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

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 gravisensing. 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.