The Ca 2+ ‐ CAMTA6 ‐ PP2C49 ‐ HKT1 ;1 signaling axis modulates ion homeostasis during Arabidopsis seed germination under salt stress

SUMMARY Soil salinization severely constrains seedling establishment by disrupting cellular Na + /K + homeostasis. Calcium (Ca 2+ ) signaling contributes to the re‐establishment of ion balance during early growth, yet the mechanisms linking Ca 2+ perception to ion transport regulation remain unclear. Here, we identify a Ca 2+ ‐responsive regulatory module in Arabidopsis thaliana comprising CALMODULIN‐BINDING TRANSCRIPTION ACTIVATOR 6 (CAMTA6), the TYPE 2C PROTEIN PHOSPHATASE PP2C49, and the HIGH‐AFFINITY K + TRANSPORTER HKT1;1, and define their coordinated roles during germination under salt stress. Spatial promoter analyses revealed that NaCl induces CAMTA6 expression at cotyledon margins, while CaCl 2 stimulates HKT1;1 transcription in the radicle, consistent with CAMTA6‐mediated repression of HKT1;1 . In camta6 mutants, PP2C49 expression expanded beyond its normal radicle‐restricted domain, indicating CAMTA6‐dependent spatial control. Promoter activation assays in planta demonstrated CAMTA6‐dependent transactivation of the HKT1;1 and PP2C49 promoters. Treatment with the PP2C inhibitor sanguinarine enhanced germination under salinity in the wild type, but not in hkt1 nor in the salt‐tolerant camta6 and pp2c49 mutants. Sanguinarine restricted CAMTA6 promoter activity to cotyledon margins, suppressed PP2C49 expression, and enhanced HKT1;1 accumulation in the radicle, collectively supporting improved Na + /K + balance. Transcriptome profiling further revealed additional Ca 2+ ‐responsive PP2C genes under CAMTA6‐dependent regulation. Together, these findings establish a Ca 2+ ‐regulated transcriptional network coordinating ion homeostasis during germination and suggest strategies to support germinating embryo performance in saline environments.