|Phyllody induced by phytoplasma infection on a coneflower (Echinacea purpurea).|
"Ca. Phytoplasma allocasuarinae"
(as of September 2016)
Phytoplasmas are obligate bacterial parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-like organisms (MLOs). Since their discovery, phytoplasmas have resisted all attempts at in vitro culture in any cell-free medium; routine cultivation in an artificial medium thus remains a major challenge. Although phytoplasmas have recently been reported to be grown in a specific artificial medium, no repetition have been reported. Phytoplasmas are characterized by the lack of a cell wall, a pleiomorphic or filamentous shape, a diameter normally less than 1 μm, and a very small genome.
Phytoplasmas are pathogens of agriculturally important plants, including coconut, sugarcane, and sandalwood, in which they cause a wide variety of symptoms ranging from mild yellowing to death. Phytoplasmas are most prevalent in tropical and subtropical regions. They are transmitted from plant to plant by vectors (normally sap-sucking insects such as leafhoppers) in which they both survive and replicate.
References to diseases now known to be caused by phytoplasmas can be found as far back as 1603 (mulberry dwarf disease in Japan.) Such diseases were originally thought to be caused by viruses, which, like phytoplasmas, require insect vectors, and cannot be cultured. Viral and phytoplasmic infections share some symptoms. In 1967, phytoplasmas were discovered in ultrathin sections of plant phloem tissue and were termed mycoplasma-like organisms (MLOs) due to their physiological resemblance The organisms were renamed phytoplasmas in 1994, at the 10th Congress of the International Organization for Mycoplasmology.
Phytoplasmas are Mollicutes, which are bound by a triple-layered membrane, rather than a cell wall. The phytoplasma cell membranes studied to date usually contain a single immunodominant protein of unknown function that constitutes most of the protein in the membrane. A typical phytoplasma is pleiomorphic or filamentous in shape and is less than 1 μm in diameter. Like other prokaryotes, phytoplasmic DNA is distributed throughout the cytoplasm, instead of being concentrated in a nucleus.
Phytoplasmas can infect and cause various symptoms in more than 700 plant species. One characteristic symptom is abnormal floral organ development including phyllody, (i.e., the production of leaf-like structures in place of flowers) and virescence (i.e., the development of green flowers attributable to a loss of pigment by petal cells). Phytoplasma-harboring flowering plants may nevertheless be sterile. The expression of genes involved in maintaining the apical meristem or in the development of floral organs is altered in the morphologically affected floral organs of phytoplasma-infected plants.
A phytoplasma infection often triggers leaf yellowing, probably due to the presence of phytoplasma cells in phloem, which can affect phloem function and carbohydrate transport, inhibit chlorophyll biosynthesis, and trigger chlorophyll breakdown. These symptoms may be attributable to stress caused by the infection rather than a specific pathogenetic process.
Many phytoplasma-infected plants develop a bushy or "witches' broom" appearance due to changes in their normal growth patterns. Most plants exhibit apical dominance but infection can trigger the proliferation of auxiliary (side) shoots and a reduction in internode size. Such symptoms are actually useful in the commercial production of poinsettias. Infection triggers more axillary shoot production; the poinsettia plants thus produce more than a single flower.
Effector (virulence) proteins
Many plant pathogens produce virulence factors (i.e., effectors) that modulate or interfere with normal host processes to the benefit of the pathogens. In 2009, a secreted protein, termed “tengu-su inducer” (TENGU), was identified from a phytoplasma causing yellowing of onions; this was the first phytoplasmal virulence factor to be described. TENGU induces characteristic symptoms (termed “tengu-su”), including witches’ broom and dwarfism. Transgenic expression of TENGU in Arabidopsis plants induced sterility in male and female flowers. TENGU contains a signal peptide at its N-terminus; after cleavage, the mature protein is only 38 amino acids in length. Although phytoplasmas are restricted to phloem, TENGU is transported from phloem to other cells, including those of the apical and axillary meristems. TENGU was suggested to inhibit both auxin- and jasmonic acid-related pathways, thereby affecting plant development. Surprisingly, the N-terminal 11 amino acid region of the mature protein triggers symptom development in Nicotiana benthamiana plants. TENGU undergoes proteolytic processing by a plant serine protease in vivo, suggesting that the N-terminal peptide (i.e., the 11 amino acid fragment) alone induces the observed symptoms. TENGU homologs have been identified in AY-group phytoplasmas. All such homologs undergo processing and can induce symptoms, suggesting that the symptom-inducing mechanism is conserved among TENGU homologs.
