Plasmid DNA content of several agronomically important Rhizobium species that nodulate alfalfa, berseem clover, or Leucaena

Abstract

Plasmids control the effective symbiotic properties of legume-nodulating Rhizobium species. Our long-term goal is to genetically improve inoculant strains using plasmid transfer, therefore, in this study, we examined the plasmid content of eight agronomically-important strains of Rhizobium meliloti from alfalfa (Medicago sativa L.), six strains of R. leguminosarum biovar trifolii from berseem clover (Trifolium alexandrinum L.), and thirteen strains of Rhizobium sp. (Leucaena) from L. leucocephala (Lam.) Dewit, isolated in Egypt. Most of the R. meliloti strains had an approximately 800-MDa cryptic megaplasmid; USDA strain 1021a and ARC strains 1, 103, B3, and B7 also contained a smaller symbiotic megaplasmid that hybridized with nifHD. These Sym plasmids varied in size from 430–615 MDa. Both strain ARC 104, with a single megaplasmid greater than 1600 MDa, and strain USDA1011, which had six plasmids ranging from 1000 to < 50 MDa, failed to hybridize with the nif gene probe. R. leguminosarum bv. trifolii USDA strains 2101, 2128, 2129 and 2131 each had a 500-MDa nif megaplasmid plus a 220 and a 150-MDa plasmid, whereas ARC strains 100 and 101 each had a 200-MDa nif plasmid and six other plasmids ranging in size from 75 to 310 MDa. Rhizobium sp. (Leucaena) strains fell into five different plasmid profile groups, most with a nif-carrying plasmid of 200, 230 or 290 MDa. Plasmid characterization will be useful for plasmid transfer studies aimed at developing improved Rhizobium inoculant strains. This is the first study to document diversity in plasmid profiles among inoculant Rhizobium strains for alfalfa and berseem clover in Egypt. It is also the first report describing plasmids and megaplasmids of Rhizobium sp. (Leucaena) strains isolated in Egypt.