Phytoplasma

colspan=2 style="text-align: center; background-color: transparent; text-align:center; border: 1px solid red;" | Phytoplasma
Phyllody induced by phytoplasma infection on a coneflower (Echinacea purpurea).
colspan=2 style="min-width:15em; text-align: center; background-color: transparent; text-align:center; border: 1px solid red;" | Scientific classification
Division:	Tenericutes
Class:	Mollicutes
Order:	Acholeplasmatales
Family:	Acholeplasmataceae
Genus:	"Candidatus Phytoplasma"
colspan=2 style="text-align: center; background-color: transparent; text-align:center; border: 1px solid red;" | Species
"Ca. Phytoplasma allocasuarinae" "Ca. Phytoplasma americanum" "Ca. Phytoplasma asteris" "Ca. Phytoplasma aurantifolia" "Ca. Phytoplasma australiense" "Ca. Phytoplasma balanitae" "Ca. Phytoplasma brasiliense" "Ca. Phytoplasma caricae" "Ca. Phytoplasma castaneae" "Ca. Phytoplasma cocosnigeriae" "Ca. Phytoplasma cocostanzaniae" "Ca. Phytoplasma convolvuli" "Ca. Phytoplasma costaricanum" "Ca. Phytoplasma cynodontis" "Ca. Phytoplasma fragariae" "Ca. Phytoplasma fraxini" "Ca. Phytoplasma graminis" "Ca. Phytoplasma japonicum" "Ca. Phytoplasma luffae" "Ca. Phytoplasma lycopersici" "Ca. Phytoplasma malasianum" "Ca. Phytoplasma mali" "Ca. Phytoplasma omanense" "Ca. Phytoplasma oryzae" "Ca. Phytoplasma palmae" "Ca. Phytoplasma palmicola" "Ca. Phytoplasma phoenicium" "Ca. Phytoplasma pini" "Ca. Phytoplasma pruni" "Ca. Phytoplasma prunorum" "Ca. Phytoplasma pyri" "Ca. Phytoplasma rhamni" "Ca. Phytoplasma rubi" "Ca. Phytoplasma solani" "Ca. Phytoplasma spartii" "Ca. Phytoplasma sudamericanum" "Ca. Phytoplasma tamaricis" "Ca. Phytoplasma trifolii" "Ca. Phytoplasma ulmi" "Ca. Phytoplasma vitis" "Ca. Phytoplasma ziziphi" (as of March 2014)

Phytoplasmas are specialised bacteria that are obligate parasites of plant phloem tissue and transmitting insects (vectors). They were 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 μm, and their very small genomes.

Phytoplasmas are pathogens of agriculturally important plants, including coconut, sugarcane, and sandalwood, causing a wide variety of symptoms that range from mild yellowing to death of infected plants. They are most prevalent in tropical and subtropical regions of the world. They 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 survive and replicate.

History

References to diseases now known to be caused by phytoplasmas occurred as far back as 1603 for mulberry 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 mycoplasmas The organisms were renamed phytoplasmas in 1994, at the 10th Congress of The International Organization for Mycoplasmology.

Morphology

Being Mollicutes, a phytoplasma lacks a cell wall and instead is bound by a triple-layered membrane. 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 μm in diameter. As prokaryotes, phytoplasmas' DNA is found throughout the cytoplasm, rather than being concentrated in a nucleus.

Symptoms

A common symptom caused by phytoplasma infection is phyllody, the production of leaf-like structures in place of flowers. Evidence suggests 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 carbohydrates.

Phytoplasma-infected plants may also suffer from virescence, the development of green flowers due to the loss of pigment in the petal cells. Phytoplasma-harboring plants which are able to flower may nevertheless be sterile. A phytoplasma effector protein (SAP54) has been identified as inducing symptoms of virescence and phyllody when expressed in plants.

Many plants infected by phytoplasmas gain a bushy or "witches' broom" appearance due to changes in their normal growth patterns. 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 poinsettias. 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, the biosynthesis of which is also inhibited.

