Legume & N-Cycle
The CILR is pleased to provide this information on where plants and animals get thier nitrogen from, what legumes are, roots and thier microbial colonisers, how legumes make nodules, and how they control nodule numbers.
In addition to the written information, some powerpoint presentations have been provided, most of which were produced by the Centre's former Education and Outreach Managers .
If using this information, please acknowledge the CILR as your source. For further information or resources, please don’t hesitate to contact us.
Where do plants and animals get their nitrogen from?
Download Powerpoint - Legumes & the Nitrogen Cycle [261K]
Plants, animals and humans need nitrogen (N) for their growth and development. Nitrogen is part of the proteins, nucleic acids (for example DNA) and other compounds necessary to build an organism. Nitrogen is plentiful in the atmosphere; 78% of the air we breathe is pure nitrogen gas. The problem is that neither we nor any plant or animal can convert gaseous nitrogen into a form that the body can use to build proteins or DNA. Nitrogen also occurs in the soil, and is available to plants mainly as nitrate (NO3-) or ammonium (NH4+). Both nitrate and ammonium are used in plant fertilisers to provide them with enough nitrogen. Plants can take up nitrate and ammonium through transporters on the root surface and transport the nitrogen into the other parts of the plant, where these forms of nitrogen are converted into proteins and DNA. Animals and humans get all their nitrogen from eating plants (or eating other animals that have eaten plants before that).
In the environment, most plants are limited in their growth by the amount of nitrogen available. One family of plants, the legumes, are special because they are the only plants that can form a symbiosis with certain bacteria from the soil, these bacteria are called rhizobia. Rhizobia are soil bacteria that are harmless to us and to plants. In fact, they help the legume plants in their battle to get enough nitrogen. Rhizobia contain an enzyme called nitrogenase, that can convert the nitrogen in the atmosphere into ammonium, a form of nitrate that plants can take up and use. With this trick, legumes are the only plants that can utilize nitrogen from the air as fertiliser.
What are legumes?
Download Powerpoint - What are Legumes? [269K]
Legumes are a large family of flowering plants. Legumes include some of the most common plants in Australia, the wattles or acacias (Acacia species). They also include many edible plants that are harvested for their seeds (these legumes are called pulses), for example peas, beans (broad beans, kidney beans, green beans, lima beans, etc), soybeans, peanuts, lentils, and chickpeas. Australia also grows a number of legume pastures, for example clover (white clover, subterranean clover, red clover, yellow clover), alfalfa or Lucerne, barrel medic and lupins.
Another well known Australian legume is the Sturt’s Desert pea!
Because of their symbiosis with rhizobia, legume seeds and leaves are high in protein which makes them valuable crop plants throughout the world.
Legumes are characterized by a seed pod that splits in two, containing the seeds. You will probably have seen pods of beans and peas, as well as Acacias.
Roots and their microbe colonisers
Download Powerpoint - Roots and Microbial Colonisers [323K]
Roots encounter hundreds of species of microorganisms in the soil, including bacteria, fungi and nematodes. Microbes colonise the outside of plant roots because roots provide a food source for microbes, e.g. sloughed off root cells, mucilage (polysaccharides produced by the root cap), exudates (organic acids, sugars, amino acids), dead roots etc. Pathogenic microbes attack plants and cause diseases like root rot and nematode galls. Others infect plants and don’t cause disease but provide nutrients for the plants. Microbes (or other organisms) that benefit the plant and in turn have a benefit by colonizing the plant, are called symbionts. For example, rhizobia, one of the most important and well studied symbionts of plants, can be free living in the soil, but have advantage of sugar supply from plants during symbiosis. They provide legumes with ammonium from which the nitrogen is built into amino acids and nucleic acids.
Other important symbionts are fungi that live in the soil and colonise most plant roots. These symbiotic fungi are called mycorrhizae. Many of the mushrooms that you can see in a forest are the fruiting bodies of these mycorrhizae.
How do legumes make nodules?
