Ralstonia solanacearum; let the most fit strain win

Bacterial wilt, caused by members of the Ralstonia solanacearum (Rs) species complex, is an economically important vascular plant disease affecting many crops. We do not understand how strains of this heterogenous group of soilborne bacteria have adapted to diverse environments. GMI1000, UW551 and K60, are Rs strains that are phylogenetically distant but can cause wilting on some of the same hosts, including tomato. While GMI1000 does not survive the cooler regions where UW551 originated and causes disease, UW551 and K60 are capable of causing disease at both temperate (24ºC day/19º night) and tropical temperatures (28ºC). Because of increased agricultural movement, new strains could be introduced to new agricultural regions, posing a threat to susceptible hosts. However, co-occurrence of strains has not been observed in the field. We hypothesized that tropical strains enjoy a competitive advantage over temperate ones if they are in the same tropical environment and that temperate strains can outcompete tropical strains in the same temperate environment.  We found that subtropical R. solanacearum strain K60 was significantly more competitive than tropical strain GMI1000 in tomato rhizospheres and stems at 28º and 24/19ºC. K60 also outcompeted UW551 at 28ºC. At 28ºC, tropical strain GMI1000 was significantly more competitive than temperate strain UW551 in tomato rhizospheres and as expected, at 24/19°C, temperate strain UW551 outcompeted GMI1000 and K60. K60's surprising superiority in the competition assays in plants was further studied with overlay competition assays in culture, where we found that K60 produced a compound that inhibited growth of GMI1000 and UW551.