European Journal of Plant Pathology 107: 95–102, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Bacterial lipopolysaccharides and plant–pathogen interactions M.-A. Newman, J.M. Dow and M.J. Daniels * The Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK; * Author for correspondence (Phone: +441603450316; Fax: +441603450011; E-mail: mike.daniels@bbsrc.ac.uk) Accepted 30 August 2000 Key words: defence, localised induced response, pepper, hydroxycinnamoyl tyramine conjugates, Xanthomonas Abstract Lipopolysaccharides are amphipathic molecules forming the outermost layer of the cell surface of Gram-negative bacteria. They are essential for protecting the cell from hostile environments and, in the case of pathogens, they play a direct role in interactions with eukaryotic host cells. Mutants with altered lipopolysaccharide structure have been obtained with several plant pathogenic bacteria; such mutants generally show reduced virulence. Purified lipopolysaccharide has several effects on plants, notably suppression of the hypersensitive response to subsequently inoculated avirulent pathogens. The suppression is strictly localized and is observed within a time ‘window’ of, typically, 10–30 h. Although infiltration of lipopolysaccharide into leaves produces no macroscopic symptoms, characteristic changes in plant gene expression can be observed. One effect is to sensitize the plant tissue to subsequent bacterial inoculation so that the sensitized tissue responds more rapidly and intensely, giving partial inhibition of bacterial growth. The synthesis of antimicrobial hydroxycinnamoyl tyramine conjugates is one facet of the process which provides an excellent biochemical model for analysing the phenomenon. Lipopolysaccharide induces the synthesis of two enzymes involved in conjugate production (tyrosine decarboxylase and tyramine- hydroxycinnamoyl transferase), but the conjugates themselves are not produced until bacteria are subsequently inoculated. Using this and other examples we discuss the mechanisms of lipopolysaccharide action on plants in the context of plant disease. Abbreviations: CT – coumaroyl tyramine; FT – feruloyl tyramine; HR – hypersensitive response; LIR – localised induced response; LPS – lipopolysaccharide; PAL – phenylalanine-ammonia lyase; THT – tyramine- hydroxycinnamoyl transferase; TyDC – tyrosine decarboxylase. Introduction The cell envelope of Gram-negative bacteria contains two bilayer membranes. The outer leaflet of the outer membrane contains lipopolysaccharide (LPS), a com- plex family of amphipathic molecules which play important roles in the interaction of bacteria with the extracellular environment, including eukaryotic hosts (Rietschel et al., 1994). LPS is a tripartite molecule. The outermost layer, exposed to the aqueous extra- cellular environment of the bacteria, is called the O antigen and consists of a chain of oligosaccharide repeat units. The O antigen is the immunodominant component of the cell surface and many serotype clas- sifications of bacteria reflect variation in O antigen structure. The O antigen is not essential for bacterial viability and mutants lacking it can be readily iso- lated. Such mutants are called ‘rough’ because of the appearance of colonies, contrasting with the ‘smooth’, more mucoid appearance of wild type colonies. In the case of plant pathogens, O antigen may not be essential for pathogenicity. For example, some strains of Xanthomonas campestris isolated from naturally diseased plants lack O antigen. The second domain of LPS comprises the inner and outer oligosaccha- ride core. Mutants can be obtained lacking the outer