Light modulates the gravitropic responses through organ-specific PIFs and HY5 regulation ofLAZY4expression inArabidopsis

Abstract

Light and gravity are two key environmental factors that control plant growth and architecture. However, the molecular basis of the coordination of light and gravity signaling in plants remains obscure. Here, we report that two classes of transcription factors, PHYTOCHROME INTERACTING FACTORS (PIFs) and ELONGATED HYPOCOTYL5 (HY5), can directly bind and activate the expression ofLAZY4, a positive regulator of gravitropism in both shoots and roots inArabidopsis. In hypocotyls, light promotes degradation of PIFs to reduceLAZY4expression, which inhibits the negative gravitropism of hypocotyls.LAZY4overexpression can partially rescue the negative gravitropic phenotype ofpifqin the dark without affecting amyloplast development. Our identification of the PIFs-LAZY4regulatory module suggests the presence of another role for PIF proteins in gravitropism, in addition to a previous report demonstrating that PIFs positively regulate amyloplast development to promote negative gravitropism in hypocotyls. In roots, light promotes accumulation of HY5 proteins to activate expression ofLAZY4, which promotes positive gravitropism in roots. Together, our data indicate that light exerts opposite regulation ofLAZY4expression in shoots and roots by mediating the protein levels of PIFs and HY5, respectively, to inhibit the negative gravitropism of shoots and promote positive gravitropism of roots inArabidopsis.