In a mycorrhizal association, how does the fungus and the plant mutually benefit each other?

The fungus provides minerals and water to the roots and the roots provide sugars and N-containing compounds to the mycorrhizae.

The fungus provides sugars and N-containing compounds to the roots and the roots provide minerals and water to the mycorrhizae.

The fungus provides minerals and water to the roots and the roots undergo photosynthesis to provide sugars and N-containing compounds to the mycorrhizae. 

The fungus protects the roots from extreme cold conditions and the roots being photosynthetic provide food to the fungus.