Phytoplasma
Encyclopedia
Phytoplasma are specialised bacteria that are obligate parasites
of plant phloem
tissue and transmitting insects (vectors). They were first discovered by scientists in 1967 and were named mycoplasma
-like organisms or MLOs. They cannot be cultured in vitro
in cell-free media. They are characterised by their lack of a cell wall
, a pleiomorphic or filamentous
shape, normally with a diameter
less than 1 micrometer
, and their very small genome
s.
Phytoplasmas are pathogens of important plants, including coconut
, sugarcane
, sandal wood, causing a wide variety of symptoms that range from mild yellowing to death of infected plants. They are most prevalent in tropical and sub-tropical regions of the world. Phytoplasmas require a vector to be transmitted from plant to plant, and this normally takes the form of sap sucking insects such as leaf hoppers in which they are also able to replicate.
dwarf disease in Japan
. Such diseases were originally thought to be caused by viruses
, which, like phytoplasmas, require insect vectors, cannot be cultured, and have some symptom similarity. In 1967 phytoplasmas were discovered in ultrathin sections of plant phloem tissue and named mycoplasma-like organisms (MLOs), because they physically resembled mycoplasma
s The organisms were renamed phytoplasmas in 1994 at the 10th congress of the International Organization of Mycoplasmology.
. The cell membranes of all phytoplasmas studied so far usually contain a single immunodominant protein
(of unknown function) that makes up the majority of the protein content of the cell membrane. The typical phytoplasma exhibits a pleiomorphic or filamentous shape and is less than 1 micrometer
in diameter. Like other prokaryotes, DNA is free in the cytoplasm
.
caused by phytoplasma infection
is phyllody
, the production of leaf
-like structures in place of flower
s. Evidence suggests that the phytoplasma downregulates a gene involved in petal
formation (AP3 and its orthologues) and genes involved in the maintenance of the apical meristem (Wus and CLV1). Other symptoms, such as the yellowing of leaves, are thought to be caused by the phytoplasma's presence in the phloem, affecting its function and changing the transport of carbohydrate
s.
Phytoplasma infected plants may also suffer from virescence, the development of green flower
s due to the loss of pigment
in the petal cells. Sometimes sterility
of the flowers is also seen.
Many phytoplasma infected plants gain a bushy
or witch's broom
appearance due to changes in normal growth patterns caused by the infection. Most plants show apical dominance
, but phytoplasma infection can cause the proliferation of auxiliary (side) shoots and an increase in size of the internodes. Such symptoms are actually useful in the commercial production of poinsettia
. The infection produces more axillary shoots, which enables production of poinsettia plants that have more than one flower.
Phytoplasmas may cause many other symptoms that are induced because of the stress
placed on the plant by infection rather than specific pathogenicity of the phytoplasma. Photosynthesis
, especially photosystem II, is inhibited in many phytoplasma infected plants. Phytoplasma infected plants often show yellowing which is caused by the breakdown of chlorophyll
, whose biosynthesis is also inhibited.
s of the families
Cicadellidea (leafhoppers), Fulgoridea (planthoppers) and Psyllidae (jumping plant lice)
,
which feed on the phloem tissues of infected plants, picking up the phytoplasmas and transmitting them to the next plant they feed on. For this reason the host range of phytoplasmas is strongly dependent upon its insect vector. Phytoplasmas contain a major antigenic protein that makes up the majority of their cell surface proteins. This protein has been shown to interact with insect microfilament complexes and is believed to be the determining factor in insect-phytoplasma interaction. Phytoplasmas may overwinter in insect vectors or perennial plants. Phytoplasmas can have varying effects on their insect hosts; examples of both reduced and increased fitness have been seen.
Phytoplasmas enter the insect's body through the stylet, move through the intestine
, and are then absorbed into the haemolymph. From here they proceed to colonise the salivary glands, a process that can take up to three weeks. Once established, phytoplasmas will be found in most major organs of an infected insect host. The time between being taken up by the insect and reaching an infectious titre in the salivary glands is called the latency period.
Phytoplasmas can also be spread via dodders cascutaceae or vegetative propagation such as the grafting
of a piece of infected plant onto a healthy plant.
s. But since they spread more slowly than solute
s, for this and other reasons, movement by passive translocation is not supported.
s such as tetracycline to see if this cured the plant was another diagnostic technique employed.
