Exogenous hydrogen peroxide reversibly inhibits root gravitropism and induces horizontal curvature of primary root during grass pea germination

During germination in distilled water (dH2O) on a horizontally positioned Petri dish, emerging primary roots of grass pea (Lathyrus sativus L.) grew perpendicular to the bottom of the Petri dish, due to gravitropism. However, when germinated in exogenous hydrogen peroxide (H2O2), the primary roots grew parallel to the bottom of the Petri dish and asymmetrically, forming a horizontal curvature. Time-course experiments showed that the effect was strongest when H2O2 was applied prior to the emergence of the primary root. H2O2 failed to induce root curvature when applied post-germination. Dosage studies revealed that the frequency of primary root curvature was significantly enhanced with increased H2O2 concentrations. This curvature could be directly counteracted by dimethylthiourea (DMTU), a scavenger of H2O2, but not by diphenylene iodonium (DPI) and pyridine, inhibitors of H2O2 production. Exogenous H2O2 treatment caused both an increase in the activities of H2O2-scavenging enzymes [including ascorbate peroxidase (APX: EC 1.11.1.11), catalase (CAT: EC 1.11.1.6) and peroxidase (POD: EC 1.11.1.7)] and a reduction in endogenous H2O2 levels and root vitality. Although grass pea seeds absorbed exogenous H2O2 during seed germination, DAB staining of paraffin sections revealed that exogenous H2O2 only entered the root epidermis and not inner tissues. These data indicated that exogenously applied H2O2 could lead to a reversible loss of the root gravitropic response and a horizontal curvature in primary roots during radicle emergence of the seedling.