Resistance mechanism to bensulfuron-methyl in biotypes of Scirpus mucronatus L. collected in Chilean rice fields

Enhanced detoxification via Cyt-P450 governs cross-tolerance to ALS-inhibiting herbicides in weed species of Centaurea

Centaurea is a genus of winter weeds with a similar life cycle and competitive traits, which occurs in small-grains production fields in the central-southern of the Iberian Peninsula. However, most of herbicides recommended for weed management in wheat show poor control of Centaurea species. This study summarizes the biology, herbicide tolerance to acetolactate synthase (ALS) inhibitors, and recommended chemical alternatives for the control of Centaurea species. Four species (C. cyanus L., C. diluta Aiton, C. melitensis L. and C. pullata L. subsp. baetica Talavera), taxonomically characterized, were found as the main important broadleaf weeds in small-grains production fields of the Iberian Peninsula. These species showed innate tolerance to tribenuron-methyl (TM), showing LD₅₀ values (mortality of 50% of a population) higher than the field dose of TM (20 g ai ha^-1). The order of tolerance was C. diluta (LD₅₀ = 702 g ha^-1) ≫ C. pullata (LD₅₀ = 180 g ha^-1) ≫ C. cyanus (LD₅₀ = 65 g ha^-1) > C. melitensis (LD₅₀ = 32 g ha^-1). Centaurea cyanus and C. melitensis presented higher foliar retention (150-180 μL herbicide solution), absorption (14-28%) and subsequent translocation (7-12%) of TM with respect to the other two species. Centaurea spp. plants were able to metabolize ¹⁴C-TM into non-toxic forms (hydroxylated OH-metsulfuron-methyl and conjugated-metsulfuron-methyl), with cytochrome P450 (Cyt-P450) monooxygenases being responsible for herbicide detoxification. Centaurea cyanus and C. mellitensis metabolized up to 25% of TM, while C. diluta and C. pullata metabolized more than 50% of the herbicide. Centaurea species showed 80-100% survival when treated with of florasulam, imazamox and/or metsulfuron-methyl, i.e., these weeds present cross-tolerance to ALS inhibitors. In contrast, auxin mimics herbicides (2,4-D, clopyralid, dicamba, fluroxypir and MCPA) efficiently controlled the four Centaurea species. In addition, the mixture of ALS-inhibitors and auxin mimics also proved to be an interesting alternative for the control of Centaurea. These results show that plants of the genus Centaurea found in the winter cereal fields of the Iberian Peninsula have an innate tolerance to TM and cross-resistance to other ALS-inhibiting herbicides, governed by reduced absorption and translocation, but mainly by the metabolization of the herbicide via Cyt-P450.

The enhanced mechanisms of Hansschlegelia zhihuaiae S113 degrading bensulfuron-methyl in maize rhizosphere by three organic acids in root exudates

The residues of bensulfuron-methyl (BSM), a sulfonylurea herbicide, in soil have caused serious damage to the rotation of susceptible crops. Many studies have reported that the removal of BSM in soil was achieved by adding degrading bacteria. However, the mechanisms used by bacteria to degrade BSM in the crop rhizosphere remain unclear. In this study, a BSM-degrading bacterium, Hansschlegelia zhihuaiae S113, was applied to investigate the enhancement of effects mediated by organic acids during the bioremediation of BSM-contaminated maize rhizosphere soil. Organic acids, such as L-malic acid, tartaric acid, and fumaric acid, identified in maize root exudates, significantly stimulated the expression of cheA, which encoded the histidine kinase in strain S113 and contributed to the chemotactic response. This process accelerated the accumulation of strain S113 around the maize roots and promoted the colonization process on maize roots. The growth of strain S113 was significantly increased by L-malic acid but not tartaric acid or fumaric acid. After the S113 suspension was root-irrigated to BSM-contaminated soil, the density of strain S113 colonizing root surfaces and in rhizosphere soil reached 1.1 × 10⁴ cells/g for roots and 4.9 × 10⁴ cells/g in dry soil at 15 d, leading to 80.9% BSM degradation efficiency. The treatment with the addition of a mixture of S113 and L-malic acid completely degraded BSM in rhizosphere soil due to the strong attraction and growth promotion of strain S113 by L-malic acid, with a higher efficiency than that with the extra addition of fumaric acid (89.7%) or tartaric acid (87.0%). This paper revealed the enhancement effects of organic acids identified in root exudates for the in situ bioremediation of BSM-contaminated rhizosphere soil.

Hydrolysis, adsorption, and biodegradation of bensulfuron methyl under methanogenic conditions

Bensulfuron methyl (BSM), one of the most widely used herbicides in paddy soils, is frequently detected in natural and artificial aquatic systems. However, BSM transformation under methanogenic conditions has not been given sufficient attention. In this study, BSM elimination and transformation by anaerobic enrichment cultures were investigated. The results showed that BSM can be mineralized to methane through hydrolysis, adsorption, and biodegradation under a methanogenic environment. The adsorption led to protein static quenching in the extracellular polymeric substances (EPSs) of the enrichment cultures. Specifically, BSM mainly reacted with the amine, amide, amino acid, and amino sugar functional groups in proteins. BSM hydrolysis and biodegradation occurred through the breakage of the sulfonylurea bridge and sulfonyl amide linkage. The cleavage of the sulfonylurea bridge occurred in both hydrolysis and biodegradation, while the cleavage of the sulfonyl amide linkage only occurred in hydrolysis. These results elucidated the complex transformation of BSM under methanogenic conditions, which will advance the studies on sulfonylurea herbicide biotransformation and hazard assessment in the environment.

