Arbuscular mycorrhizal fungi and nitrogen acquisition

How plants acquire nitrogen is one of the fundamental questions in understanding plant adaptations to low nutrient availability. For many years, plants were understood to associate with mycorrhizal fungi in order to acquire P. Later it was understood that some types of mycorrhizal fungi (ectomycorrhizal and ericoid) acquired organic N and transferred that to the host plant. Yet, for many years it was unknown whether arbuscular mycorrhizal fungi were important in the N nutrition of plants.

Leigh, Hodge, and Fitter (2009) take a big step in showing that arbuscular mycorrhizal fungi acquire N from the soil and transfer the N to plants. Building on earlier work by Hodge, the authors grew Plantago lanceolata in microcosms with two compartments. In some, plant roots had access to the second compartment which contained 13C and 15N enriched shoots. In others, only arbuscular mycorrhizal fungi could access the compartment. The authors show that the arbuscular mycorrhizal access inorganic N derived from the shoot material and transfer a large fraction of it to the plants. In one case, ~20% of the plant’s N came from the AM fungi.

In showing the ability of AM to acquire N and transfer large amounts of it to the plant, the study is an important one. The degree to which this potential is ecologically important remains to be seen. In this study, the AM fungi had access to a large, N-rich patch that the roots did not. In nature, it doesn't seem too often that an N-starved plant find's itself on the other side of a goretex barrier from a rotting carcass. Yet, if there ever is a race for a patch, can AM fungi find it before roots? Do they proliferate and therefore compete better than roots? Can they access small, rich patches that roots cannot? The experiment by Leigh, Hodge, and Fitter is an important step in our understanding. The next experiment might be the really important step.

Leigh, Hodge, and Fitter. 2009. “Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material” New Phytologist, 181: 199-207.