Identification of WNK Gene in Salvia miltiorrhiza Reveals SmWNK7 Positively Regulates Root Growth and Salt Tolerance

The dried roots and rhizomes of Salvia miltiorrhiza are widely used and economically important traditional Chinese medicinal materials. Land salinization affects the growth of S. miltiorrhiza, resulting in a decline in its quality and yield. WNK kinases belong to a unique family of serine/threonine kinases. They act as key regulators of plant growth, development and abiotic stress responses. However, this gene family has not been systematically characterized in Salvia miltiorrhiza. In this study, nine SmWNK genes were identified at the whole-genome level in S. miltiorrhiza. Phylogenetic analysis classified them into four structurally conserved subgroups. These genes are distributed across eight chromosomes and contain two pairs of intraspecific syntenic genes. Interspecific collinearity is far stronger between S. miltiorrhiza and dicots than between S. miltiorrhiza and monocots. Cis-element prediction indicated these cis-elements participate in light signaling, hormone responses, stress responses and developmental regulation. Quantitative real-time PCR revealed that eight SmWNK genes were significantly induced by salt stress, and SmWNK7 was selected as the key candidate for functional validation. Functional assays via heterologous overexpression in tobacco demonstrated that SmWNK7 overexpression promoted root elongation and enhanced salt tolerance. Compared with wild-type tobacco plants, SmWNK7-overexpressing transgenic tobacco lines had higher catalase (CAT) and peroxidase (POD) activities, lower malondialdehyde (MDA) content, and stronger root viability. These changes alleviated oxidative damage by enhancing the antioxidant defense system. Yeast two-hybrid screening yielded 40 SmWNK7-interacting annotated proteins, including 6 transcription factors and 1 protein kinase, which were enriched in 81 GO terms and 27 KEGG pathways. These findings confirm SmWNK7 positively regulates root growth and salt tolerance, laying a theoretical foundation for exploring SmWNK genes’ role in plant stress adaptation.