Ralstonia solanacearum, a widespread bacterial plant pathogen in the post-genomic era

Abstract

Ralstonia solanacearum is a soil-borne bacterium causing the widespread disease known as bacterial wilt. Ralstonia solanacearum is also the causal agent of Moko disease of banana and brown rot of potato. Since the last R. solanacearum pathogen profile was published 10 years ago, studies concerning this plant pathogen have taken a genomic and post-genomic direction. This was pioneered by the first sequenced and annotated genome for a major plant bacterial pathogen and followed by many more genomes in subsequent years. All molecular features studied now have a genomic flavour. In the future, this will help in connecting the classical field of pathology and diversity studies with the gene content of specific strains. In this review, we summarize the recent research on this bacterial pathogen, including strain classification, host range, pathogenicity determinants, regulation of virulence genes, type III effector repertoire, effector-triggered immunity, plant signalling in response to R. solanacearum, as well as a review of different new pathosystems.

Taxonomy: Bacteria; Proteobacteria; β subdivision; Ralstonia group; genus Ralstonia.

Disease symptoms: Ralstonia solanacearum is the agent of bacterial wilt of plants, characterized by a sudden wilt of the whole plant. Typically, stem cross-sections will ooze a slimy bacterial exudate. In the case of Moko disease of banana and brown rot of potato, there is also visible bacterial colonization of banana fruit and potato tuber.

Disease control: As a soil-borne pathogen, infected fields can rarely be reused, even after rotation with nonhost plants. The disease is controlled by the use of resistant and tolerant plant cultivars. The prevention of spread of the disease has been achieved, in some instances, by the application of strict prophylactic sanitation practices.

Useful websites: Stock centre: International Centre for Microbial Resources-French Collection for Plant-associated Bacteria CIRM-CFBP, IRHS UMR 1345 INRA-ACO-UA, 42 rue Georges Morel, 49070 Beaucouzé Cedex, France, http://www.angers-nantes.inra.fr/cfbp/. Ralstonia Genome browser: https://iant.toulouse.inra.fr/R.solanacearum. GMI1000 insertion mutant library: https://iant.toulouse.inra.fr/R.solanacearumGMI1000/GenomicResources. MaGe Genome Browser: https://www.genoscope.cns.fr/agc/microscope/mage/viewer.php?

Figure 1

Macro‐ and microscopic views of Ralstonia solanacearum and illustration of associated symptoms on plant bioassays. (a) Ralstonia solanacearum growing on complete BG medium (Boucher et al., 1985). The pink colour of the colonies is caused by the presence of triphenyl tetrazolium chloride in the medium. (b) Electron microscopy image of R. solanacearum rod‐shaped cells under division displaying pili structures (by the late Jacques Vasse). (c) Symptoms of bacterial wilt on Medicago truncatula plants. Inoculation in Jiffy pots with two wild‐type strains (top part). Gnotobiotic inoculation with a wild‐type strain and an hrp mutant (bottom part). (d) Symptoms of bacterial wilt on Arabidopsis thaliana plants. (e) Symptoms of bacterial wilt on tomato plants. (f) One eggplant with symptoms of bacterial wilt, and a healthy control plant.

Figure 2

Major pathways controlling the expression of Ralstonia solanacearum virulence genes. Ovals and squares indicate regulatory proteins, the latter representing the main regulatory hubs. Grey, regulatory inputs sensed by the bacterium; black, pathogenic activities controlled by this regulatory network. Arrows and T‐bars indicate activation or repression, respectively. Black lines denote control at the transcriptional level and red lines indicate post‐transcriptional effects. Full lines represent major effects and dotted lines slight transcriptional influences (modulation). For detailed explanations, see text. EPS, exopolysaccharide; HDF, Hrp‐dependent factor; 3‐OH PAME, 3‐hydroxy palmitic acid methyl ester; T3SS, type III secretion system.