Signals exchanged between legumes and Rhizobium: agricultural uses and perspectives

Legumes and rhizobia exchange at least three different, but sometimes complementary sets of signals. Amongst the variety of substances normally and continuously secreted into the rhizosphere by plants are phenolic compounds. Flavonoid components of these mixtures are especially active in inducing rhizobial nodulation genes. Many nod-genes exist. Some (e.g., nodD) serve as regulators of transcription, but most code for enzymes involved in the synthesis of a family of lipo-chito-oligosaccharides (LCOs) called Nod-factors. Nod-factors possess hormone-like properties, are key determinants in nodulation, and allow rhizobia to enter the plant. As Nod-factors also stimulate the synthesis and release of flavonoids from legume roots, the response to inoculation is amplified. Once the bacteria enter the plant, other sets of signals are exchanged between the symbionts. These include extra-cellular polysaccharides (EPSs) as well as proteins externalised via type-three secretion systems. These carbohydrates/proteins may be active in invasion of the root. At the time of writing, only flavonoids and Nod-factors have been chemically synthesised and of these only the former are available in large quantities. Field trials in North America show that seed application of flavonoids stimulates nodulation and nitrogen fixation in soybeans grown at low soil temperatures. The biological basis to these responses is discussed.