G1/S Transition and the Rb-E2F Pathway 61
totype CDK in plants. It is constitutively expressed at all phases of the cell
cycle and regulates both G1/S and G2/M transitions (Hemerly et al. 1995).
Loss-of-function mutations of CDKA inArabidopsisresulted in abnormal
pollen formation (Iwakawa et al. 2006; Nowack et al. 2006). Homozygous mu-
tant plants could not be obtained and analysis performed with heterozygous
plants revealed that the mutant pollen contains only one sperm cell, instead
of two, and consequently can fertilize only the egg cell. In the absence of
fertilization of the central cell, which is required for normal endosperm de-
velopment, the fertilized egg cell could proceed with only a limited number of
cell divisions and embryogenesis stopped early during mutant seed develop-
ment. These beautiful reverse genetic studies provide an example illustrating
the crucial function of the cell cycle machinery in plant development. Wang
and Chen (2004) previously reported thatHUA ENHANCER3 (HEN3),which
is allelic toCDKE, is required for the specification of stamen and carpel iden-
tities and for the proper termination of stem cells in the floral meristem in
Arabidopsis. Similar to that described for its homolog CDK8 in animals, the
ArabidopsisCDKE has different substrate specificity than CDKA and phos-
phorylates the carboxylterminal domain (CTD) of the largest subunit of RNA
polymerase II. However, distinct from CDK8 that associates with cyclin C, the
ArabidopsisCDKE binds D-type cyclins (Wang and Chen 2004). Moreover,
phenotypic analysis of thehen3 mutant leaves revealed that CDKE is required
for cell expansion rather than cell division. Thus, the precise role of CDKE in
cell cycle progression needs future studies.
While CDKB is expressed from late S to M phases and controls G2/M tran-
sition (Boudolf et al. 2004), the role of CDKC, CDKD and CDKF in cell-cycle
regulation is less well characterized. TheArabidopsisCDKC binds a cyclin
T homolog and was proposed to be involved in transcription elongation
(Barrôco et al. 2003). CDKD and CDKF belong to the CAK (CDK-activating
Kinase) family that appear to be involved in regulation of transcription and
cell division through phosphorylation of CDT and CDKs (Umeda et al. 2005).
Therefore, CDKA is the unique canonical CDK involved in the G1/S tran-
sition in plants, at least inArabidopsis. This contrasts the situation in mam-
malian cells where the specific CDK4 and CDK6 bind cyclin D, and CDK2
binds cyclin E and cyclin A, which together control G1/S transition and S
phase progression (Sherr and Roberts 2004).
2.2
D-Type and A-Type Cyclins
Genome-wide analysis of the cyclin family inArabidopsis(Wang et al. 2004a)
and in rice (La et al. 2006) revealed the absence of homologs to animal cyclin
E but the presence of multiple D-type and A-type cyclins, indicating that the
G1/S transition in plants is assured by D-type and A-type cyclins.Arabidop-
siscontains ten D-type cyclins that fall into seven subgroups (named CYCD1