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Azolla and other plant-cyanobacteria symbioses: Aspects of form and function

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Abstract

Nostoc, a genus of filamentous, heterocystous, cyanobacteria, is widely distributed in the free-living state. It is also the most common phycobiont in N2-fixing lichens and occurs as the N2-fixing symbiont in a small and diverse group of green plants. These include several bryophyte genera (e.g. Anthoceros and Blasia), a pteridophyte genus (Azolla; while the symbiont is referred to asAnabaena azollae, it may be aNostoc spp.), a division of gymnosperms (the 10 cycad genera) and one angiosperm genus (Gunnera). In Gunnera the Nostoc apparently penetrates into the cells of the host. In the other associations Nostoc is extracellular but specific morphological modifications and/or structures of the host plant organs create an environment which fosters interaction and metabolite interchange.

The individual group of Nostoc-green plant symbioses other than Azolla are summarized in regard to the current understanding of their establishment, perpetuation, and host-symbiont interaction. This includes available information on recognition and specificity, mode(s) of infection if applicable, and a synopsis of morphological modifications of the partners. The symbiosis withAzolla is then addressed separately with a more indepth account of the foregoing areas. In addition, the concept ofAzolla harboring a dominant, obiligately symbiotic Nostoc which has not been cultured as well as minor symbionts capable of free-living growth, the distinction between re-constituting and simply re-establishing the symbiosis, and current approaches to improving the symbiosis and to authenticating the establishment of new associations are considered.

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    Gerald A. Peters

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Peters, G.A. Azolla and other plant-cyanobacteria symbioses: Aspects of form and function. Plant Soil 137, 25–36 (1991). https://doi.org/10.1007/BF02187428

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