Recognition Systems in Plant‐Pathogen Interactions

This paper considers the surface components that may be involved in recognition of pathogenic bacteria by potential plant hosts, and how host defense reactions may be induced or prevented as a result of recognition. Most of the work carried out thus far indicates that recognition follows the initial attachment of bacteria to the host cell wall as it was clearly shown with Agrobacterium tumefaciens. Attachment may involve the interactions of extracellular polysaccharides (EPS) and lipopolysaccharides (LPS) produced by bacteria and the protein or pectic components of plant cell walls. In the tobacco‐ Pseudomonas solanacearum system, comparisons between wild‐type, fluidal and avirulent, afluidal strains (spontaneous or generated by Tn5 mutagenesis) have led to the following, tentative conclusions: Bacterial attachment may be mediated by the interaction of LPS with hydroxyproline‐rich glycoproteins (HPRGs) on plant cell walls. The binding between these two components in vitro appears to be the result of charge‐charge interactions. EPS does not inhibit the LPS‐HPRG interaction in planta , although it is a strong inhibitor of this interaction in vitro. Inhibition occurs only at very low ionic strengths, below those of the intercellular fluid. The avirulent strains that are rapidly attached to plant cell walls and prevented from multiplying have defective LPS molecules that interact strongly with cell wall HPRGs. These strains lack the 0‐antigen portion of LPS, consisting of a repeating unit of rha:xyl:glcNac in the molar ratios 4:1:1. The strains that attach weakly have a complete LPS molecule and most produce EPS. Attachment of avirulent strains to suspension‐cultured tobacco cells and to isolated tobacco cell walls initially is inhibited by high salt concentrations, but cannot be reversed by washing with 1.0 M phosphate buffer after 15 min of incubation. Eimbriae are thought to play an important role in this firm attachment of bacteria to plant cell walls.