Hydrogen peroxide mediates high-intensity blue light-induced hypocotyl phototropism of cotton seedlings.

Phototropism is a classic adaptive growth response that helps plants to enhance light capture for photosynthesis. It was shown that hydrogen peroxide (H₂O₂) participates in the regulation of blue light-induced hypocotyl phototropism; however, the underlying mechanism is unclear. In this study, we demonstrate that the unilateral high-intensity blue light (HBL) could induce asymmetric distribution of H₂O₂ in cotton hypocotyls. Disruption of the HBL-induced asymmetric distribution of H₂O₂ by applying either H₂O₂ itself evenly on the hypocotyls or H₂O₂ scavengers on the lit side of hypocotyls could efficiently inhibit hypocotyl phototropic growth. Consistently, application of H₂O₂ on the shaded and lit sides of the hypocotyls led to reduced and enhanced hypocotyl phototropism, respectively. Further, we show that H₂O₂ inhibits hypocotyl elongation of cotton seedlings, thus supporting the repressive role of H₂O₂ in HBL-induced hypocotyl phototropism. Moreover, our results show that H₂O₂ interferes with HBL-induced asymmetric distribution of auxin in the cotton hypocotyls. Taken together, our study uncovers that H₂O₂ changes the asymmetric accumulation of auxin and inhibits hypocotyl cell elongation, thus mediating HBL-induced hypocotyl phototropism.