2,4-D and dicamba resistance mechanisms in wild radish: subtle, complex and population specific?

Weed populations with resistance to synthetic auxin herbicides such as 2,4-D and dicamba are occurring worldwide. In Australia, 2,4-D resistance is known in several species but is most widespread in the economically damaging dicot weed wild radish (Raphanus raphanistrum). Despite good efforts by good researchers, relatively little is known about the mechanisms conferring auxinic herbicide resistance. This is due to the great complexity and cascades of effects of auxins on 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.

Comparison of gene expression in a susceptible and one resistant population in response to 2,4-D showed a large and rapid increase in expression of genes involved in plant defence and immunity (receptor-like kinases, MAP kinase kinase kinase 1) and in dampening of the auxin signal (IAA30). However, generalisations cannot be made because this response was not observed in the other 10 resistant populations.

In some resistant populations, there was very little translocation of radiolabelled 2,4-D out of the treated leaf, indicating reduced 2,4-D translocation as a potential resistance mechanism. However, again, generalisations cannot be made as in other resistant populations there were no differences in rates of translocation between resistant and susceptible plants. Across the 11 resistant populations, there was no correlation between resistance level and the extent of translocation of radiolabelled 2,4-D out of the treated leaf. This, plus differences in auxin selectivity between the populations, highlights the complexity of 2,4-D resistance in wild radish. Early deployment of plant defence responses may be a common factor, as illustrated by a correlation between constitutive phosphorylation (i.e. activation) of MAP kinase proteins and resistance to 2,4-D and dicamba.

This study highlights the dangers of extrapolating knowledge obtained with one resistant population to generalise about other populations/species. Understanding the mechanistic basis of auxinic herbicide resistance in plants is indeed a very difficult research problem. At least in the species wild radish, 2,4-D resistance appears to result from subtly different auxin signalling alterations in different resistant populations, supplemented by an enhanced defence response and, in some cases, reduced 2,4-D translocation.