Nodule bacteria
| Rhizobia | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Rhizobiaceae | ||||||||||||
| Conn 1938 |
Certain bacteria from the Rhizobiaceae family are known as nodule bacteria or rhizobia ( ancient Greek ῥίζα rhiza = root and βίος bios = life) . They belong to the class of Alphaproteobacteria . Rhizobia are gram-negative , can move actively by means of a polar or subpolar flagellum or 2 to 6 peritrichally arranged flagella, are aerobic and chemoorganotrophic .
Rhizobia are common and common soil bacteria. Their special importance lies in their ability to enter into a mutualistic symbiosis with plants from the legume family (Fabaceae) . The community is very close and leads to extensive morphological and physiological changes in the rhizobia and to the development of special organs in the plants. Rhizobia have the ability to bind elemental, molecular nitrogen (N 2 ) by reducing it to ammonia (NH 3 ) or ammonium (NH 4 + ) and thus making it biologically available. However, this is only possible for them in symbiosis with plants. Under natural conditions, neither legumes nor rhizobia can fix molecular nitrogen on their own. This symbiosis is of great biological as well as economic importance.
The connection between root swelling and bacterial attack was first described in 1866 by the Russian botanist Mikhail Stepanowitsch Voronin using the example of lupine , who also coined the term nodule bacteria. In 1886 Hermann Hellriegel and Hermann Wilfarth discovered the symbiosis of legumes and bacteria and their ability to convert elementary atmospheric nitrogen into nitrogen compounds that are available to plants.
Establishing the symbiosis
Plant roots give off various organic compounds. These exudates serve, among other things, for the development of a special microorganism community from bacteria and fungi in the rhizosphere , i.e. the immediate vicinity of the root. The actively moving rhizobia are chemotactically attracted by root exudates . Under certain conditions, they are able to penetrate the root (“ infection ”). A prerequisite for a successful infection is always a compatible and highly specific recognition between bacteria and plant cells at the molecular level. The infection always begins in certain root cells, the so-called root hairs .
Detection, attachment and infection
The first steps, recognition and attachment to the root hair cell, are carried out by special proteins on the surface of the bacterial and plant cells. One of the proteins formed by rhizobia is rhicadhesin , which binds to calcium compounds on the plant cell. Vegetable lectins also play a role, such as the trifoline formed by white clover, which binds to special carbohydrates in the outer cell membrane of certain rhizobia strains.
After attachment, the rhizobia begin to penetrate the root hair cell. This always takes place at the tip of the root hair, which then curves in a characteristic way and thus encloses the bacterial cell.
Differentiation of the bacteria and the infected plant tissue
Phenolic root exudates (for example luteolin ) activate a number of bacterial genes, which are called nod genes based on their function . nod stands for nodulation, which means the formation of nodules or nodules. Some of the Nod proteins formed are released to the outside, act on the plant cells and are necessary for the successful interaction between plant and bacterial cells.
The bacteria cause the root hair cells to form cellulose and induce an infection canal towards the center of the root. Adjacent cells of the root cortex are then infected along this channel . The increased release of the bacterial nod -factors causes the neighboring cells to divide and enlarge, and the resulting root cells are also often polyploid . This leads to the formation of nodular root thickenings, the "root nodules" in which the rhizobia are located.
First, the slender, rod-shaped bacteria multiply. Most of them then begin to transform into thickened, misshapen, and branched cells called bacteroids or bacterioids . These are enveloped in membranes by the infected plant cells and form cell organelles known as symbiosomes . There is evidence that each root nodule emerges from infection by a single bacterium. In this case, the bacteroids of a nodule would be clones . The bacteroids lack the outer cell membrane typical of proteobacteria . In addition, they are no longer able to reproduce or to transform themselves back into the original, viable cells. With advancing age, the degree of branching of the bacteroids increases.
Fixation of the elementary, molecular nitrogen (N 2 )
The enzymes for N 2 fixation only have the rhizobia, not the plants. However, the most important enzyme, molybdenum-containing nitrogenase , is highly sensitive to oxygen. Even low oxygen concentrations inactivate the enzyme irreversibly. However, the bacteroids are unable to live completely without oxygen. For nitrogen fixation to be successful, the oxygen concentration in the root nodules must therefore be precisely balanced. The plant cell takes on this function by forming an iron-containing protein in the nodules. This leghemoglobin binds excess oxygen, keeps its level constant and thus fulfills an oxygen buffer function. Leghemoglobin is very similar in structure to blood hemoglobin, which is also oxygen-binding (but only binds one molecule of oxygen and not four) and colors the tissue of active root nodules pale pink to blood red.
