title: On the mode of action of the herbicides cinmethylin and 5-benzyloxymethyl-1, 2-isoxazolines: putative inhibitors of plant tyrosine aminotransferase.
abstract: BACKGROUND
  The mode of action of the grass herbicides cinmethylin and 5-benzyloxymethyl-1,2-isoxazolines substituted with methylthiophene (methiozolin) or pyridine (ISO1, ISO2) was investigated.
  
  
  RESULTS
  Physiological profiling using a series of biotests and metabolic profiling in treated duckweed (Lemna paucicostata L.) suggested a common mode of action for the herbicides. Symptoms of growth inhibition and photobleaching of new fronds in Lemna were accompanied with metabolite changes indicating an upregulation of shikimate and tyrosine metabolism, paralleled by decreased plastoquinone and carotenoid synthesis. Supplying Lemna with 10 µM of 4-hydroxyphenylpyruvate (4-HPP) reversed phytotoxic effects of cinmethylin and isoxazolines to a great extent, whereas the addition of L-tyrosine was ineffective. It was hypothesised that the herbicides block the conversion of tyrosine to 4-HPP, catalysed by tyrosine aminotransferase (TAT), in the prenylquinone pathway which provides plastoquinone, a cofactor of phytoene desaturase in carotenoid synthesis. Accordingly, enhanced resistance to ISO1 treatment was observed in Arabidopsis thaliana L. mutants, which overexpress the yeast prephenate dehydrogenase in plastids as a TAT bypass. In addition, the herbicides were able to inhibit TAT7 activity in vitro for the recombinant enzyme of A. thaliana.
  
  
  CONCLUSIONS
  The results suggest that TAT7 or another TAT isoenzyme is the putative target of the herbicides.
authors.affiliation: BASF Agricultural Centre Limburgerhof, Limburgerhof, Germany. Klaus.grossmann@basf.com
authors.forename: Klaus
authors.initials: K
authors.lastname: Grossmann
authors.forename: Johannes
authors.initials: J
authors.lastname: Hutzler
authors.forename: Stefan
authors.initials: S
authors.lastname: Tresch
authors.forename: Nicole
authors.initials: N
authors.lastname: Christiansen
authors.forename: Ralf
authors.initials: R
authors.lastname: Looser
authors.forename: Thomas
authors.initials: T
authors.lastname: Ehrhardt
mesh_terms: D029064:Araceae / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000494:pharmacology*; D006540:Herbicides / Q000737:chemistry / Q000494:pharmacology*; D007555:Isoxazoles / Q000737:chemistry / Q000494:pharmacology*; D010940:Plant Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D013876:Thiophenes / Q000737:chemistry / Q000494:pharmacology*; D014444:Tyrosine Transaminase / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; D006540:Herbicides; D007555:Isoxazoles; D010940:Plant Proteins; D013876:Thiophenes; D014444:Tyrosine Transaminase; C585332:methiozolin