Identification and mapping of loci involved in motility, adsorption to wheat roots, colony morphology, and growth in minimal medium on the Azospirillum brasilense Sp7 90-MDa plasmid.

We have constructed a cosmid library of the Azospirillum brasilense Sp7 90-MDa plasmid (p90) and established the EcoRI restriction map of this plasmid. The central regions of cloned p90 DNA fragments from several recombinant cosmids were deleted by restriction endonuclease digestion and replaced by a DNA cassette encoding kanamycin resistance. Using these in vitro constructed deletions for marker exchange in Sp7, we made six different p90 deletion derivatives spanning all together 50% of the total length of p90. Comparison of the deletion derivatives with Sp7 for several properties revealed p90 loci involved in colony morphology, growth on minimal medium, motility, and adsorption to wheat roots. In analogy with the rhizobial symbiotic plasmids (pSym), we propose to denote the p90 plasmid as a rhizocoenotic plasmid (pRhico), carrying several genes involved in the A. brasilense-plant root interaction.