Fungi form mutualistic relationships with plant roots through a symbiotic association known as mycorrhiza. In this relationship, the fungi colonize the roots of the plant, forming a network that extends into the soil. The fungi help the plant by increasing its ability to absorb water and nutrients from the soil, while the plant provides the fungi with carbohydrates produced through photosynthesis. This mutually beneficial relationship helps both the plant and the fungi thrive in their environment.