Figure 1.

Proposed explanatory model of plants photoreceptors activity upon activated by light. (a) The biological active Pfr form of the phytochrome is activated by red light (R). The interaction between Pfr and PIFs triggers the rapid phosphorylation of PIFs entering protein degradation via the ubiquitin 26S proteasome pathway promoting photomorphogenesis. Pfr also induces COP1-SPA1 complex dissociation. All these pathways lead to the accumulation of major transcription factors (TFs) triggering photomorphogenesis such as HY5 (b) Two mechanisms are represented for cryptochrome signal transduction. Transcription regulation of light-activated Cry after interaction with TF CIB1 and its relatives (CIBs) leads to the activation of FT transcription, promoting floral initiation. The other mechanisms involve the interaction of cryptochromes with SPA1 proteins to suppress SPA1 activation of COP1 activity necessary for the degradation of HY5, HYH, CO, and other transcription regulators promoting photomorphogenesis. (c) ztls are activated by UV-A/blue light. Once activated, they interact with GI, resulting in the stabilization of these receptors, enabling them to act as E3 ligases which target the transcriptional regulators CDF and TOC1 for degradation. In addition, light-activated FKF1 interacts with and stabilizes CO. (d) The inactive dimeric form of UVR8 is activated to the monomeric form by UV-B, which interacts with the E3 ubiquitin ligase COP1. With the suppression of COP1, HY5 and HYH are stabilized. These two proteins, in a feed-forward loop, bind to the HY5 promoter region with further activation of transcription.