Genetics and Breeding of Herbicide Tolerance in Sunflower

Although there are some examples of gene discovery for non-target site mechanisms of tolerance in sunflower, the main focus of research and development during the last decade was directed to the discovery of altered acetohydroxyacid synthase (AHAS) genes and enzymes. In this way, several natural or induced mutant alleles of the sunflower locus were reported and characterized. Four of these alleles were utilized to develop different non-GMO traits and technologies of weed control: Clearfield®, Clearfield Plus®, Sures, and ExpressSun®. Each one of these technologies has their own characteristics, cross-tolerance pattern, benefits and drawbacks, which are briefly reviewed. Some methods to speed up the introgression of these traits into the breeding program are described, as well as the dominance relationships between some members of the multiallelic locus. Proper utilization of these technologies allowed, and will continue to allow, an excellent weed control for the sunflower crop. However, some of these genes and their allelic interactions remain to be tested and developed in the years to come in order to create novel technologies. Additionally, it is clear that only one mode of action - the inhibition of the AHAS enzyme - is being exploited so far in sunflower. This will prompt the rapid selection of tolerant weeds that may jeopardize the sustainability of all these technologies. Selection over cultivated germplasm, wild species and mutagenized libraries will allow the discovery of new sources of HT, especially to other modes of action apart from the inhibition of AHAS, in order to complement the current technologies.