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