It's pretty common knowledge that plants need several basic nutrients to survive, one of which is Phosphorus. "N-P-K," is found on every bag of fertilizer, representing the nitrogen, phosphorus, and potassium ratios of the mix.
Phosphorus is important for a variety of reasons. Phosphorus is an important part of nucleic acid compounds which control how the plant creates proteins. Without proteins, the plant cannot produce the enzymes it needs to perform basic life functions. Phosphorus is also important in energy production within the plant. The chemical ATP (adenosine triphosphate), which contains phosphorus (hence, tri-phosphate), has strong double bonds that release energy when broken.
Unfortunately for plants, phosphorus is generally available in the soil as a mineral nutrient. What this means is that only small amounts of phosphorus are dissolved in water while the rest of the mineralized phosphorus remains in place in the soil. As a consequence, plant roots must continuously explore the soil in search of more phosphorus.
Well, lucky for plants, fungi are ready to help out! Mycorrhizal fungi are fungi that engage in a mutualistic relationship with a plant, exchanging mineral nutrients with the plant for carbohydrates. Mycorrhizal fungi can attach to the plant's roots and extend its super small hyphae all throghout the soil, essentially extending the plant's root zone and providing phosphorus to the plant, among other nutrients (and water!).
It is interesting to note that if there happens to be plenty of dissolved phosphorus, or available phosphorus, to be had, then the plant might choose not to partner with mycorrhizal fungi because the cost of feeding the fungus would be greater than the benefits that the fungus would provide!
Fungi are awesome. And so are plants!