Several grass and broadleaf weed species around the world have evolved multiple-herbicide resistance at alarmingly increasing rates. Research on the biochemical and molecular resistance mechanisms of multiple-resistant weed populations indicate a prevalence of herbicide metabolism catalyzed by enzyme systems such as cytochrome P450 monooxygenases and glutathioneS-transferases and, to a lesser extent, by glucosyl transferases. A symposium was conducted to gain an understanding of the current state of research on metabolic resistance mechanisms in weed species that pose major management problems around the world. These topics, as well as future directions of investigations that were identified in the symposium, are summarized herein. In addition, the latest information on selected topics such as the role of safeners in inducing crop tolerance to herbicides, selectivity to clomazone, glyphosate metabolism in crops and weeds, and bioactivation of natural molecules is reviewed.

This article reviews, focusing on maize and soybean, previous efforts to develop nontransgenic herbicide-resistant crops (HRCs), currently available transgenic HRC traits and technologies, as well as future chemical weed management options over the horizon. Since the mid twentieth century, herbicides rapidly replaced all other means of weed management. Overreliance on ‘herbicide-only’ weed control strategies hastened evolution of HR weed species. Glyphosate-resistant (GR) crop technology revolutionized weed management in agronomic crops, but GR weeds, led by Palmer amaranth, severely reduced returns from various cropping systems and affected the bottom line of growers across the world. An additional problem was the lack of commercialization of a new herbicide mode of action since the 1990s. Auxinic HRCs offer a short-term alternative for management of GR Palmer amaranth and other weed species. New HRCs stacked with multiple herbicide resistance traits and at least two new herbicide modes of action expected to be available in the mid-2020s provide new chemical options for weed management in row crops in the next decade.

Multiple resistance to glyphosate, sethoxydim, and paraquat was previously confirmed in two Italian ryegrass [Lolium perenneL. ssp.multiflorum(Lam.) Husnot] populations,MR1andMR2, in northern California. Preliminary greenhouse studies revealed that both populations were also resistant to imazamox and mesosulfuron, both of which are acetolactate synthase (ALS)-inhibiting herbicides. In this study, three subpopulations,MR1-A(from seed ofMR1plants that survived a 16X rate of sethoxydim),MR1-P(from seed ofMR1plants that survived a 2X rate of paraquat), andMR2(from seed ofMR2plants that survived a 16X rate of sethoxydim), were investigated to determine the resistance level to imazamox and mesosulfuron, evaluate other herbicide options for the control of these multiple resistantL. perennessp.multiflorum, and characterize the underlying ALS-inhibitor resistance mechanism(s). Based on LD₅₀values, theMR1-A,MR1-P, andMR2subpopulations were 38-, 29-, 8-fold and 36-, 64-, and 3-fold less sensitive to imazamox and mesosulfuron, respectively, relative to the susceptible (Sus) population. OnlyMR1-P and MR2plants were cross-resistant to rimsulfuron, whereas bothMR1subpopulations were cross-resistant to imazethapyr. Pinoxaden (ACCase inhibitor [phenylpyrazoline 'DEN']) only controlledMR2andSusplants at the labeled field rate. However, all plants were effectively controlled (>99%) with the labeled field rate of glufosinate. Based on I₅₀values,MR1-A,MR-P, andMR2plants were 712-, 1,104-, and 3-fold and 10-, 18-, and 5-fold less responsive to mesosulfuron and imazamox, respectively, than theSusplants. Sequence alignment of theALSgene of resistant plants revealed a missense single-nucleotide polymorphism resulting in a Trp-574-Leu substitution inMR1-AandMR1-Pplants, heterozygous in both, but not in theMR2plants. An additional homozygous substitution, Asp-376-Glu, was identified in theMR1-Aplants. Addition of malathion or piperonyl butoxide did not alter the efficacy of mesosulfuron onMR2plants. In addition, the presence of 2,4-D had no effect on the response of mesosulfuron on theMR2andSus. These results suggest an altered target site is the mechanism of resistance to ALS inhibitors inMR1-AandMR1-Pplants, whereas a non–target site based resistance apparatus is present in theMR2plants.

Extrachromosomal circular DNAs (eccDNAs) are found in many eukaryotic organisms. EccDNA-powered copy number variation plays diverse roles, from oncogenesis in humans to herbicide resistance in crop weeds. Here, we report interspecific eccDNA flow and its dynamic behavior in soma cells of natural populations and F₁ hybrids of Amaranthus sp. The glyphosate-resistance (GR) trait is controlled by eccDNA-based amplification harboring the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene (eccDNA replicon), the molecular target of glyphosate. We documented pollen-mediated transfer of eccDNA in experimental hybrids between glyphosate-susceptible Amaranthus tuberculatus and GR Amaranthus palmeri. Experimental hybridization and fluorescence in situ hybridization (FISH) analysis revealed that the eccDNA replicon in Amaranthus spinosus derived from GR A. palmeri by natural hybridization. FISH analysis also revealed random chromosome anchoring and massive eccDNA replicon copy number variation in soma cells of weedy hybrids. The results suggest that eccDNAs are inheritable across compatible species, contributing to genome plasticity and rapid adaptive evolution.