PP2A regulates shade avoidance response by dephosphorylating phytochrome-interacting factor 7 in Arabidopsis

Shade avoidance is a key adaptive growth response that enables plants to compete for light under dense vegetation. In Arabidopsis thaliana, the transcription factor PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) is dephosphorylated under shade and accumulates into the nucleus to promote shade-induced elongation growth through activation of auxin biosynthesis/signaling genes, yet the molecular mechanism underlying PIF7 dephosphorylation under shade conditions has remained unclear. Using yeast two-hybrid screening and extensive biochemical, genetic, and photobiological analyses, we identified protein phosphatase 2A (PP2A) as a direct regulator of PIF7 activity. We show that four PP2A regulatory subunits, B'α, B'β, B″α, and B″β, associate with PIF7 both in vitro and in vivo. Genetic analyses demonstrate that loss of PP2A B'α, B'β, B″α, and B″β function reduces hypocotyl elongation under shade, whereas overexpression of these subunits enhances shade-induced growth. Epistasis analyses further reveal that PP2A acts in the same genetic pathway as PIF7 to regulate hypocotyl elongation during shade avoidance. Consistently, expression of PIF7 target genes is attenuated in pp2a mutants under shade conditions, and immunopurified PP2A complexes efficiently dephosphorylate PIF7 in vitro. Together, our findings establish a direct biochemical role for PP2A in PIF7 activation and reveal a new role for PP2A in regulating shade responses in plants.