When plants are shaded, production of the hormone auxin increases to initiate stem growth to help the plant regain access to sunlight. Shade is sensed as a reduction in the red to far-red light ratio (R:FR) through the photoreceptor phytochrome B (PHYB). Sunny conditions produce high R:FR, which causes PHYB to adopt a far-red–absorbing active conformation and translocate to the nucleus, where it triggers the phosphorylation and subsequent degradation of phytochrome-interacting transcription factors (PIFs). Shade conditions result in low R:FR, which shifts PHYB into the inactive, cytosolic red-absorbing form, thus allowing PIFs to promote the expression of genes associated with shade avoidance and stem growth. Li et al. explored the links between PHYB, PIF activity, and regulation of auxin signaling in Arabidopsis thaliana. Using plants expressing luciferase reporter constructs, Li et al. identified a region within the promoter of the A. thaliana shade-response marker PIL1 (PIF3-like 1) that is required for its induction upon shading. Using this region as bait in a yeast one-hybrid screen, the authors identified PIF7, a known PHYB-interacting protein, as binding to this promoter region. Shade-grown pif7 mutants exhibited short stems and other phenotypes consistent with defects in the response to shade, including the failure to up-regulate auxin-responsive genes. Application of exogenous auxin rescued the pif7 stem elongation phenotype, indicating that the mutants responded to the hormone, but shade-induced expression of auxin biosynthesis genes was reduced in pif7 mutants compared with wild-type (WT) plants. Western blot analysis demonstrated that PIF7 was present in both phosphorylated and unphosphorylated forms in WT plants grown in full light. Shade exposure increased the proportion of PIF7 that was unphosphorylated, and returning the plants to full light reversed this effect. These results suggest a model in which low R:FR leads to the retention of PHYB in the cytoplasm and accumulation of dephosphorylated PIF7 in the nucleus, where it promotes the expression of genes required for auxin synthesis. It is not clear how the phosphorylation state of PIF7 is modulated by light, but the authors propose the involvement of light-responsive kinase(s) and phosphatase(s).
L. Li, K. Ljung, G. Breton, R. J. Schmitz, J. Pruneda-Paz, C. Cowing-Zitron, B. J. Cole, L. J. Ivans, U. V. Pedmale, H.-S. Jung, J. R. Ecker, S. A. Kay, J. Chory, Linking photoreceptor excitation to changes in plant architecture. Genes Dev. 26, 785–790 (2012). [Abstract] [Full Text]