Plant Flavonoids: Signals to Legume Nodulation and Soil Microorganisms

Flavonoids released from plants play an important role as signal molecules in early stages of legume-Rhizobium symbiosis and in regulating the activities of soil microorganisms. Development of an assay system for the detection of plant-derived stimulatory compounds has led to isolation and identification of compounds responsible for activation of nodulation (nod) genes in different legumes. In addition to flavonoids, betaines (in alfalfa) and chalcones (in vetch) have been identified as nod inducers. Seeds and roots of a legume release structurally different nod inducing flavonoids. These hydroxylated flavonoid compounds are derived from phenylpropanoid pathway which are responsible for synthesis of many important plant phenolic compounds, including phytoalexin molecules, which are thought to be involved in plant defence system. Flavonoids not only regulate transcription of rhizobial nod genes, but also promote chemotaxis, growth, metabolism and symbiotic efficiency of rhizobia. Important effects on other soil microorganisms are evident as some flavonoids in seed and root exudates promote spore germination in mycorrhizal fungi. Knowledge derived from studies from how plants regulate rhizobla and other soil microorganisms with natural plant products offers a basis of defining new concepts of rhizosphere ecology.