Zinc finger transcription factor MtZPT2-2 negatively regulates salt tolerance in Medicago truncatula.

Zinc finger proteins (ZFPs) are transcription factors involved in multiple cellular functions. We identified a C2H2 type ZFP (MtZPT2-2) in Medicago truncatula and demonstrated that it localizes to the nucleus and inhibits transcription of two genes encoding high-affinity potassium transporters (MtHKT1; 1 and MtHKT1; 2). MtZPT2-2 transcripts were detected in the stem, leaf, flower, seeds and roots, with the highest level in the xylem and phloem of roots and stems. MtZPT2-2 transcription in leaves was reduced after salt stress. Compared to the wild type (WT), transgenic lines overexpressing MtZPT2-2 had decreased salt tolerance, while MtZPT2-2-knockout mutants showed increased salt tolerance. MtHKT1; 1 and MtHKT1; 2 transcripts and Na+ accumulation in shoots and roots, as well as in the xylem of all genotypes of plants, were increased after salt treatment, with higher levels of MtHKT1; 1 and MtHKT1; 2 transcripts and Na+ accumulation in MtZPT2-2-knockout mutants and lower levels in MtZPT2-2-overexpressing lines compared with the WT. K+ levels showed no significant difference among plant genotypes under salt stress. Moreover, MtZPT2-2 was demonstrated to bind with the promoter of MtHKT1; 1 and MtHKT1; 2 to inhibit their expression. Antioxidant enzyme activities and the gene transcript levels were accordingly upregulated in response to salt, with higher levels in MtZPT2-2-knockout mutants and lower levels in MtZPT2-2-overexpressing lines compared with WT. The results suggest that MtZPT2-2 regulates salt tolerance negatively through down-regulating MtHKT1; 1 and MtHKT1; 2 expression directly to reduce Na+ unloading from the xylem and regulates antioxidant defense indirectly.