Cyanobacteria in Symbiosis with Cycads
Cycads establish symbioses with filamentous cyanobacteria in highly specialised lateral roots termed “coralloid roots”. The coralloid roots, recorded in all cycad genera, show a marked negative geotropism, and grow laterally and upward toward the surface of the soil. The cyanobacteria are present in a specific cortical layer inside the root, the so-called cyanobacterial zone. The filamentous heterocystous cyanobacteria inside the root induces irreversible modifications to its growth and development due to a differentiation of elongated cycad cells, which have suggested to be responsible for the transfer of metabolites between the partners. The process of infection is still unclear. Invasion of filamentous cyanobacteria may occur at any stage of development of the root, but the precise time and location of the invasion is unpredictable. Using detailed molecular technques no genetic variation of the symbiotic cyanobacterial cells was observed within a single coralloid root. This is consistent with infection by a single cyanobiont. However, different coralloid roots from a single cycad specimen may harbour different cyanobacteria, and the same cyanobiont may be present in two different cycad specimens as well as in different cycad species. The filamentous heterocystous cyanobacteria within the cyanobacterial zone are located extracellularly between the elongated cycad cells and embedded in mucilage. All the molecular work is consistent with different Nostoc strains, the cyanobiont in cycad symbioses. The cyanobionts differentiate into vegetative cells and heterocysts, but rarely akinetes. In general, the symbiotic Nostoc filaments show only few modifications in comparison to its free-living counterparts: (1) an increased heterocyst frequency, (2) an increased level of nitrogen fixation, and (3) a transfer of fixed nitrogen from the cyanobacterial to the cycad cells. However, there is a distinct developmental gradient within the coralloid roots with “free-living like” filaments in the growing tip, which rapidly develop into symbiotic cells followed by older and metabolically less active cells in the older parts. Due to their location in coralloid roots (complete darkness), the cyanobionts are expected to have a heterotrophic mode of carbon nutrition. However, almost nothing is known about the heterotrophic metabolism in the symbiotic cells. The present review focuses on recent advances in the understanding of symbiotic cyanobacteria in cycads.
KeywordsGlutamine Synthetase Heterocystous Cyanobacterium Nostoc Strain Carbon Nutrition Symbiotic Cyanobacterium
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