Molecular diagnostic techniques for the detection of phytoplasma began to emerge in the 1980s and included ELISA
based methods. In the early 1990s, PCR
-based methods were developed that were far more sensitive than those that used ELISA, and RFLP analysis allowed the accurate identification of different strains
and species
of phytoplasma.
More recently, techniques have been developed that allow for assessment of the level of infection. Both QPCR and bioimaging have been shown to be effective methods of quantifying the titre of phytoplasmas within the plant.
Tissue culture
can be used to produce clones of phytoplasma infected plants that are healthy. The chances of gaining healthy plants in this manner can be enhanced by the use of cryotherapy, freezing the plant samples in liquid nitrogen
, before using them for tissue culture.
Work has also been carried out investigating the effectiveness of plantibodies targeted against phytoplasmas.
Tetracyclines are bacteriostatic to phytoplasmas, that is they inhibit their growth. However, without continuous use of the antibiotic, disease symptoms will reappear. Thus, tetracycline is not a viable control agent in agriculture, but it is used to protect ornamental coconut trees.
s of three phytoplasmas have been sequenced
: Aster Yellows Witches Broom, Onion Yellows (Ca. Phytoplasma asteris) and Ca. Phytoplasma australiense Phytoplasmas have very small genomes, which also have extremely low levels of the nucleotide
s G
and C
, sometimes as little as 23% which is thought to be the threshold for a viable genome. In fact Bermuda grass white leaf phytoplasma has a genome size of just 530Kb, one of the smallest known genomes of living organisms. Larger phytoplasma genomes are around 1350 Kb. The small genome size associated with phytoplasmas is due to their being the product of reductive evolution from Bacillus/Clostridium ancestors. They have lost 75% or more of their original genes, and this is why they can no longer survive outside of insects or plant phloem. Some phytoplasmas contain extrachromosomal DNA such as plasmid
s.
Despite their very small genomes, many predicted genes are present in multiple copies. Phytoplasmas lack many genes for standard metabolic
functions and have no functioning homologous recombination
pathways, but do have a sec
transport pathway. Many phytoplasmas contain 2 rRNA operon
s. Unlike the rest of the Mollicutes
, the triplet code of UGA
is used as a stop codon
in phytoplasmas, rather than to code for tryptophan
.
Phytoplasma genomes contain large numbers of transposon
genes and insertion sequence
s. They also contain a unique family of repetitive extragenic palindromes (REPs) called PhREPS whose role is unknown though it is theorised that the stem loop structures the PhREPS are capable of forming may play a role in transcription termination or genome
stability.
Acholeplasmatales.
In 1992 the Subcommittee on the Taxonomy of Mollicutes proposed the use of the name Phytoplasma in place of the use of the term MLO (Mycoplasma-like organism) "for reference to the phytopathogenic mollicutes".
In 2004 the genus name Phytoplasma was adopted and is currently at Candidatus
status which is used for bacteria
that can not be cultured. It's taxonomy
is complicated by the fact that it can not be cultured and thus methods normally used for classification of prokaryote
s are not possible. Phytoplasma taxonomic groups are based on differences in the fragment sizes produced by the restriction digest
of the 16S rRNA gene sequence (Called RFLP) or by comparison of DNA sequences from the 16s/23s spacer regions. There is some disagreement over how many taxonomic groups the phytoplasmas fall into, recent work involving computer simulated restriction digests of the 16Sr gene suggest there may be up to 28 groups where as other papers argue for less groups, but more sub-groups. Each group includes at least one Ca. Phytoplasma species, characterised by distinctive biological, phytopathological and genetic properties. The table below summaries some of the major taxonomic groups and the candidatus species that belong in them.
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
of plant phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
tissue and transmitting insects (vectors). They were first discovered by scientists in 1967 and were named mycoplasma
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
-like organisms or MLOs. They cannot be cultured in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
in cell-free media. They are characterised by their lack of a cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
, a pleiomorphic or filamentous
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...
shape, normally with a diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
less than 1 micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
, and their very small genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s.
Phytoplasmas are pathogens of important plants, including coconut
Coconut
The coconut palm, Cocos nucifera, is a member of the family Arecaceae . It is the only accepted species in the genus Cocos. The term coconut can refer to the entire coconut palm, the seed, or the fruit, which is not a botanical nut. The spelling cocoanut is an old-fashioned form of the word...
, sugarcane
Sugarcane
Sugarcane refers to any of six to 37 species of tall perennial grasses of the genus Saccharum . Native to the warm temperate to tropical regions of South Asia, they have stout, jointed, fibrous stalks that are rich in sugar, and measure two to six metres tall...
