RESUMO

Bacterial wilt-causing Ralstonia threaten numerous crops throughout the world. We studied the population structure of 196 isolates of Ralstonia solanacearum and 39 isolates of Ralstonia pseudosolanacearum, which were collected from potato- and tomato-growing areas in 19 states of Brazil. Regardless of the species, three groups of isolates were identified. One group encompassed R. pseudosolanacearum isolates. The other two groups comprise isolates of R. solanacearum (phylotype II) split according to geographic regions, one made of isolates from the North and Northeast and the other made of isolates from the Central, Southeast, and South regions (CSS). Among the isolates collected in CSS, those from tomato were genetically distinct from the potato isolates. The genetic variability in the population of R. pseudosolanacearum was lower than that of R. solanacearum, suggesting that the former was introduced in Brazil. Conversely, the high genetic variability of R. solanacearum in all regions, hosts, and times supports the hypothesis that this species is autochthonous in South America, more precisely in Brazil and Peru. For R. solanacearum, higher variability and lower migration rates were observed when tomato isolates were analyzed, indicating that the variability is caused mainly by the differences of the local, native soil population. The North subpopulation was distinct from all others, possibly because of differences in environmental features of this region. The proximity of some geographic regions and the movement of potato tubers could have facilitated migration and therefore low genetic differentiation between geographic regions. Finally, geography, which also influences host distribution, affects the structure of the population of R. solanacearum in Brazil. Despite quarantine procedures in Brazil, increasing levels of trade are a threat to biosecurity, and these results emphasize the need for improving our regional efforts to prevent the dispersal of pathogens.

Assuntos

Ralstonia solanacearum , Brasil , Estruturas Genéticas , Peru , Filogenia , Doenças das Plantas , Ralstonia

RESUMO

Mesorhizobium loti MAFF303099 has a functional type III secretion system (T3SS) that is involved in the determination of nodulation competitiveness on Lotus. The M. loti T3SS cluster contains gene y4yS (mlr8765) that codes for a protein of unknown function (Y4yS). A mutation in the y4yS gene favors the M. loti symbiotic competitive ability on Lotus tenuis cv. Esmeralda and affects negatively the secretion of proteins through T3SS. Here we localize Y4yS in the bacterial membrane using a translational reporter peptide fusion. In silico analysis indicated that this protein presents a tetratricopeptide repeat (TPR) domain, a signal peptide and a canonical lipobox LGCC in the N-terminal sequence. These features that are shared with proteins required for the formation of the secretin complex in type IV secretion systems and in the Tad system, together with its localization, suggest that the y4yS-encoded protein is required for the formation of the M. loti T3SS secretin (RhcC2) complex. Remarkably, analysis of RhcC2 in the wild-type and M. loti y4yS mutant strains indicated that the absence of Y4yS affects negatively the accumulation of normal levels of RhcC2 in the membrane.