Genomic and Evolutionary Features of the SPI-1 Type III Secretion System That Is Present inXanthomonas albilineansbut Is Not Essential for Xylem Colonization and Symptom Development of Sugarcane Leaf Scald

Abstract

Xanthomonas albilineans is the causal agent of sugarcane leaf scald. Interestingly, this bacterium, which is not known to be insect or animal associated, possesses a type III secretion system (T3SS) belonging to the injectisome family Salmonella pathogenicity island 1 (SPI-1). The T3SS SPI-1 of X. albilineans shares only low similarity with other available T3SS SPI-1 sequences. Screening of a collection of 128 plant-pathogenic bacteria revealed that this T3SS SPI-1 is present in only two species of Xanthomonas: X. albilineans and X. axonopodis pv. phaseoli. Inoculation of sugarcane with knockout mutants showed that this system is not required by X. albilineans to spread within xylem vessels and to cause disease symptoms. This result was confirmed by the absence of this T3SS SPI-1 in an X. albilineans strain isolated from diseased sugarcane. To investigate the importance of the T3SS SPI-1 during the life cycle of X. albilineans, we analyzed T3SS SPI-1 sequences from 11 strains spanning the genetic diversity of this species. No nonsense mutations or frameshifting indels were observed in any of these strains, suggesting that the T3SS SPI-1 system is maintained within the species X. albilineans. Evolutionary features of T3SS SPI-1 based on phylogenetic, recombination, and selection analyses are discussed in the context of the possible functional importance of T3SS SPI-1 in the ecology of X. albilineans.

Draft Genome Sequences of Xanthomonas sacchari and Two Banana-Associated Xanthomonads Reveal Insights into the Xanthomonas Group 1 Clade

“…Analysis of the X. albilineans genome sequence revealed that this species displays several interesting features that are quite distinct from those of the Group 2 species. For example, X. albilineans lacks the Hrp T3SS that is universally conserved and central to pathogenicity in Group 2, but it encodes an alternative non-Hrp T3SS that shares sequence similarity to the Salmonella SPI-1 T3SS [9]. This raises the question of whether these features of the X. albilineans genome are also shared with the genomes of other Group 1 Xanthomonas species.…”

Commonalities and differences of T3SSs in rhizobia and plant pathogenic bacteria

“…For example, the second T3SS cluster of P. fluorescens F113 is induced during F113-amoeba interactions and seems to be involved in resistance to bacteriovorous protists, while the role of its Hrp/Hrc1-type T3SS is yet unknown (Barret et al, 2013). Lastly, the single T3SS of Xanthomonas albilineans also belongs to the Inv/Mxi/Spa type T3SSs (Marguerettaz et al, 2011; Figures 1, 4). Although its role is yet unknown, functional analysis provided evidence that it is not required for virulence in plants.…”

The SPI-1-like Type III secretion system: more roles than you think

“…Homologs of SPI-1 T3SS have been identified and found to be essential virulence determinants in other mammalian pathogens such as Chromobacterium , Escherichia , Shigella, and Yersinia species (Barinaga, 1996; Miki et al, 2010), thus confirming its role in animal/human pathogenesis. However, recent reports in the literature have highlighted the presence of SPI-1 T3SSs outside of mammalian pathogenic bacteria, such as Sodalis (Dale et al, 2005), Erwinia (Triplett et al, 2006), Xanthomonas (Alavi et al, 2008; Marguerettaz et al, 2011), Pantoea (Correa et al, 2012), and Pseudomonas species (Barret et al, 2013a; Redondo-Nieto et al, 2013). Concurrently, the rise in genome sequences is revealing that the SPI-1 T3SS is found in a multitude of other bacterial strains ( Figure 1 ).…”

“…Phylogenetic analyses have consistently split the SPI-1 family into 2 sub-clusters (Marguerettaz et al, 2011; Abby and Rocha, 2012; Barret et al, 2013b). One sub-cluster has a greater frequency of classical enteric bacteria such as Salmonella , Shigella , and Escherichia coli , while the other contains other bacterial strains associated with plant insect or soil environments.…”