Reports

A general theory for the development of commensal and mutualistic symbioses is that they evolved from pathogenic relationships (1). In-deed, although the Rhizobium-legume symbiosis is beneficial and be-nign, the plant does respond initially with a pathogen defense response that is quickly suppressed (2, 3). The lipo-chitin nodulation factors (known as Nod factors) are substituted, acylate chitin oligomers of three to six N-acetylglucosamine residues and are the key signaling molecules for the establishment of Rhizobium-legume symbioses. These factors induce plant responses that lead to the development of the nodule, which becomes the intracellular home of the invading symbiont (4, 5). Nod factors function at nanomolar levels (6, 7), but only on specific, compat-ible legume hosts. Extension of nitrogen-fixing symbioses beyond these already compatible legume hosts could help reduce dependence on ap-plied fertilizers (8). Plant innate immunity can be triggered by the recognition of mi