ELONGATED HYPOCOTYL 5 (HY5), a basic domain/leucine zipper (bZIP) transcription factor, acts as a master regulator of transcription to promote photomorphogenesis. At present, it’s unclear whether HY5 uses additional mechanisms to inhibit hypocotyl elongation. Here, we demonstrate that HY5 enhances the activity of GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 (BIN2), a key repressor of brassinosteroid signaling, to repress hypocotyl elongation. We show that HY5 physically interacts with and genetically acts through BIN2 to inhibit hypocotyl elongation. The interaction of HY5 with BIN2 enhances its kinase activity possibly by the promotion of BIN2 Tyr²⁰⁰ autophosphorylation, and subsequently represses the accumulation of the transcription factor BRASSINAZOLE-RESISTANT 1 (BZR1). Leu¹³⁷ of HY5 is found to be important for the HY5-BIN2 interaction and HY5-mediated regulation of BIN2 activity, without affecting the transcriptional activity of HY5. HY5 levels increase with light intensity, which gradually enhances BIN2 activity. Thus, our work reveals an additional way in which HY5 promotes photomorphogenesis, and provides an insight into the regulation of GSK3 activity.

ELONGATED HYPOCOTYL 5 (HY5) 是一种基本结构域/亮氨酸拉链 (bZIP) 转录因子，充当转录的主调节因子，促进光形态发生。目前，尚不清楚 HY5 是否使用其他机制来抑制下胚轴伸长。在这里，我们证明 HY5 增强 GSK3 样激酶油菜素类固醇不敏感 2 (BIN2)（油菜素类固醇信号传导的关键抑制因子）的活性，从而抑制下胚轴伸长。我们发现 HY5 与 BIN2 发生物理相互作用，并通过 BIN2 进行遗传作用，抑制下胚轴伸长。 HY5 与 BIN2 的相互作用可能通过促进 BIN2 Tyr ²⁰⁰自磷酸化来增强其激酶活性，并随后抑制转录因子 BRASSINAZOLE-RESISTANT 1 (BZR1) 的积累。研究发现 HY5 的 Leu ¹³⁷对于 HY5-BIN2 相互作用和 HY5 介导的 BIN2 活性调节非常重要，而不影响 HY5 的转录活性。 HY5 水平随着光照强度的增加而增加，从而逐渐增强 BIN2 活性。因此，我们的工作揭示了 HY5 促进光形态发生的另一种方式，并提供了对 GSK3 活性调节的见解。