Symbiosis in biology refers to cooperation of two or more organisms where benefits are shared from the interaction. A classic example of symbiois is the formation of root nodules in plants from the legume family. The root nodule is a requirement for the plant to receive nitrogen, but nitrogen gas, N2, needs to be fixated before it can be used.
The root nodule is like a small anerobic home the plant produces specifically for harboring a specific type of Rhizobiacea bacteria, a diazotroph. Diazotrophs are organisms capable of using the nitrogen gas from air as their primary nitrogen source. The host plant uses the Rhizobiacea to generate its supply of nitrogen in the form of ammonia while the Rhizobiacea are allowed to grow comfortably within the root nodule. For this interaction to occur both the plant and bacteria both encode different genes to create the root nodule.
The rhizobiacea species that form symbiotic relationships with legumes have a separate set of genes apart from their genome known as a plasmid, in particular a pSym plasmid. This large mega plasmid produces a sect of nodulation factors (nod factors) which allow for efficient symbiosis. The plant secretes organic molecules known as flavenoids to communicate with the bacteria and nod factors D recognize the flavenoid molecules for upregulation of nod factors. The nod factors are lipochitinoligosaccharides which a chitin sugar backbone encoded by the nodABC common genes and functionally modified by nodEFG… methylating, acetylating, etc. which provides plant with a specific infection and symbiotic relationship by rhizobiacea. The nod factors provide the plant for two sets of regulation: the curling of the roothairs and calcium spiking which leads to the production of cytokinen and cell growth. The host plant then transcribes early nodule genes which included are LPS and EPS both required for host infection and when not present result in empty nodules.
In summary both the pSym plasmid from the bacteria is responsible for the root nodule formation, experiments where it was isolated and inserted into other non-rhizobiacea bacteria resulted in root nodule formation. However in this situation only rhizobiacea bacteria can symbiotically live within the legume plant. The plant requires the symbiotic relationship for a nitrogen source.