Legume nodulation and mycorrhizae formation; two extremes in host specificity meet.

Most higher plants have the ability to form arbuscular endomycorrhiza (AM); a symbiotic association of the plant root with fungi belonging to the order of Glomales. These fungi grow towards the inner cortical cells of the root where they differentiate into highly branched structures, the arbuscules (Figure 1). In AM symbiosis, the fungus also forms hyphae outside the plant and these provide a connection between the soil and the inner part of the plant and they facilitate the uptake of nutrients such as phosphate (for reviews see: Gianinazzi-Pearson, 1996; Harrison, 1997). In contrast to AM formation, only a few plant species have the ability to interact symbiotically with bacteria of the genera Azorhizobium, Bradyrhizobium, Rhizobium and Sinorhizobium (here collectively called Rhizobium). This interaction is almost completely restricted to leguminous plants and results in the formation of a completely new organ, the root nodule. In these nodules the bacteria are hosted intracellularly and there they find the ideal environment to reduce atmospheric nitrogen into ammonia, a source of nitrogen which can be used by the plant (for reviews see: Mylona et al., 1995; Long, 1996). At first glance the interactions of plants with rhizobia and AM fungi seem to have little in common. The induced morphological responses of the host plants are different. Furthermore, both interactions are extremes in terms of host specificity. Whereas in AM formation there is very little host specificity, the Rhizobium–legume interaction is highly specific. However, genetic studies have shown that several common steps are involved in establishing these symbioses (Duc et al., 1989; Bradbury et al., 1991; Gianinazzi-Pearson, 1996). Furthermore, some host genes are induced during the initial steps of both interactions. In the first part of this overview, the Rhizobium–plant interaction is described with an emphasis on factors that determine the specificity of the interaction. In the second section, AM formation is described as well as the common aspects of both symbioses.