Symbiotic characteristics and selection of autochthonous strains of Sinorhizobium meliloti populations in dierent soils Encarna VelaÂzquez a, *, Pedro F. Mateos a , Nieves Velasco a , Fernando Santos b , Pedro A. Burgos c , Pablo Villadas c , NicolaÂs Toro c , Eustoquio Mart|  nez-Molina a a Departamento de Microbiolog|  a y Gene Âtica, Facultad de Farmacia, Edi®cio Departamental, Universidad de Salamanca, Salamanca 37007, Spain b Departamento de Biolog|  a Animal, Ecolog|  a, Edafolog|  a, Parasitolog|  a y Qu|  mica Agr|  cola, Facultad de Farmacia, Universidad de Salamanca, Salamanca 37007, Spain c Departamento de Microbiolog|  a, Estacio Ân Experimental del ZaidõÂn, C.S.I.C., Profesor Albareda No. 1, Granada, Spain Abstract We studied the symbiotic characteristics of populations of Sinorhizobium meliloti present in soils with dierent cultivation histories. Usually the most 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 eectiveness of the strains bearing this nfe zone in a cryptic plasmid revealed that they form a number of nodules and ®x amounts of nitrogen signi®cantly higher than strains bearing only the symbiotic plasmid, conferring them advantages over other strains coexisting with them in the host plant rhizosphere. These ®ndings suggest the possibility of using the presence of the nfe zone as a criterion for the selection of strains that can be used as inoculants. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction An ecient rhizobia-legume symbiosis passes through a series of steps beginning with the coloniza- tion of the plant rhizosphere by the microorganism and ending with the biological ®xation of nitrogen within the nodule. In the root colonization phase, one important factor is the relative abundance of each strain in the soil, which indicates its adaptation to this environment. Although attempts have been made to increase legume production by increasing the number of bacteria sur- rounding the roots (Hossain and Alexander, 1984), the decisive factor in the ®rst stages of the process of infection is the competitiveness between strains present in the rhizosphere. Hence, the use of highly infective but not very competitive inoculants is inecient (Broughton et al., 1987a). It has been reported that nodulation ability and competitiveness may be related to the presence of cryptic plasmids (Brom®eld et al., 1985; Pankhurst et al., 1986; Toro and Olivares, 1986; Sanjua n and Olivares, 1989; Pardo et al., 1994; Hartmann et al., 1998) In S. meliloti GR4 genes involved in competitiveness, have been described called nfe, which are linked to plasmid pRmeGR4b (SanjuaÂn and Olivares, 1989; Toro and Olivares, 1986; Soto et al., 1993; Villadas et al., 1995). In some cases, the eectiveness of nitrogen ®xation has also been related to the presence of certain cryptic plasmids (Thurman et al., 1985; Pankhurst et al., 1986; Barbour and Elkan, 1989; Hynes and MacGregor, 1990; Kuykendall et al., 1994; VelaÂzquez et al., 1995). These cryptic plasmids can be transferred from one strain to others in the rhizosphere (Broughton et al., 1987b; Scho®eld et al., 1987; Rao et al., 1994). The im- portance of cryptic plasmids is related to their abun- dance in cells and their great stability (Weaver et al., 1990; Mercado-Blanco and Olivares, 1993). The role of Soil Biology and Biochemistry 31 (1999) 1039±1047 0038-0717/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(99)00018-8 PERGAMON * Corresponding author. Fax: +34-923-224876; e-mail: evp@ gugu.usal.es