Trichome cell growth in Arabidopsis thaliana can be derepressed by mutations in at least five genes

Leaf trichomes in Arabidopsis are unicellular epidermal hairs with a branch ed morphology. They undergo successive endoreduplication rounds early durin g cell morphogenesis. Mutations affecting trichome nuclear DNA content, suc h as triptychon or glabra3, alter trichome branching. We isolated new mutan ts with supernumerary trichome branches, which fall into three unlinked com plementation groups: KAKTUS and the novel loci, POLYCHOME and RASTAFARI. Th ey map to chromosomes IV, II, and V, respectively. The trichomes of these m utants presented an increased DNA content, although to a variable extent. T he spindly-5 mutant, which displays a constitutive gibberellin response, al so produces overbranched trichomes containing more nuclear DNA. We analyzed genetic interactions using double mutants and propose that two independent pathways, defined by SPINDLY and TRIPTYCHON, act to limit trichome growth. KAKTUS and POLYCHOME might have redundant actions mediating gibberellin co ntrol via SPINDLY. The overall leaf polysomaty was not notably affected by these mutations, suggesting that they affect the control of DNA synthesis i n a tissue- or cell type-specific manner. Wild-type tetraploids also produc e overbranched trichomes; they displayed a shifted polysomaty in trichomes and in the whole leaf, suggesting a developmental program controlling DNA i ncreases via the counting of endoreduplication rounds.