A domestication-associated reduction in K+-preferring HKT transporter activity underlies maize shoot K+ accumulation and salt tolerance

Maize was domesticated from Balsas teosinte c. 10 000 yr ago. Previous studies have suggested that increased tolerance to environmental stress occurred during maize domestication. However, the underlying genetic basis remains largely unknown. We used a maize (W22)-teosinte recombinant inbred line (RIL) to investigate the salt wild-type tolerance aspects of maize domestication. We revealed that ZmHKT2 is a major QTL that regulates K+ homeostasis in saline soils. ZmHKT2 encodes a K+-preferring HKT family transporter and probably reduces shoot K+ content by removing K+ ions from root-to-shoot flowing xylem sap, ZmHKT2 deficiency increases xylem sap and shoot K+ concentrations, and increases salt tolerance. A coding sequence polymorphism in the ZmHKT2(W22) allele (SNP389-G) confers an amino acid variant ZmHKT2 that increases xylem sap K+ concentration, thereby increasing shoot K+ content and salt tolerance. Additional analyses showed that SNP389-G first existed in teosinte (allele frequency 56% in assayed accessions), then swept through the maize population (allele frequency 98%), and that SNP389-G probably underwent positive selection during maize domestication. We conclude that a domestication-associated reduction in K+ transport activity in ZmHKT2 underlies maize shoot K+ content and salt tolerance, and propose that CRISPR-based editing of ZmHKT2 might provide a feasible strategy for improving maize salt tolerance.