Investigation of herbicide resistance in oriental mustard (Sisymbrium orientale L.) in Australia

Abstract

Oriental mustard (Sisymbrium orientale L.), called Indian hedge mustard in Australia, is an important broadleaf weed of southern Australia. It has become more difficult to control in field crops due to the evolution of herbicide resistance. This study investigated the extent of resistance to four different herbicide modes of action, used to control oriental mustard in Australia. Herbicide resistance status was determined in 75 populations collected in southern Australia from 2010 to 2016 with resistance confirmed to herbicides inhibiting acetolactate synthase, photosystem II, phytoene desaturase (PDS) and auxinic herbicides. Populations resistant to PS-II, PDS-inhibitors and auxinic herbicides and two known susceptible populations (S1 and S2) were used to investigate the level of resistance, its mechanism, inheritance and fitness cost associated with resistance. Populations P17 and P18 were 311 and 315-fold, respectively, more resistant to atrazine than the susceptible populations as determined by the comparisons of their LD50 values. However, there was no resistance detected in these populations to diuron. Sequencing of the chloroplastic psbA gene identified a missense mutation of serine 264 to glycine in both herbicide-resistant populations, known to confer high-level of atrazine resistance in other species. P2 and P13 populations were 81 and 67-fold more resistant to 2,4-D at the LD50 level compared to the susceptible populations, respectively. No predicted amino acid modification was detected in sequences of potential target-site genes [Auxin binding protein (ABP), Transport inhibitor response 1 (TIR 1) and Auxin F-box protein 5 (AFB5)]. Further studies showed resistant populations had reduced 2,4-D translocation compared to the susceptible populations. At 72 h after herbicide treatment, 77% of [14C]2,4-D was retained in the treated leaf in the resistant population compared to 32% of [14C]2,4-D retention in the susceptible populations. Studies on inheritance of resistance to PDS-inhibitors confirmed that resistance to diflufenican in P3 population is inherited as a single dominant gene trait. Likewise, resistance to diflufenican and picolinafen in population P40 is also due to a single dominant gene. Resistance to 2,4-D in populations P2 and P13 is inherited as a single partially dominant gene. Populations P3 and P40 were 140 and 237-fold more resistant to the PDS inhibitor diflufenican, respectively, than the susceptible populations. Both populations contained a Leu498-Val substitution in the PDS gene. An additional mutation, Glu-425-Asp, was only detected in P40, where cross-resistance to picolinafen was identified. These results suggest that Leu498 mutation alone can confer a high level of resistance to diflufenican; however, the presence of both Leu498 and Glu425 mutations increased the level of resistance to diflufenican and also conferred resistance to picolinafen. Fitness studies conducted under competition with wheat in the absence of herbicides in pots revealed that the mutant PDS genes in populations P3 and P40 did not impose any fitness costs. This means once a resistant trait occurs in the field, it will persist in the absence of herbicides.