Coordination of Cell Division and Differentiation 381
of this motif (PP/STA/TLRE) is present in type B CDKs. Others contain the
PITAIRE or the SPTAIRE motifs (Vandepoele et al. 2002).
Overexpression of ArabidopsisCDKA;1 does not have any detectable
macroscopic consequences (Hemerly et al. 1995). Likewise, overexpression of
CDKA in maize endosperm does not influence the endoreplication process
associated with endosperm development (Leiva-Neto et al. 2004). However,
overexpression of a dominant negative version of CDKA inhibits cell divi-
sion (Hemerly et al. 1995) and ploidy level (Leiva-Neto et al. 2004). Loss-of-
function of CDKA;1 produces a male gametophytic phenotype due to failure
of the generative cell during male gametogenesis (Iwakawa et al. 2006). Thus,
double fertilization does not take place and the embryo arrests early at the
globular stage. Auxin is sufficient to up regulate CDKA;1 gene expression but
cell cycle progression also requires cytokinin (Zhang et al. 1996), consistent
with the need of a concerted action, which probably impinges on the availabil-
ity of other components that modulate CDK activity. In fact, cytokinin seems
to mediate CDK activation by stimulating tyrosine dephosphorylation (Zhang
et al. 1996).
Altering the levels of the plant-specific CDKB also has developmental con-
sequences. Thus, CDKB1;1 regulates the balance between dividing and en-
doreplicating cells, by negatively regulating endocycle occurrence (Boudolf
et al. 2004a). A decrease in CDBK1;1 activity produces a short hypocotyl
phenotype which is partially rescued by increasing brassinosteroid level
(Yoshizumi et al. 1999; Hu et al. 2000). Reduction of CDKB1;1 levels also neg-
atively affects stomatal development through inhibition of cell division of the
stomatal lineage (Boudolf et al. 2004b). Interestingly, these cell cycle arrested
cells acquire a rather normal stomatal cell identity, providing one example
(among others discussed below) of uncoupling cell division and cell differen-
tiation.
Mutations in thePROPORZ1(PRZ1) gene were identified and produce
a high tendency to form calli only in presence of either auxin or cytokinin
(Sieberer et al. 2003). PRZ1 is a transcriptional adapter protein that regulates
gene expression, including that of several cell cycle genes. Thus,CDKB1;1
(as well as ofE2Fc) is up regulated in theprz1mutant grown in hormone-
containing medium, leading to the formation of undifferentiated callus-like
structures (Sieberer et al. 2003). In hormone free medium, it is the expression
of B- and D-types cyclins that is reduced in theprz1mutant.
Arabidopsis(Vandepoele et al. 2002; Wang et al. 2004) and rice (La et al.
2006) contain a large family of cyclins, suggesting that this may likely be the
case for other plant species. Expression ofCYCA3genes peaks in S-phase
(Menges et al. 2005), suggesting that they are major elements of the kinases
involved in S-phase-related events. Consistent with this idea, constitutive
overexpression of tobaccoCYCA3;2inArabidopsisleads to a suppression of
the endoreplication program associated with normal leaf development con-
comitantly with an induction of hyperplasia (Yu et al. 2003).CYCA3;2gene