Plants worldwide must defend themselves from being damaged by their enemies, including insects, deer and pathogens. Plant defenses range from physical defenses, like thorns, spines, and tough leaves, to chemical defenses such as cyanide and steroids. As plants try to defend themselves, most of them make use of nutrients that are provided to them by fungi (called mycorrhizae) that live in their roots. In what is basically a barter system, plants provide the fungi with sugar while the fungi provide plants with minerals such as phosphorus. The strength of plant defenses may vary with the amount of fungus in their roots and the identity of their fungal partners, but this has never been studied in detail.
Using common milkweed as a study plant, this research explores how the strength of plant defense varies with the fungal colonization of roots. It examines how variation in plant and fungal identity contributes to defense, and how natural variation in soil nutrients influences the barter system. Experiments also explore the effects of root fungi on insect herbivores. The results of this work will contribute to an understanding of how diverse plant species, from agricultural crops through forest trees, use root fungi in their defense strategies. It may be possible to manipulate root fungi to protect the plants that are important to humans or to render defenseless those that are invasive pests. The research will also help to train the next generation of biologists both in universities and in local area schools.
When we observe plants in nature, we see less than half of the picture. Many plants produce much more tissue belowground than they produce aboveground, and the studying the ecology of plant roots is critical for understanding how natural communities work, and how best to grow food and fiber crops. Most of land plants have fungi associated with their roots. These fungi (called AMF) are trading partners, passing nutrients like phosphorus to plants in return for sugars. This partnership between plants and AMF is both ancient and critical to the success of plants on land. Moreover, many of the traits that we observe in plants are actually expressed as a result of their interactions with AMF. We have been studying the effects of AMF on the chemical defenses of plants. Plants are attacked by a wide range of pests, including insects, mammals, and diseases. Plants produce a dazzling array of chemical defenses to help protect themselves from attack. But who is in control of making the defenses? Is it the plants or is it the fungi in their roots? Our work shows that the expression of chemical defense in plants depends fundamentally on the colonization of plant roots by AMF. The relationship appears to be hump-shaped, so that chemical defenses initially increase, and then decrease again, as colonization by AMF goes up. In other words, there is an optimal level of AMF colonization that helps to protect plants from attack. Why is the relationship curved? It turns out that when the plants host too many AMF, their costs are too high and plants don't have sufficient resources left over to make their defenses. Conversely, when plants have too few AMF, they don't have enough nutrients to help make the chemical defenses. Intermediate levels of fungal colonization provide the perfect balance of costs and benefits to make strong defenses. We examined the results of these chemcial defenses by studying the insects that fed on our experimental plants. We confirmed that AMF helped to protect plants from certain herbivores, and so reduced the loss of leaves. Our results help to explain why some plants recieve more herbivore attack in nature than do other plants, and emphasize that plant chemical defense can't be understood without reference to the fungi in roots. Our research findings can be useful in the agricultural sector. We may be able to manipulate the density, diversity and identity of fungi in soil to help protect our food and fiber crops from attack by insects. Consequently, we could use soil fungi to help reduce the inputs of harmful pesticides into our water and soil.
