In Rhizobium etli symbiotic plasmid transfer, nodulation competitivity andcellular growth require interaction among different replicons

Bacteria belonging to the genus Rhizobium are able to develop two different lifestyles, in symbiotic association with plant roots or through saprophyt ic growth. The genome of Rhizobium strains is constituted by a chromosome a nd several large plasmids. one of them containing most of the genes involve d in symbiosis (symbiotic plasmid or pSym). Our model strain Rhizobium etli CFN42 contains six plasmids. We have constructed multiple plasmid-cured de rivatives of this strain and used them to analyze the contribution of these plasmids to free-living cellular viability, competitivity for nodulation, plasmid transfer, and utilization of diverse carbon sources. Our results sh ow that the transfer of the pSym is strictly dependent on the presence of a nother plasmid: consequently under conditions where pSym transfer is requir ed, nodulation relies on the presence of a plasmid devoid of nodulation gen es. We also found a drastic decrease in competitivity for nodulation in mul tiple plasmid-cured derivatives when compared with single plasmid-cured str ains. Cellular growth and viability were greatly diminished in some multipl e plasmid-cured strains. The utilization of a number of carbon sources depe nds on the presence of specific plasmids. The results presented in this wor k indicate that Functional interactions among sequences scattered in the di fferent plasmids are required for successful completion of both lifestyles. (C) 2000 Academic Press.