Control of the Endocycle by Developmental Signals

Endoreduplication is often associated with cell differentiation in plant development, and most cell types except those in floral organs undergo several rounds of endocycle. It is well established that, in Arabidopsis, cells in leaves, hypocotyls and roots endoreduplicate up to 32C (Fig. 1), and recent genetic studies have provided key insights into the regulation of endoreduplication in the developmental context. Leaf trichome, a branched hair cell that is formed on the epidermal layer, is comprised of a single cell in Arabidopsis, and its nuclei endoreduplicate up to 32C (Hulskamp 2000). Many mutations that have increased or decreased branching have been identified and in almost all cases, these defects are associated with enhanced or reduced levels of endoreduplication, respectively (Hulskamp 2000; Table 1). For example, triptychon (try) shows increased ploidy in over-branched trichomes, suggesting that TRY protein is a negative regulator of endoreduplication (Perazza et al. 1999). TRY is a truncated MYB transcription factor that affects trichome initiation (Schellmann et al. 2002). It is thus likely that the signalling pathways that specify cell fate during trichome development also act as an upstream regulator of the endocycle. Another group of mutants such as kaktus (kak), rastifari (rfi) and polychome (pym) also produces over-branched trichomes with increased ploidy (Perazza et al. 1999). The double mutants pym kak and pym rfi, but not kak rfi, show strong additive phenotypes, suggesting that at least two independent genetic pathways, one governed by PYM and the other by KAK and RFI, negatively regulate the endocycle. KAK encodes a HECT-type ubiquitin E3 ligase that promotes ubiquitin transfer to appropriate targets for proteol-ysis (El Refy et al. 2003). How KAK represses the endocycle is not currently known but it is reasonable to predict that KAK targets some cell cycle-related protein for degradation. The pym mutation has been recently shown to be allelic to the uvi4 mutant that is isolated as a new mutation that gives resistance to UV-B irradiation (Hase et al. 2006). UVI4/PYM encodes an unknown protein that has a weak similarity to ferredoxin hydrogenase. Understanding how UVI4/PYM suppresses the endocycle requires further characterization but one possible mechanism is that UVI4/PYM is involved in maintaining the mitotic status within a cell because the UVI4/PYM expression pattern is similar to that of CYCB2;2, one of the M-phase specific cyclins (Hase et al. 2006).

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