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).
