Structure and mechanism of inhibition of plant acetohydroxyacid synthase

Acetolactate Synthase Overexpression in Mesosulfuron-Methyl-Resistant Shortawn Foxtail ( Alopecurus aequalis Sobol.): Reference Gene Selection and Herbicide Target Gene Expression Analysis

Severe infestations of shortawn foxtail ( Alopecurus aequalis Sobol.), a noxious weed in wheat and canola cropping systems in China, remain standing even after the application of the herbicides, fenoxaprop- P-ethyl and mesosulfuron-methyl. Analysis of gene expression in weed plants subjected to herbicide treatment is a key step toward more mechanistic studies. Since such an analysis often involves quantitative real-time PCR (qRT-PCR), endogenous reference genes with stable expression are required. Herein, we identified specific gene sets, suitable as references for qRT-PCR data normalization in A. aequalis plants under different experimental conditions, using geNorm, NormFinder, BestKeeper, and RefFinder software. Additionally, the reliability of reference genes was verified by analyzing the expression of genes encoding two major herbicide target enzymes: acetyl-CoA carboxylase (ACCase) and acetolactate synthase (ALS). Furthermore, functional ALS gene amplification was likely present in resistant plants, although it may make no contribution to the resistant phenotypes.

Ecotoxicological assessment of the herbicide Winner Top and its active substances-are the other formulants truly inert?

Formulants used in Plant Protection Products (PPPs) to promote their efficiency are normally undisclosed in the PPP documentation, unless they bear a human health or environmental hazardous potential per se. PPP regulation also demands the assessment of putative interactions among formulants within each product recipe and consequent effects, but these results are often unavailable. Such a case is that of the herbicide Winner Top (Selectis®, Portugal), which we selected as a model commercial formulation in the present study specifically aiming at (i) characterising its aquatic toxicity towards sensitive eco-receptors (Raphidocelis subcapitata, Chlorella vulgaris, Lemna minor and Lemna gibba), as well as that of its active substances (a.s.) nicosulfuron and terbuthylazine; (ii) comparing the ecotoxicity among the commercial formulation, the corresponding mixture of its a.s. and this a.s.'s mixture increasingly enriched with the formulants. Single chemical testing revealed that terbuthylazine was the strongest microalgae growth inhibitor and nicosulfuron was the strongest macrophyte growth inhibitor. On the other hand, the commercial formulation was consistently less toxic than the corresponding mixture of the a.s., suggesting that Winner Top formulants (72.9% of the commercial formulation) interact with the a.s., promoting less than additive effects in the selected non-target species. Importantly, this environmentally protective effect of the formulation can be apparent. Because macrophytes share most physiological features with the weeds targeted by the studied herbicide, it is likely that increased application doses are required to reach desired efficacy levels with the consequent detrimental increase of PPP residues load in edge-of-field freshwater ecosystems.

Male Sterility of an AHAS-Mutant Induced by Tribenuron-Methyl Solution Correlated With the Decrease of AHAS Activity in Brassica napus L

Tribenuron-methyl (TBM), an acetohydroxyacid synthase (AHAS)-inhibiting herbicide, can be used as an efficient chemical hybridization agent to induce male sterility for practical utilization of heterosis in rapeseed (Brassica napus L.). Utilization of rapeseed mutants harboring herbicide-resistant AHAS alleles as the male parent can simplify the hybrid seed production protocol. Here we characterized a novel TBM-resistant mutant K5 derived from an elite rapeseed variety, Zhongshuang No. 9 (ZS9), by ethyl methyl sulfonate mutagenesis. Comparative analysis of three BnAHAS genes (BnAHAS1, BnAHAS2, and BnAHAS3) between the mutant K5 and ZS9 identified a C-to-T transition at 544 from the translation start site in BnAHAS1 in K5 (This resistant allele is referred to as BnAHAS1^544T ), which resulted in a substitution of proline with serine at 182 in BnAHAS1. Both ZS9 and K5 plants could be induced complete male sterility under TBM treatment (with 0.10 and 20 mg⋅L^-1 of TBM, respectively). The relationship between TBM-induced male sterility (Y) and the relative AHAS activity of inflorescences (X) could be described as a modified logistic function, Y = 100-A/(1+Be^(-KX)) for the both genotypes, although the obtained constants A, B, and K were different in the functions of ZS9 and K5. Transgenic Arabidopsis plants expressing BnAHAS1^544T exhibited a higher TBM resistance of male reproductive organ than wild type, which confirmed that the Pro-182-Ser substitution in BnAHAS1 was responsible for higher TBM-resistance of male reproductive organs. Taken together, our findings provide a novel valuable rapeseed mutant for hybrid breeding by chemical hybridization agents and support the hypothesis that AHAS should be the target of the AHAS-inhibiting herbicide TBM when it is used as chemical hybridization agent in rapeseed.

AHAS Trp574Leu substitution in Raphanus sativus L.: screening, enzyme activity and fitness cost

BACKGROUND

Feral radish (Raphanus sativus L.) is a problematic weed that has become resistant to acetohydroxyacid synthase (AHAS) inhibitor herbicides due to the Trp574Leu mutation. An AHAS gene mutation that causes herbicide resistance may have negative pleiotropic effects on plant fitness. This study reports the effects of the Trp574Leu mutation on AHAS activity and reproductive traits of R. sativus.

RESULTS

Eight of 17 feral radish accessions presented individuals resistant to metsulfuron-methyl at 0.5% to >90.0% and all the resistant individuals analyzed showed the Trp574Leu mutation. Without herbicide selection, the AHAS activity was 3.2-fold higher in the susceptible accession than in the resistant one. The resistant accession was >9000-fold more resistant to metsulfuron-methyl and imazethapyr than the susceptible accession. Under low intraspecific competition during two growing seasons, AHAS-resistant feral radish accessions showed 22-38% and 21-47% lower seed numbers and yield per plant than the susceptible accession.

CONCLUSION

This is the first report of fitness cost associated with the AHAS Trp574Leu mutation in R. sativus populations. This fitness cost could reduce frequency of the resistant allele without herbicide selection. © 2018 Society of Chemical Industry.

Population Genomics of Herbicide Resistance: Adaptation via Evolutionary Rescue

The evolution of herbicide resistance in weed populations is a highly replicated example of adaptation surmounting the race against extinction, but the factors determining its rate and nature remain poorly understood. Here, we explore theory and empirical evidence for the importance of population genetic parameters-including effective population size, dominance, mutational target size, and gene flow-in influencing the probability and mode of herbicide resistance adaptation and its variation across species. We compiled data on the number of resistance mutations across populations for 79 herbicide-resistant species. Our findings are consistent with theoretical predictions that self-fertilization reduces resistance adaptation from standing variation within populations, but increases independent adaptation across populations. Furthermore, we provide evidence for a ploidy-mating system interaction that may reflect trade-offs in polyploids between increased effective population size and greater masking of beneficial mutations. We highlight the power of population genomic approaches to provide insights into the evolutionary dynamics of herbicide resistance with important implications for understanding the limits of adaptation.

