Role of Attachment of Rhizobium Leguminosarum Cells to Pea Root Hair Tips in Targeting Signals for Early Symbiotic Steps

Abstract

Attachment of Rhizobium and Bradvrhizobium bacteria to the tip of the root hairs of their host plants is supposed to be an essential step of the usual nodulation pathway of these organisms (e.g. Rolfe and Shine 1981). Attachment is a prerequisite for marked curling of root hairs (Yao and Vincent 1969; Van Batenburg et al 1986). Attachment is often assumed to be a determinant of host plant specificity (Bohlool and Schmidt 1974; Dazzo and Hubbell 1975) and it was hypothesized that host specificity at this level is the result of specific binding of the homologous rhizobial partner to the cell surface of the host plant’s root hair. Evidence was provided that the molecular basis for this recognition mechanism is the binding of a host plant lectin to a bacterial sugar residue, since attachment was largely reduced by the presence of lectin-haptenic monosaccharides (Dazzo et al 1976). Moreover, lectin-mediated attachment to clover root hairs was reported to be dependent on the presence of the Sym plasmid (Dazzo et al 1985), the genetic element that was known to determine host specificity (Johnston et al 1978). Consistent with this notion was also that heterologous rhizobial strains usually fail to form infection threads.