Possible function of isoleucine in the methyl jasmonate response of Arabidopsis to Phelipanche aegyptiaca

Resistance of Arabidopsis thaliana to Phelipanche aegyptiaca (syn. Orobanche aegyptiaca) can be induced by exposure of the host to low concentrations of gaseous methyl jasmonate, in a concentration-dependent fashion. Application of methyl jasmonate at marginal concentrations, 10⁻⁸ M, for 12 h reduced infection by 50%. However, if the host plants were supplied with isoleucine at 10⁻³ M, and the isoleucine removed prior to exposure to methyl jasmonate, infection was reduced to less than 20%. The presence of isoleucine throughout the experiment—before, during and after exposure to methyl jasmonate—reduced infection of the host to almost zero. These results suggest that when Arabidopsis is exposed to methyl jasmonate, full activation of the defense mechanisms, including formation of the isoleucine jasmonic acid conjugate, might be limited by the availability of isoleucine in the host. These findings add another factor to the complex interaction between infecting parasite, and evocation of host defense mechanisms.