Beyond the photocycle — how cryptochromes regulate photoresponses in plants?

Abstract

Cryptochromes (CRYs) are blue light receptors that mediate light regulation of plant growth and development. Land plants possess various numbers of cryptochromes, CRY1 and CRY2, which serve overlapping and partially redundant functions in different plant species. Cryptochromes exist as physiologically inactive monomers in darkness; photoexcited cryptochromes undergo homodimerization to increase their affinity to the CRY-signaling proteins, such as CIBs (CRY2-interacting bHLH), PIFs (Phytochrome-Interacting Factors), AUX/IAA (Auxin/INDOLE-3-ACETIC ACID), and the COP1-SPAs (Constitutive Photomorphogenesis 1-Suppressors of Phytochrome A) complexes. These light-dependent protein–protein interactions alter the activity of the CRY-signaling proteins to change gene expression and developmental programs in response to light. In the meantime, photoexcitation also changes the affinity of cryptochromes to the CRY-regulatory proteins, such as BICs (Blue-light Inhibitors of CRYs) and PPKs (Photoregulatory Protein Kinases), to modulate the activity, modification, or abundance of cryptochromes and photosensitivity of plants in response to the changing light environment.

Cryptochrome-mediated blue-light signalling modulates UVR8 photoreceptor activity and contributes to UV-B tolerance in Arabidopsis

“…Indeed, plant photoreceptors are activated upon photon absorption, which in conjunction with negative feedback regulation establishes a photoequilibrium according to the prevailing light conditions. For example, cryptochromes form an equilibrium between active homodimeric and inactive monomeric states, whereas UVR8 equilibrates between an active monomeric and an inactive homodimeric state 6,17,34,35 . Thus, both types of receptors are regulated at the level of their oligomeric state, with UVR8 "photomonomerizing" and cryptochromes "photodimerizing".…”

Plant photoreceptors and their signaling components compete for COP 1 binding via VP peptide motifs

“…We propose that in response to UV‐B light, UVR8 dimers monomerize, exposing a new interaction surface that binds to the COP1 WD40 domain and releases the UVR8 C‐terminal VP motif from structural restraints that prevent its interaction with COP1 in the absence of UV‐B (Yin et al , ; Heilmann et al , ; Camacho et al , ; Wu et al , ). Similarly, the VP motif in the CCT domain of cryptochromes may become exposed and available for interaction upon blue‐light activation of the photoreceptor (Müller & Bouly, ; Wang et al , ). Because UVR8 and CRY2 are very different in structure and domain composition, they likely use distinct interaction surfaces to target the COP1 WD40 domain, in addition to the VP peptide motifs.…”

Plant photoreceptors and their signaling components compete for binding to the ubiquitin ligase COP1 using their VP-peptide motifs

“…We propose that in response to UV-B light, UVR8 dimers monomerize, exposing a new interaction surface that binds to the COP1 WD40 domain and releases the UVR8 C-terminal VP motif from structural restraints that prevent its interaction with COP1 in the absence of UV-B (Yin et al, 2015;Heilmann et al, 2016;Wu et al, 2019;Camacho et al, 2019). Similarly, the VP motif in the CCT domain of cryptochromes may become exposed and available for interaction upon blue-light activation of the photoreceptor (Müller and Bouly, 2015;Wang et al, 2018). Because UVR8 and CRY2 are very different in structure and domain composition, they likely use distinct interaction surfaces to target the COP1 WD40 domain, in addition to the VP-peptide motifs.…”