What organisms enter into symbiosis with the roots of grasses and trees?

Mycorrhiza (fungal root), as well as mycelium mushrooms, enter into symbiosis (mutually beneficial coexistence) with the roots of trees and grasses. This coexistence is beneficial for everyone. Mycorrhiza and fungi get their share of nutrients from plants, and trees and grasses theirs. Moreover, it has long been proven that mycorrhiza for plants is not only extremely useful, but also simply necessary. If, say, a tree is planted in an environment unusual for it – in a dry steppe, and not in a forest, then it can die without mycorrhiza.