Legumes form a unique symbiotic relationship with bacteria known as rhizobia, which they allow to infect their roots. This leads to root nodule. Rhizobia are unique in that they are the only nitrogen-fixing bacteria living in a symbiotic relationship with legumes. Rhizobia sp. bacteria can be found in the root nodules of legumes. These are swellings (clusters of cells) that can be found along the roots. The Rhizobia carry .
Simona Radutoiu Legumes are able to grow in nitrogen-poor soils due to their ability to engage in symbiosis with nitrogen-fixing bacteria. There is a great interest in using the knowledge about this symbiosis, to enable transfer to other non-symbiotic plants. An international research team has come a step further to understanding this complex biological process.The Unfortunate Truth About Nitrogen Fixing Plants
Microbes, whether beneficial or harmful, play an important role in all organisms, including plants. The ability to monitor the surrounding microbes is therefore crucial for plant survival. For example, the roots of a soil-growing plant are surrounded by a microbial-rich environment and have therefore evolved sophisticated surveillance mechanisms. Unlike most other plants, legumes, such as beans, peas or lentils, are capable of growing in nitrogen-poor soils with the help of microbes.
In a process called root nodule symbiosis, legumes form a new organ called the nodule where specific soil bacteria, called rhizobia are hosted.
- New receptor involved in symbiosis between legumes and nitrogen-fixing rhizobia identified
Inside the nodule, rhizobia convert atmospheric nitrogen into ammonium that is provided to the plant increasing growth. The legume in turn, supplies the bacteria with carbon resources enabling their energetically demanding nitrogen-fixing process. The interaction between the legume plants and rhizobial bacteria is very selective. In this case, no root hair deformation is observed.
Instead the bacteria penetrate between cells, through cracks produced by lateral root emergence. Ammonium is then converted into amino acids like glutamine and asparagine before it is exported to the plant.
This process keeps the nodule oxygen poor in order to prevent the inhibition of nitrogenase activity.
Nature of the mutualism[ edit ] 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.
Nitrogen Fixation and the Nitrogen Cycle
The sanctions hypothesis[ edit ] 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. Within a nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce.
Rhizobia - Wikipedia
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. However, other studies have found no evidence of plant sanctions.
There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction perhaps by limiting oxygen supply in nodules that fix less nitrogen. Some studies support the partner choice hypothesis. 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.
To understand the evolutionary history of this symbiosis, it is helpful to compare the rhizobia-legume symbiosis to a more ancient symbiotic relationship, such as that between endomycorrhizae fungi and land plants, which dates back to almost million years ago. Instead the rhizobia simply needed to evolve mechanisms to take advantage of the symbiotic signaling processes already in place from endomycorrhizal symbiosis.
Other diazotrophs[ edit ] Many other species of bacteria are able to fix nitrogen diazotrophsbut few are able to associate intimately with plants and colonize specific structures like legume nodules.