The acetohydroxyacid synthase gene family its role in herbicide resistant sunflowers

Graduation date: 2006Resistance to acetohydroxyacid synthase (AHAS)-inhibiting herbicides has\ud been reported in over 90 weedy species, including wild sunflower biotypes, since the\ud herbicides were developed in 1982. The AHAS gene family in sunflower, consisting\ud of three paralogs AHAS1 , AHAS2, and AHAS3, has been targeted for inducing\ud herbicide resistance. A polymorphism identified in an elite sunflower line bred for\ud resistance to the class of AHAS-inhibiting herbicides - sulfonylureas (SU) allowed\ud development of a genotyping assay to be used in marker assisted selection (MAS).\ud The expression level of the AHAS gene family was assayed in eight selected sunflower\ud tissues. Diversity of the AHAS gene family was assessed among wild and\ud domesticated sunflowers. A single nucleotide polymorphism (SNP) was discovered in\ud the SU resistant sunflower. Development of a SNP assay, using fluorescently probed\ud acyclo-dNTPs, facilitated genotypic determination. Non-quantitative reverse\ud transcription-PCR (RT-PCR) and real-time RT-PCR were used to observe the\ud expression level of each gene in the AHAS family. Diversity within the AHAS gene\ud family among wild germplasm and domesticated germplasm was assessed using\ud sequence alignment of 46 different accessions. The discovery of the SNP at codon 197 in AHAS1 of the resistant line was predicted to confer resistance to the SU class of\ud AHAS-inhibiting herbicides. The mutation at codon 197 has been observed in other\ud plant species that confer resistance to the SU herbicides. Identification of the\ud presence or absence of herbicide resistance genes using the SNP assay yields reliable\ud genotyping that can be used to compliment phenotyping data. The AHAS1 paralog\ud was shown to be the most highly expressed gene in the AHAS gene family. All three\ud genes are expressed in sunflower with the highest expression of AHAS1 exhibited in\ud leaf tissue. Overall gene diversity was greatest in AHAS1. Gene diversity was\ud particularly high in the wild sunflower accessions for all three genes. The AHAS1\ud gene is likely to be the target of mutations conferring herbicide resistance because it is\ud the most highly expressed gene in the family. Ultimately, MAS for herbicide\ud resistance genes in sunflower was improved by the development of a reliable SNP\ud assay for codon 197 in AHAS1