, sandal wood, causing a wide variety of symptoms that range from mild yellowing to death of infected plants. They are most prevalent in tropical and sub-tropical regions of the world. Phytoplasmas require a vector to be transmitted from plant to plant, and this normally takes the form of sap sucking insects such as leaf hoppers in which they are also able to replicate.
History
There are references to diseases now known to be caused by phytoplasmas as far back as 1603 for MulberryMulberry
Morus is a genus of flowering plants in the family Moraceae. The 10–16 species of deciduous trees it contains are commonly known as Mulberries....
dwarf disease in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. Such diseases were originally thought to be caused by viruses
Plant virus
Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses are pathogenic to higher plants...
, which, like phytoplasmas, require insect vectors, cannot be cultured, and have some symptom similarity. In 1967 phytoplasmas were discovered in ultrathin sections of plant phloem tissue and named mycoplasma-like organisms (MLOs), because they physically resembled mycoplasma
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
s The organisms were renamed phytoplasmas in 1994 at the 10th congress of the International Organization of Mycoplasmology.
Morphology
Being mollicutes, phytoplasmas lack cell walls and instead are bound by a triple layered membraneCell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
. The cell membranes of all phytoplasmas studied so far usually contain a single immunodominant protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
(of unknown function) that makes up the majority of the protein content of the cell membrane. The typical phytoplasma exhibits a pleiomorphic or filamentous shape and is less than 1 micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
in diameter. Like other prokaryotes, DNA is free in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
.
Symptoms
A common symptomSymptom
A symptom is a departure from normal function or feeling which is noticed by a patient, indicating the presence of disease or abnormality...
caused by phytoplasma infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
is phyllody
Phyllody
Phyllody is the development of floral parts into leafy structures, generally caused by virus or phytoplasma infection such as aster yellows. Evidence suggests that the phytoplasma downregulates a gene involved in petal formation, instead causing leaves or leaflike structures to form...
, the production of leaf
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
-like structures in place of flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s. Evidence suggests that the phytoplasma downregulates a gene involved in petal
Petal
Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...
formation (AP3 and its orthologues) and genes involved in the maintenance of the apical meristem (Wus and CLV1). Other symptoms, such as the yellowing of leaves, are thought to be caused by the phytoplasma's presence in the phloem, affecting its function and changing the transport of carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
s.
Phytoplasma infected plants may also suffer from virescence, the development of green flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s due to the loss of pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
in the petal cells. Sometimes sterility
Sterility (physiology)
Sterility is the physiological inability to effect sexual reproduction in a living thing, members of whose kind have been produced sexually. The term may be used in reference to* types of organism, such as the mule, a sterile hybrid;...
of the flowers is also seen.
Many phytoplasma infected plants gain a bushy
Shrub
A shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
or witch's broom
Witch's broom
A Witch's broom is a disease or deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest....
appearance due to changes in normal growth patterns caused by the infection. Most plants show apical dominance
Apical dominance
In plant physiology, apical dominance is the phenomenon whereby the main central stem of the plant is dominant over other side stems; on a branch the main stem of the branch is further dominant over its own side branchlets....
, but phytoplasma infection can cause the proliferation of auxiliary (side) shoots and an increase in size of the internodes. Such symptoms are actually useful in the commercial production of poinsettia
Poinsettia
Euphorbia pulcherrima, commonly known as Zack Wood or noche buena, is a species of flowering plant indigenous to Mexico and Central America. The name "poinsettia" is after Joel Roberts Poinsett, the first United States Minister to Mexico, who introduced the plant into the US in 1825...
. The infection produces more axillary shoots, which enables production of poinsettia plants that have more than one flower.
Phytoplasmas may cause many other symptoms that are induced because of the stress
Stress (medicine)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...
placed on the plant by infection rather than specific pathogenicity of the phytoplasma. Photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, especially photosystem II, is inhibited in many phytoplasma infected plants. Phytoplasma infected plants often show yellowing which is caused by the breakdown of chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
, whose biosynthesis is also inhibited.
Movement between plants
The phytoplasmas are mainly spread by insectInsect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s of the families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Cicadellidea (leafhoppers), Fulgoridea (planthoppers) and Psyllidae (jumping plant lice)
,
which feed on the phloem tissues of infected plants, picking up the phytoplasmas and transmitting them to the next plant they feed on. For this reason the host range of phytoplasmas is strongly dependent upon its insect vector. Phytoplasmas contain a major antigenic protein that makes up the majority of their cell surface proteins. This protein has been shown to interact with insect microfilament complexes and is believed to be the determining factor in insect-phytoplasma interaction. Phytoplasmas may overwinter in insect vectors or perennial plants. Phytoplasmas can have varying effects on their insect hosts; examples of both reduced and increased fitness have been seen.
