Comparison of early interactions between Frankia and actinorhizal plants with different infection pathways

GABBARINI, L; WALL LG

Umea, Suecia

Simposio; 14 th Meeting on Frankia and actinorhizal plants; 2006

Different parameters - lag time for first nodule appearance, initial rate of nodulation, regulated rate of nodulation, total time elapse of nodule development, and final nodule number per plant - were defined to analyse nodulation kinetics of actinorhizal plants in two symbioses with different infection pathways, Discaria trinervis - Frankia BCU110501 and Alnus acuminata - Frankia ArI3. These symbiotic pairs are incompatible for nodulation in cross inoculation experiments. We have analysed the nodulation parameters in dose response experiments with increasing doses of the homologous Frankia and also in co-inoculation experiments with a limiting dose of the homologous Frankia and increasing doses of the heterologous one. The time elapsed until nodule appearance depends on doses of Frankia inoculum irrespectively of symbiotic recognition. Similar effect was obtained when root exudates concentration was increased in the case. The initial rate of nodulation was only increased with the doses of the homologous Frankia suggesting a specific recognition step controlling this process that could be either an effect on the rate of infection or the rate of nodule development. The time elapse of nodulation, that's it new nodule appearance and development, was modified by the doses of the heterologous Frankia in different ways depending on the infection pathway of the tested plant. In Discaria trinervis the time elapse of nodulation was increased with Frankia ArI3 co-inoculation doses. On the contrary in Alnus acuminata the time elapse of nodulation decrease with increasing doses of co-inoculated BCU110501. Thus, final number of nodules per plant was differentially affected in both plants by co-inoculation with heterologous Frankia. Although in single inoculation control assay we never observed nodulation by the heterologous Frankia, co-occupation of nodules because of co-infection was analysed by two techniques: re-inoculation with crush nodules on new plants of both species, and by RFLP-PCR of 16S rDNA. Results show that co infection does not explain the change in final number of nodules per plant because of co-inoculation suggesting that should be some complementary signal coming from the heterologous Frankia that would modify plant control for infection and nodulation. In the case of BCU110501 we have found that Diffusible Signal Factors from this strain can induce not only root hair deformation but also cortical cell division centres in Alnus acuminata roots.