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The Cycad Pages
CORALLOID ROOTS AND NITROGEN FIXATION
Juliana Medeiros and Dennis Stevenson

Specificity and Diversity

Like all other plants, cycads must exclude soil organisms from invading their root system. In order to form a symbiosis, however, cyanobacteria of the correct strain must be allowed to enter the root. Recognition between partners is accomplished by chemical signaling. Cyanobacteria have cell-surface antigens, which seem to change as the organism develops and differentiates. The plant also produces signals, facilitating interaction between the two, allowing proper timing and specificity of infection. The local soil habitat is the source of cyanobacteria, and as such, determines the strains available to form symbioses (Rai et al., 2000).

Most authors have cited strains of Nostoc as the symbiont living within cycad roots (Grilli Caiola, 1990; Lindblad and Bergman, 1988; Rai et al., 2000). Many have also included Anabaena as a symbiont of cycads (Grilli Caiola, 1990; Lindblad, 1990). Grobbelaar et al. (1986), however, identified Nostoc as the symbiotic genus, calling for reevaluation of data in which Anabaena was named. Studies by Lindblad (1990) and Grobbelaar et al. (1986) also name Calothrix as the symbiont of Encephalartos. Modern advances in genetics have been crucial in redefining cyanobacteria as prokaryotes, however, when they had previously been identified as algae. Similarly, new technologies will likely be effective in future studies concerning the taxonomic placement of endosymbiotic cyanobacteria.



The Cycad Pages

© 1998-2012 Royal Botanic Gardens Sydney
Written and maintained by Ken Hill 1998-2010
Maintained by Leonie Stanberg and Dennis Stevenson 2010-2012
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