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

The herbicide 2,4-D has several and complex impacts on plants (Grossmann 2010). Revealing the mechanisms of evolved resistance to 2,4-D, dicamba and other auxinic herbicides is complex and challenging research.

In Australia, many populations of the important dicot weed of crops Raphanus raphanistrum (wild radish) have evolved 2,4-D resistance. Individual resistant populations differ in the level of 2,4-D resistance and extent of cross-resistance to other auxinic herbicides. Additionally, there is great genetic variability between and within Raphanus populations.

AHRI research scientist Dr Danica Goggin has carefully purified more than 10 resistant Raphanus populations. While all these populations are resistant, they differ greatly in their resistance level. The resistance level of the individual R populations ranged from seven to 60-fold for 2,4-D and from two to 32 fold for dicamba.

In an earlier study, Danica had shown that in some resistant populations there is much less leaf 2,4-D translocation (Goggin et al Journal of Experimental Botany 2016). In this study, she again found populations and individuals within populations, with much less leaf 2,4-D translocation, but others did not show this reduced 2,4-D leaf translocation. Danica examined 2,4-D metabolism in all of these resistant populations and did not find evidence of differential metabolism endowing resistance. She then studied differential gene expression in S –v- R populations and the results convinced her to focus on two genes (an auxin IAA repressor and a MAP kinase kinase signalling protein). This work is ongoing.

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.