EPSPS regulates cell elongation by disrupting the balance of lignin and flavonoid biosynthesis in cotton

EPSPS is a key gene in the shikimic acid synthesis pathway that has been widely used in breeding crops with herbicide resistance.However,its role in regulating cell elongation is poorly understood.Through the overexpression of EPSPS genes,we generated lines resistant to glyphosate that exhibit an unexpected dwarf phenotype.A representative line,DHR1,exhibits a stable dwarf phenotype throughout its entire growth period.Except for plant height,the other agronomic traits of DHR1 are similar to its transgenic explants ZM24.Paraffin section observations showed that DHR1 internodes are shortened due to reduced elongation and division of the internode cells.Exogenous hormones confirmed that DHR1 is not a classical brassinolide(BR)-or gibberellin(GA)-related dwarfing mutant.Hybridization analysis and fine mapping confirmed that the EPSPS gene is the causal gene for dwarfism,and the phenotype can be inherited in different genotypes.Transcriptome and metabolome analyses showed that genes associated with the phenylpropanoid synthesis pathway are enriched in DHR1 compared with ZM24.Flavonoid metabolites are enriched in DHR1,whereas lignin metabolites are reduced.The enhancement of flavonoids likely results in differential expression of auxin signal pathway genes and alters the auxin response,subsequently affecting cell elongation.This study provides a new strategy for generating dwarfs and will accelerate advancements in light simplification in the cultivation and mechanized harvesting of cotton.

Inheritance and molecular characterization of resistance to AHAS- inhibiting herbicides in rapeseed

Rapeseed is a very important oil crop in China; however, its production is challenging due to the absence of effective weed management strategies. This is predominantly because of a shortage of herbicide resistance genes. Acetohydroxyacid synthase （AHAS） herbicides inhibit AHAS, a key enzyme involved in branched-chain amino acid synthesis that is required for plant growth. A rapeseed line designated M342 with AHAS herbicide resistance was developed through seed muta- genesis and was studied to assess the level and mode of inheritance of the resistance and to identify the molecular basis of resistance. M342 possessed a high level of cross-resistance to sulfonylureas （SUs） and imidazolinones （IMIs）. This resistance was due to AHAS insensitivity to these herbicides and was inherited as a dominant trait conferred by a single nuclear-encoded gene. Molecular analysis revealed the presence of a Trp574Leu mutation in M342, and an allele-specific cleaved amplified polymorphic sequence （AS-CAPS） marker was developed and cosegregated with herbicide resistance in the F2, BC1, and BC2 populations. This mutation altered the transcript levels of BnAHAS1 and BnAHAS3 in M342 compared with those in the wild type, but it did not affect the agronomic or quality traits. The simple genetic inheritance of this mutation and the availability of the cleaved amplified polymorphic sequence （CAPS） marker and herbicide resistance gene should facilitate the development of herbicide-resistant rapeseed cultivars for effective weed control in China.

Recent Advances in Development of Herbicide Resistant Transgenic Hybrid Rice in China

In addition to weed control in direct seeding field of hybrid rice, herbicide resistance genes were used by Chinese scientists to increase and identify the purity of hybrid seeds, and to realize the mechanization of hybrid seed production. The elite restorer lines, such as Minghui 63, R752, T461, R402, D68 and E32 were transformed directly with herbicide resistance genes, in which D68 and E32 are restorer lines of two-line system and the others are of three-line system. Because almost all of important restorer lines are indica varieties and are recalcitrant in transformation, many herbicide resistant near-isogenic restorer lines were developed by sexual hybridization of indica and japonica varieties and backcross with indica restorer lines later, such as Ce 64, Minghui 63, Teqing, Milyang 46, R402 and 9311, in which 9311 is a restorer line of two-line system. The elite photoperiod-sensitive/thermo-sensitive genic male sterile lines, such as Pei＇ai 64S, P88S, 4008S and 7001S, were transformed with herbicide resistance genes. A few herbicide resistant male sterile lines were developed through sexual hybridization and subsequently systemic selection, such as Bar1259S, Bar2172S, 05Z221A and 05Z227A. With the employment of herbicide resistant male sterile lines or herbicide resistant restorer lines, a few herbicide resistant hybrid rice combinations were developed, such as Xiang 125S/Bar 68-1 and Pei＇ai 64S/Bar 9311. Based on herbicide resistance, the research was marching on to investigate the parental lines of hybrid rice with insect resistance, drought tolerance, etc.

