Lipo-oligosaccharide nodulation factors: A new class of signaling molecules mediating recognition and morphogenesis

Jean Dbnari6 and Julie Cullimore Laboratoire de Biologie Moleculaire des Relations Plantes-Microorganismes CNRS-INRA, 31326 Castanet-Tolosan Cedex France The legume-rhizobia symbiosis is estimated to fix per an- num as much nitrogen as the fertilizer industry and is of great agronomic and ecological importance. This efficient nitrogen-fixing association occurs as a result of the forma- tion of a new specialized organ, the root nodule, which is induced by the prokaryotic symbiont on its specific plant partner (see Nap and Bisseling, 1990). The rhizobial genes that determine the host recognition and nodulation have recently been found to specify the synthesis of excreted lipo-oligosaccharide signals (Figure 1) which are capable of eliciting, at extremely low concentrations, many of the plant responses characteristic of the bacteria themselves. These responses include the initiation of cell division, in- duction of specific changes in cell morphology, and trig- gering of a plant organogenic program leading to the for- mation of the nodule. This review focuses on the role of this new class of signaling molecules in host-partner rec- ognition and nodule development in the legume-rhizobia symbiosis. Rhizobia, Prokaryotes that Elicit a Specific Plant Organogenesis Rhizobia (now classified into three genera, Rhizobium, Sradyrhizobium and Azorhizobium) are soil bacteria that can elicit the formation of nitrogen-fixing nodules on the roots of selected species of the legume family. Some of these bacteria exhibit a narrow host range of nodulation; for example three of the species discussed in this review, R. meliloti, R. leguminosarum biovar viciae and B. japoni- cum nodulate respectively alfalfa, pea/vetch and soybean and not each other’s hosts. In contrast R. sp. NGR234 has a broad host range, nodulating over 70 legume genera and even a non-legume, Parasponia (see Denarie et al., ‘I 992). Symbiotic nodule formation is based on two processes, plant infection and induction of a novel organ (Nap and Bisseling, 1990). To penetrate into their hosts, rhizobia elicit stimulation and reorientations of plant cell wall growth, presumably due to a restructuring of the cytoskele- ton. These changes result in both the entrapment of the bacteria within root hair curls and also the initiation and development of infection threads, tubular structures ,through which bacteria pass on their way down the root hair to the underlying cortical cell layers (Figure 2A). Ahead of the advancing threads a major developmental switch is elicited in the root cortex. Cortical cells are induced to dedifferentiate and divide, and a meristem is then formed, whose functioning generates the nodule. Nodules have a particular ontogeny and anatomy; they are not mere tu- mors but fully differentiated organs divided into a number of cell-types and tissues. Alfalfa nodules for example (Fig- ure 28) contain a distal meristem, an infection zone, a zone where the cells are filled with nitrogen-fixing bacteria, and vascular bundles that ensure metabolic exchanges with the rest of plant. Genetic manipulation of the prokaryotic partner has led to the identification of a set of rhizobial genes (the nod genes) required for infection, nodulation and the control of host specificity (Figure 1). The expression these genes isdependent upon plant signals, usuallyflavonoids, excreted in root exudates. In the presence of appropriate plant inducers, rhizobial NodD regulatory proteins activate the transcription of thestructural nodgenes. Among these, and present in all rhizobia, are common nodA% genes, in which mutations lead to a complete abolition of infection and nodulation. In addition, there are nod genes that are species/strain specific, such as noV De- narie et al., 1992; Fisher and Long, 1992). Nodulation Genes Specify the Production of Lipo-Oligosaccharide Nod Factors What could be the biochemical function of the rhizobial nod gene products? A link between the genetic determi-