Rhizobia (such as Sinorhizobium meliloti) are bacteria that can form a symbiotic relationship with legume plants, including crops such as soybean and alfalfa, which are of great economic importance to Canada: symbiotic nitrogen fixation. During symbiosis, S. meliloti invades nodules that it induces on the roots of specific host plants. Within these nodules, the bacteria undergo many changes, and develop into a form known as bacteroids. Bacteroids supply the host plant with a nitrogen source, which is very important in N-poor soils or in the absence of N fertilizer. In return the plant provides the bacteria with an energy source, primarily organic acids, such as malate, which are intermediates of the tricarboxylic acid (TCA) cycle. The TCA cycle allows S. meliloti to generate the large amounts of energy required for effective symbiosis. In bacteroids, the enzymes of most of the TCA cycle enzymes appear to be expressed at much higher levels than in free-living cells.
