N-Acyl Galactosamine Inhibition of Lipo- Chitooligosaccharide Action

In the rhizosphere, soil bacteria of the genus Rhizobium are induced to synthesize lipo-chitooligosaccharide (LCO) signal molecules that in turn trigger the formation of nodules on the roots of leguminous plants (Denarie and Cullimore, 1993). The important role of LCOs as plant growth regulators has stimulated considerable interest in their synthesis. Therefore, we developed simplified procedures for the synthesis of LCOs which employ both an enzymatic and a chemical step (Rohrig et al., 1995 and 1996). We demonstrated that these synthetic LCO signals, which trigger nodule organogenesis in legumes, also promote cell division of tobacco protoplasts at very low concentrations in the absence of auxin and cytokinin. Furthermore, LCOs induce the expression of AXI1 (Rohrig et al., 1995), a gene implicated in auxin signaling (Hayashi et al., 1992). More recently, using the protoplast division assay and transient expression assays with a construct in which the AXI1 promoter was linked to the GUS reporter gene, we showed that the N-octadecenoylated monosaccharide glucosamine has all structural requirements for a biological active glycolipid. The assays also revealed that the N-acylated galactosamine epimer was inactive and specifically inhibited LCO action (Rohrig et al., 1996).