Recent developments in the actinorhizal symbioses

Over 200 species of angiosperms in eight different families are capable of forming root nodule symbioses with the actinomycetal genusFrankia as endosymbiont. Several thorough reviews of the biology of these actinorhizal associations have appeared in recent years (Benson and Silvester, 1993; Schwintzer and Tjepkema, 1990; Tjepkema et al., 1986). The purpose of the present discussion is to provide a summary overview of the actinorhizal symbioses, with an emphasis on recent research activities. A few areas of comparative interest with other symbiotic diazotrophs will be highlighted, especially regarding the question of oxygen protection and nitrogen fixation.