Effector (virulence) proteins

Many plant pathogens produce virulence factors (or effectors) that modulate or interfere with normal host processes in a way that is beneficial to the pathogen. TCP transcription factors normally regulate plant development and control the expression of lipoxygenase (LOX) genes that are required for the biosynthesis of jasmonate. In infected Arabidopsis plants (and plants that express SAP11 transgenically), jasmonate levels are decreased. The downregulation of jasmonate production is beneficial to the phytoplasma because jasmonate is involved in plant defence against herbivorous insects such as leafhoppers, and leafhoppers have been shown to lay more eggs on AY-WB-infected plants at least in part because of SAP11. For example, the leafhopper Macrosteles quadrilineatus lays 30% more eggs on plants that express SAP11 transgenically, and 60% more eggs on plants infected with AY-WB. Phytoplasmas cannot survive in the external environment and are dependent upon insects such as leafhoppers for transmission to new (healthy) plants. Thus, by interfering with jasmonate production, SAP11 'encourages' leafhoppers to lay more eggs on phytoplasma-infected plants, thereby ensuring that newly hatching leafhopper nymphs feed upon infected plants and become vectors for the bacteria.

Transmission

Movement between plants

Phytoplasmas are mainly spread by insects 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 on which they feed. So, 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 there they proceed to colonise the salivary glands, a process that can take up to three weeks. Once established, phytoplasmas are 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 (Cuscuta) or vegetative propagation such as the grafting 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 elements. But since they spread more slowly than solutes, 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 antibiotics 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, polymerase chain reaction-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 quantitative PCR 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 resistant varieties of crops (believed to the most economically viable option) and by the control of the insect vector.

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. However, without continuous use of the antibiotic, disease symptoms reappear. Thus, tetracycline is not a viable control agent in agriculture, but it is used to protect ornamental coconut trees.

Genetics

The genomes 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 nucleotides 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 530 kb, 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, so can no longer survive outside of insects or plant phloem. Some phytoplasmas contain extrachromosomal DNA such as plasmids.

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 two rRNA operons. Unlike the rest of the Mollicutes, the triplet code of UGA is used as a stop codon in phytoplasmas.

Phytoplasma genomes contain large numbers of transposon genes and insertion sequences. They also contain a unique family of repetitive extragenic palindromes 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.

Taxonomy

Phytoplasmas belong to the monophyletic order 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 cannot be cultured. Its taxonomy is complicated because it can not be cultured, thus methods normally used for classification of prokaryotes 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 (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 whereas other papers argue for less groups, but more subgroups. Each group includes at least one Ca. Phytoplasma species, characterised by distinctive biological, phytopathological, and genetic properties.

• List of Ca. Phytoplasma species, major groups and subgroups published with their common name and GenBank accession number of 16S rRNA gene sequence

• Note: The Ca. Phytoplasma species names given in square brackets are provisional (16S rRNA sequences similarity <97.5%) and not yet published separately as new species. ‘Undetermined’ denotes Ca. Phytoplasma species’ group or subgroup status pending. 'Not assigned’ denotes Ca. Phytoplasma species status pending (16S rRNA sequences similarity >97.5%) as symptoms shown by the host plant/ insect were significantly different when infected with described strain of phytoplasma as compared to strain of same species or group.

