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[交流] [优秀文章推荐]PNAS：揭示光信号重要组分参与植物低温应答已有1人参与

在低温和黑暗条件下，泛素连接酶EBF1/EBF2发生降解，使PIF3蛋白积累并抑制CBF基因的表达

近日，中国农业大学植物生理生化国家重点实验室杨淑华课题组在最新一期的国际著名刊物Proceedings of the National Academy of Sciences (PNAS)发表了标题为“PIF3 is a negative regulator of the CBF pathway and freezing tolerance in Arabidopsis”的研究论文。该研究发现光信号转导通路关键转录因子PIF3参与负调控植物抗冻性，同时E3泛素化连接酶EBF1/EBF2参与了低温介导的PIF3蛋白稳定性调控，揭示了光信号参与负调控植物抗冻性的分子机理。

植物生长发育需要适宜的环境温度条件。低温是影响植物生长发育及作物产量最主要的逆境胁迫之一，因此植物进化出了冷驯化（cold acclimation）能力来应对低温逆境胁迫。杨淑华课题组一直致力于植物低温胁迫的信号转导通路的研究。该课题组相继发现蛋白激酶OST1能被低温激活，从而正调控ICE1–CBF低温关键信号通路和植物抗冻性（Dev Cell, 2015）；CBF信号通路同时会被CRPK1–14-3-3等负调控信号途径所抑制（Mol Cell, 2017），以此精细调控植物的生长发育与抗逆性之间的平衡。

PIF3 interacts with EBF1/2 in vitro and in vivo

在自然条件下，植物需要整合光和温度信号等环境因子来适应环境的变化，但是光信号在植物冷响应过程中的作用目前尚不清楚。该团队在筛选植物抗冻突变体的过程中，鉴定到光信号关键转录因子PIF3负调控植物抗冻性。pif3突变体表现出抗冻表型，而PIF3过表达转基因植株则具有明显的冻敏感表型。进一步研究发现，PIF3能够直接结合在转录因子CBFs的启动子区，抑制CBFs及其下游冷响应基因表达。此外，该研究还发现低温可以增强PIF3蛋白的稳定性。此前的研究表明，在常温光照条件下PIF3蛋白会被LRBs E3连接酶识别并降解。该研究发现了两个新的E3泛素连接酶EBF1和EBF2能够与PIF3发生相互作用，并使其通过26S蛋白酶体途径降解。低温及黑暗条件可以促进EBFs蛋白降解，从而使PIF3蛋白更加稳定。遗传证据进一步表明EBF1和EBF2是植物冷响应途径的正调控因子，ebf1和ebf2单突变体在冷驯化前后均表现出冻敏感表型，而pif3突变体可以回复ebf1突变体表型。该研究通过揭示PIF3在低温响应过程中的新功能和调控机制，阐明了光和温度在植物冷驯化过程的关系密不可分，共同平衡着植物的抗冻性和生长发育过程。

PHYTOCHROME-INTERACTING FACTORS (PIFs) are central integrators of plants’ responses to various environmental signals. In this study, we show that PIF3 acts as a negative regulator of plant cold acclimation by directly repressing the expression of CBF genes, whereas its protein stability is negatively regulated by two F-box proteins, EBF1 and EBF2, via the 26S proteasome pathway. Moreover, EBF1 and EBF2 are degraded under cold stress, which enhances the stability of PIF3 protein. Collectively, our study establishes an important regulatory paradigm for PIF3 in preventing runaway expression of the CBF genes at low temperature, which allows plants to adapt to and withstand harsh environments.

PIF3 negatively regulates freezing tolerance in Arabidopsis

Light and temperature are major environmental factors that coordinately control plant growth and survival. However, how plants integrate light and temperature signals to better adapt to environmental stresses is poorly understood. PHYTOCHROME-INTERACTING FACTOR 3 (PIF3), a key transcription factor repressing photomorphogenesis, has been shown to play a pivotal role in mediating plants’ responses to various environmental signals. In this study, we found that PIF3 functions as a negative regulator of Arabidopsis freezing tolerance by directly binding to the promoters of C-REPEAT BINDING FACTOR (CBF) genes to down-regulate their expression. In addition, two F-box proteins, EIN3-BINDING F-BOX 1 (EBF1) and EBF2, directly target PIF3 for 26S proteasome-mediated degradation. Consistently, ebf1 and ebf2 mutants were more sensitive to freezing than were the wild type, and the pif3 mutation suppressed the freezing-sensitive phenotype of ebf1. Furthermore, cold treatment promoted the degradation of EBF1 and EBF2, leading to increased stability of the PIF3 protein and reduced expression of the CBF genes. Together, our study uncovers an important role of PIF3 in Arabidopsis freezing tolerance by negatively regulating the expression of genes in the CBF pathway.

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