DECREASED SYMBIOTIC EFFECTIVENESS OF RHIZOBIUM-LEGUMINOSARUM STRAINS CARRYING PLASMID RP4

The presence of derivatives of the broad host range plasmid RP4 in str ains of Rhizobium leguminosarum biovar viciae severely inhibited nitro gen fixation by these strains in nodules on cultivars of pea (Pisum sa tivum). The strains formed small white nodules. Yield and total nitrog en values were comparable with those obtained for plants inoculated wi th a non-nodulating mutant. Strains carrying the same derivatives gave rise to nitrogen fixing nodules when inoculated on cultivars or lenti ls (Lens culinaris). Similar results were observed with plasmid R702 b ut not with R751, suggesting that the effect is limited to plasmids of the IncP alpha classification. Histological examination of nodules in duced by strains carrying RP4 indicated that there are fewer infected cells and starch granules are organised unusually in the infected cell s. Tn5 mutagenesis of plasmid RP4-4 was undertaken and Tn5 inserts wer e screened for abolition of the effect on nitrogen fixation. Eight mut ants, having no effect on nitrogen fixation, were isolated. Seven of t hese had lost the ability to transfer by conjugation and the eighth wa s greatly reduced in conjugation frequency. Physical analysis of the t ransposon inserts revealed that they were located in the Tra regions o f RP4. (C) 1998 Federation of European Microbiological Societies. Publ ished by Elsevier Science B.V.