Microsymbionts Isolated from Pea Root Nodules – the Study of Population Diversity

Aims: The study of strains belonging to local rhizobial population, concerning their diversity in the
genetic, metabolic and symbiotic properties, and their prevalence in the microsymbiont population.
Methodology: 257 rhizobial isolates recovered from nodules of five pea (Pisum sativum cv. Ramrod)
plants grown at one site were classified using PCR-RFLP analysis of 16-23S rRNA ITS. After that, for
representative group of 55 strains, 16-23S rRNA ITS region was sequenced, nodA-F region was
analyzed by PCR-RFLP and sequencing, metabolic capabilities were studied using Biolog`s and
growth tests and symbiotic performance in plant tests were assayed.
Results: Individual plants were infected by numerous and diverse strains, however, in the entire
sampled population of microsymbionts, only three large clusters of strains (one similar to Rhizobium
pisi and two similar to Rhizobium leguminosarum) could be distinguished on the basis of PCR-RFLP
and sequence analyses of 16S-23S rRNA ITS region. Rhizobium strains belonging to different groups
varied in plasmid number and the amount of plasmid DNA, utilization of carbon and energy sources,
growth on soil extract-based media and the ability for symbiotic plant growth promotion. The most
numerous group of the isolates was characterized by the high plasmid DNA content, low number of
utilized sugar substrates, and comprised numerous strains with low symbiotic efficiency.
Conclusion: Sampled population of pea microsymbionts had its own characteristic structure with
clearly distinguishable sub-populations, composed of numerous strains - probably descendants of a
few old lineages, which diversified in the lapse of time. These strains are still competing during root
nodule colonization, resulting in the symbiosis of individual pea plants with broad spectrum of different
Rhizobium strains.