Agrobacterium adherence involves the pectic portion of the host cell wall and is sensitive to the degree of pectin methylation

Cell walls isolated from dicotyledon tissues compete with natural plant host sites for Agrobacterium tumefaciens (strain B6) when co‐inoculated with infectious bacteria, thereby reducing tumor initiation. Removal of the pectic fraction from the cell walls results in loss of inhibition and the soluble pectic fraction is inhibitory. On treatment with pectin methyl transferase plus S‐adenosyl‐L‐methionine these cell walls become less inhibitory and this change is reversible by pectinesterase. Cell walls isolated from monocotyledons, crown gall tumors or embryonic dicotyledons do not compete for Agrobacterium in the infection assay. These cell walls become inhibitory on treatment with pectinesterase and this is partially reversed by pectin methyl transferase. These data indicate that the pectic portion of the host cell wall is involved in the Agrobacterium‐host adherence which is essential for tumor initiation and that the degree of methylation of polygalacturonic acid is critical to this adherence.