The Endoreduplication Cell Cycle: Regulation and Function 87
endoreduplication correlates with its branching pattern and size, with highly
endoreduplicated trichomes being more branched and larger (Hulskamp
et al. 1999) (Fig. 2). InCDC6-andCDT1-overexpressing plants, endoredu-
plication is enhanced in trichome-forming cells, which eventually display
a more highly branched pattern (Castellano et al. 2004). RBR1 inactivation
by WDV RepA protein also resulted in trichomes with increased nuclear size
and larger number of branches (Desvoyes et al. 2006). The closely related
CYCB1;1andCYCB1;2genes are not normally expressed in trichomes. How-
ever, ectopic expression ofCYCB1;2,butnotCYCB1;1, in trichomes promoted
a switch to cell division resulting in a multicellular phenotype (Schnittger
et al. 2002b). Interestingly, although trichome cells appeared to switch from
an endoreduplication to a full mitotic cycle uponCYCB1;2expression, the
total number of cell cycles did not change during trichome development, in-
dicating the presence of upstream constraints on the number of cell cycle
rounds. The multicellular phenotype became enhanced whenCYCB1;2was
expressed in asiamesemutant background (which itself results in multicel-
lular trichomes), suggesting thatSIMinteracts genetically withCYCB1;2and,
Fig. 2Simplified diagram illustrating the relationship between endoreduplication and tri-
chome cell size and differentiation inArabidopsis. Wild-type trichomes are unicellular,
have three to four branches and an average DNA content of 32C. In certain mutants en-
doreduplication is stimulated up to a DNA content of 256C, resulting in large cells with
a complex branched morphology. These mutants identify negative regulatory genes of
endoreduplication. In other mutants endoreduplication is inhibited, indicating that the
corresponding genes play a positive role. In addition, expression of certain cell cycle
genes can promote or repress endoreduplication, the latter generally resulting in smaller
trichomes with a simplified morphology (as in the case ofICK1/KPR1). Expression of
CYCB1;2andCYCD3;1in trichomes can induce the mitotic program, resulting in multi-
cellular structures. TheSIAMESE(SIM) gene appears to stimulate endoreduplication by
repressingCYCB1;2andCYCD3;1