Significance of Leguminous Plant and Bacteria Relationship
The relationship between a leguminous plant and bacteria in an ecosystem and how it affects agriculture has been recognized and utilized by man for many hundred of years. The early Greeks knew the value of rotating legumes with other crops. The true nature of the effect has been worked out only within the last seventy-five years
The partners concerned are members of the flowering plant family papilionaceae (leguminosae) and the bacterium Rhizobium legumino-sarum [Bacilus radicicola] [pseudomonas radicicola]. Only certain strains of the bacterium can unite with a particular species of legume and altogether there are about seven groups of legumes recognized as forming effective unions with seven different strains of the bacterium.
The micro-organism exists freely in the soil in a non-motile coccoid (spherical) form for most of its time, but when in proximity with the roots of a leguminous plant, an oval uniflagellate motile conditions is formed which effect penetration of the root. It appears that the changes undergoes by the bacterium are in response to some substance secreted by the roots. Penetration is made through root-hairs but from the numbers of recognizable infections, it is obvious that the event must be comparatively rare. After entry, the bacterium rapidly multiplies and changes form yet again to become multiflagellate and a deeper invasion into the cortical cells of the root occurs.
The effect of the presence of the bacterium in the root cortex of a leguminous plant is to stimulate the cells to very active division and in the immediate vicinity of infection a tuberous swelling or nodule appears on the root. The bacteria possibly secrete a substance of the nature of β-indolyl acetic acid to cause this rapid proliferation of cells by the root. Internally, the nodule can be see to be composed of a central mass of large cortical cells containing great numbers of the bacteria in still yet another form. This is the “banded-rod” form of various Y and T shapes and the structures are known as bacteroids. The banded appearance becomes apparent only in stained preparations and as yet there is no clear explanation of the condition. Surrounding the central
Mass is a layer of uninfected tissue, some parts of which are differentiated into vascular tissues continuous with those of the main root at the distal end of the nodule, a zone of cells retains its meristematic activity for some time and the nodule gradually increases In size. As more cells from this region, differentiate, they become infected by the bacteria. As a module ages, each bacterium-infected cell vacuolated and the bacteroids become densely crowded at the cell periphery. They then digest the cell contents and attack the cell wall. The nodule gradually breaks away from the root to disintegrate and release the bacteria into the soil. The alliance is undoubtedly beneficial to both participants from a nutritional standpoint. The higher plant is able to assimilate carbon dioxide and the bacterium benefits from this by obtaining carbohydrates from the root cells.
The effect of cutting off the carbohydrate supply to the bacteria by growing the legume in the dark, is to convert the bacteria into active parasites, which dissolve the cell walls of the host and obtain carbohydrate in that way. From such observation it might be conjectured that the association could originally have been a parasitic one, but under natural conditions has become one of almost perfect symbiosis.
The agricultural implication of a symbiotic relationship between a leguminous plant and a bacterium is that soil fertility is enhanced ad atmospheric nitrogen is fixed by the bacteria enriching the soil with nitrogen which is an important mineral component of fertile soils.
Leguminous plants such as peas and beans form a symbiotic relationship with bacteria that live in the soil. The bacteria invade the plant roots and form organs called nodules, where they convert nitrogen from the air into compounds the plant uses for growth.
A new study shows that leguminous plants select for bacteria that are efficient at converting nitrogen. This could help us engineer legumes with enhanced nitrogen-fixing capacity that can boost crop yields under stress conditions.
Nitrogen Fixation by Bacteria
Nitrogen is one of the most important plant nutrients but it’s also one of the most difficult to absorb from the soil. Legume plants, such as clovers and alfalfa, have a unique ability to partner with soil bacteria called Rhizobia to create symbiotic nitrogen-fixing nodules on their roots. The bacteria pull atmospheric nitrogen gas and convert it to plant-available ammonia in a process known as biological nitrogen fixation. In return, the legume plants supply carbon from photosynthesis and other organic compounds to the bacteria to sustain bacterial growth and nodule development.
In the field, a healthy symbiosis between rhizobia and legume plants can result in significant benefits for both. The nodules are able to capture and hold nitrogen from the air, which can reduce the need for chemical fertilizers. In addition, the symbiotic nitrogen fixation can increase root mass and enhance crop productivity by supplying nitrogen in a form that is more easily absorbed by the roots.
It’s important to note that not all legume plants develop nodules and that the formation of nodules is highly dependent on soil conditions. In general, a soil that is too dry, cold or acidic can hinder the formation of nodules and reduce the amount of biologically fixed nitrogen produced by the legume crop. To help ensure that your legume crops are developing a healthy symbiosis and are able to produce nitrogen, it’s best to inoculate the seed with live rhizobia.
The best way to tell if your legume plants have properly inoculated seed is to inspect the roots for nodules. When a legume is well established, the nodules will be visible and can be identified by their dark color and fingerlike appearance. Nodules on perennial legumes like clovers and alfalfa are long-lived and will continue to fix nitrogen throughout the growing season. However, annual legumes, such as beans, peanuts and soybeans, will lose their ability to fix nitrogen once they start producing pods.
To check if the symbiotic relationship between legume and rhizobia is working, dig up a legume that has matured and cut open a nodule. The inside of the nodule should be a reddish-pink color, which indicates healthy bacteria that are generating plant-available nitrogen for the legume crop.
Root Nodule Development of Leguminous Plants
Most leguminous plants form a highly profitable symbiosis with nitrogen fixing soil bacteria known as rhizobia. This symbiosis is known as nodulation, and it provides the plant with a source of nitrogen for growth but requires high levels of energy from the host to initiate and sustain nodule formation. For this reason, nodulation is tightly regulated by the legume host to minimize energy costs and maximize benefits.
