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Amaranthus hybridus is one of the main weed species in Córdoba, Argentina. Until recently, this weed was effectively controlled with recurrent use of glyphosate. However, a population exhibiting multiple resistance (MR2) to glyphosate and imazamox appeared in a glyphosate resistant (GR) soybean field, with levels of resistance up to 93 and 38-fold higher to glyphosate and imazamox, respectively compared to the susceptible (S) population. In addition to imidazolinones, MR2 plants showed high resistance levels to sulfonylamino-carbonyl (thio) benzoates and moderate resistance to sulfonylureas and triazolopyrimidines. Multiple amino acid substitutions were found in both target genes, acetolactate synthase (ALS) and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), responsible for conferring high herbicides resistance levels in this A. hybridus population. In the case of EPSPS, the triple amino acid substitution TAP-IVS was found. In addition, MR2 plants also showed increased EPSPS gene expression compared to susceptible plants. A Ser653Asn substitution was found in the ALS sequence of MR2, explaining the pattern of cross-resistance to the ALS-inhibitor herbicide families found at the ALS enzyme activity level. No other mutations were found in other conserved domains of the ALS gene. This is the first report worldwide of the target site resistance mechanisms to glyphosate and ALS inhibitors in multiple herbicide resistance Amaranthus hybridus.

Euphorbia heterophylla is a weed species that invades extensive crop areas in subtropical regions of Brazil. This species was previously controlled by imazamox, but the continuous use of this herbicide has selected for resistant biotypes. Two biotypes of E. heterophylla from southern Brazil, one resistant (R) and one susceptible (S) to imazamox, were compared. The resistance of the R biotype was confirmed by dose-response assays since it required 1250.2 g ai ha -1 to reduce the fresh weight by 50% versus 7.4 g ai ha -1 for the S biotype. The acetolactate synthase (ALS) enzyme activity was studied using ALS-inhibiting herbicides from five different chemical families. The R biotype required the highest concentrations to reduce this enzyme activity by 50%. A Ser653Asn mutation was found in the ALS gene of the R biotype. The experiments carried out showed that imazamox absorption and metabolism were not involved in resistance. However, greater 14 C-imazamox root exudation was found in the R biotype (~70% of the total absorbed imazamox). Target site mutation in the ALS gene is the principal mechanism that explains the imazamox resistance of the R biotype, but root exudation seems to also contribute to the resistance of this biotype.

Weed control in wheat is one of the major goals of farmers in their efforts toward obtaining the highest crop yields for human foods. Several studies (dose-response, enzyme activity, absorption-translocation and metabolism) were conducted to characterize the resistance level of two new wheat cultivars called Rafalín (Triticum aestivum) and Antoñín (T. durum) that were obtained by conventional breeding based on Clearfield® technology; they are resistant (R) to imazamox compared to their sensitive (S) counterparts (Gazul and Simeto, respectively). The R-cultivars were 93.7-fold (Rafalín) and 43.7-fold (Antoñín) more resistant than their respective S-cultivars. The acetolactate synthase (ALS) enzyme activity revealed high resistance to imidazolinone (IMI) herbicides in R-cultivars, but no cross-resistance to other ALS herbicides was found. The Ser653Asn mutation that confers resistance to IMI herbicides was identified in the imi1 and imi2 genes of Rafalín and only in the imi1 gene of Antoñín. The 14 C-imazamox absorption did not differ between the R- and S-cultivars. Imazamox was metabolized by Cyt-P450 into imazamox-hydroxyl and imazamox-glucoside in the R-cultivars, altering their translocation patterns. The differential sensitivity to imazamox between R-cultivars was due to the number of resistance genes that carry each genotype. The R-cultivars Rafalín and Antoñín could be excellent weed control tools.