Cinmethylin, a pre-emergence herbicide inhibiting fatty acid thioesterase activity, has recently been introduced to Australian cereal cropping for the control of Lolium rigidum Gaud. (annual ryegrass). To date, there have been no confirmed cases of cinmethylin resistance identified in this species, but some populations exhibit reduced sensitivity to this herbicide.
The research by AHRI PhD student Jinyi Chen (now Dr Jinyi Chen!) documents both target site and non-target site resistance to dinitroaniline herbicides in Lolium individuals. We are long accustomed to documenting multiple mechanisms of resistance existing within Lolium individuals/populations because of the high genetic diversity and obligate cross-pollination between Lolium which enables multiple resistance mechanisms to be a frequent occurrence. In this particular Lolium population, resistance to dinitroaniline herbicides is due to a mutation in the alpha tubulin gene (Val-202-Phe) and non-target site enhanced rates of dinitroaniline herbicide trifluralin metabolism. Unequivocal evidence for the target site Val-202-Phe mutation endowing resistance was obtained by expressing in transgenic rice, where it endowed resistance.
An overall finding of this study of auxinic herbicide resistance, at least in Raphanus R populations, is that conclusions on mechanisms cannot be made from studying just a few R populations. There are very clear differences between and within resistant populations. This research is ongoing in an attempt to reveal the important mechanisms that can endow resistance to 2,4-D and dicamba in plants.
In an Australian Research Council funded Linkage project with Nufarm as the industry partner, AHRI researcher Danica Goggin combined a transcriptomic and biochemical approach to investigate the diversity of 2,4-D resistance mechanisms in 11 resistant populations of wild radish. All of these wild radish populations had a relatively high level of resistance to 2,4-D and dicamba, although there were differences between populations in the level of resistance.
Arelatively low number of weed species have evolved resistance to auxinic herbicides despite their use for almost 70 years. This inheritance study with two Raphanus raphanistrum populations multiple-resistant 2,4-D and the ALS-inhibiting herbicide chlorsulfuron determined the number of genes and genetic dominance of 2,4-D resistance and investigated the association between traits conferring resistance to the two herbicide modes of action. Levels of 2,4-D phenotypic resistance and resistance segregation patterns were assessed in parental populations, F1 and F2 families.
When used at effective doses, weed resistance to auxinic herbicides has been slow to evolve when compared with other modes of action. Here we report the evolutionary response of a herbicide-susceptible population of wild radish (Raphanus raphanistrum L.) and confirm that sublethal doses of 2,4-dichlorophenoxyacetic acid (2,4-D) amine can lead to the rapid evolution of 2,4-D resistance and cross-resistance to acetolactate synthase (ALS)-inhibiting herbicides.