Cell division is a vital process that requires orderly progression. Endogenous hormones such as auxin, abscisic acid, gibberellins and brassinosteroids as well as environmental factors all regulate progression through the cell cycle. The cycle is divided into several phases an initial gap (G1), synthesis of DNA (S), a second gap (G2) and the final mitotic nuclear and cellular division (M) to result in two identical daughter cells. As a result of rigorous control, cell cycle events take place with clock-like precision. Recent studies have demonstrated that cell cycles in plants and animals are regulated by similar mechanisms. A group of highly conserved serine/threonine kinases called cyclin-dependent kinases (CDKs) has been found to play a key role in guiding the cell cycle process.
In the August, 2001 issue of Trends in Plant Science (6:359-364), Hilde Stals and Dirk Inzé (Universiteit Gent, Belgium) review the current status of knowledge about plant cell cycle control mechanisms at the G1-S and G2-M transitions points. The authors describe similarities and differences between animal and plant cell cycle regulatory mechanisms, the roles of different recently reported cell cycle machinery proteins, and areas of future research.
The authors illustrate a model of the plant mitotic cell cycle, which shares some regulatory features in common with mammalian cell cycles. The first step in the process is the transition from G1 to S phase. Recent studies on cell cycle mechanisms in plants and animals indicate similarities in the control of progression through this checkpoint. According to the authors, these studies suggest that the regulatory mechanisms might have developed in a primitive multicellular eukaryote before the dichotomy of the plant and animal kingdoms. Parallel studies in yeast indicate that the proteins that control the G1-S transition in this organism are unrelated to those present in multicellular eukaryotes.
The authors describe the G1-S progression in detail. During cell division, D-type cyclins (CycD) become active in the G1 phase and are regulated by several growth factors such as auxin, cytokinin, gibberellin (GA), brassinosteroid (BR), abscisic acid (ABA) and sugars. CycD subunits interact with a catalytic subunit, cyclin-dependent-kinase A (CDKA). The transition to the S phase requires the removal of CDK inhibitory protein (CIK), which binds to the CDKA/CycD complex. Expression of CIK is induced by ABA. So long as CIK remains bound to the CDKA/CycD complex, the cycle will not progress into S phase. Another regulatory mechanism is the phosphorylation state of CDKA, which is a substrate for CDK activating kinase CDKD;1. CDKD;1 is itself up-regulated by GA.
Progression from G1 to S phase involves a mechanism that includes the retinoblastoma (RB) protein. When CDKA is phosphorylated and thus in active form, the CDKA/CytD complex can initiate phosphorylation of RB in late G1 phase. When RB is unphosphorylated, it interacts with two proteins called E2F and DP. E2F is a transcription factor. When phosphorylated, RB dissociates from it and E2F/DP facilitate transcription of genes that are needed for entry into the S phase.
In animals, the retinoblastoma tumor suppressor protein not only controls the cell-cycle progression but also participates in the process of programmed cell death. Prohibitins are a group of proteins that control G1-S cell cycle progression, senescence, and tumor suppression in many organisms. Prohibins inhibit DNA synthesis and thus entry into the S phase. The recent report of plant homologs of prohibitins raises the possibility of similar mechanisms of cell death regulation in plants and animals.
Following replication of DNA, the G2-M transition is also a major control point. Both A-type and B-type CDKs are thought to be involved in this process. A-type CDKs are expressed constitutively, while the plant-specific B-type CDKs increase expression during the G2-M transition point. CDKA and CDKB subunits interact with their respective cyclin partners (CycA/B). Both the expression of CDKA/B and CycA/B genes and the activity of CDKA/B kinases are affected by plant hormones. Auxin, GA and cytokinin increase expression of these genes, and cytokinin can also induce removal of an inhibitory phosphate group (T14/Y15) on CDKA/B subunits. In an additional level of control, phosphorylation at a separate site on CDKA/B subunits (T160) can induce activity of these subunits.
The conclusion of additional iterations of the cell cycle occurs when B-type cyclins are degraded by a ubiquitin-dependant pathway. In animals and yeast, mitotic cyclins contain a destruction box motif that targets them for degradation. Plant mitotic cyclins have a similar destruction box, indicating that exit from mitosis may occur in plants in a mechanism analogous to that in animal cells. This ubiquitin-dependant process occurs by action of the anaphase-promoting complex (APC).
Genomic studies in Arabidopsis are beginning to allow surveys of entire cyclin gene families. Twenty-seven cyclin genes have been identified in Arabidopsis representing four different types (A, B, D and H) based on similarity to mammalian homologs. Interestingly, some differences have been noted in D-type cyclin gene expression patterns between mammals and plants. In animals, D-type cyclins show constitutive expression patterns. In synchronized tobacco BY-2 cells, however, two CycD homologs increase expression during mitosis. This could indicate that D-type cyclins are needed for entry into mitosis; alternatively, it could be an artifactual expression pattern caused by long-term culturing in BY-2 cell lines.
The authors describe additional ways in which various hormones regulate the plant cell cycle. Recently, it has been recognized that the ubiquitin protein ligase SCF complex contains a subunit encoded by the auxin transport resistant 1 (TIR1) gene. Both auxin signaling and pericycle cell division is required to form lateral roots, and tir1-1 mutants have a reduced number of lateral roots. This indicates that a functional SCF complex is needed to elicit pericycle cell division. Auxin also increases the expression of CDKA;1 and mitotic cyclins, although application of exogenous auxin alone is not sufficient to induce cell division.
Auxin and cytokinins are both necessary for progression through the G1-S and G2-M transitions as demonstrated in a variety of cultured plant cells. Cytokinin can also increase the expression and kinase activity of CDKA;1 by a mechanism involving removal of an inhibitory phosphate group on the kinase. In addition, CycD3;1 expression is also up-regulated by cytokinins. CycD3;1 appears to be a critical regulator of G1-S progression, as constitutive expression of this gene in cells allows them to be cultured without application of cytokinins.
Brassinosteroids (BR) may influence cell division in a manner similar to cytokinins, as BR also increases expression of CycD3;1 and can substitute for cytokinins in cell culture. BR signaling does not, however, require protein phosphorylation but does require protein synthesis.
Gibberellic acid (GA) generally promotes cell division, and during water submergence in deepwater rice GA has been shown to induce the expression of CycA1;1 and CDKB1;1 at the G1-S transition. This induction is followed by an increase in expression of mitotic B-type cyclins.
Not all hormones promote progression through the cell cycle, however. The stress hormone abscisic acid (ABA) inhibits cell division under unfavorable environmental conditions. This may occur through up-regulation of expression of the cyclin kinase inhibitor ICK1 and down-regulation of expression and activity levels of CDKA;1.
There are significant differences between signaling in plants and other organisms. Plants differ from animals with regard to growth factor receptors. In animals, tyrosine kinases serve as receptors, while plants appear to lack equivalent proteins. Ser/Thr kinases may serve this role in plants. Downstream pathways in plants probably consist of mitogen-activated kinases (MAPK) cascades, though a MAPK pathway involved in regulation of the cell cycle has not yet been described.
In conclusion, the authors emphasize that future efforts will focus on understanding how environmental signals are transduced to regulate the cell cycle. Areas of particular investigation include clarification of regulatory mechanisms, elucidation of signal transduction pathways, and investigations into the entry and exit from cell division in certain cell types. The authors feel that practical applications will arise from this research, such as alteration of plant architecture and growth traits. They anticipate that tobacco BY-2 cells and Arabidopsis will continue to be excellent model systems for cell cycle research.