Herbicide Resistance in Weed Management

Herbicides are the most efficient and cost-effective means of weed management [...]

Herbicides are the most efficient and cost-effective means of weed management. Over the years, intensive use of herbicides, their misapplication, and inappropriate use, as well as stricter herbicide registration requirements and environmental regulations, resulted in a drastic decline in available active ingredients, which has led to strong selection pressure and evolution of herbicide-resistant weeds (http://www.weedscience.com) throughout the world, especially in intensive agro-systems (e.g., monoculture, and herbicide-tolerant crops). Herbicide resistance is driven by either target site (TS) and non-target site (NTS) mechanisms. TS resistance involves alteration of herbicide efficacy via structural modifications of the binding site or overexpression of its target gene [1]. NTS resistance is associated with various anatomical, cellular and physiological processes, such as reduced absorption and translocation, modified subcellular distribution, and detoxification of the herbicide. The evolution of resistance to a specific herbicide comes with a fitness penalty (i.e., fitness cost), characterized by a reduction in the ability of an organism to survive and/or reproduce when the selective pressure is removed [2]. Yet, at the same time, the fitness of herbicide-resistant populations may also result in an advantage related to altered activity of in planta processes under different environmental conditions. The fitness penalty associated with TS mutations is mainly related to reduced substrate affinity due to structural modification, while in the NTS resistance mechanism, the fitness may originate at the expense of other metabolic functions.
Managing resistant weeds is one of the major challenges in ensuring future global food security and sustainability. The papers that comprise this Special Issue on "Herbicide Resistance in Weed Management", address various aspects of this pressing challenge, including herbicide resistance reports, and elucidation of resistance mechanisms and the fitness of resistant weed species under different environmental conditions (G × E interaction). Moreover, they offer alternative management practices to minimize the impact of resistant populations.
Identification and characterization of new cases of herbicide-resistant weed populations play a key role in understanding the distribution and severity of this problem, and in mitigating failed weed control efforts. Herbicide resistance studies should include elucidation of related resistance mechanisms to broaden our understanding of its evolution. Vijayarajan et al. [3] reported on the first case of Avena fatua resistance to ACCase inhibitors in Ireland. Variability in the levels of resistance and cross-resistance to various ACCase inhibitors (e.g., pinoxaden, propaquizafop and cycloxydim) was identified and characterized. Matzrafi et al. [4] tested the resistance of two Mediterranean weeds, Diplotaxis erucoides and Erucaria hispanica, to various acetolactate synthase (ALS) inhibitors. Herbicide-resistant populations demonstrated resistance to all tested ALS inhibitors associated with various TS mutations in the ALS gene sequence. Auxinic herbicides, 2,4-D, and mecoprop-P, were suggested as a potential alternative for the management of these resistant populations. Pandian et al. [5] reported on the first case of TS resistance to ALS inhibitors in a Poly-gonum convolvulus population from KS, USA. McKenzie-Gopsill et al. [6] described TS resistance of Chenopodium album to metribuzin, a photosystem II-inhibiting herbicide. The authors also demonstrated alternative herbicide application for weed control in the Atlantic Canadian potato agro-system to overcome the development of herbicide resistance. Mendez et al. [7] investigated the response of Euphorbia heterophylla to imazamox in Brazil. High levels of resistance (>123-fold) were found, and were associated with a Trp574Leu mutation in the ALS gene sequence. Travlos et al. [8] evaluated the efficacy of penoxsulam, profoxydim, cycloxydim, cyhalofop-butyl, florpyrauxifen-benzyl, and glyphosate in controlling Echinochloa colona, and reported on the first glyphosate-resistant populations in Greece. Overall, these studies provide fundamental information on the evolutionary development of herbicide-resistant weeds and highlight the selection process involved in the development of the resistance.
Several papers in this Special Issue present alternative weed management tools, which, in most cases, involve substitute herbicides and alternative methods of application. After testing potential weed management strategies to control glyphosate-resistant Lolium perenne ssp. multiflorum in bread wheat (Triticum aestivum), Bararpour et al. [9] proposed that early post-emergence application of flufenacet/metribuzin, followed by late post-emergence application of pinoxaden may be used as an alternative treatment. Furthermore, the authors highlighted the importance of seedbank suppression as a long-term strategy to prevent the buildup of an herbicide-resistant field population. Matzrafi et al. [10] assessed the potential of sub-lethal glufosinate rates to select for reduced susceptibility of L. perenne ssp. multiflorum, and cross-resistance to herbicides with different modes of actions. After three generations of selection, the progeny survival rates were higher than those of the parental population, suggesting that reduced susceptibility to glufosinate can evolve in weed populations following repeated applications of low herbicide rates. Kelly et al. [11] examined the effect of integrated weed management using post-emergence herbicide application and strategic defoliation by mowing on the control of Hordeum murinum subsp. glaucum in Australia. Propaquizafop was found to be effective for the control of this weed, as shown by a 98% decrease in H. murinum subsp. glaucum survival and seed production. Integrated with herbicide application, a single repeated mowing treatment resulted in a 95% decline in H. murinum subsp. glaucum seedling emergence. Both means, herbicide use and mowing, were affected by application time and environmental conditions. Loureiro et al. [12] studied the potential addition of herbicide-tolerant maize to an integrated weed management program in Spain. Reduced herbicide application and changes in weed community composition were observed using glyphosate-tolerant maize. Herbicide resistance may also correlate with changes in other life-history traits, such as germination, flowering initiation time, and response to environmental cues. In their evaluation of the possible interaction between herbicide resistance and salinity on germination of Italian weedy rice and cultivated rice (Oryza sativa), Fogliatto et al. [13] found that the resistant line was more affected by salinity and showed reduced seed germination at high salt concentrations.
Several aspects should be further explored in the context of herbicide resistance and reduced herbicide sensitivity. Studies relating to more basic scientific questions such as regulatory elements, epigenetic alterations, and transcription factors, major players in the evolution of herbicide-resistance, are needed to enhance our understanding of resistance evolution [14]. Furthermore, there is mounting evidence suggesting that extreme weather events play a crucial role in the determination of herbicide efficacy [15]. In light of the predicted global climate change, studies exploring these interactions are of high importance.
While this focus has been of great interest as of late, further research on the development of integrated weed management approaches to address herbicide-resistant weeds is required [16][17][18]. We are pleased to report that two papers on this topic are presented in the current Special Issue [11,12]. However, more efforts should be pointed toward this urgent issue in the near future.
Emerging technologies offer improved opportunities to modify or develop technology and machinery for use in weed seed destruction, for example [19], and offer cost-effective solutions that were previously not feasible, such as weed management through the use of lasers or by electrocution. As new and improved non-chemical control options have received greater attention in recent research across the globe, integrated weed management systems that utilize these tools must be considered, especially for cropping systems with a high probability of herbicide resistance evolution, as in roadside, broadacre, and horticultural cropping systems, or those employing herbicide-tolerant crops.
Author Contributions: All authors equally contributed to the conceptualization, original draft preparation, and review and editing of this manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This study was supported by the Chief Scientist of the Israeli Ministry of Agriculture and Rural Development.

Conflicts of Interest:
The authors declare no conflict of interest.