Arachidonic acid as an elicitor of the plant defense response to phytopathogens

Review summarizes both literature and own experimental data on the application of arachidonic acid (AA, C-20:4 omega-6) as an elicitor of the plant defense reactions to phytopathogens. The elicitation activity of AA was shown to depend considerably on its concentration. High AA concentrations (over 10(-5) M) induce necrosis of plant tissues and accumulation of antimicrobial compounds (phytoalexins), whereas low AA concentrations (10(-7)-10(-7) M) elicit systemic and prolonged resistance to the phytopathogen infections that is similar to immunization process. Biochemical mechanisms responsible for the elicitor activity of AA involve the whole complex of reactions including reconstruction of the cell ultrastructure, an increase in the amounts of certain enzymes and protective substances, a decrease in sterol content, redirection of isoprenoid biosynthesis from sterol derivatives toward sesquiterpenoid phytoalexins, and appearance of signal molecules, which are spread all over the plant tissues making them immunized. Field experiments revealed that the treatment of potato, tomato, sugar beet, and vine plants with low concentrations of pure AA or AA-containing preparations isolated from Phytophtora and Mortierella fungi increased plant resistance to diseases (late blight, common scrab, rhizoctonoise, cercosporose, powdery mildew, etc.) and thus enhanced the harvest yield. The application of AA-containing preparations is an alternative to the use of environmentally dangerous chemical fungicides. The processes of microbiological AA production from renewable inexpensive raw substrates are considered.