In higher plants, organ formation occurs throughout their life via the maintenance of a reservoir of stem cells in the shoot and root apical regions called meristems (Doerner 2007; Scofield and Murray 2006; Sharma et al. 2003). Plant growth regulation is tightly tuned with the genetic setup and environmental conditions. In meristems undifferentiated cells are produced by cell proliferation, and when these cells stop dividing, as they leave the meristematic region, they differentiate into specific tissues. During differentiation, plant cells frequently increase their DNA content by a modified mitotic cycle called endoreduplication, a process of continuous DNA synthesis without intervening mitosis (Inze and De Veylder 2006). Cell division and differentiation are well co-ordinated events during plant life, but the molecular mechanisms that maintain the balance between these processes are still not well understood.

The plant cell cycle is regulated by conserved molecular elements; pivotal among them are cyclins and cyclin-dependent kinases (CDKs) that drive the cell through the cell cycle control points. The canonical CDK, CDKA1 is an essential gene in Arabidopsis and is required both for gamete and somatic cell proliferation (Dissmeyer et al. 2007). The CDK family of genes have largely expanded with around 30 members in Arabidopsis. CDKBs are par ticularly specialized in the regulation of mitosis, while others might work outside the cell cycle, such as in the regulation of transcription. The family of cyclins is also expanded in plants where they are similarly grouped to D-, A-, and B-types as in animals (Inze and De Veylder 2006). CDK activity is regulated by inhibitors of CDK (ICKs), also known as Kip-related proteins (KRPs) in plants (De Veylder et al. 2001; Wang et al. 1997). Recently a new class of CDK inhibitor genes was discovered, SIAMESE (SIM), a regulator of trichome development through endoreduplication (Churchman et al. 2006). CDK is also regulated through activating phosphorylation at the T-loop by a CDK activation kinase (CAK) (Umeda et al. 2005) and through inhibitory phosphorylation by the WEE1 kinase (Inze 2005). The identity of the phosphatase that removes this inhibitory phosphorylation from CDKA is currently debated (Boudolf et al. 2006).

This chapter focusses on the role of the RB-E2F pathway in plant growth control. Readers are directed to excellent recent reviews on general description of the plant cell cycle control (Dewitte and Murray 2003; Inze 2005; Inze and De Veylder 2006; Ramirez-Parra et al. 2005). Our knowledge about E2Fs and RBs comes from animal studies. Models of how the RB-E2F pathway controls the G1-S transition can be found in every textbook of molecular biology. However, recent discoveries have put these models under serious challenge (Rowland and Bernards 2006). Therefore first I will summarize the available data on the RB-E2F pathway in animal cells and later focus on the study of this pathway in plants.

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