Sulphated lipo-oligosaccharide signals of Rhizobium meliloti elicit root nodule organogenesis in alfalfa

Abstract

RHIZOBIUM meliloti is a symbiotic bacterium that elicits the morphogenesis of nitrogen-fixing nodules, specific organs on the roots of alfalfa (Medicago sativa)1. In R. meliloti a series of nodulation (nod) genes have been identified which are involved in root-hair curling and infection and in nodule formation1. The nodABC genes, common to all Rhizobium sp., and the host-range nodH and nodPQ genes are involved in the production of an excreted root-hair deforming factor, NodRm-1, which is a sul-phated and acylated glucosamine oligosaccharide2–7. Here we report that purified NodRm-1 and a related compound, Ac-Nod Rm-1, at concentrations in the micromolar–nanomolar range, elicit cortical cell divisions and the formation of genuine nodules on aseptically grown seedlings of alfalfa. Chemical modifications of NodRm-1, such as the removal of the sulphate group, reduction of the terminal sugar or hydrogenation of the acyl chain result in a strong decrease in the morphogenic activity. A highly specific prokaryotic lipo-oligosaccharide signal is thus able to trigger a genuine organogenesis in a higher plant.