Tropical Strains of Ralstonia solanacearum Outcompete Race 3 Biovar 2 Strains at Lowland Tropical Temperatures

Abstract

Bacterial wilt, caused by members of the heterogenous Ralstonia solanacearum species complex, is an economically important vascular disease affecting many crops. Human activity has widely disseminated R. solanacearum strains, increasing their global agricultural impact. However, tropical highland race 3 biovar 2 (R3bv2) strains do not cause disease in tropical lowlands, even though they are virulent at warm temperatures. We tested the hypothesis that differences in temperature adaptation and competitive fitness explain the uneven geographic distribution of R. solanacearum strains. Using three phylogenetically and ecologically distinct strains, we measured competitive fitness at two temperatures following paired-strain inoculations of their shared host, tomato. Lowland tropical strain GMI1000 was only weakly virulent on tomato under temperate conditions (24°C for day and 19°C for night [24/19°C]), but highland tropical R3bv2 strain UW551 and U.S. warm temperate strain K60 were highly virulent at both 24/19°C and 28°C. Strain K60 was significantly more competitive than both GMI1000 and UW551 in tomato rhizospheres and stems at 28°C, and GMI1000 also outcompeted UW551 at 28°C. The results were reversed at cooler temperatures, at which highland strain UW551 generally outcompeted GMI1000 and K60 in planta. The superior competitive index of UW551 at 24/19°C suggests that adaptation to cool temperatures could explain why only R3bv2 strains threaten highland agriculture. Strains K60 and GMI1000 each produced different bacteriocins that inhibited growth of UW551 in culture. Such interstrain inhibition could explain why R3bv2 strains do not cause disease in tropical lowlands. Ralstonia solanacearum, which causes bacterial wilt disease of many crops, is found on six continents, and its hosts include plants in over 50 dicot and monocot families. The pathogen forms a heterogenous species complex that contains thousands of distinct strains (1, 2). No single strain can attack all known hosts, but most members of the species complex can wilt tomato. The best way to manage bacterial wilt is by planting resistant crop varieties, but the high variability of the pathogen complicates breeding efforts because wilt resistance is often strain specific (3, 4). Sequencing over 20 diverse strains revealed that R. solanacearum has a fluid genome with evidence of extensive horizontal gene transfer (5-7). However, the specific mechanisms by which this pathogen has adapted to diverse environments and hosts are unknown. For example, most strains of R. solanacearum are tropical or subtropical, but one highly adapted group, known historically and for regulatory purposes as race 3 biovar 2 (R3bv2), causes economically important wilt of both potato and tomato in cool highland tropics (8).R. solanacearum strains fall into four genetically distinct phylotypes that correspond to their geographic origin: phylotypes I (Asia), II (the Americas), III (Africa), and IV (Indonesia) (1) However, because of human activity, multiple phylotypes are now commonly...

Genetic Diversity and Pathogenicity of Ralstonia solanacearum Causing Tobacco Bacterial Wilt in China

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“…Among the seven sequevars, sequevar 15 constituted the largest group and was widely distributed in all four regions in China, indicating that either sequevar 15 strains have more adaptability to the environment (temperature, humidity, and habitats) than that of other sequevars or they may produce bacteriocins that inhibited growth of other sequevars under temperate conditions (Huerta et al 2015). Therefore, this sequevar should be prioritized for monitoring in China.…”

Metabolomics of tomato xylem sap during bacterial wilt reveals Ralstonia solanacearum produces abundant putrescine, a metabolite that accelerates wilt disease

“…The lack of growth inhibition of healthy xylem sap was confirmed on solid media using an overlay inhibition assay (Huerta et al, 2015). Briefly, a suspension of R. solanacearum cells in semisolid CPG agar was overlaid on solid CPG agar.…”

Engineering bacteriocin‐mediated resistance against the plant pathogen Pseudomonas syringae

“…Bacteriocin production is not exclusive to the genus Pseudomonas and so this strategy should in principle be applicable to a wide variety of important phytopathogens, such as Xanthomonas spp. (which causes the greening and blight of rice and banana; Pham et al , ; Ghequire et al , ); Ralstonia solanacearum (which causes potato brown rot and bacterial wilt of tomato; Huerta et al , ) and Pectobacterium and Dickeya spp. (which cause potato soft rot and blackleg; Roh et al , ; Chan et al , ; Grinter et al , ; Czajkowski et al , ) .…”