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Plant and non-plant species possess cryptochrome (CRY) photoreceptors to mediate blue-light regulation of development or the circadian clock. The blue light-dependent homooligomerization of Arabidopsis CRY2 is a known early photoreaction necessary for its functions, but the photobiochemistry and function of light-dependent homooligomerization and heterooligomerization of cryptochromes, collectively referred to as CRY photooligomerization, have not been well-established. Here we show that photooligomerization is an evolutionarily conserved photoreaction characteristic of the CRY photoreceptors in plant and some non-plant species. Our analyses of the kinetics of the forward and reverse reactions of photooligomerization of Arabidopsis CRY1 and CRY2 provide a previously unrecognized mechanism underlying the

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Cryptochromes (CRYs) are photoreceptors or components of the molecular clock in various evolutionary lineages, and they are commonly regulated by polyubiquitination and proteolysis. Multiple E3 ubiquitin ligases regulate CRYs in animal models, and previous genetics study also suggest existence of multiple E3 ubiquitin ligases for plant CRYs. However, only one E3 ligase, Cul4COP1/SPAs, has been reported for plant CRYs so far. Here we show that Cul3LRBs is the second E3 ligase of CRY2 in Arabidopsis. We demonstrate the blue light-specific and CRY-dependent activity of LRBs (Light-Response Bric-a-Brack/Tramtrack/Broad 1, 2 & 3) in blue-light regulation of hypocotyl elongation. LRBs physically interact with photoexcited and phosphorylated CRY2, at the CCE domain of CRY2, to facilitate polyubiquitination and degradation of CRY2 in response to blue light. We propose that Cul4COP1/SPAs and Cul3LRBs E3 ligases interact with CRY2 via different structure elements to regulate the abundance of CRY2 photoreceptor under different light conditions, facilitating optimal photoresponses of plants grown in nature.