The diversity of actinorhizal symbiosis

Filamentous aerobic soil actinobacteria of the genus Frankia can induce the formation of nitrogen-fixing nodules on the roots of a diverse group of plants from eight dicotyledonous families, collectively called actinorhizal plants. Within nodules, Frankia can fix nitrogen while being hosted inside plant cells. Like in legume/rhizobia symbioses, bacteria can enter the plant root either intracellularly through an infection thread formed in a curled root hair, or intercellularly without root hair involvement, and the entry mechanism is determined by the host plant species. Nodule primordium formation is induced in the root pericycle as for lateral root primordia. Mature actinorhizal nodules are coralloid structures consisting of multiple lobes, each of which represents a modified lateral root without a root cap, a superficial periderm and with infected cells in the expanded cortex. In this review, an overview of nodule induction mechanisms and nodule structure is presented including comparisons with the corresponding mechanisms in legume symbioses.