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#herbicide tolerance

3 public questions tagged with this topic.

Transgenic Plant -1

Sulfonyl urea resistance is due to mutation in:

Acetolactate synthase also called acetohydroxyacid synthase catalyzes first common step of branched-chain amino acid biosynthesis: condensation of two pyruvate molecules to acetolactate leading to valine and leucine, and condensation of pyruvate and 2-ketobutyrate to acetohydroxybutyrate leading to isoleucine. Sulfonylurea herbicides such as chlorsulfuron and tribenuron bind in substrate access channel of ALS, blocking access to catalytic site containing thiamine pyrophosphate. Resistance evolves via non-synonymous mutations in ALS gene altering herbicide binding pocket without abolishing catalytic residues. Classic mutations include Pro197 to Ser, Leu, Arg, Trp and Trp574 to Leu conferring cross-resistance to multiple ALS inhibitor classes including imidazolinones. Engineered crops express mutated ALS from tobacco or Arabidopsis as selectable marker and agronomic trait. Mechanism exemplifies target-site resistance where single amino acid substitution reduces herbicide affinity, preserving amino acid synthesis under otherwise lethal herbicide doses applied for weed management. Molecular basis of sulfonylurea resistance studied via heterologous expression of mutated ALS in Escherichia coli showing retains catalytic activity for acetolactate synthesis but 1000-fold reduced herbicide binding. Residue Pro197 located in herbicide binding tunnel near catalytic center, mutation to Ser enlarges pocket reducing interaction. Commercial traits like Clearfield wheat developed through EMS mutagenesis selecting Pro197 mutants. Understanding ALS mutation mechanism

Ref: LaRossa Trends Biotech 1984 ALS mutation; Tranel Weed Sci 2002 ALS Pro197; NCBI NBK21601; PubMed 14769484 sulfonylurea resistance mechanism review.

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Transgenic Plant -1

Bialaphos resistance in plants is conferred by:

Bialaphos tripeptide herbicide consists of phosphinothricin plus two alanine residues. In plant tissue peptidases release active phosphinothricin, structural analog of glutamate that inhibits glutamine synthetase essential for assimilation of ammonia into glutamine. Inhibition causes ammonia accumulation, photosynthesis disruption, and rapid cell death. Bacterium Streptomyces hygroscopicus produces bialaphos and protects itself via bar gene encoding phosphinothricin acetyltransferase. Enzyme transfers acetyl group from acetyl-CoA to free amino group of phosphinothricin, producing N-acetyl phosphinothricin unable to bind glutamine synthetase active site. Expression of bar under control of CaMV 35S promoter in transformed crops confers detoxification mediated resistance to glufosinate, ammonium salt of phosphinothricin used as Liberty herbicide. Method offers alternative weed management option, widely used in transgenic canola, corn, cotton, and as selectable marker in transformation laboratories thanks to efficient selection of resistant callus on phosphinothricin containing medium. Formulation of glufosinate herbicide marketed as Liberty 200 g per liter glufosinate ammonium provides broadleaf weed control in tolerant crops. Resistance mechanism enzymatic acetylation prevents binding to glutamine synthetase active site, maintaining ammonia assimilation via GS/GOGAT cycle. Transgenic plants accumulate N-acetyl glufosinate sequestered in vacuole without toxic effects. Gene bar widely used as selectable marker in transformation due to low escape rate and

Ref: Murakami Mol Gen Genet 1986 bar Streptomyces; De Block EMBO J 1987 Basta resistance; NCBI NBK131103 bar function; PubMed 2884102.

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Transgenic Plant -1

Roundup Ready crops are resistant to:

Roundup Ready is brand name for transgenic crops tolerant to glyphosate based herbicide Roundup. Native crop EPSPS inhibited by glyphosate application, blocking aromatic amino acid synthesis. Incorporation of insensitive EPSPS variant allows continued pathway function despite spray, enabling post-emergence weed control without crop injury. Crops carrying cp4-epsps gene survive broadcast over-the-top application at commercial rates, simplifying weed management and facilitating conservation tillage reducing soil erosion. Soybean, cotton, maize, canola, alfalfa, and sugar beet versions deregulated globally. Resistance not based on detoxification but target site insensitivity, so herbicide still translocates systemically killing weeds. Development of Roundup Ready revolutionized agriculture in late 1990s, dominating transgenic acreage. Stewardship challenges include evolution of glyphosate resistant weeds after continuous monoculture and reliance on single mode of action. Therefore Roundup Ready crops are resistant to glyphosate herbicide through expression of tolerant EPSPS enzyme. Adoption history began 1996 with Roundup Ready soybean event GTS 40-3-2 developed by Monsanto, followed by cotton, maize. Global planted area exceeded 90 million hectares for soybean alone by 2018. Benefits include flexibility of post-emergence weed control and enablement of no-till preserving soil moisture and reducing erosion. Concerns about herbicide over-reliance led to development of stacked herbicide tolerance with dicamba and

Ref: Dill GM Crops 2005 Roundup Ready; Funke PNAS 2006 CP4; NCBI NBK131103; ISAAA Pocket K 10 herbicide tolerance mechanism.

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