Studies on the nodulation preferente in Phaseolus vulgaris-rhizobium coevolution

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

Varenna, Italia

Congreso; Phaseomics V; 2007

Phaseolus vulgaris is preferentially nodulated by R. etli lineages which are geographically related to the centre of host diversification.  This is shown by 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, kinetics of infection threads formation and nodulation were performed using four strains of each lineage. The number of infection threads and 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. Currently, the role of these genes in the interaction is being assessed by RNAi silencing and over-expression using composite plants of common beans.