20.2.3  Cross-resistance

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Cross-resistance is the phenomenon of a weed population developing simultaneous resistance to more than one chemical class of herbicide. The simplest case of target site cross-resistance has been considered for ALS-inhibiting herbicides in Section 20.2.1(b). This occurs when herbicides of dissimilar chemistry bind to identical or overlapping domains of the same target site. Cross-resistance occurs when mutations within the target enzyme endow resistance to herbicides from various chemical classes that inhibit that target site. For example, plants with different mutations conferring resistance to ALS-inhibiting herbicides have varying patterns of target site cross-resistance to three chemically dissimilar classes of ALS-inhibiting herbicides. Each herbicide has a subtly different binding domain within ALS. Similar target site cross-resistance occurs for ACCase herbicides. Clearly, cross-resistance is likely when contrasting herbicides target a single site. However, the patterns of cross-resistance resulting from mutations within ALS and ACCase are not predictable as they depend upon the particular mutation that has occurred.

Non-target site cross-resistance occurs when a population becomes simultaneously resistant to the chemistry of more than one herbicide due to a mechanism other than a resistant target site. Populations of L. rigidum often exhibit non-target site cross-resistance through enhanced herbicide metabolism, mediated by microsomal oxidases. For example, an L. rigidum population that is resistant following selection with a triazine herbicide is also resistant to the chemically dissimilar substituted-urea herbicides. Equally, resistance to substituted-urea herbicides confers triazine resistance. Both populations metabolise both triazine and substituted-urea herbicides at enhanced rates. Similarly, plants resistant to an ACCase herbicide also have non-target site resistance to ACCase and ALS herbicides. In this case the enhanced metabolism detoxifies three distinct herbicides with two different target sites. Inhibitor experiments suggest that such complex metabolism is achieved by the action of several cytochrome P450-dependent microsomal oxidases rather than a single multifunctional enzyme.

Cross-resistance in L. rigidum is a serious practical problem in the field because plants can potentially develop resistance to any herbicide molecule that the cytochrome P450-dependent microsomal oxidases can degrade. Plants might even develop resistance to undiscovered herbicides!