The Virulence Factors of the Bacterial Wilt Pathogen Ralstonia solanacearum

One of the most important virulence factors is a heterogeneous polymer of N-acetylated extracellular polysaccharide I (EPS I) [8]. EPS I-deficient mutants are nearly avirulent and do not colonize plant xylem vessels as efficient as wild type [4,9]. It has been suggested that EPS I directly causes wilting by physically blocking the vascular system and thereby alters water movement [8]. It has also been hypothesized that EPS protects R. solanacearum from plant antimicrobial defenses by cloaking bacterial surface features that could be recognized by hosts [4,9]. Interestingly, it has been recently found that R. solanacearum EPS I plays different roles in resistant and susceptible hosts [10]. In susceptible tomato plants, the wild-type and EPS I-deficient mutant induced generally similar defense responses; but in resistant tomato plants, the wild-type induced significantly greater defense responses than the EPS I-deficient mutants, suggesting that the EPS I itself is a specific elicitor of plant defense responses [10]. The Type III Secretion System

Ralstonia solanacearum, previously named Pseudomonas solanacearum and Burkholderia solanacearum, is a soil-borne gramnegative bacterium that causes bacterial wilt disease on more than 200 plant species from 50 botanical families, including important crops such as potato, tomato, eggplant, pepper, tobacco and banana [1,2]. R. solanacearum is considered a species complex-a heterogeneous group of related but genetically distinct strains [3]. This bacterium infects plants through root wounds or at sites of secondary root emergence, then colonizes the xylem vessels and spreads rapidly to aerial parts of the plant through the vascular system. In xylem vessels, the bacterial population can multiply extensively and rapidly reach very high levels (>10 10 cells/cm of stem in tomato) [4,5]. Typical disease symptoms include browning of the xylem, chlorosis, stunting, wilting, and the infected plants usually die rapidly. Bacterial wilt is considered one of the most destructive bacterial plant diseases because of its extreme aggressiveness, world-wide geographic distribution, and unusually broad host range [6]. In fact, R. solanacearum was ranked 2 nd in a list of the top 10 most scientifically/economically important plant pathogenic bacterial pathogens in 2012 [7].
R. solanacearum has been widely accepted as a model organism for the study of bacterial virulence and pathogenicity in plants [1]. To date, many factors have been found to contribute to the virulence of R. solanacearum, however, due to the limited space, this paper will only discuss the major virulence factors in this pathogen.
The Type III Secretion System (T3SS) has a central role in pathogenesis of many bacterial pathogens of plants and animals [11]. In R. solanacearum, the T3SS is encoded by the hrp gene cluster, which spans a 23-kb region on the mega plasmid [12]. As in other major groups of Gram-negative bacteria, R. solanacearum hrp genes are key determinants for disease development on compatible hosts and for induction of the defensive hypersensitive response (HR) on resistant plants [13]. R. solanacearum is estimated to produce 70-80 type III effectors [14]. The completely nonpathogenic phenotype of R. solanacearum T3SS-defective mutants illustrates the collective importance of the effector proteins that are injected into plant cells by the system, although mutants lacking single effectors are usually fully virulent [11,15]. The T3SS of R. solanacearum contributes greatly to pathogenesis, but hrp mutants retain the ability to invade tomato roots and systemically colonize the vascular system, although the population size of T3SS mutants in infected tissues was reduced by 10 to 1000 fold compared to wild-type strains [16,17]. Recently, in planta transcriptome study and qRT-PCR tests by Jacobs et al. [18] and in planta expression study using green fluorescent protein reporter fusions by Monterio et al. [19] found that the T3SS is still active even after R. solanacearum has taken over the xylem, suggesting that the T3SS is functional throughout disease. These results changed the wide spread view from in vitro studies that T3SS is only active at the first stage of infection and is not needed when bacteria reach high cell densities [20,21].
swimming motility contributes to virulence in the early stage of host colonization and invasion [22,23]. However, when R. solanacearum grows in plant xylem, virtually all the bacterial cells are nonmotile [22,23]. Interestingly, recently it is reported that a hypermotile motN mutant of R. solanacearum is also reduced in virulence [26], indicating the importance of precise regulation of motility in this bacterium. R. solanacearum strains with mutations in pilQ, pilT or pliA lost twitching soil-drench and cut-petiole inoculation [24,25]. Furthermore, the pilA mutant was also affected in biofilm formation, adherence to multiple surfaces and natural transformation [24]. Together, these results demonstrate that type IV pili and twitching motility are important for several stages of wilt disease development.

Other Virulence Factors
, The virulence factors of R. solanacearum are controlled by a complex regulatory signal transduction pathway that responds to both environmental signals and quorum sensing molecule 3-hydroxypalmitic acid methyl ester (3-OH PAME) [20,42]. For details, the readers are referred to a very comprehensive review on regulation of virulence and pathogenicity genes written by Schell [43]. Over the years, substantial progress has been made in studying R. solanacearum and bacterial wilt disease [43]. With genomes of more than 10 strains from R. solanacearum species complex being available now [44][45][46][47][48][49][50], our knowledge about this pathogen will be broadened further. Based on comparative genomic analysis of eight sequenced strains, it has been proposed that the R. solanacearum core genome comprises ~2,850 conserved genes, whereas the variable genome contains ~3,100 genes and the numbers of strain-specific genes vary from strain to strain [46,47]. This great genetic variation may account for the broad host range of R. solanacearum species complex and makes it more challenging to determine which genes are responsible for host-range speciation. A better understanding of the R. solanacearum virulence factors and their complex regulation will lead to novel avenues for research and effective disease control strategies.
Besides the above mentioned virulence factors, R. solanacearum also has other factors that contribute to its virulence. For example, R. solanacearum encounters reactive oxygen species (ROS) during bacterial wilt pathogenesis and expresses diverse oxidative stress response genes to detoxify ROS or otherwise tolerate this oxidative environment [34][35][36]. Inactivation of oxyR, which encodes the only identified regulator of oxidative stress gene in R. solanacearum significantly reduced virulence [37]. Two genes, acrA and dinF encoding multidrug efflux pumps, were also found to contribute to bacterial wilt virulence [38]. Some metabolic pathways also appear to be required during pathogenesis of R. solanacearum, and deletion of metER methionine biosynthesis genes produced significantly reduced disease symptoms without causing auxotrophy or affecting growth inside the plant [39,40]. R. solanacearum was also found to need Flp pili for virulence on potato [41].