Symbiotic characteristics and selection of autochthonous strains of Sinorhizobium meliloti populations in different soils

We studied the symbiotic characteristics of populations of Sinorhizobium me liloti present in soils with different cultivation histories. Usually the m ost abundant strains of S. meliloti did not bear cryptic plasmids. However, most of the very competitive strains bare other plasmids as well as pSym ( the symbiotic plasmid). In a high proportion of these plasmids the presence of nfe genes was detected; these genes are involved in the competitiveness of S. meliloti strains. Studies on effectiveness of the strains bearing th is nfe zone in a cryptic plasmid revealed that they form a number of nodule s and fix amounts of nitrogen significantly higher than strains bearing onl y the symbiotic plasmid, conferring them advantages over other strains coex isting with them in the host plant rhizosphere. These findings suggest the possibility of using the presence of the nfe zone as a criterion for the se lection of strains that can be used as inoculants. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.