Scarabaeid Larvae- and Herbicide-Resistant Transgenic Perennial Ryegrass (Lolium perenne L.) Obtained by Agrobacterium tumefaciens-Mediated Transformation of cry8Ca2, cry8Ga and bar Genes

Insect pest and weeds are two major problems for forage and turf grasses. In this study, scarab larvae- and herbicide-resistant transgenic perennial ryegrass （Lolium perenne L.） was obtained by transforming it with cry and bar genes simultaneously via the Agrobacterium-mediated method. To optimize the callus induction and plant regeneration conditions, various concentrations of 2,4-dichlorophenoxyacetic acid and 6-benzylaminopurine were assayed. The transformation efficiencies of different Agrobacterium suspension media, used during Agrobacterium-mediated transformation, were compared. Then, plasmids of pCAMBIA3301 containing cry gene （cry8Ca2 or cry8Ga） and bar gene, driven by ubiquitin promoter, were transformed into perennial ryegrass. The transformants were generated and confirmed by both Southern hybridization analysis and Western hybridization analysis. Further, the resistance of transgenic perennial ryegrass plants to scarab larvae and herbicide were analyzed. After 30 d of co-cultivation with scarab larvae, the damage to the root system of transgenic plants was less than that of non-transgenic control plants. Additionally, the leaves of transgenic plants were resistant to Basta, while leaves of the wild plants wilted after Basta spraying. These results show that cry gene and bar gene were successfully transferred into perennial ryegrass by the Agrobactgerium-mediated method, and convey resistance to scarab larvae and herbicide in transgenic perennial ryegrass plants.

Control of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.) with Isoxaflutole and Metribuzin Tankmix

Five field trials were conducted over a two-year period (2013, 2014) to determine the control of glyphosate-resistant (GR) giant ragweed with isoxaflutole (IFT) and metribuzin (MTZ) applied alone and in combination. Treatments were designed to assess the dose response of an IFT plus MTZ tank-mix as well as each chemical applied alone to classify the response using Flint’s adaptation of Colby’s equation. Two factor factorial experiments were performed in the growth room to ascertain the response of IFT versus glyphosate, IFT versus MTZ, and IFT plus MTZ versus glyphosate on single plants. Field experiments evaluated the control of GR giant ragweed with IFT plus MTZ in tank-mix in a 1:4 ratio. The rate of IFT plus MTZ for 80% control of GR giant ragweed at 4 and 8 weeks after application (WAA) was 518 (104 g a.i. ha-1 IFT + 414 g a.i. ha-1 MTZ) and 631 g a.i. ha-1 (126 g a.i. ha-1 IFT + 505 g a.i. ha-1 MTZ), respectively. A rate of 668 and 467 g a.i. ha-1 was required to reduce GR giant ragweed density and biomass by 80%, respectively. Field experiments evaluating the control of GR giant ragweed with tank-mixes of IFT plus MTZ, where glyphosate was a constant tank-mix partner, were mostly synergistic. However, the low tank-mix rate (52.5 + 210 g a.i. ha-1) had an additive response for GR giant ragweed biomass reduction. When tested in the greenhouse and growth room, glyphosate susceptible (GS) giant ragweed showed some antagonism with glyphosate and isoxaflutole tank-mixes at rates less than commercial field rates. GR giant ragweed showed an additive response across all treatments in the growth room. Greenhouse experiments evaluating IFT versus MTZ and IFT plus MTZ versus glyphosate revealed all tank-mix treatments to be synergistic at 2 WAA.

A new method to identify and improve the purity of hybrid rice with herbicide resistant gene

There is a close relationship between the hy—brid rice production and seed purity.Two-linehybrid rice with higher heterosis is producedthrough the hybridization between a photo-thermo sensitive genetic male sterile(GMS) rice line and a paternal variety.But the fertili-

Detection of Acetyl-CoA Carboxylase （ACCase） Inhibitor Herbicides Resistance in Sterile Wild Oat （Avena sterilis L.） Using Agar Quick Test

