Resumo

Rhizobium tropici strain PRF 81 is an important nitrogen fixing symbiont of common bean (Phaseolus vulgaris L.) in tropical acids soils; however the molecular mechanisms related to R. tropici symbiosis are still poorly studied. In addition to the nodulation (nod) factors, the rhizobial surface polysaccharides, such as exo- (EPS) lipo- (LPS) and capsular polysaccharides (KPS) are key molecules for the establishment of the Rhizobium-legume symbiosis. In R. tropici, very few studies have been performed so far elucidating the structures of surface polysaccharides, but its roles in symbiosis is not elucidated yet. Recently, a significant contribution to the knowledge of R. tropici specie was achieved by genome sequencing of the strain PRF81, which revealed new genes involved in biosynthesis of surface polysaccharide (our unpublished data). Here, the transcription levels of lpxA, lpxE and rkpI genes of R. tropici strain PRF 81 were investigated after cell exposure to common bean exudates containing nodulation genes (nod) inducers (anthocyanidins and flavonols). RNA pools were obtained from cultures (three replicates) at distinct induction times and then, used for quantification of expression by reverse-transcription quantitative PCR (RT-qPCR). RT-qPCR analysis revealed, by the first time, that inducers compounds of exudates stimulated strongly the expression of all three surface polysaccharide genes of R. tropici PRF81. In the first assay, in which cells were grown for 48 hours in the presence of seed common bean exudates, a significant increase of about twofold in the transcription level of lpxE, lpxA and rkpI genes was observed. However, in the second test, after only five minutes of exposure to seed exudates, the transcription level of lpxE had increased about fiftyfold, and about thirtyfold for lpxA and rkpI, reaching levels statistically significant (p<0.05) as compared to the control. The lpxA and lpxE genes are involved in biosynthesis and modification of lipid A, the hydrophobic anchor of LPS, while the rkpI gene is related to a lipid carrier required for KPS synthesis. The up regulation of lpxE, lpxA and rkpI genes by common bean exudates suggests that inducer compounds released from seed exudates promotes changes in the R. tropici cell wall surface, which may be necessary to the establishment of the R. tropici-Phaseolus symbiosis.