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An Echinochloa colona population from Western Australia has evolved resistance to glyphosate. This current study investigates the physiological, molecular and biochemical basis of glyphosate resistance in this population. To minimise genetic differences the susceptible (S) and resistance (R) phenotypes were isolated from within this resistant population. The S phenotype was found to accumulate significantly more shikimate in the leaf tissue than the R phenotype following glyphosate treatment. Target-site EPSPS gene sequencing revealed no resistance mutations and EPSPS gene expression was similar between the S and R phenotypes. Thus, glyphosate resistance in this population is unlikely target-site based. Similarly, there were no significant differences between the S and R phenotypes in glyphosate leaf uptake and translocation at the whole plant level. It is also unlikely that the resistance is associated with glyphosate metabolism as no major glyphosate metabolites were detected in leaf tissue of the S and R phenotypes. Despite much effort the exact glyphosate resistance mechanisms in this R population remain unclear, and novel resistance mechanisms are to be determined. [ABSTRACT FROM AUTHOR]

A relatively low number of weed species have evolved resistance to auxinic herbicides despite their use for almost 70 years. This inheritance study with two Raphanus raphanistrum populations multiple-resistant 2,4-D and the ALS-inhibiting herbicide chlorsulfuron determined the number of genes and genetic dominance of 2,4-D resistance and investigated the association between traits conferring resistance to the two herbicide modes of action. Levels of 2,4-D phenotypic resistance and resistance segregation patterns were assessed in parental populations, F 1 and F 2 families. [ABSTRACT FROM AUTHOR]

Highlights: [•] Semi-dominant alleles segregate at one major locus for pyroxasulfone resistance. [•] Cross-resistance is also conferred by major locus(i). [•] A combination of herbicide options can reduce pyroxasulfone selection pressure. [•] This study will enable more accurate predictions by computational modelling. [•] Inheritance studies of novel herbicides can help sustain their longevity. [ABSTRACT FROM AUTHOR]

Abstract: The potential for low rates of diclofop-methyl to result in rapid evolution of herbicide resistance in a herbicide-susceptible Lolium rigidum (annual ryegrass) population was demonstrated in a recent crop-field study. In this present study, the data from the crop-field study was used together with simulation modelling to identify possible genetics of the herbicide resistance that was selected for. This analysis clearly indicated that the herbicide resistance was polygenic. Subsequently, the estimated genetic possibilities were used to parameterise a model of herbicide resistance evolution in a simulated crop-field situation, and the potential of different rates of diclofop-methyl (ACCase herbicide) to cause herbicide-resistance evolution in L. rigidum was explored and compared using the calibrated model. The calibrated model outputs indicated that the evolution of diclofop-methyl resistance would generally be faster at low herbicide rates than at higher rates due to the rapid selection of minor gene herbicide resistance traits at low rates and their subsequent recombination by cross-pollination. The results of the study therefore indicate potential risks in herbicide rate cutting and highlight the need for careful scientific evaluation of any herbicide use rate for its potential to select for minor gene herbicide resistance from a weed population. [Copyright &y& Elsevier]

• Herbicide resistance gene flow in weeds is under-estimated and under-appreciated. • The consequences are increasing occurrence of multiple-resistant weed populations. and dwindling efficacy of remaining herbicide tools. • The best mitigation strategy is weed seed bank management. Interest in the magnitude and consequences of intra- and inter-specific herbicide resistance gene flow, particularly that mediated by pollen, increased in the mid-1990s with the introduction of herbicide-resistant (HR) transgenic crops. During that time, less attention was paid to the movement of HR alleles via pollen or seed among weed populations. Incidence of HR weeds in a region is often attributed to independent evolution through herbicide selection; the role or contribution of HR allele movement via pollen, seeds and/or vegetative propagules is often under-estimated and under-appreciated. Once a new HR weed biotype has been confirmed in a jurisdiction, how often have we been surprised at its rapid areawide expansion? In genotypic studies of HR weed populations, the contribution of gene flow to incidence of resistance is frequently similar or greater than that of independent evolution. Simulation models have consistently predicted that frequent widespread applications of highly effective herbicides (e.g., acetolactate synthase inhibitors, glyphosate) provide connective high-fitness habitats across the landscape, which facilitate a rapid increase in the frequency and movement of an HR trait within and among populations. Such habitats, characterized by minimal heterogeneity but high selection intensity due to frequent herbicide 'on' vs. 'off' exposure periods, strongly favour the fitness of HR propagules at great disadvantage of herbicide-susceptible individuals. The unanticipated speed of areawide expansion of some HR weed biotypes has spurred numerous calls over the past decade for a collective community or regional response to mitigate this unhindered spread of HR alleles. The best mitigation strategy is minimizing weed population abundance and seed bank replenishment in fields and adjacent ruderal areas. This goal is difficult, but necessary for preserving the remaining public good and common pool resource of herbicide susceptibility. [ABSTRACT FROM AUTHOR]