Abstract
Synthetic auxin herbicides are designed to mimic indole-3-acetic acid (IAA), an integral plant hormone affecting cell growth, development, and tropism. In this review, we explore target site genes in the auxin signaling pathway including SCFTIR1/AFB, Aux/IAA, and ARFs that are confirmed or proposed mechanisms for weed resistance to synthetic auxin herbicides. Resistance to auxin herbicides by metabolism, either by enhanced cytochrome P450 detoxification or by loss of pro-herbicide activation, is a major non-target-site resistance pathway. We speculate about potential fitness costs of resistance due to effects of resistance-conferring mutations, provide insight into the role of polyploidy in synthetic auxin resistance evolution, and address the genetic resources available for weeds. This knowledge will be the key to unlock the long-standing questions as to which components of the auxin signaling pathway are most likely to have a role in resistance evolution. We propose that an ambitious research effort into synthetic auxin herbicide/target site interactions is needed to 1) explain why some synthetic auxin chemical families have activity on certain dicot plant families but not others and 2) fully elucidate target-site cross-resistance patterns among synthetic auxin chemical families to guide best practices for resistance management.
Highlights
Introduction to synthetic auxin herbicidesSynthetic auxin herbicides (Weed Science Society of America/ Herbicide Resistance Action Committee Group 4/O) are a class of herbicides that mimic the activity of the plant hormone auxin
We propose that an ambitious research effort into synthetic auxin herbicide/target site interactions is needed to 1) explain why some synthetic auxin chemical families have activity on certain dicot plant families but not others and 2) fully elucidate target-site cross-resistance patterns among synthetic auxin chemical families to guide best practices for herbicide rotation and mixture in resistance management
We propose that the factorial combinations of target-site auxin receptor/coreceptor complexes in key weeds need further characterization with regards to binding of synthetic auxin herbicide from different chemical groups, across the large gene family of Aux/indole-3-acetic acid (IAA) co-receptors
Summary
Synthetic auxin herbicides (Weed Science Society of America/ Herbicide Resistance Action Committee Group 4/O) are a class of herbicides that mimic the activity of the plant hormone auxin (indole-3acetic acid, IAA). Research on efficacy and weed management with synthetic auxin herbicides have been at consistent levels at WSSA over the last 10 years, exemplifying the longterm interest in studying this growing weed science issue. Despite the importance of this mode of action for weed management, only one molecular resistance mechanism in a weed species has been functionally validated [11]. Due to this lack of information, our scope of understanding of the resistance mechanisms in weedy species for synthetic auxins is relatively poor. We discuss the canonical and non-canonical auxin signaling pathways in model plants and consider potential candidate genes which, if mutated, could be the keys to conferring resistance to synthetic auxins in weeds. We identify research needed to understand the differential efficacy of synthetic auxins on different plant families and speculate as to the basis of cross-resistance patterns to chemically dissimilar families of these herbicides
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