Symbioses are natural biotechnological systems in which soil bacteria, named rhizobia, form interactions with legumes, resulting in nitrogen fixation. Rhizobium leguminosarum bv. trifolii TA1 (RtTA1) genome is partitioned into chromosome and four megaplasmids. The macrorestriction map of RtTA1 genome was constructed and over 100 genetic markers on chromosome and plasmids were localized. In the RtTA1 genome, five large regions and many separate potential polysaccharide biosynthesis genes were found. All regions showed high degree of sequence identity and conserved synteny between R. leguminosarum bv. viciae (Rlv) and Rhizobium etli (Rhe). Comparative analyses of genetic maps of close relatives RtTA1, Rlv and Rhe showed significant chromosome colinearity, providing a basis for comprehensive analysis of R. leguminosarum genome evolution.
Successful nodulation of legumes by rhizobia is a complex process that in open field depends on various environmental and biological factors. Generally legume productivity may be improved by inoculation with selected, highly effective in diazotrophy root nodule bacteria. However, field legume inoculation with Rhizobiaceae species is very often unsuccessful due to the presence of native strains in soil which are better adapted and usually dominate over introduced bacteria. The ability of one strain to outnumber others in nodule occupancy is commonly termed competitiveness. This feature of strain is genetically regulated by numerous bacterial genes, as well as it is highly dependent on host plant genotype and environmental cues. The competitiveness of endogenous strains is critical for the successful use of inocula to introduce the quality strains. In this paper we describe ways and means which should be considered in order to manipulate both established and introduced strains ecologically, edaphically and genetically to improve legume productivity and, as the consequence, soil fertility.
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