Advances in Actinorhizal Symbiosis: Host Plant- Frankia Interactions, Biology, and Applications in Arid Land Reclamation. A Review

Symbiotic association of the N 2 -fixing actinomycete Frankia with the roots of more than 200 tree species from 24 genera of 8 families of angiosperms has been studied since 1829. The first successful isolation of the microsymbiont and reinfection in the host plant was achieved in 1978. Marked advances in research and understanding of Frankia biology, its actinorhizal hosts and their interactions have been made since then, although the studies on Frankia have been hampered by difficulties of isolation and their slow growth rate in vitro. Exponential growth with high biomass yields within three to four days has been obtained for a number of strains isolated from Casuarina spp. Use of BAP medium, supplemented with avian phospholipid mixtures and certain fatty acids at controled O 2 access, optimizes growth. Monosporal cultures are scarce; recently a few became available for biochemical and genetic studies. Research using exponentially growing cultures has yielded information on a complex proteolytic system, including proteasomes, endo- and extracellular proteinases and aminopeptidases, and also on esterases, dehydrogenases, and extracellular DNAses. Molecular tools have revealed a marked genetic diversity of Frankia soil populations and have enabled the definition of four clades in the Frankia phylogenetic tree. Studies on Frankia -host plant interactions have detected molecular signal exchange preceeding the establishment of symbiosis. Similarly, there is progress in research on transgenic actinorhizal plants and on actinorhizal-specific genes and proteins (actinorhizins) involved in symbiotic interactions, infectivity, and host specificity. Actinorhizal plants are rapidly growing species, able to develop in N-poor soils, and for certain species, in harsh environmental stress conditions. They increase the fertility of agroforestry ecosystems, and have an economic potential for timber, fuelwood production, land reclamation, and amenity planting. The Casuarina spp. are of especial value in arid environments.