62 W.-H. Shen
to CYCD7). Overexpression ofArath;CYCD1;1orArath;CYCD2;1in tobacco
or inArabidopsisplants increased growth rate likely by shortening the G1
phase (Cockroft et al. 2000; Masubelele et al. 2005), whereas overexpression
ofArath;CYCD3;1led to cytokinin-independent growth and resulted in hy-
perplasia in leaves but not in roots (Riou-Khamlichi et al. 1999; Dewitte et al.
2003; Masubelele et al. 2005). Loss-of-function mutations ofArath;CYCD1;1
orArath;CYCD4;1delayed the onset of cell proliferation upon root protru-
sion during seed germination (Masubelele et al. 2005), a process by which
the plant embryo resumes growth, after a period of quiescence, through G1/S
transition (Barrôco et al. 2005). In suspension-culturedArabidopsisand to-
bacco BY2 cells, overexpression ofNicta;CYCD3;3(Nakagami et al. 2002),
Antma;CYCD1;1(Koroleva et al. 2004) orArath;CYCD3;1(Menges et al. 2006)
accelerated G1/S transition but also directly or indirectly perturbed other
phases of the cell cycle. Together with previous molecular data showing that
expression of CYCDs is responsive to nutrient availability and to phytohor-
mones and that CYCD proteins bind CDKA forming active kinases (reviewed
in Shen 2001; Oakenfull et al. 2002), these studies demonstrate that CYCD-
CDKA kinases are essential for G1/S transition and that different CYCDs
could have specific functions playing different roles in different tissues or cell
types during plant development.
In contrast to animals where only a single A-type cyclin is present in
invertebrates and two (with one of them expressed only in germ cells) in ver-
tebrates, plants hold a higher complexity of A-type cyclins, comprising ten
members inArabidopsisthat fall into three subgroups (CYCA1 to CYCA3)
(Chaubet-Gigot 2000; Wang et al. 2004a). In synchronized tobacco BY2 cells,
different CYCAs are expressed sequentially at different time points from late
G1 till mid M phase (Reichheld et al. 1996), suggesting that plant CYCAs
could exert functions spanning G1/S to G2/M transitions. Local and tran-
sient induction ofNicta;CYCA3;2expression in transgenic tobacco activated
cell division in shoot apical meristem and leaf primordia (Wyrzykowska et al.
2002), whereas down-regulation by antisense expression induced defects in
embryo formation and impaired callus regeneration in vitro from leaf disks
(Yu et al. 2003). In transgenicArabidopsis, Nicta;CYCA3;2 and CDKA form
active kinase complex and overexpression ofNicta;CYCA3;2upregulated ex-
pression of an S-phase-specific histone gene but inhibited cell differentiation
and endoreplication (Yu et al. 2003). Together with the fact that expression
ofNicta;CYCA3;2occurs from late G1 and peaks at the S phase (Reichheld
et al. 1996), it appears that Nicta;CYCA3;2 could function in an analogous
manner as the animal cyclin E in G1/S transition (Yu et al. 2003). Interest-
ingly, inducible overexpression ofArath;CYCA2;3in transgenicArabidopsis
resulted in a similar cellular phenotype as that observed withNicta;CYCA3;2
whereas null mutation ofArath;CYCA2;3oppositely, as expected, promoted
endoreplication (Imai et al. 2006). It is worth noting that during the G1/S
transition of seed germinationArath;CYCA2;3was classified in the same