Design, Synthesis, and Herbicidal Activity of Pyrimidine-Biphenyl Hybrids as Novel Acetohydroxyacid Synthase Inhibitors

The issue of weed resistance to acetohydroxyacid synthase (EC 2.2.1.6, AHAS) inhibitors has become one of the largest obstacles for the application of this class of herbicides. In a continuing effort to discover novel AHAS inhibitors to overcome weed resistance, a series of pyrimidine-biphenyl hybrids (4aa-bb and 5aa-ah) were designed and synthesized via a scaffold hopping strategy. Among these derivatives, compounds 4aa ( K_i = 0.09 μM) and 4bb ( K_i = 0.02 μM) displayed higher inhibitory activities against Arabidopsis thaliana AHAS than those of the controls bispyribac ( K_i = 0.54 μM) and flumetsulam ( K_i = 0.38 μM). Remarkably, compounds 4aa, 4bb, 5ah, and 5ag exhibited excellent postemergence herbicidal activity and a broad spectrum of weed control at application rates of 37.5-150 g of active ingredient (ai)/ha. Furthermore, 4aa and 4bb showed higher herbicidal activity against AHAS inhibitor-resistant Descurainia sophia, Ammannia arenaria, and the corresponding sensitive weeds than that of bispyribac at 0.94-0.235 g ai/ha. Therefore, the pyrimidine-biphenyl motif and lead compounds 4aa and 4bb have great potential for the discovery of novel AHAS inhibitors to combat AHAS-inhibiting herbicide-resistant weeds.

Characterization of an acetohydroxy acid synthase mutant conferring tolerance to imidazolinone herbicides in rice (Oryza sativa)

The acetohydroxy acid synthase S627N mutation confers herbicide tolerance in rice, and the rice variety containing this mutation produces good yields. This variety is commercially viable at Shanghai and Jiangsu regions in China. Weedy rice is a type of rice that produces lower yields and poorer quality grains than cultivated rice. It plagues commercial rice fields in many countries. One strategy to control its proliferation is to develop rice varieties that are tolerant to specific herbicides. Acetohydroxy acid synthase (AHAS) mutations have been found to confer herbicide tolerance to rice. Here, we identified a single mutation (S627N) in AHAS from an indica rice variety that conferred tolerance against imidazolinone herbicides, including imazethapyr and imazamox. A japonica rice variety (JD164) was developed to obtain herbicide tolerance by introducing the mutated indica ahas gene. Imidazolinone application was sufficient to efficiently control weedy rice in the JD164 field. Although the imazethapyr treatment caused dwarfing in the JD164 plants, it did not significantly reduce yields. To determine whether the decrease of the ahas mRNA expression caused the dwarfism of JD164 after imazethapyr application, we detected the ahas mRNA level in plants. The abundance of the ahas mRNA in JD164 increased after imidazolinone application, thus excluding the mRNA expression level as a possible cause of dwarfism. Activity assays showed that the mutated AHAS was tolerant to imidazolinone but the catalytic efficiency of the mutated AHAS decreased in its presence. Moreover, the activity of the mutated AHAS decreased more in the presence of imazethapyr than in the presence of imazamox. We observed no difference in the AHAS secondary structures, but homology modeling suggested that the S627N mutation enabled the substrate to access the active site channel in AHAS, resulting in imidazolinone tolerance. Our work combined herbicides with a rice variety to control weedy rice and showed the mechanism of herbicide tolerance in this rice variety.

Autophagy contributes to sulfonylurea herbicide tolerance via GCN2-independent regulation of amino acid homeostasis

Sulfonylurea (SU) herbicides inhibit branched-chain amino acid (BCAA) biosynthesis by targeting acetolactate synthase. Plants have evolved target-site resistance and metabolic tolerance to SU herbicides; the GCN2 (general control non-repressible 2) pathway is also involved in SU tolerance. Here, we report a novel SU tolerance mechanism, autophagy, which we call 'homeostatic tolerance,' is involved in amino acid signaling in Arabidopsis. The activation and reversion of autophagy and GCN2 by the SU herbicide tribenuron-methyl (TM) and exogenous BCAA, respectively, confirmed that TM-induced BCAA starvation is responsible for the activation of autophagy and GCN2. Genetic and biochemical analyses revealed a lower proportion of free BCAA and more sensitive phenotypes in atg5, atg7, and gcn2 single mutants than in wild-type seedlings after TM treatment; the lowest proportion of free BCAA and the most sensitive phenotypes were found in atg5 gcn2 and atg7 gcn2 double mutants. Immunoblotting and microscopy revealed that TM-induced activation of autophagy and GCN2 signaling do not depend on the presence of each other, and these 2 pathways may serve as mutually compensatory mechanisms against TM. TM inhibited the TOR (target of rapamycin), and activated autophagy in an estradiol-induced TOR RNAi line, suggesting that TM-induced BCAA starvation activates autophagy, probably via TOR inactivation. Autophagy and GCN2 were also activated, and independently contributed to TM tolerance in plants conferring metabolic tolerance. Together, these data suggest that autophagy is a proteolytic process for amino acid recycling and contributes to GCN2-independent SU tolerance, probably by its ability to replenish fresh BCAA.

Autophagy contributes to sulfonylurea herbicide tolerance via GCN2-independent regulation of amino acid homeostasis

Sulfonylurea (SU) herbicides inhibit branched-chain amino acid (BCAA) biosynthesis by targeting acetolactate synthase. Plants have evolved target-site resistance and metabolic tolerance to SU herbicides; the GCN2 (general control non-repressible 2) pathway is also involved in SU tolerance. Here, we report a novel SU tolerance mechanism, autophagy, which we call 'homeostatic tolerance,' is involved in amino acid signaling in Arabidopsis. The activation and reversion of autophagy and GCN2 by the SU herbicide tribenuron-methyl (TM) and exogenous BCAA, respectively, confirmed that TM-induced BCAA starvation is responsible for the activation of autophagy and GCN2. Genetic and biochemical analyses revealed a lower proportion of free BCAA and more sensitive phenotypes in atg5, atg7, and gcn2 single mutants than in wild-type seedlings after TM treatment; the lowest proportion of free BCAA and the most sensitive phenotypes were found in atg5 gcn2 and atg7 gcn2 double mutants. Immunoblotting and microscopy revealed that TM-induced activation of autophagy and GCN2 signaling do not depend on the presence of each other, and these 2 pathways may serve as mutually compensatory mechanisms against TM. TM inhibited the TOR (target of rapamycin), and activated autophagy in an estradiol-induced TOR RNAi line, suggesting that TM-induced BCAA starvation activates autophagy, probably via TOR inactivation. Autophagy and GCN2 were also activated, and independently contributed to TM tolerance in plants conferring metabolic tolerance. Together, these data suggest that autophagy is a proteolytic process for amino acid recycling and contributes to GCN2-independent SU tolerance, probably by its ability to replenish fresh BCAA.

Tribenuron-methyl resistance and mutation diversity of the AHAS gene in shepherd's purse (Capsella bursa-pastoris (L.) Medik.) in Henan Province, China

Shepherd's purse is a troublesome dicot weed that occurs in the major wheat-producing areas in China. Twenty-eight shepherd's purse populations were collected from winter wheat-planting areas in Henan Province and used to evaluate tribenuron-methyl resistance and acetohydroxyacid synthase (AHAS) gene-mutation diversity. The results indicate that all 28 shepherd's purse populations were resistant to tribenuron-methyl at different levels compared with the susceptible population. Mutation of the 197 codon (CCT) changed proline (Pro) into tyrosine (Tyr), histidine (His), leucine (Leu), serine (Ser), arginine (Arg), alanine (Ala) and threonine (Thr), whereas mutation of the 574 codon (TGG) changed tryptophan (Trp) into leucine (Leu). Among these amino acid changes, a co-concurrence of Pro197Leu and Trp574Leu substitutions was identified for the first time in resistant weed species. Furthermore, Pro197Tyr, Pro197Arg and Pro197Ala substitutions have not been previously reported in shepherd's purse. The results of the in vitro AHAS assay suggest that an insensitive AHAS is likely involved in the resistance to tribenuron-methyl in the R populations with AHAS gene mutations, and the non-target-site based resistance might exist in some populations.

