[337]. In the Arabidopsisroot epidermis, hairless cells are longer than hair cells at maturity. The cell di-
vision rate in the hairless cells slows down, allowing the cells to reach their normal larger size [338]. Fur-
thermore, mutation of a gene controlling hair cell fate (TTG) causes ectopic root hair formation and all
cell sizes are similar. Coupled with the finding that growth and morphogenesis can be uncoupled from
cell division, this indicates that developmental processes control cell division rather than cell division
controlling development. Organogenesis in flowering plants results from patterned control of the number,
place, and plane of cell divisions. Of the cloned mutants that affect the various modes of cell division in
mersistems, none appear to be homologues of the major cell cycle control genes, and so pattern control
genes may be acting at some distance to regulate the cell cycle machinery [339].
252 REDDY AND DAYFigure 4 Model of plant cell cycle control. (A) General proteins involved in cyclin/cyclin-dependent kinase
regulation. A cyclin and its Cdk partner form a complex. Negative regulation is by phosphorylation of Thr14
and Tyr15 by a Weel-type kinase as isolated in maize [184]. Activating phosphorylation is on a Thr in the
RTFTHEV (Table 4) motif by either a CakAt-like kinase [141] or an R2-like CAK [36]. The negative phos-
phates on Thr14 and Tyr 15 are removed by a phosphatase that has yet to be identified but whose existence is
expected [143,144]. Active cyclin/Cdk phosphorylates appropriate substrates. (B) Cell cycle progression in
plants. During G 1 , D-type cyclins may be induced by hormones and other mitogens [149,221]. Some plant Cdks
have also been shown to be induced by auxins [111,122,123]. Formation of active CyclinD/Cdk (Cdc2a in Ara-
bidopsis[156]) complexes results in phosphorylation of Rb protein [227–230] causing the release of E2F tran-
scription factors [233], which, in turn, activate the genes necessary for S-phase progression. Inhibitors of cy-
clin/Cdk (CKI) such as the ABA-inducible protein ICK1 isolated in Arabidopsis[236] may inhibit various
cyclin/Cdk complexes in the cell cycle. A-type cyclins appear at S phase and B-type toward G 2
[151,164,167,169]. No specific Cdk partners have been established; however, different timing of expression of
genes encoding differend Cdks has been reported [99,100]. Activated cyclin/Cdk complexes phosphorylate
substrates involved in cell cycle progression. At the end of mitosis cyclin is marked for destruction by the ubiq-
uitin/proteasome pathway possibly involving SKP1 protein [183,340].