Physiology and genetic bases of nodulation preference in common beans (Phaseolus vulgaris)-rhizobium coevolution

BEKER M.P., BRUZESSE D., MESCHINI E, BLANCO F., ZANETTI M.E, KÜSTER H, PÜHLER A., FAVELUKES G., AGUILAR O.M

Sorrento, Italia

Congreso; . XIII International Congress on Molecular Plant-Microbe Interactions 2007; 2007

Common beans are preferentially nodulated by geographically related R. etli lineages from the same centre of host diversification.  This is associated with polymorphism of the bacterial gene nodC, being nodCá and nodCd the most common alleles in nodules of Mesoamerican and Andean beans, respectively. To gain insight into the basis of this coevolution, nodulation kinetics was performed using four strains of each lineage. The number of nodules formed on the Mesoamerican bean cultivar Negro Xamapa by nodCá strains was significantly higher during the first days after inoculation. R. etli strains carrying the alleles nodCá and nodCd were differentially tagged with DsRed and GFP and used in competition assays. Two days after inoculation, only infection threads produced by nodCá strain were detected, whereas nodCd strains initiated the formation of this structure later, about four days after inoculation. These results provide evidence that the mechanisms underlying nodulation preference begins very early in the symbiotic association. In order to identify genes responsible for the selectivity in the symbiotic association, we constructed a cDNA macroarray containing 2108 non-redundant sequences corresponding to TCs of root hair from Negro Xamapa inoculated with R. etli SC15 (nodCá) or 55N1 (nodCä). Macroarrays were hybridized to cDNA of control (mock treated), SC15 or 55N1 inoculated beans. Differential clones were selected and confirmed by qRT-PCR. They showed homology to a diversity of genes involved in primary and secondary metabolism, signal transduction pathways, protein transport and folding, transcription factors, pathogen defence, etc. The role of these genes in the interaction is being assessed by RNAi silencing and overexpression using composite plants of common beans.