TCP transcription factors normally regulate plant development, controlling the expression of lipoxygenase genes required for jasmonate biosynthesis. Jasmonate levels are decreased in phytoplasma-infected Arabidopsis plants and plants that transgenically express SAP11, an effector of AY-WB phytoplasmas. The downregulation of jasmonate production is beneficial to phytoplasmas because jasmonate is involved in plant defenses against herbivorous insects such as leafhoppers. Leafhoppers lay increased numbers of eggs on AY-WB-infected plants, at least in part because of SAP11 production. For example, the leafhopper Macrosteles quadrilineatus laid 30% more eggs on plants that expressing SAP11 transgenically than control plants, 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 compromising jasmonate production, SAP11 'encourages' leafhoppers to lay more eggs on phytoplasma-infected plants, thereby ensuring that newly hatched leafhopper nymphs feed upon infected plants to become phytoplasma vectors.
A phytoplasma effector protein, SAP54, has been shown to induce virescence and phyllody when expressed in plants. SAP54 homologs have been identified in various phytoplasma species. Two SAP54 homologs, PHYL1 of the onion yellows phytoplasma and PHYL1PnWB of the peanut witches’ broom phytoplasma, induce phyllody-like floral abnormalities. These results suggest that PHYL1, SAP54, and their homologs form a phyllody-inducing gene family, the members of which are termed phyllogens. MADS-box transcription factors (MTFs) of the ABCE model play critical roles in floral organ development in Arabidopsis. Phyllogens interact directly with class A and class E MTFs, inducing protein degradation in a ubiquitin/proteasome-dependent manner. The accumulation of mRNAs encoding class B MTFs, the transcription of which is positively regulated by class A and class E MTFs, is drastically decreased in Arabidopsis constitutively expressing PHYL1. Phyllogens induce abnormal floral organ development by inhibiting the functions of these MTFs.
Movement between plants
Phytoplasmas are spread principally by insects of the families Cicadellidae (leafhoppers), Fulgoridae (planthoppers), and Psyllidae (jumping plant lice) , which feed on the phloem of infected plants, ingesting phytoplasmas and transmitting them to the next plant on which they feed. Thus, the host range of phytoplasmas is strongly dependent upon that of the insect vector. Phytoplasmas contain a major antigenic protein constituting most of the cell surface protein. This protein associates with insect microfilament complexes and is believed to control insect-phytoplasma interactions. Phytoplasmas can 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 noted.
Phytoplasmas enter the insect body through the stylet, pass through the intestine, and then move to the hemolymph and colonize the salivary glands: the entire process that can take up to 3 weeks. Once established in an insect host, phytoplasmas are found in most major organs. The time between ingestion by the insect and attainment of an infectious titer in the salivary glands is termed the latency period.
Movement within plants
Phytoplasmas move within phloem from a source to a sink, and can pass through sieve tubes. However, as phytoplasmas spread more slowly than solutes, and for other reasons, passive translocation within plants is thought to be unimportant
Detection and diagnosis
Before the molecular era, the diagnosis of phytoplasma-caused diseases was difficult because the organisms could not be cultured. Thus, classical diagnostic techniques, including symptom observation were used. Ultrathin sections of phloem tissue from plants with suspected phytoplasma-infections were also studied. The empirical use of antibiotics such as tetracycline was additionally employed.
Molecular diagnostic techniques for phytoplasma detection began to emerge in the 1980s and included enzyme-linked immunosorbent assay (ELISA)-based methods. In the early 1990s, polymerase chain reaction (PCR)-based techniques were developed: these are far more sensitive than ELISAs, and restriction fragment length polymorphism (RFLP) analysis allowed the accurate identification of various phytoplasma strains and species.
More recent techniques allow infection levels to be assessed. Both quantitative PCR and bioimaging can effectively quantify phytoplasma titers within plant. In addition, loop-mediated isothermal amplification (a sensitive, simple, and rapid diagnostic method) is now available as a commercial kit allowing all known phytoplasma species to be detected in about 1 h, including the DNA extraction step.
Phytoplasmas are normally controlled by the breeding and planting of disease-resistant crop varieties (perhaps the most economically viable option) and by the control of insect vectors.
Tissue culture can be used to produce healthy clones of phytoplasma-infected plants. Cryotherapy (i.e., the freezing of plant samples in liquid nitrogen) prior to tissue culture increases the probability of producing healthy plants in this manner.
Tetracyclines are bacteriostatic to phytoplasmas. However, disease symptoms reappear in the absence of continuous antibiotic application. Thus, tetracycline is not a viable agricultural control agent, but it is used to protect ornamental coconut trees.