Comparison of ALS functionality and plant growth in ALS-inhibitor susceptible and resistant Myosoton aquaticum L

Herbicide target-site resistance mutations may cause pleiotropic effects on plant ecology and physiology. The effect of several known (Pro197Ser, Pro197Leu Pro197Ala, and Pro197Glu) target-site resistance mutations of the ALS gene on both ALS functionality and plant vegetative growth of weed Myosoton aquaticum L. (water chickweed) have been investigated here. The enzyme kinetics of ALS from four purified water chickweed populations that each homozygous for the specific target-site resistance-endowing mutations were characterized and the effect of these mutations on plant growth was assessed via relative growth rate (RGR) analysis. Plants homozygous for Pro197Ser and Pro197Leu exhibited higher extractable ALS activity than susceptible (S) plants, while all ALS mutations with no negative change in ALS kinetics. The Pro197Leu mutation increased ALS sensitivity to isoleucine and valine, and Pro197Glu mutation slightly increased ALS sensitivity to isoleucine. RGR results indicated that none of these ALS resistance mutations impose negative pleiotropic effects on relative growth rate. However, resistant (R) seeds had a lowed germination rate than S seeds. This study provides baseline information on ALS functionality and plant growth characteristics associated with ALS inhibitor resistance-endowing mutations in water chickweed.

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.

Understanding the Differential Response of Setaria viridis L. (green foxtail) and Setaria pumila Poir. (yellow foxtail) to Pyroxsulam

Green foxtail [Setaria viridis (L) Beauv.] and yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.] are among the most abundant and troublesome annual grass weeds in cereal crops in the Northern Plains of the United States and the Prairie Provinces of Canada. Greenhouse and laboratory experiments were conducted to examine the differential responses of both weed species to foliar applications of the new triazolopyrimidine sulfonamide acetolactate synthase-inhibiting herbicide, pyroxsulam, and to determine the mechanism(s) of differential weed control. Foliar applications of pyroxsulam resulted in >90% control of yellow foxtail at rates between 7.5 and 15 g ai ha^-1, whereas the same rates resulted in a reduced efficacy on green foxtail (≤81%). The absorption and translocation of [¹⁴C]pyroxsulam in green and yellow foxtail were similar and could not explain the differential whole-plant efficacy. Studies with [¹⁴C]pyroxsulam revealed a higher percentage of absorbed pyroxsulam was metabolized into an inactive metabolite in the treated leaf of green foxtail than in the treated leaf of yellow foxtail. Metabolism studies demonstrated that, 48 h after application, 50 and 35% of pyroxsulam in the treated leaf was converted to 5-hydroxy-pyroxsulam in green and yellow foxtail, respectively. The acetolactate synthase (ALS) inhibition assay showed that ALS extracted from green foxtail was more tolerant to pyroxsulam than the enzyme extracted from yellow foxtail was. The in vitro ALS assay showed IC₅₀ values of 8.39 and 0.26 μM pyroxsulam for green and yellow foxtail, respectively. The ALS genes from both green and yellow foxtail were sequenced and revealed amino acid differences; however, the changes are not associated with known resistance-inducing mutations. The differential control of green and yellow foxtail following foliar applications of pyroxsulam was attributed to differences in both metabolism and ALS sensitivity.

Discovery of Putative Herbicide Resistance Genes and Its Regulatory Network in Chickpea Using Transcriptome Sequencing

Background: Chickpea (Cicer arietinum L.) contributes 75% of total pulse production. Being cheaper than animal protein, makes it important in dietary requirement of developing countries. Weed not only competes with chickpea resulting into drastic yield reduction but also creates problem of harboring fungi, bacterial diseases and insect pests. Chemical approach having new herbicide discovery has constraint of limited lead molecule options, statutory regulations and environmental clearance. Through genetic approach, transgenic herbicide tolerant crop has given successful result but led to serious concern over ecological safety thus non-transgenic approach like marker assisted selection is desirable. Since large variability in tolerance limit of herbicide already exists in chickpea varieties, thus the genes offering herbicide tolerance can be introgressed in variety improvement programme. Transcriptome studies can discover such associated key genes with herbicide tolerance in chickpea. Results: This is first transcriptomic studies of chickpea or even any legume crop using two herbicide susceptible and tolerant genotypes exposed to imidazoline (Imazethapyr). Approximately 90 million paired-end reads generated from four samples were processed and assembled into 30,803 contigs using reference based assembly. We report 6,310 differentially expressed genes (DEGs), of which 3,037 were regulated by 980 miRNAs, 1,528 transcription factors associated with 897 DEGs, 47 Hub proteins, 3,540 putative Simple Sequence Repeat-Functional Domain Marker (SSR-FDM), 13,778 genic Single Nucleotide Polymorphism (SNP) putative markers and 1,174 Indels. Randomly selected 20 DEGs were validated using qPCR. Pathway analysis suggested that xenobiotic degradation related gene, glutathione S-transferase (GST) were only up-regulated in presence of herbicide. Down-regulation of DNA replication genes and up-regulation of abscisic acid pathway genes were observed. Study further reveals the role of cytochrome P450, xyloglucan endotransglucosylase/hydrolase, glutamate dehydrogenase, methyl crotonoyl carboxylase and of thaumatin-like genes in herbicide resistance. Conclusion: Reported DEGs can be used as genomic resource for future discovery of candidate genes associated with herbicide tolerance. Reported markers can be used for future association studies in order to develop marker assisted selection (MAS) for refinement. In endeavor of chickpea variety development programme, these findings can be of immense use in improving productivity of chickpea germplasm.

Crystal structure of plant acetohydroxyacid synthase, the target for several commercial herbicides

Acetohydroxyacid synthase (AHAS, EC 2.2.1.6) is the first enzyme in the branched-chain amino acid biosynthesis pathway. Five of the most widely used commercial herbicides (i.e. sulfonylureas, imidazolinones, triazolopyrimidines, pyrimidinyl-benzoates and sulfonylamino-cabonyl-triazolinones) target this enzyme. Here we have determined the first crystal structure of a plant AHAS in the absence of any inhibitor (2.9 Å resolution) and it shows that the herbicide-binding site adopts a folded state even in the absence of an inhibitor. This is unexpected because the equivalent regions for herbicide binding in uninhibited Saccharomyces cerevisiae AHAS crystal structures are either disordered, or adopt a different fold when the herbicide is not present. In addition, the structure provides an explanation as to why some herbicides are more potent inhibitors of Arabidopsis thaliana AHAS compared to AHASs from other species (e.g. S. cerevisiae). The elucidation of the native structure of plant AHAS provides a new platform for future rational structure-based herbicide design efforts.

DATABASE

The coordinates and structure factors for uninhibited AtAHAS have been deposited in the Protein Data Bank (www.pdb.org) with the PDB ID code 5K6Q.

