Insight Into The Mechanism Of H₂O₂-induced Horizontal Bending In Primary Pea Root

In recent years, biochemical and genetic researches indicate that H2O2 has been identified as a signaling molecule to regulate biotic and abiotic stresses in plant. H2O2 signaling regulates a number of biochemical and physiological processes in plants, such as stomatal closure, formation of adventitious root, PCD and roots growth, but with regard to its impact on the early primary root growth of plants is also rarely reported. Recent studies have shown that exogenous H2O2 can induce the loss of geotropism and bending growth of grass pea primary root. This paper used pea seeds(Long Wan 1st) as the test material, through a variety of physiological and biochemical methods to study the effects of exogenous H2O2 on the bending induction of primary pea root and physiological indicators, as well as the mitigative effect of plant hormones and Ca2 + on H2O2-induced the horizontal bending of plants primary roots, in order to explore the physiological mechanisms of horizontal bending of the primary root and mitigative effects. Test results were as follows:1. Exogenous H2O2 induced horizontal bending of pea primary root Pea seeds treated with different concentrations of exogenous H2O2, found that exogenous H2O2 not only inhibited the normal root growth and induced horizontal bending, and the bending degree and curvature increased with the increase of H2O2 concentration, namely, there existed a dose effect.2. Exogenous H2O2 induced the hydrolysis of starch in root cap of pea primary root The starch content, α-amylase activity and ABA, gibberellin3(GA3) content were determined with treatment of exogenous H2O2. The results found that, GA3 content and α-amylase activity increased, the concent of ABA and starch decreased. The mechanism may be that the decrease of ABA as well as increase of GA3 activated α-amylase activity, and thus root starch degraded. According to starch-statolith hypothesis, roots lost the perception of gravity with the hydrolysis of starch in root cap, providing condition for the horizontal bending growth of the primary root.3. Changes of endogenous hormones in bending primary root induced by exogenousH2O2 Changes of hormone content in bending root were determined in this experiment, and results indicated that auxin(IAA), cytokinin(CTK) and abscisic acid(ABA) levels in root were reduced, while the GA3 content increased, and the IAA, GA3, ABA and CTK contents were significant difference in both sides of curved root, namely that they were asymmetric distribution. Asymmetric distribution of these hormones can lead to the occurrence of bending rowth of root.4. Exogenous H2O2 rather than endogenous H2O2 induced the bending primary root Endogenous H2O2 content in primary root was determined with treatment of exogenous H2O2, and found that exogenous H2O2 could reduce endogenous H2O2 content in the root, indicating that there was some connection between the two kind of H2O2. In order to explore the reason leading root bending, the experiment applied H2O2 scavenger, DMTU, and an inhibitor of H2O2 produce, pyridine, and observed inhibition effect of root bending. The results illustrated that, DMTU could significantly inhibit the growth of root bending, pyridine had no significant inhibition effect. The above results indicated that exogenous H2O2 rather than endogenous H2O2 induced the occurrence of root bending.5. The mitigation effect of exogenous hormones on bending primary root induced by H2O2 By applying exogenous hormones of IAA, ABA and CTK, we found that IAA and ABA could alleviate the curvature of primary root induced by H2O2, but CTK had no mitigation effect. And when exogenous GA3 treatment alone, curvature of primary pea root also occurred at a certain degree. This showed that ABA, IAA and GA3 may play a crucial role in geotropism bending of root. 6. The mitigation effect of exogenous Ca2+ on bending primary root induced by H2O2 Ca2+, a second messenger in plant, impacts the metabolism of plant deeply. In this experiment, when pea seeds treated by exogenous Ca2+, we found that Ca2+ had the mitigation effect on primary bending root induced by H2O2, and the mitigation effect increased with increasing concentrations of Ca2+. And exogenous Ca2+ treatments could relieve the increase or decrease of hormone in roots, and thereforeCa2+ might serve as a signal substances and act upstream of the hormone, and thus adjusted the growth of the primary root.