• In Arabidopsis thaliana, phytochrome B (phyB) is the dominant receptor of photomorphogenic development under red light. Phytochrome B interacts with a set of downstream regulatory proteins, including PHYTOCHROME INTERACTING FACTOR 3 (PIF3). The interaction between PIF3 and photoactivated phyB leads to the rapid phosphorylation and degradation of PIF3 and also to the degradation of phyB, events which are required for proper photomorphogenesis.
• Here we report that PIF3 is SUMOylated at the Lys13 (K13) residue and that we could detect this posttranslational modification in a heterologous experimental system and also in planta.
• We also found that the SUMO acceptor site mutant PIF3(K13R) binds more strongly to the target promoters than its SUMOylated, wild‐type counterpart. Seedlings expressing PIF3(K13R) show an elongated hypocotyl response, elevated photoprotection and higher transcriptional induction of red‐light responsive genes compared with plantlets expressing wild‐type PIF3.
• These observations are supported by the lower level of phyB in plants which possess only PIF3(K13R), indicating that SUMOylation of PIF3 also alters photomorphogenesis via the regulation of phyB levels. In conclusion, whereas SUMOylation is generally connected to different stress responses, it also fine‐tunes light signalling by reducing the biological activity of PIF3, thus promoting photomorphogenesis.

中文翻译：

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植物色素相互作用因子 3 的 SUMOylation 促进拟南芥的光形态发生

• 在拟南芥中，光敏色素 B (phyB) 是红光下光形态发生发育的主要受体。光敏色素 B 与一组下游调节蛋白相互作用，包括光敏色素相互作用因子 3 (PIF3)。PIF3 和光活化的 phyB 之间的相互作用导致 PIF3 的快速磷酸化和降解以及 phyB 的降解，这是适当的光形态发生所必需的事件。
• 在这里，我们报告 PIF3 在 Lys13 (K13) 残基处被 SUMO 化，我们可以在异源实验系统和植物中检测到这种翻译后修饰。
• 我们还发现，SUMO 受体位点突变体 PIF3(K13R) 与目标启动子的结合比其 SUMO 化的野生型对应物更强。与表达野生型 PIF3 的幼苗相比，表达 PIF3(K13R) 的幼苗显示出延长的下胚轴反应、增强的光保护和更高的红光反应基因转录诱导。
• 这些观察得到了仅具有 PIF3(K13R) 的植物中较低水平的 phyB 的支持，这表明 PIF3 的 SUMO 化也通过调节 phyB 水平来改变光形态发生。总之，虽然 SUMO 化通常与不同的应激反应有关，但它还通过降低 PIF3 的生物活性来微调光信号，从而促进光形态发生。