Instability of Cryptic Plasmids Affects the Symbiotic Effectivity of Rhizobium leguminosarum bv. viceae Strains. Werner Selbitschka. Institut für Mikrobiologie und Biochemie der Universität Erlangen-Nürnberg, Lehrstuhl Mikrobiologie, Staudtstrasse 5, D-8520 Erlangen, Germany. Wolfgang Lotz. Institut für Mikrobiologie und Biochemie der Universität Erlangen-Nürnberg, Lehrstuhl Mikrobiologie, Staudtstrasse 5, D-8520 Erlangen, Germany.. MPMI 4:608-618. Accepted 25 June 1991. Copyright 1991 The American Phytopathological Society.

A total of fifty-two strains of Rhizobium leguminosarum biovar viceae were newly isolated from effective root nodules of faba bean plants (Vicia faba ‘Herz Freya’). When tested with Pisum sativum ‘Poneka’, 20 of the 52 strains showed a significantly reduced ability to fix nitrogen. Such strains were classified as Fix' for this host. Fully effective nodules developed at a frequency of about 2% on pea plants infected with Fix' strains. Segregant strains isolated from such nodules were stably Nod⁺Fix⁺ with P. sativum and V. faba and usually had an altered plasmid profile (e.g., deletions of different sizes in a cryptic plasmid). Conjugal transfer of a mobilizable derivative of the genetically unstable cryptic plasmid pRleF41b of Fix' strain R. leguminosarum bv. viceae F41 into the Fix⁺ strain P13 resulted in the co-transfer of the Fix' phenotype. A gene library of the Fix' wild type strain F41 was constructed. Overlapping cosmid clones carrying the wild type DNA sequences from the genetically unstable region of plasmid pRleF41b have been identified. Several lines of experimental evidence suggest that determinants for a host-dependent reduction of nitrogen fixation (hrf) are located on a 13-kb BamHI fragment of this 210-kb plasmid.