Peanut agglutinin-induced structural changes in cowpea Rhizobia as revealed by freeze-etching

The ultrastructure of two strains of cowpeaRhizobium species, JLn(c) and NC-92, was studied especially with regards to the effect of peanut agglutinin on the cell envelope. Freezeetching electron microscopy revealed the structural details of the outer and the cytoplasmic membrane. The topology of both membranes resembled that of other Gram-negative bacteria. The cells displayed marked polarity, expressed in the form of an extracellular polar body (EPB) near one end and a periplasmic bay at the opposite end. The EPB was seen as a polar outgrowth of the outer membrane and resembled in structure the outer membrane. The existence of a polar periplasmic bay as a characteristic feature of cowpea rhizobia is reported here for the first time and has been implicated as a probable site for polysaccharide synthesis. Ruthenium red staining showed a polar mode of capsule initiation in the early exponential phase. Peanut agglutinin was found to enhance the incidence of EPB which is necessary for adhesion of rhizobia to root hairs. This suggests that lectins serve as an important biological stimulus which preconditions the symbiont for successful attachment to root hairs.