The Influence of the Plant Growth Regulator Maleic Hydrazide on Egyptian Broomrape Early Developmental Stages and Its Control Efficacy in Tomato under Greenhouse and Field Conditions

Broomrapes (Phelipanche spp. and Orobanche spp.) are holoparasitic plants that cause tremendous losses of agricultural crops worldwide. Broomrape control is extremely difficult and only amino acid biosynthesis-inhibiting herbicides present an acceptable control level. It is expected that broomrape resistance to these herbicides is not long in coming. Our objective was to develop a broomrape control system in tomato (Solanum lycopersicum L.) based on the plant growth regulator maleic hydrazide (MH). Petri-dish and polyethylene-bag system experiments revealed that MH has a slight inhibitory effect on Phelipanche aegyptiaca seed germination but is a potent inhibitor of the first stages of parasitism, namely attachment and the tubercle stage. MH phytotoxicity toward tomato and its P. aegyptiaca-control efficacy were tested in greenhouse experiments. MH was applied at 25, 50, 75, 150, 300, and 600 g a.i. ha-1 to tomato foliage grown in P. aegyptiaca-infested soil at 200 growing degree days (GDD) and again at 400 GDD. The treatments had no influence on tomato foliage or root dry weight. The total number of P. aegyptiaca attachments counted on the roots of the treated plants was significantly lower at 75 g a.i. ha-1 and also at higher MH rates. Phelipanche aegyptiaca biomass was close to zero at rates of 150, 300, and 600 g a.i. ha-1 MH. Field experiments were conducted to optimize the rate, timing and number of MH applications. Two application sequences gave superior results, both with five split applications applied at 100, 200, 400, 700, and 1000 GDD: (a) constant rate of 400 g a.i. ha-1; (b) first two applications at 270 g a.i. ha-1 and the next three applications at 540 g a.i. ha-1. Based on the results of this study, MH was registered for use in Israel in 2013 with the specified protocol and today, it is widely used by most Israeli tomato growers.

The Effects of Herbicides Targeting Aromatic and Branched Chain Amino Acid Biosynthesis Support the Presence of Functional Pathways in Broomrape

It is not clear why herbicides targeting aromatic and branched-chain amino acid biosynthesis successfully control broomrapes-obligate parasitic plants that obtain all of their nutritional requirements, including amino acids, from the host. Our objective was to reveal the mode of action of imazapic and glyphosate in controlling the broomrape Phelipanche aegyptiaca and clarify if this obligatory parasite has its own machinery for the amino acids biosynthesis. P. aegyptiaca callus was studied to exclude the indirect influence of the herbicides on the parasite through the host plant. Using HRT - tomato plants resistant to imidazolinone herbicides, it was shown that imazapic is translocated from the foliage of treated plants to broomrape attachments on its roots and controls the parasite. Both herbicides inhibited P. aegyptiaca callus growth and altered the free amino acid content. Blasting of Arabidopsis thaliana 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) and acetolactate synthase (ALS) cDNA against the genomic DNA of P. aegyptiaca yielded a single copy of each homolog in the latter, with about 78 and 75% similarity, respectively, to A. thaliana counterparts at the protein level. We also show for the first time that both EPSPS and ALS are active in P. aegyptiaca callus and flowering shoots and are inhibited by glyphosate and imazapic, respectively. Thus leading to deficiency of those amino acids in the parasite tissues and ultimately, death of the parasite, indicating the ability of P. aegyptiaca to synthesize branched-chain and aromatic amino acids through the activity of ALS and EPSPS, respectively.

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.

Cross-resistance patterns to acetolactate synthase (ALS)-inhibiting herbicides of flixweed (Descurainia sophia L.) conferred by different combinations of ALS isozymes with a Pro-197-Thr mutation or a novel Trp-574-Leu mutation

Acetolactate synthase (ALS) is the common target of ALS-inhibiting herbicides, and target-site ALS mutations are the main mechanism of resistance to ALS-inhibiting herbicides. In this study, ALS1 and ALS2 genes with full lengths of 2004bp and 1998bp respectively were cloned in individual plants of susceptible (S) or resistant (R) flixweed (Descurainia sophia L.) populations. Two ALS mutations of Pro-197-Thr and/or Trp-574-Leu were identified in plants of three R biotypes (HB24, HB30 and HB42). In order to investigate the function of ALS isozymes in ALS-inhibiting herbicide resistance, pHB24 (a Pro-197-Thr mutation in ALS1 and a wild type ALS2), pHB42 (a Trp-574-Leu mutation in ALS1 and a wild type ALS2) and pHB30 (a Trp-574-Leu mutation in ALS1 and a Pro-197-Thr mutation in ALS2) subpopulations individually homozygous for different ALS mutations were generated. Individuals of pHB30 had mutations in each isozyme of ALS and had higher resistance than pHB24 and pHB42 populations containing mutations in only one ALS isozyme. Moreover, the pHB24 had resistance to SU, TP and SCT herbicides, whereas pHB24 and pHB42 had resistance to these classes of herbicides as well as IMI and PTB herbicides. The sensitivity of isolated ALS enzyme to inhibition by herbicides in these populations correlated with whole plant resistance levels. Therefore, reduced ALS sensitivity resulting from the mutations in ALS was responsible for resistance to ALS-inhibiting herbicides in flixweed.

Genetic dissection of metabolite variation in Arabidopsis seeds: evidence for mQTL hotspots and a master regulatory locus of seed metabolism

To gain insight into genetic factors controlling seed metabolic composition and its relationship to major seed properties, an Arabidopsis recombinant inbred line (RIL) population, derived from accessions Col-0 and C24, was studied using an MS-based metabolic profiling approach. Relative intensities of 311 polar primary metabolites were used to identify associated genomic loci and to elucidate their interactions by quantitative trait locus (QTL) mapping. A total of 786 metabolic QTLs (mQTLs) were unequally distributed across the genome, forming several hotspots. For the branched-chain amino acid leucine, mQTLs and candidate genes were elucidated in detail. Correlation studies displayed links between metabolite levels, seed protein content, and seed weight. Principal component analysis revealed a clustering of samples, with PC1 mapping to a region on the short arm of chromosome IV. The overlap of this region with mQTL hotspots indicates the presence of a potential master regulatory locus of seed metabolism. As a result of database queries, a series of candidate regulatory genes, including bZIP10, were identified within this region. Depending on the search conditions, metabolic pathway-derived candidate genes for 40-61% of tested mQTLs could be determined, providing an extensive basis for further identification and characterization of hitherto unknown genes causal for natural variation of Arabidopsis seed metabolism.

Copy Number Variation in Acetolactate Synthase Genes of Thifensulfuron-Methyl Resistant Alopecurus aequalis (Shortawn Foxtail) Accessions in Japan

Severe infestations of Alopecurus aequalis (shortawn foxtail), a noxious weed in wheat and barley cropping systems in Japan, can occur even after application of thifensulfuron-methyl, a sulfonylurea (SU) herbicide. In the present study, nine accessions of A. aequalis growing in a single wheat field were tested for sensitivity to thifensulfuron-methyl. Seven of the nine accessions survived application of standard field rates of thifensulfuron-methyl, indicating that severe infestations likely result from herbicide resistance. Acetolactate synthase (ALS) is the target enzyme of SU herbicides. Full-length genes encoding ALS were therefore isolated to determine the mechanism of SU resistance. As a result, differences in ALS gene copy numbers among accessions were revealed. Two copies, ALS1 and ALS2, were conserved in all accessions, while some carried two additional copies, ALS3 and ALS4. A single-base deletion in ALS3 and ALS4 further indicated that they represent pseudogenes. No differences in ploidy level were observed between accessions with two or four copies of the ALS gene, suggesting that copy number varies. Resistant plants were found to carry a mutation in either the ALS1 or ALS2 gene, with all mutations causing an amino acid substitution at the Pro197 residue, which is known to confer SU resistance. Transcription of each ALS gene copy was confirmed by reverse transcription PCR, supporting involvement of these mutations in SU resistance. The information on the copy number and full-length sequences of ALS genes in A. aequalis will aid future analysis of the mechanism of resistance.