This study was conducted using seeds in Petri dish containing agar medium in order to determine acetyl-CoA carboxylase （ACCase） herbicides resistance （R） in Avena sterilis that was grown in wheat fields at Adana province, Turkey. Seeds were collected from one large suspected field, where clodinafop-propargyl （Aryloxyphenoxypropionate “FOPs”） and pinoxaden （Phenylpyrazoline “DEN”） have been applied for many years. Susceptible （S） population was collected from the road side on the same region. Agar media of concentration 14 g/L was prepared and it was melted in microwave. Then the amount of 20 mL agar media was added into each Petri dish. Five seeds were placed on agar mediums containing discriminating dose of clodinafop and pinoxaden. Petri dishes were placed in growth incubator operating at 10 °C. After 15 d, both radicle and hypocotyl length were measured. The percentage of germinated seed and dose-response curves were determined. At these different concentration levels, there were more than 50% of R and less than 40% of S seed germinated for pinoxaden. However, for clodinafop, more than 60% of R and less than 50% of S seeds were germinated. At higher concentration levels, the populations of resistant and susceptible were not germinated for both herbicides. The resistance value of R population was then compared with that of the S biotype. From the resistance index （RI）, the population was more resistant to pinoxaden （7.43 for radicle and 2.47 for hypocotyl） than the clodinafop-propagyl （1.39 for radicle and 3.77 for hypocotyl）. The method provided a simple, quick and cost effective way to identify ACCase herbicides resistance in most grass weeds.

Allelopathic Potential of Wheat on Sourgrass Resistant to Glyphosate

We aimed with this study to assay the allelopathic potential of wheat genotypes in inhibiting germination and initial growth of sourgrass (Digitaria insularis) resistant to the herbicide glyphosate. Two experiments were installed under greenhouse conditions: the first one aimed to assay the potential of wheat genotypes in inhibiting emergence and initial growth of two test plants, and the second aimed to evaluate the potential of those wheat genotypes which performed better at the first experiment, inhibiting the emergence and initial growth of sourgrass biotypes resistant to the herbicide glyphosate. Five doses of aqueous cool-extracted extract from 32 wheat genotypes were applied to tomato and cucumber (Experiment 1) aiming to identify the ones with higher inhibitory effect. The five most inhibitory wheat genotypes were selected for Experiment 2, where the same doses of extract were applied on sourgrass. For both experiments, shoot fresh and dry mass, as well as water content, were assessed. Sourgrass germination is affected by wheat extracts, but initial seedling growth seems not to be affected;wheat genotypes differ in terms of their allelopathic potential in inhibiting sourgrass germination;wheat presents little inhibitory effect on sourgrass.

Pyramiding of multiple genes generates rapeseed introgression lines with clubroot and herbicide resistance, high oleic acid content, and early maturity

Clubroot and herbicide resistance,high oleic acid(OA)content,and early maturity are targets of rapeseed(Brassica napus L.)breeding.The objective of this study was to develop new male-fertility restorer lines by pyramiding favorable genes to improve these traits simultaneously.Seven elite alleles for the four traits were introduced into the restorer line 621R by speed breeding with marker-assisted and phenotypic selection.Six introgression lines(ILs)were developed with four-to seven-gene combinations and crossed with two elite parents to develop hybrids.All ILs and their corresponding hybrids displayed high resistance to both clubroot pathotype 4 and sulfonylurea herbicides.Three ILs and their hybrids showed large increases in OA contents and four showed earlier maturity.These new ILs may be useful in rapeseed hybrid breeding for the target traits.

Construction of Maize Universal Expression Vector PGM-35Sbar and Maize Transformation

[Objective] The paper was to construct maize universal expression vector, in order to provide basis for using transgenic methods to improve abiotic stress tolerance of maize. [Method] Based on the transformation of existing pGreen0229 plant expression vector, phosphinothricin-resistant selectable marker-bar gene driving by CaMv35S promoter was constructed, which could be used to connect target gene of maize expression vector PGM-35Sbar, and transform Ji444 maize inbred lines by pollen tube pathway. [Result] The universal expression vector for PGM-35Sbar maize had been successfully constructed. When the maize plants were transformed, 14 resistant plants were obtained, and 12 plants were identified to be positive plants by PCR detection. [Conclusion] The study provided basis for rapid construction of maize expression vector containing specific target gene.

