Indian Journal of Experimental Biology Vol. 41, October 2003, pp. 1205-1208 11th International Congress on Molecular Plant-Microbe Interactions held at St. Petersburg - A report Barry G Rolfe*, Ulrike Mathesius, Michael A Djordjevic, Peter M Gresshoff" ARC Centre of Excellence for Integrative Legume Research and the Genome Interactions Group, Research School of Biological Sciences, The Australian National University, GPO Box 475, Canberra ACT 2601, Australia aCILR and Department of Botany, University of Queensland, St Lucia, Qld 4072, Australia The report is a short summary of the most interesting presentations at the n" International Congress on Molecular Plant-Microbe Interactions held during July 18-27,2003 at St. Petersburg, Russia. The key elements from several sessions on the DCBA legume-Rhizobium interactions have been discussed. Keywords: Autoregulation, Calcium spiking, Legume, Nodule, Plant-microbe interactions, Proteomic analysis, Rhizobium Legumes are very special plants. They form symbioses as a result of their roots being invaded by nitrogen-fixing soil bacteria, called rhizobia, which induce a root outgrowth called a nodule. The exchange of signals between the partners leads to specific developmental pathways being expressed. The plant provides carbon while the rhizobia produce fixed nitrogen which the legumes use for their metabolic needs. This report will only summarise key elements from several sessions on the legume- Rhizobium interactions. How do legumes recognise and distinguish Rhizobium The laboratory of Jens Stougaard I has used several new mutants of Lotus japonicus to generate new insights of the Rhizobium-legume interaction. Mutants Ljsyml and LjsymS are really nfrl and nfrS and are LysM receptor kinases, known to interact with oligosaccharide repeat units. The nfrS gene is also PssymJO in pea and nfrS may be Pssym2 of peas. Combining the data from both model legumes, L. japonicus and Medicago truncatula, a proposed signal transduction pathway might be: nfrl, nfrS, symRK, dmil , dmi2, (Ca Spiking), dmi3, npsl, nps2, (ccd gene expression), nin, ... nodulation. The two nfrl and nfr5 are receptor kinases and act before the NORK! symRK gene activity. The cloned gene NORK is a receptor kinase (a dmi2 ortholgue, G B Kiss) and is a member of a gene family called NSL (NORK Sequence Like) broadly distributed in the plant *For correspondence: Phone: +61 261254054; Fax: +61 261250754; E-mail: ro1fe@rsbs.anu.edu.au kingdom. One can imagine a cluster of receptor kinases at the root hair surface forming some type of heteromeric complex which in turn can bind the Rhizobium Nod Factors. The consequence of this binding could lead to ion fluxes, membrane depolarisation and ultimately NIN gene activation and the initiation of nodulation. However, it is the isolation of spontaneous nodulation mutants, that is nodulation in the absence of Rhizobium inoculation, that provides new insights to nodulation control 2. It is mutant, nar 1, which is a single recessive mutant that generates spontaneous nodules on L. japonicus at about the same levels as the wild type parent plant inoculated with Rhizobium. This has profound implications for control of nodulation. The narl mutant will form Fix + nodules if inoculated with Rhizobium strains and it does show the autoregulation phenotype. It is in a new gene and the proposal is that nar 1 is a repressor of cortical cell division (ccd). This repression normally would be alleviated by Rhizobium inoculation and hence cortical cell division and nodule formation. The site of action of the repression system could be at the nucleic acid level where it would be inactivated by the Rhizobium- generated transduction signal or at the proposed kinase complex level. At this level the repression system might act as a phosphatase enzyme blocking the transmission of the phosphorylation of the signal transduction system. A second nar mutant, nar2, is a gain of function mutant which is dominant and produces a constitutive nodulation phenotype. Both these mutants can be regulated by added nitrate and illustrate the complexity of the multiple controls of ccd nodule primordia in legumes.