Abstract
All higher plants possess multiple phytochrome photoreceptors, with phytochrome A (phyA) being light labile and other members of the family being relatively light stable (phyB-phyE in Arabidopsis [Arabidopsis thaliana]). phyA also differs from other members of the family because it enables plants to deetiolate in far-red light-rich environments typical of dense vegetational cover. Later in development, phyA counteracts the shade avoidance syndrome. Light-induced degradation of phyA favors the establishment of a robust shade avoidance syndrome and was proposed to be important for phyA-mediated deetiolation in far-red light. phyA is ubiquitylated and targeted for proteasome-mediated degradation in response to light. Cullin1 and the ubiquitin E3 ligase constitutive photomorphogenic1 (COP1) have been implicated in this process. Here, we systematically analyze the requirement of cullins in this process and show that only CULLIN1 plays an important role in light-induced phyA degradation. In addition, the role of COP1 in this process is conditional and depends on the presence of metabolizable sugar in the growth medium. COP1 acts with SUppressor of phytochrome A (SPA) proteins. Unexpectedly, the light-induced decline of phyA levels is reduced in spa mutants irrespective of the growth medium, suggesting a COP1-independent role for SPA proteins.
Highlights
All living organisms need to perceive and respond to changes in the environment in order to adapt their growth and development to fluctuating conditions
SUPPRESSOR OF PHYA (SPA) proteins, which are proposed to act with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) (Lau and Deng, 2012) are required for phyA degradation in the light irrespective of the presence of sucrose. These results indicate a primary importance for CUL1 in phyA degradation and suggest that SPA proteins may act independently of COP1
PhyA has been shown to be ubiquitylated in the light and its degradation is inhibited by proteasome inhibitors (Jabben et al, 1989; Seo et al, 2004; Debrieux and Fankhauser, 2010)
Summary
All living organisms need to perceive and respond to changes in the environment in order to adapt their growth and development to fluctuating conditions. Light-stable phytochromes predominantly control R/FR-reversible light responses while phyA has a specific mode of action allowing it to control seed germination and seedling deetiolation even when only a very minor fraction of the photoreceptor is in its active Pfr conformation (Quail, 2002; Franklin and Quail, 2010; Kami et al, 2010) Such conditions are encountered under deep vegetational cover and it has been argued that the appearance of phyA class phytochromes has provided a competitive advantage to flowering plants when plant cover on earth became important (Mathews, 2006). Despite the importance of light-induced phyA degradation the molecular mechanism underlying this crucial regulatory event are still poorly understood
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