Herbicide Resistance

Regulation and functional validation of GSTs involved in detoxification of herbicides mainly applied in pre-emergence

Herbicides are important, reliable, and cost-effective tools to control weeds (Owen, 2016) in combination with non-chemical tools in an Integrated Weed Management approach (Byrne et al., 2018). Because of their convenience to use, farmers have relied, since at least 3 decades, mainly on leaf acting (post-emergent) herbicides to control grass weeds such as Alopecurus myosuroides and Lolium spp. in cereal based cropping systems, especially in Europe (Powles & Yu, 2010). The lack of diversity in weed control has resulted in the evolution of resistance to these active ingredients, in particular ALS- and ACCase inhibitors (Heap, 2018). The application of pre-emergent (soil applied), early post-emergent and post-emergent herbicides with different MoAs in the right combinations is one solution to increase diversity from the chemical perspective, and mitigate the evolution of resistance. Weed resistance mechanisms are multiple (Powles and Yu, 2010). Among these, metabolic resistance to herbicides has been a growing problem for successful weed management and also poses a threat to herbicides that will be discovered in the future due to the potential for cross-resistance to a broad spectrum of chemical classes. Among other genes, the up-regulation of cytochrome P450 monooxygenases, glycosyltransferases and glutathione-S-transferases (GST) has mainly been implicated so far. Previous research, in particular conducted at the Weed Resistance Competence Center, investigated the molecular mechanism of metabolic resistance to flufenacet and several other herbicides applied in blackgrass and ryegrass in pre-emergence (Busi, 2014; Dücker, 2014; Parcharidou, 2019). In several studies AmGSTF1, a glutathione-S-transferase was described as a marker for metabolism-based resistance in black grass (Cummins et al., 2013; Tétard-Jones et al., 2018). Although, the most recent studies showed that resistance could be based on post-transcriptional mechanism of the protein (Cummins et al., 2013; Wortley 2013).  Through the identification of metabolites, the detoxification pathway was confirmed to happen via GST conjugation and it was shown that resistant individuals had the ability to detoxify flufenacet at a faster rate than susceptible individuals (Dücker et al., 2019; Parcharidou, 2019). RNA-Sequencing experiment with ryegrass and blackgrass populations identified seventeen GSTs to be overexpressed among other genes. The results of expression in E.coli, of four recombinant GSTs out of the eleven genes overexpressed in ryegrass showed that one tau class GST was able to detoxify flufenacet fast while two phi class GSTs detoxified it at a slower rate. The tau class GST was also able to detoxify S-metolachlor at a slow rate but not pyroxasulfone, both inhibors of Very Long Chain Fatty Acid synthesis like flufenacet (Dücker, 2019). None of the GSTs were able to detoxify diflufenican or mesosulfuron-methyl. Thus, it was concluded that at least one main GST in addition with two other confers resistance to flufenacet in these populations. Despite several single nucleotide polymorphisms (SNPs) found in the GSTs, no SNPs were found to co-segregate with resistance over all populations analyzed. It was confirmed that the enhanced detoxification of the herbicides was correlated with enhanced GST enzyme activity (Dücker 2019). This suggests that overexpression of GST genes(s) is responsible for enhanced detoxification of the herbicides. Continue the work aiming to functionally characterize the different overexpressed GST genes and a better understanding of the molecular mechanisms of herbicide resistance regulation and evolution are the next steps in 1) the discovery of possible markers for herbicide resistance diagnostics, 2) to better define new strategies to overcome resistance, and 3) to allow improvement of herbicide use recommendations to customers to mitigate the evolution of resistance.

Funding Institution: Bayer AG, Department of Crop Sciences, WRCC (Weed Resistance Competence Center)

Investigator: M.Sc. Evlampia Parcharidou

Supervisors: Prof. Andreas von Tiedemann, Dr. Roland Beffa, Prof. Petr Karlovsky