Development of herbicide resistance genes and their application in rice

Rice is one of the most important food crops in the world.Weeds seriously affect the rice yield and grain quality.In recent years,there are tremendous progresses in the research and application of herbicideresistant genes in rice worldwide.This article reviews the working mechanisms of six herbicides(glyphosate,glufosinate,acetolactate synthase inhibitor herbicides,acetyl-Co A carboxylase inhibitor herbicides,hydroxyhenylpyruvate dioxygenase(HPPD)inhibitor herbicides and dinitroaniline herbicides),the resistance mutations of the corresponding herbicide-target genes,and the herbicide detoxification mechanisms by non-target genes.Examples are provided on herbicide-resistant rice materials obtained by transformation of exogenous resistance genes,by artificial mutagenesis and mutant screening,and by modifying the target genes through gene editing.This paper also introduces the current application of herbicide-resistant rice,points out problems that may be caused by utilization of herbicide resistant rice and solutions to the problems,and discusses the future prospects for the development of herbicideresistant rice.

Agrobacterium-mediated transformation of herbicide resistance in creeping bentgrass and colonial bentgrass

Embryogenic calli were induced from the seeds of creeping bentgrass ( Agrostis palustris Huds.) cv. Regent and colonial bentgrass ( Agrostis Tenuis Sibth. Fl. Oxen.) cv. Tiger. The embryogenic calli were precultured on fresh medium for 4-7 days and then co cultivated with Agrobacterium tumefaciens , LBA4404, which contains plasmid vector pSBGM harboring bar coding region, synthetic green fluorescent protein (sGFP) coding region and matrix attachment region (MAR). After 3 days of co cultivation, the calli were washed thoroughly and transferred to MS medium containing 2 mg/L of 2, 4 D, 12-15 mg/L phosphinothricin (PPT) and 250 mg/L of cefotaxime. After 2-3 months of selection, the actively growing calli of 'Regent' and 'Tiger' were transferred to MS medium with 12-15 mg/L PPT and 250 mg/L cefotaxime for regeneration. The putative transformants were maintained on MS medium with 3 mg/L PPT for long period but control died within 1 month. After establishing in greenhouse, the transformants also showed strong resistance to 0.4% of herbicide Basta but control plants died within 2 weeks. Under confocal microscope, both young leaves and roots showed significant GFP expression. PCR analysis revealed the presence of a DNA fragment of GFP gene at the expected size (380 bp) in the transformants and its absence in a randomly selected control plant.

Transgenic rice expressing a novel phytase-lactoferricin fusion gene to improve phosphorus availability and antibacterial activity

The developing trends of livestock production are efficiency availability of phytate phosphorus and abuse of antibiotics. safety and sustainability, which face two major challenges： low As a solution phytases and antimicrobial peptides are applied as feed additives. However, phytases and antimicrobial peptides are susceptible to proteases, costly by fermentation and potential toxic to production hosts. We transformed an optimized phytase-lactoferricin fusion gene PhyLfdriven by an en- dosperm-specific promoter Gt13aP and Bar （bialaphos resistance） gene as a selection maker into rice. The Bar and PhyLf genes were integrated into the rice genome, stably inherited and expressed. Their phosphinothricin acetyl transferase （PAT） protein content oftransgenic plants with glufosinate resistance varied between 50.45-93.39 IJg g-l. Fusion protein expressed especially in the seeds of transgenic rice had a summit phytase activity at 32.30 U g-l, which increased by 61.71-fold com- pared to the control/check group （CK） and 7.54-fold compared to un-optimized transgenic plant. The highest inorganic phosphorus （Pi） content of the transgenic seeds reached 13.15 mg g-~, increased by 12.77-fold compared to that of CK. Preliminary antibacterial experiments showed that the enterokinase hydrolysate product of fusion protein could inhibit the growth of Escherichia coil DH5a. These results indicated that the protein PhyLf has the potential to increase availability of feed phytate phosphorus, improve consumer＇s immunity and reduce the use of antibiotics.

Functional and numerical responses of Cyrtorhinus lividipennis to eggs of Nilaparvata lugens are not affected by genetically modified herbicide-tolerant rice

To safely and sustainably utilize genetic breeding techniques for crop production, greater understanding of the potential effects of genetically modified herbicide-tolerant（GMHT） crops on the ecological functions of predators is required. In the laboratory, we examined the functional and numerical responses of Cyrtorhinus lividipennis Reuter to eggs of brown planthopper（BPH）, Nilaparvata lugens（St？l）, which were reared on GMHT rice Bar68-1; the untransformed parental cultivar, D68; or a BPH-susceptive rice variety, Taichung Native 1. All stages of nymphs and female adult of C. lividipennis, either on GMHT rice or control plants, exhibited typical type II functional responses when fed on BPH eggs; the attacking rate and handling time of C. lividipennis on GMHT rice Bar68-1 was not significantly different from that on D68. The numerical responses of C. lividipennis on GMHT rice or controls fit Beddington＇s model; there were no significant differences in the parameters of numerical responses between GMHT rice Bar68-1 and D68. The results indicated that the functional and numerical responses of C. lividipennis to BPH eggs are not affected by GMHT rice Bar68-1.

