The excellent facilities at SCRI and the broad repertoire of skills within the plant pathology programme allow for in-depth investigation into the Pectobacterium atrosepticum (Pba)-plant interaction. Our current research employs genomics approaches to identify and investigate pathogenicity determinants that are novel to this group of pathogens (particularly phytotoxins, regulators and secretion systems), and to determine regulatory networks involved in pathogenesis (including those associated with type III secretion and quorum sensing).

We are also interested in how these determinants are used by the pathogen to interact with its host. For example, Pba is considered a necrotroph, an organism that kills part or all of another organism before deriving nutrients from it, in contrast to biotrophic pathogens such as Pseudomonas syringae, that derive nutrients from the living tissues of another organism. We are investigating the potential role of biotrophy in the interaction between Pba and potato by focusing on i) the type III secretion system and its effectors, ii) putative phytotoxic compounds such as coronafacic acid conjugates, and their effects on basal resistance in potato.

Quorum sensing

Quorum sensing (QS) is a population density-dependent regulatory mechanism controlled by the production of N-acyl homoserine lactone (AHL) – synthesised by the ExpI protein. Using whole-genome Pba microarrays (Agilent Technologies), we have generated global gene expression data based on a comparison between wild-type Pba and an expI^? mutant strain during infection of potato tubers. In our study, we have shown that, in planta, QS appears to control i) all major plant cell wall degrading enzymes; ii) a number of known or putative virulence-associated genes; iii) over 70 regulators; iv) the type I, II and other secretion systems; and a number of genes involved in a range of other processes.

Our data demonstrates that the QS system has a far broader influence on the interaction between Pba and its host potato than has previously been shown. We are now investigating some of these genes to determine their possible role in pathogenicity, particularly with a view to identifying potential novel effectors that may be involved in manipulating plant defences.

Type III secretion

The type III secretion system of Pba is being used to investigate the Pba-potato interaction. Mutants in structural genes and putative effectors both reduce pathogenicity on potato and trigger defence genes in Nicotiana species.

We have developed a plant response microarray (in association with Agilent Technologies) with over 6000 plant genes either known or thought to be associated with resistance. On challenge of the plant with wild type Pba, mutants disabled in phytotoxin production or type III secretion, or plant chemical signals such as salicylic acid and methyl jasmonate, we have identified a number of candidate defence genes and mechanisms involved in the early host response to Pba.

One of these (encoding a DNA binding protein), believed to be a key regulator in the early plant response, has been constitutively expressed in a susceptible potato plant and this provides total resistance to Pba. High levels of expression of this gene have been observed naturally in a cultivated potato species with a high level of resistance to Pba and other plant pathogens. The link between this gene and resistance is now being investigated.

Phytotoxins

The production of coronafacoyl phytotoxins is essential for full virulence of Pba, and cfa biosynthetic genes are significantly up-regulated during the later stages of tuber infection. We have shown that the two main coronafacoyl conjugates produced by Pba are structurally analogous to the plant defence-signalling hormone methyl jasmonate, indicating that plant defences may be suppressed in the latter stages of infection during production of plant cell wall degrading enzymes.