Regulation of Arabidopsis HFR1 Activity in Light Signaling

植物的固位性要求其生长发育对周围环境具有高度的可塑性，而光是控制植物发育许多方面的主要环境信号之一。拟南芥幼苗光形态发生已被用作分子遗传学研究的模式系统，以剖析植物光信号的调控机制。两个蓝/UV-A吸收隐色素(Cry1和Cry2)和五个红/远红光吸收光敏色素(Phya-PhyE)是介导拟南芥幼苗光形态发生的主要光感受器。HFR1是一种bHLH转录因子，在PhyA介导的远红光和Cry1介导的蓝光信号中都起着正向调节作用。通过观察到HFR1与COP1和SPA1之间的物理相互作用，提出了HFR1活性的调节机制。COP1是一种E3泛素连接酶，以光形态建成的正调控因子为靶点进行降解，从而抑制光形态建成。SPA1是PHYA信号的负调控因子，在结构上与COP1相关。假设COP1在SPA1调控的过程中泛素化HFR1，并通过26S蛋白酶体途径靶向HFR1降解，从而减敏光信号。实验的目的是在遗传、生理和生化水平上证实这种工作假说。预计这项研究的成果将：1)提供重要的见解，了解HFR1在PhyA和Cry1信号传导中的调控机制，更好地了解COP1和SPA1的功能，以及受调控的蛋白质分解在调控植物发育中的作用；2)为基因工程提供指导，以微调具有农艺重要性的作物的光响应生长性状(如高度、叶面积、花时间和播种密度)；3)为不同水平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，重点关注纳入未被充分代表的群体。

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.