Phytoplasmas enter the insect's body through the stylet, move through the intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
, and are then absorbed into the haemolymph. From here they proceed to colonise the salivary glands, a process that can take up to three weeks. Once established, phytoplasmas will be found in most major organs of an infected insect host. The time between being taken up by the insect and reaching an infectious titre in the salivary glands is called the latency period.
Phytoplasmas can also be spread via dodders cascutaceae or vegetative propagation such as the grafting
Grafting
Grafting is a horticultural technique whereby tissues from one plant are inserted into those of another so that the two sets of vascular tissues may join together. This vascular joining is called inosculation...
of a piece of infected plant onto a healthy plant.
Movement within plants
Phytoplasmas are able to move within the phloem from source to sink, and they are able to pass through sieve tube elementSieve tube element
Sieve tubes are mainly to transport sugars and nutrients up and down the plant. In plant anatomy, sieve vascular tissue tube elements, also called sieve tube members, are a type of elongated sclerenchyma cells in phloem tissue. The ends of these cells are connected with other sieve elements, and...
s. But since they spread more slowly than solute
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s, for this and other reasons, movement by passive translocation is not supported.
Detection and Diagnosis
Before molecular techniques were developed, the diagnosis of phytoplasma diseases was difficult because they could not be cultured. Thus classical diagnostic techniques, such as observation of symptoms, were used. Ultrathin sections of the phloem tissue from suspected phytoplasma infected plants would also be examined for their presence. Treating infected plants with antibioticAntibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
s such as tetracycline to see if this cured the plant was another diagnostic technique employed.
Molecular diagnostic techniques for the detection of phytoplasma began to emerge in the 1980s and included ELISA
ELISA
Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
based methods. In the early 1990s, PCR
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
-based methods were developed that were far more sensitive than those that used ELISA, and RFLP analysis allowed the accurate identification of different strains
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...
and species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of phytoplasma.
More recently, techniques have been developed that allow for assessment of the level of infection. Both QPCR and bioimaging have been shown to be effective methods of quantifying the titre of phytoplasmas within the plant.
Control
Phytoplasmas are normally controlled by the breeding and planting of disease resistance varieties of crops (believed to the most economically viable option) and by the control of the insect vector.Tissue culture
Tissue culture
Tissue culture is the growth of tissues or cells separate from the organism. This is typically facilitated via use of a liquid, semi-solid, or solid growth medium, such as broth or agar...
can be used to produce clones of phytoplasma infected plants that are healthy. The chances of gaining healthy plants in this manner can be enhanced by the use of cryotherapy, freezing the plant samples in liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
, before using them for tissue culture.
Work has also been carried out investigating the effectiveness of plantibodies targeted against phytoplasmas.
Tetracyclines are bacteriostatic to phytoplasmas, that is they inhibit their growth. However, without continuous use of the antibiotic, disease symptoms will reappear. Thus, tetracycline is not a viable control agent in agriculture, but it is used to protect ornamental coconut trees.
Genetics
The genomeGenome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s of three phytoplasmas have been sequenced
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....
: Aster Yellows Witches Broom, Onion Yellows (Ca. Phytoplasma asteris) and Ca. Phytoplasma australiense Phytoplasmas have very small genomes, which also have extremely low levels of the nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s G
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
and C
Cytosine
Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
, sometimes as little as 23% which is thought to be the threshold for a viable genome. In fact Bermuda grass white leaf phytoplasma has a genome size of just 530Kb, one of the smallest known genomes of living organisms. Larger phytoplasma genomes are around 1350 Kb. The small genome size associated with phytoplasmas is due to their being the product of reductive evolution from Bacillus/Clostridium ancestors. They have lost 75% or more of their original genes, and this is why they can no longer survive outside of insects or plant phloem. Some phytoplasmas contain extrachromosomal DNA such as plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
s.