Download Powerpoint - How Legumes Form Nodules [1261K]
Certain species of rhizobia only form symbioses with certain species of legumes. The specificity is based on specific chemical signal molecules. Plant roots exude flavonoids, which have a structure that is specific for the species of legume producing it. Flavonoids are chemical compounds in the plant. Some flavonoids are found in flowers and give them their typical blue and purple colours. Other flavonoids have health benefits and are found in high levels in soybean products. Flavonoids produced by legume roots stimulate rhizobia in the soil to move towards the legume roots and to start making a nodule and colonising it. First, rhizobia infect the root hairs and form an infection thread that grows in to the inside of the root. The rhizobia multiply inside the infection thread, so that the initial infection of a few bacteria can cause a large colony of bacteria to build up inside the plant. Some cells inside the root (called cortex cells) start to divide in response to the rhizobia. This is similar to the growth of a tumour, but much more structured and controlled.
Where has the Rhizobium-legume symbiosis evolved from?
It is known that only legumes form a symbiosis with rhizobia. This symbiosis is “only” about 65 million years old. Recent findings suggest that the Rhizobium-legume symbiosis might have evolved from the more ancient symbiosis between plants and mycorrhizal fungi. This symbiosis has been established for the last 450 million years. How is this possible? The current idea is that mycorrhizae infect plant roots similarly to rhizobia and that the receptors necessary for a symbiotic microbe to colonise the roots have evolved in legumes so that legume plants are infected by rhizobia instead of the fungus. This has not happened in most other plants, so that they cannot form a symbiosis with rhizobia.
How do legumes control the number of nodules on their roots?
Download Powerpoint - Nodule Number Control [226K]
When rhizobia and plants get together in the soil, they don’t automatically cause the formation of as many nodules on the roots as possible. Instead, the plant has a tight control over the bacteria, it only allows them to make nodules under certain conditions.
Why is that?
The bacteria don’t live inside the roots “for free”. The plant needs to nourish the bacteria with sugars to provide them with enough energy to fix nitrogen for the plant. Because the plant cannot easily make unlimited amount of sugar, the symbiosis is not just a pure benefit for the plant but also a cost. Therefore, the plant needs a mechanism to control nodule numbers on its roots. This mechanism is called autoregulation.
How does it work?
When rhizobia first infect roots, a signal travel to the leaves of the plant. From there, this first signal triggers a second signal (the “autoregulation signal”) that moves from the leaves back down to the roots. This autoregulation signal then stops further nodules from forming, so that the initial nodules develop and fix nitrogen, but further nodules are suppressed. The following pictures demonstrate what happens when a legume loses the ability to regulate nodule formation through a mutation in a gene that makes the autoregulation signal. This example shows that the phenotype (the visible characteristics) of the plant is determined (at least partly) by its genes or genotype.
The characteristics (phenotype) of the plant is not only determined by its genes (or mutations in its genes), but also by the environment. An important environmental factor for plants is the level of nutrients in the soil. You will all be familiar with the fact that if plants are lacking nutrients, they will be stunted in their growth. If the same plant, with the same genotype, is provided with extra nutrients, they will grow bigger, have greener leaves and might form more flowers. The nodule numbers of a legume are another characteristic that is influenced by the nutrient levels. Under circumstances where the soil already contains enough nitrogen in the form of nitrate or ammonium (for example in a fertilized field), it is “cheaper” for the plant for take up the nitrogen from the soil than to form a symbiosis with nitrogen fixing bacteria. In general, the more nitrogen freely available for a legume in the soil, the fewer nodules it will form. Therefore, nitrogen levels in the soil are environmental factors that affect the phenotype of the plant.
What happens when a supernodulating mutant is grown in soil that contains nitrate levels that would inhibit nodulation in a wild type plant?
People found that nitrate has a much reduced inhibitory effect on nodule numbers in the mutant compared to the wild type. Why is that? The detailed reason is not known, but researchers think that nitrate inhibits nodulation in the wild type by increasing autoregulation. If autoregulation is defective, then it is irrelevant if nitrate is present or not because autoregulation cannot be increased if it is defective.
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