Light modulates the gravitropic responses through organ-specific PIFs and HY5 regulation of <i>LAZY4</i> expression in <i>Arabidopsis</i>.

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 of <i>LAZY4</i>, a positive regulator of gravitropism in both shoots and roots in <i>Arabidopsis</i> In hypocotyls, light promotes degradation of PIFs to reduce <i>LAZY4</i> expression, which inhibits the negative gravitropism of hypocotyls. <i>LAZY4</i> overexpression can partially rescue the negative gravitropic phenotype of <i>pifq</i> in the dark without affecting amyloplast development. Our identification of the PIFs-<i>LAZY4</i> regulatory 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 of <i>LAZY4</i>, which promotes positive gravitropism in roots. Together, our data indicate that light exerts opposite regulation of <i>LAZY4</i> expression 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 in <i>Arabidopsis</i>.