A Mutualistic Fungus of Tall Fescue Grass

Mutualism is an interaction between two species in which both benefit. A classic example of this relationship is between tall fescue grass and a fungus named Epichloé coenophiala . That fungus is completely internal and grows intercellularly (between the cells) in the above-ground portion of the grass. The grass supplies all the nutritional needs of the fungus. Infected plants exhibit no external symptoms of the fungus and no disease symptoms. In contrast, pathogenic (disease-causing) fungi result in symptoms that include wilt, leaf spots, deformed plant structures, or death of plant tissue. Pathogenic fungi often invade plant host cells and absorb the cells’ contents (thereby eventually killing the cells) or produce chemicals that kill them outright.

Tall fescue is extensively used as both a pasture grass and a turf or lawn grass. It is a hardy, vigorously growing grass but has been associated with various health problems in cattle and horses. Beef cattle often show reduced weight gains when grazing on tall fescue pastures, and dairy cattle experience lower milk yields. Female horses may be more likely to lose their offspring during pregnancy and may not produce milk following a live birth. Reports of such adverse effects resulted in microscopic investigation of leaf and stem tissue that revealed the presence of the Epichloe coenophiala fungus. Surveys of infected pastures found a positive correlation between the percentage of infected plants and the severity of animal symptoms. The fungus in the grass was found to produce various alkaloids (toxic nitrogen-containing organic molecules). One type of alkaloid (ergot alkaloids) contributes to the livestock’s symptoms and reduced grazing, whereas another type (loline alkaloids) makes the grass more resistant to insects. Interestingly, the insect-deterring alkaloids are not produced when the fungus is grown in the laboratory apart from the plant. Because these chemicals defend the grass as well as the fungus from consumption by both livestock and insects, the mutualistic relationship has been termed defensive mutualism.

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