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.

中文翻译：

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NAD激酶OsNADK1影响细胞内的氧化还原平衡并增强水稻对干旱的耐受性。

背景技术NAD激酶（NADK）是唯一已知的能直接磷酸化NAD（H）在不同亚细胞区室产生NADP（H）的酶。它们参与多种生活活动，例如调节NADP / NAD比率，维持细胞内氧化还原平衡并响应环境压力。但是，单个NADK在植物中的功能仍在研究中。在此，鉴定了水稻NADK，即OsNADK1，并通过使用一系列转基因植物系分析了其在植物生长调节和胁迫耐受性中的功能。结果OsNADK1是水稻中胞浆定位的NADK。它在所有检查过的水稻组织中都有表达，并且其转录表达可以通过许多环境胁迫处理来刺激。与野生型水稻相比，抽穗期剔除OsNADK1的突变植物osnadk1在抽穗期是矮化的，其NADP（H）/ NAD（H），抗坏血酸（ASA）/脱氢抗坏血酸（DHA）降低，谷胱甘肽（GSH）/氧化型谷胱甘肽减少（ GSSG）比率，从而导致水稻细胞中的氧化态增加和对干旱敏感。此外，与野生型相比，某些胁迫相关基因在正常生长和干旱胁迫条件下在osnadk1中显示差异表达模式。在这些基因中，OsDREB1B和几个WRKY家族转录因子，例如OsWRKY21和OsWRKY42，在osnadk1中与OsNADK1相关联的共表达模式，而植物在OsNADK1中过表达或过表达OsNADK1，这暗示了这些转录因子在OsNADK1介导的过程中的作用。此外，OsNADK1的过表达增强了水稻的抗旱性，而基因功能的丧失则降低了抗旱性。此外，在干旱条件下，过表达OsNADK1的品系叶片中脯氨酸含量显着增加。结论总之，结果表明OsNADK1介导的细胞内氧化还原平衡与水稻对干旱的耐受性有关。