Copyright © 1991, American Society for Microbiology Antigenic Changes in Lipopolysaccharide I of Rhizobium leguminosarum bv. viciae in Root Nodules of Vicia sativa subsp. nigra Occur during Release from Infection Threads

Three different monoclonal antibodies raised against the 0 antigen-containing lipopolysaccharide (LPS I) of free-living cells were used in an immunocytochemical study to follow the fate of LPS I on the outer membrane of Rhizobium leguminosarum bv. viciae 248 during the nodulation of Vicia sativa subsp. nigra. After immunogold labeling, the LPS I epitopes were detected on the outer membrane of bacteria present in infection threads throughout the nodule. Epitopes were not detectable on bacteria released from the infection thread. The data show that the LPS I epitopes present on rhizobia in infection droplets disappear shortly before or during endocytosis of the bacteria into the host plant cell cytoplasm. The abruptness of the change suggests an active degradation or modification of LPS I epitopes rather than only a repression of their synthesis. Members of the soil bacterium genus Rhizobium are capable of fixing atmospheric nitrogen when living in sym-biosis with a leguminous plant. The bacteria enter the plant root through a tip-growing structure of plant origin called the infection thread. They multiply in the infection thread and reach the young nodule cells, which are meanwhile prolifer-ating. From an infection droplet (14) at the tip of th