Differential expression of acetohydroxyacid synthase genes in sunflower plantlets and its response to imazapyr herbicide

Exploring the influence of invasive weed biochar on the sorption and dissipation dynamics of imazethapyr in sandy loam soil

The management of invasive weeds on both arable and non-arable land is a vast challenge. Converting these invasive weeds into biochar and using them to control the fate of herbicides in soil could be an effective strategy within the concept of turning waste into a wealth product. In this study, the fate of imazethapyr (IMZ), a commonly used herbicide in various crops, was investigated by introducing such weeds as biochar, i.e., Parthenium hysterophorus (PB) and Lantana camara (LB) in sandy loam soil. In terms of kinetics, the pseudo-second order (PSO) model provided the best fit for both biochar-mixed soils. More IMZ was sorbed onto LB-mixed soil compared to PB-mixed soil. When compared to the control (no biochar), both PB and LB biochars (at concentrations of 0.2% and 0.5%) increased IMZ adsorption, although the extent of this effect varied depending on the dosage and type of biochar. The Freundlich adsorption isotherm provided a satisfactory explanation for IMZ adsorption in soil/soil mixed with biochar, with the adsorption process exhibiting high nonlinearity. The values of Gibb's free energy change (ΔG) were negative for both adsorption and desorption in soil/soil mixed with biochar, indicating that sorption was exothermic and spontaneous. Both types of biochar significantly affect IMZ dissipation, with higher degradation observed in LB-amended soil compared to PB-amended soil. Hence, the findings suggest that the preparation of biochar from invasive weeds and its utilization for managing the fate of herbicides can effectively reduce the residual toxicity of IMZ in treated agroecosystems in tropical and subtropical regions.

Evaluating and Validating Sunflower Reference Genes for Q-PCR Studies Under High Temperature Condition

Background

Q-PCR is the method of choice for PCR- based transcriptomics and validating microarray-based and RNA-seq results. Proper application of this technology requires proper normalization to correct as much as possible errors propagating during RNA extraction and cDNA synthesis.

Objectives

The investigation was performed to find stable reference genes in sunflower under shifting in ambient temperature.

Materials and Methods

Sequences of five well-known reference genes of Arabidopsis (Actin, Ubiquitin, Elongation factor-1, GAPDH, and SAND) and one well-known reference gene inhuman, Importin, were subjected to BLASTX against sunflower databases and the relevant genes were subjected to primer designing for q-PCR. Two sunflower inbred lines were cultivated at two dates so that anthesis occurred at nearly 30 °C and 40 °C (heat stress). The experiment was repeated for two years. Q-PCR was run on samples taken for two planting date separately at the beginning of anthesis for each genotype from leaf, taproots, receptacle base, immature and mature disc flowers and on pooled samples comprising of the tissues for each genotype, planting dates and also all tissues for both genotypes and both planting dates. Basic statistical properties of each candidate gene across all the samples were calculated. Furthermore, gene expression stability analysis was done for six candidate reference genes on Cq mean of two years using three independent algorithms, geNorm, Bestkeeper, and Refinder.

Results

Designed primers for Actin2, SAND, GAPDH, Ubiquitin, EF-1a, and Importin yielded a single peak in melting curve analysis indicating specificity of the PCR reaction. Basic statistical analysis showed that Actin2 and EF-1a had the highest and lowest expression levels across all the samples, respectively. Actin2 appeared to be the most stable reference gene across all the samples based on the three used algorithms. Pairwise variation analysis revealed that for samples taken under ambient temperature of 30 °C, Actin2, EF-1a, SAND and for those taken under ambient temperature of 40 °C, Actin2, EF-1a, Importin and SAND have to be used for normalization in q-PCR studies. Moreover, it is suggested that normalization to be based on Actin2, SAND and EF-1a for vegetative tissues and Actin2, EF-1a, SAND and Importin for reproductive tissues.

