Structural and functional principles of low potassium signaling and the integration of nutrient sensing and adaptation in Arabidopsis

植物对钾、硝、铵等养分的有效感知和吸收是必不可少的。植物发展了广泛的适应性反应，由感知外部营养可用性的传感系统触发。然而，植物如何感知和适应波动的营养条件的分子机制才刚刚开始出现。对于K+摄取至关重要的是K+通道AKT1及其钙（Ca2+）依赖性激活CBL1/CBL9-CIPK23复合物。然而，尽管钾离子感知和吸收的基本重要性，植物如何感知低钾离子条件和Ca2+信号对这一过程的潜在贡献的机制在很大程度上仍然是谜。与AKT1类似，硝酸盐转运体NPF6.3（也称为NRT1.1/CHL1.5）受CBL1/9-CIPK23调控，并被报道为硝酸盐受体。此外，CIPK23激酶调节其他几种离子通道和转运体，在拟南芥中确立了CIPK23作为营养敏感激酶的地位，可能在协调和整合植物的一般营养稳态中发挥核心作用。然而，允许CIPK23发挥如此核心和复杂作用的机制基础是完全未知的。我们的初步工作和数据表明(a) K+通道AKT1本身形成或是拟南芥中主要低K+传感器受体的一部分，(b)确定了CBL1/9-CIPK23复合物对AKT1磷酸化的要求，以实现其传感和运输功能。(c)分离并鉴定了具有非偶联传感和运输缺陷以及调节激酶CIPK23中多个顺式和反式磷酸化位点的新型akt1突变等位基因；(d)首次鉴定并表征了植物根系中低K+诱导的Ca2+信号的发生。根据我们的初步数据，拟议项目的具体目标是：(1)识别和描述与低钾+感知相关的成分和信号事件；(2)阐明这些过程背后的结构和机制原理；(3)研究调节CIPK23功能和活性以实现养分感知整合的结构和调控机制；(4)了解这些适应过程如何转化为对根系生长和养分分布的适应性控制和调控。通过结合现有的突变体，新分离的抑制系，将K+传感与K+运输分离，并结合植物表型分析，生化/结构方法和细胞生物学分析，我们打算确定和表征初级低K+传感的基本分子机制和结构原理。由此产生的植物生长适应性，以及了解植物如何整合和多种机制，这些机制允许多种必需营养素的协调和严格调节的吸收和稳态。

Efficient sensing and uptake of various nutrients like potassium, nitrate and ammonium is essential for plants. Plants developed a wide range of adaptive responses triggered by sensing systems that perceive the external nutrient availability. However, the molecular mechanisms how plants sense and adapt to fluctuating nutrient conditions are only beginning to emerge.Crucial for K+ uptake is the K+ channel AKT1 and its calcium (Ca2+) dependent activation by CBL1/CBL9-CIPK23 complexes. However, despite the fundamental importance of K+ sensing and uptake, the mechanisms how plants sense low K+ conditions and the potential contribution of Ca2+ signaling to this process have remained largely enigmatic.Similar to AKT1, the nitrate transporter NPF6.3 (also known as NRT1.1/CHL1.5) is regulated by CBL1/9-CIPK23 and has been reported to function as nitrate transceptor. In addition, the kinase CIPK23 regulates several other ion channels and transporters, establishing CIPK23 as nutrient sensing kinase in Arabidopsis that likely fulfills a central role in coordinating and integrating the general nutritional homeostasis in plants. However, the mechanistic basis that would allow CIPK23 to fulfill such central and complex role is completely unknown. Our preliminary work and data, indicated (a) that the K+ channel AKT1 itself forms or is part of the primary low K+ sensor-receptor in Arabidopsis, (b) identified the requirement of AKT1 phosphorylation by CBL1/9-CIPK23 complexes for its sensing and transport function, (c) isolated and identified novel akt1 mutant alleles with uncoupled sensing- and transport defects as well as multiple cis- and trans-phosphorylation sites in the regulatory kinase CIPK23 and (d) for the first time identified and characterized the occurrence of low K+ induced Ca2+ signals in plants roots. Based on our preliminary data the proposed project specifically aims to: (i) identify and characterize the components and signaling events that are involved in low K+ sensing, (ii) elucidate the structural and mechanistic principles underlying these processes, (iii) investigate the structural and regulatory mechanisms that modulate CIPK23 function and activity to allow nutrient sensing integration and (iv) to understand how these adaptation processes are translated into the adaptive control and regulation of root growth and nutrient distribution. By combining our available mutants, newly isolated suppressor lines that uncouple K+ sensing from K+ transport and by using a combination of phenotypic plant analyses, biochemical / structural approaches, and cell biological assays, we intend to identify and characterize the fundamental molecular mechanisms and structural principles that underlie primary low K+ sensing, the resulting growth adaptation of plants and to achieve an understanding of how plants integrate and the multiple mechanisms that allow for coordinated and tightly regulated uptake and homeostasis of multiple essential nutrients.