The Plant Pathogen Pseudomonas syringae pv. tomato Is Genetically Monomorphic and under Strong Selection to Evade Tomato Immunity

Abstract

Recently, genome sequencing of many isolates of genetically monomorphic bacterial human pathogens has given new insights into pathogen microevolution and phylogeography. Here, we report a genome-based micro-evolutionary study of a bacterial plant pathogen, Pseudomonas syringae pv. tomato. Only 267 mutations were identified between five sequenced isolates in 3,543,009 nt of analyzed genome sequence, which suggests a recent evolutionary origin of this pathogen. Further analysis with genome-derived markers of 89 world-wide isolates showed that several genotypes exist in North America and in Europe indicating frequent pathogen movement between these world regions. Genome-derived markers and molecular analyses of key pathogen loci important for virulence and motility both suggest ongoing adaptation to the tomato host. A mutational hotspot was found in the type III-secreted effector gene hopM1. These mutations abolish the cell death triggering activity of the full-length protein indicating strong selection for loss of function of this effector, which was previously considered a virulence factor. Two non-synonymous mutations in the flagellin-encoding gene fliC allowed identifying a new microbe associated molecular pattern (MAMP) in a region distinct from the known MAMP flg22. Interestingly, the ancestral allele of this MAMP induces a stronger tomato immune response than the derived alleles. The ancestral allele has largely disappeared from today's Pto populations suggesting that flagellin-triggered immunity limits pathogen fitness even in highly virulent pathogens. An additional non-synonymous mutation was identified in flg22 in South American isolates. Therefore, MAMPs are more variable than expected differing even between otherwise almost identical isolates of the same pathogen strain.

Receptor Kinases in Plant-Pathogen Interactions: More Than Pattern Recognition

“…This unique feature can be exploited to rapidly identify new effectors or microbial patterns by interrogating microbial genome sequences. As such, a second 28-amino acid epitope (flgII-28) of bacterial flagellin and Ave1 of Verticillium dahliae were found to be subject to strong positive selection (Cai et al, 2011;de Jonge et al, 2012). These findings directly led to the discovery of flgII-28 as a pattern recognized by LRR-RK FLS3 (see below) and Ave1 as an effector recognized by LRR-RLP Ve1 in tomato plants.…”

A Subset of Ubiquitin-Conjugating Enzymes Is Essential for Plant Immunity

“…A luminol-horseradish peroxidase assay was used to quantify ROS induction as described previously (Cai et al, 2011) with small modifications. In brief, leaf discs of 4 mm in diameter were punched out with a cork borer from N. benthamiana plants and floated adaxial side up in 200 mL of distilled, deionized water at room temperature overnight in wells of a 96-well Nunc white plate (Thermo Scientific).…”

Plant Innate Immunity: Perception of Conserved Microbial Signatures

“…For this reason, we use the term conserved microbial signatures throughout this review. Endogenous elicitors that are released from the host by enzymatic or mechanical processes controlled by the pathogen are now widely referred to as danger-associated molecular patterns (DAMPs) (20,142 (27,200). In the case of Ax21, tyrosine sulfation is required for XA21 binding and immunity (133).…”

“…The recognized domain within flagellin is not necessarily the same for all plant species. For example, in addition to the classical flg22 peptide, tomato is able to recognize a shorter version (called flg15) and a second, newly identified, conserved 28-amino-acid region just C-terminal of flg22 (called flgII-28) (14,27,172). Similarly, rice is able to recognize flg22 but is more responsive to full-length flagellin protein (204).…”