Abstract
Pseudomonas syringae is a ubiquitous plant pathogen, infecting both woody and herbaceous plants and resulting in devastating agricultural crop losses. Characterized by a remarkable specificity for plant hosts, P. syringae pathovars utilize a number of virulence factors including the type III secretion system and effector proteins to elicit disease in a particular host species. Here, two Pseudomonas syringae strains were isolated from diseased Populus trichocarpa seeds. The pathovars were capable of inhibiting poplar seed germination and were selective for the Populus genus. Sequencing of the newly described organisms revealed similarity to phylogroup II pathogens and genomic regions associated with woody host-associated plant pathogens, as well as genes for specific virulence factors. The host response to infection, as revealed through metabolomics, is the induction of the stress response through the accumulation of higher-order salicylates. Combined with necrosis on leaf surfaces, the plant appears to quickly respond by isolating infected tissues and mounting an anti-inflammatory defense. This study improves our understanding of the initial host response to epiphytic pathogens in Populus and provides a new model system for studying the effects of a bacterial pathogen on a woody host plant in which both organisms are fully genetically sequenced.
Highlights
Pseudomonas syringae, a plant pathogen with at least sixty known host-specific pathovars, is one of the most broad-ranging and agriculturally relevant pathogens known [1]
While Populus spp. have been extensively studied as a bioenergy crop, relatively few bacterial pathogens have far been described for use in model studies
Two strains of Pseudomonas syringae were isolated from Populus catkins in the field and characterized for host range, virulence genes, and host response
Summary
Pseudomonas syringae, a plant pathogen with at least sixty known host-specific pathovars, is one of the most broad-ranging and agriculturally relevant pathogens known [1]. As a result of its ubiquity and simultaneous strain limitation, P. syringae is a useful model pathogen for studying plant host specificity as well as host immune response [2,3]. A complex system of secretion machinery, effector proteins, and chaperones, the T3SS genes are grouped together in a genomic region of the bacterial chromosome called the hypersensitivity response and pathogenicity (Hrp) island [6]. The T3SS effector proteins (TTEs), encoded by the avirulence (avr) genes, are secreted through the needlelike export system into the host cell cytosol and therein elicit the plant hypersensitivity response (HR) [7,8]
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