21 Vesicular-arbuscular mycorrhizal fungi in nitrogen-fixing systems

The microbial interactions concerning vesicular-arbuscular mycorrhiza or vesicular-arbuscular mycorrhizal fungi and nitrogen-fixing bacteria are of relevance because they can improve plant establishment, development and nutrient acquisition. Taking this into account, two main groups of experimental approaches can be distinguished: (1) studies concerning the establishment of micro-organisms in the rhizosphere (or mycorrhizosphere) and (2) manipulation of these microbial association as a biotechnological tool to improve plant growth. It is well known that a range of soil bacteria possess the ability to cycle nitrogen from the atmosphere to the biosphere by means of the so-called nitrogen-fixing process. All of these bacteria (with the exception of Sesbania sp., which also nodulates on stems) live either in the endorhizosphere (forming root nodules), in intimate association with the root surface or rhizoplane, or in the rhizosphere. Therefore, they must coexist with vesicular-arbuscular mycorrhizal fungi and/or vesiculararbuscular mycorrhiza in the ecosystem. Coexistence commonly involves interaction and there is evidence that such interactions occur, either at the colonization and/or at the functional and nutritional levels. The chapter describes experiments to assess the determinants of symbiotic functional compatibility for particular fungus-plant bacteria-environment combinations.Peer reviewe