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

Hydrogen peroxide (H2O2) is the key factor in many physiological and metabolic processes in plants. During seed germination, exogenous H2O2 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 H2O2 on the gravitropic response of primary roots during Arabidopsis seed germination was studied using histology and molecular biology approaches. Appropriate H2O2 treatment not only restrained primary root growth, but also disrupted gravitropism and induced root curvature. Histological staining and molecular analysis demonstrated that exogenous H2O2 correlated with lack of starch-dense amyloplasts in root tip columella cells, which ultimately results in the lack of grayisensing. Detection of calcium ion (Ca2+) by a fluorescent probe showed that Ca2+ distribution changed and intracellular Ca2+ concentration increased in H2O2-treated primary root, which was consistent with alterations in auxin distribution and concentration triggered by H2O2 treatment. Furthermore, the normally polar localization of Pin-formed 1 (PIN1) and PIN2 became uniformly distributed on root tip cell membranes after treatment with H2O2. This leads to speculation that the IAA signaling pathway was affected by exogenous H2O2, causing asymmetrical distribution of IAA on both sides of the primary root, which would influence the gravitropic response.