THE CYANOBACTERIUM‐ZAMIA SYMBIOSIS: AN ULTRASTRUCTURAL STUDY

Abstract

SUMMARYFilamentous cyanobacteria are found intercellularly in a zone between inner and outer cortex of coralloid roots, growing below the soil surface, from the cycad Zamia skinneri Warsc. The zone is transversed by elongated Zamia cells interconnecting the two cortex layers. These host cells show characteristics of transfer cells, i.e. considerably expanded cell walls, numerous mitochondria and amyloplasts. An involvement of these cells in the transfer of solutes between cyanobacterium and host is therefore suggested. The cyanobacterium belongs to the genus Nostoc and consists of vegetative cells, heterocysts and a few akinetes. Healthy vegetative cells contain thylakoids with phycobilisomes, carboxysomes, cyanophycin and glycogen granules. Thus, no distinct subcellular modifications due to a symbiotic mode of life in the absence of light were observed.

Regulation of Cellular Differentiation in Filamentous Cyanobacteria in Free-Living and Plant-Associated Symbiotic Growth States

“…Cavities of Azolla are exceptional in supporting other bacteria (26) that may play an important role in the symbiosis (60). Finally, the symbiotic cavities respond to the presence of cyanobacteria by elaborating long, finger-like cells (103,163) that may serve to increase the surface area for nutrient exchange.…”

Cyanobacterial Associations

“…It has repeatedly been shown that the heterocyst frequencies are enhanced in cyanobionts of cycads (Lindblad et al, 1985;Lindblad and Bergman, 1990;Costa and Lindblad, 2002). The heterocyst frequencies increase from the young towards the basal parts of the coralloid roots and, in the mean time, they undergo some structural changes.…”

“…This idea was later abandoned. Indeed, cyanobacterial infection, and the absence of other bacteria, in the coralloid roots was repeatedly verified (see Schaede, 1944;Wittman et al, 1965;Lindblad et al, 1985;Costa and Lindblad, 2002). Coralloid roots are highly modified roots that develop from the pericycle of the primary roots Ahern and Staff, 1994).…”

“…The former epidermal cells present within the cyanobacterial zone eventually elongate and start to display transfer-cell morphology (Lindblad et al, 1985). A role in exchange of nutrients via these cells, extending radially across the cyanobacteriafilled zone, has therefore been inferred.…”

Occurrence and Localization of Phycoerythrin in Symbiotic Nostoc of Cycas revoluta and in the Free-Living Isolated Nostoc 7422

“…It is known that phycobiliproteins are synthesized in intact filaments of the cyanobionts of the lichen Peltigera (3,10,22), the water-fern Azolla (1,11,25,26) and the cycad Cycas circinalis (17). Moreover, phycobilisomes have been shown to be present in Nostoc vegetative cells of the cycad Zamia skinneri (13). ' Supported by the Swedish Natural Science Research Council, The Crafoord Fund, and Marianne and Marcus Wallenberg Foundation.…”