Fungal endophytes in herbaceous plants: mutualists or antagonists?

'Endophyte' is the term used to describe a fungus that lives inside the living tissues of a plant, while causing no visible signs of infection. In grasses and trees, endophytes can protect the host plant against vertebrate and invertebrate herbivores and pathogenic fungi, by the production of chemicals, active against the attacker. However, in herbaceous plants, the number of endophyte fungi may be ten times that found in a grass or a tree. Furthermore, herbaceous endophytes are known to produce a wide variety of toxic chemicals when grown on agar in the laboratory. Remarkably, these fungi have escaped attention by ecologists. Do they exist in a mutualistic relationship with their hosts, protecting plants against attack through their chemical production? Alternatively, if they cause no symptoms of infection, does this mean they suppress the plant's defence system, rendering it more susceptible to herbivore attack? It is the aim of this research to answer these questions, using a model system of creeping thistle (Cirsium arvense), and its associated endophytes and insects. In this project, we will examine whether insect herbivores can transmit these fungi from plant to plant, either directly on their bodies, or by allowing fungal entry to tissues, through wounds in the plant created by their feeding. We will see if biotic and abiotic factors in the soil that are known to increase plant size (symbiotic fungi and nutrients) can affect the numbers of endophytes that infect a plant. We will also examine the reverse interaction: if endophytes consume plant resources then they might compete for carbon in the plant, thereby reducing the growth of the symbiotic fungi. Finally, when we have discovered the factors that affect endophyte occurrence, we will grow plants that are endophyte-free or infected to see what effects fungal infection has on the growth and survival of a sucking, chewing and a galling insect. We will also use the most modern techniques of chemical analysis to detect and identify the toxic chemicals produced in infected plants and compare them to those in uninfected plants and the fungi themselves. Why is this work important? Because it has the potential to change the way in which we think about plant-insect interactions and could lead to many novel forms of pest or weed control. If at least some of the chemicals in plants are produced by these fungi, rather than the plants themselves, this could have profound implications for the use of endophytes as insect biological control agents. We could inoculate the fungi into crop plants and so protect the plant from the inside out. Also, C. arvense is one of the world's worst weeds and all attempts to use insects or pathogenic fungi as biological control agents of it have failed. We think that the presence of invisible, and hitherto unconsidered, endophytes may be the reason. If we show that they protect the plant against insects, then in weeds, we can start to manipulate fungal occurrence, so that biological control agents stand a better chance of success.