Fitness of ALS-Inhibitors Herbicide Resistant Population of Loose Silky Bentgrass (Apera spica-venti)

Herbicide resistance is an example of plant evolution caused by an increased reliance on herbicides with few sites of action to manage weed populations. This micro-evolutionary process depends on fitness, therefore the assessment of fitness differences between susceptible and resistant populations are pivotal to establish management strategies. Loose silky bentgrass (Apera spica-venti) is a serious weed in Eastern, Northern, and Central Europe with an increasing number of herbicide resistant populations. This study examined the fitness and growth characteristics of an ALS resistant biotype. Fitness and growth characteristics were estimated by comparing seed germination, biomass, seed yield and time to key growth stages at four crop densities of winter wheat (0, 48, 96, and 192 plants m^-2) in a target-neighborhood design. The resistant population germinated 9-20 growing degree days (GDD) earlier than the susceptible population at 10, 16, and 22°C. No differences were observed between resistant and susceptible populations in tiller number, biomass, time to stem elongation, time to first visible inflorescence and seed production. The resistant population reached the inflorescence emergence and flowering stages in less time by 383 and 196 GDD, respectively, at a crop density of 96 winter wheat plants m^-2 with no differences registered at other densities. This study did not observe a fitness cost to herbicide resistance, as often hypothesized. Inversely, a correlation between non-target site resistance (NTSR), earlier germination and earlier flowering time which could be interpreted as fitness benefits as these plant characteristics could be exploited by modifying the timing and site of action of herbicide application to better control ALS NTSR populations of A. spica-venti.

Physiological, biochemical and molecular characterization of an induced mutation conferring imidazolinone resistance in wheat

The Clearfield(®) wheat cultivars possessing imidazolinone (IMI)-resistant traits provide an efficient option for controlling weeds. The imazamox-resistant cultivar Pantera (Clearfield(®) ) was compared with a susceptible cultivar (Gazul). Target and non-target mechanisms of resistance were studied to characterize the resistance of Pantera and to identify the importance of each mechanism involved in this resistance. Pantera is resistant to imazamox as was determined in previous experiments. The molecular study confirmed that it carries a mutation Ser-Asn627 conferring resistance to imazamox in two out of three acetolactate synthase (ALS) genes (imi1 and imi2), located in wheat on chromosomes 6B and 6D, respectively. However, the last gene (imi3) located on chromosome 6A does not carry any mutation conferring resistance. As a result, photosynthetic activity and chlorophyll content were reduced after imazamox treatment. Detoxification was higher in the resistant biotype as shown by metabolomic study while imazamox translocation was higher in the susceptible cultivar. Interestingly, imazamox metabolism was higher at higher doses of herbicide, which suggests that the detoxification process is an inducible mechanism in which the upregulation of key gene coding for detoxification enzymes could play an important role. Thus, the identification of cultivars with a higher detoxification potential would allow the development of more resistant varieties.

Genetics of metabolic resistance

Herbicide resistance has become a major issue for many weeds. Metabolic resistance refers to the biochemical processes within organisms that degrade herbicides to less toxic compounds, resulting in a shift of the dose response curve. This type of resistance involves polygenic inheritance. A model is presented linking the biochemical pathway of amino acid synthesis and the detoxifying pathway of an inhibitor of the key enzyme ALS. From this model, resistance factors for each biotype are derived, which are then applied to a polygenic population genetic model for an annual weed plant. Polygenic inheritance is described by a new approach based on tensor products of heredity matrices. Important results from the model are that low dose regimes favour fast emergence of resistant biotypes and that the emergence of resistant biotypes occurs as abrupt outbreaks. The model is used to evaluate strategies for the management of metabolic resistance.

The Regulation of Essential Amino Acid Synthesis and Accumulation in Plants

Although amino acids are critical for all forms of life, only proteogenic amino acids that humans and animals cannot synthesize de novo and therefore must acquire in their diets are classified as essential. Nine amino acids-lysine, methionine, threonine, phenylalanine, tryptophan, valine, isoleucine, leucine, and histidine-fit this definition. Despite their nutritional importance, several of these amino acids are present in limiting quantities in many of the world's major crops. In recent years, a combination of reverse genetic and biochemical approaches has been used to define the genes encoding the enzymes responsible for synthesizing, degrading, and regulating these amino acids. In this review, we describe recent advances in our understanding of the metabolism of the essential amino acids, discuss approaches for enhancing their levels in plants, and appraise efforts toward their biofortification in crop plants.

Target-site basis for resistance to imazethapyr in redroot amaranth (Amaranthus retroflexus L.)

Experiments were conducted to confirm imazethapyr resistance in redroot amaranth (Amaranthus retroflexus L.) and study the target-site based mechanism for the resistance. Whole-plant response experiments revealed that the resistant (R) population exhibited 19.16 fold resistance to imazethapyr compared with the susceptible (S) population. In vitro ALS activity assay demonstrated that the imazethapyr I50 value of the R population was 21.33 times greater than that of the S population. However, qRT-PCR analysis revealed that there is no difference in ALS gene expression between the R and S populations. Sequence analysis revealed an Asp-376-Glu substitution in ALS in the R population. In order to verify that the imazethapyr resistance was conferred by Asp-376-Glu mutation, the ALS-R and ALS-S genes were fused to the CaMV 35S promoter and introduced into Arabidopsis respectively. The expression of ALS-R in transgenic Arabidopsis plants exhibited 13.79 fold resistance to imazethapyr compared to ALS-S transgenic Arabidopsis.

Broad resistance to acetohydroxyacid-synthase-inhibiting herbicides in feral radish (Raphanus sativus L.) populations from Argentina

BACKGROUND

Soon after the commercial release of sunflower cultivars resistant to imidazolinone herbicides, several uncontrolled feral radish (Raphanus sativus L.) populations were found in south-eastern Buenos Aires, Argentina. These populations were studied in field, glasshouse and laboratory experiments aiming to characterise their resistance profile and to develop management tools.

RESULTS

Three feral radish accessions were highly resistant to ten active ingredients of five families of acetohydroxyacid synthase (AHAS)-inhibiting herbicides. Sequence analysis of the AHAS gene detected a Trp574Leu mutation in all resistant accessions. One accession with an intermediate level of resistance was heterozygous for this mutation, probably owing to gene exchange with a susceptible subpopulation located in the field margin. Herbicide-resistant and herbicide-susceptible radish could be controlled in sunflower by alternative herbicides.

CONCLUSION

This is the first report of feral radish with resistance to herbicides belonging to all the AHAS-inhibiting herbicide families, conferred by Trp574Leu mutation in the AHAS gene. An appropriate herbicide rotation with alternative herbicides such as fluorochloridone or aclonifen and an increase in the diversity of cropping systems are important for minimising the prevalence of these biotypes.

Redirection of the Reaction Specificity of a Thermophilic Acetolactate Synthase toward Acetaldehyde Formation

Acetolactate synthase and pyruvate decarboxylase are thiamine pyrophosphate-dependent enzymes that convert pyruvate into acetolactate and acetaldehyde, respectively. Although the former are encoded in the genomes of many thermophiles and hyperthermophiles, the latter has been found only in mesophilic organisms. In this study, the reaction specificity of acetolactate synthase from Thermus thermophilus was redirected to catalyze acetaldehyde formation to develop a thermophilic pyruvate decarboxylase. Error-prone PCR and mutant library screening led to the identification of a quadruple mutant with 3.1-fold higher acetaldehyde-forming activity than the wild-type. Site-directed mutagenesis experiments revealed that the increased activity of the mutant was due to H474R amino acid substitution, which likely generated two new hydrogen bonds near the thiamine pyrophosphate-binding site. These hydrogen bonds might result in the better accessibility of H⁺ to the substrate-cofactor-enzyme intermediate and a shift in the reaction specificity of the enzyme.

