The hrpB and hrpG Regulatory Genes of Ralstonia solanacearum Are Required for Different Stages of the Tomato Root Infection Process

Abstract

hrp genes, encoding type III secretion machinery, have been shown to be key determinants for pathogenicity in the vascular phytopathogenic bacterium Ralstonia solanacearum GMI1000. Here, we show phenotypes of R. solanacearum mutant strains disrupted in the prhJ, hrpG, or hrpB regulatory genes with respect to root infection and vascular colonization in tomato plants. Tests of bacterial colonization and enumeration in tomato plants, together with microscopic observations of tomato root sections, revealed that these strains display different phenotypes in planta. The phenotype of a prhJ mutant resembles that of the wild-type strain. An hrpB mutant shows reduced infection, colonization, and multiplication ability in planta, and induces a defense reaction similar to a vascular hypersensitive response at one protoxylem pole of invaded plants. In contrast, the hrpG mutant exhibited a wild-type level of infection at secondary root axils, but the ability of the infecting bacteria to penetrate into the vascular cylinder was significantly impaired. This indicates that bacterial multiplication at root infection sites and transit through the endodermis constitute critical stages in the infection process, in which hrpB and hrpG genes are involved. Moreover, our results suggest that the hrpG gene might control, in addition to hrp genes, other functions required for vascular colonization.

A luminescent reporter evidences active expression of Ralstonia solanacearum type III secretion system genes throughout plant infection

“…2, top pictures). Focal dissection of the images proved that the fluorescence originated inside the root and localized in the xylem vessels, as expected for this vascular pathogen (Vasse et al, 2000). Three days later, the fluorescence became brighter and had extended towards upper positions of the root.…”

“…In support of the latter is the observation that a functional HrpX, the HrpB orthologue in X. campestris, is required to inhibit a hypersensitive response at the vascular level in crucifers (Kamoun et al, 1992). Similarly, degeneration of cells flanking the protoxylem has also been observed in tomato plants challenged with an hrpB-mutated R. solanacearum strain (Vasse et al, 2000).…”

“…This soil-borne b-proteobacterium is the causative agent of bacterial wilt on a wide range of plant hosts, including economically important species such as tomato, potato, peanut and eggplant (Hayward, 2000). R. solanacearum invades plants through root wounds and rapidly colonises the xylem vessels, where it multiplies extensively and produces large amounts of exopolysaccharide (EPS) (Kao et al, 1992;Vasse et al, 2000). EPS accumulation in the vascular system and the ensuing collapse of the water flow causes the wilting symptoms and eventually plant death.…”

Reactive oxygen species and ethylene play a positive role in lateral root base nodulation of a semiaquatic legume

“…The diazotrophic endophyte Azoarcus forms colonies in the aerenchyma of f lood-tolerant Kallar grass (55,56). Intercellular colonies are formed during root invasion by pathogenic Ralstonia solanacearum (57) and in the upper parts of host plants by the biotrophic phytopathogen, Rhodococcus fascians (58). Our data provide a basis for future investigations of the differences and similarities in pathogenic and beneficial intercellular colonization strategies, as a way to understand the subtle edge between symbiosis and pathogenicity.…”

The Virulence Factors of the Bacterial Wilt Pathogen Ralstonia solanacearum

“…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.…”