Two consecutive aspartic acid residues conferring herbicide resistance in tobacco acetohydroxy acid synthase

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.

Bacterial Branched-Chain Amino Acid Biosynthesis: Structures, Mechanisms, and Drugability

The eight enzymes responsible for the biosynthesis of the three branched-chain amino acids (l-isoleucine, l-leucine, and l-valine) were identified decades ago using classical genetic approaches based on amino acid auxotrophy. This review will highlight the recent progress in the determination of the three-dimensional structures of these enzymes, their chemical mechanisms, and insights into their suitability as targets for the development of antibacterial agents. Given the enormous rise in bacterial drug resistance to every major class of antibacterial compound, there is a clear and present need for the identification of new antibacterial compounds with nonoverlapping targets to currently used antibacterials that target cell wall, protein, mRNA, and DNA synthesis.

Somatic cell selection for chlorsulfuron-resistant mutants in potato: identification of point mutations in the acetohydroxyacid synthase gene

Background

Somatic cell selection in plants allows the recovery of spontaneous mutants from cell cultures. When coupled with the regeneration of plants it allows an effective approach for the recovery of novel traits in plants. This study undertook somatic cell selection in the potato (Solanum tuberosum L.) cultivar ‘Iwa’ using the sulfonylurea herbicide, chlorsulfuron, as a positive selection agent.

Results

Following 5 days’ exposure of potato cell suspension cultures to 20 μg/l chlorsulfuron, rescue selection recovered rare potato cell colonies at a frequency of approximately one event in 2.7 × 10⁵ of plated cells. Plants that were regenerated from these cell colonies retained resistance to chlorsulfuron and two variants were confirmed to have different independent point mutations in the acetohydroxyacid synthase (AHAS) gene. One point mutation involved a transition of cytosine for thymine, which substituted the equivalent of Pro-197 to Ser-197 in the AHAS enzyme. The second point mutation involved a transversion of thymine to adenine, changing the equivalent of Trp-574 to Arg-574. The two independent point mutations recovered were assembled into a chimeric gene and binary vector for Agrobacterium-mediated transformation of wild-type ‘Iwa’ potato. This confirmed that the mutations in the AHAS gene conferred chlorsulfuron resistance in the resulting transgenic plants.

Conclusions

Somatic cell selection in potato using the sulfonylurea herbicide, chlorsulfuron, recovered resistant variants attributed to mutational events in the AHAS gene. The mutant AHAS genes recovered are therefore good candidates as selectable marker genes for intragenic transformation of potato.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0371-4) contains supplementary material, which is available to authorized users.

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.

Characterization of sulfonylurea-resistant Schoenoplectus juncoides having a target-site Asp(376)Glu mutation in the acetolactate synthase

Schoenoplectus juncoides, a noxious weed for paddy rice, is known to become resistant to sulfonylurea (SU) herbicides by a target-site mutation in either of the two acetolactate synthase (ALS) genes (ALS1 and ALS2). SU-resistant S. juncoides plants having an Asp376Glu mutation in ALS2 were found from a paddy rice field in Japan, but their resistance profile has not been quantitatively investigated. In this study, dose-response of the SU-resistant accession was compared with that of a SU-susceptible accession at in vivo whole-plant level as well as at in vitro enzymatic level. In whole-plant tests, resistance factors (RFs) based on 50% growth reduction (GR50) for imazosulfuron (ISF), bensulfuron-methyl (BSM), metsulfuron-methyl (MSM), bispyribac-sodium (BPS), and imazaquin (IMQ) were 176, 40, 14, 5.2 and 1.5, respectively. Thus, the accession having an Asp376Glu mutation in ALS2 was highly resistant to the three SU herbicides and moderately resistant to BPS, but was not substantially resistant to IMQ. This is slightly different from the earlier results reported from other weeds with an Asp376Glu mutation, in which the mutation confers resistance to broadly all the chemical classes of ALS-inhibiting herbicides. In enzymatic tests, ALS2 of S. juncoides was expressed in E. coli; the resultant ALS2 was subjected to an in vitro assay. RFs of the mutated ALS2 based on 50% enzymatic inhibition (I50) for ISF, BSM, MSM, BPS, and IMQ were 3699, 2438, 322, 80, and 4.8, respectively. The RFs of ALS2 were highly correlated with those of the whole-plant; this suggests that the Asp376Glu mutation in ALS2 is a molecular basis for the whole-plant resistance. The presence of two ALS genes in S. juncoides can at least partially explain why the whole-plant RFs were less than those of the expressed ALS2 enzymes.