Acetohydroxyacid synthase FgIlv2 and FgIlv6 are involved in BCAA biosynthesis, mycelial and conidial morphogenesis, and full virulence in Fusarium graminearum

In this study, we characterized FgIlv2 and FgIlv6, the catalytic and regulatory subunits of acetohydroxyacid synthase (AHAS) from the important wheat head scab fungus Fusarium graminearum. AHAS catalyzes the first common step in the parallel pathways toward branched-chain amino acids (BCAAs: isoleucine, leucine, valine) and is the inhibitory target of several commercialized herbicides. Both FgILV2 and FgILV6 deletion mutants were BCAA-auxotrophic and showed reduced aerial hyphal growth and red pigmentation when cultured on PDA plates. Conidial formation was completely blocked in the FgILV2 deletion mutant ΔFgIlv2-4 and significantly reduced in the FgILV6 deletion mutant ΔFgIlv6-12. The auxotrophs of ΔFgIlv2-4 and ΔFgIlv6-12 could be restored by exogenous addition of BCAAs but relied on the designated nitrogen source the medium contained. Deletion of FgILV2 or FgILV6 also leads to hypersensitivity to various cellular stresses and reduced deoxynivalenol production. ΔFgIlv2-4 lost virulence completely on flowering wheat heads, whereas ΔFgIlv6-12 could cause scab symptoms in the inoculated spikelet but lost its aggressiveness. Taken together, our study implies the potential value of antifungals targeting both FgIlv2 and FgIlv6 in F. graminearum.

Target-site resistance to bensulfuron-methyl in Sagittaria trifolia L. populations

Sagittaria trifolia L. is one of the most serious weeds in paddy fields in northeast of China and cannot be controlled effectively by bensulfuron-methyl in recent years. In this study, two suspected resistant S. trifolia populations (R1 and R2) were collected in Liaoning province of China. Whole-plant dose-response studies showed that R1 and R2 were highly resistant to bensulfuron-methyl, with the GR50 R/S ratios of 76.99 and 49.94 respectively. In vitro acetolactate synthase (ALS) assays revealed that resistance was due to reduced sensitivity of the ALS to bensulfuron-methyl inhibition, with I50 R/S ratios of 81.86 and 67.48 for R1 and R2, respectively. Total ALS activity was similar for the S and R2 populations, whereas the R1 population displayed significantly higher ALS activity than did the S population. The mutations Pro-197-Leu and Pro-197-Ser were identified in the ALS gene of the R1 and R2 populations, respectively. This is the first report examining bensulfuron-resistant S. trifolia in Liaoning province, China. The Pro197 mutation is likely responsible for resistance to bensulfuron-methyl in S. trifolia populations.

Molecular basis of resistance to imazethapyr in redroot pigweed (Amaranthus retroflexus L.) populations from China

Three putative resistant Amaranthus retroflexus L. populations were collected in Heilongjiang province in China. Whole plant bioassays indicated high resistance (RI > 10) to imazethapyr in the three populations. In vitro acetolactate synthase (ALS) assays revealed that ALS from populations H3, H17 and H39 was less sensitive to imazethapyr inhibition compared to the susceptible population H76. The half-maximal inhibitory concentration (I50) values for H3, H17 and H39 were 14.83, 15.27 and 268 times greater, respectively, than that of the susceptible population H76. Three nucleotide mutations resulted in three known resistance-endowing amino acid substitutions, Ala-205-Val, Trp-574-Leu and Ser-653-Thr in the three resistant populations respectively. Therefore, ALS target-site mutations in resistant A. retroflexus could be responsible for imazethapyr resistance.

Multiple resistance to ACCase and AHAS-inhibiting herbicides in shortawn foxtail (Alopecurus aequalis Sobol.) from China

Shortawn foxtail (Alopecurus aequalis) is a troublesome grass weed infesting winter wheat and oilseed rape productions in China. Fenoxaprop-p-ethyl and mesosulfuron-methyl failed to control shortawn foxtail of AHSX-1 population collected from a wheat field in Shou County, Anhui province. Molecular analyses revealed that Asp2078Gly mutation of ACCase and Trp574Leu mutation of AHAS were present in plants of the AHSX-1 population. The homozygous plants were isolated and cultured until seed maturity. Whole-plant herbicide bioassays were conducted in the greenhouse using the purified seeds of F1 generation. Dose-response experiments showed that the AHSX-1 population has evolved a very high level resistance to fenoxaprop-p-ethyl (RI = 275) and mesosulfuron-methyl (RI = 788). To determine the sensitivity to other herbicides, assays were conducted at the single recommended rate of each herbicide. Based on the results, the AHSX-1 population was considered to be highly resistant to clodinafop-propargyl, pyroxsulam and flucarbazone-sodium, moderately or highly resistant to quizalofop-p-ethyl, clethodim, sethoxydim and pinoxaden, and susceptible to isoproturon and chlorotoluron. This is the first report of Asp2078Gly mutation in shortawn foxtail and the two robust dCAPS markers designed could quickly detect Asp2078 and Trp574 mutations in ACCase and AHAS gene of shortawn foxtail, respectively.

Characterization of acetohydroxyacid synthase from the hyperthermophilic bacterium Thermotoga maritima

Acetohydroxyacid synthase (AHAS) is the key enzyme in branched chain amino acid biosynthesis pathway. The enzyme activity and properties of a highly thermostable AHAS from the hyperthermophilic bacterium Thermotoga maritima is being reported. The catalytic and regulatory subunits of AHAS from T. maritima were over-expressed in Escherichia coli. The recombinant subunits were purified using a simplified procedure including a heat-treatment step followed by chromatography. A discontinuous colorimetric assay method was optimized and used to determine the kinetic parameters. AHAS activity was determined to be present in several Thermotogales including T. maritima. The catalytic subunit of T. maritima AHAS was purified approximately 30-fold, with an AHAS activity of approximately 160±27 U/mg and native molecular mass of 156±6 kDa. The regulatory subunit was purified to homogeneity and showed no catalytic activity as expected. The optimum pH and temperature for AHAS activity were 7.0 and 85 °C, respectively. The apparent K_m and V_max for pyruvate were 16.4±2 mM and 246±7 U/mg, respectively. Reconstitution of the catalytic and regulatory subunits led to increased AHAS activity. This is the first report on characterization of an isoleucine, leucine, and valine operon (ilv operon) enzyme from a hyperthermophilic microorganism and may contribute to our understanding of the physiological pathways in Thermotogales. The enzyme represents the most active and thermostable AHAS reported so far.

•

First report of AHAS from a hyperthermophilic bacterium.

•

Catalytic and regulatory subunits of AHAS of T. maritima was expressed in E. coli.

•

Recombinant proteins were purified using a simplified procedure.

•

Enzyme represents the most active and thermostable AHAS reported so far.

•

Kinetic parameters were determined for the purified recombinant enzyme

ALOMYbase, a resource to investigate non-target-site-based resistance to herbicides inhibiting acetolactate-synthase (ALS) in the major grass weed Alopecurus myosuroides (black-grass)

Background

Herbicide resistance in agrestal weeds is a global problem threatening food security. Non-target-site resistance (NTSR) endowed by mechanisms neutralising the herbicide or compensating for its action is considered the most agronomically noxious type of resistance. Contrary to target-site resistance, NTSR mechanisms are far from being fully elucidated. A part of weed response to herbicide stress, NTSR is considered to be largely driven by gene regulation. Our purpose was to establish a transcriptome resource allowing investigation of the transcriptomic bases of NTSR in the major grass weed Alopecurus myosuroides L. (Poaceae) for which almost no genomic or transcriptomic data was available.

