Plant Cell Cycle

Abstract

In plants, just like animals, proliferative cells divide mostly mitotically or sometimes meiotically, where chromosome number is either maintained or halved, respectively. This is achieved in the cell cycle, an ordered sequence of events that enables a healthy cell to divide. Cell‐cycle regulation is crucial for plant development due to the link between cell proliferation in meristems and differentiation or altered plane of division of descendants. Regulation of cell division is also a major way whereby plants are able to respond to an ever‐changing environment. While generic cyclin‐dependent kinase (CDK) regulation operates in the plant cell cycle, there are distinct differences from animals. Prominently these are very large cyclin families, the plant‐specific class of B‐type CDKs and the absence of phosphoregulation of CDKs by CDC25 and WEE1 except under checkpoint conditions. Cell division in meristems and gametophytes is regulated by feedback loops involving transcription factors, cell‐cycle activators and repressors.

Key Concepts

  • There are many more members in cell‐cycle regularity families in plants.
  • In an unperturbed cell cycle, phosphoregulation of higher plant CDKs does not require WEE1 or CDC25.
  • Plant cell‐cycle checkpoints lack CHK1 but engage WEE1 and KRPs.
  • MYB transcription factors promote and repress G2/M expressed genes.
  • Induction of DNA replication is similar in plants and animals but there are more E2F and RB‐related proteins in plants.
  • Negative feedback loops regulate organising centres in plant meristems.
  • Coordination between cell division and cell differentiation is spatially and temporally different in RAMs and SAMs.
  • Similar to cell growth, organ growth by cell proliferation can be subdivided into the proliferation rate, the direction of cell divisions determined by the orientation of the mitotic spindle and the duration of cell divisions.
  • ICK4/KRP6 and CDKA are major regulators of cell division in gametophytes.

Keywords: proliferation; endoreduplication; mitosis; meiosis; cell growth; cyclin‐dependent kinase; cyclins; checkpoints

