Fine structure of nitrogen-fixing leguminous root nodules from the Canadian Arctic

Effective (nitrogen-fixing) root nodules of Oxytropis maydelliana Trautv., O. arctobia Bunge and Astragulus alpinus L. were collected in the high Arctic tundra and subsequently processed for structural studies. The cylindrically-shaped perennial nodules consisted of the following tissues: nodule cortex, nodule meristem, nodular vascular bundles, an active central region with uninfected and infected cells at various stages of development, and a proximal region of senescent cells. The active central region was dark red-coloured due to the presence of the pigment leghemoglobin. The host cells became infected by the growth of infection threads into cells recently derived from the nodule meristem and the subsequent endocytotic release of rhizobia from unwalled membrane-bound regions of the infection thread. The host plasma membrane adjacent to the unwalled regions of infection thread gave rise to the peribacteroid membrane which surrounded the released bacteria. Thus, nodule development and the basic tissue arrangement of the arctic nodules was similar to that of cylindrically-shaped nodules formed on temperate species of legumes.
The arctic legume nodules are unique in having large numbers of lipid droplets present in the cytoplasm of the nodule cortex and uninfected cells of the central active region. Newly infected cells also have lipid droplets. More developed infected cells lack lipid droplets but often contain amyloplasts. Mature differentiated bacteria were spherically-shaped and contained electron-dense inclusions. Electron-dense material was also present in vesicles formed from dilated endoplasmic reticulum and in the peribacteroid space. The lipid droplets present in the host cytoplasm of the nodule cortex and uninfected cells of the central tissue may be storage products which are used to support nitrogen-fixation in nodules growing under cool temperatures of this harsh environment.