Genetic basis and origin of resistance to acetolactate synthase inhibitors in Amaranthus palmeri from Spain and Italy

A New Pro-197-Ile Mutation in Amaranthus palmeri Associated with Acetolactate Synthase-Inhibiting Herbicide Resistance

Palmer amaranth (Amaranthus palmeri S. Watson), native to North America, is one of the most prominent invasive weed species on agricultural land. Acetolactate synthase (ALS)-resistant A. palmeri (Amaranthus palmeri) is widespread, while the research focus on resistance pattern and molecular basis of A. palmeri to imazethapyr is seldom documented in China. An A. palmeri population that survived the recommended rate of imazethapyr was collected in Shandong Province, China. The resistant mechanism and pattern of A. palmeri to imazethapyr was investigated. Dose-response assay showed that the resistant (R) population displayed a high resistance level (292.5-fold) to imazethapyr compared with the susceptible (S) population. Sequence analysis of the ALS gene revealed that nucleotide mutations resulted in three resistance-conferring amino acid substitutions, Pro-197-Ile, Trp-574-Leu, and Ser-653-Asp, in the individual plants of the R population. An in vitro enzyme assay indicated that the ALS was relatively unsusceptible to imazethapyr in the R population, showing a resistance index of 88.6-fold. ALS gene expression and copy number did not confer resistance to imazethapyr in the R population. Pro-197-Ile is the first reported amino acid substitution conferring ALS resistance to A. palmeri. This is the first case of an imazethapyr-resistant A. palmeri biotype in China.

Characterization of acetolactate synthase genes and resistance mechanisms of multiple herbicide resistant Lolium multiflorum

Combining imidazolinone-tolerant wheat with imazamox presents an effective solution to combat weed resistance. However, Lolium multiflorum, a troublesome resistant weed infesting wheat fields, may have developed resistance to imazamox, and the potential resistance mechanisms are intriguing. In this study, we explored the susceptibility of L. multiflorum to imazamox and investigated the resistance mechanisms, including the contribution of the target enzyme acetolactate synthase (ALS) to resistance and the presence of non-target-site resistance (NTSR). Eight L. multiflorum populations suspected of being resistant to imazamox were collected, and six populations exhibited resistance, ranging from 2.45-fold to 16.32-fold. The LmALS1 gene from susceptible population D3 plants and multiple copies of the LmALS gene (LmALS1, LmALS2, LmALS2α, LmALS3, LmALS3α, LmALS3β) from resistant populations D5 and D8 plants were separately amplified. Two mutations (Pro/Gln197 to Thr, Trp574 to Leu) were found in LmALS1 in the resistant populations. Compared to D3, LmALS1 was overexpressed in D5 but not in D8. The presence of LmALS1 mutants (LmALS1-Thr197 and LmALS1- Leu574), along with LmALS2, LmALS3, and their subunits, contribute to the resistance phenotype by increasing bonding energies, weakening hydrogen bonds, or decreasing protein binding pocket volumes and surface area. Additionally, D5 and D8 populations exhibited multiple resistance (>40-fold) to three other ALS inhibitors: pyroxsulam, flucarbazone-sodium, and mesosulfuron-methyl. Pre-treatment with malathion and 4-chloro-7-nitrobenzoxadiazole (cytochrome P450 monooxygenase and glutathione S-transferase inhibitors respectively) reversed the resistance of the D8 population and partially reversed the resistance of the D5 population to imazamox. This study characterizes ALS genes and extends our knowledge into the ALS resistance mechanisms involved in L. multiflorum. It also deepens our understanding of the complex diversification resistance mechanisms, thereby facilitating advances in weed resistance management strategies in wheat fields.

Development of a rapid detection assay for acetolactate synthase inhibitors resistance in three Amaranthus weed species through loop-mediated isothermal amplification

BACKGROUND

The early detection of herbicide resistance in weeds is a key factor to avoid herbicide waste and improve agriculture sustainability. The present study aimed to develop and validate an allele-specific loop-mediated isothermal amplification (AS-LAMP) assay for the quick on-site detection of the resistance-endowing point mutation Trp-574-Leu in the acetolactate synthase (ALS) gene in three widely diffused Amaranthus weed species: Amaranthus retroflexus, Amaranthus hybridus and Amaranthus tuberculatus.

RESULTS

The AS-LAMP protocol was developed on wild-type and ALS-mutant plants of the three species and revealed that the amplification approach with only the primer set specific for the mutant allele (574-Leu) was the most promising. The validation and estimation of the AS-LAMP performance evaluated by comparing the results with those of the molecular marker (cleaved amplified polymorphic sequences) indicated that, although the sensitivity and specificity were relatively high in all species (overall 100 and > 65%, respectively), precision was high for A. hybridus L. and A. retroflexus L. (75 and 79%, respectively), but quite low for A. tuberculatus (Moq.) J. D. Sauer (59%). The LAMP assay was also effective on crude genomic DNA extraction, allowing the quick detection of mutant plants in field situation (on site resistance detection).

CONCLUSION

The proposed AS-LAMP method has proven to be a promising technique for rapid detection of resistance as a result of Trp-574-Leu on the two monoecious weedy Amaranthus species but resulted less effective in the genetically variable dioecious species A. tuberculatus. © 2024 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

ALS gene overexpression and enhanced metabolism conferring Digitaria sanguinalis resistance to nicosulfuron in China

Crabgrass (Digitaria sanguinalis) is a common malignant weed in corn fields in China. Recently, the acetolactate synthase (ALS) inhibitor, nicosulfuron, has shown decreasing efficacy against crabgrass. In order to elucidate the molecular basis of resistance to nicosulfuron in crabgrass, we conducted bioassays, combined with gene sequence analysis, relative expression and relative copy number analysis, to characterize resistance in crabgrass populations collected from Beijing, Heilongjiang, Jilin and Anhui provinces. Whole-plant dose-response results indicated that only population collected in Heilongjiang province (HLJ) had developed low level of resistance to nicosulfuron compared with the sensitive population (SD22). No known resistant mutation of ALS gene was found in HLJ population. The real-time fluorescence quantitative PCR results showed that the ALS gene copy number did not differ significantly between the HLJ and SD22 populations. However, the ALS gene expression in the HLJ was 2.07-fold higher than that of the SD22 population at 24 h after treatment with nicosulfuron. Pretreatment with the cytochrome P450 (CYP450) inhibitor malathion, piperonyl butoxide (PBO), and the glutathione S-transferase (GST) inhibitor 4-Chloro-7-nitro-1,2,3-benzoxadiazole (NBD-Cl) all partially reversed HLJ resistance. Among them, the synergistic effect of PBO and nicosulfuron is the most significant. This is the first report of resistance to nicosulfuron in crabgrass through ALS gene overexpression and possible metabolic resistance.