Morphology and anatomy of root nodules of Retama monosperma (L.)Boiss.

In spite of the importance of Retama species for dune stabilization and re-vegetation and the contribution to the bio-fertilization of semi-arid and arid ecosystems, the symbiotic interaction of Retama species with rhizobia remains largely unstudied. In this paper, we aim to provide the first detailed study on nodule morphology and anatomy of Retama monosperma. We collected nodules from coastal areas nearby Oran (Algeria) and studied in detail their anatomy and ultrastructure by light and electron microscopy. First, we confirmed the likely identity of the microsymbiont as B. retamae and found that nodules of R. monosperma belong to the genistoid type of indeterminate nodules. Infection threads, typical for most nodules of legumes, are absent in nodules of R. monosperma and bacterial spread is associated with plant cell division. The nitrogen fixation zone is homogenous with only invaded cells and a network of non-invaded cells found in many nodules, is absent. Moreover, endoreduplication does not take place in bacteroids in nodules of R. monosperma. The features observed in this study are compared to the morphology and anatomy of nodules of other legumes and the possible consequences for nodule functioning and the mode of infection during the establishment of the interaction are discussed.