Toxicological effects of active and inert ingredients of imazethapyr formulation Verosil® against Scenedesmus vacuolatus (Chlorophyta)

Imazethapyr, a selective systemic herbicide, is widely used in agriculture and it is frequently detected in water bodies close to application areas. Like other agrochemicals, imazethapyr is commercialized in formulations containing a mixture of additives that increase the effectiveness of the active ingredient. These complex mixtures may cause adverse effects on non-target primary producers, such as microalgae, when they reach freshwater bodies. The aim of this study was to assess the effects, separately, of the formulation Verosil^®, the formulation additives, and technical-grade imazethapyr, in the acidic form or as ammonium salt, on the microalga Scenedesmus vacuolatus (Chlorophyta). Verosil^®, formulation additives, and acid imazethapyr significantly inhibited the growth of S. vacuolatus (Verosil^® > formulation additives > acid imazethapyr) and caused morphological alterations from 2 mg L^-1, 4 mg L^-1, and 60 mg L^-1 onwards, respectively. Verosil^® and formulation additives caused the most adverse effect including membrane disorganization, cytoplasm contraction, cell wall thickening, thylakoidal membrane disaggregation, and starch granule accumulation. In addition, Verosil^® and formulation additives increased the chl a/chl b ratio, indicating possible alterations in photosystems as a stress response. The carotene/chl a ratio was also increased in microalgae exposed to both Verosil^® and formulation additives, suggesting an antioxidant response to these toxic compounds. All these results support the hypothesis that the formulation additives contribute significantly to the toxicity and alterations caused by the commercial formulation Verosil^® on S. vacuolatus.

Cultivar variation for imazamox resistance in wheat (Triticum aestivum L.): Insights into enzymatic assays for early selection

Acetohydroxyacid synthase (AHAS, E.C. 2.2.1.6) is the target site of several herbicide classes including imidazolinones. Imidazolinone resistance in wheat is conferred by two major genes AhasL-D1 and AhasL-B1. The objective of this work was to evaluate the in vitro and in vivo AHAS activity and plant growth in response to imazamox of nine wheat cultivars. Dose-response curves for two-gene resistant cultivars were significantly different from the single-gene resistant and susceptible cultivars in the in vitro AHAS assay. Resistance levels at the in vivo AHAS and whole-plant assays for resistant cultivars were >10-fold higher than susceptible cultivars. Moreover, in vivo dose-response curves showed differences among cultivars with the same number of resistance genes. It was concluded that in the in vitro AHAS assay cultivar variability was due to differences in target-site sensitivity while the in vivo AHAS assay reflected the resistance at whole-plant level. Both in vitro and in vivo AHAS dose-response curves could be useful tools when exploring mechanisms involved in imidazolinone resistance in different wheat genetic backgrounds and for the selection of higher resistant genotypes.

Sublethal application of various sulfonylurea and imidazolinone herbicides favors outcrossing and hybrid seed production in oilseed rape

BACKGROUND

Acetolactate synthase (ALS)-inhibiting herbicides from the chemical families of sulfonylureas and imidazolinones are used worldwide. However, drift or sprayer contamination from some sulfonylurea herbicides causes a high level of male sterility in cruciferous species, especially oilseed rape (OSR). In this paper, we evaluated the gametocidal effects of 27 ALS-inhibiting herbicides that were sprayed on OSR plants at the bolting stage.

RESULTS

OSR anther development was very sensitive to sublethal exposure to most ALS-inhibiting herbicides. The application of 18 out of the 20 tested sulfonylureas (except ethametsulfuron and ethoxysulfuron), two imidazolinones (imazethapyr and imazamox), and one sulfonylamino-carbonyltriazolinone (flucarbazone-sodium) at suitable rates could induce male sterility. Eight of the herbicides, including chlorsulfuron (at application rates of 60-120 mg/ha), halosulfuron-methyl (300-600 mg/ha), sulfosulfuron (400-600 mg/ha), triflusulfuron-methyl (500-750 mg/ha), pyrazosulfuron-ethyl (150-225 mg/ha), nicosulfuron (200-300 mg/ha), imazethapyr (750-1125 mg/ha), and imazamox (400-800 mg/ha), could induce over 90% male sterility and over 60% relative outcrossed seed set in six cultivars with different origins. These eight chemicals could be used as new gametocides for hybrid seed production. This study also examined the possibility of external application of these gametocides on several unstable Polima cytoplasmic male sterile and thermosensitive genic male sterile lines. Although the outcrossed seed set of the treated lines was slightly reduced, the gametocide application significantly increased the seed purity of the resulting hybrid.

