Cultivar-Specific Carbohydrate-Binding Properties of Lectins from Broad-Bean Seeds

Lectins were isolated and purified from three broad bean (Vicia faba L.) cultivars differing in the effectiveness of their symbiosis with root nodule bacteria (Rhizobium leguminosarum bv. viciae). From seeds of symbiotically effective cvs. Aushra and Daiva, we isolated only one lectin from each cultivar, whereas two lectins, Yu-1 and Yu-2, were isolated from seeds of symbiotically ineffective cv. Yugeva. Lectins from cvs. Aushra and Daiva were more active than lectins from cv. Yugeva and exhibited similar carbohydrate specificity. Methyl-α-D-mannopyranoside and trehalose were the most potent inhibitors of their hemagglutination activity. Lectin Yu-1 resembled them in its carbohydrate-binding properties. However, D-mannose, trehalose, and melecitose were its most effective inhibitors. Lectin Yu-2 differed substantially from these lectins. It exhibited an affinity for D-glucuronic acid, D-glucosamine, and 2-deoxy-D-glucose. In addition, it could interact with carbohydrates of the galactose family (2-deoxy-D-galactose, D-galactosamine, and lactose) and also with D-xylose and 2-deoxy-D-talose. Thus, lectins from cvs. Aushra and Daiva and also Yu-1 can be considered D-mannose/D-glucose-specific lectins, whereas Yu-2 lectin exhibited a combined carbohydrate specificity. The affinity of Yu-1 and Yu-2 lectins for their natural receptors, exopolysaccharides and lipopolysaccharides of broad-bean nodule bacteria, was twice as low as that of lectins from cvs. Aushra and Daiva. We believe that properties of seed lectins are an important cultivar-specific trait that determines host-plant (broad beans) specificity during the establishment of legume–rhizobia symbiosis.