Wall Frankia diffusible signal factor(s) can modify bacterial activity for nodulation and susceptibility of Discaria trinervis root for infection and nodule development

GABBARINI, L; WALL, LG

New Hampshire, USA.

Simposio; XIII Symposium on Frankia and Actinorhizal Plants; 2004

One of our major interests is to uncover signal molecules involved in the early interactions between Frankia and actinorhizal plants. As a first approach to this goal, we analyzed the influence of different sources of diffusible signal factors on the nodulation rate and nodule distribution profile of Discaria trinervis taproots inoculated with Frankia isolate BCU110501.Nodulation rate was modified by co-inoculation with non infective Frankia isolates or Streptomyces colecoide, but was not affected by co-inoculation with other Gram- or Gram+ rhizospheric bacteria, as Azospirillum, Pseudomonas or Bacillus. Nodulation rate increased with the Frankia inoculum dose starting from a threshold concentration until a plateau was attained, suggesting the saturation of a putative receptor for Frankia signals. This enhancement of nodulation rate could also be obtained by pre-treating a diluted BCU110501 inoculum for 36h with high concentrations of the same Frankia strain, or with non Discaria infective strain ArI3, or even with Streptomyces colecoide contained within dialysis bags, suggesting the existence of an diffusible and low molecular weight signal common to these actinomycetes. The phenomenon of cell-to-cell activation for nodulation of Frankia BCU110501 required viable cells and increased from 0h to 96h of contact with no further increment after 156h of bacterial pretreatment. The nodulation profile on the tap root was shifted basipetaly when roots where pretreated with Frankia BCU110501 dialyzates before inoculation with BCU110501 cells. The magnitude of the shift correlated with the concentration of the diffusible Frankia factor(s). Considering our knowledge of the regulation of nodulation in Discaria trinervis, these results suggest that a Frankia diffusible signal can modify the transient susceptibility for infection and nodulation of the host, suggesting a recognition phenomenon during early interactions between symbionts. We do not know yet if the detected bacteria-bacteria and bacteria-plant interactions involve the same diffusible Frankia factor or different bacterial signals.