Scientific Correspondence The Roles of Photoreceptor Systems and the COP1- Targeted Destabilization of HY5 in Light Control of Arabidopsis Seedling Development

Arabidopsis seedlings display contrasting developmental patterns depending on the light environment, e.g. photomorphogenesis in the light and skotomorphogenesis (etiolation) in darkness. Recent studies have implicated protein degradation as a means of regulating this developmental switch (Osterlund et al., 2000). For example, HY5, a positive photomorphogenic regulator that acts to promote the light developmental pattern (Koornneef et al., 1980; Oyama et al., 1997), is subject to the control of negative photomorphogenic regulators such as COP1 and COP9 signalosome at the level of protein stability (Osterlund et al., 1999, 2000). Light controls this process by modulating the subcellular localization of COP1. COP1 is predominantly cytoplasmic in the light, but accumulates in the nuclei in the darkness where it directly interacts with HY5 (von Arnim and Deng, 1994; Ang et al., 1998). A nuclear COP1-HY5 interaction is necessary for the degradation of HY5 (Osterlund et al., 2000). HY5 is a basic Leu zipper-type transcription factor and is localized in the nucleus constitutively (Oyama et al., 1997; Ang et al., 1998). It has been shown that HY5 binds to the G-box motif of multiple lightinducible promoters and is necessary for optimal expression of the corresponding genes (Ang et al., 1998; Chattopadhyay et al., 1998a, 1998b). The darkdependent degradation of HY5 provides a reasonable means by which the activity of HY5 and ultimately HY5-mediated gene expression could be regulated in responsive to light. We set out to examine HY5mediated gene expression as a measure of its activity in correlation with HY5 protein levels.