Testing for mechanisms of inducible resistance in endophyte-infected grass
The grass tall fescue (Schedonorus arundinaceus) often harbors the fungal endophyte Neotyphodium coenophialum. The fungus lives in shoot tissues and forms a mutualistic relationship with its host, providing resistance to biotic and abiotic stresses. Resistance to herbivores is often mediated by alkaloids produced by the fungus. The fungus provides wound-inducible resistance via upregulation of a loline alkaloid biosynthesis gene following natural and simulated (clipping) damage to shoots of tall fescue. This raises the question of how the fungus determines that its host has been damaged. We tested if plant hormones associated with regrowth serve as signals for upregulation to the fungus. Tall fescue infected or uninfected with N. coenophialum were exposed to methyl jasmonate, salicylic acid, gibberellic acid, or 3-indoleacetic acid. Rhopalosiphum padi aphids, which are sensitive to lolines, were used as a bioassay on plants following exposure. We subsequently conducted a damage experiment that tested the hypothesis that direct physical damage to the fungus, not a chemical messenger from the plant, causes loline upregulation. This was done by damaging the distal half of leaf blades (a component of the shoots that has low levels of endophyte hyphae) or the roots (where the fungus does not live).
In the first set of experiments we found that application of plant hormones did not enhance resistance to aphids. In fact, methyl jasmonate actually lowered resistance. In the second experiment, when infected plants were cut in areas where the fungus lives (across pseudostems of tillers), the fewest aphids survived. In contrast, damaging plants in areas where fungal hyphal densities are low (distal parts of leaf blades) or nonexistent (roots) resulted in higher resistance levels to aphids and similar to those in undamaged plants. Uninfected plants showed no consistent response to damage treatments with respect to resistance to aphids. We conclude that N. coenophialum may not respond to plant biochemical signals for the upregulation of loline synthesis following clipping, but rather to direct physical damage to hyphae that is coincident with clipping of tillers.