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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 plantMn homeostasis. [ABSTRACT FROM AUTHOR]

Increasing soil salinity is a major problem in global crop production. The detrimental effect of saline soils on plant growth results from ionic toxicity of the Na+ ion in combination with osmotic perturbation. However, the mechanisms of Na uptake in roots, its transport to other tissues as well as the distribution of Na+ in specific plant tissues and cell layers are still not fully understood. The hyphenation of laser ablation (LA) and inductively coupled plasma mass spectrometry (ICP-MS) provides a well-suited tool for elemental bioimaging in plant tissues. Here, we present a novel LA-ICP-MS method which allows the generation of meaningful images of the Na distribution in Arabidopsis thaliana roots at the cellular level. For this purpose, a sample preparation protocol was developed which maintains the native Na distribution of the sample while avoiding contamination with Na+ from other sources. The ICP-MS was optimized to provide a high sensitivity for Na by applying cold plasma conditions. Plasma cooling was achieved by reducing the plasma power and delivering an additional wet aerosol to the ICP. By this means, the limit of detection was reduced by a factor of two in comparison with the standard mode. Images of the Na+ distribution in roots indicative of a distinctive accumulation of Na+ in defined A. thaliana root cell layers are thus obtained with a spatial resolution of down to 4 μm. Our analysis provides the first evidence for sharp Na concentration differences in roots, which are established between different cell layers and result in maximal Na accumulation in the cortex cells of the root organ. [ABSTRACT FROM AUTHOR]

In plants, potassium (K+) homeostasis is tightly regulated and established against a concentration gradient to the environment. Despite the identification of Ca2+-regulated kinases as modulators of K+ channels, the immediate signaling and adaptation mechanisms of plants to low-K+ conditions are only partially understood., To assess the occurrence and role of Ca2+ signals in Arabidopsis thaliana roots, we employed ratiometric analyses of Ca2+ dynamics in plants expressing the Ca2+ reporter YC3.6 in combination with patch-clamp analyses of root cells and two-electrode voltage clamp (TEVC) analyses in Xenopus laevis oocytes., K+ deficiency triggers two successive and distinct Ca2+ signals in roots exhibiting spatial and temporal specificity. A transient primary Ca2+ signature arose within 1 min in the postmeristematic stelar tissue of the elongation zone, while a secondary Ca2+ response occurred after several hours as sustained Ca2+ elevation in defined tissues of the elongation and root hair differentiation zones. Patch-clamp and TEVC analyses revealed Ca2+ dependence of the activation of the K+ channel AKT1 by the CBL1- CIPK23 Ca2+ sensor-kinase complex., Together, these findings identify a critical role of cell group-specific Ca2+ signaling in low K+ responses and indicate an essential and direct role of Ca2+ signals for AKT1 K+ channel activation in roots. [ABSTRACT FROM AUTHOR]

Ca2+ signatures are central to developmental processes and adaptive responses in plants. However, high-resolution studies of Ca2+ dynamics using genetically encoded Ca2+ indicators ( GECIs) such as Yellow Cameleon ( YC) proteins have so far not been conducted in important model crops such as rice ( Oryza sativa)., We conducted a comparative study of 35S and ubiquitin-10 ( UBQ10) promoter functionality in Arabidopsis thaliana and O. sativa plants expressing the Ca2+ indicator Yellow Cameleon 3.6 ( YC3.6) under control of the UBQ10 or 35S promoter. Ca2+ signatures in roots of both species were analyzed during exposure to hyperpolarization/depolarization cycles or in response to application of the amino acid glutamate., We found a superior performance of the UBQ10 promoter with regard to expression pattern, levels and expression stabilities in both species. We observed remarkable differences between the two species in the spatiotemporal parameters of the observed Ca2+ signatures. Rice appeared in general to respond with a lower maximal signal amplitude but greatly increased signal duration when compared with Arabidopsis., Our results identify important advantages to using the UBQ10 promoter in Arabidopsis and rice and in T- DNA mutant backgrounds. Moreover, the observed differences in Ca2+ signaling in the two species underscore the need for comparative studies to achieve a comprehensive understanding of Ca2+ signaling in plants. [ABSTRACT FROM AUTHOR]

