Genome Wide Association Mapping of Grain Cadmium in Hard Winter Wheat.

Pollution-safe cultivars of bread wheat (Triticum aestivum L.) can reduce cadmium consumption and the corresponding risks to human health.  Cadmium contamination of agricultural soils is widespread and rapidly worsening in China, raising concern about the safety of the Chinese grain supply since cereals are dominant sources of Cd in human diets.  Variation in Cd concentration in durum wheat (Triticum turgidum L. var. durum Desf.) is well documented and has been associated with a major QTL on chromosome 5B, but genetic variation in Cd accumulation in hexaploid bread wheat is less well characterized.  Grain mineral concentration was measured in a 299-genotype hard winter wheat association mapping panel grown in 2012 and 2013 near Ithaca, NE (on the site of a decommissioned munitions factory) and Tipton, OK.  In Nebraska trials, grain Cd concentration averaged 0.230 mg kg^-1 in 2012 and 0.157 mg kg^-1 in 2013; Cd concentrations ranged from 0.080 to 0.580 mg kg^-1 in 2012.  In contrast, mean Cd concentration in Oklahoma trials was 0.014 mg kg^-1.  The low-Cd phenotype was heritable and amenable to selection in the high-Cd Nebraska environment.  Genome-wide association analysis with 92K SNP markers identified a major QTL on chromosome 5A in each environment and across environments.  Syntenic relationships with the barley genome sequence were used to determine that this QTL likely is homeologous to the 5B QTL for Cdu1 in durum wheat.   The high-Cd allele was present in 29% of the genotypes in the AM panel.  A second QTL on 5A, present in only 9% of the genotypes in the panel, had a significant additive effect on grain Cd concentration.   In the highest Cd trial (2012), the two QTLs had a significant epistatic interaction.  These results provide a foundation for further genetic studies exploring grain Cd accumulation in bread wheat.