SUMMARY Cells maintain integrity despite changes in their mechanical properties elicited during growth and environmental stress. How cells sense their physical state and compensate for cell-wall damage is poorly understood, particularly in plants. Here we report that FERONIA (FER), a plasma-membrane-localized receptor kinase from Arabidopsis, is necessary for the recovery of root growth after exposure to high salinity, a widespread soil stress. The extracellular domain of FER displays tandem regions of homology with malectin, an animal protein known to bind diglucose in vitro and important for protein quality control in the endoplasmic reticulum. The presence of malectin-like domains in FER and related receptor kinases has led to widespread speculation that they interact with cell-wall polysaccharides and can potentially serve a wall-sensing function. Results reported here show that salinity causes softening of the cell wall and that FER is necessary to sense these defects. When this function is disrupted in the fer mutant, root cells explode dramatically during growth recovery. Similar defects are observed in the mur1 mutant, which disrupts pectin cross-linking. Furthermore, fer cell-wall integrity defects can be rescued by treatment with calcium and borate, which also facilitate pectin cross-linking. Sensing of these salinity-induced wall defects might therefore be a direct consequence of physical interaction between the extracellular domain of FER and pectin. FER-dependent signaling elicits cell-specific calcium transients that maintain cell-wall integrity during salt stress. These results reveal a novel extracellular toxicity of salinity, and identify FER as a sensor of damage to the pectin-associated wall.

SUMMARYDuring adaptation and developmental processes cells respond through nonlinear calcium-decoding signaling cascades, the principal components of which have been identified. However, the molecular mechanisms generating specificity of cellular responses remain poorly understood. Calcineurin B-like (CBL) proteins contribute to decoding calcium signals by specifically interacting with a group of CBL-interacting protein kinases (CIPKs). Here, we report the subcellular localization of all 10 CBL proteins from Arabidopsis and provide a cellular localization matrix of a plant calcium signaling network. Our findings suggest that individual CBL proteins decode calcium signals not only at the plasma membrane and the tonoplast, but also in the cytoplasm and nucleus. We found that distinct targeting signals located in the N-terminal domain of CBL proteins determine the spatially discrete localization of CBL/CIPK complexes by COPII-independent targeting pathways. Our findings establish the CBL/CIPK signaling network as a calcium decoding system that enables the simultaneous specific information processing of calcium signals emanating from different intra-and extracellular stores, and thereby provides a mechanism underlying the specificity of cellular responses.

Stimulus-specific accumulation of second messengers like reactive oxygen species (ROS) and Ca(2+) are central to many signaling and regulation processes in plants. However, mechanisms that govern the reciprocal interrelation of Ca(2+) and ROS signaling are only beginning to emerge. NADPH oxidases of the respiratory burst oxidase homolog (RBOH) family are critical components contributing to the generation of ROS while Calcineurin B-like (CBL) Ca(2+) sensor proteins together with their interacting kinases (CIPKs) have been shown to function in many Ca(2+)- signaling processes. In this study, we identify direct functional interactions between both signaling systems. We report that the CBL-interacting protein kinase CIPK26 specifically interacts with the N-terminal domain of RBOHF in yeast two-hybrid analyses and with the full-length RBOHF protein in plant cells. In addition, CIPK26 phosphorylates RBOHF in vitro and co-expression of either CBL1 or CBL9 with CIPK26 strongly enhances ROS production by RBOHF in HEK293T cells. Together, these findings identify a direct interconnection between CBL-CIPK-mediated Ca(2+) signaling and ROS signaling in plants and provide evidence for a synergistic activation of the NADPH oxidase RBOHF by direct Ca(2+)-binding to its EF-hands and Ca(2+)-induced phosphorylation by CBL1/9-CIPK26 complexes.

Pollen tubes grow rapidly by very fast rates and reach extended lengths to bring about fertilization during plant reproduction. The pollen tube grows exclusively at its tip. Fundamental for such local, tip-focused growth are the presence of internal gradients and transmembrane fluxes of ions. Consequently, vegetative pollen tube cells are an excellent single cell model system to investigate cell biological processes of vesicle transport, cytoskeleton reorganization and regulation of ion transport. The second messenger Ca(2+) has emerged as a central and crucial modulator that not only regulates but also integrates the coordination each of these processes. In this review we reflect on recent advances in our understanding of the mechanisms of Ca(2+) function in pollen tube growth, focusing on its role in basic cellular processes such as control of cell growth, vesicular transport and intracellular signaling by localized gradients of second messengers. In particular we discuss new insights into the identity and role of Ca(2+) conductive ion channels and present experimental addressable hypotheses about their regulation. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Stomatal movements rely on alterations in guard cell turgor. This requires massive K + bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K + into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K + / H + exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K + and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K + accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K + is a requirement for stomatal opening and a critical component in the overall K + homeostasis essential for stomatal closure, and suggest that vacuolar K + fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.stomata | luminal pH control T he rapid accumulation and release of K + and of organic and inorganic anions by guard cells controls the opening and closing of stomata and thereby gas exchange and transpiration of plants. The intracellular events that underlie stomatal opening start with plasma membrane hyperpolarization caused by the activation of H + -ATPases, which induces K + uptake through voltage-gated inwardly rectifying K + in channels (1). Potassium uptake is accompanied by the electrophoretic entry of the counterions chloride, nitrate, and sulfate, and by the synthesis of malate. These osmolytes, together with sucrose accumulation, increase the turgor in guard cells and thereby drive stomatal opening. Stomatal closure is initiated by activation of the plasma membrane localized chloride and nitrate efflux channels SLAC1 and SLAH3 that are regulated by the SnRK2 protein kinase OST1 and the Ca 2+ -dependent protein kinases CPK21 and 23 (2, 3). CPK6 also activates SLAC1 and coordinately inhibits rectifying K + in channels to hinder stomatal opening (4, 5). Sulfate and organic acids exit the guard cell through R-type anion channels. The accompanying reduction in guard cell turgor results in stomatal closure (1).Despite the established role of plasma membrane transport in guard cell function and stomatal movement, ion influx into the cytosol represents only a transit step to the vacuole, as more than 90% of the solutes released from guard cells originate from vacuoles (6). In contrast to the plasma membrane, knowledge of the transport processes occurring in intracellular compartments of guard cells during stomatal movements is less advanced (7). Only recently, AtALMT9 has been shown to act as a malateinduced chloride channel at the tonoplast that is required for stomatal opening (8). Vacuoles govern turgor-driven cha...