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

Abstract During germination in distilled water (dH 2 O) 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 (H 2 O 2 ), 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 H 2 O 2 was applied prior to the emergence of the primary root. H 2 O 2 failed to induce root curvature when applied post-germination. Dosage studies revealed that the frequency of primary root curvature was significantly enhanced with increased H 2 O 2 concentrations. This curvature could be directly counteracted by dimethylthiourea (DMTU), a scavenger of H 2 O 2 , but not by diphenylene iodonium (DPI) and pyridine, inhibitors of H 2 O 2 production. Exogenous H 2 O 2 treatment caused both an increase in the activities of H 2 O 2 -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 H 2 O 2 levels and root vitality. Although grass pea seeds absorbed exogenous H 2 O 2 during seed germination, DAB staining of paraffin sections revealed that exogenous H 2 O 2 only entered the root epidermis and not inner tissues. These data indicated that exogenously applied H 2 O 2 could lead to a reversible loss of the root gravitropic response and a horizontal curvature in primary roots during radicle emergence of the seedling.