Diversity of plant methionine sulfoxide reductases B and evolution of a form specific for free methionine sulfoxide

Methionine can be reversibly oxidized to methionine sulfoxide (MetO) under physiological conditions. Organisms evolved two distinct methionine sulfoxide reductase families (MSRA & MSRB) to repair oxidized methionine residues. We found that 5 MSRB genes exist in the soybean genome, including GmMSRB1 and two segmentally duplicated gene pairs (GmMSRB2 and GmMSRB5, GmMSRB3 and GmMSRB4). GmMSRB2 and GmMSRB4 proteins showed MSRB activity toward protein-based MetO with either DTT or thioredoxin (TRX) as reductants, whereas GmMSRB1 was active only with DTT. GmMSRB2 had a typical MSRB mechanism with Cys121 and Cys 68 as catalytic and resolving residues, respectively. Surprisingly, this enzyme also possessed the MSRB activity toward free Met-R-O with kinetic parameters similar to those reported for fRMSR from Escherichia coli, an enzyme specific for free Met-R-O. Overexpression of GmMSRB2 or GmMSRB4 in the yeast cytosol supported the growth of the triple MSRA/MSRB/fRMSR (Δ3MSRs) mutant on MetO and protected cells against H2O2-induced stress. Taken together, our data reveal an unexpected diversity of MSRBs in plants and indicate that, in contrast to mammals that cannot reduce free Met-R-O and microorganisms that use fRMSR for this purpose, plants evolved MSRBs for the reduction of both free and protein-based MetO.

Structural and functional evaluation of three well-conserved serine residues in tobacco acetohydroxyacid synthase

The first step in the common pathway for the biosynthesis of branched-chain amino acids (BCAAs) is catalyzed by acetohydroxyacid synthase (AHAS). The roles of three well-conserved serine residues (S167, S506, and S539) in tobacco AHAS were determined using site-directed mutagenesis. The mutations S167F and S506F were found to be inactive and abolished the binding affinity for cofactor FAD. The Far-UV CD spectrum of the inactive mutants was similar to that of wild-type enzyme, indicating no major conformational changes in the secondary structure. However, the active mutants, S167R, S506A, S506R, S539A, S539F and S539R, showed lower specific activities. Further, a homology model of tobacco AHAS was generated based on the crystal structure of yeast AHAS. In the model, the S167 and S506 residues were identified near the FAD binding site, while the S539 residue was found to near the ThDP binding site. The S539 mutants, S539A and S539R, showed strong resistance to three classes of herbicides, NC-311 (a sulfonylurea), Cadre (an imidazolinone), and TP (a triazolopyrimidine). In contrast, the active S167 and S506 mutants did not show any significant resistance to the herbicides, with the exception of S506R, which showed strong resistance to all herbicides. Thus, our results suggest that the S167 and S506 residues are essential for catalytic activity by playing a role in the FAD binding site. The S539 residue was found to be near the ThDP with an essential role in the catalytic activity and specific mutants of this residue (S539A and S539R) showed strong herbicide resistance as well.

Structure and mechanism of inhibition of plant acetohydroxyacid synthase

Plants and microorganisms synthesize valine, leucine and isoleucine via a common pathway in which the first reaction is catalysed by acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This enzyme is of substantial importance because it is the target of several herbicides, including all members of the popular sulfonylurea and imidazolinone families. However, the emergence of resistant weeds due to mutations that interfere with the inhibition of AHAS is now a worldwide problem. Here we summarize recent ideas on the way in which these herbicides inhibit the enzyme, based on the 3D structure of Arabidopsis thaliana AHAS. This structure also reveals important clues for understanding how various mutations can lead to herbicide resistance.

Herbicide-binding sites revealed in the structure of plant acetohydroxyacid synthase

The sulfonylureas and imidazolinones are potent commercial herbicide families. They are among the most popular choices for farmers worldwide, because they are nontoxic to animals and highly selective. These herbicides inhibit branched-chain amino acid biosynthesis in plants by targeting acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This report describes the 3D structure of Arabidopsis thaliana AHAS in complex with five sulfonylureas (to 2.5 A resolution) and with the imidazolinone, imazaquin (IQ; 2.8 A). Neither class of molecule has a structure that mimics the substrates for the enzyme, but both inhibit by blocking a channel through which access to the active site is gained. The sulfonylureas approach within 5 A of the catalytic center, which is the C2 atom of the cofactor thiamin diphosphate, whereas IQ is at least 7 A from this atom. Ten of the amino acid residues that bind the sulfonylureas also bind IQ. Six additional residues interact only with the sulfonylureas, whereas there are two residues that bind IQ but not the sulfonylureas. Thus, the two classes of inhibitor occupy partially overlapping sites but adopt different modes of binding. The increasing emergence of resistant weeds due to the appearance of mutations that interfere with the inhibition of AHAS is now a worldwide problem. The structures described here provide a rational molecular basis for understanding these mutations, thus allowing more sophisticated AHAS inhibitors to be developed. There is no previously described structure for any plant protein in complex with a commercial herbicide.