The <i>Gunnera— Nostoc</i> Symbiosis

Abstract

Among angiosperms, a restricted number of plants are capable of forming symbioses with nitrogen fixing organisms. The best-studied examples of symbiosis are those between legumninous plants and nitrogen-fixing rhizobia. Although cyanobacteria are the most potent nitrogen-fixing microsymbionts in terms of variations in the taxonomy of the hosts, ranging from fungi to higher plants, extant cyanobacterial symbioses encompass only one an giosperm family. This monogeneric family, the Gunneraceae, has an unclear taxonomic affiliation. It comprises about 50 species, which show great variation in size. The larger, rhubarb-like, stand forming species (up to 6m high) are typical of areas ranging from Hawaii to Central America and South America, whereas smaHer, more slender species are found in New Zealand, South-East Asia and the southernmost parts of South America. The eleven endemic species of New Zealand are all small, often < 10cm high, and form stolons, whereas the larger species are rhizomatous. The natural distribution of Gunnera is restricted to the southern hemisphere, but naturalised Gunnera (G. tinctoria) is now known from some European sites, for instance the west coast of Ireland, south-western England, northern France and the Azores. A characteristic of all Gunnera species is their preference for wet and humid areas. Ever since cyanobacteria were discovered in side cells of Gunnera at the end of the nineteenth century, they have been classified as belonging to the genus Nostoc. This is a widespread and globally common filamentous genus capable of advanced cell differentiation. Under nitrogen limitation, het erocysts (sites for the nitrogen-fixing enzyme intro genase) are formed, constituting 5-10% of the total cell population, whereas other adverse conditions may give rise to motile hormogonia or resting spores (akinetes). The differentiation of hormogo nia (the de facto infection units in symbioses) and heterocysts is crucial to the formation of the Gunnera symbioses. Although possibly restricted to the genus Nostoc, several species and strains are capable of forming symbioses with Gunnera, including those compatible with plants such as liverworts and cycads. These aspects of cyano bacterial specificity in symbioses are fairther discussed by Rasmussen and Johansson (this volume).