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We compared putrescine metabolism in roots of tolerant maize cultivar (Karoon) to a susceptible cultivar (260) under drought stress. The experiment was conducted in growth chamber under controlled condition in 80 pots. Drought stress led to significant increase in root growth indices, H2O2 content and activity of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase). The qPCR analysis revealed that expression of maize polyamine oxidase (ZmPAO) genes including ZmPAO1, ZmPAO2, ZmPAO4 and ZmPAO6 as well as ornithine decarboxylase (ZmODC) significantly increased in root of both cultivars in response to drought. However, up-regulation of ZmPAO3 and ZmPAO5 was only observed in root of tolerant cultivar. Consistently, drought caused increased polyamine oxidase activity in the tolerant cultivar. Transcription of spermidine synthase (ZmSPDS) was down-regulated by drought stress. Transcript levels of ZmPOA, ZmODC and ZmSPDS were more affected by drought in tolerant cultivar compared to susceptible cultivar. Increase in putrescine content under drought stress was more prominent in roots of tolerant cultivar, compared to susceptible cultivar. We attribute the accumulation of putrescine to increased rate of putrescine synthesis, due to higher expression of ZmPAOs and ZmODC and decreased rate of its depletion for biosynthesis of higher polyamines due to down-regulation of ZmSPDS.

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