Methyl jasmonate and methyl salicylate, but not cis-jasmone, evoke defenses against infection of Arabidopsis thaliana by Orobanche aegyptiaca

The mechanism by which some plant species develop resistance to the root parasite, broomrape (Orobanche aegyptiaca), is still not clear. Resistance to other pathogens can be induced by methyl jasmonate and systemic acquired resistance can be induced by treatment with salicylic acid, while cis-jasmone can act as a signaling molecule in plant-insect interactions. The three compounds studied, methyl jasmonate, cis-jasmone, and methyl salicylate, were applied to Arabidopsis thaliana seedlings that were then transferred to Nunc cell culture plates and exposed to the germinating seeds of O. aegyptiaca. The number of infections of the roots of single seedlings of A. thaliana was then quantified. Exposure for 24 h to very low concentrations of methyl jasmonate or methyl salicylate, which were then removed, effectively induced resistance to infection of A. thaliana by O. aegyptiaca, reducing attachment and tubercle formation by 90%. cis-Jasmone was far less effective in inducing a similar resistance to infection. These results support the view that methyl jasmonate can induce almost full resistance to infection by broomrape. The fact that such resistance is not observed under normal conditions of infection supports the idea that the root parasite does not evoke the full defensive response in the host plant.