the central regulators of the cell cycle are cyclin‑dependent kinases (CDKs) and their regulatory subunits – cyclins. The activity of the CDK–cyclin complex determines the G₁/S and G₂/M transitions, as well as progression through mitosis. In plants, this mechanism possesses several features (Dewitte & Murray 2003; Sablowski & Gutierrez 2022):

For fine‑tuning CDK activity and arresting the cycle at critical points, plants use two families of inhibitors:

The expression levels of KRP and SMR are controlled by hormones, sugars, and stress factors, making them important integrators of external signals.

A key mechanism linking external signals to the G₁/S transition is the retinoblastoma pathway (Sablowski & Gutierrez 2022; Dewitte & Murray 2003). In plants, the single homologue of the retinoblastoma protein is called RBR1. In an unstimulated cell, RBR1 binds to E2F and DP transcription factors, repressing genes required for S‑phase (DNA polymerase, thymidine kinase, histones, CDC6, CDT1). When CDKA–cyclin D is activated (by cytokinins, sugars, brassinosteroids), RBR1 is phosphorylated, releases E2F/DP, and they initiate transcription of S‑phase genes. Thus, RBR1 acts as the “gatekeeper” for S‑phase entry.

Mutations in the RBR1 gene lead to uncontrolled proliferation, impaired differentiation, and ultimately lethality. Heterozygotes exhibit abnormalities in stomatal and root hair development (Lee & Torii 2026). Interestingly, RBR1 also interacts with proteins that determine cell identity (e.g., SCARECROW and FAMA), thereby linking the cell cycle to cell fate (Sablowski & Gutierrez 2022).

For irreversible transition from one cycle phase to the next, destruction of key regulators (cyclins, CDK inhibitors) is required. This is accomplished by two major E3 ubiquitin ligase complexes (Dewitte & Murray 2003; Sablowski & Gutierrez 2022):

Disruption of APC/C or SCF function leads to accumulation of cyclins or inhibitors, cell‑cycle arrest, and ultimately cell death.

Phytohormones are the major external signals controlling proliferation in meristems. Their action is mediated by changes in the expression of cyclins and CDK inhibitors (Dewitte & Murray 2003; Lee & Torii 2026):

Cell division requires energy and building blocks. A key integrator of metabolic status is the target of rapamycin (TOR) – a conserved kinase activated by high levels of sugars, amino acids, and ATP (Sablowski & Gutierrez 2022). TOR phosphorylates the ribosomal protein S6K and elongation factor eEF2, stimulating translation. In addition, TOR activates the RBR1‑dependent G₁/S transition. Under starvation, TOR is inhibited and cells arrest in G₁. Interestingly, glucose itself can induce D‑cyclin expression even through TOR‑independent pathways.

Nitrogen nutrition (nitrate, ammonium) also affects division: nitrogen deficiency reduces D‑cyclin expression and increases KRP levels. Phosphorus is required for nucleotide synthesis and protein phosphorylation; its deficiency leads to a G₂/M delay.

In meiosis, plants must control not only entry into division but also the frequency and distribution of crossovers. As discussed in sections 3.2 and 5, this control is achieved by a balance between pro‑recombination (ZMM, HEI10, MLH1/3) and anti‑recombination (FANCM, RECQ4, FIGL1, TOP3α) factors (Chen et al. 2025; Zou et al. 2024). The expression level of HEI10, for example, dose‑dependently determines the number of crossovers and their interference. Genetic variation in HEI10 promoters could be used to increase recombination in breeding.

Extreme temperatures, drought, salinity, excess or deficiency of light, and pathogen attack can all affect cell division (Zou et al. 2024). As a rule, severe stress inhibits division, shifting cells into a G₀‑like state or inducing endoreduplication. However, mild stresses may conversely stimulate division (e.g., moderate warming accelerates mitosis in grass intercalary meristems). The mechanisms of stress‑induced regulation include:

Regulation of cell division is not linear; it involves multiple feedback loops. For example, activated E2F stimulates transcription of its own inhibitor E2Fc, thereby limiting the duration of S‑phase (Sablowski & Gutierrez 2022). D‑cyclins are induced not only by hormones but also by the very fact of cell‑cycle entry (positive feedback). High CDK activity in G₂/M phosphorylates and activates its own positive regulators (MYB₃R), creating another amplification loop.

Thus, the regulation of plant cell division is a complex, hierarchical system that includes transcriptional control, post‑translational modifications, ubiquitin‑dependent degradation, and integrates hormonal, metabolic, and stress signals. Understanding these mechanisms opens avenues for agronomic intervention – from growth stimulation with hormones to targeted modification of recombination frequency in breeding.

The rate and frequency of mitotic divisions in meristems determine plant growth rates. Agronomic practices affect the cell cycle through resource supply and regulation of hormonal status (Dewitte & Murray 2003; Lee & Torii 2026).

Some herbicides disrupt plant mitosis or cytokinesis, making them effective for weed control. Understanding the mechanisms of division allows the design of selective compounds (Mauseth 2017; Beck 2010).

The ability of somatic cells to dedifferentiate and return to the mitotic cycle underlies microclonal propagation (Andreeva & Rodman 2002; Lee & Torii 2026). Plant fragments (explants) are placed on a nutrient medium supplemented with auxins and cytokinins at a specific ratio. This induces divisions in parenchyma cells and the formation of callus – a mass of unorganised dividing cells. Under the influence of hormones, shoots or roots are then formed from the callus, and subsequently a whole plant.

The method allows:

For potato tubers, bulbs, root vegetables, and fruits, an important agronomic practice is the suppression of sprouting – i.e., inhibition of mitosis in meristems (potato eyes, apical buds of bulbs). Inhibitors (chlorpropham, maleic hydrazide) are used that:

Suppressing division extends storage life and preserves product quality.

Understanding the mechanisms of meiotic recombination allows acceleration of the breeding process (Zou et al. 2024; Chen et al. 2025).

One of the most ambitious applications is the creation of synthetic apomixis (seed propagation without fertilisation, preserving the maternal genotype). Combining mitotic (MiMe) and haploid‑inducing technologies allows the production of clonal seeds in hybrid crops (rice, maize, tomato) (Chen et al. 2025). Such seeds maintain the heterosis of the F₁ hybrid over generations, dramatically reducing the cost of annual hybrid seed production. Although the efficiency of current systems is still modest (5–30% clonal seeds), progress in gene editing is rapidly improving performance.

Spontaneous or induced polyploidy is often accompanied by increased cell, organ, and fruit size. Polyploid forms of tetraploid rye, triticale, strawberry, watermelon, and many ornamental crops possess economically valuable traits. Polyploidy induction is achieved by treating meristems with colchicine – an alkaloid that binds tubulin and blocks chromosome segregation in anaphase (Mauseth 2017). DNA replication proceeds, but cytokinesis is suppressed, resulting in a doubling of the chromosome set.

Cell division is highly sensitive to toxicants, mutagens, and radiation. The ana‑telophase method (counting cells with bridges and fragments in anaphase‑telophase in root meristems) is used to assess the mutagenic activity of chemicals, soil and water pollution, and to screen potential herbicides. This method is cheap, fast, and correlates well with more complex tests on animals.

In agronomic education, knowledge of the cell cycle and division is the basis for understanding growth physiology, genetics, breeding, biotechnology, and plant protection. Without this knowledge, it is impossible to properly interpret the effects of fertilisers, herbicides, growth regulators, or to apply modern genome editing methods. Thus, cell division is a bridging link between fundamental botany and practical agriculture.