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 (HO) 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 HO in cotton hypocotyls. Disruption of the HBL-induced asymmetric distribution of HO by applying either HO itself evenly on the hypocotyls or HO scavengers on the lit side of hypocotyls could efficiently inhibit hypocotyl phototropic growth. Consistently, application of HO on the shaded and lit sides of the hypocotyls led to reduced and enhanced hypocotyl phototropism, respectively. Further, we show that HO inhibits hypocotyl elongation of cotton seedlings, thus supporting the repressive role of HO in HBL-induced hypocotyl phototropism. Moreover, our results show that HO interferes with HBL-induced asymmetric distribution of auxin in the cotton hypocotyls. Taken together, our study uncovers that HO changes the asymmetric accumulation of auxin and inhibits hypocotyl cell elongation, thus mediating HBL-induced hypocotyl phototropism.