Conclusions

In the present research, proper reference genes for normalization of gene expression studies under heat stress conditions were introduced. Moreover, the presence of genotype-by- planting date interaction effects and tissue specific gene expression pattern on the behavior of the most three stable reference genes was indicated.

Target-Site Mutations and Expression of ALS Gene Copies Vary According to Echinochloa Species

The sustainability of rice cropping systems is jeopardized by the large number and variety of populations of polyploid Echinochloa spp. resistant to ALS inhibitors. Better knowledge of the Echinochloa species present in Italian rice fields and the study of ALS genes involved in target-site resistance could significantly contribute to a better understanding of resistance evolution and management. Using a CAPS-rbcL molecular marker, two species, E. crus-galli (L.) P. Beauv. and E. oryzicola (Vasinger) Vasing., were identified as the most common species in rice in Italy. Mutations involved in ALS inhibitor resistance in the different species were identified and associated with the ALS homoeologs. The relative expression of the ALS gene copies was evaluated. Molecular characterization led to the identification of three ALS genes in E. crus-galli and two in E. oryzicola. The two species also carried different point mutations conferring resistance: Ala122Asn in E. crus-galli and Trp574Leu in E. oryzicola. Mutations were carried in the same gene copy (ALS1), which was significantly more expressed than the other copies (ALS2 and ALS3) in both species. These results explain the high resistance level of these populations and why mutations in the other ALS copies are not involved in herbicide resistance.

Enantioselectivity effects of imazethapyr enantiomers to metabolic responses in mice

Imazethapyr (IMZT) is a typical chiral pesticide with two enantiomers with the R-IMZT having the main herbicidal activity. However, the enantioselectivity of the effects of IMZT enantiomers on human and animals is still unclear. In this study, a nuclear magnetic resonance (NMR)-based metabolomics method and determination of oxidative stress were used to evaluate the enantioselectivity of IMZT enantiomers in mice. The results showed that the R-IMZT caused larger disturbances of endogenous metabolites and the S-IMZT had stronger interferences to oxidation defense system. The significantly perturbed metabolic pathways in mice exposed to the R-enantiomer were the valine, leucine and isoleucine biosynthesis pathway as well as the phenylalanine, tyrosine and tryptophan biosynthesis pathway. However, exposure of mice to the S-enantiomer did not significantly affect the metabolic pathways, but exposure led to an increase of catalase (CAT) activity and an increase in malondialdehyde (MDA) content in the liver. These results indicate that we need to conduct a more comprehensive assessment of the health risks of pesticide monomers in the future. In a word, these results provide more evidence for assessing the differences in health risks of IMZT enantiomers to mammals as well as provide more references for the promotion and use of pesticide monomers in the future.

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.

Effect of Ahasl1-1 and Ahasl1-4 alleles on herbicide resistance and its associated dominance in sunflower

BACKGROUND

Acetohydroxyacid synthase large subunit 1 (Ahasl1) is a multiallelic locus involved in herbicide resistance in sunflower. Ahasl1-1 and Ahasl1-4 alleles harbor different point mutations that lead to different amino acid substitutions (Ala205Val and Trp574Leu, respectively). The objectives of this work were to evaluate the effect of these alleles at the enzymatic and whole-plant levels, and to determine the dominance relationships for imazapyr and metsulfuron-methyl herbicides.

RESULTS

Resistant near-isogenic lines showed significantly lower specific AHAS activity than susceptible near-isoline. However, kinetic studies indicated that mutations did not change AHAS pyruvate affinity. Dose-response for six near-isolines carrying different combinations of Ahasl1-1 and Ahasl1-4 alleles and two herbicides (imazapyr and metsulfuron-methyl) were evaluated at whole-plant and enzymatic levels. Ahasl1-1 allele conferred moderate resistance to imazapyr and low resistance to metsulfuron-methyl. Conversely, Ahasl1-4 allele endowed high levels of resistance for both herbicides. Dominance of resistance at whole-plant level showed a semi-dominant behavior among the alleles for both herbicides.

