The Old Arbuscular Mycorrhizal Symbiosis in the Light of the Molecular Era

Abstract

Mycorrhiza, from the Greek terms myco (=fungus) and rhiza (=root), is the predominant root symbiosis. More than 90% of land plants form mycorrhiza with soil fungi belonging to three different phyla (Smith and Read 1997). A few plant families (e.g. Cruciferae, Chenopodiaceae, Proteaceae) as well as some genera (e.g. Lupinus) have been described as non-mycorrhizal. It is speculated that the mycorrhizal character has possibly arisen several times during evolution (Trappe 1987), but the mechanisms of mycorrhiza exclusion have not yet been identified. From the different types of mycorrhiza existing in nature arbuscular mycorrhizas are the oldest and most widespread symbiosis (Remy et al. 1994). Recent fossil studies and molecular data have tracked the presence of this symbiosis all the way to the Ordovician era, i.e. to be at least 460 million years old (Redecker et al. 2000b). The fungi forming this mutualistic symbiosis have been recently recognized as belonging to an independent phylum, the Glomeromycota (Schüßler et al. 2001b) with a monophyletic origin. The permanence of this mutualistic association during evolution reflects its importance in nature. The reciprocal benefit, achieved by the nutrient exchange between both partners in intimate contact, is possibly the reason for this durability. However, the benefits of the symbiosis to the maintenance of natural ecosystems cannot be only estimated in terms of improved plant nutrition.