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The objective of this study was to examine the role of NADPH oxidase on superoxide radical production under waterlogging in mung bean (Vigna radiata) cvs. T 44 (tolerant) and Pusa Baisakhi (PB) (susceptible), and wild species Vigna luteola. Two days of waterlogging caused decline in superoxide radical (O2 .-) contents in all the genotypes, however, further waterlogging up to 8 d caused significant increase in O2 .- contents. In control and revived plants O2 .- contents were higher in PB, while under waterlogging stress T 44 and V. luteola showed greater increases in the O2 .- contents. During waterlogging the increase in O2 .- content was found to be due to the diphenylene iodonium chloridesensitive NADPH oxidase (NOX). This was further confirmed by the waterlogging induced increase in NOX activity, which was higher in tolerant genotypes T 44 and V. luteola compared with PB. Gene expression studies showed enhanced expression of NOX in the roots of waterlogged V. luteola and T 44, while little expression was observed in control or treated plants of PB. PCR band products were cloned and sequenced, and partial cDNAs of NOX was obtained. Results suggest that increase in O2 .- content during waterlogging could be due to the induction of membrane linked NOX.