The selectivity and metabolism of sulfonamide herbicide safeners in crops

Abstract

Crop protection is increasingly challenged by demanding regulations, limited discovery of new herbicide/pesticide modes of action and increasing pest and weed resistance. Safeners, a diverse group of agrochemicals, have been developed to diversify the application of existing herbicides by selectively enhancing tolerance in large-grained cereal crops. While their exact mode of action remains to be determined, safener treatment results in the induction of xenobiotic detoxifying enzymes and associated transport proteins collectively termed the xenome. Key inducible xenome enzymes include glutathione S-transferases (GSTs) and cytochromes P450s (CYPs). This study’s aim was to investigate the molecular mechanism and selectivity of the recently developed sulfonamide safener cyprosulfamide [N-[4- (cyclopropylcarbamoyl)phenylsulfonyl]-2-methoxybenzamide] in maize (Zea Mays L.), wheat (Triticum Aestivum) and soybean (Glycine Max). The safening activity of cyprosulfamide (CSA) in protecting these crops from herbicide damage was assessed following exposure to herbicides thiencarbazone-methyl (TCM) and tembotrione (TBT) in greenhouse trials, where CSA protected maize, but not wheat and soybean from injury by both herbicides. This correlated with the relative degree of enhanced detoxification observed with herbicide TCM, where CSA enhanced its metabolism specifically in maize. To study the comparative activity with other sulfonamide safeners, the same plants were treated with metcamifen [2-methoxyN-{[4-(3-methylureido)phenyl]sulfonyl}benzamide]. In contrast to CSA, metcamifen was active in herbicide safening in maize and wheat, but not in soybean when tested in the greenhouse against the same herbicides, which was also associated with enhanced TCM metabolism in both maize and wheat with the safener. To examine the basis of the differential safening by CSA, its uptake, translocation and metabolism were studied in maize and wheat. The safener displayed increased mobility and translocation in wheat compared to maize but this did not correlate with activity. Metabolism studies showed more rapid metabolism in maize than in wheat, with the presence of a specific metabolite correlating with activity. Primary metabolites of CSA and metcamifen were identified and synthesised for activity testing in the greenhouse and in TCM metabolism studies. With the exception of one metcamifen metabolite, these primary biotransformation products were found to be inactive. Gene expression studies were designed in order to determine if CSA induced xenobiotic detoxifying enzymes prior to its metabolism. ZmGSTL1 a gene widely used as a marker for safening, was induced prior to the appearance of CSA metabolites in maize. To investigate the roles of CSA metabolism and safening in greater detail, Matrix Assisted Laser Desorption Ionisation (MALDI) Imaging Mass spectrometry was performed in maize, where CSA and TCM were applied on the same and on different leaves. The experiment showed that TCM metabolism was elevated when compounds were applied on the same leaf, indicating that the two compounds should be present in the same tissue for safening to be induced. The characterisation of the relationship between herbicide safeners and plant signalling mechanisms appears to be complex territory, but new insights provided by this study can help lead to the design of improved safeners, which will play an important role in the future of weed control.