Comparative Genome Analysis Provides Insights into the Evolution and Adaptation of Pseudomonas syringae pv. aesculi on Aesculus hippocastanum

Abstract

A recently emerging bleeding canker disease, caused by Pseudomonas syringae pathovar aesculi (Pae), is threatening European horse chestnut in northwest Europe. Very little is known about the origin and biology of this new disease. We used the nucleotide sequences of seven commonly used marker genes to investigate the phylogeny of three strains isolated recently from bleeding stem cankers on European horse chestnut in Britain (E-Pae). On the basis of these sequences alone, the E-Pae strains were identical to the Pae type-strain (I-Pae), isolated from leaf spots on Indian horse chestnut in India in 1969. The phylogenetic analyses also showed that Pae belongs to a distinct clade of P. syringae pathovars adapted to woody hosts. We generated genome-wide Illumina sequence data from the three E-Pae strains and one strain of I-Pae. Comparative genomic analyses revealed pathovar-specific genomic regions in Pae potentially implicated in virulence on a tree host, including genes for the catabolism of plant-derived aromatic compounds and enterobactin synthesis. Several gene clusters displayed intra-pathovar variation, including those encoding type IV secretion, a novel fatty acid biosynthesis pathway and a sucrose uptake pathway. Rates of single nucleotide polymorphisms in the four Pae genomes indicate that the three E-Pae strains diverged from each other much more recently than they diverged from I-Pae. The very low genetic diversity among the three geographically distinct E-Pae strains suggests that they originate from a single, recent introduction into Britain, thus highlighting the serious environmental risks posed by the spread of an exotic plant pathogenic bacterium to a new geographic location. The genomic regions in Pae that are absent from other P. syringae pathovars that infect herbaceous hosts may represent candidate genetic adaptations to infection of the woody parts of the tree.

Aggressive Emerging Pathovars of Xanthomonas arboricola Represent Widespread Epidemic Clones Distinct from Poorly Pathogenic Strains, as Revealed by Multilocus Sequence Typing

“…actinidiae on kiwi fruit. A pattern comparable to the one in this study is seen with epidemic clonal strains responsible for canker symptoms, whereas phylogenetically related strains can only cause leaf damage (84,85). Comparative genomics will provide clues about the evolutionary mechanisms underlying these adaptation processes and potential common functions of pathogens of woody hosts.…”

“…Comparative and population genomics will help to validate this hypothesis and to elucidate the evolutionary events that have led to the emergence of these threatening tree pathogens. Within Pseudomonas syringae, phylogenetic clades made up of only tree pathogens have also been observed (84,85), suggesting a sequential adaptation, with the bacterial adaptation to tree biology being the first step that occurs before host specialization. Comparative genomics might decipher common adaptive traits of tree pathogens.…”

Endophytes in changing environments - do we need new concepts in forest management?

“…has not been investigated up to now in this context. Several authors have tried to explain the infection pathway and history of the so called "novel" serious diseases, like ash dieback (Kowalski 2006), bleeding cancer in Horse chestnut (Aesculus hippocastanum - Green et al 2010) or Cryptostroma corticale the sooty bark disease of sycamore (Acer pseudoplatanus - Bevercombe & Rayner 1984). They hypothesized that climatic changes might induce stress reactions in the host plants which may favor the spread of aggressive pathogens.…”

Using experimental evolution to explore natural patterns between bacterial motility and resistance to bacteriophages

“…pv. aesculi 6617 and 6623; Green et al, 2010), to amplify numbers of phage particles, and separated from the bacteria using chloroform. The 16 phage isolates were mixed to produce a stock inoculum, representing a subset of the naturally occurring phage community in the leaf environment, and stored at 4 1C.…”