FIGURE 1.

Simplified overview of light signal transduction via phytochromes (PHY) and cryptochromes (CRY) and their main interacting proteins. (1) Upon far-red light (FRL) or red light (RL) irradiation, cytosolic PHYA_Pr and PHYB_Pr, respectively, are converted into their active Pfr forms, which migrate to the nucleus. (2) In the cell nucleus, PHY_Pfr represses CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) E3-complex (“COP1 complex”), which is responsible for targeting positive components of the light signal transduction pathway such as LONG HYPOCOTYL 5 (HY5) and LONG HYPOCOTYL IN FAR-RED 1 (HFR1) to proteasomal degradation. The expression of genes coding for HY5 and HFR1 is also promoted by PHY_Pfr and these transcription factors are responsible for stimulating the expression of numerous photomorphogenesis-associated genes. (3) In parallel, PHY_Pfr activates an unidentified E3 Ubiquitin (Ub) ligase responsible for targeting light signal transduction repressor proteins such as PHYTOCHROME-INTERACTING FACTORS (PIFs) to degradation via 26S proteasome. (4) PIFs are also known to stimulate HFR1 and HFR1 blocks PIFs transcriptional activity. (5) Upon blue light (BL) or UV-A exposure, nuclear-localized CRY is converted into active CRY*, which also repress “COP1 complex” action.