Morning glory trees (Ipomoea murucoides) form beneficial relationships with fungal partners that live inside their tissues and produce toxic compounds called alkaloids. These alkaloids accumulate in the tree’s seeds and protect them from beetle damage. Trees hosting the common fungal partner Ceramothyrium produce more of the protective alkaloid swainsonine and suffer less seed damage than those with a different fungal partner, demonstrating how this natural partnership helps the plant defend its offspring.