Evolved resistance to the herbicide glyphosate has been reported in eleven weed species, including Lolium multiflorum. Two glyphosate-resistant L. multiflorum populations were collected, one from Chile (SF) and one from Oregon, USA (OR), and the mechanisms conferring glyphosate resistance were studied. Based on a Petri dish dose-response bioassay, the OR and the SF populations were two and fivefold more resistant to glyphosate when compared to the susceptible (S) population, respectively; however, based on a whole-plant dose-response bioassay, both OR and SF populations were fivefold more resistant to glyphosate than the S population, implying that different resistance mechanisms might be involved. The S population accumulated two and three times more shikimic acid in leaf tissue 96 h after glyphosate application than the resistant OR and SF populations, respectively. There were no differences between the S and the glyphosate-resistant OR and SF populations in 14C-glyphosate leaf uptake; however, the patterns of 14C-glyphosate translocation were significantly different. In the OR population, a greater percentage of 14C-glyphosate absorbed by the plant moved distal to the treated section and accumulated in the tip of the treated leaf. In contrast, in the S and in the SF populations, a greater percentage of 14C-glyphosate moved to non-treated leaves and the stem. cDNA sequence analysis of the EPSP synthase gene indicated that the glyphosate-resistant SF population has a proline 106 to serine amino acid substitution. Here, we report that glyphosate resistance in L. multiflorum is conferred by two different mechanisms, limited translocation (nontarget site-based) and mutation of the EPSP synthase gene (target site-based).

Development of herbicide-resistant crops has resulted in significant changes to agronomic practices, one of which is the adoption of effective, simple, low-risk, crop-production systems with less dependency on tillage and lower energy requirements. Overall, the changes have had a positive environmental effect by reducing soil erosion, the fuel use for tillage, and the number of herbicides with groundwater advisories as well as a slight reduction in the overall environmental impact quotient of herbicide use. However, herbicides exert a high selection pressure on weed populations, and density and diversity of weed communities change over time in response to herbicides and other control practices imposed on them. Repeated and intensive use of herbicides with the same mechanisms of action (MOA; the mechanism in the plant that the herbicide detrimentally affects so that the plant succumbs to the herbicide; e.g., inhibition of an enzyme that is vital to plant growth or the inability of a plant to metabolize the herbicide before it has done damage) can rapidly select for shifts to tolerant, difficult-to-control weeds and the evolution of herbicide-resistant weeds, especially in the absence of the concurrent use of herbicides with different mechanisms of action or the use of mechanical or cultural practices or both.

A naturally occurring prickly lettuce biotype resistant to a 5:1 formulated mixture of chlorsulfuron:metsulfuron (DPX-G8311) was identified in a no-till winter wheat field near Lewiston, ID, in April, 1987. Field and greenhouse studies were established to evaluate its resistance to other sulfonylureas, imidazolinones, and herbicides with alternate sites of action. The resistant biotype resisted eight sulfonylurea herbicides; resisted the imidazolinone herbicides, imazapyr and imazethypyr, but not imazaquin; and resisted no other herbicides included in the studies. The resistant biotype was identified in seven of nine fields on the farm where it was discovered.

Jointed goatgrass is a major weed in the wheat-producing areas of the western U.S. It shares the D genome with wheat, and interspecific hybrids between the two species occur in the field. The objective of this research was to determine if wheat X jointed goatgrass hybrids could serve to transfer genes from wheat to jointed goatgrass. A backcrossing program was initiated in the greenhouse between wheat X jointed goatgrass hybrids and either jointed goatgrass or wheat to determine the potential for seed set and the restoration of self-fertility. Seed was set by backcrossing with either species as the recurrent parent. Female fertility increased from 2% in the hybrid to 37% in the BC2 plants with jointed goatgrass as the recurrent parent. Partial self-fertility was restored in the second backcross (BC2) generation using jointed goatgrass as the recurrent parent. This indicates that genes could be transferred between wheat and jointed goatgrass after only two backcrosses. The number of bivalents observed in the plants during meiosis appeared to be key to increasing female fertility and self-fertility. Based on the results of this study, it is possible for genes to move from wheat to jointed goatgrass. Any release of a herbicide-resistant wheat should be accompanied by a management plan that would minimize the potential for gene movement between these species.

Concerns about genetically modified (GM) crops include transgene flow to compatible wild species and unintended ecological consequences of potential transgene introgression. However, there has been little empirical documentation of establishment and distribution of transgenic plants in wild populations. We present herein the first evidence for escape of transgenes into wild plant populations within the USA; glyphosate-resistant creeping bentgrass (Agrostis stolonifera L.) plants expressing CP4 EPSPS transgenes were found outside of cultivation area in central Oregon. Resident populations of three compatible Agrostis species were sampled in nonagronomic habitats outside the Oregon Department of Agriculture control area designated for test production of glyphosate-resistant creeping bentgrass. CP4 EPSPS protein and the corresponding transgene were found in nine A. stolonifera plants screened from 20,400 samples (0.04 +/- 0.01% SE). CP4 EPSPS-positive plants were located predominantly in mesic habitats downwind and up to 3.8 km beyond the control area perimeter; two plants were found within the USDA Crooked River National Grassland. Spatial distribution and parentage of transgenic plants (as confirmed by analyses of nuclear ITS and chloroplast matK gene trees) suggest that establishment resulted from both pollen-mediated intraspecific hybridizations and from crop seed dispersal. These results demonstrate that transgene flow from short-term production can result in establishment of transgenic plants at multi-kilometre distances from GM source fields or plants. Selective pressure from direct application or drift of glyphosate herbicide could enhance introgression of CP4 EPSPS transgenes and additional establishment. Obligatory outcrossing and vegetative spread could further contribute to persistence of CP4 EPSPS transgenes in wild Agrostis populations, both in the presence or absence of herbicide selection.