Xanthomonas is a large genus of Gram-negative, yellow-pigmented bacteria that cause disease in nearly 400 plant hosts, including economically important crops such as rice, citrus, banana, cabbage, tomato, pepper and bean.
Currently, the complete genome sequences of 11 Xanthomonas spp. strains have been determined and draft genomes of a further seven strains are available, in total comprising seven species and nine pathovars that are distinguished by their host range.
Large-scale comparative analysis has revealed extraordinary genome plasticity within the genus Xanthomonas. Additional genetic variation comes from the presence of plasmids and large sets of insertion sequence (IS) elements in some strains.
The determination of genome sequences has greatly accelerated functional analyses that aim to understand the molecular basis of virulence of Xanthomonas spp.
Comparative genomics in Xanthomonas spp. has led to a better understanding of genome evolution and has identified candidate genes that may determine host and tissue specificity.
Xanthomonas is a large genus of Gram-negative bacteria that cause disease in hundreds of plant hosts, including many economically important crops. Pathogenic species and pathovars within species show a high degree of host plant specificity and many exhibit tissue specificity, invading either the vascular system or the mesophyll tissue of the host. In this Review, we discuss the insights that functional and comparative genomic studies are providing into the adaptation of this group of bacteria to exploit the extraordinary diversity of plant hosts and different host tissues.
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The work of the authors has been supported in part by grants awarded by the Science Foundation of Ireland (SFI 07/IN.1/B955 to J.M.D. and SFI 09/SIRG/B1654 to R.P.R.) and a European Society of Clinical Microbiology and Infectious Diseases (ESCMID) research grant (to R.R.P). The authors thank Y. McCarthy for helpful comments on the manuscript.
The authors declare no competing financial interests.
Pathogenic variants within a species that are defined by a characteristic host range and/or tissue specificity.
Plant tissue in the leaf mesophyll that has a diverse range of functions, including photosynthesis, storage, secretion and short-distance transport.
Secretory organs in leaves, usually of angiosperms, that are located at the leaf margin and that secrete guttation fluid, which may contain a variety of organic and inorganic solutes.
Members of the family Xanthomonadaceae, which is a family of Gram-negative bacteria that includes species from the genera Xanthomonas and Xylella (which cause plant diseases) and species from the genus Stenotrophomonas (one of which, Stenotrophomonas maltophilia, is an opportunistic human pathogen).
- Type II secretion system
A two-step secretion system that secretes proteins which are first translocated across the inner membrane by the general secretion pathway.
- Type III secretion system
A multisubunit protein apparatus that is used to secrete or inject effector proteins which contribute to interactions with eukaryotic cells.
- Type III effectors
Bacterial proteins that are delivered into a host cell through a type III secretion system, which is required for pathogenesis. Contributions of individual effectors to disease vary, and some trigger host defence.
- Type IV secretion systems
Secretion systems typically comprising a macromolecular complex that spans the bacterial inner and outer membranes and can also span the membrane of eukaryotic host cells. These secretion systems contribute to various biological functions, including the exchange of genetic material with other bacteria and the translocation of oncogenic DNA and effector proteins into eukaryotic host cells.
- Two-component regulators
Part of a mechanism that allows bacteria to sense and respond to changes in many different environmental cues. These systems typically consist of a membrane-bound histidine kinase that senses a specific environmental stimulus, and the corresponding response regulator that mediates the cellular response.
- TonB-dependent transporters
A family of proteins in the outer membrane of Gram-negative bacteria that sense environmental signals or substrates and thus trigger changes in gene transcription or uptake of the substrate across the outer membrane. These functions are dependent on the energy-transducing protein TonB.
- Cyclic di-GMP
A bacterial second messenger that is involved in the regulation of a wide variety of cellular processes.
- Non-host resistance
A host–pathogen interaction in which all members of a plant species exhibit resistance to all strains of a pathogen.
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Ryan, R., Vorhölter, FJ., Potnis, N. et al. Pathogenomics of Xanthomonas: understanding bacterium–plant interactions. Nat Rev Microbiol 9, 344–355 (2011). https://doi.org/10.1038/nrmicro2558
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