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).Figure 1. Pea root cortex infected by the mycorrhizal fungus Glomus intraradices ( A ) and a Rhizobium leguminosarum bv viciae induced infection thread in a vetch‐root hair ( B ). The AM fungus has entered the root intercellularly and it has formed an arbuscle [(A), Trypan blue staining]. In contrast, Rhizobium enters its host plant intracellularly via an infection thread [(B), Vetch‐root hair with an infection thread containing R.leguminosarum bv viciae bacteria expressing green fluorescent protein (GFP) (Spaink et al ., 1998)].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 …