The Asymmetric Expression of <i>SAUR</i> Genes Mediated by ARF7/19 Promotes the Gravitropism and Phototropism of Plant Hypocotyls

Abstract

Gravitropism and phototropism are important for sessile plants to adapt to various environments, and the asymmetric distribution of auxin leads to the bending growth of hypocotyls during these tropic responses. However, the signaling events downstream of auxin that direct the bending growth remain unclear. Here, we observe asymmetric expression of Arabidopsis SAUR19 subfamily genes during gravitropic and phototropic responses induced by lateral redistribution of auxin. Ectopic expression of SAUR19 weakened these tropic responses, indicating that the asymmetric expression of SAUR19 subfamily genes is critical for hypocotyl bending. The auxin responsive transcription factor ARF7 may directly bind the AuxRE element of the SAUR19 promoter and activate SAUR19 expression. Furthermore, over-expression of SAUR19 under its own promoter rescues the impaired gravitropic and phototropic responses of the arf7 arf19 double mutant. Taken together, our results reveal that gravity- or light-triggered asymmetric auxin distribution induces asymmetric expression of SAUR19 subfamily genes via ARF7 and ARF19 in the hypocotyls, which leads to bending growth during gravitropic and phototropic responses.