The NAD kinase OsNADK1 affects the intracellular redox balance and enhances the tolerance of rice to drought

BACKGROUND: NAD kinases (NADKs) are the only known enzymes that directly phosphorylate NAD(H) to generate NADP(H) in different subcellular compartments. They participate in multiple life activities, such as modulating the NADP/NAD ratio, maintaining the intracellular redox balance and responding to environmental stresses. However, the functions of individual NADK in plants are still under investigation. Here, a rice NADK, namely, OsNADK1, was identified, and its functions in plant growth regulation and stress tolerance were analysed by employing a series of transgenic plant lines. RESULTS: OsNADK1 is a cytosol-localized NADK in rice. It was expressed in all rice tissues examined, and its transcriptional expression could be stimulated by a number of environmental stress treatments. Compared with wild-type (WT) rice, the mutant plant osnadk1 in which OsNADK1 was knocked out was a dwarf at the heading stage and had decreased NADP(H)/NAD(H), ascorbic acid (ASA)/dehydroascorbate (DHA) and reduced glutathione (GSH)/oxidized glutathione (GSSG) ratios, which led to increased oxidation states in the rice cells and sensitivity to drought. Moreover, certain stress-related genes showed differential expression patterns in osnadk1 under both normal growth and drought-stress conditions compared with WT. Among these genes, OsDREB1B and several WRKY family transcription factors, e.g., OsWRKY21 and OsWRKY42, showed correlated co-expression patterns with OsNADK1 in osnadk1 and the plants overexpressing or underexpressing OsNADK1, implying roles for these transcription factors in OsNADK1-mediated processes. In addition, overexpression of OsNADK1 enhanced the drought tolerance of rice plants, whereas loss of function of the gene reduced the tolerance. Furthermore, the proline content was dramatically increased in the leaves of the OsNADK1-overexpressing lines under drought conditions. CONCLUSIONS: Altogether, the results suggest that an OsNADK1-mediated intracellular redox balance is involved in the tolerance of rice plants to drought.