The peanut-rhizobia interaction: diversity in the peanut nodulating rhizobial population of Cordoba, Argentina, and role of bacterial molecules

FABRA, A.; TAURIAN, T.; ANGELINI, J.; MORGANTE, C.; DARDANELLI, M.; IBAÑEZ, F.; CASTRO, S.; AGUILAR, M.

Miguel Pereira Río de Janeiro, Brazil

Conferencia; RELAR 2004; 2004

Peanut (Arachis hypogaea L.) is an important crop all over the world whose domestication has been proposed to occur in an area between the North of Argentina and the South of Bolivia. More than 90% of the Argentinian peanut production is concentrated in the province of Cbrdoba. It is well known that the nodule morphogenesis, the mode of infection and the uptake of rhizobia in nodule cells are characteristics determined by the host plant. Arachis hypogaea L. and Stylosanthes spp are the only two hosts that differ from other herbaceous legumes in that rhizobia penetration into the root and in the cortex as well as spreading inside the nodules occur without infection threads formation and involves intercellular penetration (crack entry). Thus, unlike other host plant-rhizobia symbiosis, several events of the interaction do not occur in peanut. So far, bacteria that nodulate peanut in natural environments have been classified as Bradyrhizobium (Arachis hypogaea) spp but species names have not been defined yet. In the course of a project aimed to examine the diversity of peanut  odulating rhizobia, a collection of 39 isolates, obtained from nodules or from soil samples collected in Cbrdoba, was characterized henotypically (rate of growth, acid production, tolerance to acidity, sodium chloride and high temperatures, symbiotic effectiveness) and genotypically (ERIC and ARDRA fingerprints, 16s rRNA gene sequencing). These studies have shown firstly that peanut is nodulated by fast and slow growers rhizobia in field conditions, and secondly that each group incompasses heterogeneity among their members, having distinct rep fingerprints. The 16s rRNA sequencing data from two isolates, that represent slow and fast growers, respectively, revealed to be closely related to members of genera Bradyrhizobium and Rhizobium and Sinorhizobium, respectively. Currently, we are pursuing the molecular characterization of other isolates in order to assess the prevalence of each group. Compared with other symbiotic association, the information on the peanut-rhizobia interaction is rather limited. Thus, the second goal of our laboratory is to gain information on the symbiotic signalling compounds that are involved in the events that lead to nodule formation. Biochemical and molecular approaches were used to identify the profile of flavonoid exudated by peanut, the rhizobial Nod factor structure, and investigate the significance of bacterial adhesins and exopolysaccharides in the peanut infection process. We identified a 14 KDa protein that has properties identical to those reported for rhicadhesin and seems to be involved in the attachment process of rhizobia to peanut root. Using a peanut nodulating rhizobia strain impaired in the exopolysaccharide production we have examined the nature of its symbiotic deficiency. Data obtained have important implications for understanding how bacterial molecules are functioning in the plant-rhizobia interactions leading to the formation of determinate nodules, specially in legumes that are infected without the infection threads formation.