Exogenous hydrogen peroxide inhibits primary root gravitropism by regulating auxin distribution during Arabidopsis seed germination.

Hydrogen peroxide (HO) is the key factor in many physiological and metabolic processes in plants. During seed germination, exogenous HO application influences gravitropism and induces curvature of the primary root in grass pea and pea seedlings. However, it remains unclear whether and how this happens in the model plant Arabidopsis thaliana. In the present study, the effect of exogenous HO on the gravitropic response of primary roots during Arabidopsis seed germination was studied using histology and molecular biology approaches. Appropriate HO treatment not only restrained primary root growth, but also disrupted gravitropism and induced root curvature. Histological staining and molecular analysis demonstrated that exogenous HO correlated with lack of starch-dense amyloplasts in root tip columella cells, which ultimately results in the lack of gravisensing. Detection of calcium ion (Ca) by a fluorescent probe showed that Ca distribution changed and intracellular Ca concentration increased in HO-treated primary root, which was consistent with alterations in auxin distribution and concentration triggered by HO treatment. Furthermore, the normally polar localization of Pin-formed 1 (PIN1) and PIN2 became uniformly distributed on root tip cell membranes after treatment with HO. This leads to speculation that the IAA signaling pathway was affected by exogenous HO, causing asymmetrical distribution of IAA on both sides of the primary root, which would influence the gravitropic response.