NOA1 Functions in a Temperature-Dependent Manner to Regulate Chlorophyll Biosynthesis and Rubisco Formation in Rice

NITRIC OXIDE-ASSOCIATED1 (NOA1) encodes a circularly permuted GTPase (cGTPase) known to be essential for ribosome assembly in plants. While the reduced chlorophyll and Rubisco phenotypes were formerly noticed in both NOA1-supressed rice and Arabidopsis, a detailed insight is still necessary. In this study, by using RNAi transgenic rice, we further demonstrate that NOA1 functions in a temperature-dependent manner to regulate chlorophyll and Rubisco levels. When plants were grown at 30°C, the chlorophyll and Rubisco levels in OsNOA1-silenced plants were only slightly lower than those in WT. However, at 22°C, the silenced plants accumulated far less chlorophyll and Rubisco than WT. It was further revealed that the regulation of chlorophyll and Rubisco occurs at the anabolic level. Etiolated WT seedlings restored chlorophyll and Rubisco accumulations readily once returned to light, at either 30°C or 15°C. Etiolated OsNOA1-silenced plants accumulated chlorophyll and Rubisco to normal levels only at 30°C, and lost this ability at low temperature. On the other hand, de-etiolated OsNOA1-silenced seedlings maintained similar levels of chlorophyll and Rubisco as WT, even after being shifted to 15°C for various times. Further expression analyses identified several candidate genes, including OsPorA (NADPH: protochlorophyllide oxidoreductase A), OsrbcL (Rubisco large subunit), OsRALyase (Ribosomal RNA apurinic site specific lyase) and OsPuf4 (RNA-binding protein of the Puf family), which may be involved in OsNOA1-regulated chlorophyll biosynthesis and Rubisco formation. Overall, our results suggest OsNOA1 functions in a temperature-dependent manner to regulate chlorophyll biosynthesis, Rubisco formation and plastid development in rice.

一氧化氮关联蛋白1（NITRIC OXIDE-ASSOCIATED1, NOA1）编码一种环排列GTP酶（circularly permuted GTPase, cGTPase），已知该酶在植物核糖体组装过程中发挥关键作用。此前研究曾在NOA1沉默的水稻与拟南芥中观察到叶绿素及核酮糖-1,5-二磷酸羧化酶/加氧酶（Rubisco）含量降低的表型，但目前仍缺乏对其详细的机制解析。本研究通过RNA干扰（RNA interference, RNAi）转基因水稻，进一步证实OsNOA1以温度依赖性方式调控叶绿素与Rubisco含量：当植株在30℃条件下培养时，OsNOA1沉默植株的叶绿素与Rubisco含量仅略低于野生型（wild type, WT）；而在22℃条件下，沉默植株的叶绿素与Rubisco积累量则远低于野生型。研究还发现，对叶绿素与Rubisco的调控发生在合成代谢层面。黄化野生型幼苗无论在30℃还是15℃条件下，一旦恢复光照即可快速恢复叶绿素与Rubisco的积累；而黄化OsNOA1沉默植株仅在30℃条件下才能将叶绿素与Rubisco积累至正常水平，在低温条件下则丧失该能力。另一方面，脱黄化的OsNOA1沉默幼苗即便转移至15℃条件下培养不同时长，其叶绿素与Rubisco含量仍与野生型植株保持一致。进一步的表达分析筛选得到多个候选基因，包括OsPorA（NADPH：原叶绿素酸酯氧化还原酶A）、OsrbcL（Rubisco大亚基）、OsRALyase（核糖体RNA无嘌呤位点特异性裂解酶）及OsPuf4（Puf家族RNA结合蛋白），这些基因可能参与OsNOA1调控的叶绿素生物合成与Rubisco形成过程。综上，本研究结果表明，水稻OsNOA1通过温度依赖性方式调控叶绿素生物合成、Rubisco形成以及质体发育。