Photoregulatory Protein Kinases Fine-tune Plant Photomorphogenesis by Directing a Bifunctional Phospho-code on HY5 in Arabidopsis

Photomorphogenesis is a light-dependent plant growth and development program. As the core regulator of photomorphogenesis, ELONGATED HYPOCOTYL 5 (HY5) is affected by dynamic changes in its transcriptional activity and protein stability; however, little is known about the mediators of these processes. Here, we identified PHOTOREGULATORY PROTEIN KINASE 1 (PPK1), which interacts with and phosphorylates HY5 in Arabidopsis, as one such mediator. The phosphorylation of HY5 by PPK1 is essential to establish high-affinity binding with B-BOX PROTEIN 24 and CONSTITUTIVE PHOTOMORPHOGENIC 1, which inhibit the transcriptional activity and promote the degradation of HY5, respectively. As such, PPKs regulate not only the binding of HY5 to its target genes under light conditions, but also HY5 degradation when plants are transferred from light to dark. Our data identify a PPK-mediated phospho-code on HY5 that integrates the molecular mechanisms underlying the regulation of HY5 to precisely control plant photomorphogenesis.

光形态建成（Photomorphogenesis）是一类依赖光照的植物生长发育程序。作为光形态建成的核心调控因子，下胚轴伸长蛋白5（ELONGATED HYPOCOTYL 5，HY5）的活性受其转录活性与蛋白质稳定性的动态变化调控，但目前对介导这两类过程的分子因子仍知之甚少。本研究在拟南芥（Arabidopsis）中鉴定得到光调控蛋白激酶1（PHOTOREGULATORY PROTEIN KINASE 1，PPK1），其可与HY5互作并对其进行磷酸化，即为这类介导因子之一。PPK1对HY5的磷酸化修饰是其与B盒蛋白24（B-BOX PROTEIN 24）以及组成型光形态建成蛋白1（CONSTITUTIVE PHOTOMORPHOGENIC 1）建立高亲和力结合的必要前提；其中，B盒蛋白24可抑制HY5的转录活性，而组成型光形态建成蛋白1则可促进HY5的降解。由此可见，光调控蛋白激酶家族（PPKs）不仅可调控光照条件下HY5与其靶基因的结合，还可在植物从光照环境转移至黑暗环境时调控HY5的降解过程。本研究的数据揭示了HY5蛋白上由PPK介导的磷酸化密码，该密码整合了HY5的调控分子机制，从而实现对植物光形态建成的精准调控。