Curing of a non-symbiotic plasmid of the Rhizobium tropici strain CIAT 899 affected nodule occupancy and competitiveness of the bacteria in symbiosis with common beans

Functional analyses of rhizobial plasmids have concentrated mainly on symbiotic plasmids (pSym) that carry genes required for nitrogen fixation and nodule formation. However, information on the other plasmids, termed cryptic or non-symbiotic, is still scarce. In this work, a collection of sequential cured derivatives with different combinations of plasmids was generated from the Rhizobium tropici strain CIAT 899 to study their functions in symbiosis with common bean plants. PCR analysis of the nif genes from all cured derivatives indicated that plasmid b is the symbiotic plasmid (pSym) while a and c are the cryptic plasmids. This genotype was confirmed by the presence of nodules in common bean plants inoculated with derivatives containing plasmid b. However, when plasmid a was missing in the derivative containing the pSym, nodules were formed but in lower numbers and were smaller in size. The derivative cured of plasmid a formed nodules with a large quantity of starch and crystals and showed a significant decrease in nitrogenase activity. In addition, the presence of bacteria in the intercellular spaces of the nodules was observed. A co-inoculation experiment involving both the wild-type CIAT 899 strain and derivative CIAT 899a suggested that the cryptic plasmid a contains genes involved in the competition for nodulation of Phaseolus vulgaris.

► It was obtained derivatives from CIAT 899 strain cured of plasmid a, b, ab and abc.
► Curing plasmid a from Rhizobium tropici CIAT 899 strain affects its symbiosis with beans.
► CIAT 899a cured derivative formed lower number of nodules and reduced nitrogenase activity.
► Plasmid a contains genes involved in the competition for nodulation in Phaseolus vulgaris.