Effects of methyl jasmonate and an endophytic fungus on plant resistance to insect herbivores.

Tall fescue (Lolium arundinaceum) forms a mutualistic relationship with the fungal endophyte Neotyphodium coenophialum. The endophyte provides constitutive resistance to herbivores through its production of alkaloid compounds. Moreover, herbivore attack induces elevated synthesis of loline alkaloids, that is, the fungus also provides wound-inducible resistance for its host. Jasmonic acid and its conjugates are key signaling compounds in many plant species and play a role systemically in the upregulation of defensive compounds within plants following attack by herbivores. The purpose of our study was to determine if and how the plant and fungus respond to methyl jasmonate (MJ) exposure and if these responses interact in antagonistic or synergistic ways. Plants were exposed to MJ via gaseous diffusion within a controlled environment chamber. Response to MJ was assessed with an herbivore bioassay, gas chromatography-mass spectrometry to quantify alkaloids, and real-time reverse transcriptase-polymerase chain reaction to quantify mRNA from a loline alkaloid biosynthesis gene. We found that MJ hindered endophyte-infected tall fescue's resistance against aphids by downregulating transcription of the LolC gene. The opposite pattern was observed for endophyte-free tall fescue; its exposure to MJ resulted in a significant increase in resistance to aphids, apparently through stimulating defense compounds produced by the plant. These results indicate that, when tall fescue lacks fungal infection, MJ induces the plant to produce its own defensive compounds. In contrast, while endophyte-infected plants are defended from herbivores by fungally produced lolines, this defense is compromised by MJ.