Frankia – from soil to symbiosis.

CHAIA E; WALL LG; HUSS-DANELL K

Bariloche, Argentina

Congreso; 15th International Frankia and Actinorhizal Plants Meeting; 2008

Frankia are N2-fixing actinomycetes forming root nodule symbioses with some 200 plants, so called actinorhizal plants, mainly shrubs and trees from temperate regions. All eight families of actinorhizal plants are placed together in the rosid clade together with legumes and Parasponia, which are infected by rhizobia. Symbiosis with Frankia is not obligate for actinorhizal plant, nor all Frankia strains bound to soil carrying their host plants.    Occurrence and diversity of Frankia in soil has been demonstrated by DNA based methods but, so far, Frankia can hardly be isolated from soil, only from root nodules. Using plants to trap infective Frankia is therefore an important tool to study distribution and dispersal of Frankia into soils of different characteristics as well as competition among Frankia strains and interactions with other soil organisms. Comparisons of molecular assays and nodulation assays for enumeration of Frankia units in soil indicate that not all Frankia in soil may be infective in bioassays. Cultured strains of Frankia are grouped into three clusters with different and sometimes overlapping host specificity. The infection pathway for Frankia into a root is determined by the host plant and goes either via infection of root hairs or via direct infection of root cells. Given the usefulness of actinorhizas for as diverse functions as wood, fuel, food, soil restoration, wind protection, shade, chemical compounds and landscaping, there is a continued need for understanding physiology and recognition phenomena as well as ecology of Frankia in relation to hosts.