Despite their very small genomes, many predicted genes are present in multiple copies. Phytoplasmas lack many genes for standard metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
functions and have no functioning homologous recombination
Homologous recombination
Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of DNA, known as double-strand breaks...
pathways, but do have a sec
Secretory pathway
The secretory pathway is a series of steps a cell uses to move proteins out of the cell; a process known as secretion. The path of a protein destined for secretion has its origins in the rough endoplasmic reticulum, a membrane-bound compartment in the cell...
transport pathway. Many phytoplasmas contain 2 rRNA operon
Operon
In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...
s. Unlike the rest of the Mollicutes
Mollicutes
The Mollicutes are a class of bacteria distinguished by the absence of a cell wall. The word "Mollicutes" is derived from the Latin mollis , and cutis . They are parasites of various animals and plants, living on or in the host's cells. Individuals are very small, typically only 0.2–0.3 μm in size...
, the triplet code of UGA
Stop codon
In the genetic code, a stop codon is a nucleotide triplet within messenger RNA that signals a termination of translation. Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide...
is used as a stop codon
Stop codon
In the genetic code, a stop codon is a nucleotide triplet within messenger RNA that signals a termination of translation. Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide...
in phytoplasmas, rather than to code for tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
.
Phytoplasma genomes contain large numbers of transposon
Transposon
Transposable elements are sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations and alter the cell's...
genes and insertion sequence
Insertion sequence
An insertion sequence is a short DNA sequence that acts as a simple transposable element...
s. They also contain a unique family of repetitive extragenic palindromes (REPs) called PhREPS whose role is unknown though it is theorised that the stem loop structures the PhREPS are capable of forming may play a role in transcription termination or genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
stability.
Taxonomy
Phytoplasmas are mollicutes and within this group belong to the monophyletic orderOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Acholeplasmatales.
In 1992 the Subcommittee on the Taxonomy of Mollicutes proposed the use of the name Phytoplasma in place of the use of the term MLO (Mycoplasma-like organism) "for reference to the phytopathogenic mollicutes".
In 2004 the genus name Phytoplasma was adopted and is currently at Candidatus
Candidatus
Candidatus is in scientific classification a component of the taxonomic name for a bacterium that cannot be maintained in a Bacteriology Culture Collection. It is an interim taxonomic status for noncultivable organisms. An example would be "Candidatus Phytoplasma allocasuarinae"...
status which is used for bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that can not be cultured. It's taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
is complicated by the fact that it can not be cultured and thus methods normally used for classification of prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s are not possible. Phytoplasma taxonomic groups are based on differences in the fragment sizes produced by the restriction digest
Restriction digest
A restriction digest is a procedure used in molecular biology to prepare DNA for analysis or other processing. It is sometimes termed DNA fragmentation...
of the 16S rRNA gene sequence (Called RFLP) or by comparison of DNA sequences from the 16s/23s spacer regions. There is some disagreement over how many taxonomic groups the phytoplasmas fall into, recent work involving computer simulated restriction digests of the 16Sr gene suggest there may be up to 28 groups where as other papers argue for less groups, but more sub-groups. Each group includes at least one Ca. Phytoplasma species, characterised by distinctive biological, phytopathological and genetic properties. The table below summaries some of the major taxonomic groups and the candidatus species that belong in them.
| 16Sr Group | Group Name | Species | Reference |
|---|---|---|---|
| 16SrI | Aster yellows Aster yellows Aster yellows is a chronic, systemic plant disease caused by a bacterium-like organism called a phytoplasma. The aster yellows phytoplasma affects 300 species in 38 families of broad-leaf herbaceous plants, primarily in the aster family. Symptoms are variable and can include phyllody, virescence,... |
Ca. Phytoplasma asteris Ca. Phytoplasma japonicum |
|
| 16SrII | Peanut witch's broom Witch's broom A Witch's broom is a disease or deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest.... |
Ca. Phytoplasma aurantifolia | |
| 16SrIII | X-disease | Ca. Phytoplasma pruni | |
