Figure 1. The cell cycle and its regulation. (a) Overview of the cell cycle with indicated mitosis (M), phase of DNA synthesis (S) and two gap phases (G1 and G2). The transition from G1 to S phase and from G2 to M phase are two major check points that are controlled by CDK-cyclin complexes. (b) The principle of CDK-gated progression through the cell cycle is conserved between all eukaryotes. The activity of CDK–cyclin complexes (inner circle) is controlled by many layers of regulation. Immediate factors affecting CDK activity are the binding of further positive cofactors, such as CKS, or negative factors, i.e. CDK inhibitors (CKIs). Further on, CDK activity is controlled by positive and negative phosphorylation at the residues Thr14 and Tyr15 mediated by CDC25 and WEE1, respectively. In addition, for full CDKs activity, a conserved Thr in the T-loop (in humans Thr161) needs to be phosphorylated. Important is also the degradation of cyclins and other cofactors, such as CKIs, by controlled protein degradation mediated by APC/C or SCF activity. Finally, extrinsic or environmental cues and intrinsic or developmental cues impinge on the activity and abundance of these CDK regulators. Owing to these manifold regulatory input and the different possibilities to influence CDK activity, CDKs resemble microprocessors integrating many different inputs and generating an unambiguous output, i.e. progression in the next cell-cycle phase or arrest. The numbers in the white boxes indicate how many potential homologues of the animal regulators have been found in Arabidopsis. For CDC25 only one putative homologue is known, therefore indicated by a dotted line. Also which of the four possible kinases, function as the Arabidopsis CAK is not clear at the moment, indicated with a dotted line.

Figure 2. The RBR-E2F pathway. Factors like sugar, cytokinin, auxin and wounding can induce the expression of CYCD, which binds CDKA. The retinoblastoma protein (RBR) is bound to the transcriptional regulator E2F and keeps it in an inactive state. The CDK-cyclin complex phosphorylates RBR, by which it dissociates from E2F. E2F is now active and can promote the transcription of factors involved in DNA replication, for example CDC6.

Figure 3. The cell cycle in various plant tissues. (a) The cell cycle in the shoot apical meristem (SAM), the ultimate source for all above ground cells of the plant, and in the root apical meristem (RAM), the region where in roots new cells originate, show many similarities. Cells in the periphery of the SAM (light green) or in the more proximal regions in the RAM (light brown) undergo very rapid cell cycles to produce many daughter cells whereas the stem cells divide rarely, only for self-renewal. The stem cells (yellow) are kept in an undifferentiated state with help of the organizing centre (blue). The new cells of the SAM are incorporated into the growing stems (to give height to the plant) and into developing primordia of lateral organs. At the base of a growing leaf cell divisions take place to increase the size of the leaf disc. In the distal part of a growing leaf, cells exit the mitotic cell cycle and terminally differentiate. The new cells formed by the RAM can be found in the root cap for protection of the meristem, and in all layers of the growing root. Often before withdrawal from the cell cycle many differentiating root and shoot cells switch to an alternate cell-cycle programme, called endoreplication, to amplify their DNA content without undergoing cell division. (b) The reproductive system of plants requires both haploid (1n) and diploid (2n) generations. The diploid stage is the flower-producing sporophyte. In the reproductive organs, meiosis takes place to create cells with only one set of chromosomes, the haploid megaspores (female) and microspores (male). The megaspore undergoes three rounds of free nuclear divisions to create the female gametophyte bearing the central cell (2n, pink), the egg cell (1n, pink), two synergids flanking the egg cell and three antipodals at the opposite side of the embryo sac (1n, black). The microspores undergo two rounds of mitosis to create male gametophytes consisting of a large vegetative cell with in it two sperm cells (1n, blue). Upon fertilization, one of the two sperm cells fuses with the nucleus of the central cell to give rise to the 3n endosperm, the other fuses with the egg cell to form a 2n embryo.