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

SANNAZZARO, ANALIA INÉS; CASTAGNO, LUIS NAZARENO; RUIZ, OSCAR A; PISTORIO, MARIANO; TORRES TEJERIZO, GONZALO; LAGARES, ANTONIO; ESTRELLA, MARIA JULIA

Bariloche

Congreso; International Plasmid Biology Conference; 2010

IBBM - Universidad Nacional de La Plata

The utilization of Lotus tenuis to obtain a high productivity of pastures in soils of the Flooding Pampa (Buenos Aires, Argentina) is an useful agronomic strategy for cattle production, given that restrictive soils of this region limit the growth of traditional legumes such as clover and lucerne. Mesorhizobium loti was traditionally assumed as the common symbiont of L. tenuis. In a previous study we have taxonomically characterized native rhizobia isolated from L. tenuis in constrained environments of the Flooding Pampa (Estrella et al., 2009). A majority of these isolates were related with different species of Mesorhizobium genera and some of these strains have been  selected by their superior symbiotic N2-fixing abilities under laboratory conditions and proved to be useful to increase the yield of L. tenuis (Sannazzaro et al., 2010). Intriguingly, some of L. tenuis symbionts were closely related to rhizobial species that have been shown to be common symbionts of Phaseolus vulgaris. This was the first time that L. tenuis was identified as the primary host of a Rhizobium sp. strain. This work presents a preliminary survey about the occurrence of plasmids in native strains isolated from L. tenuis growing in the Flooding Pampa. Preliminary results indicate that in all the studied Mesorhizobium isolates the symbiotic functions seem to be located on the chromosomein a symbiosis island adjacent to a Phe tRNA gene, suggesting that the extrachromosomal elements observed in this work would be cryptic plasmids. We intend to elucidate whether these elements could modulate the interaction between the symbionts, nodulation efficiency, competitiveness or adaptation and survival under different environmental conditions. Thus far, our results suggest that functions related with NaCl tolerance of isolates in free living conditions are not exclusively plasmid-related. The evidence of a putative symbiosis island in a Rhizobium isolate suggests the occurrence of a lateral transference event of chromosomal symbiotic elements to Rhizobium species, a genus in which symbiotic functions are usually located on plasmidic elements