Apoplastic reactive oxygen species mediated escape growth of root during illumination in Vigna radiata (L.) Wilczek seedlings

Besides gravity, roots are also guided by light to grow deep into the soil and sensitivity of roots to light is evidently due to presence of photoreceptors like phototropins. Such light-induced root growth (light-escape growth) presumably involves reactive oxygen species (ROS). Present study explores the possibility of ROS action in this event during early seedling growth of Vigna radiata based on pharmacological evidences. Germinated (20 h) seeds were incubated in dark or light in presence of general ROS scavenger (propyl gallate), specific scavengers of O-2(center dot-) (copper chloride; CuCl2), H2O2 [dimethylthiourea (DMTU) and potassium iodide (KI)] and (OH)-O-center dot (sodium benzoate) and ROS-producing enzyme inhibitors [zinc chloride (ZnCl2), inhibitor of NADPH oxidase (NOX); diethyldithiocarbamate (DEDTC), inhibitor of superoxide dismutase (SOD) and salicylhydroxamic acid (SHAM), inhibitor of peroxidase]. Light-induced root growth of 3-day seedlings diminished significantly in case of all the treatments suggesting for a positive role of ROS in light-escape growth. This is supported by elevated level of apoplastic ROS in light grown roots as evident from ROS-specific staining [nitroblue tetrazolium chloride (NBT) for O-2(center dot-) and 3,3,5,5-tetramethylbenzidine (TMB) for H2O2] and spectrophotometric estimation of apoplastic ROS production (O-2(center dot-) and H2O2). In addition, higher activity of membrane bound NOX (producing O-2(center dot-)) and apoplastic class III peroxidase (Prx, producing (OH)-O-center dot) in light grown roots further corroborates the view that apoplastic ROS (initiated with NOX-generated O-2(center dot-), which is converted, either spontaneously or by the activity of SOD, to H2O2 and further metabolized by Prx to (OH)-O-center dot that participates in cell wall relaxation required for growth) is instrumental in light-escape growth of roots.

Automated summary

In addition to gravity, roots are guided by light to grow deep into the soil due to photoreceptors like phototropins. The involvement of reactive oxygen species (ROS) in light-induced root growth in Vigna radiata seedlings was explored in this study. Pharmacological evidence suggests a positive role of ROS in light-escape growth, with elevated levels of apoplastic ROS in light grown roots indicating their instrumental role in cell wall relaxation required for growth.