Role of rhizobial lipo-oligosacharides in root nodule formation on leguminous plants

Abstract

During recent years signals leading to the early stages of nodulation of legumes by rhizobia have been identified. Plant flavonoids induce rhizobial nod genes that are essential for nodulation. Most of the nod gene products are involved in the biosynthesis of lipo-oligosaccharide molecules. The common nodABC genes are minimally required for the synthesis of all lipo-oligosaccharides. Host-specific nod gene products in a given Rhizobium species are responsible for synthesis or addition of various moieties to those basic lipo-oligosaccharide molecules. For example, in R. leguminosarum, the nodFEL operon is involved in the production of lipo-oligosaccharide signals that mediate host specificity. A nodFE-determined highly unsaturated fatty acid (trans-2, trans-4, trans-6, cis-11-octadecatetraenoic acid) is essential for inducing nodule meristems and pre-infection thread structures on the host plant Vicia sativa. Lipo-oligosaccharides also trigger autoregulation of nodulation in pea and, if applied in excessive amounts to a legume, can prevent nodulation and thereby might play a role in competition. During our studies on the biosynthesis of lipo-oligosaccharides, we discovered that, besides the lipo-oligosaccharides, other metabolites are synthesized de novo after induction of the nod genes. These novel metabolites appeared to be phospholipids, containing either one of the three fatty acids which are made by the action of NodFE in R. leguminosarum.