Abstract
During their initial association with plant hosts, pathogenic bacteria interact with plant cell walls. The results of this interaction appear to determine whether bacterial multiplication will take place. With one group of bacterial plant pathogens (e.g. Agrobacterium tumefaciens), attachment to the host surface appears essential for pathogenesis. With another group (e.g. Pseudomonas solanacearum), only those strains that do not attach to the host cell wall are able to multiply in the intercellular spaces. Attachment of many incompatible strains to tobacco mesophyll cell walls leads to a rapid hypersensitive response (HR) and a drastic reduction in bacterial multiplication. Our working hypothesis is that these differences in host response to strains of P. solanacearum are the result of a recognition response in which surface components of both host and pathogen play important roles. Our approach is based on the use of spontaneous or transposon (Tn5)-generated mutants of strains K60 (virulent) and B1 (avirulent) that differ in surface components and in their ability to attach to host cells and to induce the HR. A study of the surface components of bacterial and tobacco cell walls has led to the tentative conclusion that bacterial lipopolysaccharide (LPS) and plant hydroxyproline-rich glycoproteins mediate initial attachment, apparently as a result of charge-charge interaction. This initial attachment is reversed by high salt concentrations during the first 15 min, but not thereafter. Firm attachment appears to depend on hydrophobic interactions mediated by bacterial pili. At the normal ionic strength of intercellular fluids, extracellular polysaccharide (EPS) appears to inhibit only the pili-mediated attachment. Several HR- mutants of strain B1 have been obtained by Tn5 insertion, but they remain avirulent on tobacco. We are examining the EPS, LPS and pili production, and the attachment characteristics of these strains.
Highlights
Bacteria adhere to many different types of surfaces
This hypothesis received some support when: (a) bacterial agglutination by this fraction was shown to be inhibited in the presence of chitotriose, and (b) the localization of the agglutinin in plant cell walls was demonstrated by immuno fluorescence (Leach, Cantrell & Sequeira, 1982a)
Pili and plant cell wall glycoproteins, are known to be hightly resistant to proteolysis. These results suggest that binding of B 1 bacteria to plant cell walls is a two-step process in which ionic forces, presumably involving the LPS in the outer membrane, are involved initially
Summary
During their initial association with plant hosts, pathogenic bacteria interact with plant cell walls. With one group of bacterial plant pathogens (e.g. Agrobacterium tumefaciens), attachment to the host surface appears essential for pathogenesis With another group (e.g. Pseudomonas solanacearum), only those strains that do not attach to the host cell wall are able to multiply in the intercellular spaces. A study of the surface components of bacterial and tobacco cell walls has led to the tentative conclusion that bacterial lipopolysaccharide (LPS) and plant hydroxyproline-rich glycoproteins mediate initial attachment, apparently as a result of charge-charge interaction. This initial attachment is reversed by high salt concentrations during the first 15min, but not thereafter. We are examining the EPS, LPS and pili production, and the attachment characteristics of these strains
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