Rhizobium meliloti produces lipochitooligosaccharide nodulation NodRm factors that are required for nodulation of legume hosts. NodRm factors are O-acetylated and N-acylated by specific Cl6-unsaturated fatty acids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhlbited a significantly reduced capacity to elicit infection thread (IT) formation ln alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodFlnodL 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 cortical cell activation at a distance. NodRm factor structural requirements are thus clearly more stringent for bacterial entry than for the elicitation of developmental plant responses.

Rhizobia elicit the formation of nitrogen‐fixing nodules on specific legume hosts. Rhizobium meliloti nodulation (nod) genes control a signal exchange between the two symbiotic partners during infection and the early steps of nodulation. The regulatory nodD1, nodD2 and nodD3 genes are involved in the specific perception of different plant and environmental signals and activate the transcription of the nod operons. Once activated, the structural nod genes specify the synthesis of diffusible lipo‐oligosaccharides, the Nod factors, which signal back to the plant. R. meliloti Nod factors are sulfated chito‐oligosaccharides which are mono‐N‐acylated by unsaturated C16 fatty acids or by a series of C18 to C26 (omega‐1)‐hydroxylated fatty acids. In this paper we show that the regulatory nodD3 gene and another symbiotic regulatory gene, syrM, which mediate bacterial responses to plant signals that differ from those involving nodD1 and nodD2, determine the synthesis of Nod factors with different acyl moieties. nodD3 and syrM are required for the synthesis of Nod factors N‐acylated by the (omega‐1)‐hydroxylated fatty acids. This regulatory mechanism makes possible the qualitative adaptation of bacterial Nod signal production to plant signals in the course of the symbiotic process.

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