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''Frankia alni'' is a Gram-positive species of actinomycete filamentous bacterium that lives in
symbiosis Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can fo ...
with actinorhizal plants in the genus '' Alnus''. It is a
nitrogen-fixing Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...
bacterium and forms nodules on the roots of alder trees.


Distribution

''Frankia alni'' forms a symbiotic relationship exclusively with trees in the genus ''Alnus''. These are widely distributed in temperate regions of the northern hemisphere. One species, '' Alnus glutinosa'', is also found in Africa and another, the Andean alder, '' Alnus acuminata'', extends down the mountainous spine of Central and South America as far as Argentina. Evidence suggests that this alder may have been exploited by the
Inca The Inca Empire, officially known as the Realm of the Four Parts (, ), was the largest empire in pre-Columbian America. The administrative, political, and military center of the empire was in the city of Cusco. The History of the Incas, Inca ...
s and used to increase soil fertility and stabilize terrace soils in their upland farming systems. Actinomycetota, like ''Frankia alni'', need a flagellum to be mobile, but ''F. alni'' does not have one, and is immobile. ''Alnus'' species grow in a wide range of habitats that include glacial till, sand hills, the banks of water courses, bogs, dry volcanic lava flows and ash alluvium.


Infection process

The first symptom of infection by ''Frankia alni'' is a branching and curling of the root hairs of the alder as the bacterium moves in. The bacterium becomes encapsulated with a material derived from the plant cell wall and remains outside the host's cell membrane. The encapsulation membrane contains pectin,
cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...
and
hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls. Cellulose is crystalline, strong, an ...
. Cell division is stimulated in the hypodermis and cortex, which leads to the formation of a "prenodule". The bacterium then migrates into the cortex of the root while the nodule continues to develop in the same way as a lateral root. Nodule lobe primordia develop in the pericycle, endodermis or cortex during the development of the prenodule and finally the bacterium enters the cells of these to infect the new nodule.


Nitrogen fixation

left, Common alder, ''Alnus glutinosa'' In nitrogen-free culture and often in symbiosis, ''Frankia alni'' bacteria surround themselves in "vesicles". These are roughly spherical cellular structures that measure two to six millimetres in diameter and have a laminated
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
envelope. The vesicles serve to limit the diffusion of
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
, thus assisting the reduction process that is catalysed by the enzyme nitrogenase. This enzyme bonds each atom of
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
to three
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
atoms, forming
ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
(NH3). The energy for the reaction is provided by the hydrolysis of
Adenosine triphosphate Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...
(ATP). Two other enzymes are also involved in the process, glutamine synthetase and glutamate synthase. The final product of the reactions is
glutamate Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...
, which is thus normally the most abundant free amino acid in the cell cytoplasm. A by-product of the process is gaseous hydrogen, one molecule of which is produced for every molecule of nitrogen reduced to ammonia, but the bacterium also contains the enzyme hydrogenase, which serves to prevent some of this energy being wasted. In the process, ATP is recovered and oxygen molecules serve as the final electron acceptor in the reaction, leading to the lowering of ambient oxygen levels. This is to the benefit of the nitrogenases, which only function anaerobically. As a result of their mutually beneficial relationship with ''Frankia'', alder trees improve the
fertility Fertility in colloquial terms refers the ability to have offspring. In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to reproduce, which is termed fecundity. The fertility rate ...
of the
soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
s in which they grow and are considered to be a
pioneer species Pioneer species are resilient species that are the first to colonize barren environments, or to repopulate disrupted biodiverse steady-state ecosystems as part of ecological succession. Various kinds of events can create good conditions for pi ...
, making the soil more fertile and thus enabling other successional species to become established.


Dispersal

In culture and in some root nodules, multilocular
sporangia A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...
containing many
spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...
s are produced. The sporangia are non-motile but the spores can migrate to infect new host plants. A Swedish study found that root nodules developed on transplanted seedlings of the grey alder, ''Alnus incana'', planted in meadow soil that had not grown actinorhizal plants for nearly sixty years. A similar experiment planting seedlings in deep layers of
peat Peat is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. ''Sphagnum'' moss, also called peat moss, is one of the most ...
where the surface layer had been removed, did not produce nodulation. This seems to have been because there were no infective
propagule In biology, a propagule is any material that functions in propagating an organism to the next stage in its life cycle, such as by dispersal. The propagule is usually distinct in form from the parent organism. Propagules are produced by organisms ...
s of ''Frankia alni'' deep in the peat. No air-borne dispersal of ''Frankia alni'' was detected and it was thought that movement of water might account for the dispersal of the bacteria in peat soils.


References


External links


''Frankia'' and actinorhizal plants

Type strain of ''Frankia alni'' at Bac''Dive'' - the Bacterial Diversity Metadatabase
{{Taxonbar, from=Q5490884 Actinomycetia Bacteria described in 1866