Diffusible factors from Frankia modify nodulation kinetics in Discaria trinervis, an intercellular root-infected actinorhizal symbiosis
Frankia BCU110501 induces nitrogen-fixing root nodules in Discaria trinervis (Gillies ex Hook. & Arn.) Reiche (Rhamnaceae) via intercellular colonisation, without root hair deformation. It produces diffusible factors (DFs) that might be involved in early interactions with the D. trinervis roots, playing a role in the nodulation process. The induction of root nodule development in actinorhizal symbiosis would depend on the concentration of factors produced by the bacteria and the plant. A detailed analysis of nodulation kinetics revealed that these DFs produce changes at the level of initial rate of nodulation and also in nodulation profile. Diluted Frankia BCU110501 inoculum could be activated in less than 96 h by DFs produced by Frankia BCU110501 cells that had been previously washed. Biochemical characterisation showed that Frankia BCU110501 DFs have a molecular weight of <12 kDa, are negatively charged at pH 7.0 and seem to contain a peptide bond necessary for their activity. Frankia BCU110501, belonging to Frankia Clade 3, does not induce nodules in Alnus acuminata H.B.K. ssp. acuminata but is able to deform root hairs, as do Frankia strains from Clade 1. The root hair deforming activity of Frankia BCU110501 DFs show the same biochemical characteristics of the DFs involved in nodulation of D. trinervis. These results suggest that Frankia symbiotic factors have a basic structure regardless of the infection pathway of the host plant.