A salt stress-activated GSO1-SOS2-SOS1 module protects the Arabidopsis root stem cell niche by enhancing sodium ion extrusion.

Soil salinity impairs plant growth reducing crop productivity. Toxic accumulation of sodium ions is counteracted by the Salt Overly Sensitive (SOS) pathway for Na+ extrusion, comprising the Na+ transporter SOS1, the kinase SOS2, and SOS3 as one of several Calcineurin-B-like (CBL) Ca2 + sensors. Here, we report that the receptor-like kinase GSO1/SGN3 activates SOS2, independently of SOS3 binding, by physical interaction and phosphorylation at Thr16. Loss of GSO1 function renders plants salt sensitive and GSO1 is both sufficient and required for activating the SOS2-SOS1 module in yeast and in planta. Salt stress causes the accumulation of GSO1 in two specific and spatially defined areas of the root tip: in the endodermis section undergoing Casparian strip (CS) formation, where it reinforces the CIF-GSO1-SGN1 axis for CS barrier formation; and in the meristem, where it creates the GSO1-SOS2-SOS1 axis for Na+ detoxification. Thus, GSO1 simultaneously prevents Na+ both from diffusing into the vasculature, and from poisoning unprotected stem cells in the meristem. By protecting the meristem, receptor-like kinase-conferred activation of the SOS2-SOS1 module allows root growth to be maintained in adverse environments.

Ca2+-dependent phosphorylation of NRAMP1 by CPK21 and CPK23 facilitates manganese uptake and homeostasis in Arabidopsis.

Homeostasis of the essential micronutrient manganese (Mn) is crucially determined through availability and uptake efficiency in all organisms. Mn deficiency of plants especially occurs in alkaline and calcareous soils, seriously restricting crop yield. However, the mechanisms underlying the sensing and signaling of Mn availability and conferring regulation of Mn uptake await elucidation. Here, we uncover that Mn depletion triggers spatiotemporally defined long-lasting Ca2+ oscillations in Arabidopsis roots. These Ca2+ signals initiate in individual cells, expand, and intensify intercellularly to transform into higher-order multicellular oscillations. Furthermore, through an interaction screen we identified the Ca2+-dependent protein kinases CPK21 and CPK23 as Ca2+ signal-decoding components that bring about translation of these signals into regulation of uptake activity of the high-affinity Mn transporter natural resistance associated macrophage proteins 1 (NRAMP1). Accordingly, a cpk21/23 double mutant displays impaired growth and root development under Mn-limiting conditions, while kinase overexpression confers enhanced tolerance to low Mn supply to plants. In addition, we define Thr498 phosphorylation within NRAMP1 as a pivot mechanistically determining NRAMP1 activity, as revealed by biochemical assays and complementation of yeast Mn uptake and Arabidopsis nramp1 mutants. Collectively, these findings delineate the Ca2+-CPK21/23-NRAMP1 axis as key for mounting plant Mn homeostasis.

Ca2+ signaling in plant responses to abiotic stresses.

Adverse variations of abiotic environmental cues that deviate from an optimal range impose stresses to plants. Abiotic stresses severely impede plant physiology and development. Consequently, such stresses dramatically reduce crop yield and negatively impact on ecosystem stability and composition. Physical components of abiotic stresses can be, for example, suboptimal temperature and osmotic perturbations, while representative chemical facets of abiotic stresses can be toxic ions or suboptimal nutrient availability. The sheer complexity of abiotic stresses causes a multitude of diverse components and mechanisms for their sensing and signal transduction. Ca2+ , as a versatile second messenger, plays multifaceted roles in almost all abiotic stress responses in that, for a certain abiotic stress, Ca2+ is not only reciprocally connected with its perception, but also multifunctionally ensures subsequent signal transduction. Here, we will focus on salt/osmotic stress and responses to altered nutrient availability as model cases to detail novel insights into the identity of components that link stress perception to Ca2+ signal formation as well as on new insights into mechanisms of Ca2+ signal implementation. Finally, we will deduce emerging conceptual consequences of these novel insights and outline arising avenues of future research on the role of Ca2+ signaling in abiotic stress responses in plants.

The potassium channel GhAKT2bD is regulated by CBL-CIPK calcium signalling complexes and facilitates K+ allocation in cotton.

