Structural similarity of the membrane envelopes of rhizobial bacteroids and the host plasma membrane as revealed by freeze-fracturing.

The freeze-fracture technique was used to study the host plasma membrane and the membrane envelope of bacteroids in rhizobial root nodules of three host-rhizobium combinations. In all three combinations studied, the membrane envelopes of bacteroids are structurally similar to their host plasma membrane. However, the membrane appears to be reversed, because the number and arrangement of particles in the outer fractured face (face A, concave) and in the inner fractured face (face B, convex) of the host plasma membrane are seen, respectively, in the inner fractured face (face B, convex) and in the outer fractured face (face A, concave) of the membrane envelope of the bacteroids at an early stage. This reversion of the membrane surface is consistent with the hypothesis that the membrane envelopes of bacteroids are derived from the host plasma membrane during endocytotic engulfment.