Rhizobial and Actinorhizal Symbioses: What Are the Shared Features?

In endophytic symbioses, microorganisms live intercellularly or intracellularly inside a plant, in a mutually beneficial rela? tionship. The host plant (the macrosymbiont) usually supplies the endophyte (the microsymbiont) with photosynthates, whereas the benefit of these associations for the plant is an improvement in access to important mineral nutrients whose supply would otherwise be growth limiting. With the exception of cyanobacterial symbioses, in most endophytic symbioses involving higher plants, microorganisms colonize the plant root (reviewed in Quispel, 1992). In mycorrhizal sym? bioses, the fungus improves the absorption of minerals, particularly phosphorus, by the roots (Read et al., 1992; Harrison and van Buuren, 1995; see also Gianinazzi-Pearson, 1996, in this issue). In root nodule symbioses involving eubacteria of the family Rhizobiaceae or actinomycetes of the genus Frankia, the microsymbiont supplies the plant host with am? monium, the product of biological nitrogen fixation. To provide an ecological niche for the endophyte, the mac? rosymbiont has to undergo symbiosis-specific differentiation. In nitrogen-fixing root nodule symbioses, the microsymbionts are hosted in special organs, the root nodules, which develop on roots upon infection by the microsymbiont (reviewed in Benson and Silvester, 1993; Mylona et al., 1995). In root sym? bioses with fungi, plant differentiation is less dramatic. In ectomycorrhizal symbioses, for example, the fungus somewhat alters root growth, triggering the formation of short thick infected roots (Martin and Hilbert, 1991), whereas in endomycorrhizal symbioses, the plant root structure is altered at the cellular level but not macroscopically (Koide, 1991; Gianinazzi-Pearson, 1996, in this issue). In this review, we discuss the recent progress in research on endophytic symbioses involving plant roots. Special attention is given to nitrogen-fixing symbiotic interactions, and we summarize our current knowledge of nodule formation and function. The last part of this review focuses on the develop? ment of model systems and the relationships between different endophytic symbioses involving plant roots.