Reactive Oxygen Species-Mediated Promotion of Root Growth Under Mild Water Stress During Early Seedling Stage of Vigna radiata (L.) Wilczek

Plants often show promotion of root growth under water stress while inhibiting shoot growth. Regulation of such differential growth response is not very clearly understood. Our earlier observation on the role of reactive oxygen species (ROS) in embryonic axis growth during seed germination prompted us to explore a possible role for ROS in water stress-induced root growth promotion during early seedling development of Vigna radiata. Normal and water stress-induced root growth were dependent on generation of ROS, for example, superoxide (O-2(center dot-)), hydrogen peroxide (H2O2) and hydroxyl radical (OH center dot), as evident from pharmacological experiments using ROS scavengers and inhibitors. However, in the case of the hypocotyl, neither normal growth nor growth inhibition under water stress was associated with ROS. Apoplastic O-2(center dot-) and H2O2 production monitored by spectrophotometric analysis of the bathing medium using XTT and xylenol orange, respectively and localization of O-2(center dot-) and H2O2 by NBT and TMB, respectively confirmed the involvement of ROS in root growth. Increased NADPH oxidase (NOX) activity (putative plasma membrane located apoplastic O-2(center dot-) producer) in the membrane fraction, as determined by in-gel assay and concomitant increase in apoplastic O-2(center dot-) and H2O2 generation, as evident by NBT and TMB stain in water stressed roots corroborated the view that promotion of root growth in the early seedling stage is effected by ROS being initiated with O-2(center dot-) generation by NOX in the apoplastic space.