Similarities between legume-rhizobium communication and steroid-mediated intercellular communication in vertebrates

Regulation of the actions of flavonoids and Nod factors in legume-rhizobia communication has several interesting similarities with that of steroid-mediated actions in vertebrates. Oxidation or reduction of flavonoids and Nod factors modifies their biological activity just as, for example, oxidation of an alcohol at C11 on hydrocortisone regulates its biological activity. Second, some flavonoids are anti-inducers, functioning like steroid antagonists to negate the actions of inducer flavonoids. Amino acid sequence analyses show that human 17 beta-hydroxysteroid dehydrogenase, which catalyzes the interconversion of the alcohol and ketone at C17 on estrogens and androgens, and rat 11 beta-hydroxysteroid dehydrogenase, which catalyzes the interconversion of the alcohol and ketone at C11 of glucocorticoids, and Rhizobium meliloti NodG and Bradyrhizobium japonicum. FixR are derived from a common ancestor. Just as steroid-metabolizing enzymes can regulate steroid-mediated gene transcription, enzymes that modify substituents on flavonoids and Nod factors may have a similar role in regulating signalling between legumes and rhizobia. The enzymes that modify flavonoids and Nod factors have not yet been identified. However, NodG and FixR are two likely candidates to have this role in regulating legume-rhizobia signalling.