Association of P1B‑ATPase HMA3-B1 with Phenotypic Variation in Grain Cadmium Concentration in Durum Wheat.

Cadmium (Cd) is an environmental hazard that can negatively affect human health. The dominant source of human exposure to environmental Cd is through contaminated staple foods. Cultivars of durum wheat (Triticum turgidum subsp. durum) display genetic variation in propensity to accumulate Cd in grain, a trait that has been associated with restricted root-to-shoot translocation of Cd. A single major locus controlling grain Cd concentration (Cdu-B1) was reported on chromosome 5B, but the genetic factor conferring the low Cd phenotype was previously unknown. We have identified allelic variation in HMA3-B1, a P_1B-type ATPase predicted to have Cd transport activity, which could account for differences in grain Cd accumulation in durum wheat. Sequencing of HMA3-B1 from high-Cd and low-Cd lines isogenic for Cdu-B1 revealed a 17 bp duplication in exon 1 of high-Cd accumulators that generates a premature stop codon and non-functional HMA3-B1 protein. Fine mapping in more than 3,500 F₂ plants co-localized Cdu-B1 and HMA3-B1 to a 0.28 cM interval, and a marker for the 17 bp duplication segregated with grain Cd phenotype in a diverse set of 96 durum wheat cultivars and breeding lines. Heterologous expression of the functional allele, HMA3-B1a, phenotypically complemented yeast strains defective in vacuolar sequestration of Cd (Δycf1) and zinc (Δzrc1Δcot1). HMA3-B1a-GFP localizes at the tonoplast, consistent with a role for HMA3-B1 in the influx of Cd into the vacuole. The Cd-specific phenotype of Cdu-B1 is attributable to interaction of HMA3-B1 with homoeologue HMA3-A1, which transports Zn but not Cd. Given these data, HMA3-B1 is a strong candidate gene for Cdu-B1 in durum wheat and is currently the target of detailed functional analysis.