The compound 2,4-D (2,4-dichlorophenoxyacetic acid) is a synthetic auxin used as a herbicide. Resistance to the herbicide is increasing in a range of weed species and has evolved in populations of wild radish (Raphanus raphanistrum), a major weed in southern Australia and other regions with a Mediterranean-type climate. While studies have ruled out several common modes of resistance such as reduced uptake or metabolic detoxification, the mechanism at work in wild radish is still unknown. The action of auxin in plants is highly complex because large gene families are involved in its perception, signalling, transport, and metabolism, making unravelling evolved resistance a puzzle. However, previous research has suggested that the plasma membrane is likely a key site for 2,4-D resistance in wild radish.
Danica E. Goggin and colleagues recently published an article in Annals of Botany that aims to identify which components of the leaf plasma membrane proteome are contributing to 2,4-D resistance. They compared the plasma membrane proteome of a susceptible population with that of a resistant population under control and herbicide-treated conditions. Identifying eight proteins that differed in abundance between the two groups, they then looked at the amounts of these proteins in the plasma membrane and their possible functions.
The authors identified three proteins as being related to 2,4,-D resistance: two receptor-like kinases of unknown function, as well as an ATP-binding cassette transporter, ABCB19, which acts as an auxin efflux transporter, moving the compound throughout the plant. The results of this study suggest, the authors write, “that the resistant populations are less receptive to the presence of these herbicides at the plasma membrane, but this hypothesis needs to be tested.” This is the first time receptor-like kinases have been implicated in 2,4-D resistance. However, they are unlikely to represent the sole mechanism; the high level of genetic diversity within and between wild radish populations, as well as the finding that genes and proteins of interest in one population seem to be less important in others, is leading researchers to the conclusion that each population may rely on a slightly different resistance mechanism. “It is likely that auxinic herbicide resistance in wild radish is mediated by multiple processes that may be present in different proportions in different populations or even between individuals within a population,” the authors write.
Next steps for this line of research will include biochemical and physiological studies to determine the precise roles of the receptor-like kinases and auxin efflux transporter highlighted in this paper.