Extensive remodeling of the Pseudomonas syringae pv. avellanae type III secretome associated with two independent host shifts onto hazelnut.
BACKGROUND
Hazelnut (Corylus avellana) decline disease in Greece and Italy is caused by the convergent evolution of two distantly related lineages of Pseudomonas syringae pv. avellanae (Pav). We sequenced the genomes of three Pav isolates to determine if their convergent virulence phenotype had a common genetic basis due to either genetic exchange between lineages or parallel evolution.


RESULTS
We found little evidence for horizontal transfer (recombination) of genes between Pav lineages, but two large genomic islands (GIs) have been recently acquired by one of the lineages. Evolutionary analyses of the genes encoding type III secreted effectors (T3SEs) that are translocated into host cells and are important for both suppressing and eliciting defense responses show that the two Pav lineages have dramatically different T3SE profiles, with only two shared putatively functional T3SEs. One Pav lineage has undergone unprecedented secretome remodeling, including the acquisition of eleven new T3SEs and the loss or pseudogenization of 15, including five of the six core T3SE families that are present in the other Pav lineage. Molecular dating indicates that divergence within both of the Pav lineages predates their observation in the field. This suggest that both Pav lineages have been cryptically infecting hazelnut trees or wild relatives for many years, and that the emergence of hazelnut decline in the 1970s may have been due to changes in agricultural practice.


CONCLUSIONS
These data show that divergent lineages of P. syringae can converge on identical disease etiology on the same host plant using different virulence mechanisms and that dramatic shifts in the arsenal of T3SEs can accompany disease emergence.
Department of Cell and Systems Biology, University of Toronto, 25 Willcocks St., Toronto, ON, M5S 3B2, Canada. heath.obrien@gmail.com
Heath E
HE
O'Brien
Shalabh
S
Thakur
Yunchen
Y
Gong
Pauline
P
Fung
Jianfeng
J
Zhang
Lijie
L
Yuan
Pauline W
PW
Wang
Choseung
C
Yong
Marco
M
Scortichini
David S
DS
Guttman
D058947:Bacterial Secretion Systems / Q000235:genetics*; D031211:Corylus / Q000382:microbiology*; D004269:DNA, Bacterial / Q000737:chemistry* / Q000235:genetics; D016680:Genome, Bacterial; D006115:Greece; D007558:Italy; D008969:Molecular Sequence Data; D010935:Plant Diseases / Q000382:microbiology*; D044224:Pseudomonas syringae / Q000235:genetics* / Q000302:isolation & purification; D017422:Sequence Analysis, DNA; D037521:Virulence Factors / Q000235:genetics*
D058947:Bacterial Secretion Systems; D004269:DNA, Bacterial; D037521:Virulence Factors