The cleavage of the nitrogen triple bond for subsequent fixation requires a great deal of energy. To break down one molecule of nitrogen, 16 ATP and 4 NADH are required. In order to generate 16 ATP, at least four molecules of oxygen must be reduced in the respiratory chain . This means that optimal fixation would require a gas mixture made up of four parts oxygen and one part nitrogen. It is known that the ratio of these two gases in the atmosphere is reversed. Oxygen is therefore “in short supply” in the cells. In fact, the leghemoglobin increases the oxygen flow to the bacteroids due to the significantly better solubility of the leghemoglobin-oxygen complex compared to the simple oxygen molecule and that despite the reduced diffusion speed due to the enormous increase in volume and mass.
The oxygen-sensitive, nitrogen-fixing enzyme, nitrogenase , is protected by the bacteroids themselves, since the respiratory chain in which the ATP is formed is located in its membrane, i.e. outside. There the oxygen is consumed and so cannot damage the nitrogenase inside the bacteroids, unless, for some reason, insufficient oxygen can be consumed in the respiratory chain.
Until recently, it was assumed that the components for the formation of functional hemoglobin were produced cooperatively: the protein component, i.e. globin , would be synthesized by the plant cell, while the heme component with the porphyrin ring would be formed by the bacteroids and exported to the plant cell would. The complete leghemoglobin is then assembled in the plant cell with the incorporation of the iron ion. However, recent findings show that the blueprint for leghemoglobin is located exclusively on the DNA of the plant and that the leghemoglobin is only synthesized there.
Mass transfer
The bacteroids are nutritionally dependent on the plant. The plant provides organic carbon compounds such as succinate , malate and fumarate , i.e. intermediate products of the citric acid cycle , to meet the energy requirements for nitrogen fixation . These substances originally come from photosynthesis in plants. By breaking down the compounds, the bacteroids gain energy in the form of ATP and a reducing agent, in this case pyruvate , to split and reduce the nitrogen molecule. This reaction is extremely energy-intensive. At least 16 molecules of ATP are required to convert a single N 2 molecule. In return, the bacteroids mainly supply the plant cells with ammonia (NH 3 ) as the first stable product of nitrogen fixation, which is converted into ammonium ions (NH 4 + ) in an aqueous environment . However, ammonia is a powerful cell poison. In order to avoid an accumulation of ammonia in the plant cells, this is immediately used for the synthesis of glutamine and glutamic acid ( ammonia assimilation ).
Genetics and Compatibility Groups
The bacterial genes for the establishment of the symbiosis and also for the nitrogen fixation are usually not in the bacterial genome, but on a plasmid , the sym plasmid.
The genes that are necessary for the formation of nodules and primarily affect the plant cells are called nod genes ( nod stands for nodulation). The genes for nitrogen fixation are called nif genes ( nif stands for nitrogen fixation ).
In addition, there are genes that are necessary for the highly specific recognition between bacterial and plant cells: Rhizobia always belong to a special compatibility group. Under natural conditions, a certain bacterial strain can usually only enter into a symbiosis with a certain plant species. Bacteria that have specialized in clover ( Trifolium sp. ), For example , cannot establish a successful symbiosis with other legumes. In this case, one speaks of "biovars": Rhizobium leguminosarum biovar trifolii is a strain that can only be found on clover. However, if the corresponding compatibility genes are transferred, other plants can also serve as symbionts.
ecology
Nitrogen is essential for all organisms. It is an essential component of amino acids and thus of proteins , but also of nucleic acids , i.e. DNA and RNA .
Animals usually get their nitrogen by taking in complete amino acids by feeding on other organisms. You therefore have sufficient nitrogen sources.
Most plants and many microorganisms, on the other hand, can only assimilate inorganic nitrogen. Bound nitrogen, i.e. nitrogen that is present in compounds and is therefore biologically available, occurs as nitrate , urea or ammonium in waters and soils. Such nitrogen compounds are usually scarce under natural conditions and limit the growth of these organisms. The largest nitrogen supply is in elementary, molecular form (N 2 ) in the air (78 percent by volume) and dissolved in water, but in this form it is not usable for animals, plants, fungi and most microorganisms. Only some bacteria and archaea have the enzymes with which N 2 can be reduced and converted into a form that can be used by other organisms (nitrogen fixation). The bacterial nitrogen fixation is therefore of fundamental importance for life and the global nitrogen cycle .
