Rhizobia in diverse genera may contain small plasmids and/or large plasmids, termed as “megaplasmids”. Megaplasmids are categorized as the symbiotic plasmid (pSym) or the cryptic plasmid. In some rhizobial strains, the symbiotic plasmid and the cryptic plasmid may coexist together. Although the symbiotic plasmids malinly encode symbiotic genes, some symbiotic plasmids also carry genes involved in various other functions. The symbiotic plasmids have been found in some genera of rhizobia including Ensifer, Mesorhizobium, Rhizobium and Sinorhizobium. Although the cryptic plasmids mainly carry genes encoding basic metabolic functions, utilization of nutrients and synthesis of cellular components, symbiotic-related genes are found to be located on some cryptic plasmids. Megaplasmid profiles of rhizobia vary greatly, both in size and number of plasmids. It is one of the characteristics used to examine genotypic diversity of rhizobia but it is less valuable for taxonomic proposes. The relation between megaplasmid profiles and some other genotypic characteristics has been reported.
Keywords: Cryptic plasmid, Genotypic diversity, Megaplasmid, Plasmid profile, Plasmid stability, Plasmid transfer, Replicator gene (rep), Rhizobia, Symbiosis island, Symbiotic plasmid (pSym).