FRIEND OR FOE? THE CONDITIONALITY OF MYCORRHIZAE-CONFERRED RESISTANCE TO INSECT HERBIVORES

Abstract

Plants face a constant struggle to acquire nutrients and defend themselves against herbivores. Association with soil microbes like mycorrhizal fungi can increase plant growth, and alter resistance to herbivores. Mycorrhizae are traditionally seen as mutualists that increase plant growth, and as such are used in agriculture. However, the effects of mycorrhizae on resistance to herbivores are variable. The conditions that drive either mycorrhizae-conferred resistance or mycorrhizae-conferred susceptibility to herbivores are not well understood. To determine the conditions under which mycorrhizae confer resistance, I conducted a series of greenhouse experiments testing the effects of different abiotic and biotic conditions on mycorrhizae-conferred resistance, specifically manipulating intraspecific plant competition, fertilization, plant domestication, and plant species identity. For each of these experiments, I measured resistance traits within the plant to identify potential mechanisms by which mycorrhizae might change resistance. In my first experiment, mycorrhizae increased susceptibility to herbivores when the plants were not in competition, but had no effect in competition. I also showed that mycorrhizae induced jasmonic acid-mediated decreases in foliar nitrogen, a novel mechanism by which mycorrhizae affect resistance to herbivores. In my second experiment, I investigated mycorrhizae-conferred resistance along a gradient of fertilization treatments. I found that mycorrhizae only conferred resistance to herbivores at medium levels of fertilization. Increased resistance was again correlated with changes in the plant’s foliar nitrogen content. In my third experiment looking at the effects of domestication on mycorrhizae-conferred resistance to three different herbivores, I found that mycorrhizae changed the growth and resistance of undomesticated plants to a larger degree than domesticated plants. The change in mycorrhizae-conferred resistance in undomesticated, mycorrhizal plants corresponded with an increase in protease inhibitors, a class of chemical defenses. By changing the defensive chemistry and nutrient content of their host plants, mycorrhizae can shift plant resistance to herbivores. While mycorrhizae are traditionally seen as mutualists, under many conditions, and when viewed in a tri-trophic context, they can act parasitically. My research demonstrates the limitation of mycorrhizae as an agricultural tool and provides insights into ways that mycorrhizae can manipulate aboveground herbivore community composition. FRIEND OR FOE? THE CONDITIONALITY OF MYCORRHIZAE-CONFERED RESITANCE TO INSECT HERBIVORES