ABSTRACT
Accelerating genetic gain in crop improvement is required to ensure improved yield and yield stability under increasingly challenging climatic conditions. This case study demonstrates the effective confluence of innovative breeding technologies within a collaborative breeding framework to develop and rapidly introgress imidazolinone Group 2 herbicide tolerance into an adapted Australian chickpea genetic background. A well-adapted, high-yielding desi cultivar PBA HatTrick was treated with ethyl methanesulfonate to generate mutations in the ACETOHYDROXYACID SYNTHASE 1 (CaAHAS1) gene. After 2 years of field screening with imidazolinone herbicide across >20 ha and controlled environment progeny screening, two selections were identified which exhibited putative herbicide tolerance. Both selections contained the same single amino acid substitution, from alanine to valine at position 205 (A205V) in the AHAS1 protein, and KASP™ markers were developed to discriminate between tolerant and intolerant genotypes. A pipeline combining conventional crossing and F2 production with accelerated single seed descent from F2:4 and marker-assisted selection at F2 rapidly introgressed the herbicide tolerance trait from one of the mutant selections, D15PAHI002, into PBA Seamer, a desi cultivar adapted to Australian cropping areas. Field evaluation of the derivatives of the D15PAHI002 × PBA Seamer cross was analyzed using a factor analytic mixed model statistical approach designed to accommodate low seed numbers resulting from accelerated single seed descent. To further accelerate trait introgression, field evaluation trials were undertaken concurrent with crop safety testing trials. In 2020, 4 years after the initial cross, an advanced line selection CBA2061, bearing acetohydroxyacid synthase (AHAS) inhibitor tolerance and agronomic and disease resistance traits comparable to parent PBA Seamer, was entered into Australian National Variety Trials as a precursor to cultivar registration. The combination of cross-institutional collaboration and the application of novel pre-breeding platforms and statistical technologies facilitated a 3-year saving compared to a traditional breeding approach. This breeding pipeline can be used as a model to accelerate genetic gain in other self-pollinating species, particularly food legumes.
ABSTRACT
BACKGROUND: Faba bean (Vicia faba L.) is an important crop in Australian farming systems, however, weed control is a major constraint due to a lack of in-crop broadleaf herbicide options. To address this, we developed acetohydroxyacid synthase (AHAS) inhibitor herbicide tolerance in faba bean using mutagenesis techniques. Dose-response experiments, agronomic field evaluation and DNA sequencing of the AHAS gene were used to quantify and validate tolerance traits. RESULTS: Four M2 faba bean single-plant biotypes (IMI-1, IMI-2, IMI-3 and IMI-4) at a frequency of 3.63 × 10-6 were successfully recovered. Molecular characterisation of the AHAS gene identified two known target site mutations (resulting in protein substitutions Ala205Val and Ser653Asn) conferring tolerance. Phenotypic characterisation found that both mutations conferred high levels of tolerance to the imidazolinone herbicide imazapyr. However, although the Ala205Val substitution showed improved levels of cross-tolerance to a range of sulfonylurea chemistries, the Ser653Asn substitution did not. In the field, IMI-3 showed the highest level of agronomic tolerance across a range of imidazolinone herbicides. CONCLUSIONS: Mutagenesis techniques were successful in the development of tolerance to AHAS inhibitor herbicides in faba bean, and could facilitate the first safe in-crop broadleaf herbicide control option in Australian faba bean production. © 2019 Society of Chemical Industry.
