The evolution of nodulation

In this review we will first describe the different steps leading to nodule formation, and these will be compared with processes of non-symbiotic plan t development and growth. In general, aspects of both actinorhizal as well as rhizobial symbiosis are described, but in several cases, the emphasis wi ll be on the Rhizobium-legume symbiosis because more knowledge of this syst em is available. Subsequently, the phylogeny of nodulating plants is descri bed and a comparison is made between several aspects of legume and actinorh izal nodulation. At the end of this paper the relationship between nodule s ymbiosis and endomycorrhizal symbiosis is described, and it is discussed to what extent the development of root nodules involves unique properties, or whether processes and genes have been recruited from common plant developm ent and the endomycorrhizal symbiosis.