Non-Target Site Mechanisms Endow Resistance to Glyphosate in Saltmarsh Aster (Aster squamatus)

Abstract

Of the six-glyphosate resistant weed species reported in Mexico, five were found in citrus groves. Here, the glyphosate susceptibility level and resistance mechanisms were evaluated in saltmarsh aster (Aster squamatus), a weed that also occurs in Mexican citrus groves. The R population accumulated 4.5-fold less shikimic acid than S population. S plants hardly survived at 125 g ae ha−1 while most of the R plants that were treated with 1000 g ae ha−1, which suffered a strong growth arrest, showed a vigorous regrowth from the third week after treatment. Further, 5-enolpyruvylshikimate-3-phosphate basal and enzymatic activities did not diverge between populations, suggesting the absence of target-site resistance mechanisms. At 96 h after treatment, R plants absorbed ~18% less glyphosate and maintained 63% of the 14C-glyphsoate absorbed in the treated leaf in comparison to S plants. R plants metabolized twice as much (72%) glyphosate to amino methyl phosphonic acid and glyoxylate as the S plants. Three non-target mechanisms, reduced absorption and translocation and increased metabolism, confer glyphosate resistance saltmarsh aster. This is the first case of glyphosate resistance recorded for A. squamatus in the world.

Reduced Glyphosate Movement and Mutation of the EPSPS Gene (Pro106Ser) Endow Resistance in Conyza canadensis Harvested in Mexico

“…canadensis studied form part of the glyphosate-resistant weeds reported in this citrus-growing region of the Gulf of Mexico. The implementation of a generalized weed management strategy based only on chemical tools and repeated every agricultural cycle has selected glyphosate-resistant weeds in the region such as Leptochloa virgata, Bidens pilosa, Eleusine indica, Parthenium hysterophorus, and Aster squamatus. ,− According to the whole plant assays, the LD 50 values for the four R populations were higher than the field dose (720 g ae ha –1 ) recommended for weed control in Mexican citrus orchards. The estimated values for GR 50 and LD 50 of the R populations covered a wide range of resistance, but these values can be divided into two groups: the R1 and R4 populations with high values of resistance and the other group formed by R2 and R3, which had medium resistance values.…”

“…indica, P. hysterophorus, and Aster squamatus collected from nearby agricultural areas had this same NTSR resistance mechanism. ,− However, increasing glyphosate rates when a species survives at previously controlled rates is a practice that places greater selection pressure on weeds in this agricultural region, and this practice has led C. canadensis to add a TSR resistance mechanism, similar to what happened with other weed species studied in this region of the Gulf of Mexico. − …”