Target-site and non-target-site resistance mechanisms to ALS inhibiting herbicides in Papaver rhoeas

Target-site and non-target-site resistance mechanisms to ALS inhibitors were investigated in multiple resistant (tribenuron-methyl and 2,4-D) and only 2,4-D resistant, Spanish corn poppy populations. Six amino-acid replacements at the Pro197 position (Ala197, Arg197, His197, Leu197, Thr197 and Ser197) were found in three multiple resistant populations. These replacements were responsible for the high tribenuron-methyl resistance response, and some of them, especially Thr197 and Ser197, elucidated the cross-resistant pattern for imazamox and florasulam, respectively. Mutations outside of the conserved regions of the ALS gene (Gly427 and Leu648) were identified, but not related to resistance response. Higher mobility of labeled tribenuron-methyl in plants with multiple resistance was, however, similar to plants with only 2,4-D resistance, indicating the presence of non-target-site resistance mechanisms (NTSR). Metabolism studies confirmed the presence of a hydroxy imazamox metabolite in one of the populations. Lack of correlation between phenotype and genotype in plants treated with florasulam or imazamox, non-mutated plants surviving imazamox, tribenuron-methyl translocation patterns and the presence of enhanced metabolism revealed signs of the presence of NTSR mechanisms to ALS inhibitors in this species. On this basis, selection pressure with ALS non-SU inhibitors bears the risk of promoting the evolution of NTSR mechanisms in corn poppy.

A high-throughput, modified ALS activity assay for Cyperus difformis and Schoenoplectus mucronatus seedlings

Cyperus difformis L. (CYPDI) and Schoenoplectus mucronatus (L.) Palla (SCHMU) are major weeds of California (CA) rice, where resistance to acetolactate synthase (ALS)-inhibitors was identified in several CYPDI and SCHMU populations that have also evolved resistance to photosystem II (PSII)-inhibiting herbicides. The mechanism of ALS resistance in these populations remains to be clarified but this information is crucial in a weed management program, especially in a scenario where resistance to multiple herbicides has been identified. ALS activity assays are commonly used to diagnose resistance to ALS-inhibitors, but protocols currently available are burdensome for the study of CYPDI and SCHMU, as they require large amounts of plant material from young seedlings and have low yields. Our objective was to investigate the ALS resistance mechanism in suspected ALS-resistant (R) CYPDI and SCHMU biotypes using a modified ALS activity assay that requires less plant material. ALS enzymes from suspected R biotypes were at least 10,000-fold less sensitive to bensulfuron-methyl than susceptible (S) cohorts, indicating ALS resistance that is likely due to an altered target-site. Protein concentration (mgg^-1 tissue) did not differ between R and S biotypes within each species, suggesting that R biotypes do not over produce ALS enzymes. CYPDI biotypes had up to 4-fold more protein per mg of tissue than SCHMU biotypes, but up to 7-fold more acetoin per mg^-1 protein was quantified in SCHMU, suggesting greater ALS catalytic ability in SCHMU biotypes, regardless of their herbicide resistance status. Our optimized protocol to measure ALS activity allowed for up to a 3-fold increase in the number of assays performed per g of leaf tissue. The modified assay may be useful for measuring ALS activity in other weed species that also produce small amount of foliage in early growth stages when protein in tissue is most abundant.

Biodegradation of bensulfuron-methyl and its effect on bacterial community in paddy soils

Bensulfuron-methyl (BSM) is a new kind of sulfonylurea herbicide widely used to control broad-leaf weeds in rice paddies. The aim of this work was to study BSM biodegradation in paddy soils with BSM-degrading bacteria Bacillus megaterium L1 and Brevibacterium sp. BH and its effect on the structures of soil bacterial community. More than 90 % of BSM could be degraded in paddy soils with 0.0355 mg kg⁻¹ BSM concentration. Addition of BSM-degrading bacterial strains Bacillus megaterium L1 into BSM contaminated paddy soil could have the half-life time of BSM compared to treatment without Bacillus megaterium L1 inoculation. Denaturing gradient gel electrophoresis and principle component analysis indicated that the diversity of the soil microbial community structure changed along with the addition of BSM, which recovered at the end of the experiment (5 weeks). Addition of BSM-degrading bacteria Bacillus megaterium L1 enriched the diversity of soil microbial community structure in paddy soils. This study provides information on the biodegradation of BSM and BSM's influences on the soil bacteria microbial community structures.

Mechanism of resistance to ACCase-inhibiting herbicides in wild oat (Avena fatua ) from Latin America

Whole-plant response of two suspected resistant Avena fatua biotypes from Chile and Mexico to ACCase-inhibiting herbicides [aryloxyphenoxypropionate (APP), cyclohexanedione (CHD), and pinoxaden (PPZ)] and the mechanism behind their resistance were studied. Both dose-response and ACCase enzyme activity assays revealed cross-resistance to the three herbicide families in the biotype from Chile. On the other hand, the wild oat biotype from Mexico exhibited resistance to the APP herbicides and cross-resistance to the CHD herbicides, but no resistance to PPZ. Differences in susceptibility between the two biotypes were unrelated to absorption, translocation, and metabolism of the herbicides. PCR generated fragments of the ACCase CT domain spanning the potential mutations sited in the resistant and susceptible biotypes were sequenced and compared. A point mutation was detected in the aspartic acid triplet at the amino acid position 2078 in the Chilean biotype and in isoleucine at the amino acid position 2041 in the Mexican wild oat biotype, which resulted in a glycine triplet and an asparagine triplet, respectively. On the basis of in vitro assays, the target enzyme (ACCase) in these resistant biotypes contains a herbicide-insensitive form. This is the first reported evidence of resistance to pinoxaden in A. fatua.