Plant-fungus mycorrhizal association is:
Mycorrhizae join fungal hyphae with plant roots in a reciprocal exchange. The fungus expands the effective absorptive surface, improving acquisition of phosphorus, nitrogen, micronutrients, and water; it can also enhance pathogen resistance and drought tolerance. The plant supplies photosynthetically fixed carbon and lipids that fuel fungal growth. Many plants and especially arbuscular mycorrhizal fungi depend strongly on this association, supporting its treatment here as obligate mutualism. Dependence is not universal, because some plant-fungus pairs are facultative and benefit can shift with soil fertility. Even so, commensalism, parasitism, and amensalism do not capture the characteristic reciprocal resource trade emphasized by mycorrhizal ecology. Ecological categories are simplified models, yet they remain valuable when their assumptions are stated. The selected description captures the dominant net effect, while real systems may vary with density, habitat, life stage, and environmental conditions. This reasoning connects organismal behavior with broader ecological consequences. A mechanism that changes encounter rates, resource use, or reproductive success can scale up to alter population trajectories and the structure of species interactions.
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 8-13