Nodule development of Leguminous plants involves synchronized differentiation of the host plant nodule cells and the bacterial symbionts. Nodule cells arise from cell division in the inner cortex of the nodule and the symbionts arise from cell division in the symbiotic zone, which surrounds the nodule cell masses. The nodule has a structure similar to that of the root, with a meristem zone, an infection zone, a nitrogen fixing zone, and a senescence zone. Indeterminate nodules such as those of alfalfa and clovers grow from a persistent apical meristem and are cylindrical in shape, while determinate nodules such as those of beans, peas, and soybeans are spherical (Nap and Bisseling, 1990).
The Nod signaling system has co-evolved between the rhizobial symbionts and their compatible legume hosts. The bacterium encodes Nod genes, which produce signal molecules called Nod factors that are recognized by host proteins named Nod receptors. The recognition systems involved are highly specific, involving differences in the chemical properties of Nod factors and in the structures of the receptor proteins.
When Nod signals are recognized, the host cells respond by secreting a signaling molecule that changes normal polar root hair growth so that they curve down toward the symbionts and open to create a cavity for colonization. This triggering of a change in nodule growth is important for efficient symbiosis, as it reduces the time needed to develop a nodule and increases the number of nodules that are formed.
Studies of the legume-rhizobial symbiosis have been facilitated by genetic approaches that allow researchers to manipulate the interaction. For example, introducing mutations into the cellular receptors that mediate Nod factor recognition allows them to be tested for their role in nodule formation. In addition, the bacterial nod gene system has been subjected to experimental evolution to determine the factors that affect symbiotic compatibility.
Nitrogen Fixing Bacteria
The bacterium Rhizobium forms symbiotic associations, or mutualisms, with legumes that result in the plants absorbing nitrogen from the soil. The bacteria do this by converting free, molecular nitrogen gas (N2) into the organic compound, nitrate nitrogen (NO3), that can be used as a plant nutrient. This is known as biological nitrogen fixation. The rhizobia also receive sugars from the legume, which they use to fuel their growth. This symbiotic relationship between legumes and rhizobia is called a diazotrophic system.
Legumes have evolved to send out hair-like cells that track the rhizobia to their roots, where the bacteria are able to take up residence in nodules. Inside the nodules, bacteria work anaerobically to combine hydrogen and nitrogen in a process called nitrogen fixation. This produces ammonia, which is absorbed by the legume in the form of protein and amino acids that are vital to its growth.
This process is incredibly efficient. While the amount of nitrogen fixed varies from legume to legume, a typical soybean field produces about 200 lb of nitrogen per acre. The level of nitrogen fixed is affected by the amount of soil nitrogen, the rhizobia strain infecting the legume, how much the legume grows, and how long it grows.
Most legumes require a specific strain of Rhizobium to be effective. For example, arrowleaf clover requires the bacterium Rhizobia inoculated into its seed, while red and white clovers, alfalfa, vetch, and annual medics require different strains. Leguminous plants that are well-inoculated with an effective rhizobia strain produce more nodules with darker pink centers than less-effective strains of rhizobium, and they have higher rates of nitrogen fixation.
Inoculating legume seeds with the appropriate rhizobia can be a significant economic and environmental benefit, especially for farmers who grow high-yielding crops like corn. However, there are a number of hurdles to overcome before legumes can be used for the purpose of providing commercial fertilizer. If a legume is grown in an environment that is too salty, for example, it will be harder for the rhizobia to infect the root and perform nitrogen fixation, and so fewer nodules will develop and a lower amount of nitrogen will be fixed.
Nitrogen Fixing Leguminous Plants
Rhizobia bacteria are the key to legumes’ ability to fix nitrogen in the soil. They convert nitrogen gas (N2) from the air into a usable form of plant nitrogen (ammonium, NH3). In return, the legume plants supply the bacteria with energy in the form of carbohydrates. The process is referred to as a symbiotic relationship because both organisms benefit from the partnership.
Rhizobium bacteria invade the root hairs of leguminous plants and induce, in a specific manner, the formation of nodules in which they fix nitrogen. In the early steps of this symbiotic interaction, the legume plants emit a nodulation peptide that stimulates bacterial expression of nodulation genes. These genes encoding for extracellular polysaccharide (“EPS”) signals with specific structural properties, such as substituted and N-acylated chitin oligosaccharides, promote nodule development. The EPS signals are recognized by a plant receptor protein known as Nodulin 3.
Once the nodules have developed, they provide an efficient way for the legumes to absorb fixed nitrogen from the atmosphere. The amount of fixed nitrogen is correlated to the rate of plant growth. Anything that slows the legume’s growth, such as drought or low temperature, reduces the amount of fixed nitrogen produced. In addition, the legume’s nutrient requirements also affect how much fixed nitrogen is made.
The rate of nitrogen fixation is also dependent on how many rhizobia are present in the soil. Soil conditions that encourage the presence of rhizobia include adequate moisture, temperature and fertility, and the availability of nitrogen-fixing minerals such as calcium, phosphorus and potassium.
While some fixed nitrogen is leaked from the legume into the soil, the majority of the fixed nitrogen goes into the plant for use in proteins and amino acids. The rest of the nitrogen is eventually re-released into the soil for non-legume plants when the legume’s leaves, fruits and roots die and decompose.
The best way to insure a high level of nitrogen fixation is to purchase rhizobia-inoculated legume seeds. Your local county extension office can recommend the types of rhizobia that work best with your crop. The inoculant should be coated onto the legume seed and mixed thoroughly.
