Mycorrhizal fungi, or mycorrhizae, live inside or attached to plant roots. The two types of organisms help each other to survive; that is, they are symbiotic. Fungi help plants to uptake soil nutrients in exchange for sugars produced by the plants. In forests, mycorrhizae form long strands called hyphae that run between trees, acting as connectors. This giant underground transportation network is called the “common mycorrhizal network” or CMN. The CMN uses chemical communication to exchange nutrients between trees on an “as-needed” basis. Besides nutrients, the CMN also helps trees get water that their own roots would not be able to reach.
When deep soils are moist and shallow soils are dry, trees pull water upward through their tap roots, which are deep in the soil, up to the shallow roots. Evaporation through the leaves and from the soil surface act like a drinking straw, pulling water through the plant and soil. Mycorrhizal fungi can grab that water coming up from the deep tap root of one tree, send it along the highways of the CMN, and deposit it in the roots of a distant tree. With the mycorrhizae’s help, trees with roots too shallow to suck up water from deep soil can still get the water they need. Whether trees are stressed from drought or are just a tiny seedling competing for water in a forest of giant adults, mycorrhizae help them survive. Mycorrhizae can move water between trees of the same species, and between trees of different species. By creating cooperation between species, the invisible underground network shapes the community of forest trees that we see above ground.
We do not yet know exactly how this microscopic highway in the soil works to move water around. We do know that the active area of the fungus is the rhizomorph. This is a special structure where nutrient and water exchange between the fungus and its host plants happens. Both fungi and plants have proteins called aquaporins in their cell membranes. Aquaporins act like gates to allow water in or out of the cell. If both the rhizomorph aquaporins and the plant cell’s aquaporins are open at the same time, water can move between them. There remains many mysteries to be uncovered about how symbiotic fungi and plants interact to distribute water in a forest.
Climate change is presenting us with challenges related to water, food, and nutrient availability. Some places may have shortages, while others have excess. The relationship between forest trees and the fungi that help them share resources based on need might help us solve these problems.
Learn more about mycorrhizal networks in this video by NOVA’s Gross Science.Edit Summary
“Plant roots may be linked by shared or common mycorrhizal networks (CMNs) that constitute pathways for the transfer of resources among plants…Our results suggest that the movement of water by CMNs is potentially important to plant survival during drought, and that the functional ecophysiological traits of individual mycorrhizal fungi may be a component of this mechanism.” (Egerton-Warburton et al. 2007:1473)
Common mycorrhizal networks provide a potential pathway for the transfer of hydraulically lifted water between plantsJournal of Experimental BotanyMarch 10, 2007
“Our results demonstrate large positive effects of overstory mycorrhizal networks on seedling survival, along with simultaneous negative effects of tree roots…. Physiological and leaf chemistry measurements suggest that seedlings connected to common mycorrhizal networks benefited from increased access to soil water. The similar magnitude of the positive and negative overstory effects on seedlings and the ubiquity of mycorrhizal networks in forests suggest that mycorrhizal networks fundamentally influence the demographic and community dynamics of forest trees.” (Booth and Hoeksema, 2010:2294)
Mycorrhizal networks counteract competitive effects of canopy trees on seedling survival. Ecology 91(8): 2294–2302.
“Fungal aquaporins regulate the acquisition and release of water by the hyphae… Because of their significance in the processes of water entry and exit of the symplastic [interconnected plasma membrane of cells] pathway, understanding their role is key to elucidate the precise pathways for water transport from the fungal partner to the host roots in mycorrhizal associations. Firstly, fungal aquaporins regulate the amount of water acquired by the hyphae at mycelial front and subsequently transported into rhizomorphs [fungal structures that function in colonization, and transportation of water and nutrients]. Secondly, the abundance and activity of fungal aquaporins could impact water availability in root extracellular space ….Ultimately, by changing the hydration in the apoplastic [intercellular] space, it can influence root aquaporin regulation and the overall root water uptake (Javot and Maurel, 2002; Marjanović et al., 2005; Lee et al., 2010; Dietz et al., 2011; Navarro-Ródenas et al., 2013; Xu et al., 2015; Calvo-Polanco et al., 2019).” (Xu and Zwiazek, 2020)