CONCLUSION

The finding of the gametocidal effects of most sulfonylureas and imidazolinones are of great importance for developing new functions for ALS-inhibiting herbicides. The application of gametocides will also greatly promote the safe utilization of environment-sensitive male sterility in hybrid seed production. Unexpectedly, the application of three triazolopyrimidines (florasulam, flumetsulam, and penoxsulam) and one pyrimidinylthiobenzoate (bispyribac-sodium) did not cause male sterility, although these herbicides obviously inhibited the activity of ALS and plant growth. This result suggests that inhibition of ALS activity does not always lead to male sterility in plants, and these gametocides may also inhibit other biological functions vital for microspore development.

Chemotactic screening of imidazolinone-degrading bacteria by microfluidic SlipChip

The group of imidazolinone herbicides, widely used for weed control, is hazardous to some sensitive rotational crops. Thus, rapid elimination of imidazolinones from contaminated soil is significant for the environment. Biodegradation studies have demonstrated the ability of chemotaxis to enhance the biodegradation of pollutants. In this study, we used our newly developed SlipChip device for chemotactic sorting and a microfluidic streak plate device for bacterial cultivation as a new pipeline for screening imidazolinone degraders. The degradation efficiencies of an enrichment consortium and a chemotaxis consortium were determined by HPLC-MS/MS. Both consortia degraded all tested imidazolinones, with the highest efficiency (71.8%) for imazethapyr, and the chemotaxis consortium degraded these compounds approximately 10% more efficiently than the enrichment consortium. Moreover, the community diversities of the enrichment consortium and the chemotaxis consortium were analyzed by 16S rRNA gene amplicon sequencing. The results indicated that members of genus Ochrobactrum primarily contribute to the degradation of imidazolinones. This work proved that chemotaxis toward biodegradable pollutants increases their bioavailability and enhances the biodegradation rate. It also provided a new way to screen effective pollutant degraders and can be applied for the selective isolation of other chemotactic species from environmental samples.

Validation and application of a QuEChERS based method for estimation of half-life of imidazolinone herbicides in soils by LC-ESI-MS/MS

A modified QuEChERS (quick, easy, cheap, effective, rugged and safe) method was validated and applied for the estimation of half-lives of two imidazolinone group herbicides, imazethapyr and imazapic, in the Dystric Plinthosol (FX) and Ferralsol (LVA) agricultural soils using liquid chromatography tandem mass spectrometry. The solutions were prepared in a matrix extract to avoid matrix effects. The analytical method showed satisfactory specificity, selectivity, linearity (R² > 0.99), recoveries (range 85.0-117.0%), with RSD between 6.7% and 16.6%, and precision (range 94.7-108.5%), with RSD between 6.0% and 11.9%. The limit of detections for imazethapyr and imazapic in the soils were 2.2 µg kg^-1 and 2.0 µg kg^-1, and the limit of quantifications were 6.6 µg kg^-1 and 6.1 µg kg^-1. The half-lives of imazethapyr (35.7 and 97.9 days) and imazapic (40.4 and 64.4 days), in the FX and LVA soils, respectively, indicate that they are medium-persistence herbicides with possibility of leaching into groundwater. In addition, high concentrations of imazethapyr and imazapic were found in the soil samples after the time of application, meaning that there is a potential for prolonged soil residual activity due to carryover.

Development of imidazolinone herbicide tolerant borage (Borago officinalis L.)

Borage (Borago officinalis) is an annual herb that produces a high level of gamma-linolenic acid (GLA) in its seed oil. Due to the recognized health benefits of GLA, borage is now commercially cultivated worldwide. However, an herbicide-tolerant variety for effective weed management has not yet been developed. Here we report the generation and characterization of ethyl methanesulfonate (EMS) induced borage mutant lines tolerant to the herbicide imidazolinone. An EMS-mutagenized borage population was generated by using a series of concentrations of EMS to treat mature borage seeds. Screening of the M2 and M3 borage plants using an herbicide treatment resulted in the identification of two imidazolinone-tolerant lines. Sequence analysis of two acetohydroxyacid synthase (AHAS) genes, AHAS1 and AHAS2, from the mutant (tolerant) and wild type (susceptible) borage plants showed that single nucleotide substitutions which resulted in amino acid changes occurred in AHAS1 and AHAS2, respectively in the two tolerant lines. A KASP marker was then developed to differentiate the homozygous susceptible, homozygous tolerant and heterozygous borage plants. An in vitro assay showed that homozygous tolerant borage carrying the AHAS1 mutation retained significantly higher AHAS activity than susceptible borage across different imazamox concentrations. A herbicide dose response test indicated that the line with the AHAS1 mutation could tolerate four times the normally used field concentration of "Solo" herbicide.