Excessive Na + in soils inhibits plant growth. Here, we report that Na + stress triggers primary calcium signals specifically in a cell group within the root differentiation zone, thus forming a "sodium-sensing niche" in Arabidopsis. The amplitude of this primary calcium signal and the speed of the resulting Ca 2+ wave dose-dependently increase with rising Na + concentrations, thus providing quantitative information about the stress intensity encountered. We also delineate a Ca 2+ -sensing mechanism that measures the stress intensity in order to mount appropriate salt detoxification responses. This is mediated by a Ca 2+ -sensor-switch mechanism, in which the sensors SOS3/CBL4 and CBL8 are activated by distinct Ca 2+ -signal amplitudes. Although the SOS3/CBL4-SOS2/CIPK24-SOS1 axis confers basal salt tolerance, the CBL8-SOS2/CIPK24-SOS1 module becomes additionally activated only in response to severe salt stress. Thus, Ca 2+ -mediated translation of Na + stress intensity into SOS1 Na + /H + antiporter activity facilitates fine tuning of the sodium extrusion capacity for optimized salt-stress tolerance., Competing Interests: Declaration of interests After completion of this work in the laboratory of J.K., K.H.E. became an employee of Yara GmbH & Co. KG, a company that offers products based on calcium as a stress alleviator., (Copyright © 2022 Elsevier Inc. All rights reserved.)

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 di-glucose 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., (Copyright © 2018 The Authors. Published by Elsevier Ltd.. All rights reserved.)

In plants, potassium (K + ) homeostasis is tightly regulated and established against a concentration gradient to the environment. Despite the identification of Ca 2+ -regulated kinases as modulators of K + channels, the immediate signaling and adaptation mechanisms of plants to low-K + conditions are only partially understood. To assess the occurrence and role of Ca 2+ signals in Arabidopsis thaliana roots, we employed ratiometric analyses of Ca 2+ dynamics in plants expressing the Ca 2+ reporter YC3.6 in combination with patch-clamp analyses of root cells and two-electrode voltage clamp (TEVC) analyses in Xenopus laevis oocytes. K + deficiency triggers two successive and distinct Ca 2+ signals in roots exhibiting spatial and temporal specificity. A transient primary Ca 2+ signature arose within 1 min in the postmeristematic stelar tissue of the elongation zone, while a secondary Ca 2+ response occurred after several hours as sustained Ca 2+ elevation in defined tissues of the elongation and root hair differentiation zones. Patch-clamp and TEVC analyses revealed Ca 2+ dependence of the activation of the K + channel AKT1 by the CBL1-CIPK23 Ca 2+ sensor-kinase complex. Together, these findings identify a critical role of cell group-specific Ca 2+ signaling in low K + responses and indicate an essential and direct role of Ca 2+ signals for AKT1 K + channel activation in roots., (© 2016 The Authors. New Phytologist © 2016 New Phytologist Trust.)

Ca(2+) signatures are central to developmental processes and adaptive responses in plants. However, high-resolution studies of Ca(2+) dynamics using genetically encoded Ca(2+) indicators (GECIs) such as Yellow Cameleon (YC) proteins have so far not been conducted in important model crops such as rice (Oryza sativa). We conducted a comparative study of 35S and ubiquitin-10 (UBQ10) promoter functionality in Arabidopsis thaliana and O. sativa plants expressing the Ca(2+) indicator Yellow Cameleon 3.6 (YC3.6) under control of the UBQ10 or 35S promoter. Ca(2+) signatures in roots of both species were analyzed during exposure to hyperpolarization/depolarization cycles or in response to application of the amino acid glutamate. We found a superior performance of the UBQ10 promoter with regard to expression pattern, levels and expression stabilities in both species. We observed remarkable differences between the two species in the spatiotemporal parameters of the observed Ca(2+) signatures. Rice appeared in general to respond with a lower maximal signal amplitude but greatly increased signal duration when compared with Arabidopsis. Our results identify important advantages to using the UBQ10 promoter in Arabidopsis and rice and in T-DNA mutant backgrounds. Moreover, the observed differences in Ca(2+) signaling in the two species underscore the need for comparative studies to achieve a comprehensive understanding of Ca(2+) signaling in plants., (© 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.)