Calcium regulation of theArabidopsisNa+/K+transporter HKT1;1 improves seed germination under salt stress

Abstract

ABSTRACTCalcium is known to improve seed-germination rates under salt stress. We investigated the involvement of calcium ions (Ca2+) in regulatingHIGH-AFFINITY K+TRANSPORTER 1(HKT1;1), which encodes a Na+/K+transporter, and its post-translational regulatorTYPE 2C PROTEIN PHOSPHATASE 49(PP2C49), in germinatingArabidopsis thalianaseedlings. Germination rates ofhkt1mutant seeds under salt stress remained unchanged by CaCl2treatment in wild-typeArabidopsis, whereaspp2c49mutant seeds displayed improved salt-stress tolerance in the absence of CaCl2supplementation. Analysis ofHKT1;1andPP2C49promoter activity revealed that CaCl2treatment results in radicle-focused expression ofHKT1;1and reduction of the native radicle-exclusive expression ofPP2C49. Ion-content analysis indicated that CaCl2treatment improves K+retention in germinating wild-type seedlings under salt stress, but not inhkt1seedlings. Transgenic seedlings designed to exclusively expressHKT1;1in the radicle during germination displayed higher germination rates under salt stress than the wild type in the absence of CaCl2treatment. Transcriptome analysis of germinating seedlings treated with CaCl2, NaCl, or both revealed 118 upregulated and 94 downregulated genes as responsive to the combined treatment. Bioinformatics analysis of the upstream sequences of CaCl2–NaCl-treatment-responsive upregulated genes revealed the abscisic acid response element CACGTGTC, a potential CaM-binding transcription activator-binding motif, as most prominent. Our findings suggest a key role for Ca2+in mediating salt-stress responses during germination by regulating genes that function to maintain Na+and K+homeostasis, which is vital for seed germination under salt stress.