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The maintenance of sodium/potassium (Na+/K+) homeostasis in plant cells is essential for salt tolerance. Plants export excess Na+ out of cells mainly through the Salt Overly Sensitive (SOS) pathway, activated by a calcium signal; however, it is unknown whether other signals regulate the SOS pathway and how K+ uptake is regulated under salt stress. Phosphatidic acid (PA) is emerging as a lipid signaling molecule that modulates cellular processes in development and the response to stimuli. Here, we show that PA binds to the residue Lys57 in SOS2, a core member of the SOS pathway, under salt stress, promoting the activity and plasma membrane localization of SOS2, which activates the Na+/H+ antiporter SOS1 to promote the Na+ efflux. In addition, we reveal that PA promotes the phosphorylation of SOS3‐like calcium‐binding protein 8 (SCaBP8) by SOS2 under salt stress, which attenuates the SCaBP8‐mediated inhibition of Arabidopsis K+ transporter 1 (AKT1), an inward‐rectifying K+ channel. These findings suggest that PA regulates the SOS pathway and AKT1 activity under salt stress, promoting Na+ efflux and K+ influx to maintain Na+/K+ homeostasis.

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) Ca 2+ 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.

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.

Systemic signaling is an essential hallmark of multicellular life. Pathogen encounter occurs locally but triggers organ-scale and organismic immune responses. In plants, elicitor perception provokes systemically expanding Ca2+and H2O2signals conferring immunity. Here, we identify a Ca2+sensing bi-kinase module as becoming super-activated through mutual trans-phosphorylation and imposing synergistically enhanced NADPH oxidase activation. A combined two-layer bi-kinase/substrate phospho-code allows for sensitized signaling initiation already by near-resting elevations of Ca2+concentration at the infection site. Subsequently, it facilitates further signal wave proliferation with minimal amplitude requirement, triggering protective defense responses throughout the plant. Our study reveals how plants build and perpetuate trans-cellular immune signal proliferation while avoiding disturbance of ongoing cellular signaling along the path of response dissemination.