Evidence, Mechanism and Alternative Chemical Seedbank-Level Control of Glyphosate Resistance of a Rigid Ryegrass (Lolium rigidum) Biotype from Southern Spain

Abstract

Rigid ryegrass (Lolium rigidum) is one of the most troublesome weeds in different crops in the Mediterranean region. A rigid ryegrass biotype from an olive grove in Jaén province (Andalusía, southern Spain), potentially resistant to glyphosate (RG), was tested for its resistance level through dose-response assays using a susceptible biotype (SG). To test the hypothesis of a non-target-site-based resistance, as point mutations are far less common mechanisms of glyphosate resistance, studies were also conducted to elucidate whether resistance was associated with biochemical, metabolism, molecular and/or physiological mechanisms. Alternative herbicide-based control options, including single-herbicide or herbicide mixtures with glyphosate, applied at seedling, tillering or full heading stages, were tested in field experiments for 2 years for their efficacy against rigid ryegrass plants and their effects on the soil seed bank. Resistance levels of the RG biotype were 23- (LD50) and 7-fold (GR50) higher compared to the SG biotype. The SG biotype exhibited a significantly greater shikimic acid accumulation than the RG one. At 96 HAT, 58 and 89% of applied 14C-glyphosate was up taken by leaves of RG and SG biotype plants, respectively, and, at this time, a significantly higher proportion of the glyphosate taken up by the treated leaf remained in its tissue in RG plants compared to the SG ones. The RG biotype did not reveal any point mutation in the glyphosate target site EPSP synthase. Overall, results confirmed reduced glyphosate uptake and translocation as being the mechanism involved in glyphosate resistance in the RG biotype. RG biotype responses to the alternative treatments tested in situ indicated that herbicide applications at the later growth stage tended to be less effective in terms of immediate effects on population size than earlier applications, and that only in some cases, the removal of at least 85% of the RG biotype was achieved. However, with few exceptions, the alternative treatments tested appeared to be highly effective in reducing the seed bank irrespective of the growth stage. The frequency of the resistant phenotype in the progeny of surviving plants of the RG biotype was dependent on treatment. Results suggest that a potential exists for effective management of glyphosate-resistant rigid ryegrass in olive groves in southern Spain.

Multiple target site resistance to glyphosate in junglerice (Echinochloa colona) lines from California orchards

“…Absorption and movement of glyphosate within the plant was similar among resistant and susceptible plants in this study and, thus, was not considered a primary mechanism of resistance in these E. colona lines. Altered translocation of glyphosate has not been observed as a mechanism of resistance in Echinochloa species to date, despite being identified in other weed genera including Conyza , Lolium and Chloris …”

Mechanisms of glyphosate resistance and response to alternative herbicide‐based management in populations of the three Conyza species introduced in southern Spain

“…Experiments were carried out following Fernandez‐Moreno et al . A solution of 14 C‐glyphosate (American Radiolabeled Chemicals, Inc., Saint Louis, MO, USA) was prepared by adding radiolabeled glyphosate to commercially formulated glyphosate, with the final specific activity of 0.834 kBq µL −1 .…”

“…Percentage seed germination was recorded after 14 days under conditions described in the plant material section. Relative effectiveness in reducing seed production was estimated as the product of percentage cover at 60 DAT, an estimate of the relative production of capitula, by percentage germination, relative to untreated plots, and finally expressed as the complementary percentages . In doing so, we assumed that the number of flowers per capitulum, i.e.…”

“…Most herbicides are designed to target specific enzymes or proteins and thus TSR is mostly monogenic and involves a point‐mutated target enzyme . NTSR, described as a collection of mechanisms that are by definition not TSR, results from reduced uptake and/or translocation, increased vacuolar sequestration or metabolism to non‐toxic compounds, with consequent decreased levels of glyphosate interacting with EPSPS . TSR mechanisms involve one or more mutations in the DNA encoding the target protein of the herbicide, which leads to changes in amino acids or conformational changes in protein folding .…”

Multiple resistance to thifensulfuron-methyl and fomesafen in redroot pigweed ( Amaranthus retroflexus L.) from China

“…Unlike TSR, only conferring resistance to herbicides targeting enzyme concerned, NTSR confers unpredictable resistance levels to herbicides with different modes of action (Petit et al, 2010). Up to now, the typical cases in NTSR include Bromus rigidus (Owen et al, 2012), Lolium rigidum (Yu et al, 2013;Fernández-Moreno et al, 2017) and Echinochloa phyllopogon (Iwakami et al, 2014).…”