title: Insights into the molecular mechanism of tolerance to carboxylic acid amide (CAA) fungicides in Pythium aphanidermatum.
abstract: BACKGROUND
  Tolerance to the oomycete-specific carboxylic acid amide (CAA) fungicides is a poorly understood mechanism in Pythium species. The root-rot and damping-off causative agent Pythium aphanidermatum and the CAA fungicide mandipropamid (MPD) were used to investigate the molecular basis of CAA tolerance.
  
  
  RESULTS
  Five genes putatively involved in carbohydrate synthesis were identified and characterised: one chitin synthase gene, PaChs, and four cellulose synthase genes PaCesA1 to PaCesA4, of which PaCesA3 encodes the MPD target enzyme. These genes were differentially expressed throughout the life cycle of P. aphanidermatum. Mycelium treated with MPD concentrations slightly affecting mycelial growth did not cause a change in PaCesA3 expression nor a strong upregulation of PaCesA homologues. The high tolerance level of P. aphanidermatum and the lack of PaCesA upregulation imply that MPD tolerance is the result of a specific amino acid configuration in the cellulose synthase 3 (CesA3) target enzyme. Indeed, P. aphanidermatum displays the amino acid L1109 which is also associated with MPD resistance in artificial mutants of Phytophthora species.
  
  
  CONCLUSIONS
  It is concluded that MPD tolerance in P. aphanidermatum is not caused by compensatory mechanisms but most likely by an inherent target-site configuration in PaCesA3 that hinders MPD binding to the enzyme pocket.
authors.affiliation: Institute of Botany, Section Plant Physiology, University of Basel, Switzerland. mathias.blum@gmx.ch
authors.forename: Mathias
authors.initials: M
authors.lastname: Blum
authors.forename: Ulrich
authors.initials: U
authors.lastname: Gisi
mesh_terms: D000577:Amides / Q000494:pharmacology*; D002264:Carboxylic Acids / Q000494:pharmacology*; D025141:Drug Resistance, Fungal; D004798:Enzymes / Q000235:genetics / Q000378:metabolism; D005659:Fungicides, Industrial / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects; D010935:Plant Diseases / Q000469:parasitology; D011775:Pythium / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics* / Q000502:physiology
chemical_list: D000577:Amides; D002264:Carboxylic Acids; D004798:Enzymes; D005659:Fungicides, Industrial