Rapid hyperosmotic-induced Ca2+ responses in Arabidopsis thaliana exhibit sensory potentiation and establish involvement of plastidial KEA transporters

AbstractPlants experience hyperosmotic stress when faced with saline soils and possibly drought stress, but it is currently unclear how plants perceive this stress in an environment of dynamic water availabilities. Hyperosmotic stress induces a rapid rise in intracellular Ca2+ concentrations ([Ca2+]i) in plants, and this Ca2+ response may reflect the activities of osmo-sensory components. Here, we find in the reference plant Arabidopsis thaliana that the rapid hyperosmotic-induced Ca2+ response exhibited enhanced response magnitudes after pre-exposure to an intermediate hyperosmotic stress. We term this phenomenon \"osmo-sensory potentiation\". The initial sensing and potentiation occurred in intact plants as well as in roots. Having established a quantitative understanding of WT responses, we investigated effects of pharmacological inhibitors and candidate channel/transporter mutants. Quintuple MSL channel mutants as well as double MCA channel mutants did not affect the response. However interestingly, double mutations in the plastid KEA transporters, kea1kea2, and a single mutation that does not visibly affect chloroplast structure, kea3, impaired the rapid hyperosmotic-induced Ca2+ responses. These mutations did not significantly affect sensory potentiation of the response. These findings suggest that plastids may play an important role in the early steps mediating the response to hyperosmotic stimuli. Together, these findings demonstrate that the plant osmosensory components necessary to generate rapid osmotic-induced Ca2+ responses remains responsive under varying osmolarities, endowing plants with the ability to perceive the dynamic intensities of water limitation imposed by osmotic stress.\n\nSignificance StatementThe sensitivity ranges of biological sensors determine when- and to what extent responses to environmental stimuli are activated. Plants may perceive water limitation imposed by soil salinity or drought in the form of osmotic stress, among other mechanisms. Rapid osmotic stress-induced Ca2+ responses provide the opportunity to quantitatively characterize the responses to osmotic stress under environmental and genetic perturbations. This report describes a phenomenon whereby prior exposure to osmotic stress increases the sensitivity of the rapid responses to subsequent stress. Further, mutations in specific plastidial transporters were found to reduce the stress response. These findings inform the reader of new avenues for understanding osmotic stress responses in plants.