The genomes of four phytoplasmas have been sequenced: "onion yellows", "aster yellows witches' broom" (Candidatus [Ca] Phytoplasma asteris), Ca. Phytoplasma australiense, and Ca. Phytoplasma Mali. Phytoplasmas have very small genomes, with extremely small amount of G and C nucleotides (sometimes as little as 23%, which is thought to be the lower threshold for a viable genome). In fact, the Bermuda grass white-leaf phytoplasma has a genome size of only 530 kb, one of the smallest known genomes of all living organisms. The larger phytoplasma genomes are around 1350 kb in size. The small genome size of phytoplasma is attributable to reductive evolution from Bacillus/Clostridium ancestors. Phytoplasmas have lost ≥75% of their original genes, and can thus no longer survive outside of insects or plant phloem. Some phytoplasmas contain extrachromosomal DNA such as plasmids.
Despite their small genomes, many predicted phytoplasma genes are present in multiple copies. Phytoplasmas lack many genes encoding standard metabolic functions and have no functioning homologous recombination pathway, but they do have a sec transport pathway. Many phytoplasmas contain two rRNA operons. Unlike other Mollicutes, the triplet code of UGA is used as a stop codon in phytoplasmas.
Phytoplasma genomes contain large numbers of transposons and insertion sequences and also contain a unique family of repetitive extragenic palindromes termed PhREPS for which no role is known. However, it is theorized that the stem-loop structures in PhREPS play a role in transcription termination or genome stability.
Phytoplasmas belong to the monophyletic order Acholeplasmatales. In 1992, the Subcommittee on the Taxonomy of Mollicutes proposed the use of "Phytoplasma" rather than "MLO" "for reference to the phytopathogenic mollicutes". In 2004, the generic name phytoplasma was adopted and is currently of Ca. status (used for bacteria that cannot be cultured). Phytoplasma taxonomy is complicated because the organisms cannot be cultured; methods normally used to classify prokaryotes are thus not available. Phytoplasma taxonomic groups are based on differences in fragment sizes produced by restriction digests of 16S rRNA gene sequences (RFLPs) or by comparisons of DNA sequences from 16s/23s spacer regions. The actual number of taxonomic groups remains unclear; recent work on computer-simulated restriction digests of the 16Sr gene suggested up to 28 groups, whereas others have proposed fewer groups, but more subgroups. Each group includes at least one Ca. Phytoplasma species, characterized by distinctive biological, phytopathological, and genetic properties.
A grape vine with "flavescence dorée" phytoplasma disease
Coconut palms dying of lethal yellowing disease
Trees dying of ash yellows phytoplasma
Symptoms of sweet potato little leaf phytoplasma on Catharanthus roseus
Phyllody of goldenrod
Sugarcane grassy shoot disease 
A cabbage tree killed by Phytoplasma australiense
- Doi Y, Teranaka M, Yora K, Asuyama H (1967). "Mycoplasma or PLT-group-like organisms found in the phloem elements of plants infected with mulberry dwarf, potato witches' broom, aster yellows or paulownia witches' broom". Annals of the Phytopathological Society of Japan. 33 (4): 259–266. doi:10.3186/jjphytopath.33.259.
- Contaldo, N.; Bertaccini, A.; Paltrinieri, S.; Windsor, H.M.; Windsor, G.D. (2012). "Axenic culture of a plant pathogenic Phytoplasma". Phytopathologia Mediterranea. 244: 607–617. doi:10.14601/Phytopathol_Mediterr-11773.
- Okuda, S (1972). "Occurrence of diseases caused by mycoplasma-like organisms in Japan". Plant Protection. 26: 180–183.
- Hogenhout, SA; Oshima K; Ammar E-D; Kakizawa S; Kingdom HN; Namba S (2008). "Phytoplasmas: bacteria that manipulate plants and insects". Molecular Plant Pathology. 9 (4): 403–423. PMID 18705857. doi:10.1111/j.1364-3703.2008.00472.x. Retrieved 2008-07-04.
- Bertamini, M; Grando M. S; Nedunchezhian N (2004). "Effects of Phytoplasma Infection on Pigments, Chlorophyll-Protein Complex and Photosynthetic Activities in Field Grown Apple Leaves". Biologia Plantarum. Springer Netherlands. 47 (2): 237–242. doi:10.1006/pmpp.2003.0450.
- Berg, M; Davis DL; Clark MF; Vetten HJ; Maier G; Marcone C; Seemuller E (1999). "Isolation of the gene encoding an immunodominant membrane protein of the apple proliferation phytoplasma, and expression and characterization of the gene product". Microbiology. Society for General Microbiology. 145: 1939–1943. PMID 10463160. doi:10.1099/13500872-145-8-1937.
