A Peanut Nodule Lectin in Infected Cells and in Vacuoles and the Extracellular Matrix of Nodule Parenchyma

Abstract

Root nodules on peanut (Arachis hypogaea L.) accumulate a galactose/lactose-binding lectin that is similar, but not identical, to the major seed lectin in peanut. The function of the peanut nodule lectin (PNL) is not known. In the current study, we have investigated the location of lectin in the nodule using immunogold labeling and enzyme-linked immunosorbant assays (ELISA). Lectin was most abundant in the nodule parenchyma, where it accumulated in vacuoles, suggesting a possible role as a vegetative storage protein. Lectin was also detected in the extracellular matrix in the nodule parenchyma, a location that corresponds to the tissue layer forming a barrier to oxygen diffusion. The potential for interactions between PNL and other cell wall components, including a previously described high-molecular weight glycoprotein that co-localizes with PNL, is discussed. Within infected cells, lectin was not detectable by immunogold labeling within the cytoplasm, but light labeling was suggestive of lectin localization within the symbiosome lumen. Analysis of fractionated symbiosomes by the more sensitive ELISA technique confirmed that lectin was present within the symbiosome, but was not bound to bacteroids. Our results indicate that PNL probably plays several roles in this nitrogen-fixing symbiosis.