Plants subjected to cold stress have been observed to accumulate proline, but the underlying regulatory mechanism remains to be elucidated. In this study, we identified a pyrroline-5-carboxylate synthetase (P5CS)-encoding gene (CtrP5CS1) from trifoliate orange (Citrus trifoliata L.), a cold-hardy citrus species, as a critical gene for cold-induced proline accumulation. CtrTGA2 bound directly to the TGACG motif of the CtrP5CS1 promoter and activated its expression. Moreover, CtrTGA2 functioned positively in cold tolerance via modulation of proline synthesis by regulating CtrP5CS1 expression. Up-regulation of CtrP5CS1 and CtrTGA2 under cold stress was dependent on salicylic acid (SA) biosynthesis. CtrTGA2 directly regulated the expression of CtrICS1, a gene encoding isochorismate synthase (ICS) involved in SA biosynthesis, forming a positive feedback loop to intensify the CtrTGA2-mediated transcriptional activation of CtrP5CS1. The cold-induced SA receptor NONEXPRESSOR OF PATHOGENESIS-RELATED GENES3 (CtrNPR3) interacted with CtrTGA2 to inhibit its transcriptional activation activity; however, the inhibition was released by SA. Our results uncover the CtrTGA2-CtrP5CS1/CtrICS1 regulatory module that orchestrates the SA signal to regulate proline synthesis, giving important insights into the transcriptional mechanism underlying proline accumulation in plants under cold stress.

中文翻译：

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转录因子 TGA2 协调水杨酸信号以调节寒冷诱导的柑橘脯氨酸积累


已观察到受到寒冷胁迫的植物积累脯氨酸，但潜在的调节机制仍有待阐明。在这项研究中，我们从耐寒柑橘物种三叶橙 （Citrus trifoliata L.） 中鉴定了一个吡咯啉-5-羧酸合成酶 （P5CS） 编码基因 （CtrP5CS1），作为寒冷诱导脯氨酸积累的关键基因。CtrTGA2 直接与 CtrP5CS1 启动子的 TGACG 基序结合并激活其表达。此外，CtrTGA2 通过调节 CtrP5CS1 表达调节脯氨酸合成，在耐寒性中发挥积极作用。在低温胁迫下 CtrP5CS1 和 CtrTGA2 的上调依赖于水杨酸 （SA） 的生物合成。CtrTGA2 直接调节 CtrICS1 的表达，CtrICS1 是一种编码参与 SA 生物合成的异胆酸合酶 （ICS） 的基因，形成正反馈回路以加强 CtrTGA2 介导的 CtrP5CS1 转录激活。冷诱导的 SA 受体 NONEXPRESSOR OF PATHOGENESIS-RELATED GENES3 （CtrNPR3） 与 CtrTGA2 相互作用以抑制其转录激活活性;然而，抑制是由 SA 释放的。我们的结果揭示了 CtrTGA2-CtrP5CS1/CtrICS1 调节模块，该模块协调 SA 信号以调节脯氨酸合成，为低温胁迫下植物脯氨酸积累的转录机制提供了重要见解。