Involvement of putrescine and nitric oxide in aluminum tolerance by modulating citrate secretion from roots of red kidney bean

Polyamines and nitric oxide (NO) are two important molecules modulating numerous environment stresses in plants. This study was to investigate the roles of polyamines and NO in aluminum (Al) tolerance in red kidney bean. The interaction between putrescine (Put) and NO under Al stress was examined. NO donor and scavenger were used to further examine the role of NO in Al-induced citrate secretion from roots by high performance liquid chromatography. Al stress caused increase of endogenous free Put, and exogenous Put alleviated Al-induced inhibition of root elongation and Al accumulation. In addition, Put induced NO production and nitrate reductase (NR) activity under Al stress. Al- and Put-induced NO production could be reversed by NR inhibitor. Furthermore, Al stress stimulated citrate secretion from roots, and this response was stimulated by NO donor, whereas NO scavenger inhibited Al-induced citrate secretion from roots. Concomitantly, NO donor reduced Al accumulation in root apexes, while NO scavenger further enhanced Al accumulation. Al-induced inhibition of root growth was significantly improved by exogenous citrate treatment. Put and NO enhanced Al tolerance by modulating citrate secretion from roots, and NO may act downstream of Put in red kidney bean under Al stress.