Plasmid profiles of rhizobia nodulating Lotus tenuis in different soils of the Flooding Pampa

SANNAZZARO, A.; CASTAGNO, L.; RUIZ, O.; PISTORIO, M.; TORRES TEJERIZO, G. A.; LAGARES, A.; ESTRELLA, M.J.

Bariloche

Congreso; International Plasmid Biology Conference 2010; 2010

International Society for Plasmid Biology and other Mobile Genetic Elements

Background and aims: The utilization of Lotus tenuis to obtain a high productivity of pastures in soils of the Flooding Pampa (FP, Buenos Aires, Argentina) becomes a very relevant topic for cattle production; in view of that region have restrictive soils that limit the growth of traditional legumes such as clover and lucerne. In a previous study we have taxonomically characterized native rizobia isolated from L. tenuis in constrained environments of the FP (Estrella et al., 2009). Some of these strains have been screened and selected for their superior symbiotic N2-fixing abilities under laboratory conditions and could be useful to increase the yield of L. tenuis (Sannazzaro et al., 2010). A majority of these isolates were related to species of the genera Mesorhizobium and Rhizobium.While in Mesorhizobium spp. the genes required for symbiosis are usually located on chromosomal symbiotic islands, in,Rhizobium spp. the homolog genes are located on (mega) plasmids. However, most rhizobia have in addition cryptic plasmids that, though not essential for the establishment of symbiosis, can modulate the interaction between the symbionts by changing  nodulation efficiency, competitiveness, or adaptation and survival under different environmental conditions. This work presents a preliminary survey on the occurrence of cryptic plasmids in native rhizobia isolated from L. tenuis growing in the FP. Methods: Rhizobial strains used in this work belong to a previously characterized collection of isolates recovered from nodulated L. tenuis from the FP (Estrella et al. 2009). Plasmids of rhizobia were separated on horizontal agarose gels by following the in situ lisis procedure described by Eckhardt (1978).Results and conclusions: The number of plasmids varied between zero and five, with most strains containing only one plasmid. The comparison of infective and non-infective isolates revealed no evident correlation between this trait and particular plasmid profiles. Similarly, the presence of plasmids could not be associated with any particular stress tolerance. These preliminary results encourage us to further explore the presence of plasmid-borne genes involved in the symbiosis with L. tenuis and/or environmental adaptative functions.