Role of Pili (Fimbriae) in Attachment of Bradyrhizobium japonicum to Soybean Roots

Abstract

Pili (fimbriae) were observed on cells of each of the five strains of Bradyrhizobium japonicum and the one strain of Rhizobium trifolii examined. Pili on B. japonicum were about 4 nm in diameter and polarly expressed. Piliated cells were estimated by transmission electron microscopy and hydrophobic attachment to polystyrene to constitute only a small percentage of the total population. The proportion of piliated cells in these populations was dependent on culture age in some strains. Piliated B. japonicum cells were selectively and quantitatively removed from suspension when cultures were incubated with either soybean roots or hydrophobic plastic surfaces, indicating that pili were involved in the attachment of the bacteria to these surfaces. Pili from B. japonicum 110 ARS were purified and found to have a subunit molecular weight of approximately 21,000. Treatment of B. japonicum suspensions with antiserum against the isolated pili reduced attachment to soybean roots by about 90% and nodulation by about 80%. Pili appear to be important mediators of attachment of B. japonicum to soybean roots under the conditions examined.

Molecular mechanisms of attachment of Rhizobium bacteria to plant roots

“…Data supporting the lectin-recognition hypothesis were also reported for other (Brady)rhizobium-\egume combinations (Kato etai, 1980;Stacey etai, 1980). In contrast to these data, several studies did not demonstrate specific lectin-mediated binding of homologous rhizobia to legume root hairs (Badenoch-Jones et ai, 1985;Mills and Bauer, 1985;Smit etai, 1986Smit etai, , 1987Vesper and Bauer, 1986;Vesper et al, 1987). Heterologous rhizobia were found to adhere equally well to host and non-host plant surfaces in comparison with homologous rhizobia.…”

“…For B. japonicum, bacteriai fimbriae were reported to mediate firm attachment to soybean roots (Vesper and Bauer, 1986;Vesper et ai, 1987). This type of binding is specificaily inhibited in the presence of gaiactose, suggesting the presence of a fimbriae-associated iectin en the bacterial ceii surface.…”

“…This question becomes even more obvious in view of the observation that R. leguminosarum bacteria form ceiiuiose fibriis even under carbon-iimiting conditions (Smit et ai, 1986). A number cf studies have shown that adhesion to piant ceiis is positiveiy correiated with non-specific adhesion to abiotic surfaces (e.g.. Vesper and Bauer, 1986;Smit et ai. 1987).…”

Factors affecting the attachment of rhizospheric bacteria to bean and soybean roots

“…Other studies have demonstrated an active adsorption to roots by polymeric fibrillar bridges or via protein bridges. Factors affecting the attachment between the symbionts have also been the subject of debate, including culture age, bacterial and root pretreatment conditions, bacterial chemotaxis and motility, presence and extension of bacterial fimbriation (Vesper & Bauer, 1986), and bacterial surface polysaccharides (EPS and LPS).…”

Role of haemagglutinins in adhesion of Erwinia carotovora to potato tissue

“…It has since been reported that the attachment of KI. pneumoniae and Enterobacter agglomerans to Poa pratensis is mediated by fimbriae (Haahtela et al 1985) and attachment of Bradyrhizobium japonicum to soybean roots is also dependent on the presence of fimbriae (Vesper and Bauer 1986).…”