The actinorhizal symbiosis is entered by nitrogen-fixing actinobacteria of the genus Frankia and a large group of woody plant species distributed among eight dicot families. The actinorhizal symbiosis, as well as the legume-rhizobia symbiosis, involves the stable intracellular accommodation of the microsymbionts in special organs called root nodules. Within the nodules, the nitrogen-fixing bacteria are provided with carbon sources by the host plant while supplying the plant with fixed nitrogen, which is often a limiting factor in plant growth and development.

Datisca glomerata (C. Presl.) Baill. (Datiscaceae, Cucurbitales) is a suffruticose plant with a relatively short generation time of six months, and therefore represents the actinorhizal species most suited as a genetic model system. In order to obtain an overview of nodule development and metabolism, the nodule transcriptome was analyzed. Comparison of nodule vs. root transcriptomes allowed identification of potential marker genes for nodule development. The activity of the promoters of two of these genes was studied in planta. Furthermore, auxins and cytokinins were quantified in roots and nodules, and the auxin responses in roots were compared in D. glomerata and the model legume Medicago truncatula.

Our results indicate that in actinorhizal plants signaling in the root epidermis leading to nodule organogenesis follows the common symbiosis pathway described for the legume-rhizobia symbiosis and arbuscular mycorrhiza. Moreover, we discovered a group of nodule-specific genes encoding defensin-like peptides with an unusual C-terminal domain that had not been found in other plant species. A possible role in the symbiosis-specific differentiation of the microsymbiont and in nodule development was suggested for these cysteine-rich peptides. Finally, we showed that D. glomerata and M. truncatula differ in their auxin and cytokinin requirements for the development of both nodules and lateral roots.