Efficient allocation of the essential nutrient potassium (K+ ) is a central determinant of plant ion homeostasis and involves AKT2 K+ channels. Here, we characterize four AKT2 K+ channels from cotton and report that xylem and phloem expressed GhAKT2bD facilitates K+ allocation and that AKT2-silencing impairs plant growth and development. We uncover kinase activity-dependent activation of GhAKT2bD-mediated K+ uptake by AtCBL4-GhCIPK1 calcium signalling complexes in HEK293T cells. Moreover, AtCBL4-AtCIPK6 complexes known to convey activation of AtAKT2 in Arabidopsis also activate cotton GhAKT2bD in HEK293T cells. Collectively, these findings reveal an essential role for AKT2 in the source-sink allocation of K+ in cotton and identify GhAKT2bD as subject to complex regulation by CBL-CIPK Ca2+ sensor-kinase complexes.

Ca2+-dependent successive phosphorylation of vacuolar transporter MTP8 by CBL2/3-CIPK3/9/26 and CPK5 is critical for manganese homeostasis in Arabidopsis.

Manganese (Mn) is an essential micronutrient for all living organisms. However, excess Mn supply that can occur in acid or waterlogged soils has toxic effects on plant physiology and development. Although a variety of Mn transporter families have been characterized, we have only a rudimentary understanding of how these transporters are regulated to uphold and adjust Mn homeostasis in plants. Here, we demonstrate that two calcineurin-B-like proteins, CBL2/3, and their interacting kinases, CIPK3/9/26, are key regulators of plant Mn homeostasis. Arabidopsis mutants lacking CBL2 and 3 or their interacting protein kinases CIPK3/9/26 exhibit remarkably high Mn tolerance. Intriguingly, CIPK3/9/26 interact with and phosphorylate the tonoplast-localized Mn and iron (Fe) transporter MTP8 primarily at Ser35, which is conserved among MTP8 proteins from various species. Mn transport complementation assays in yeast combined with multiple physiological assays indicate that CBL-CIPK-mediated phosphorylation of MTP8 negatively regulates its transport activity from the cytoplasm to the vacuole. Moreover, we show that sequential phosphorylation of MTP8, initially at Ser31/32 by the calcium-dependent protein kinase CPK5 and subsequently at Ser35 by CIPK26, provides an activation/deactivation fine-tuning mechanism for differential regulation of Mn transport. Collectively, our findings define a two-tiered calcium-controlled mechanism for dynamic regulation of Mn homeostasis under conditions of fluctuating Mn supply.

A potassium-sensing niche in Arabidopsis roots orchestrates signaling and adaptation responses to maintain nutrient homeostasis.

Organismal homeostasis of the essential ion K+ requires sensing of its availability, efficient uptake, and defined distribution. Understanding plant K+ nutrition is essential to advance sustainable agriculture, but the mechanisms underlying K+ sensing and the orchestration of downstream responses have remained largely elusive. Here, we report where plants sense K+ deprivation and how this translates into spatially defined ROS signals to govern specific downstream responses. We define the organ-scale K+ pattern of roots and identify a postmeristematic K+-sensing niche (KSN) where rapid K+ decline and Ca2+ signals coincide. Moreover, we outline 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. The resulting ROS signals simultaneously convey HAK5 K+ uptake-transporter induction and accelerated Casparian strip maturation. Collectively, these mechanisms synchronize developmental differentiation and transcriptome reprogramming for maintaining K+ homeostasis and optimizing nutrient foraging by roots.

Tissue-specific accumulation of pH-sensing phosphatidic acid determines plant stress tolerance.

The signalling lipid phosphatidic acid (PA) is involved in regulating various fundamental biological processes in plants. However, the mechanisms of PA action remain poorly understood because currently available methods for monitoring PA fail to determine the precise spatio-temporal dynamics of this messenger in living cells and tissues of plants. Here, we have developed PAleon, a PA-specific optogenetic biosensor that reports the concentration and dynamics of bioactive PA at the plasma membrane based on Förster resonance energy transfer (FRET). PAleon was sensitive enough to monitor physiological concentrations of PA in living cells and to visualize PA dynamics at subcellular resolution in tissues when they were challenged with abscisic acid (ABA) and salt stress. PAleon bioimaging revealed kinetics and tissue specificity of salt stress-triggered PA accumulation. Compared with wild-type Arabidopsis, the pldα1 mutant lacking phospholipase Dα1 (PLDα1) for PA generation showed delayed and reduced PA accumulation. Comparative analysis of wild type and pldα1 mutant indicated that cellular pH-modulated PA interaction with target proteins and PLD/PA-mediated salt tolerance. Application of the PA biosensor PAleon uncovered specific spatio-temporal PA dynamics in plant tissues. Our findings suggest that PA signalling integrates with cellular pH dynamics to mediate plant response to salt stress.