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Quorum sensing (QS) in vitro controls production of plant cell wall degrading enzymes (PCWDEs) and other virulence factors in the soft rotting enterobacterial plant pathogen Pectobacterium atrosepticum (Pba). Here, we demonstrate the genome-wide regulatory role of QS in vivo during the Pba-potato interaction, using a Pba-specific microarray. We show that 26% of the Pba genome exhibited differential transcription in a QS (expI-) mutant, compared to the wild-type, suggesting that QS may make a greater contribution to pathogenesis than previously thought. We identify novel components of the QS regulon, including the Type I and II secretion systems, which are involved in the secretion of PCWDEs; a novel Type VI secretion system (T6SS) and its predicted substrates Hcp and VgrG; more than 70 known or putative regulators, some of which have been demonstrated to control pathogenesis and, remarkably, the Type III secretion system and associated effector proteins, and coronafacoyl-amide conjugates, both of which play roles in the manipulation of plant defences. We show that the T6SS and a novel potential regulator, VirS, are required for full virulence in Pba, and propose a model placing QS at the apex of a regulatory hierarchy controlling the later stages of disease progression in Pba. Our findings indicate that QS is a master regulator of phytopathogenesis, controlling multiple other regulators that, in turn, co-ordinately regulate genes associated with manipulation of host defences in concert with the destructive arsenal of PCWDEs that manifest the soft rot disease phenotype.
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Genomics research is changing the way we study all forms of life, and the interaction between plant pathogens and their hosts is benefiting greatly from the genomics revolution. To have the "genetic blueprint" for a pathogen is of immeasurable value as it lays bare the potential capabilities of that organism and allows comparisons to be made with others. Although genomics opens up the field of research it also has the potential to target key genes and mechanisms precisely. In 2004 the first genome of an enterobacterial plant pathogen (Pectobacterium atrosepticum Pba – formerly Erwinia carotovora subsp. atroseptica) was sequenced and several more are now available, including Erwinia amylovora. Based on this genome sequence, microarrays, bioinformatics, comparative genomics and systems biology have all been used to advance our knowledge of the pathogen, together with new discoveries in potential alternative hosts in the environment and comparisons to Pba's human and animal enterobacterial cousins E. coli and Salmonella. INTRODUCTION In 2004, Pectobacterium atrosepticum (Pba) became the first fully sequenced genome of an enterobacterial plant pathogen (Bell et al., 2004). Since then, related pathogens have been sequenced (including Dickeya dadantii (Dda) – formerly Erwinia chrysanthemi) or are being sequenced (Erwinia amylovora, Pantoea stewartii subsp. stewartii and Pantoea ananatis). Using powerful functional and comparative genomics tools, these genome sequences can be exploited to enable the discovery of new factors and mechanisms involved in the interaction between these organisms and their hosts or their life in the environment when not causing disease. Pba is a pathogen of potato, causing blackleg of plants in the field and soft rot of tubers in storage (Perombelon, 2002). It produces large quantities and different types of plant cell wall degrading enzymes (PCWDEs) during the latter stages of infection (Toth et al., 2003). These enzymes are co-ordinately regulated by a mechanism called quorum sensing (QS), which is activated when the signalling molecule N-acyl homoserine lactone (AHL) reaches a threshold level at high bacterial cell densities (Barnard and Salmond, 2007). Most investigations on QS have been related to the production of these PCWDEs, but there are likely to be other genes and mechanisms controlled by QS that remain to be determined. The massive and coordinated production of PCWDEs towards the end of the infection process has led to Pba being described as a "brute force" necrotrophic pathogen, causing death of host tissue through a concerted attack on the plant cell wall, followed by colonisation of the dead substrate. However, relatively little is known about whether Pba undergoes a more "stealth-like" biotrophic phase of infection, interacting with live host cells and possibly suppressing host defences (as in the case of Pseudomonas syringae, which uses effectors secreted by a Type III secretion system and the phytotoxin coronatine to manipulate plant defences) (Toth et al., 2005). Although Pba is well known for its role in disease on potato, little is known about its life when not causing disease (Perombelon, 2002). This has important consequences to potato production, as preventing early contamination of high grade potato seed stocks during their propagation in the field could significantly reduce the incidence of disease. However, while efforts have been made to relate pathogen numbers in soil and ground Proc. XI th IW on Fire Blight Eds.: K.B. Johnson and V.O. Stockwell Acta Hort. 793, ISHS 2008