ABSTRACT
Although dinitrogen (N2) comprises 78% of the air, it is frequently one of the most limiting nutrients for plant growth in natural ecosystems. However, under N-limiting conditions, five members of the gram-negative family Rhizobiaceae- Rhizobium, Bradyrhizobium, Mesorhizobium, Azorhizobium, and Sinorhizobium, interact with certain dicotyledonous plants to establish symbiotic associations. The ability to establish an N2-fixing symbiosis with rhizobia is restricted to the family Fabaceae, with one exception, Parasponia, a member of the family Ulmaceae. The N fixed by the bacteria as ammonia is subsequently assimilated into amino acids, which are incorporated into other N-containing compounds. Ethylene has long been known to be inhibitory for nodulation. During the early events of nodule development, root cortical cells, located between the central vascular cylinder (the stele) and the root epidermis, respond to Nod factors or Rhizobium infection differently depending on their radial positions.
