RHIZOBIUM-MELILOTI LIPOOLIGOSACCHARIDE NODULATION FACTORS - DIFFERENTSTRUCTURAL REQUIREMENTS FOR BACTERIAL ENTRY INTO TARGET ROOT HAIR-CELLS AND INDUCTION OF PLANT SYMBIOTIC DEVELOPMENTAL RESPONSES

Rhizobium meliloti produces lipochitooligosaccharide nodulation NodRm factors that are required for nodulation of legume hosts. NodRm factor s are O-acetylated and N-acylated by specific C16-unsaturated fatty ac ids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhibite d a significantly reduced capacity to elicit infection thread (IT) for mation in alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodF/nod L mutants were unable to penetrate into legume hosts and to form ITs. Nevertheless, these mutants induced widespread cell wall tip growth in trichoblasts and other epidermal cells and were also able to elicit c ortical cell activation at a distance. NodRm factor structural require ments are thus clearly more stringent for bacterial entry than for the elicitation of developmental plant responses.