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

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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.