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

The asymmetric distribution of auxin leads to the bending growth of hypocotyls during gravitropic and phototropic responses, but the signaling events downstream of auxin remain unclear. Here, we identify many SAUR genes showing asymmetric expression in soybean hypocotyls during gravistimulation and then study their homologs in Arabidopsis. SAUR19 subfamily genes have asymmetric expression in Arabidopsis hypocotyls during gravitropic and phototropic responses, induced by the lateral redistribution of auxin. Both the mutation of SAUR19 subfamily genes and the ectopic expression of SAUR19 weaken these tropic responses, indicating the critical role of their asymmetric expression. The auxin-responsive transcription factor ARF7 may directly bind the SAUR19 promoter and activate SAUR19 expression asymmetrically in tropic responses. Taken together, our results reveal that a gravity- or light-triggered asymmetric auxin distribution induces the asymmetric expression of SAUR19 subfamily genes by ARF7 and ARF19 in the hypocotyls, which leads to bending growth during gravitropic and phototropic responses. [Display omitted] •Gravity triggers asymmetric expression of SAUR family genes in soybean hypocotyls•SAUR19 subfamily genes are expressed asymmetrically in Arabidopsis hypocotyls in tropisms•SAUR19 subfamily genes are critical for hypocotyl gravitrophic and phototropic responses•ARF7 and ARF19 mediate the expression of SAUR19 subfamily genes in tropic responses Gravitropism and phototropism are important for sessile plants to adapt to various environments. Wang et al. show that gravity- or light-triggered asymmetric distribution of auxin induces the asymmetric expression of SAUR19 subfamily genes by ARF7 and ARF19, which directs the bending growth of Arabidopsis hypocotyls.