Frankia alni
Encyclopedia
Frankia alni is a species of actinomycete filamentous
bacterium that lives in symbiosis
with actinorhizal plant
s in the genus Alnus
. It is a nitrogen-fixing
bacterium and forms nodules
on the roots of alder trees.
, 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 Incas and used to increase soil fertility and stabilize terrace soils in their upland farming systems. 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.
, cellulose
and hemicellulose
. 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.
envelope. The vesicles serve to limit the diffusion of oxygen
, thus assisting the reduction process that is catalysed by the enzyme nitrogenase
. This enzyme bonds each atom of nitrogen
to three hydrogen
atoms, forming ammonia
(NH3). The energy for the reaction is provided by the hydrolysis of Adenosine triphosphate
(ATP). Two other enzymes are also involved in the process, glutamine synthetase
and glutamate synthase
. The final product of the reactions is glutamate
, 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 anaerobic
ally.
As a result of this mutually beneficial relationship, alder trees improve the fertility of the soils in which they grow and are considered to be a pioneer species
, making the soil more fertile and thus enabling other successional
species to become established.
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
where the surface layer had been removed, did not produce nodulation. This seems to have been because there were no infective propagule
s of Frankia alna 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.
Filamentation
Filamentation is the anomalous growth of certain bacteria, such as E. coli, in which cells continue to elongate but do not divide . Bacterial filamentation is often observed as a result of bacteria responding to various stresses, including DNA damage or inhibition of replication...
bacterium that lives in symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
with actinorhizal plant
Actinorhizal plant
Actinorhizal plants are a group of angiosperms characterized by their ability to form a symbiosis with the nitrogen fixing actinobacteria Frankia...
s in the genus Alnus
Alder
Alder is the common name of a genus of flowering plants belonging to the birch family . The genus comprises about 30 species of monoecious trees and shrubs, few reaching large size, distributed throughout the North Temperate Zone and in the Americas along the Andes southwards to...
. It is a nitrogen-fixing
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
bacterium and forms nodules
Root nodule
Root nodules occur on the roots of plants that associate with symbiotic nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia...
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 acuminataAlnus acuminata
Alnus acuminata Alnus acuminata Alnus acuminata (also called Alder is a species of tree in the Betulaceae family. It is found in Argentina, Bolivia, Colombia, Costa Rica, Ecuador, Guatemala, Mexico, Panama, Peru and Venezuela.-References:...
, 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 Incas and used to increase soil fertility and stabilize terrace soils in their upland farming systems. 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 pectinPectin
Pectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
, cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
and hemicellulose
Hemicellulose
A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...
. Cell division is stimulated in the hypodermis
Subcutaneous tissue
The hypodermis, also called the hypoderm, subcutaneous tissue, or superficial fascia is the lowermost layer of the integumentary system in vertebrates. Types of cells that are found in the hypodermis are fibroblasts, adipose cells, and macrophages...
and cortex
Cortex (anatomy)
In anatomy and zoology the cortex is the outermost layer of an organ. Organs with well-defined cortical layers include kidneys, adrenal glands, ovaries, the thymus, and portions of the brain, including the cerebral cortex, the most well-known of all cortices.The cerebellar cortex is the thin gray...
, 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
Pericycle
The pericycle is a cylinder of parenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants.Although it is composed of non-vascular parenchyma cells, it is still considered part of the vascular cylinder because it arises from the procambium as do the...
, endodermis
Endodermis
The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances to restrict apoplastic flow of water to the inside...
or cortex during the development of the prenodule and finally the bacterium enters the cells of these to infect the new nodule.
Nitrogen fixation
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 lipidLipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
envelope. The vesicles serve to limit the diffusion of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, thus assisting the reduction process that is catalysed by the enzyme nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
. This enzyme bonds each atom of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
to three hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atoms, forming ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
(NH3). The energy for the reaction is provided by the hydrolysis of Adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(ATP). Two other enzymes are also involved in the process, glutamine synthetase
Glutamine synthetase
Glutamine synthetase is an enzyme that plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine:Glutamate + ATP + NH3 → Glutamine + ADP + phosphate...
and glutamate synthase
Glutamate dehydrogenase
Glutamate dehydrogenase is an enzyme, present in most microbes and the mitochondria of eukaryotes, as are some of the other enzymes required for urea synthesis, that converts glutamate to α-Ketoglutarate, and vice versa. In animals, the produced ammonia is, however, usually bled off to the urea...
. The final product of the reactions is glutamate
Glutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
, 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
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
, 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 anaerobic
Anaerobic
Anaerobic is a word which literally means without oxygen, as opposed to aerobic.In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.Anaerobic may refer to:*Anaerobic...
ally.
As a result of this mutually beneficial relationship, alder trees improve the fertility of the soils in which they grow and are considered to be a pioneer species
Pioneer species
Pioneer species are species which colonize previously uncolonized land, usually leading to ecological succession. They are the first organisms to start the chain of events leading to a livable biosphere or ecosystem...
, making the soil more fertile and thus enabling other successional
Ecological succession
Ecological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...
species to become established.
Dispersal
In culture and in some root nodules, mulitlocular sporangia containing many sporeSpore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
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
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
where the surface layer had been removed, did not produce nodulation. This seems to have been because there were no infective propagule
Propagule
In horticulture, a propagule is any plant material used for the purpose of plant propagation. In asexual reproduction, a propagule may be a woody, semi-hardwood, or softwood cutting, leaf section, or any number of other plant parts. In sexual reproduction, a propagule is a seed or spore...
s of Frankia alna 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.