The Role of Gene Duplication and Allelic Diversity in Adaptation for High Soil Boron

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. The major tolerance locus in wheat, Bo1, is located on 7BL while four QTL for tolerance have been mapped in the barley genome.  The wheat loci and three of the four barley loci have been cloned and illustrate the processes that can now be used to identify and deploy variation. In selecting for performance, early farmers around the Mediterranean, through the Middle East and into India and China developed wheat landraces with varying levels of tolerance. A similar process was occurring in barley but the results were very different. Our data reveals divergent evolution of B tolerance in wheat compared to barley.  While in both species transcriptional regulation of B transporter genes is a common mechanism, tissue specificity differs and, in barley, tolerance is achieved by tandem gene duplication while in wheat gene duplication was combined with the generation of allelic diversity.

The success of breeding for B tolerant wheat versus barley may lie in the narrow allelic variation seen in barley compared to wheat. Matching B tolerance alleles to the level of B in the environment appears to be critical but we now have germplasm, markers and information on the mechanism of B tolerance.