Cyanobacteria in Symbiosis with Cycads

Abstract

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 negativegeotropism, and grow laterally and upward toward the surface of the soil. The cyanobacteria are presentin a specific cortical layer inside the root, the so-called cyanobacterial zone. The filamentous heterocystouscyanobacteria inside the root induces irreversible modifications to its growth and development due to a differentiationof elongated cycad cells, which have suggested to be responsible for the transfer of metabolites betweenthe partners. The process of infection is still unclear. Invasion of filamentous cyanobacteria may occurat 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 observedwithin 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 thesame cyanobiont may be present in two different cycad specimens as well as in different cycad species. Thefilamentous heterocystous cyanobacteria within the cyanobacterial zone are located extracellularly betweenthe elongated cycad cells and embedded in mucilage. All the molecular work is consistent with differentNostoc strains, the cyanobiont in cycad symbioses. The cyanobionts differentiateinto vegetative cells and heterocysts, but rarely akinetes. In general, the symbiotic Nostocfilaments show only few modifications in comparison to its free-living counterparts: (1) an increasedheterocyst frequency, (2) an increased level of nitrogen fixation, and (3) a transfer offixed nitrogen from the cyanobacterial to the cycad cells. However, there is a distinct developmentalgradient within the coralloid roots with ``free-living like'' filaments in the growing tip, whichrapidly develop into symbiotic cells followed by older and metabolically less active cells in the olderparts. Due to their location in coralloid roots (complete darkness), the cyanobionts are expected to havea heterotrophic mode of carbon nutrition. However, almost nothing is known about the heterotrophicmetabolism in the symbiotic cells. The present review focuses on recent advances in the understanding ofsymbiotic cyanobacteria in cycads.