Figure 1. Mitotic and meiotic cell cycles. (a) When a proliferative G2 cell traverses mitosis to G1, it maintains the diploid number of chromosomes (2n) but nuclear DNA amount (C value) is halved (4C to 2C). When the G1 cell traverses synthetic (S)‐phase to G2, nuclear DNA amount is doubled. (b) A meiocyte cell traversing meiosis undergoes two divisions, meiosis I, where chromosome number and nuclear DNA amount are halved (2n to n and 4C to 2C) and meiosis II, where chromosome number is maintained as haploid (n) but nuclear DNA amount is halved again (2C to 1C). 1C is defined as the amount of nuclear DNA in the unreplicated haploid genome of a gamete.
Figure 2. Cell‐cycle control at G2/mitosis. (a) CDKA is repressed by ICK1. Activation of CDKA is through phosphorylation ( ) by CAK and cyclin binding, and phosphorylation of ICK1 by CDKB, causing ICK1 to release CDKA. Based on model by Boudolf et al., 2006. (b) At metaphase, the cyclin is ubiquitinated by the APC/C (anaphase promotion complex/cycleosome CULLIN‐RINGfinger E3 ligase) complex and degraded en route to the 26S proteasome. Model based on observations by Weingartner et al., () that a nondegradable cyclin permits normal mitosis until metaphase followed by mitotic catastrophe and that M‐phase degradative complex colocalises with mitotic spindles at metaphase (Farras et al., ). CDK = cyclin‐dependent kinase; ICK1 = interactor/inhibitor of CDK; CAK = cyclin‐dependent activating kinase.
Figure 3. DNA repair and replication checkpoints in plants. In the repair pathway, ultraviolet light (UVB) or ionising radiation (IR) induces the formation of ATM that phosphorylates WEE1 that, in turn, represses CDKA. The omega form of 14‐3‐3 protein binds to and protects the phosphorylated site of WEE1. In the replication pathway, hydroxyurea (HU) or reactive oxygen species (ROS) activates ATR, which, in turn, upregulates SOG1 that itself activates SMR5/7. The latter complex then represses CDKA. ATR can also phosphorylate WEE1. ATM = ataxia telangiectasia mutated; ATR= ATM‐related; SOG1 = suppressor of γ1; SMR = Siamese/Siamese‐related. Based on Yi et al. () and Sorrell et al. ().
Figure 4. Activation of DNA replication. A CDKA‐cyclin D complex forms at G1/S and hyperphosphorylates RBR of the RBR/E2F complex. RBR releases three E2Fs, transcription factors that are stabilised by DPa and b. This complex, in turn, transcriptionally upregulates CDC6 that coordinates the activation of DNA replication at a replication origin. RBR = retinoblastoma‐related, E2F = DNA‐binding protein essential for E1A‐dependent activation of adeno virus E2 promoter; CDC = cell division cycle; DP = dimerisation protein.
Figure 5. A summary of cell cycles in meristems. (a) In the vegetative shoot apical meristem, three histological zones can be ascribed: the central zone (CZ), large vacuolated cells with long cell cycles (288 h), a peripheral zone (PZ), small cells with shorter cell cycles (157 h) and a pith rib meristem (PRM), with intermediate cell size and cell cycle times. WUS expression (which is positively regulated by STIP) occurs in the lower region of the CZ and confers stem‐cell identity on neighbouring cells above. CLV3 expression in apical cells of the CZ marks a stem cell region. CLV3 encodes a small polypeptide acting as a ligand for CLV1, which then negatively regulates WUS. MGO expression is restricted to the PZ while ANT, which promotes primordial outgrowth, is expressed solely in the new leaf primordium of rapidly cycling cells (20 h). In the youngest formed primordium, cell division stops with cells arresting in G1 or G2 or becoming endopolyploid (see shoot apical meristem). WUS = Wuschel, CLV = Clavata, STIP = Stimpy, MGO = Mgoun, ANT = Aintegumenta. Cell‐cycle data are from the vegative SAM of Sinapis alba (Bodson, ). (b) In the root apical meristem is the quiescent centre that locates at the very tip of the RAM. It comprises noncycling cells at its midpoint, surrounded by very slowly cycling stem cells (390 h). On the margins of the QC, are apical initials that cycle much more rapidly (18 h) and result in cell lineages forming. Subtending the distal apical initials is the root cap in which cells slowly cease proliferating and eventually slough off. A longitudinal file of cells is shown stretching along the epidermis. At the transition point, the cell elongates but exits the cell cycle, demarking the proximal limit of the meristem. SHR and SCR are transcription factors that are expressed in the quiescent centre to maintain the stem cell population. A feedback control of cell proliferation is depicted in the side box. Here, a close relative to CLV3, CLE40, is secreted from cap columella cells into the QC and represses WOX5 (related to WUS) via ACR4, which is expressed in the distal meristem and restricts cell division. SCR = Scarerow; SHR = short root; CLE = CLV3/Endosperm surrounding region; ACR = Arabidopsis crinkly; WOX = Wuschel‐related homoebox. Cell‐cycle data are from Clowes ().
Figure 6. Generic model of gametogenesis, and fertilisation in higher plants. In the reproductive organs of the sporophyte flower, diploid (2n) meiocyctes undergo meiosis to generate four haploid (n) spores. Each haploid male microspore undergoes a mitotic division, generating the vegetative and generative cell and the latter undergoes a further mitosis yielding two sperm cells/nuclei. The female meiocyte yields four haploid cells, three of which degenerate. The remaining haploid cell (megaspore) then undergoes three rounds of mitosis. Six of the resulting eight haploid nuclei cellularise to form three antipodals at one end and, a central cell flanked by two synergids at the other. Two polar nuclei localise in the centre. Fertilisation occurs when the pollen tube penetrates one of the synergids, which then degrades, allowing one of the male nuclei to fuse with the nucleus of the egg cell to form the diploid zygote (2n). The other male nucleus fuses with two polar nuclei of the female gametophyte. This is the triploid (3n) progenitor of the endosperm. See also: Plant Reproduction
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Further Reading

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Francis, Dennis(Dec 2015) Plant Cell Cycle. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020111.pub2]