Rapid hyperosmotic-induced Ca ²⁺ responses in Arabidopsis thaliana exhibit sensory potentiation and involvement of plastidial KEA transporters

Abstract

Plants experience hyperosmotic stress when faced with saline soils and possibly with drought stress, but it is currently unclear how plant roots perceive this stress in an environment of dynamic water availabilities. Hyperosmotic stress induces a rapid rise in intracellular Ca 2+ concentrations ([Ca 2+ ] i ) in plants, and this Ca 2+ response may reflect the activities of osmo-sensory components. Here, we find in the reference plant Arabidopsis thaliana that the rapid hyperosmoticinduced Ca 2+ response exhibited enhanced response magnitudes after preexposure 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 wild-type responses, we investigated effects of pharmacological inhibitors and candidate channel/transporter mutants. Quintuple mechano-sensitive channels of small conductance-like (MSL) plasma membrane-targeted channel mutants as well as double mid1-complementing activity (MCA) channel mutants did not affect the response. Interestingly, however, double mutations in the plastid K + exchange antiporter (KEA) transporters kea1kea2 and a single mutation that does not visibly affect chloroplast structure, kea3, impaired the rapid hyperosmotic-induced Ca 2+ responses. These mutations did not significantly affect sensory potentiation of the response. These findings suggest that plastids may play an important role in early steps mediating the response to hyperosmotic stimuli. Together, these findings demonstrate that the plant osmosensory components necessary to generate rapid osmotic-induced Ca 2+ responses remain responsive under varying osmolarities, endowing plants with the ability to perceive the dynamic intensities of water limitation imposed by osmotic stress.osmotic sensing | calcium | salt stress | plastid | abscisic acid P lants exhibit a wide range of physiological responses to cope with water deprivation by drought and salinity stress (1-3). The properties of biological sensors determine the circumstances and extent to which these coping mechanisms are activated, but the early sensory mechanisms and components regulating the osmotic sensory components in plants are not well understood (see ref. 4 for review). Pioneering studies have demonstrated that Arabidopsis seedlings expressing the bioluminescent Ca 2+ reporter protein aequorin exhibit a rapid rise in intracellular Ca 2+ ([Ca 2+ ] i ) within seconds upon stimulation by NaCl solution (5, 6). This rapid osmotic-induced Ca 2+ response has been observed in plant species ranging from rice (7) to the basal-branching moss taxon Physcomitrella patens (8), indicating that this response may be conserved across the Plantae kingdom. Solutions of either NaCl or isoosmotic mannitol/sorbitol induce nearly identical rapid Ca 2+ responses, indicating that the nature of this rapid stimulus is largely osmotic rather than ionic (5, 9, 10). Individual seedling responses tend to be quite hetero...

Cryo-EM Structure of OSCA1.2 from Oryza sativa: Mechanical basis of hyperosmolality-gating in Plants

“…(A) From left to right, (1) Parallel to membrane plane view of unsharpened cryoEM density map used for initial chain tracing, (2–4) sharpened 4.9 Å map used for model building and refinement: (2) membrane plane view, (3) extracellular view, and (4) intracellular view. (B) Protein topology of OsOSCA1.2.…”

“…Hyperosmolarity and osmotic stress are among the first physiological responses to changes in salinity and drought. Hyperosmolality triggers increases in cytosolic free Ca 2+ concentration and thereby initiates an osmotic stress-induced signal transduction cascade in plants (1–3). Salinity and drought stress trigger diverse protective mechanisms in plants enabling enhanced drought tolerance and reduction of water loss in leaves.…”

Genomic novelty and process-level convergence in adaptation to whole genome duplication

“…2), a K + antiporter that modulates osmoregulation, ion, and pH homeostasis 44 . Recent evidence indicates that KEA2 is important for eliciting a rapid hyperosmotic-induced Ca 2+ response to water limitation imposed by osmotic stress 45 . The KEA2 locus in autotetraploid C. amara features an exceptional ten fineMAV-outlier SNPs (Table S3, S5), indicating that the sweep contains a run of radical amino acid changes at high allele frequency difference between the ploidies, strongly suggesting a ploidy-selected functional change.…”

The battle of two ions: Ca2+ signalling against Na+ stress

“…Overall, despite establishing Ca 2+ signals as occurring during salt stress responses, the limited light emission of aequorin impeded high spatial and temporal resolution analyses. Moreover, it remained little understood whether the cue inducing this rapid Ca 2+ response is constituted by physical osmotic or chemical ionic parameters, or rather represents a combination of the two (Stephan et al 2016). Nevertheless, a hyperosmolarity gated Ca 2+ permeable channel (OSCA1) has been recently identified as a strong candidate for involvement in the perception of saline (and/or) drought stress conditions (Yuan et al 2014).…”