CONCLUSION

Ahasl1-4 allele confers higher resistance levels than Ahasl1-1 when evaluated with imazapyr and metsulfuron-methyl. Dominance estimations suggested that both parental lines should carry a resistance trait when developing hybrids. © 2018 Society of Chemical Industry.

Genotoxicity of the herbicide imazethapyr in mammalian cells by oxidative DNA damage evaluation using the Endo III and FPG alkaline comet assays

We evaluated the role of oxidative stress in the genotoxic damage induced by imazethapyr (IMZT) and its formulation Pivot® in mammalian CHO-K1 cell line. Using the alkaline comet assay, we observed that a concentration of 0.1 μg/mL of IMZT or Pivot® was able to induce DNA damage by increasing the frequency of damaged nucleoids. To test whether the DNA lesions were caused by oxidative stress, the DNA repair enzymes endonuclease III (Endo III) and formamidopyrimidine-DNA glycosylase (Fpg), which convert base damage to strand breaks, were used. Our results demonstrate that after treatment of CHO-K1 cells with the pure active ingredient as well as the commercial formulation Pivot®, an increase in DNA strand breaks was observed after incubation of both Endo III and Fpg enzymes, indicating that both compounds induce DNA damage involving both pyrimidine and purine-based oxidations, at least in CHO-K1 cells. Our findings confirm the genotoxic potential of IMZT and suggest that this herbicide formulation must be employed with great caution, especially not only for exposed occupational workers but also for other living species.

Toxic and genotoxic effects of the imazethapyr-based herbicide formulation Pivot H® on montevideo tree frog Hypsiboas pulchellus tadpoles (Anura, Hylidae)

Acute lethal and sublethal toxicity of the imidazolinone imazethapyr (IMZT)-based commercial formulation herbicide Pivot H® (10.59% IMZT) was evaluated on Hypsiboas pulchellus tadpoles. Whereas mortality was used as the end point for lethality, frequency of micronuclei (MNs) and other nuclear abnormalities as well as DNA single-strand breaks evaluated by the single cell gel electrophoresis assay were employed to test genotoxicity. Behavioral, growth, developmental, and morphological abnormalities were also employed as sublethal end points. Mortality studies revealed equivalent LC50 (96h) values of 1.49mg/L (confidence limit, 1.09-1.63) and 1.55mg/L (confidence limit, 1.51-1.60) IMZT for Gosner stage (GS) 25 and GS36, respectively. Behavioral changes, i.e., irregular swimming and immobility, as well as a decreased frequency of keratodonts were observed. The herbicide increased the frequency of MNs in circulating erythrocytes of tadpoles exposed for 48h to the highest concentration assayed (1.17mg/L). However, regardless of the concentration of the herbicide assayed, an enhanced frequency of MNs was observed in tadpoles exposed for 96h. The herbicide was able to induce other nuclear abnormalities, i.e., blebbed and notched nuclei, only when tadpoles were exposed for 96h. In addition, we observed that exposure to IMZT within the 0.39-1.17mg/L range increased the genetic damage index in treatments lasting for both 48 and 96h. This study represents the first evidence of acute lethal and sublethal effects exerted by IMZT on amphibians. Finally, our findings highlight the properties of this herbicide that jeopardize nontarget living species exposed to IMZT.