Through the symbiosis with rhizobia, N 2 is indirectly available for the plants . Legumes therefore have a clear selection advantage on low-nitrogen soils . Many butterflies also play an important role as pioneer plants on sand and rubble, heaps and clearcuts.
Importance of symbiosis
The amount of nitrogen fixed annually is estimated at over 120 million tons. Plants that thrive in this way on less valuable soils make up a large part of agriculture and our food source. Legumes and their fruits are often rich in protein, which is certainly due to the good supply of nitrogen. In addition, legumes are cultivated as green manure for the natural enrichment of the soil with nitrogen.
Examples of plant genera that can enter into a symbiosis with rhizobia:
| Useful plants (selection) | ||||
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Soy ( Glycine sp. ) Pea ( Pisum sp. ) Lentil ( Lens sp. ) Chickpea ( Cicer sp. ) Bean ( Phaseolus sp. ) Field bean ( Vicia faba ) Clover ( Trifolium sp. ) Peanut ( Arachis sp. ) Alfalfa ( Medicago sp. ) |
Soybean ( Glycine max ), ready-to-harvest plants
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Field bean ( Vicia faba )
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White clover ( Trifolium repens )
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Alfalfa ( Medicago sativa )
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| Wild, medicinal and ornamental plants | ||||
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Vetch ( Vicia sp. ) Kidney Vetch ( Anthyllis sp. ) Birdsfoot trefoil ( Lotus sp. ) Pea ( Lathyrus sp. ) Laburnum ( Laburnum sp. ) Broom ( Genista sp., Cytisus sp. ) Black locust ( Robinia sp. ) Lupine ( Lupinus sp . ) |
Vetch ( Vicia sepium )
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Common Goldregen ( Laburnum anagyroides )
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Common black locust ( Robinia pseudoacacia )
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Lupine ( Lupinus sp.)
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and others, including many tropical genera.
The best-known plant species that live in symbiosis with rhizobia belong to the butterfly family (Faboideae), but symbioses with rhizobia are also found within the closely related carob family (Caesalpinioideae) and mimosa family (Mimosoideae). Important rhizobia species are Rhizobium leguminosarum , R. tropici, R. loti, R. trifolii, R. meliloti, R. fredii, Bradyrhizobium japonicum , B. elkanii, Azorhizobium caulinodans (tropical, forms stem nodules ).
The symbiosis between plants and nitrogen-fixing bacteria is not limited to legumes and rhizobia. Close communities can also be found, for example, between alders and actinomycetes of the genus Frankia , here the symbiosis is called actinorrhiza . Another example is the symbiosis of the tropical swimming fern Azolla with cyanobacteria of the genus Anabaena .
Accumulations of nitrogen-fixing bacteria could also be detected in the rhizosphere of various grasses. Carbohydrates, which the plant roots secrete, serve as an energy source here.
There are also free-living bacteria that - without entering into a symbiosis - assimilate N 2 only for their own use (for example Azotobacter and some cyanobacteria ). This type of nitrogen assimilation, the “nitrogen fixation”, is strictly regulated due to the high energy requirement and only takes place when no other nitrogen source is available.
literature
- MS Voronin: About the swelling of the roots of the black alder (Alnus glutinosa) and the common garden lupine (Lupinus mutabilis) . Mémoires de l'Academie Impériale des Sciences de St. Pétersbourg, VII Series, vol. X. 1866.
- Wolfgang Böhm : The fixation of elementary nitrogen by the root nodules of legumes. In memory of Hermann Hellriegel's epoch-making discovery in 1886 . In: Angewandte Botanik Vol. 60, 1986, pp. 1-5 (with picture).
- Lincoln Taiz, Eduardo Zeiger: Plant Physiology - the original with translation aids . 4th edition. Spectrum Akademischer Verlag / Springer-Verlag, Heidelberg a. a. O. 2007, ISBN 3-8274-1865-8 .
- Gerhard Richter: Metabolic Physiology of Plants . 6th edition, Thieme Verlag, Stuttgart 1998, ISBN 3-13-442006-6 .
- Hans W. Heldt and Birgit Piechulla: Plant biochemistry . 4th edition, Spektrum Akademischer Verlag, Heidelberg 2008. ISBN 978-3-8274-1961-3 ; Pp. 295-309
- Bob B. Buchanan, Gruissem, Jones: Biochemistry & Molecular Biology of Plants 6th Edition, American Society of Plant Physiologists, Rockville 2006, ISBN 0-943088-39-9 .
