The UV-A/blue light photoreceptor cry2 plays a fundamental role in the transition from the vegetative to the reproductive phase in the facultative long-day plant Arabidopsis thaliana by stabilizing the photoperiodic pathway element CO and modulating FT expression under long-day conditions. The level of cry2 protein itself is regulated by light with a strong decrease upon transition of etiolated seedlings to blue light. Both, cry2 signaling and its degradation occur in the nucleus. Degradation of cry2 is preceded by phosphorylation and polyubiquitination and mediated by the 26S proteasome. Although, the E3 ubiquitin ligase COP1 has been identified as one element involved in cry2 degradation, several cop1 mutants show only some reduction but no prevention of cry2 degradation. We thus reasoned that SPA proteins, which act in concert with COP1, could also have a role in cry2 degradation. To test this hypothesis we analyzed cry2 protein levels in spa single, triple and quadruple mutants. cry2 degradation under continuous blue light was alleviated in a fluence rate-dependent fashion in all spa mutants analyzed. Consistent with a role of SPA proteins in phyA signaling we found enhanced levels of cry2 in the phyA mutant in particular under low fluence rate blue light. Moreover, in vitro pull-down studies showed that photoactive cry2 interacts directly with SPA1 and SPA4. Together our results suggest that cry2 is under control of SPA and phyA thus adding another piece of information on the molecular mechanisms of interaction between cryptochrome and phytochrome photoreceptors.
Weidler, Guido: Die Rolle der SPA Proteine auf den lichtregulierten Abbau von Arabidopsis thaliana Cryptochrom 2. : Philipps-Universität Marburg 2012-07-11. DOI: https://doi.org/10.17192/z2012.0491.
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