Sr. no.	Common name	Phytoplasma species	Group/subgroup	Acc. no.	Reference
1	Nigerian awka disease	[Ca. P. cocosnigeriae]	16SrXXII-A	Y14175	Firraro, 2004
2	Tanzanian lethal disease	[Ca. P. cocostanzaniae]	Undetermined	X80117	Firraro, 2004
3	Loofah witches' broom	[Ca. P. luffae]	16SrVIII-A	AF086621	Firraro, 2004
4	Palm lethal yellowing	[Ca. P. palmae]	16SrIV-A	U18747	Firraro, 2004
5	Western X-disease	[Ca. P. pruni]	16SrIII-A	L04682	Firraro, 2004
6	Stolbur phytoplasma	[Ca. P. solani]	16SrXII-A	AF248959	Firraro, 2004
7	Flavescence doree	[Ca. P. vitis]	16SrV-C	AF176319	Firraro, 2004
8	Papaya yellow crinkle	Ca. P. aurantifolia	16SrII-D	Y10097	Davis et al., 1997
9	Allocasuarina yellows	Ca. P. allocasuarinae	Undetermined	AY135523	Marcone et al., 2004
10	Potato purple top wilt	Ca. P. americanum	16SrXVIII	DQ174122	Lee et al., 2006
11	Lime witches' broom	Ca. P. aurantifolia	16SrII-B	U15442	Zriek et al., 1995
12	Grapevine yellows	Ca. P. australience	16SrXII	L76865	Davis et al., 1997
13	Balanites witches' broom	Ca. P. balanitae	16SrV	AB689678	Win et al., 2012
14	Hibiscus witches' broom	Ca. P. brasiliense	16SrXV	AF147708	Montano et al., 2001
15	Papaya bunchy top	Ca. P. caricae	16SrXVII	AY725234	Arocha et al., 2005
16	Chestnut witches' broom	Ca. P. castaneae	16SrXIX	AB054986	Jung et al., 2002
17	Bindweed yellows	"Ca. P. convolvuli"	16SrXII	JN833705	Martini et al., 2012
18	Soybean stunt	Ca. P. costaricanum	16SXXXI	HQ225630	Lee et al., 2011
19	Sugarcane white leaf	Ca. P. cynodontis	16SrXI	AB052874	Jung et al., 2003
20	Areca palm yellow leaf	Ca. P. cynodontis	16SrXI-A	JN967909	Ramaswamy et al., 2012
21	Sugarcane white leaf	Ca. P. cynodontis	16SrXI-C	X76432	Lee et al., 1998
22	Leafhopper bourne BVK	Ca. P. cynodontis	16SrXIV	X76429	Semuller et al., 2004
23	Bermuda grass white leaf	Ca. P. cynodontis	16SrXIV	AJ550985	Marcone et al., 2004
24	Cynodon white leaf	Ca. P. cynodontis	Undetermined	AF509321	Blanche et al., 2003
25	Sorghum grassy shoot	Ca. P. cynodontis	Undetermined	AF509324	Blanche et al., 2003
26	Sorghum grassy shoot	Ca. P. cynodontis	Undetermined	AF509325	Blanche et al., 2003
27	Strawberry witches' broom yellows	Ca. P. fragariae	16SrXII-E	DQ086423	Valiunas et al., 2006
28	Ash yellows	Ca. P. fraxini	16SrVII-A	AF092209	Griffiths et al., 1999
29	Sugarcane Yellow Leaf	"Ca. P. graminis"	16SrXVI	AY725228	Arocha et al., 2005
30	Hydrangea phyllody	Ca. P. japonicum	16SrXII-D	AB010425	Sawayanagi et al., 1999
31	Parsley leaf of tomato	Ca. P. lycopersici"	Undetermined	EF199549	Arocha et al., 2007
32	Periwinkle phyllody	Ca. P. malasianum	16SrXXXII	EU371934	Nejat et al., 2012
33	Apple proliferation	Ca. P. mali	16SrX-A	AJ542541	Semuller et al., 2004
34	Cassia italica witches' broom	Ca. P. omanense	16SrXXIX	EF666051	Saddy et al., 2008
35	Rice yellow dwarf	Ca. P. oryzae	16SrXI-A	D12581	Namba et al., 2009
36	Rice yellow dwarf	Ca. P. oryzae	16SrXI-A	AB052873	Jung et al., 2003
37	Almond lethal disease	Ca. P. phoenicium	16SrIX-D	AF515636	Verdin et al., 2003
38	Pine shoot proliferation	Ca. P. pini	16SrXXI	AJ632155	Schneider et al., 2005
39	Prunus X-disease	Ca. P. pruni	16SrIII-A	JQ044393	Davis et al., 2012
40	European stone fruit	Ca. P. prunorum	16SrX-F	AJ542544	Semuller et al., 2004
41	Pear decline	Ca. P. pyri	16SrX-C	AJ542543	Semuller et al., 2004
42	Buckthorn witches' broom	Ca. P. rhamni	16SrXX	X76431	Marcone et al., 2004
43	Rubus stunt	Ca. P. rubi	16SrV	AY197648	Maher et al., 2011
44	Bois nor	Ca. P. solani	16SrXII	JQ730746	Quaglino et al., 2013
45	Spartium witches' broom	Ca. P. spartii	16SrX-D	X92869	Marcone et al., 2004
46	Passiflora witches' broom	Ca. P. sudamericanum	16SrIII-V	GU292082	Davis et al., 2012
47	Passiflora witches' broom	Ca. P. sudamericanum	16SrVI	GU292081	Davis et al., 2012
48	Salt cedar witches' broom	Ca. P. tamaricis	16SrXXX	FJ432664	Zhao et al., 2009
49	Clover proliferation	Ca. P. trifolii	16SrVI-A	AY390261	Hiruki et al., 2004
50	Elm yellows	Ca. P. ulmi	16SrV-A	AY197655	Lee et al., 2004
51	Jujube witches' broom	Ca. P. ziziphi	16SrV-B	AB052876	Jung et al., 2003
52	Aster yellows	Ca. P. asteris	16SrI	M30790	Lee et al., 2004
53	MexiCan periwinkle virescence	Not assigned	16SrXIII-A	AF248960	Wei et al. 2007
54	Bermuda grass white leaf	Not assigned	16SrXIV	AJ550984	Marcone et al., 2004
55	Grapevine yellow	Not assigned	16SrXXIII-A	AY083605	Wei et al., 2007
56	Sorghum bunchy shoot	Not assigned	16SrXXIV-A	AF509322	Wei et al., 2007
57	Weeping tea witches' broom	Not assigned	16SrXXV-A	AF521672	Wei et al., 2007
58	Sugarcane yellows	Not assigned	16SrXXVI-A	AJ539179	Wei et al., 2007
59	Sugarcane yellows	Not assigned	16SrXXVII-A	AJ539180	Wei et al., 2007
60	Derbid phytoplasma	Not assigned	16SrXXVIII-A	AY744945	Wei et al., 2007
61	Chinaberry yellows	Not assigned	Undetermined	AF495882	Wei et al., 2007

