Mechanistic insights into host adaptation, virulence and epidemiology of the phytopathogen Xanthomonas.

Xanthomonas is a well-studied genus of bacterial plant pathogens whose members cause a variety of diseases in economically important crops worldwide. Genomic and functional studies of these phytopathogens have provided significant understanding of microbial-host interactions, bacterial virulence and host adaptation mechanisms including microbial ecology and epidemiology. In addition, several strains of Xanthomonas are important as producers of the extracellular polysaccharide, xanthan, used in the food and pharmaceutical industries. This polymer has also been implicated in several phases of the bacterial disease cycle. In this review, we summarise the current knowledge on the infection strategies and regulatory networks controlling virulence and adaptation mechanisms from Xanthomonas species and discuss the novel opportunities that this body of work has provided for disease control and plant health.

Management of a pet dog after exposure to a human patient with Ebola virus disease.

In October 2014, a health-care worker who had been part of the treatment team for the first laboratory-confirmed case of Ebola virus disease imported to the United States developed symptoms of Ebola virus disease. A presumptive positive reverse transcription PCR assay result for Ebola virus RNA in a blood sample from the worker was confirmed by the CDC, making this the first documented occurrence of domestic transmission of Ebola virus in the United States. The Texas Department of State Health Services commissioner issued a control order requiring disinfection and decontamination of the health-care worker's residence. This process was delayed until the patient's pet dog (which, having been exposed to a human with Ebola virus disease, potentially posed a public health risk) was removed from the residence. This report describes the movement, quarantine, care, testing, and release of the pet dog, highlighting the interdisciplinary, one-health approach and extensive collaboration and communication across local, county, state, and federal agencies involved in the response.

Top 10 plant pathogenic bacteria in molecular plant pathology.

Many plant bacteriologists, if not all, feel that their particular microbe should appear in any list of the most important bacterial plant pathogens. However, to our knowledge, no such list exists. The aim of this review was to survey all bacterial pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate the bacterial pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 458 votes from the international community, and allowed the construction of a Top 10 bacterial plant pathogen list. The list includes, in rank order: (1) Pseudomonas syringae pathovars; (2) Ralstonia solanacearum; (3) Agrobacterium tumefaciens; (4) Xanthomonas oryzae pv. oryzae; (5) Xanthomonas campestris pathovars; (6) Xanthomonas axonopodis pathovars; (7) Erwinia amylovora; (8) Xylella fastidiosa; (9) Dickeya (dadantii and solani); (10) Pectobacterium carotovorum (and Pectobacterium atrosepticum). Bacteria garnering honourable mentions for just missing out on the Top 10 include Clavibacter michiganensis (michiganensis and sepedonicus), Pseudomonas savastanoi and Candidatus Liberibacter asiaticus. This review article presents a short section on each bacterium in the Top 10 list and its importance, with the intention of initiating discussion and debate amongst the plant bacteriology community, as well as laying down a benchmark. It will be interesting to see, in future years, how perceptions change and which bacterial pathogens enter and leave the Top 10.

Diversification of the function of cell-to-cell signaling in regulation of virulence within plant pathogenic xanthomonads.

The virulence of plant pathogenic bacteria belonging to the genera Xanthomonas and Xylella depends upon cell-to-cell signaling mediated by the diffusible signal molecule DSF (Diffusible Signaling Factor). Synthesis and perception of the DSF signal require products of the rpf gene cluster. The synthesis of DSF depends on RpfF, whereas the RpfC/RpfG two-component system is implicated in DSF perception and signal transduction. The sensor RpfC acts to negatively regulate synthesis of DSF. In Xanthomonas campestris, mutation of rpfF or rpfC leads to a coordinate down-regulation in synthesis of virulence factors and a reduction in virulence. In contrast, in Xylella fastidiosa, the causal agent of Pierce's disease of grape, mutation of rpfF and rpfC have opposite effects on virulence, with rpfF mutants exhibiting a hypervirulent phenotype. The findings suggest that different xanthomonads have adapted the perception and function of similar types of signaling molecule to fit the specific needs for colonization of different hosts.

The Induction and Modulation of Plant Defense Responses by Bacterial Lipopolysaccharides.

Lipopolysaccharides (LPSs) are ubiquitous, indispensable components of the cell surface of Gram-negative bacteria that apparently have diverse roles in bacterial pathogenesis of plants. As an outer membrane component, LPS may contribute to the exclusion of plant-derived antimicrobial compounds promoting the ability of a bacterial plant pathogen to infect plants. In contrast, LPS can be recognized by plants to directly trigger some plant defense-related responses. LPS can also alter the response of plants to subsequent bacterial inoculation; these delayed effects include alterations in the expression patterns of genes coding for some pathogenesis-related (PR) proteins, promotion of the synthesis of antimicrobial hydroxycinnamoyl-tyramine conjugates, and prevention of the hypersensitive reaction caused by avirulent bacteria. Prevention of the response may allow expression of resistance in the absence of catastrophic tissue damage. Recognition of LPS (and other nonspecific determinants) may initiate responses in plants that restrict the growth of nonpathogenic bacteria, whereas plant pathogens may possess hrp gene-dependent mechanisms to suppress such responses.

The putative permease PhlE of Pseudomonas fluorescens F113 has a role in 2,4-diacetylphloroglucinol resistance and in general stress tolerance.

2,4-Diacetylphloroglucinol (PHL) is the primary determinant of the biological control activity of Pseudomonas fluorescens F113. The operon phlACBD encodes enzymes responsible for PHL biosynthesis from intermediate metabolites. The phlE gene, which is located downstream of the phlACBD operon, encodes a putative permease suggested to be a member of the major facilitator superfamily with 12 transmembrane segments. PhlE has been suggested to function in PHL export. Here the sequencing of the phlE gene from P. fluorescens F113 and the construction of a phlE null mutant, F113-D3, is reported. It is shown that F113-D3 produced less PHL than F113. The ratio of cell-associated to free PHL was not significantly different between the strains, suggesting the existence of alternative transporters for PHL. The phlE mutant was, however, significantly more sensitive to high concentrations of added PHL, implicating PhlE in PHL resistance. Furthermore, the phlE mutant was more susceptible to osmotic, oxidative and heat-shock stresses. Osmotic stress induced rapid degradation of free PHL by the bacteria. Based on these results, we propose that the role of phlE in general stress tolerance is to export toxic intermediates of PHL degradation from the cells.

Ubiquitin ligase-associated protein SGT1 is required for host and nonhost disease resistance in plants.

Homologues of the yeast ubiquitin ligase-associated protein SGT1 are required for disease resistance in plants mediated by nucleotide-binding site/leucine-rich repeat (NBS-LRR) proteins. Here, by silencing SGT1 in Nicotiana benthamiana, we extend these findings and demonstrate that SGT1 has an unexpectedly general role in disease resistance. It is required for resistance responses mediated by NBS-LRR and other R proteins in which pathogen-derived elicitors are recognized either inside or outside the host plant cell. A requirement also exists for SGT1 in nonhost resistance in which all known members of a host species are resistant against every characterized isolate of a pathogen. Our findings show that silencing SGT1 affects diverse types of disease resistance in plants and support the idea that R protein-mediated and nonhost resistance may involve similar mechanisms.