Abstract
BackgroundNAD 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.ResultsOsNADK1 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.ConclusionsAltogether, the results suggest that an OsNADK1-mediated intracellular redox balance is involved in the tolerance of rice plants to drought.
Highlights
NAD kinases (NADKs) are the only known enzymes that directly phosphorylate NAD(H) to generate NADP(H) in different subcellular compartments
Expression profiles of OsNADK1 in different tissues and under various external conditions OsNADK1 expression profiles in different tissues and under various stress conditions were examined by quantitative real-time RT-PCR
A 2000-bp nucleotide sequence upstream of the OsNADK1 coding sequence that contains the OsNADK1 promoter region was cloned into the pCAMBIA1301 vector, and histochemical staining of GUS activity in the OsNADK1 promoter: GUS transgenic plants was detected
Summary
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. In Arabidopsis, a single mutation in one of the three NADKs is not lethal, but it greatly affects plant growth [15, 16, 22]. Loss of function of OsNADK3 leads to dwarfism and sterility [18] These results support the fundamental roles of NADKs in the development of most living organisms
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