ABSTRACT
Signals and substrates from plant roots are equally important for the microbial populations as well as for their interaction with plant roots. Several hundred secondary metabolites from plants have been identified during the last decades in the large groups of ftavonoids, non-proteinogenic amino acids, and alkaloids. With the discovery of ftavonoids and isoflavonoids as specific signal molecules in the legume-rhizobia symbiosis, interest in these groups of organic molecules in the rhizosphere has very much increased, since it has transferred this group from the status of "secondary" plant metabolites to a primary position in developmental biology. Comprehensive reviews about structures and some functions of flavonoids have been published (1-3).
