Alnus glutinosa

Branch of Filao. The leaves are reduced to scales, which permits to limit transpiration (photo S. Svistoonoff)

Among higher plants, the association between Rhizobium and Legumes is not the only one to have resulted in a nitrogen-fixing root symbiosis. Alder and Casuarina, more commonly called filao, are actinorhizal trees, that is able to live in symbiosis with a nitrogen-fixing actinobacterium called Frankia. As in the case of the interaction between Rhizobium and Legumes, Frankia reduces molecular nitrogen into ammonium in specialised organs, the nodule. Actinorhizal nodules, also called actinorhizae, are most of the time on the roots, or more rarely on the stems. In general, actinorhizal plants have a nitrogen-fixing capacity as efficient as that of Legumes and, even though they have been less studied, they nevertheless constitute the second group of nitrogen-fixing plants, dominating in nitrogen-poor ecosystems of temperate regions.

Expression of GFP under control of the Casuarina cg12 gene promotor in prenodules. This gene is involved in the infection process of the root by Frankia (photo S. Svistoonoff)

Actinorhizal plants are for the most part trees and shrubs. Contrary to Legumes that belong to a single family, plants nodulated by Frankia are characterised by a large taxonomic distribution; there are about 260 species of actinorhizal plants distributed in 24 genera and eight families of angiosperms. Their worldwide distribution is vast; they are found on all continents, save Antarctica. After glacial periods, these plants have initiated ecological successions, permitting soils enrichment through high nitrogen and organic matter inputs, and then were replaced by non nitrogen-fixing competitive plants. Today, they are found in disturbed environments such as volcanic substrata or mine spoils, and of course on nitrogen-poor soils such as glacial moraines or sand soils.