Actinorhizal, mycorhizal and rhizobial symbioses: how much do we know?

The symbiotic association between certain plants and microorganisms plays an important role in soil fertilization, and improves their growth and mineral nutrition. Microorganisms implicated in this symbiotic interaction are from two groups: bacteria and fungi. The bacteria group is implicated on nitrogen fixation (for review see Pawlowski and Bisseling, 1996), while the fungal group is involved in the uptake of nutrients with low mobility (Diop, 1996; Gianinazzi-Pearson, 1996). Among the bacteria which establish symbiotic association with dicotyledonous plants, nitrogen fixation is exclusively carried out by rhizobia and Frankia in a specialized organ, the root nodule where atmospheric nitrogen is reduced to ammonium. Rhizobia and Frankia are soil bacteria which are unicellular gram-negative and filamentous branching gram-positive, respectively. Rhizobia exist in symbiotic association with legumes and one species member of Ulmaceae family, Parasponia andersonii. In contrast, Frankia can interact with diverse group of dicotyledonous plants which are called actinorhizal plants. More than 90% of terrestrial plants live symbiotically with arbuscular mycorhizal fungi. Plants with few or short root hairs are very mycotrophic and depend on mycorrhizae for nutrition and growth (Baylis, 1975). Observations of fossil plants from Devonian indicate that arbuscular mycorhizal fungi played an important role in plant colonization of land (Pirozinski and Malloch, 1975). Legume and actinorhizal plants can establish at the same time a symbiotic association with the arbuscular mycorrhizal fungi of the order Glomales. Recent studies conducted in Gymnostoma have shown that root nodules can also be colonized by arbuscular