Results

RNA-Seq was performed from plants in one F2 population that were sensitive or expressing NTSR to herbicides inhibiting acetolactate-synthase. Cloned plants were sampled over seven time-points ranging from before until 73 h after herbicide application. Assembly of over 159M high-quality Illumina reads generated a transcriptomic resource (ALOMYbase) containing 65,558 potentially active contigs (N50 = 1240 nucleotides) predicted to encode 32,138 peptides with 74 % GO annotation, of which 2017 were assigned to protein families presumably involved in NTSR. Comparison with the fully sequenced grass genomes indicated good coverage and correct representation of A. myosuroides transcriptome in ALOMYbase. The part of the herbicide transcriptomic response common to the resistant and the sensitive plants was consistent with the expected effects of acetolactate-synthase inhibition, with striking similarities observed with published Arabidopsis thaliana data. A. myosuroides plants with NTSR were first affected by herbicide action like sensitive plants, but ultimately overcame it. Analysis of differences in transcriptomic herbicide response between resistant and sensitive plants did not allow identification of processes directly explaining NTSR. Five contigs associated to NTSR in the F2 population studied were tentatively identified. They were predicted to encode three cytochromes P450 (CYP71A, CYP71B and CYP81D), one peroxidase and one disease resistance protein.

Conclusions

Our data confirmed that gene regulation is at the root of herbicide response and of NTSR. ALOMYbase proved to be a relevant resource to support NTSR transcriptomic studies, and constitutes a valuable tool for future research aiming at elucidating gene regulations involved in NTSR in A. myosuroides.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1804-x) contains supplementary material, which is available to authorized users.

Utilizing next-generation sequencing to study homeologous polymorphisms and herbicide-resistance-endowing mutations in Poa annua acetolactate synthase genes

BACKGROUND

Detection of single nucleotide polymorphisms (SNPs) related to herbicide resistance in non-model polyploid weed species is fraught with difficulty owing to the gene duplication and lack of reference sequences. Our research seeks to overcome these obstacles by Illumina HiSeq read mapping, SNP calling and allele frequency determinations. Our focus is on the acetolactate synthase (ALS) gene, the target site of ALS-inhibiting herbicides, in Poa annua, an allotetraploid weed species originating from two diploid parents, P. supina and P. infirma.

RESULTS

ALS contigs with complete coding regions of P. supina, P. infirma and P. annua were assembled and compared with ALS genes from other plant species. The ALS infirma-homeolog of P. annua showed higher levels of nucleotide sequence variability than the supina-homeolog. Comparisons of read mappings of P. annua and a simulated P. supina × P. infirma hybrid showed high resemblance. Two homeolog-specific primer pairs were designed and used to amplify a 1860 bp region covering all resistance-conferring codons in the ALS gene. Four P. annua populations, GN, RB, GW and LG, showed high resistance to two ALS inhibitors, bispyribac-sodium and foramsulfuron, and two populations, HD and RS, showed lower resistance in the rate-response trial. Mutations conferring Trp-574-Leu substitution were observed in the infirma-homeolog of GN and RB and in the supina-homeolog of GW and LG, but no resistance-conferring mutation was observed in the two populations of lower resistance, HD and RS.

CONCLUSION

In this study we have demonstrated the use of NGS data to study homeologous polymorphisms, parentage and herbicide resistance in an allotetraploid weed species, P. annua. Complete coding sequences of the ALS gene were assembled for P. infirma, P. supina, infirma-homeolog and supina-homeolog in P. annua. A pipeline consisting of read mapping, SNP calling and allele frequency calculation was developed to study the parentage of P. annua, which provided a new perspective on this topic besides the views of morphology, karyotype and phylogeny. Our two homeolog-specific primer pairs can be utilized in future research to separate the homeologs of the ALS gene in P. annua and cover all the codons that have been reported to confer herbicide resistance.

Molecular basis of ALS- and/or ACCase-inhibitor resistance in shortawn foxtail (Alopecurus aequalis Sobol.)

Alopecurus aequalis, a predominant weed species in wheat and oilseed rape fields, can no longer be controlled by mesosulfuron-methyl application after continuous use over several years. Based on dose-response studies, the putative resistant populations, JTJY-1 and JHHZ-1, were found to be resistant to mesosulfuron-methyl, with resistance index values of 5.5 and 14, respectively. Sensitivity assays of the mesosulfuron-methyl-resistant populations to other herbicides revealed that the JTJY-1 population had moderate or high cross resistance to sulfonylureas (SUs) and triazolopyrimidines (TPs), but displayed a low level resistance to imidazolinones (IMIs). JTJY-1 also had high multi-resistance to ACCase inhibitors, but remained susceptible to photosystem II inhibitors. The JHHZ-1 population was resistant to all ALS inhibitors tested, but was sensitive to ACCase inhibitors and photosystem II inhibitors. To clarify the molecular basis of resistance in JTJY-1 and JHHZ-1 population, the ALS and ACCase gene were sequenced. Two ALS mutations (Pro-197-Thr or Trp-574-Leu) were detected in the mesosulfuron-methyl-resistant plants. The ACCase gene analysis revealed that the resistant JTJY-1 population had an Ile-1781-Leu mutation. Furthermore, the presence of two different target site resistance (TSR) mechanisms (ALS and ACCase mutations) existing simultaneously in individual A. aequalis was firstly documented in the presented study.

Generation and characterization of tribenuron-methyl herbicide-resistant rapeseed (Brasscia napus) for hybrid seed production using chemically induced male sterility

Identification and molecular analysis of four tribenuron-methyl resistant mutants in Brassica napus , which would be very useful in hybrid production using a Chemically induced male sterility system. Chemically induced male sterility (CIMS) systems dependent on chemical hybridization agents (CHAs) like tribenuron-methyl (TBM) represent an important approach for practical utilization of heterosis in rapeseed. However, when spraying the female parents with TBM to induce male sterility the male parents must be protected with a shield to avoid injury to the stamens, which would otherwise complicate the seed production protocol and increase the cost of hybrid seed production. Here we report the first proposed application of a herbicide-resistant cultivar in hybrid production, using a CIMS system based on identifying four TBM-resistant mutants in Brassica napus. Genetic analysis indicated that the TBM resistance was controlled by a single dominant nuclear gene. An in vitro enzyme activity assay for acetohydroxyacid synthase (AHAS) suggested that the herbicide resistance is caused by a gain-of-function mutation in a copy of AHAS genes. Comparative sequencing of the mutants and wild type BnaA.AHAS.a coding sequences identified a C-to-T transition at either position 535 or 536 from the translation start site, which resulted in a substitution of proline with serine or leucine at position 197 according to the Arabidopsis thaliana protein sequence. An allele-specific dCAPS marker developed from the C536T variation co-segregated with the herbicide resistance. Transgenic A. thaliana plants expressing BnaA.ahas3.a conferred herbicide resistance, which confirmed that the P197 substitution in BnaA.AHAS.a was responsible for the herbicide resistance. Moreover, the TBM-resistant lines maintain normal male fertility under TBM treatment and can be of practical value in hybrid seed production using CIMS.

A novel Pro197Glu substitution in acetolactate synthase (ALS) confers broad-spectrum resistance across ALS inhibitors

Water chickweed (Myosoton aquaticum L.), a competitive broadleaf weed, is widespread in wheat fields in China. Tribenuron and pyroxsulam failed to control water chickweed in the same field in Qiaotian Village in 2011 and 2012, respectively. An initial tribenuron resistance confirmation test identified a resistant population (AH02). ALS gene sequencing revealed a previously unreported substitution of Glu for Pro at amino acid position 197 in resistant individuals. A purified subpopulation (WRR04) that was individually homozygous for the Pro197Glu substitution was generated and characterized in terms of its response to different classes of ALS inhibitors. A whole-plant experiment showed that the WRR04 population exhibited broad-spectrum resistance to tribenuron (SU, 318-fold), pyrithiobac sodium (PTB, > 197-fold), pyroxsulam (TP, 81-fold), florasulam (TP, > 36-fold) and imazethapyr (IMI, 11-fold). An in vitro ALS assay confirmed that the ALS from WRR04 showed high resistance to all the tested ALS inhibitors. These results established that the Pro197Glu substitution endows broad-spectrum resistance across ALS inhibitors in water chickweed. In addition, molecular markers were developed to rapidly identify the Pro197Glu mutation.

