Regulation of Arabidopsis HFR1 Activity in Light Signaling

植物的固着性要求其生长和发育具有高度的可塑性，以响应周围环境，光是控制植物发育许多方面的主要环境信号之一。拟南芥幼苗光形态建成是研究植物光信号调控机制的分子遗传学模型。两种吸收蓝光/UV-A的隐花色素（cry 1和cry 2）和五种吸收红光/远红光的光敏色素（phyA-phyE）是介导拟南芥幼苗光形态建成的主要光受体。HFR 1是一种bHLH转录因子，在phyA介导的远红光信号和cry 1介导的蓝光信号中起正调节作用。通过观察到的HFR 1与COP 1和SPA 1之间的物理相互作用，提出了HFR 1活性的调节机制。COP 1是一种E3遍在蛋白连接酶，靶向光形态建成的一组正调节因子进行降解，从而抑制光形态建成。SPA 1是phyA信号转导的负调节因子，在结构上与COP 1相关。据推测，COP 1在由SPA 1调节的过程中泛素化HFR 1，并通过26 S蛋白酶体途径靶向HFR 1降解，从而使光信号转导脱敏。实验的目的是在遗传、生理和生化水平上证实这样一个工作假设。本研究的结果有望：1）对HFR 1在phyA和cry 1信号转导中的调控机制、COP 1和SPA 1的功能以及蛋白水解在植物发育调控中的作用提供重要的认识; 2）为基因工程提供指导，以微调光响应生长性状（如高度、叶面积、开花时间和播种密度）; 3）为不同层次的科学家提供良好的培训机会，包括博士后研究员，研究生和本科生/高中生/高中教师，重点是纳入代表性不足的群体。

The sessile nature of plants demands a high degree of plasticity in their growth and development in response to the ambient environment, and light is one of the major environmental signals that controls many aspects of plant development. Arabidopsis seedling photomorphogenesis has been used as a model system for molecular genetic studies to dissect the mechanisms regulating light signaling in plants. Two blue/UV-A absorbing cryptochromes (cry1 and cry2) and five red/far-red light absorbing phytochromes (phyA-phyE) are the primary photoreceptors responsible for mediating Arabidopsis seedling photomorphogenesis. HFR1, a bHLH transcription factor, acts as a positive regulator in both phyA-mediated far-red and cry1-mediated blue light signaling. A regulatory mechanism of HFR1 activity was suggested by the observed physical interactions between HFR1 with COP1 and SPA1. COP1 is an E3 ubiquitin ligase targeting a subset of positive regulators of photomorphogenesis for degradation, thus repressing photomorphogenesis. SPA1 is a negative regulator of phyA signaling and is structurally related to COP1. It is hypothesized that COP1 ubiquitinates HFR1 in a process modulated by SPA1, and targets HFR1 for degradation via the 26S proteasome pathway, thus desensitizing light signaling. Experiments are designed to substantiate such a working hypothesis at the genetic, physiological and biochemical levels. The outcome of this research is expected to: 1) provide significant insights into the regulatory mechanisms of HFR1 function in both phyA and cry1 signaling, a better understanding of COP1 and SPA1 function, and the role of regulated proteolysis in regulating plant development in general; 2) provide guidance for genetic engineering to fine-tune light-responsive growth traits (such as height, leaf area, flowering time and sowing density) in crops of agronomic importance; 3) provide excellent training opportunities for scientists at different levels, including postdoctoral fellows, graduate and undergraduate/high school students/high school teachers, with a focus on the inclusion of underrepresented groups.