MOLECULAR DISSECTION AND IMPROVEMENT OF THE NODULE SYMBIOSIS IN LEGUMES

The initiation and development of nitrogen (N-2) fixing nodules in the roots of leguminous plants occurs by the induction of cell division a nd redifferentiation in the root cortex, followed by the formation of a meristem and progressive differentiation of specialized cells and ti ssues. During this process, competent rhizobia invade the root and bec ome specialized N-2-fixing endosymbionts. The onset of the symbiosis i s largely mediated by an exchange of diffusible signals, bacterial lip o-oligosaccharides being the main determinants of specificity and the initial inducers of plant responses. It is however the host which cont rols most facets of the nodulation process, including nodule morpholog y, efficiency, specificity and function. The dissection of plant mecha nisms underlying signal-transduction during nodulation may be crucial to understand and then manipulate the symbiosis. Positional cloning or gene targeting offer strategies that promise the identification of cr ucial plant genes determining nodulation. The search for the nts - 1 g ene that controls nodulation in soybean illustrates the challenges and limitations of positional cloning. It also shows how biotechnology ca n offer tools to help in the breeding of plant traits important to agr iculture. Molecular dissection of the symbiosis will ultimately be use d to improve N-2 fixation by molecular breeding and genetic engineerin g in legumes. (C) 1997 Elsevier Science B.V.