SYBR~ Green qPCR Screening Methods for Detection of Anti-herbicide Genes in Genetically Modified Processed Products

The use of genetically modified organisms （GMOs） as food products becomes more and more widespread. The European Union has implemented a set of very strict procedures for the approval to grow, import and/or utilize GMOs as food or food ingredients. Thus, analytical methods for detection of GMOs are necessary in order to verify compliance with labelling requirements. There are few effective screening methods for processed GM （genetically modified） products. Three anti-herbicide genes （CP4- EPSPS, BAR and PAT） are common exogenous genes used in commercialized transgenic soybean, maize and rice, In the present study, a new SYBR Green qPCR screening method was developed to simultaneously detect the three exogenous anti-herbicide genes and one endogenous gene in a run. We tested seven samples of representative processed products （soya lecithin, soya protein powder, chocolate beverage, infant rice cereal, maize protein powder, maize starch, and maize jam） using the developed method, and amplicons of endogenous gene and transgenic fragments were obtained from all the processed products, and the sensitivity was 0.1%. These results indicated that SYBR Green qPCR screening method was appropriate for qualitative detection of transgenic soybean, maize and rice in processed products.

Precise base editing of non-allelic acetolactate synthase genes confers sulfonylurea herbicide resistance in maize

Single-nucleotide polymorphisms contribute to phenotypic diversity in maize. Creation and functional annotation of point mutations has been limited by the low efficiency of conventional methods based on random mutation. An efficient tool for generating targeted single-base mutations is desirable for both functional genomics and precise genetic improvement. The objective of this study was to test the efficiency of targeted C-to-T base editing of two non-allelic acetolactate synthase(ALS) in generating sulfonylurea herbicide-resistant mutants. A CRISPR/Cas9 nickase-cytidine deaminase fused with uracil DNA glycosylase inhibitor(UGI) was employed to achieve targeted conversion of cytosine to thymine in ZmALS1 and ZmALS2. Both protoplasts and recovered mutant plants showed the activity of the cytosine base editor, with an in vivo efficiency of up to 13.8%. Transgene-free edited plants harboring a homozygous ZmALS1 mutation or a ZmALS1 and ZmALS2 double mutation were tested for their resistance at a dose of up to 15-fold the recommended limit of chlorsulfuron, a sulfonylurea herbicide widely used in agriculture. Targeted base editing of C-to-T per se and a phenotype verified in the generated mutants demonstrates the power of base editing in precise maize breeding.

The EPSPS Pro106Ser substitution solely accounts for glyphosate resistance in a goosegrass(Eleusine indica) population from Tennessee, United States

Previous studies have documented the occurrence of glyphosate-resistant （GR） goosegrass （Eleusine indica （L.） Gaertn.） and, in at least some cases, resistance is due to an altered target site. Research was performed to determine if an altered target site was responsible for GR in a Tennessee, United States goosegrass population （TennGR）. DNA sequencing revealed a mutation in TennGR plants conferring the Prol06Ser 5-enolpyruvylshikimate-3-phosphate synthase （EPSPS） substitution previously identified in other GR populations. F2 populations were derived from TennGR plants crossed with plants from a glyphosate-susceptible population （TennGS） and analyzed for their response to glyphosate and genotyped at the EPSPS locus. Plants from the F2 populations segregated 1：2：1 sensitive：intermediate：resistant in response to a selec- tive dose of glyphosate, and these responses co-segregated with the EPSPS genotypes （PP106, PS106, and SS106）. To separately investigate the effect of the Prol06Ser substitution on GR, glyphosate dose-response curves and 50% effective dose （EDso） values were compared among the three genotypes and the two parental populations. The SS106 genotype was 3.4-fold resistant relative to the PP106 genotype, identical to the resistance level obtained when comparing the resistant and susceptible parental populations. We conclude that the mutation conferring a Prol06Ser EPSPS mutation is solely responsible for GR in the TennGR goosegrass population.

