THE GUNNERA/NOSTOC SYMBIOSIS: AN ULTRASTRUCTURAL COMPARISON OF AGING GLANDS AND NODULES

The ultrastructure of the Gunnera/Nostoc symbiosis has been examined with emphasis upon the possible mechanisms of the cyanobacterium's entry into the cells of its host, G. kaalensis (Krajina) St. John. Mucilage glands consisting of parenchyma cells and found on the upper portions of Gunnera stems are entry sites for the Nostoc. Gland cells have many features in common with other secretory cells, most conspicuously numerous dictyosomes which produce large mucilage-containing vesicles. The passage of mucilage to the exterior of the cell is via granulocrine secretion. Nostoc cells are present within the mucilage, in the channels which separate the papillae, and within cavities at the base of the gland. Just below the gland, the symbiont is intracellular.;Morphometric data show that thickness of the encapsulating layer is not significantly different with respect to position in the host stem, suggesting that walling-off of the symbiont occurs primarily at the time of infection. However, deposition of the encapsulating layer in stages may occur since sometimes wall stratification can be seen. The similarity of wall proliferations in Nostoc-containing cells and the extensive wall labyrinths found in transfer cells suggest that nodule cells represent transfer cells.;Symbiotic Nostoc cells resembled isolates which have been kept in darkness. When isolates are illuminated the cell ultrastructure changes as does cell shape. Heterocyst frequency does not seem to be influenced by light conditions. Illuminated isolates form long filaments in contrast to remaining aseriate or in short filaments, characteristic for symbionts and dark-grown isolates. This suggests that the symbiont can undergo a developmental cycle similar to some free-living species. Although cell degeneration occurs in both the symbiont and isolates, only the former shows osmiophilic reactions of the protoplasts associated with vesiculation, breakdown of the Nostoc cell wall, and a negatively-stained appearance of the thylakoids. Such degeneration processes may be host-influenced.;It seems that entry into the host cells is by a combination of three processes in which: (a) areas of apparent cell wall invagination are present in some Nostoc-containing host cells; (b) the host cell walls often appear to be modified in ways which suggest cellulolytic and pectolytic activity; and (c) other microorganisms may be involved in entry since both fungal and bacterial cells are found within and between the host cell walls in some glands.