Athb-12, a homeobox-leucine zipper domain protein from Arabidopsis thaliana, increases salt tolerance in yeast by regulating sodium exclusion

An Arabidopsis cDNA clone that encodes Athb-12, a homeobox-leucine zipper domain protein (HD-Zip), was isolated by functional complementation of the NaCl-sensitive phenotype of a calcineurin (CaN)-deficient yeast mutant (cnbΔ, regulatory subunit null). CaN, a Ca ²⁺/calmodulin- dependent protein phosphatase, regulates Na ⁺ ion homeostasis in yeast. Expression of Athb-12 increased NaCl tolerance but not osmotic stress tolerance of these cnbΔ cells. Furthermore, expression of two other HD-Zip from Arabidopsis, Athb-1 and -7, did not suppress NaCl sensitivity of cnbΔ cells. These results suggest that Athb-12 specifically functions in Na ⁺ ion homeostasis in yeast. Consistent with these observations, expression of Athb-12 in yeast turned on transcription of the NaCl stress-inducible PMR2A, which encodes a Na ⁺/Li ⁺ translocating P-type ATPase, and decreased Na ⁺ levels in yeast cells. To investigate the biological function of Athb-12 in Arabidopsis, we performed Northern blot analysis. Expression of Athb-12 was dramatically induced by NaCl and ABA treatments, but not by KCl. In vivo targeting experiments using a green fluorescent protein reporter indicated that Athb-12 was localized to the nucleus. These results suggest that Athb-12 is a putative transcription factor that may be involved in NaCl stress responses in plants.