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 plant development and growth. In general, aspects of both actinorhizal as well as rhizobial symbiosis are described, but in several cases, the emphasis will be on the Rhizobium-legume symbiosis because more knowledge of this system is available. Subsequently, the phylogeny of nodulating plants is described and a comparison is made between several aspects of legume and actinorhizal nodulation. At the end of this paper the relationship between nodule symbiosis 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 development and the endomycorrhizal symbiosis

Modulation of Host Defence Systems

Arbuscular mycorrhizal (AM) fungi extensively invade host root tissues. This raises the question of how host plants contend with them; they must exert some kind of control over fungal proliferation since it is confined to a specific root tissue, the parenchymal cortex. Defence processes, which are triggered as a general plant response to microbial invasion, are modulated during root-fungus interactions in arbuscular mycorrhizas. This chapter presents an up-dated review of data on plant defence elicitation in these symbiotic systems and discusses possible mechanisms whereby defence reactions are maintained at a low level, as well as their implication in the phenomenon of bioprotection by AM fungi against soil-borne pathogens