382 C. Gutierrez
expression is up regulated at the G1/S transition and excess expression pre-
vents cell differentiation and regeneration from leaf disks (Yu et al. 2003).
CYCA2;3 acts also as a major repressor of endoreplication. However, as re-
vealed by the phenotype of null mutants, endocycle occurrence is promoted
and ploidy level is increased, but the number of cells undergoing endocycle
does not change (Imai et al. 2006).
Adequate levels of CYCB are also important for correct development.
CYCB1 degradation depends on CCS52, a Fizzy-related (Fzr) activator of the
anaphase-promoting complex (APC; Capron et al. 2003), originally identified
in alfalfa (Cebolla et al. 1999). The relevance of CYCB1 is further supported
by studies where ectopic expression ofCYCB1;2(but not ofCYCB1;1)in
Arabidopsistrichomes results in the appearance of multicellular structures,
containing 2C nuclei, instead of the normal unicellular, polyploid branched
trichomes (Schnittger et al. 2002). Interestingly, mutations in other APC com-
ponents also lead to CYCB1 accumulation and to mitosis-related developmen-
tal defects (Blilou et al. 2002; Capron et al. 2003; Kwee and Sundaresan 2003).
Overexpression ofArabidopsis CYCD3;1(Riou-Khamlichi et al. 1999; De-
witte et al. 2003), but not ofCYCD2(Cockcroft et al. 2000), allows cytokinin-
independent growth and induces ectopic divisions producing leaves with
more but smaller cells. Consistent with this,CYCD3;1is up regulated in the
Arabidopsis siamesemutants, which has multicellular trichomes (Walker et al.
2000). Furthermore, overexpression ofAINTEGUMENTA(ANT;Mizukami
and Fischer 2000) or the auxin-inducibleARGOS(Hu et al. 2003) genes induce
cell proliferation, leading to an increase in organ size, an effect also mediated
by upregulatingCYCD3expression. It should be pointed out that the tran-
scriptional activation ofCYCD3by cytokinins is the main evidence for the
involvement of cytokinin in G1/S regulation (Soni et al. 1995). A recent study
implicates the microRNAJAW-Das a direct regulator ofTCPgenes (Palatnik
et al. 2003), a repressor ofCYCD3(Gaudin et al. 2000).
Seed germination is another process where D-type cyclins play crucial
roles. Before root emergence, expression of sixCYCD(and twoCYCA) genes
is activated (Masubelele et al. 2005). Current transcriptomic data support
a model in which the cumulative action (rather than functional redun-
dancy), of several CYCD determine cell cycle activation. A role in germination
has been also shown for maize CYCD2, based on its cytokinin-dependent
transcriptional activation (Gutierrez et al. 2005). Furthermore, the impor-
tance of hormones (benzyladenine, abscisic acid) during maize germina-
tion, most likely controlling the activity of different CDK-cyclin complexes,
has been also demonstrated (Sanchez et al. 2005). CYCD4;1, a distinct type
of cyclin that lacks the RBR-binding motif, forms active kinase complexes
with CDKA;1 and its function is required to promote callus proliferation in
hypocotyl explants (Kono et al. 2006).
Arabidopsis KRP1andKRP2are expressed mainly in endoreplicating cells
(Wang et al. 2000; De Veylder et al. 2001). Overexpression ofKRP1orKRP2