The space-dwelling fungus cultivated aboard the USS Discovery, whose spores give access to the mycelial network and provide the fuel for the ship's spore drive.
Prototaxites stellaviatori is a fungal species whose root system, the mycelium, extends through subspace across the whole universe. Paul Stamets and his research partner Straal studied it for years before Starfleet turned their work to military use.
In Star Trek: Discovery the fungus is grown in a cultivation bay next to engineering, first seen in Context Is for Kings, and its harvested spores are loaded in canisters into the reaction chamber for each jump.
The species remains essential to Discovery in the 32nd century, when the spore drive becomes the only means of rapid travel not dependent on dilithium.