CP31A, a Rice Chloroplast Ribonucleoprotein Gene, Regulates Chloroplast mRNA Stability Conferring Drought and Cold Tolerance

Chloroplast ribonucleoproteins (cpRNPs) are nuclear-encoded and highly abundant proteins that are proposed to function in chloroplast RNA metabolism. However, the molecular mechanisms underlying the regulation of chloroplast RNAs involved in stress tolerance are poorly understood. Here, we demonstrate that OsCP31A, a rice (Oryza sativa) cpRNP gene, is essential for stabilization of RNAs from the NAD(P)H dehydrogenase (NDH) complex, which in turn enhances drought and cold stress tolerance. OsCP31A is expressed specifically in green tissues throughout all developmental stages, and the corresponding protein is localized in the chloroplasts. An RNA-immunoprecipitation assay revealed that OsCP31A is associated with a set of chloroplast RNAs. Transcript profiling indicated that the mRNA levels of genes from the NDH complex significantly increased in the OsCP31 overexpressing compared to non-transgenic plants, whereas the pattern in OsCP31 RNAi plants were opposite. The increase in mRNA stability of the NDH complex genes was verified by measuring the mRNA half-life during drought stress in the presence of the transcriptional inhibitor, cordycepin. Importantly, the OsCP31A overexpressing plants showed a higher cyclic electron transport (CET) activity, which is essential for elevated levels of ATP for photosynthesis, as well as plant fitness, under abiotic stress conditions. Additionally, overexpression of OsCP31A resulted in significantly enhanced drought and cold stress tolerance with higher ATP levels compared to wild type. Thus, our findings suggest that overexpression of OsCP31 stabilizes a set of mRNAs from genes of the NDH complex involved in increasing CET activity and production of ATP, which consequently confers enhanced drought and cold tolerance.

叶绿体核糖核蛋白（chloroplast ribonucleoproteins, cpRNPs）是一类核编码且高度富集的蛋白质，被认为在叶绿体RNA代谢中发挥功能。然而，参与胁迫耐受调控的叶绿体RNA分子机制仍不甚明晰。本研究证实，水稻（Oryza sativa）cpRNP基因OsCP31A对于稳定NAD(P)H脱氢酶（NAD(P)H dehydrogenase, NDH）复合体相关RNA至关重要，进而提升植株的干旱与低温胁迫耐受性。OsCP31A在所有发育阶段的绿色组织中特异性表达，其编码蛋白定位于叶绿体。通过RNA免疫沉淀实验（RNA-immunoprecipitation assay）发现，OsCP31A与一组叶绿体RNA相结合。转录组分析（transcript profiling）结果显示，相较于非转基因植株，OsCP31过表达植株中NDH复合体相关基因的mRNA水平显著上调，而OsCP31 RNA干扰植株则呈现相反的表达模式。通过在转录抑制剂虫草素（cordycepin）存在的条件下检测干旱胁迫期间的mRNA半衰期，证实了NDH复合体基因的mRNA稳定性得到提升。尤为重要的是，在非生物胁迫条件下，OsCP31A过表达植株展现出更高的环式电子传递（cyclic electron transport, CET）活性——这对于光合作用所需ATP的合成以及植株适应性均不可或缺。此外，相较于野生型植株，OsCP31A过表达植株的ATP水平更高，且干旱与低温胁迫耐受性显著增强。综上，本研究结果表明，OsCP31过表达能够稳定一组NDH复合体相关基因的mRNA，进而提升CET活性与ATP生成量，最终赋予植株更强的干旱与低温胁迫耐受能力。