'Plants and Fungi Play the "Underground Market" '

Test plant, Medicago truncatula,
related to Alfalfa.

Underground roots get phosphorus from fungi, and fungi get carbohydrates from roots. This study shows both roots and fungi are picky about how much fuel they trade.  The best suppliers of phosphorus are given the most carbohydrate and vice versa.  This was tracked by radioactive tags added to the plant carbon and the fungus phosphorus.   Science Daily News.

Photo of fungi connecting
roots of plant hosts. Three
Glomus species measured.