157 ACTINORHIZAL SYMBIOSES Pawlowski Katharina 1 , Zdyb Anna 1 , Hause Bettina 2 , Göbel Cornelia 1 , Feussner Ivo 1 , Demchenko Kirill 1,3 1 Plant Biochemistry, Göttingen University, Justus-von-Liebig-Weg 11, 37077 Göttingen, Germany, 2 Leibniz Institute for Plant Biochemistry, Weinberg 3, 06120 Halle, Germany, and 3 Komarov Botanical Institute, Russian Academy of Sciences, St.-Petersburg 197376, Russia Two groups of plants can enter root nodule symbioses with nitrogen-fixing soil bacteria. Gram-negative, unicellular rhizobia induce nodules on legume roots, while Gram- positive mycelial actinomycetes of the genus Frankia induce nodules on the roots of plants from eight angiosperm families, mostly woody shrubs, collectively called actinorhizal plants. Molecular phylogenetic studies indicate that legumes and actinorhizal plants belong to the same clade (Soltis et al., 1995), suggesting that the common ancestor aquired the underlaying capacity to develop a root nodule symbiosis originated, probably in a single evolutionary event. To identify this underlaying capacity, we compared different aspects of the symbiosis in three diverse systems, one legume (Medicago truncatula) and two actinorhizal plants (Casuarina glauca and Datisca glomerata). In order to develop an intracellular symbiosis, plants had to evolve a mechanism to internalize the microsymbionts and to control the nutrient exchange, i.e. to enclose the microsymbionts in a plant-derived membrane. Bacterial microsymbionts (Frankia and rhizobia) can enter the root of the host plant either intercellularly (Coriariaceae, Datiscaceae, Elaeagnaceae, Rhamnaceae, Rosaceae) or intracellularly (Betulaceae, Casuarinaceae, Myricaceae, most legumes). During intercellular entry in actinorhizal symbioses, bacteria enter the plant root beween epidermal cell and colonize the apoplastic spaces, before they are internalized in primordium cells by the formation of intensely branching infection threads in these cells. During intracellular entry, infection threads are formed in root hairs and grow across cortical cells toward the incipient nodule primordium. Hence, the formation of infection threads that cross cells is an ability of (a subset of) root cells. When the bacteria have reached nodule primordium cells, they can infect them. In actinorhizal nodules, this happens by intense branching of infection threads within the infected cell; in most legume nodules, bacteria are internalized via a complete endocytotic process during which they are enclosed in a peribacteroid membrane derived from the plasma membrane of the plant cell. Infected Y-P. Wang et al, (eds.), Biological Nitrogen Fixation, Sustainable Agriculture and the Environment, 157-160. © 2005 Springer. Printed in the Netherlands.