Characterization of Mutants Reveal Novel Roles for Reactive Oxygen Species in Modulating Not Only Root Gravitropism but Also Hypoxia Tolerance in Rice Seedlings.

Tolerance to submergence-induced hypoxia is an important agronomic trait especially for crops in lowland and flooding-affected areas. Although rice () is considered a flood-tolerant crop, only limited cultivars display strong tolerance to prolonged submergence and/or hypoxic stress. Therefore, characterization of hypoxic resistant genes and/or germplasms have important theoretical and practical significance for rice breeding and sustained improvements. Previous investigations have demonstrated that loss-of-function of , a gene encoding an auxin efflux transporter, results in the loss of root gravitropism due to disrupted auxin transport in the root tip. In this study, we revealed a novel connection between and reactive oxygen species (ROS) in modulating root gravitropism and hypoxia tolerance in rice. It is shown that the mutant had decreased accumulation of ROS in root tip, due to the downregulation of glycolate oxidase encoding gene , one of the main HO sources. The morphological defects of root including waved rooting and agravitropism in mutant may be rescued partly by exogenous application of HO. The mutant exhibited increased resistance to ROS toxicity in roots due to treatment with HO. Furthermore, it is shown that the mutant had increased tolerance to hypoxic stress accompanied by lower ROS accumulation in roots, because the hypoxia stress led to over production of ROS in the roots of the wild type but not in that of mutant. Accordingly, the anoxic resistance-related gene showed differential expression in the root of the WT and mutant in response to hypoxic conditions. Notably, compared with the wild type, the mutant displayed a different pattern of auxin distribution in the root under hypoxia stress. It was shown that hypoxia stress caused a significant increase in auxin distribution in the root tip of the WT but not in that of the mutant. In summary, these results suggested that may play a role in regulating ROS accumulation probably via mediating auxin transport and distribution in the root tip, affecting root gravitropism and hypoxic tolerance in rice seedlings. These findings may contribute to the genetic improvement and identification of potential hypoxic tolerant lines in rice.