Survey of Genomes - Ralstonia solanacearum

Bacterial pathogens don’t just attack humans and other animals. There are many important bacterial pathogens of plants and Sam Hitchcock, working with just his “off hand” (his dominant hand was in a cast), tells us about a pathogen that can attack over 200 different kinds of plants - Ralstonia solanacearum.

Welcome to Genomics Revolution. This is Sam Hitchcock from the 2019 Hiram College Genetics course and I will be hosting this episode on the genome of Ralstonia solanacearum GMI1000. R. solanacearum is a beta-proteobacterium, which is a class of proteobacteria that occupy diverse environments as pathogens living within hosts. R. solanacearum is no different, this pathogen is soil borne and infects roots. Different strains of R. solanacearum are able to attack 200 different hosts. The first strain of R. solanacearum to have its genome sequenced was Ralstonia solanacearum GMI1000, this took place in 2002 (3). The complete genome was sequenced in order to better understand pathogenicity and host response.

The genome of Ralstonia solanacearum GMI1000 is 5,810,922 base pairs long. 3,716,413 of which lie in a circular chromosome. The other 2,094,509 base pairs are in a circular plasmid. It was found that both of these replicons have a mosaic structure, which provides evidence for acquisition of genes through horizontal gene transfer (2). The genome was also found to encode 5,129 predicted proteins. It was found that many of the encoded proteins were equivalent to known pathogenetic proteins in other organisms. This further supported the idea that Ralstonia solanacearum GMI1000 is a pathogen. Studying the pathogenicity of this organism was also one of the main goals for sequencing this organism (2). 

 Ralstonia solanacearum GMI1000 strain most notably targets tobacco plants and tomato plants, when it attacks these plant it causes them to wilt. The genome sequence of this strain was used to study what caused this wilting. It was found that a double inactivation of both the avrA gene and the popP1 gene allowed GMI1000 to wilt the tobacco plants. Both avrA and popP1 genes are type III secretion system effectors (1). One interesting fact that the genome of GMI1000 showed was that there were over 40 possible type III secreted effector proteins identified. Then by comparing with other genomes it was found that bacterial plant pathogens and animal pathogens have their own distinct arrays of specialized type III dependent factors. Meaning that the effector proteins in this organism could not be found in an animal pathogen (2).

The sequencing of Ralstonia solanacearum GMI1000 allowed scientists the opportunity to study plant pathogens and host response at a genomic level. It also showed how this strain was able to cause wilt in many host plants. Then through later genome sequencing of different R. solanacearum strains researchers found that almost all genes encoding type III secretion system effectors are conserved. However, the promoters differ between differing strains. In the future these differences can be looked at to understand why certain strains attack certain plants (3).
Thank You for watching.

References:
 (1) Poueymiro et al., 2009. International Society for Molecular Plant-Microbe Interactions (5):538-50. Two type III secretion system effectors from Ralstonia solanacearum GMI1000 determine host-range specificity on tobacco.
(2) Salanoubat et al., 2002. Nature 415, 497-502. Genome sequence of the plant pathogen Ralstonia solanacearum.
(3) Ying et al., 2017. Frontiers in Microbiology 8:974. Genome Sequencing of Ralstonia solanacearum CQPS-1, a Phylotype I Strain Collected from a Highland Area with Continuous Cropping of Tobacco.