Maccaferri et al. 2019 — Durum wheat reference genome; TdHMA3-B1 gene controls grain Cd accumulation
Maccaferri et al. present the first high-quality reference genome assembly for durum wheat (Triticum turgidum ssp. durum), comprising 10.45 Gb across the A and B sub-genomes. The paper is primarily a genomics resource paper, but contains a critical finding for heavy metal contamination: the gene TdHMA3-B1 (encoding a heavy metal P-type ATPase on chromosome 5B) is identified as the primary determinant of cadmium accumulation in durum wheat grain. Modern high-yielding durum wheat varieties carry a non-functional allele of TdHMA3-B1 that fails to sequester Cd in root vacuoles, allowing Cd to translocate to grain. Older landraces and some wild relatives carry a functional allele that strongly limits grain Cd accumulation. This explains why modern commercial durum (used in pasta, semolina, couscous) tends to have higher grain Cd than ancient wheat varieties and bread wheat, and it points to a breeding solution.
Key numbers
- TdHMA3-B1 locus on chromosome 5B controls approximately 80–90% of the variance in grain Cd accumulation in durum wheat mapping populations
- Modern commercial durum varieties: non-functional TdHMA3-B1 allele; grain Cd typically 50–400 µg/kg depending on soil Cd
- Landraces carrying functional TdHMA3-B1: grain Cd reduced by approximately 3–5× relative to commercial varieties on equivalent soils
- Genome assembly: 10.45 Gb total; 66,559 high-confidence gene models
- Reference genome enables CRISPR and marker-assisted breeding targeting TdHMA3-B1 restoration
Methods (brief)
Reference genome assembly using PacBio long-read sequencing, 10x Genomics linked reads, and Bionano optical mapping; gene annotation using RNA-seq; functional validation of TdHMA3-B1 by QTL analysis in biparental crosses and natural accessions; grain Cd phenotyping by ICP-MS across mapping populations.
Implications
Certification: TdHMA3-B1 non-functional allele is why pasta/semolina products made from commercial durum wheat carry elevated Cd relative to what is agronomically achievable. Breeding programs targeting this locus could substantially reduce Cd in commercial pasta without changing origin. Courses: Illustrates that cultivar genetics — not just soil Cd — determines grain Cd in durum; the same high-Cd Italian soils growing modern vs. landrace durum produce very different grain Cd levels. App: Durum wheat-derived ingredients (semolina, pasta, couscous) should carry an elevated Cd signal relative to bread wheat, reflecting the systematic difference in HMA3 allele status.