ISOLATION, CHEMICAL STRUCTURES AND BIOLOGICAL-ACTIVITY OF THE LIPO-CHITIN OLIGOSACCHARIDE NODULATION SIGNALS FROM RHIZOBIUM-ETLI

Rhizobium etli is a microsymbiont of plants of the genus Phaseolus. Us ing mass spectrometry we have identified the lipo-chitin oligosacchari des (LCOs) that are produced by R. etli strain CE3. They are N-acetylg lucosamine pentasaccharides of which the non-reducing residue is N-met hylated and N-acylated with cis-vaccenic acid (C18:1) or stearic acid (C18:0) and carries a carbamoyl group at C4. The reducing residue is s ubstituted at the C6 position with O-acetylfucose. Analysis of their b iological activity on the host plant Phaseolus vulgaris shows that the se LCOs can elicit the formation of nodule primordia which develop to the stage where vascular bundles are formed. The formation of complete nodule structures, including an organized vascular tissue, is never o bserved. Considering the very close resemblance of the R. etli LCO str uctures to those of R. loti (I.M. Lopez-Lara, J.D.J. van den Berg, J.E . Thomas Gates, J. Glushka, B.J.J. Lugtenberg, H.P. Spaink, Mol Microb iol 15: 627-638, 1995) we tested the ability of R. etli strains to nod ulate various Lotus species and of R. loti to nodulate P. vulgaris. Th e results show that R. etli is indeed able to nodulate Lotus plants. H owever, several Lotus species are only nodulated when an additional fl avonoid independent transcription activator (FITA) nodD gene is provid ed. Phaseolus plants can also be nodulated by R. loti bacteria, but on ly when the bacteria contain a FITA nodD gene. Apparently, the type of nod gene inducers secreted by the plants is the major basis for the s eparation of Phaseolus and Lotus into different cross inoculation grou ps.