Role of Lectins in Plant-Microorganism Interactions

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Abstract
Highly purified soybean lectin (SBL) was labeled with fluorescein isothiocyanate (FITC-SBL) or tritium (3H-SBL) and repurified by affinity chromatography. FITC-SBL bound to living cells of 15 of the 22 Rhizobium japonicum strains tested. The lectin did not bind to cells of the other 7 R. japonicum strains, or to cells of any of the 9 Rhizobium strains tested which do not nodulate soybean. The binding of the lectin to the SBL-positive strains of R. japonicum was specific and reversible by hapten inhibition with D-galactose or N-acetyl-D-galactosamine. The lectin-binding properties of the SBL-positive R. japonicum strains changed substantially with culture age. The percentage of cells in a population exhibiting fluorescence after exposure to FITC-SBL varied between 0 and 70%. The average number of SBL molecules bound per cell varied between 0 and 2 .times. 106. While most strains had their highest percentage of SBL-positive cells and maximum number of SBL-binding sites per cell in the early and midlog phases of growth, 1 strain had a distinctly different pattern. The SBL-negative strains did not bind lectin at any stage of growth. Quantitative binding studies with 3H-SBL indicated that the affinity constant for binding of SBL to its receptor sites on R. japonicum is approximately 4 .times. 107 M-1. Many of the binding curves were biphasic. An inhibitor of SBL binding was present in R. japonicum culture filtrates.