How pathogens coevolve with and adapt to their hosts are critical to understanding how host jumps and/or acquisition of novel traits can lead to new disease emergences. The Xanthomonas genus includes Gram-negative plant-pathogenic bacteria that collectively infect a broad range of crops and wild plant species. However, individual Xanthomonas strains usually cause disease on only a few plant species and are highly adapted to their hosts, making them pertinent models to study host specificity. This review summarizes our current understanding of the molecular basis of host specificity in the Xanthomonas genus, with a particular focus on the ecology, physiology, and pathogenicity of the bacterium. Despite our limited understanding of the basis of host specificity, type III effectors, microbe-associated molecular patterns, lipopolysaccharides, transcriptional regulators, and chemotactic sensors emerge as key determinants for shaping host specificity.

Insects belonging to the recent orders of the endopterygote clade (Lepidoptera, Diptera, Hymenoptera and Coleoptera) respond to bacterial challenge by the rapid and transient synthesis of a battery of potent antibacterial peptides which are secreted into their haemolymph. Here we present the first report on inducible antibacterial molecules in the sap-sucking bug Pyrrhocoris apterus, a representative species of the Hemiptera, which predated the Endoptergotes by at least 50 million years in evolution. We have isolated and characterized from immune blood of this species three novel peptides or polypeptides: (i) a 43-residue cysteine-rich anti-(Gram-positive bacteria) peptide which is a new member of the family of insect defensins; (ii) a 20-residue proline-rich peptide carrying an O-glycosylated substitution (N-acetylgalactosamine), active against Gram-negative bacteria; (iii) a 133-residue glycine-rich polypeptide also active against Gram-negative bacteria. The proline-rich peptide shows high sequence similarities with drosocin, an O-glycosylated antibacterial peptide from Drosophila, and also with the N-terminal domain of diptericin, an inducible 9 kDa antibacterial peptide from members of the order Diptera, whereas the glycine-rich peptide has similarities with the glycine-rich domain of diptericin. We discuss the evolutionary aspects of these findings.

Background: TheXanthomonadaceaefamily contains two xylem-limited plant pathogenic bacterial species,Xanthomonas albilineansandXylella fastidiosa.X. fastidiosawas the first completely sequenced plant pathogen. It is insect-vectored, has a reduced genome and does not possesshrpgenes which encode a Type III secretion system found in most plant pathogenic bacteria.X. fastidiosawas excluded from theXanthomonasgroup based on phylogenetic analyses with rRNA sequences.Results: The complete genome ofX. albilineanswas sequenced and annotated.X. albilineans, which is not known to be insect-vectored, also has a reduced genome and does not possesshrpgenes. Phylogenetic analysis usingX. albilineansgenomic sequences showed thatX. fastidiosabelongs to theXanthomonasgroup. Order of divergence of theXanthomonadaceaerevealed thatX. albilineansandX. fastidiosaexperienced a convergent reductive genome evolution during their descent from the progenitor of theXanthomonasgenus. Reductive genome evolutions of the two xylem-limitedXanthomonadaceaewere compared in light of their genome characteristics and those of obligate animal symbionts and pathogens.Conclusion: The two xylem-limitedXanthomonadaceae, during their descent from a common ancestral parent, experienced a convergent reductive genome evolution. Adaptation to the nutrient-poor xylem elements and to the cloistered environmental niche of xylem vessels probably favoured this convergent evolution. However, genome characteristics ofX. albilineansdiffer from those ofX. fastidiosaand obligate animal symbionts and pathogens, indicating that a distinctive process was responsible for the reductive genome evolution in this pathogen. The possible role in genome reduction of the unique toxin albicidin, produced byX. albilineans, is discussed.