<P> The legume--rhizobium symbiosis is a classic example of mutualism--rhizobia supply ammonia or amino acids to the plant and in return receive organic acids (principally as the dicarboxylic acids malate and succinate) as a carbon and energy source . However, because several unrelated strains infect each individual plant, a classic tragedy of the commons scenario presents itself . Cheater strains may hoard plant resources such as polyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen . Given the costs involved in nodulation and the opportunity for rhizobia to cheat, it may be surprising that this symbiosis should exist at all . </P> <P> There are two main hypotheses for the mechanism that maintains legume - rhizobium symbiosis (though both may occur in nature). The sanctions hypothesis theorizes that legumes cannot recognize the more parasitic or less nitrogen fixing rhizobia, and must counter the parasitism by post-infection legume sanctions . In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules . These sanctions include, but are not limited to reduction of nodule growth, early nodule death, decreased carbon supply to nodules, or reduced oxygen supply to nodules that fix less nitrogen . Within a nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce . Therefore, with the development of a symbiotic relationship, if the host sanctions hypothesis is correct, the host sanctions must act toward whole nodules rather than individual bacteria because individual targeting sanctions would prevent any reproducing rhizobia from proliferating over time . This ability to reinforce a mutual relationship with host sanctions pushes the relationship toward a mutualism rather than a parasitism and is likely a contributing factor to why the symbiosis exists . </P> <P> However, other studies have found no evidence of plant sanctions . </P> <P> The partner choice hypothesis proposes that the plant uses prenodulation signals from the rhizobia to decide whether to allow nodulation, and chooses only noncheating rhizobia . There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction (perhaps by limiting oxygen supply) in nodules that fix less nitrogen . Likewise, wild lupine plants allocate fewer resources to nodules containing less - beneficial rhizobia, limiting rhizobial reproduction inside . This is consistent with the definition of sanctions, although called "partner choice" by the authors . Some studies support the partner choice hypothesis . While both mechanisms no doubt contribute significantly to maintaining rhizobial cooperation, they do not in themselves fully explain the persistence of the mutualism . The partner choice hypothesis is not exclusive from the host sanctions hypothesis, as it is apparent that both of them are prevalent in the symbiotic relationship . </P>

Where does rhizobium live how does it help the plant