Molecular mechanisms of sensing of and adaptation to K+ deprivation

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Sensing and uptake of potassium is essential for plants. The availability of K+ in soils dramatically fluctuates in natural conditions making K+-limiting conditions a frequently encountered environmental stress. Organismic K+ homeostasis is determined by a complex interplay of processes that convey root K+ uptake, transport processes within the root and from root to shoot as well as K+ storage mechanisms. Plants possess a wide range of adaptive responses mechanisms, which are triggered by sensing systems that perceive K+ availability and subsequently trigger different adaptive reactions. However, the molecular mechanisms how plants sense and adapt to fluctuating potassium conditions are only beginning to emerge. The hardwiring of K+ transport meaning the identity and regulation of K+ channels and transporters that bring about K+ uptake and distribution of this ion is relatively well understood. However, how and where fluctuations in K+ availability are mechanistically sensed has remained far less understood. Moreover, which signaling and regulatory processes convey the establishment of K+ fluxes and homeostasis at the tissue and organ scale has remained enigmatic. Previous work of both applicants: (a) defined the organ scale K+ pattern of roots, (b) identified a postmeristematic K+-sensing niche (KSN) where rapid K+ decline and Ca2+ signals coincide, (c) identified a bifurcating low-K+ signaling axis of CIF peptide-activated SGN3-LKS4/SGN1 receptor complexes that convey low-K+-triggered phosphorylation of the NADPH oxidases RBOHC, RBOHD and RBOHF, (d) uncovered that the resulting ROS signals simultaneously convey HAK5 K+ uptake transporter induction and accelerated Casparian strip maturation. Collectively, these findings revealed how plants synchronize developmental differentiation and transcriptome reprogramming for maintaining K+ homoeostasis and optimizing nutrient foraging by roots. Based on our preliminary data the proposed project specifically aims to: (i) define the landscape and dynamics of K+ distribution in roots, (ii) uncover the molecular mechanisms and components that generate the K+ signal in the postmeristematic K+ sensing niche (KSN), (iii) explore how KSN-born Ca2+/K+ signals mechanistically convey intensified CIF2 signaling, (iv) investigate the molecular function of membrane polarization and the role of PM ATPases in LK signaling and adaptation, and (v) delineate the functional interrelation of the secondary Ca2+ elevation with ROS signal formation and of the mechanisms that bring about specificity in LK-induced NOX activation. By combining genetic approaches, biosensorics analyses and by using a combination of phenotypic plant analyses, biochemical approaches, and novel reporter assays, we intend to identify and characterize fundamental molecular mechanisms that underlie LK sensing and signaling to achieve integrated adaptation responses.