Arbuscular Mycorrhizal Symbiosis

Arbuscular mycorrhizal symbiosis

 Over 80% of plants obtain their mineral nutrients with the help of symbiotic fungi that grow in and around their roots. These root-fungal partnerships (mycorrhizas) evolved around the time that plants first invaded the land, over 400 million years ago and today are found across all major taxa of plants and in ecosystems across the planet. Billions of tons of carbon flow annually from leaves to mycorrhizas to support this relationship, which is responsible for huge nutrient uptake fluxes into plants worldwide. Despite the importance of these fluxes to plant ecology, evolution and function, we have only recently begun to describe the pathways involved. 

Our work has traced the routes by which carbon flows from roots to fungi and how nitrogen flows in the other direction. Using isotopic labeling, spectroscopic analysis, and gene characterization and expression measurements we have developed working models for carbon and nitrogen metabolism and transport (illustrated below). We are working to extend these models, which are now largely accepted, to address unanswered questions about their mechanisms and to include regulation.

Photo of one of our mycorrhizal cultures taken by Stacy LaClair

              

These images show the fungal arbuscules growing inside living plant root cells. These structures give the symbiosis its name and function as an interface for the exchange of nutrients between the partners.  From Mark Brundrett: http://mycorrhizas.info/resource.html

 

The life cycle of an Arbuscular mycorrhizal fungus. The germination and hyphal growth from asexual spores in the soil is stimulated by signaling compounds released by roots (top right). These hyphae form infection structures (appresoria) on the surface of host roots the fungus grows into the root forming hyphae between cells and arbuscules that penetrate cell walls without killing the plant cells. Hyphae also grow out into the soil forming a branched mycelium that functions to explore the soil and take up mineral nutrients. Spores are formed by this external mycelium, completing the life cycle.

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 The movement of carbon in the AM symbiosis. We have used stable isotopic labeling experiments supported by gene expression and microscopy measurements to elucidate the processes shown above. Carbon is transferred from the plant to the fungus inside the roots as hexose and this is made into short term fungal storage carbohydrates (glycogen and trehalose). The fungus makes triacylglycerol (TAG) in the root which is exported within the fungus to the external mycelium and to fungal mycelium in other roots (but carbon is not transferred to roots). TAG is broken down in the external mycelium and the glyoxylate cycle is used to produce storage and structural carbohydrates (cell walls) as well as energy via the TCA cycle and other products including amino acids (AA).

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The movement of Nitrogen in the AM symbiosis. Based on previous work we proposed the scheme shown here by which nitrogen moves from soil to plant via arbuscular mycorrhizal fungi (Bago et al 2001). We subsequently provided evidence from labeling and gene expression that this is how N moves through the symbiosis (Govindarajulu et al 2005; Jin et al 2005). Inorganic N (NO3 and NH4+) is taken up by the external mycelium, assimilated and converted to arginine, which is transported (probably in association with Polyphosphate) within the fungus to the fungal mycelium inside plant roots. There the arginine is broken down to release ammonium which is transferred to the plant without carbon. More recently we have identified most of the fungal metabolic genes involved in N movement and begun to characterize the regulation of the pathway.