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.

Figure 1

The homodimerization-dependent photoactivation and negative feedback regulation of plant cryptochromes. Cryptochromes exist as physiologically inactive monomers in darkness. Photoexcited CRY molecules undergo homodimerization to become physiologically active. The CRY homodimer or oligomers interact with the COP1/SPA complex to inhibits ubiquitylation and degradation of transcription regulators, such as HY5, Accumulation of HY5 promotes transcriptional changes of light-responsive genes, including increased transcription of BICs in response to light. BIC proteins interact with CRYs to inhibit CRY homodimerization, CRY activity, and photosignaling.

Figure 2

PPKs catalyze blue light-dependent phosphorylation of CRY2 to trigger its polyubiquitylation by E3 ubiquitin ligases (E3) and degradation by the 26S proteasome.

Figure 3

A model depicting interaction of CRY with three types of CRY-signaling proteins to transduce light signals. First, photoexcited CRY homodimer/oligomer interacts with transcription factors (TFs), such as CIBs and PIFs, to regulate transcription directly. Second, photoexcited CRY homodimer/oligomer interacts with transcription regulators, such as AUX/IAA, to regulate transcription indirectly. Third, photoexcited CRY homodimer/oligomer interacts with the COP1/SPA complex to suppress ubiquitylation and degradation of transcription factors and to regulate transcription indirectly. It is hypothesized that both unphosphorylated and phosphorylated CRY (depicted by negative changes) are active but the latter may have higher activity.