The Function of Lectin in the Seeds and Nodules of the Legume Arachis hypogaea
In the best known examples of legume-bacteria symbiosis, rhizobia infect the plant via the root hair. The symbiont of peanut, however, a species of Bradyrhizobium, enters the root through breaks in the epidermis of the primary root. It has been suggested that lectins play a role in the initiation of symbiosis and development of infection threads by agglutination and binding of rhizobia to root hairs. Although there is no evidence of bradyrhizobial infection through root hairs or infection thread formation in peanuts, lectin has been detected in extracts of peanut seed and nodule proteins. Extracts of seed and nodule proteins were separated by one and two-dimensional gel electrophoresis, Western blotted and immunostained to characterize differences between nodule and seed lectins. One-dimensional analysis showed seed lectin to be approximately 1 kD larger than nodule lectin. Two-dimensional analysis showed that seed lectin had a small basic component which nodule lectin lacked. Two different microscopic techniques were employed in an attempt to localize lectin within sections of seed and nodule: immunogold labelling with silver enhancement and immunofluorescence labelling. Results were inconclusive due to non-specific silver deposition during enhancement and autofluorescence of tissue sections. Although the results are not yet clear, it is possible that both seed and nodule lectins are products of the same gene, nodule lectin undergoing additional post-translational processing. Seed lectin may serve as a nitrogen storage protein, providing a source of nitrogen for nucleic and amino acids in the developing plant. Nodule lectin could serve a similar function, serving as a nitrogen reserve to be tapped by the plant after senescence of the root nodules. Nodule lectin could also play a role in initiation of symbiosis by masking bacterial cell-surface constituents that could trigger plant defense responses.