Early interactions between Frankia and actinorhizal plants

WALL, LUIS GABRIEL

Miguel Pereira, Río de Janeiro, Brazil

Conferencia; XXII Reunión Latinoamericana de Rizobiología (RELAR) y I RBFBN; 2004

RELAR

The interaction between the actinomycete bacteria Frankia and more than 220 plant species distributed in 25 genera and 8 families, induces the development of nitrogen fixing root nodules that fixes atmospheric nitrogen. This association is named actinorhizal symbiosis. Most of its physiological features resamble legume-rizobia symbioses, as it happens with the regulation of  root infection and nodule development, the regulation of  nitrogen fixation, specificity and recognition phenomena between symbionts, and both root hair infection or intercellular invasion strategies. Nevertheless, there are some features related to actinorhizal symbiosis that  differ from  rizobia-legume symbioses, for example actinorhizal nodules are modified lateral roots with central vascular bundles. The molecular basis for recognition and specificity in actinorhizal symbiosis is not well known. Detailed studies have demostrated that rizobial LCOs or Nod factors can not replace  Frankia root hair deformation factors, and they are not able to interfere with actinorhizal nodulation. These results open the question about what kind of signals have been selected during evolution of actinorhizal plants for plant-bacteria recognition. During the last years  we have described the intercellular infection pathway, nodule development and the regulation of nodulation in Discaria trinervis - Frankia BCU110501 symbiosis. Discaria trinervis (Rhamneceae) is a native southamerican actinorhizal plant that belongs to a different cross inoculation group than Alnus acuminata (Betulaceae), another southamerican native actinorhizal plant which is infected via root hair deformation. After a serie of coinoculation experiments of D. trinervis with Frankia BCU110501  plus different bacteria, we concluded that a Frankia extracellualr factor, also produced by other actinomycetes, is able to stimulate nodulation rate by activating Frankia in a dose response fashion, resambling quorum sensing phenomenon. This Frankia activation for nodulation can be obtained by pretreating Frankia in  absence of the plant. In adition,  it seems that a Frankia extracellular factor (probably different from the previous one) is able to activate the plant for nodulation, expresing its effect as a basipetally shift of the nodulation profile of the main root, also in a dose response fashion. When searching for local Frankia isolates from field Alnus acuminata nodules, we  found at least three different actinomycetes which are able to grow in nitrogen free media and  are not typical Frankia. Each of them can induce different responses on A. acuminata, as root hair deformation, prenodule formation, nodule formation, or inhibition of nodulation by a control Frankia strain.The most  remarcable fact  was the finding that some of these isolates could be phisyological complemented for root nodulation of A. acuminata when they were co-inoculated with Frankia BCU110501, which alone is not able to nodulate Alnus. These biological assays of Frankia activation for nodulation, plant activation for nodulation, and complementation for nodulation of deffective Frankia (or whatever they are) strains will help to search for the molecules involved in actinorhizal symbiotic recognition.