Fungi are usually considered bad news for plants—Phytophthora infestans caused the Irish potato famine, and powdery mildew is the wine industry’s enemy No. 1. Yet scientists keep finding tons of fungi inside healthy plants, squeezed into nooks and crannies between cells. “What we call a plant isn’t just a plant. It’s usually a mosaic of plant and fungal tissues,” says evolutionary ecologist Allen Herre. What are all these fungi up to, he wondered, if they’re not up to no good?
Herre, along with Elizabeth Arnold and other colleagues at the Smithsonian Tropical Research Institute in Panama, decided to find out by studying Theobroma cacao, the tree that gives us chocolate. In the tropical forests of Panama, cacao plants are normally saturated with a diverse mix of seemingly harmless fungi, or endophytes. Only in the greenhouse could the Smithsonian researchers create pure plant seedlings. They then injected half the leaves with the usual cacao fungi and exposed the plants to another, pathogenic fungus that causes black pod, an ancient scourge. Leaves without the endophytes were three times more likely to die. Somehow, the good fungi crowded out or warded off the bad ones. “You can think of these fungi as an environmentally acquired immune system,” Herre says.
That finding raises prospects for eco-sensitive disease control—spraying cacao trees with beneficial fungi, for instance, or planting them alongside endophyte donor plants. Keith Clay, an ecologist at Indiana University, thinks scientists need to rethink the relationship between plants and their alleged parasites. “It opens a door on a type of plant-microbial interaction that we’re going to find all over the plant world,” he says.