All higher plants possess multiple phytochrome (phy) photoreceptors with phyA being light-labile and other members of the family being relatively light stable (phyB-phyE in Arabidopsis). phyA also differs from other members of the family because it enables plants to de-etiolate in far-red (FR) 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 de-etiolation in FR light. phyA is ubiquitylated and targeted for proteasome-mediated degradation in response to light. Cullin1 and the ubiquitin E3 ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (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 media. COP1 acts with SUPPRESSOR OF PHYA (SPA) proteins. Unexpectedly the light-induced decline of phyA levels is reduced in spa mutants irrespective of the growth media suggesting a COP1-independent role for SPA proteins.

PMID: 

23391578 

[PubMed – as supplied by publisher]