Cryptochromes suppress leaf senescence in response to blue light in Arabidopsis

Blue light photoreceptors, cryptochromes, suppress the progression of leaf senescence in response to blue light through the regulation of HY5 and PIF4/5 transcriptional factors. The induction and progression of leaf senescence are effectively changed according to the light environment. The leaf senescence response is enhanced when plants are grown under a dense shade cast by neighboring vegetation. Although the fluence rate of the red and blue regions in the light spectrum is strongly attenuated under shade, photosensory mechanisms that underpin the blue light response are still unclear. In this study, we analyzed leaf senescence in response to blue light in Arabidopsis (Arabidopsis thaliana). We found that leaf senescence was promoted by the elimination of active phytochrome Pfr by pulsed far-red (FR) light, whereas irradiation with blue light suppressed leaf senescence in the wild type but not in the cryptochrome (CRY)-deficient mutant, cry1 cry2. Hence, two light-sensing modes contributed to the suppression of leaf senescence that was dependent on light spectrum features. First was the leaf senescence response to blue light, which was mediated exclusively by cryptochromes. Second was the phytochrome-mediated leaf senescence response to red/FR light. Physiological analysis of transgenic plants expressing green fluorescent protein (GFP)-tagged CRY2 revealed that photo-activation of cryptochromes was required to suppress leaf senescence in response to blue light. Transcriptomic analysis further uncovered the molecular and cellular processes involved in the regulation of leaf senescence downstream of cryptochromes. Furthermore, analysis of cryptochrome-downstream components indicated that ELONGATED HYPOCOTYL 5 (HY5) and PHYTOCHROME INTERACTING FACTOR (PIF) 4 and PIF5 were required for suppression and promotion of leaf senescence, respectively.