Molecular Genetic Analysis of Rhizobium tropici CIAT899 genes involved in symbiosis and environmental adaptation

Participants : Keilor Rojas-Jiménez (former PhD student of Pablo), Christian Sohlenkamp, Otto Geiger, Dietrich Werner, Esperanza Martínez-Romero and Pablo Vinuesa.


R. tropici CIAT899 is a N 2 -fixing microsymbiont of Phaseolus vulgaris and other legumes that was originally isolated from common bean growing in tropical acid soils of Colombia . This strain is highly tolerant to acidity and other environmental stress factors such as high temperature or elevated concentrations of aluminum and other metals.We have used Tn 5 insertion mutagenesis to identify genetic loci involved in acid tolerance by selecting mutants unable to grow on acidified media. Interestingly, the genetic analysis of three of these loci resulted in the discovery of genes not previously known to be involved in symbiosis or stress tolerance Some of them are involved in the modification of membrane lipids (i.e. lpiA, olcC ), others control the flux of ions through it ( sycA ) while others (i.e. atvA ) are orthologs of virulence genes with unknown biochemical function. As a matter of fact, s everal of these loci are orthologs of virulence genes, highlighting common underlying mechanisms in symbiosis and pathogenesis. (Fig. 4)

Fig. 4. Phaseolus vulgaris plant nodulated by Rhizobium tropici CIAT899 21 dpi (A). Macroscopic view of the nodules (B) and CLSM of a hand-cut nodule section showing the infection of the central tissue by CIAT899 expressing a constitutive gfp fusion.
Environmental and Symbiotic Microbiology Group