Making metabolism accessible and meaningful: is the definition of a central metabolic dogma within reach?

Intermediary metabolism, a dominant research area before the emergence of molecular biology, is attracting renewed interest for fundamental and applied reasons as documented here. Nonetheless, the field may appear to be a thicket precluding entry to all but the most determined. Here we present a metabolic overview that makes this important and fascinating area accessible to a broad range of the molecular biological and biotechnological communities that are being attracted to biological problems crying out for metabolic solutions. This is accomplished by identifying seven key concepts, a so-called metabolic central dogma, that provide a core understanding analogous to the "Central Dogma of Molecular Biology" which focused upon maintenance and flow of genetic information.

Precision phenotyping of imidazolinone-induced chlorosis in sunflower

Chlorosis level is a useful parameter to assess imidazolinone resistance in sunflower (Helianthus annuus L.). The aim of this study was to quantify chlorosis through two different methods in sunflower plantlets treated with imazapyr. The genotypes used in this study were two inbred lines reported to be different in their resistance to imidazolinones. Chlorosis was evaluated by spectrophotometrical quantification of photosynthetic leaf pigments and by a bioinformatics-based color analysis. A protocol for pigment extraction was presented which improved pigment stability. Chlorophyll amount decreased significantly when both genotypes were treated with 10 μM of imazapyr. Leaf color was characterized using Tomato Analyzer(®) color test software. A significant positive correlation between color reduction and chlorophyll concentration was found. It suggests that leaf color measurement could be an accurate method to estimate chlorosis and infer chlorophyll levels in sunflower plants. These results highlight a strong relationship between imidazolinone-induced chlorosis and variations in leaf color and in chlorophyll concentration. Both methods are quantitative, rapid, simple, and reproducible. Thus, they could be useful tools for phenotyping and screening large number of plants when breeding for imidazolinone resistance in this species.

Molecular and phenotypic characterization of Als1 and Als2 mutations conferring tolerance to acetolactate synthase herbicides in soybean

BACKGROUND

Sulfonylurea (SU) herbicides are effective because they inhibit acetolactate synthase (ALS), a key enzyme in branched-chain amino acid synthesis required for plant growth. A soybean line known as W4-4 was developed through rounds of seed mutagenesis and was demonstrated to have a high degree of ALS-based resistance to both post-emergence and pre-emergence applications of a variety of SU herbicides. This report describes the molecular and phenotypic characterization of the Als1 and Als2 mutations that confer herbicide resistance to SUs and other ALS inhibitors.

RESULTS

The mutations are shown to occur in two different ALS genes that reside on different chromosomes: Als1 (P178S) on chromosome 4 and Als2 (W560L) on chromosome 6 (P197S and W574L in Arabidopsis thaliana).

CONCLUSION

Although the Als1 and Als2 genes are unlinked, the combination of these two mutations is synergistic for improved tolerance of soybeans to ALS-inhibiting herbicides.

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.

From dusk till dawn: the Arabidopsis thaliana sugar starving responsive network

Plant growth and development are tightly controlled by photosynthetic carbon availability. The understanding of mechanisms governing carbon partitioning in plants will be a valuable tool in order to satisfy the rising global demand for food and biofuel. The goal of this study was to determine if sugar starvation responses were transcriptionally coordinated in Arabidopsis thaliana. A set of sugar-starvation responsive (SSR) genes was selected to perform a co-expression network analysis. Posteriorly, a guided-gene approach was used to identify the SSR-network from public data and to discover candidate regulators of this network. In order to validate the SSR network, a global transcriptome analysis was realized on three A. thaliana starch-deficient mutants. The starch-deficient phenotype in leaves induces sugar starvation syndrome at the end of the night due to the absence of photosynthesis. Promoter sequences of genes belonging to the SSR-network were analyzed in silico reveling over-represented motifs implicated in light, abscisic acid, and sugar responses. A small cluster of protein encoding genes belonging to different metabolic pathways, including three regulatory proteins, a protein kinase, a transcription factor, and a blue light receptor, were identified as the cornerstones of the SSR co-expression network. In summary, a large transcriptionally coordinated SSR network was identified and was validated with transcriptional data from three starch-deficient mutant lines. Candidate master regulators of this network were point out.

Resistance to AHAS inhibitor herbicides: current understanding

Acetohydroxyacid synthase (AHAS) inhibitor herbicides currently comprise the largest site-of-action group (with 54 active ingredients across five chemical groups) and have been widely used in world agriculture since they were first introduced in 1982. Resistance evolution in weeds to AHAS inhibitors has been rapid and identified in populations of many weed species. Often, evolved resistance is associated with point mutations in the target AHAS gene; however non-target-site enhanced herbicide metabolism occurs as well. Many AHAS gene resistance mutations can occur and be rapidly enriched owing to a high initial resistance gene frequency, simple and dominant genetic inheritance and lack of major fitness cost of the resistance alleles. Major advances in the elucidation of the crystal structure of the AHAS (Arabidopsis thaliana) catalytic subunit in complex with various AHAS inhibitor herbicides have greatly improved current understanding of the detailed molecular interactions between AHAS, cofactors and herbicides. Compared with target-site resistance, non-target-site resistance to AHAS inhibitor herbicides is less studied and hence less understood. In a few well-studied cases, non-target-site resistance is due to enhanced rates of herbicide metabolism (metabolic resistance), mimicking that occurring in tolerant crop species and often involving cytochrome P450 monooxygenases. However, the specific herbicide-metabolising, resistance-endowing genes are yet to be identified in resistant weed species. The current state of mechanistic understanding of AHAS inhibitor herbicide resistance is reviewed, and outstanding research issues are outlined.

Involvement of threonine deaminase FgIlv1 in isoleucine biosynthesis and full virulence in Fusarium graminearum

In this study we characterized FgIlv1, a homologue of the Saccharomyces cerevisiae threonine dehydratase (TD) from the important Fusarium head blight fungus Fusarium graminearum. TD catalyzes the first step in the biosynthesis pathway of isoleucine (Ile) for conversion of threonine (Thr) to 2-ketobutyrate (2-KB). The FgILV1 deletion mutant ΔFgIlv1-3 was unable to grow on minimal medium or fructose gelatin agar which lacked Ile. Exogenous supplementation of Ile or 2-KB but not Thr rescued the mycelial growth defect of ΔFgIlv1-3, indicating the involvement of FgIlv1 in the conversion of Thr to 2-KB in Ile biosynthesis. Additionally, exogenous supplementation of Methionine (Met) could also rescue the mycelial growth defect of ΔFgIlv1-3, indicating a crosstalk between Ile biosynthesis and Met catabolism in F. graminearum. Deletion of FgILV1 also caused defects in conidial formation and germination. In addition, ΔFgIlv1-3 displayed decreased virulence on wheat heads and a low level of deoxynivalenol (DON) production in wheat kernels. Taken together, results of this study indicate that FgIlv1 is an essential component in Ile biosynthesis and is required for various cellular processes including mycelial and conidial morphogenesis, DON biosynthesis, and full virulence in F. graminearum. Our data indicate the potential of targeting Ile biosynthesis for anti-FHB management.