Phytochrome A inhibits shade avoidance responses under strong shade through repressing the brassinosteroid pathway in Arabidopsis.

In dense canopy, a reduction in red to far-red (R/FR) light ratio triggers shade avoidance responses (SARs) in Arabidopsis thaliana, a shade avoiding plant. Two red/far-red (R/FR) light photoreceptors, PHYB and PHYA, were reported to be key negative regulators of the SARs. PHYB represses the SARs under normal light conditions; however, the role of PHYA in the SARs remains elusive. We set up two shade conditions: Shade and strong Shade (s-Shade) with different R/FR ratios (0.7 and 0.1), which allowed us to observe phenotypes dominated by PHYB- and PHYA-mediated pathway, respectively. By comparing the hypocotyl growth under these two conditions with time, we found PHYA was predominantly activated in the s-Shade after prolonged shade treatment. We further showed that under s-Shade, PHYA inhibits hypocotyl elongation partially through repressing the brassinosteroid (BR) pathway. COP1 and PIF4,5 act downstream of PHYA. After prolonged shade treatment, the nuclear localization of COP1 was reduced, while the PIF4 protein level was much lower in the s-Shade than that in Shade. Both changes occurred in a PHYA-dependent manner. We propose that under deep canopy, the R/FR ratio is extremely low, which promotes the nuclear accumulation of PHYA. Activated PHYA reduces COP1 nuclear speckle, which may lead to changes of downstream targets, such as PIF4,5 and HY5. Together, these proteins regulate the BR pathway through modulating BES1/BZR1 and the expression of BR biosynthesis and BR target genes.