Resistance evaluation for herbicide resistance-endowing acetolactate synthase (ALS) gene mutations using Raphanus raphanistrum populations homozygous for specific ALS mutations

Acetolactate synthase (ALS)-inhibiting herbicide resistance is common in Raphanus raphanistrum (wild radish) populations across the Western Australian (WA) grain belt. This study investigates the molecular and biochemical basis of ALS herbicide resistance in five R.similar to raphanistrum populations. Five known ALS herbicide resistanceendowing mutations (Pro-197-Ala, Pro-197-Thr, Pro-197-Ser, Asp-376-Glu and Trp-574-Leu) were identified, and their resistance spectrum to ALS-inhibiting herbicides was determined using purified populations individually homozygous for each mutation (except for Pro-197-Ala). Plants homozygous for ALS mutations at Pro-197 were found to be cross-resistant to ALS-inhibiting sulfonylurea (SU) and triazolopyrimidine (TP) herbicides, while plants homozygous for Trp-574-Leu were resistant to SU, TP and imidazolinone (IMI) ALS herbicide classes. The Asp-376-Glu mutation is reported here for the first time in R.similar to raphanistrum populations and characterised at both the whole-plant and enzyme level. Plants homozygous for Asp-376-Glu were highly resistant to SU and TP herbicides, based on LD50 R/S ratios (>130 and 128 respectively) and I50 R/S ratios (170 and >110 respectively). In contrast, these plants were moderately resistant to the IMI imazamox (LD50 R/S ratio of 8, I50 R/S ratio of 3) and imazethapyr (I50 R/S ratio of 8) and susceptible to imazapyr (I50 R/S ratio of 0.76). A novel observation in this study is that resistance of homozygous Glu-376 plants is associated with a remarkable growth reduction in the presence of the ALS herbicides tested, making early resistance diagnosis and management difficult.