Trichomes are singled-celled hairs on the epidermis of leaves, stems and sepals that are thought to protect plants from damage by pathogens, light and drought. Wild-type Arabidopsis leaf trichomes have three branches, while stem trichomes are unbranched. Trichome branch number depends on GA signalling, nuclear DNA content (ploidy) and microtubules (Hulskamp et al. 1999). Plants with elevated GA levels or increased ploidy have overbranched trichomes, while plants with reduced GA signalling develop trichomes with fewer branches.
KAKTUS/UPL3 is an Arabidopsis HECT-Arm protein (Fig. 1; Downes et al. 2003; El Refy et al. 2003). The kaktus/upl3 mutants produce overbranched trichomes with increased ploidy (Perazza et al. 1999). The kaktus/upl3 mutants are also hypersensitive to GA-dependent hypocotyl cell elongation, and have many cells with elevated ploidy (Downes et al. 2003; El Refy et al. 2003). This suggests that, in contrast to PHOR1, KAKTUS/UPL3 is a negative regulator of GA signalling. KAKTUS/UPL3 also negatively regulates DNA content in a variety of cell types.
However, some GA responses are normal in kaktus/upl3 mutants, suggesting that KAKTUS/UPL3 is involved only in specific aspects of GA signalling or that another protein can substitute for KAKTUS/UPL3 function in certain circumstances. Arabidopsis KLI5/UPL4 is 54% similar to KAK-TUS/UPL3 (Downes et al. 2003; El Refy et al. 2003); however, the two genes are not entirely redundant given that upl4 mutants have no trichome phe-notype (Downes et al. 2003). As yet, KAKTUS/UPL3 has not been shown to function as a bona fide ubiquitin ligase, but the current hypothesis is that KAKTUS/UPL3 degrades protein(s) that are positive regulators of trichome branching, DNA replication and/or GA signalling.
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