Multiple-herbicide resistance in Echinochloa crus-galli var. formosensis, an allohexaploid weed species, in dry-seeded rice

Biotypes of Echinochloa crus-galli var. formosensis with resistance to cyhalofop-butyl, an acetyl-CoA carboxylase (ACCase) inhibitor, have been found in dry-seeded rice fields in Okayama, Japan. We collected two lines with suspected resistance (Ecf27 and Ecf108) from dry-seeded rice fields and investigated their sensitivity to cyhalofop-butyl and other herbicides. Both lines exhibited approximately 7-fold higher resistance to cyhalofop-butyl than a susceptible line. Ecf108 was susceptible to penoxsulam, an acetolactate synthase (ALS) inhibitor. On the other hand, Ecf27 showed resistance to penoxsulam and two other ALS inhibitors: propyrisulfuron and pyriminobac-methyl. The alternative herbicides butachlor, thiobencarb, and bispyribac-sodium effectively controlled both lines. To examine the molecular mechanisms of resistance, we amplified and sequenced the target-site encoding genes in Ecf27, Ecf108, and susceptible lines. Partial sequences of six ACCase genes and full-length sequences of three ALS genes were examined. One of the ACCase gene sequences encodes a truncated aberrant protein due to a frameshift mutation in both lines. Comparisons of the genes among Ecf27, Ecf108, and the susceptible lines revealed that none of the ACCases and ALSs in Ecf27 and Ecf108 have amino acid substitutions that are known to confer herbicide resistance, although a single amino acid substitution was found in each of three ACCases in Ecf108. Our study reveals the existence of a multiple-herbicide resistant biotype of E. crus-galli var. formosensis at Okayama, Japan that shows resistance to cyhalofop-butyl and several ALS inhibitors. We also found a biotype that is resistant only to cyhalofop-butyl among the tested herbicides. The resistance mechanisms are likely to be non-target-site based, at least in the multiple-herbicide resistant biotype.

Analysis of acetohydroxyacid synthase1 gene in chickpea conferring resistance to imazamox herbicide

Chickpea (Cicer arietinum L.) production in the Canadian prairies is challenging due to a lack of effective weed management mainly because of poor competition ability of the crop and limited registered herbicide options. Chickpea genotype with resistance to imidazolinone (IMI) herbicides has been identified. A point mutation in the acetohydroxyacid synthase1 (AHAS1) gene at C581 to T581, resulting in an amino acid substitution from Ala194 to Val194 (position 205, standardized to arabidopsis), confers the resistance to imazamox in chickpea. However, the molecular mechanism leading to the resistance is not fully understood. In many plant species, contrasting transcription levels of AHAS gene has been implicated in the resistant and susceptible genotypes in response to IMI. The objectives of this research were to compare the AHAS gene expression and AHAS enzyme activity in resistant and susceptible chickpea cultivars in response to imazamox herbicide treatment. Results from RT–qPCR indicated that there is no significant change in the transcript levels of AHAS1 between the susceptible and the resistant genotypes in response to imazamox treatment. Protein hydrophobic cluster analysis, protein-ligand docking analysis, and AHAS enzyme activity assay all indicated that the resistance to imazamox in chickpea is due to the alteration of interaction of the AHAS1 enzyme with the imazamox herbicide.

Acetohydroxyacid synthase activity and transcripts profiling reveal tissue-specific regulation of ahas genes in sunflower

Acetohydroxyacid synthase (AHAS) is the target site of several herbicides and catalyses the first step in the biosynthesis of branched chain amino acid. Three genes coding for AHAS catalytic subunit (ahas1, ahas2 and ahas3) have been reported for sunflower. The aim of this work was to study the expression pattern of ahas genes family and AHAS activity in sunflower (Helianthus annuus L.). Different organs (leaves, hypocotyls, roots, flowers and embryos) were evaluated at several developmental stages. The transcriptional profile was studied through RT-qPCR. The highest expression for ahas1 was shown in leaves, where all the induced and natural gene mutations conferring herbicide resistance were found. The maximal expression of ahas2 and ahas3 occurred in immature flowers and embryos. The highest AHAS activity was found in leaves and immature embryos. Correlation analysis among ahas gene expression and AHAS activity was discussed. Our results show that differences in ahas genes expression are tissue-specific and temporally regulated. Moreover, the conservation of multiple AHAS isoforms in sunflower seems to result from different expression requirements controlled by tissue-specific regulatory mechanisms at different developmental stages.