Karen Cornelis

The plant pathogen Rhodococcus fascians colonizes the exterior and interior of the aerial parts of plants.

Rhodococcus fascians is a plant-pathogenic bacterium that causes malformations on aerial plant parts, whereby leafy galls occur at axillary meristems. The colonization behavior on Nicotiana tabacum and Arabidopsis thaliana plants was examined. Independent of the infection methods, R. fascians extensively colonized the plant surface where the bacteria were surrounded by a slime layer. R. fascians caused the collapse of epidermal cells and penetrated intercellularly into the plant tissues. The onset of symptom development preceded the extensive colonization of the interior. The meristematic regions induced by pathogenic strain D188 were surrounded by bacteria. The nonpathogenic strain, D188-5, colonized the exterior of the plant equally well, but the linear plasmid (pFiD188) seemed to be involved in the penetration efficiency and colonization of tobacco tissues.

The att locus of Rhodococcus fascians strain D188 is essential for full virulence on tobacco through the production of an autoregulatory compound

The ability of Rhodococcus fascians strain D188 to provoke leafy gall formation on a variety of plant species is correlated with the linear plasmid pFiD188, on which different pathogenicity loci were identified. The att locus affects the severity of symptom development on tobacco, whereas the fas locus is essential for virulence. To gain insight into the function of the att locus, sequence and expression analyses were performed. The att locus contains nine open reading frames homologous to arginine and β‐lactam biosynthetic genes. att gene expression is transcriptionally induced by leafy gall extracts, but not by extracts of uninfected plants, and depends on the attR gene that encodes a LysR‐type transcriptional regulator. The att locus proves to be essential for the formation of inducing factors (IFs) that are present in gall extracts. Because the induction of the fas locus also requires the presence of IFs in gall extracts, the att locus is proposed to play an important role in regulating the expression of the virulence loci of R. fascians.

Virulence Genes of the Phytopathogen Rhodococcus fascians Show Specific Spatial and Temporal Expression Patterns During Plant Infection

The phytopathogenic bacterium Rhodococcus fascians provokes shoot meristem formation and malformations on aerial plant parts, mainly at the axils. The interaction is accompanied by bacterial colonization of the plant surface and tissues. Upon infection, the two bacterial loci required for full virulence, fas and att, were expressed only at the sites of symptom development, although their expression profiles differed both spatially and temporally. The att locus was expressed principally in bacteria located on the plant surface at early stages of infection. Expression of the fas locus occurred throughout infection, mainly in bacteria that were penetrating, or had penetrated, the plant tissues and coincided with sites of meristem initiation and proliferation. The implications for the regulation of virulence genes of R. fascians during plant infection are discussed.

Metabolic Colonisation: the Fate of a Leafy Gall

The Gram-positive plant pathogen Rhodococcus fascians has a broad host range and induces malformations without killing its host (7). To provoke symptoms the bacteria require the action of virulence genes located on a linear plasmid (pFiDl88). Three distinct pathogenicity loci have been characterised on pFiD188: fas, att and hyp (1). The fas operon is essential for disease development and its expression is tightly regulated by several regulatory proteins and a multitude of environmental signals. The fas locus encodes the enzymatic machinery for the synthesis of a signal molecule that disrupts the hormone balance of the host plant (2,6), which ultimately results in the formation of the most typical symptom a leafy gall. The two other loci are required for balanced disease: mutants in the att region have an attenuated virulence, while mutants in the hyp locus are hypervirulent.