| 16SrIV | Coconut Coconut The coconut palm, Cocos nucifera, is a member of the family Arecaceae . It is the only accepted species in the genus Cocos. The term coconut can refer to the entire coconut palm, the seed, or the fruit, which is not a botanical nut. The spelling cocoanut is an old-fashioned form of the word... lethal yellowing Lethal yellowing thumb|Palm tree dying of lethal yellowingLethal Yellowing is a phytoplasma disease that attacks many species of palms, including some commercially important species such as the Coconut and Date Palm. It is spread by the planthopper Haplaxius crudus which is native to Florida, parts of the... |
Ca. Phytoplasma palmae Ca. Phytoplasma castaneae Ca. Phytoplasma cocosnigeriae |
|
| 16SrV | Elm Elm Elms are deciduous and semi-deciduous trees comprising the genus Ulmus in the plant family Ulmaceae. The dozens of species are found in temperate and tropical-montane regions of North America and Eurasia, ranging southward into Indonesia. Elms are components of many kinds of natural forests... yellows |
Ca. Phytoplasma ziziphi Ca. Phytoplasma vitis Ca. Phytoplasma ulmi |
|
| 16SrVI | Clover Clover Clover , or trefoil, is a genus of about 300 species of plants in the leguminous pea family Fabaceae. The genus has a cosmopolitan distribution; the highest diversity is found in the temperate Northern Hemisphere, but many species also occur in South America and Africa, including at high altitudes... proliferation |
Ca. Phytoplasma trifolii | |
| 16SrVII | Ash Ash tree Fraxinus is a genus flowering plants in the olive and lilac family, Oleaceae. It contains 45-65 species of usually medium to large trees, mostly deciduous though a few subtropical species are evergreen. The tree's common English name, ash, goes back to the Old English æsc, while the generic name... yellows |
Ca. Phytoplasma fraxini | |
| 16SrVIII | Luffa Luffa The luffa, loofah, or lufah are tropical and subtropical vines comprising the genus Luffa, the only genus of the subtribe Luffinae of the plant family Cucurbitaceae... witch's-broom |
Ca. Phytoplasma luffae | |
| 16SrIX | Pidgeon pea witch's broom | Ca. Phytoplasma phoenicium | |
| 16SrX | Apple Apple The apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family . It is one of the most widely cultivated tree fruits, and the most widely known of the many members of genus Malus that are used by humans. Apple grow on small, deciduous trees that blossom in the spring... proliferation |
Ca. Phytoplasma mali Ca. Phytoplasma pyri Ca. Phytoplasma prunorum Ca. Phytoplasma spartii Ca. Phytoplasma rhamnii Ca. Phytoplasma allocasuarinae |
|
| 16SrXI | Rice Yellow Dwarf Sugarcane Grassy Shoot Disease Sugarcane Grassy Shoot Disease Sugarcane grassy shoot disease , caused by small, parasitic bacteria, contributes to losses of 5% to 20% in the main crop of sugarcane, and these losses are higher in the ratoon crop... (SCGS) |
Ca. Phytoplasma oryzae | |
| 16SrXII | Stolbur | Ca. Phytoplasma solani Ca. Phytoplasma australiense |
|
| 16SrXIII | Mexican periwinkle Catharanthus Catharanthus is a genus of eight species of herbaceous perennial plants, six endemic to the island of Madagascar, the seventh and eighth native to the Indian subcontinent in southern Asia... virescence |
Undefined | |
| 16SrXIV | Bermuda white leaf | Ca. Phytoplasma cynodontis | |
| 16SrXV | Hibiscus Hibiscus Hibiscus is a genus of flowering plants in the mallow family, Malvaceae. It is quite large, containing several hundred species that are native to warm-temperate, subtropical and tropical regions throughout the world... witch's-broom |
Ca. Phytoplasma brasiliense |
See also
- MollicutesMollicutesThe Mollicutes are a class of bacteria distinguished by the absence of a cell wall. The word "Mollicutes" is derived from the Latin mollis , and cutis . They are parasites of various animals and plants, living on or in the host's cells. Individuals are very small, typically only 0.2–0.3 μm in size...
- MycoplasmaMycoplasmaMycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
- Polymerase Chain ReactionPolymerase chain reactionThe polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
or PCR - ELISAELISAEnzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
- Sugarcane Grassy Shoot DiseaseSugarcane Grassy Shoot DiseaseSugarcane grassy shoot disease , caused by small, parasitic bacteria, contributes to losses of 5% to 20% in the main crop of sugarcane, and these losses are higher in the ratoon crop...
(SCGS)
External links
- First International Phytoplasmologist Working Group Meeting published in Vol. 60-2 2007 of Bulletin of Insectology
- Photo gallery about plants infected of phytoplasma
- Phytoplasma Resource and phytoplasma classification database
- Ohio State University publishes an informative site on this topic.
- First Internet Conference of Phytopathogenic Mollicutes includes several interesting articles on this topic.
- Phytoplasma Genome Projects.
- The Centre for Information on Coconut Lethal Yellowing (CICLY) with an associated Yahoo discussion group.
- Video of Melia yellows symptoms
- Video of maize bushy stunt symptoms
- Current research on Phytoplasmas at the Norwich Research Park