Nitrate reductase-mediated production of nitric oxide enhances cold tolerance in rice seedlings.

The function of NR in modulating NO production and cold tolerance was elucidated through the analysis of physiological changes in NR mutants. The results indicated that the cold tolerance of osnia1 and osnia1nia2 was diminished, compared with the wild type (WT) and osnia2, and these mutants also showed lower NR activity. The hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels were higher in the osnia1 and osnia1nia2 rice seedlings compared with WT and osnia2. The proline, ascorbic acid (AsA), and glutathione (GSH) contents, as well as antioxidant and AsA-GSH cycle-related enzyme activities, were reduced. The application of nitric oxide synthase (NOS) inhibitor (L-NAME) and NO scavenger (cPTIO) promoted the accumulation of H2O2 and MDA while decreasing the antioxidant content and enzyme activities. Conversely, sodium nitroprusside (SNP) administration induced opposite effects. The NO content exhibited significant or highly significant positive correlations with the proline, AsA, GSH contents, and the activities of ascorbate peroxidase (APX) and glutathione reductase (GR). In contrast, it exhibited negative correlations with the levels of hydrogen peroxide and MDA, and the activities of related reactive oxygen species enzymes. In summary, the NR gene OsNia1 is a crucial regulator of NR activity and NO production. Low temperature stress can directly or indirectly induce NR activity through reactive oxygen species (ROS), subsequently mediating NO production. The production of NO alleviates oxidative stress under low temperature conditions and enhances the antioxidant defense system, thereby improving the tolerance of rice seedlings to low temperature stress.