The action mechanisms of plant cryptochromes

Abstract

Cryptochromes (CRY) are blue-light receptors that mediate various light responses in plants. The photoexcited CRY molecules undergo several biophysical and biochemical changes, including electron transfer, phosphorylation and ubiquitination, resulting in conformational changes to propagate light signals. Two modes of CRY signal transduction have recently been discovered: the cryptochrome-interacting basic-helix-loop-helix 1 (CIB)-dependent CRY2 regulation of transcription; and the SUPPRESSOR OF PHYA1/CONSTITUTIVELY PHOTOMORPHOGENIC1 (SPA1/COP1)-dependent cryptochrome regulation of proteolysis. Both CRY signaling pathways rely on blue light-dependent interactions between the CRY photoreceptor and its signaling proteins to modulate gene expression changes in response to blue light, leading to altered developmental programs in plants.

Figure 1

Photoexcitation of cryptochromes. (a) Five possible redox forms of flavins. The two different forms of semiquinone radicals: anion radical (e.g. FAD^•—) and neutral radical (e.g. FADH^•), and two forms of reduced flavins: protonated hydroquinone (e.g. FADH₂) and anionic hydroquinone (e.g. FADH^—) are shown. R: side groups of flavins. (b) The photolyase-like cyclic electron shuttle model of cryptochrome photoexcitation. In this model, the resting state of a cryptochrome contains the anion radical semiquinone (FAD^•—). Upon photon absorption, the excited FAD^•— transfers an electron to ATP, triggering phosphotransfer and autophosphorylation of the cryptochrome. The electron is subsequently transferred back to flavin to complete the cycle. The putative locations of phosphorous group (red circle) and electron transfer path (red arrows) are indicated.

Figure 2

Signal transduction of cryptochromes.(a) Photoexcited cryptochrome change conformation to initiate signal transduction by interacting with signaling proteins. This model depicts cryptochrome homodimerization via the PHR domains, light-dependent phosphorylation (negative charges shown), changes of protein conformation by the disengagement of the PHR and CCE domains, and interaction with partner proteins, including CIBs, SPAs, COP1 and other yet to be identified CRY-interacting proteins (X and Y). (b) Two mechanisms of cryptochrome signal transduction: regulation of transcription via light-dependent interaction of cryptochromes with transcription factors CIB1 and its relatives (CIBs), and post-translational regulation of proteolysis via light-dependent interaction of cryptochromes with SPA1 and its relatives (SPAs). The cryptochrome-interacting CIBs activate FT transcription to promote floral initiation. Cryptochromes interact with SPA proteins to suppress the SPA activation of COP1 activity that is required for the degradation of HY5, HYH, CO, and other transcription regulators, resulting in changes of transcription of light-regulated genes (LRG) and photomorphogenesis.