Cloning the Chromosome 7BL Boron Toxicity Tolerance Locus Bo1 and Analysis of Allelic Variation 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 previously identified tolerance QTL have been priorities for marker assisted selection in breeding programs across southern Australia. We report the positional cloning and characterisation of the gene underlying the major boron tolerance locus (Bo1) in wheat on chromosome 7BL. High-resolution genetic and physical mapping in 2,200 F2’s derived from a cross between Halberd and Cranbrook defined the tolerance locus and enabled identification of the tolerance gene TaBot1L. The gene shows most similarity to a family of anion exchange proteins, and has 82% amino acid identity to the major boron tolerance gene in barley (Hordeum vulgare L.) located on chromosome 4H, HvBot1 (Sutton et al. 2007 Science 318:1446). Heterologous expression and functional analysis of the protein in yeast confirms its role as a boron efflux transporter, and suggests that the favourable Halberd allele is a more efficient boron efflux transporter than the barley protein. Independent Halberd-EMS mutations provide evidence that TaBot1L is the Bo1 locus gene. Allelic variation is consistent with the observed boron tolerance phenotype. In the highly boron tolerant line G61450 an additional highly similar TaBot1L transporter is found on 4AL. To date multiple alleles in bread and durum wheat have been characterized. No 7A version of the TaBot1L transporter has been found. The D-genome version produces a truncated, non-functional protein.