Index >> Rhizobium and Legume Root Nodulation >>Structure of the Nodule

In all ineffective associations, it has been shown that starch accumulates in the uninfected cell and dextran in the infected cell with glycogen in the bacteroid. Effective nodules are generally large and pink (due to leghaemoglobin) with well developed and organized bacteroid tissue.

A fully-developed bacteroid has no flagella and is surrounded by three unit membranes. There exists an intracytoplasmic membrane system in the bacteroids of nodules in subterranean clover. The nuclear region of bacteroids appears fragmented and is associated with granular cytoplasm. Bacteroids can be produced in vitro on a medium containing 3.5% of yeast extract. Caffeine and several other alkaloids also encourage bacteroid production on artificial media. Depending on the legume, each bacteroid or groups of bacteroids are surrounded by membrane envelopes whose identity has been variously interpreted, possibly because of the different techniques used in fine structure studies.

The membrane envelope surrounding the bacteroids is also known as the peribacteroid membrane. The earlier concept that the peribacteroid membrane is only a slightly modified plasma membrane has to be viewed differently in the light of later findings. The multiplication of bacteroids and the formation of the peribacteroid membrane may not be synchronized cell events which result in the variability in the number of bacteroids enclosed by the membrane. The microsymbiont (Rhizobium) strain determines the particle density and the protein and fatty acid composition of the membrane. The peribacteroid membrane contains nodulins which may play special roles in the two-way transport of metabolites between the symbionts.