Mechanisms of Cryptochrome-Mediated Photoresponses in Plants

Abstract

Cryptochromes are blue-light receptors that mediate photoresponses in plants. The genomes of most land plants encode two clades of cryptochromes, CRY1 and CRY2, which mediate distinct and overlapping photoresponses within the same species and between different plant species. Photoresponsive protein-protein interaction is the primary mode of signal transduction of cryptochromes. Cryptochromes exist as physiologically inactive monomers in the dark; the absorption of photons leads to conformational change and cryptochrome homooligomerization, which alters the affinity of cryptochromes interacting with cryptochrome-interacting proteins to form various cryptochrome complexes. These cryptochrome complexes, collectively referred to as the cryptochrome complexome, regulate transcription or stability of photoresponsive proteins to modulate plant growth and development. The activity of cryptochromes is regulated by photooligomerization; dark monomerization; cryptochrome regulatory proteins; and cryptochrome phosphorylation, ubiquitination, and degradation. Most of the more than 30 presently known cryptochrome-interacting proteins are either regulated by other photoreceptors or physically interactingwith the protein complexes of other photoreceptors. Some cryptochrome-interacting proteins are also hormonal signaling or regulatory proteins. These two mechanisms enable cryptochromes to integrate blue-light signals with other internal and external signals to optimize plant growth and development.

Keywords: Arabidopsis; CRY1; CRY2; blue light; cryptochrome; photomorphogenesis; photoreceptor; protein–protein interactions; proteolysis; transcription.

Figure 1

Photoreceptors mediate light regulation of gene expression to modulate plant growth and development. The diagram depicts the general action mechanisms of most plant photoreceptors, except phototropins (PHOTs), which act by modulating gene expression to alter plant growth and development. The ultraviolet (UV)-B receptor UVR8, blue/UV-A light receptors cryptochromes (CRYs), PHOTs, the LOV-domain/F-box proteins ZTL/FKF1/LKP2 represented by ZLF, and red/far-red (FR) light receptors phytochromes (PHYs) have the respective absorption spectra that are indicated by their positions under the light wavelength (nm). Known or potential regulatory mechanisms of gene expression are indicated. PHOTs are found primarily in plasma membrane such that regulation of gene expression is not depicted as the main mechanism of PHOTs. PHYs, especially phyA, are known to absorb blue light, in addition to red/FR light, and to regulate blue light responses.

Figure 2

CRY2 photooligomerization and dark-reversion. The Flag- and Myc-tagged Arabidopsis CRY2 recombinant proteins were coexpressed in human embryonic kidney cells (HEK293). (a) HEK293 cells were irradiated with 100 μmol m⁻²s⁻¹ blue light for the indicated time at 21°C; the kinetics of CRY2 homooligomerization was measured by co-immunoprecipitation at the indicated time after illumination. (b) The kinetics of CRY2 monomerization or dark-reversion was measured by co-immunoprecipitation at the indicated time after light-treated HEK293 cells were transferred to darkness. Results of this experiment indicate that photooligomerization of CRY2 is much faster (~30x) than its dark reversion under the experimental conditions used.

Figure 3

CRY photoactivation, signaling, and inactivation mechanisms. Cryptochromes exist as inactive monomers in darkness. Photoexcited cryptochromes undergo homooligomerization to become biochemically and physiologically active. The cryptochrome homooligomers interact with cryptochrome-interacting proteins. The presently known cryptochrome-interacting proteins, or components of the CRY complexome, include transcription regulators CIB proteins, PIF proteins, AUX/IAA proteins, the cryptochrome regulators BICs and PPKs, the E3 ubiquitin ligase complex COP/SPA, and the BRs (including BZR1, BES1, BIN2, and BIM1). The CRY complexome mediates blue-light regulation of transcription or protein stability. For example, the CRY-COP1-SPA interaction positively regulates the abundance of the HY5 protein, which promotes transcription of the BIC genes. The BIC proteins interact with photoexcited cryptochromes to inhibit cryptochrome homooligomerization and activity. The PPK protein kinases catalyze blue-light-dependent phosphorylation of cryptochromes to positively regulate not only cryptochrome activity but also cryptochrome polyubiquitination. The COP1/SPA proteins and another E3 ubiquitin ligase (E3X) catalyze the polyubiquitination and degradation of phosphorylated cryptochromes. Abbreviations: AUX/IAA, auxin/indole-3-acetic acid; ARFs, auxin response factors; BIC, blue-light inhibitor of cryptochromes; BIM1, bisindolylmaleimide-based protein kinase C inhibitor; BIN2, bridging integrator 2; BR, brassinosteroid regulator; BZR1, brassinosteroid signaling positive regulator; BES1, BRI1-EMS-SUPPRESSOR 1; BIM1, BES1-INTERACTING MYC-LIKE1; BIN2, BR-INSENSITIVE 2; CIB, cryptochrome-interacting basic helix-loop-helix; CO, CONSTANS; COP1, CONSTITUTIVE PHOTOMORPHOGENESIS 1; CRY, cryptochrome; E3X, unknown E3 ubiquitin ligases; HY5, LONG HYPOCOTYL 5; PIF, phytochrome-interacting factor; PPK, photoregulatory protein kinase; SPA, suppressor of phytochrome A.