Expression of the Xanthomonas campestris pv. vesicatoria hrp gene cluster, which determines pathogenicity and hypersensitivity on pepper and tomato, is plant inducible

Abstract

The hrp gene cluster from Xanthomonas campestris pv. vesicatoria determines functions necessary not only for pathogenicity on the host plants pepper and tomato but also for the elicitation of the hypersensitive reaction on resistant host and nonhost plants. Transcriptional orientation and expression of the hrp loci were determined with hrp::Tn3-gus fusions. In addition, expression of the hip loci was studied by RNA hybridization experiments. Expression of the hrp genes was not detectable after growth of the bacteria in complex medium or in minimal medium. However, high levels of induction of hrp gene expression were measured during growth of the bacteria in the plant. To search for a plant molecule responsible for this induction, we examined a variety of materials of plant origin for their ability to induce hrp gene expression. Filtrates from plant suspension cultures induced hrp genes to levels comparable to those induced in the plant. The inducing molecule(s) was found to be heat stable and hydrophilic and to have a molecular mass of less than 1,000 daltons.The molecular mechanisms involved in plant-bacterium interactions during pathogenesis are complex and far from being understood. In the last few years, a number of bacterial genes that determine the outcome of the interaction between the bacterium and the plant have been identified and isolated. Most notable are two classes of genes required for basic compatibility: disease-specific (dsp) genes, which are associated with disease development in host plants but not with the induction of a hypersensitive response (HR) in nonhost plants (7, 27); and hrp genes, which are required for both the pathogenic interaction with host plants and the induction of the HR in resistant host and nonhost plants. hrp genes have been cloned from a number of different species of gram-negative phytopathogenic bacteria, e.g., Erwinia amylovora, Pseudomonas solanacearum, and pathovars of Pseudomonas syringae and ofXanthomonas campestris (for a review, see reference 35). Genetic analysis of hrp genes from these different organisms indicates that they determine basic pathogenicity functions necessary for any interaction with the plant. The elucidation of their biochemical function and their role in the plant-bacterium interaction is expected to lead to a molecular understanding of the mechanisms underlying bacterial plant diseases.We have chosen the interaction between X. campestris pv. vesicatoria, the causal agent of bacterial spot disease, and its host plants, pepper (Capsicum annuum L.) and tomato (Lycopersicum esculentum Mill.), as a system for the analysis of hrp genes. After invasion of the plant via stomata or wounds, X. campestris pv. vesicatoria multiplies in the intercellular spaces of the leaf tissue, giving rise to disease symptoms (29). Depending on the susceptibility of the particular plant cultivar, two different types of reactions can be observed. In the susceptible plant, water-soaked lesions occur (compatible interaction). In the resistant plant, avirulent strains in...

Virulence of the Phytopathogen Pseudomonas syringae pv. Maculicola Is rpoN Dependent

“…Similarly, in Xanthomonas campestris pv. vesicatoria, hrp gene expression is regulated by an OmpR homolog, HrpG, which in turn activates HrpX, another AraC-like activator that activates the remaining hrp genes (79,92). This latter regulatory cascade is consistent with the fact that rpoN is not required for hrp expression or pathogenicity in X. campestris pv.…”

Characterization of the fimA gene encoding bundle-forming fimbriae of the plant pathogen Xanthomonas campestris pv. vesicatoria

“…Promoter fusions to the ␤-glucuronidase (gusA) gene were obtained after random mutagenesis of pTMO7 in E. coli with Tn3-gus (6) or cloning a fragment in front of the promoterless gusA gene in pLAFR6GUSB (21). For ␤-glucuronidase assays, bacterial cells grown in NYB medium to the exponential growth phase were harvested, and activity was measured as described previously (40).…”

Identification of differentially expressed genes of Xanthomonas axonopodis pv. citri by representational difference analysis of cDNA

“…This strain was cultured in nutrient yeast glycerol (NYG) medium (Daniels et al, 1984) with and without agar addition, at 28°C. The modified MM1 medium (Schulte and Bonas, 1992) containing 20 mM NaCl, 10 mM (NH 4 ) 2 SO 4 , 5 mM MgSO 4 , 1 mM CaCl 2 , 0.16 mM KH 2 PO 4 , 0.32 mM K 2 HPO 4 and 10 mM sucrose was used as the basal medium. For the induction experiments, the bacterium was grown in NYG medium until an A 600 = 1.2 was reached in order to obtain cellular mass.…”

“…This protein regulates directly or indirectly genes implicated in pathogenicity, which are usually repressed in complex media in Xanthomonas (Schulte and Bonas, 1992;Wei et al, 1992). de Crecy-Lagard et al (1990) analyzed a mutant of X. campestris pv.…”

“…citri in response to different growth conditions by cDNA RDA. The modified MM1 medium (Schulte and Bonas, 1992), described in the induction of the hrp genes in X. campestris, was used as a basal medium. cDNA from the bacterium grown in the presence of leaf extract from a susceptible host (sweet orange) as well as in the host plant leaves and complex medium was used in successive rounds of subtractive hybridizations against cDNA from the MM1 treatment.…”