In response to far-red light (FR), FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) transports the photoactivated phytochrome A (phyA), the primary FR photoreceptor, into the nucleus, where it initiates FR signaling in plants. Light promotes the 26S proteasome-mediated degradation of FHY1, which desensitizes FR signaling, but the underlying regulatory mechanism remains largely unknown. Here, we show that reversible SUMOylation of FHY1 tightly regulates this process. Lysine K32 (K32) and K103 are major SUMOylation sites of FHY1. We found that FR exposure promotes the SUMOylation of FHY1, which accelerates its degradation. Furthermore, we discovered that ARABIDOPSIS SUMO PROTEASE 1 (ASP1) interacts with FHY1 in the nucleus under FR and facilitates its deSUMOylation. FHY1 was strongly SUMOylated and its protein level was decreased in the asp1-1 loss-of-function mutant compared with that in the wild type under FR. Consistently, asp1-1 seedlings exhibited a decreased sensitivity to FR, suggesting that ASP1 plays an important role in the maintenance of proper FHY1 levels under FR. Genetic analysis further revealed that ASP1 regulates FR signaling through an FHY1- and phyA-dependent pathway. Interestingly, We found that continuous FR inhibits ASP1 accumulation, perhaps contributing to the desensitization of FR signaling. Taken together, these results indicate that FR-induced SUMOylation and ASP1-dependent deSUMOylation of FHY1 represent a key regulatory mechanism that fine-tunes FR signaling.

响应远红光（FR），远红加长型油菜籽素1（FHY1）将主要的FR光感受器光活化的植物色素A（phyA）转运到细胞核中，从而在植物中启动FR信号传导。光促进FHY1的26S蛋白酶体介导的降解，这使FR信号脱敏，但基本的调节机制仍是未知的。在这里，我们表明FHY1的可逆SUMOylation紧密调节这一过程。赖氨酸K32（K32）和K103是FHY1的主要SUMOylation位点。我们发现，FR暴露促进FHY1的SUMOylation，加速其降解。此外，我们发现阿拉比多斯SUMO蛋白酶1（ASP1）在FR下与细胞核中的FHY1相互作用，并促进其去SUMOylation。FHY1被强烈SUMO化，其蛋白水平下降。与FR条件下的野生型相比，asp -1功能丧失的突变体与野生型相比。一致的是，asp1-1幼苗对FR的敏感性降低，这表明ASP1在维持FR下适当的FHY1水平方面起着重要作用。遗传分析进一步表明，ASP1通过FHY1和phyA依赖性途径调节FR信号传导。有趣的是，我们发现连续的FR抑制了ASP1的积累，可能有助于FR信号的脱敏。综上所述，这些结果表明FHY1的FR诱导的SUMOylation和ASP1依赖的deSUMOylation代表了微调FR信号传导的关键调节机制。