Allelic Variation and Gene Duplication Determine Boron Toxicity Tolerance in Bread and Durum Wheat

Boron has the narrowest range between deficient and toxic soil solution concentration of all plant nutrients, and boron deficiency and toxicity both severely limit crop production worldwide. In wheat, genetic variation exists for tolerance to soil boron, and the utilisation of a locus on the long arm of chromosome 7BL named Bo1 has been a long term priority for marker assisted selection in wheat breeding programs in Australia. To assist breeding for this trait we defined the tolerance locus using a population of 2100 individuals derived from a cross between Cranbrook (boron intolerant) and Halberd (boron tolerant) and used a map based cloning approach to identify the Bo1-locus gene. We generated independent Halberd EMS mutations to confirm the identity of the tolerance protein as a membrane bound boron efflux transporter, which is similar but not orthologous to a barely boron tolerance gene HvBot1. An allelic series of genes at the locus was identified from a panel of bread and durum wheats originating from diverse agro-ecological zones, enabling correlation between allele distribution and varietal dominance within an Australian breeding context. In addition to variants at the 7BL locus, a rare allele is also found in the highly boron tolerant line G61450 on chromosome 4AL. D genome variation ranges from a functional allele identified in Aegilops tauschii AL8/78, to a truncated non-functional allele in Chinese Spring. No 7A version of the boron transporter has been found. Knowledge of allele functionality provides opportunities for informed breeding regarding this trait and the utilisation of tightly linked traits of agronomic significance.