Resistance Evolution to EPSPS Inhibiting Herbicides in False Barley (Hordeum murinum) Harvested in Southern Spain

Distribution of Glyphosate-Resistance in Echinochloa crus-galli Across Agriculture Areas in the Iberian Peninsula

The levels of resistance to glyphosate of 13 barnyard grass (Echinochloa crus-galli) populations harvested across different agriculture areas in the Southern Iberian Peninsula were determined in greenhouse and laboratory experiments. Shikimate accumulation fast screening separated the populations regarding resistance to glyphosate: susceptible (S) E2, E3, E4, and E6 and resistant (R) E1, E5, E7, E8, E9, E10, E11, E12, and E13. However, resistance factor (GR₅₀ E1-E13/GR₅₀ E6) values separated these populations into three groups: (S) E2, E3, E4, and E6, (R) E1, E5, E7, E8, and E9, and very resistant (VR) E10, E11, E12, and E13. ¹⁴C-glyphosate assays performed on two S populations (E2 and E6) showed greater absorption and translocation than those found for R (E7 and E9) and VR (E10 and E12) populations. No previous population metabolized glyphosate to amino methyl phosphonic acid (AMPA) and glyoxylate, except for the E10 population that metabolized 51% to non-toxic products. The VR populations showed two times more 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) activity without herbicide than the rest, while the inhibition of the EPSPS activity by 50% (I₅₀) required much higher glyphosate in R and VR populations than in S populations. These results indicated that different target-site and non-target-site resistance mechanisms were implicated in the resistance to glyphosate in E. crus-galli. Our results conclude that resistance is independent of climate, type of crop, and geographic region and that the level of glyphosate resistance was mainly due to the selection pressure made by the herbicide on the different populations of E. crus-galli studied.

First Case of Multiple Resistance to EPSPS and PSI in Eleusine indica (L.) Gaertn. Collected in Rice and Herbicide-Resistant Crops in Colombia

Eleusine indica is a highly competitive and difficult-to-control plant in annual and perennial crops. In Colombia, broad-spectrum herbicides, such as paraquat and glyphosate, have begun to present poor levels of control for this weed. The multiple resistance to glyphosate and paraquat, the increase in herbicide performance with adjuvants (Retenol® and Trend® 90), and alternative herbicides were evaluated in a resistant (R) population of E. indica collected in rice fields, which is rotated with herbicide-resistant (HR) crops. Based on plant mortality, the R population was 9.8 and 7.2 times more resistant than susceptible (S) plants to glyphosate and paraquat, respectively. R plants accumulated 4.2 less shikimic acid and had at least 70% less electrolyte leakage than S plants when they were exposed to glyphosate and paraquat, respectively. Both adjuvants increased the foliar retention of herbicides. In addition, adjuvants also increased the performance of glyphosate effectiveness between 22% and 58% and that of paraquat from 61% to 100%. Alternative herbicides (atrazine, clethodim, imazamox, diuron, flazasulfuron, glufosinate, oxyfluorfen, quizalofop, and tembotrione) provided high levels of control in both populations of E. indica. This is the first case of multiple resistant E. indica confirmed in Colombia. Adjuvants improved the leaf retention and efficacy of glyphosate and paraquat. In summary, the alternative herbicides evaluated in this study should be adopted by Colombian farmers and provide additional herbicide modes-of-action to combat future resistance.

Resistance to Fomesafen, Imazamox and Glyphosate in Euphorbia heterophylla from Brazil

Euphorbia heterophylla is a species of weed that was previously controlled by fomesafen, imazamox and glyphosate, but continued use of these herbicides has selected resistant populations from the Rio Grande do Sul (Brazil). One resistant (R) strain and one susceptible (S) strain to fomesafen, imazamox and glyphosate were compared, the latter by recurrent selection. Dose-response tests showed multiple resistance to these herbicides. The required imazamox concentration to inhibit ALS by 50% was approximately 16 times greater in the R population than in the S population. Based on the EPSPS activity results, the R population was 10 fold less sensitive to glyphosate than the S counterpart. In addition, basal EPSPS activity from R plants was 3.3 fold higher than the level detected on S plants. The Proto IX assays showed high resistance to fomesafen in the R population that accumulated less Proto IX than the S population. Malathion assays showed the participation of CytP450 in fomesafen resistance, but a molecular mechanism could also be involved. To our knowledge, this is the first characterisation of multiple resistance to these three groups of herbicides in E. heterophylla in the world.