Gallery

• Vitis vinifera phytoplasma.jpg

  A grape vine with "bois noir" phytoplasma disease

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  A grape vine with "flavescence dorée" phytoplasma disease

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  Coconut palms dying of lethal yellowing disease

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  Symptoms of aster yellows on marigold

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  Tephrosia purpurea witches' broom

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  Symptoms of elm phloem necrosis phytoplasma

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  Trees dying of ash yellows phytoplasma

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  Phyllody caused by phytoplasma infection on Cosmos spp.

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  Symptoms of sweet potato little leaf phytoplasma on Catharanthus roseus

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  Phyllody of goldenrod

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  Sugarcane grassy shoot disease

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  A palm tree dying of lethal yellowing phytoplasma

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  A cabbage tree killed by Phytoplasma australiense

See also

• Mollicutes
• Mycoplasma
• Polymerase chain reaction
• Sugarcane grassy shoot disease

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

• Chen et al. (1992) Detection and identification of plant and insect mollicutes. In The Mycoplasmas, editor RF Whitcomb and JG Tully 5: 393-424
• Wang et al. (2007) Effective elimination of sweet potato little lead by cryotherapy of shoot tips. Plant Pathology online early edition.
• Drug for Humans Checks Palm Trees Disease. New York Times, July 19 1983
• Dikinson, M. Molecular Plant Pathology (2003) BIOS Scientific Publishers
• Subcommittee on the Taxonomy of Mollicutes. Minutes of the Interim Meetings, 1 and 2 August, 1992, Ames, Iowa Int. J. of Syst. Bact. April 1993, p. 394-397; Vol. 43, No. 2 (see minutes 10 and 25)
• The IRPCM Phytoplasma/Spiroplasma Working Team - Phytoplasma taxonomy group: Candidatus Phytoplasma, a taxon for the wall-less, non-helical prokaryotes that colonize plant phloem and insects. Int. J. Syst. Evol. Microbiol., 2004, 54, 1243-1255.[1]