Alleles contributing to acetyl coenzyme A carboxylase(ACCase)resistance in keng stiffgrass(Pseudosclerochloa kengiana)populations from China

Keng stiffgrass is a grass weed that affects wheat-rice cropping systems in China. The extensive reliance on acetyl coen- zyme A carboxylase （ACCase）-inhibiting herbicides has resulted in keng stiffgrass developing resistance to these herbicides. The objective of this research was to evaluate the resistance level of the putative resistant keng stiffgrass populations to ACCase-inhibiting herbicides and to identify their molecular resistance mechanism. Whole-plant dose-response experi- ments demonstrated that SD-4 （R）, SD-11 （R）, and JS-25 （R） populations were highly resistant to fenoxaprop, clodinafop, and fluazifop, moderately resistant to diclofop, had low resistance to sethoxydim and pinoxaden, but were sensitive to clethodim. Partial chloroplasticACCase sequences showed that there were two copies ofACCase gene in keng stiffgrass, and all homoeologous genes were expressed. The results of sequence analyses of the ACCase CT domain revealed an isoleucine-to-asparagine substitution at position 2041 in SD-4 （R） and SD-11 （R） populations, and a tryptophan-to-cysteine substitution at position 2027 in the JS-25 （R） population. To our knowledge, this is the first report of Ile-2041-Asn and Trp- 2027-Cys mutations in ACCase-resistant keng stiffgrass. In addition, three robust （derived） cleaved amplified polymorphic sequence （（d）CAPS） markers have been developed to rapidly identify these mutations in the ACCase gene of keng stiffgrass.

Genetics, Development, and Application of Cytoplasmic Herbicide Resistance in Foxtail Millet

The effect of cytoplasmic herbicide resistant gene in millet plants was studied. The heterozygous populations and isogenic lines with homocaryotic alloplasmic genes were obtained by crossing and reciprocal crossing of cytoplasmic herbicide resistant plants with susceptive plants of foxtail millet. The characters of F1, F2, backcross and composite cross groups, and the growth and development of isogenic lines were compared. The cytoplasmic herbicide resistant gene slowed the development of seedling, delayed heading, and shortened the milking stage in the foxtail millet plant. Yield capacity and main agronomic characters were all affected by the cytoplasmic herbicide resistant gene in most of the backcross, composite cross, and F2 populations. However, there was stronger hybrid vigor in F1. The backcrosses, composite crosses, and F2 populations were widely separated and some of them had good characters similar to those of susceptive groups. The plant characters and development of foxtail millet were negatively affected by the cytoplasmic herbicide resistant gene. The authors proposed a method of using hybrid vigor to obtain high yield and avoid the negative effects of herbicide resistance cytoplasm in plant growth. The expected results could be obtained by selecting individuals in separate populations of fast developed seedlings, well-developed roots, and with capacities of early heading and fast milking. Guided by the principal mentioned above, many high yield lines and hybrid crosses of foxtail millet with herbicide resistant cytoplasm were obtained.

Analysis of a Critical Residue Determining Herbicide Efficiency Sensitivity in Carboxyltransferase Domain of Acetyl-CoA Carboxylase from Poaceae by Homology Modeling and Free Energy Simulation

Carboxyltransferase domain（CT） of acetyl-coenzyme A carboxylase（ACCase, EC 6.4.1.2） from a family of Poaceae is an important target of commercial herbicide APPs for controlling grass weed growth. As the abuse of APPs herbicides, the resistant ACCase due to the mutation of a single residue（Ile→Leu）, which is located in CT active site, is emergent in many populations and species of Poaceae. So it is urgent to understand the resistant mechanism so as to design new effect herbicides. Herein lies the complex of CT dimmer from Lolium rigidum and herbicide haloxyfop successfully constructed for wild type enzyme and Ile/Leu mutant, respectively, providing a basis for explaining the resistance from microscopic structure. Moreover, the binding free energy difference between wild type and mutant enzymes was predicted in good agreement with the known observation, and the various contributions to it were analyzed, by Molecular mechanics-Poisson-Boltzmann surface area（MM-PBSA） method. The results indicate the van der Waals interaction difference between the protein and inhibitor, -22.94 kJ/mol of CT wild type lower than that of mutant, is the major reason for resistance. Structure analysis further suggests that van der Waals interaction difference is originated from the steric hindrance between the side chain of mutated residue Leu and the chiral methyl group of haloxyfop. All these findings enhance the understanding of resistant mechanism of ACCase to herbicide by Ile/Leu mutation and provide an important clue for the rational design of high effective herbicides.