- Lee, IM; Davis RE; Gundersen-Rindal DE (2000). "Phytoplasma: Phytopathogenic Mollicutes". Annual Review of Microbiology. 54: 221–255. PMID 11018129. doi:10.1146/annurev.micro.54.1.221.
- Pracros, P; Renaudin J; Eveillard S; Mouras A; Hernould M (2006). "Tomato Flower Abnormalities Induced by Stolbur Phytoplasma Infection Are Associated with Changes of Expression of Floral Development Genes". Molecular Plant Microbe Interactions. APS Press. 19 (1): 62–68. PMID 16404954. doi:10.1094/MPMI-19-0062.
- Himeno, M; Neriya Y; Minato N; Miura C; Sugawara K; Ishii Y; Yamaji Y; Kakizawa S; Oshima K; Namba S (2011). "Unique morphological changes in plant pathogenic phytoplasma-infected petunia flowers are related to transcriptional regulation of floral homeotic genes in an organ-specific manner". Plant Journal. 67 (6): 971–979. PMID 21605209. doi:10.1111/j.1365-313X.2011.04650.x.
- Muast BE, Espadas F, Talavera C, Aguilar M, Santamaría JM, Oropeza C (2003). "Changes in carbohydrate metabolism in coconut palms infected with the lethal yellowing phytoplasma". Phytopathology. 93 (8): 976–981. PMID 18943864. doi:10.1094/PHYTO.2003.93.8.976.
- Lee, IM; Klopmeyer M; Bartoszyk IM; Gundersen-Rindal DE; Chou TS; Thomson KL; Eisenreich R (1997). "Phytoplasma induced free-branching in commercial poinsettia cultivars". Nature Biotechnology. Nature Publishing Group. 15 (2): 178–182. PMID 9035146. doi:10.1038/nbt0297-178.
- Hoshi, A; Oshima K; Kakizawa S; Ishii Y; Ozeki J; Hashimoto M; Komatsu K; Kagiwada S; Yamaji Y; Namba S (2009). "A unique virulence factor for proliferation and dwarfism in plants identified from a phytopathogenic bacterium". Proceedings of the National Academy of Sciences. 106 (15): 6416–6421. PMC . PMID 19329488. doi:10.1073/pnas.0813038106.
- Minato, N; Himeno M; Hoshi A; Maejima K; Komatsu K; Takebayashi Y; Kasahara H; Yusa A; Yamaji Y; Oshima K; Kamiya Y; Namba S (2014). "The phytoplasmal virulence factor TENGU causes plant sterility by downregulating of the jasmonic acid and auxin pathways". Scientific Reports. 4: 7399. PMC . PMID 25492247. doi:10.1038/srep07399.
- Sugawara, K; Honma Y; Komatsu K; Himeno M; Oshima K; Namba S (2013). "The alteration of plant morphology by small peptides released from the proteolytic processing of the bacterial peptide TENGU". Plant Physiology. 162 (4): 2004–2015. PMC . PMID 23784461. doi:10.1104/pp.113.218586.
- Sugio, A; Kingdom HN; MacLean AM; Grieve VM; Hogenhout SA (2011). "Phytoplasma protein effector SAP11 enhances insect vector reproduction by manipulating plant development and defense hormone biosynthesis". Proceedings of the National Academy of Sciences. 108 (48): E1254–E1263. PMC . PMID 22065743. doi:10.1073/pnas.1105664108.
- MacLean, A. M.; Sugio, A.; Makarova, O. V.; Findlay, K. C.; Grieve, V. M.; Toth, R.; Nicolaisen, M.; Hogenhout, S. A. (2011). "Phytoplasma effector SAP54 induces indeterminate leaf-like flower development in Arabidopsis plants". Plant Physiology. 157 (2): 831–841. PMC . PMID 21849514. doi:10.1104/pp.111.181586.
- Maejima, K; Iwai R; Himeno M; Komatsu K; Kitazawa Y; Fujita N; Ishikawa K; Fukuoka M; Minato N; Yamaji Y; Oshima K; Namba S (2014). "Recognition of floral homeotic MADS-domain transcription factors by a phytoplasmal effector, phyllogen, induces phyllody". Plant Journal. 78 (4): 541–554. PMC . PMID 24597566. doi:10.1111/tpj.12495.
- Yang, CY; Huang YH; Lin CP; Lin YY; Hsu HC; Wang CN; Liu LY; Shen B; Lin SS (2015). "MiR396-targeted SHORT VEGETATIVE PHASE is required to repress flowering and is related to the development of abnormal flower symptoms by the PHYL1 effector". Plant Physiology. 168 (4): 1702–1716. PMC . PMID 26103992. doi:10.1104/pp.15.00307.
- MacLean, AM; Orlovskis Z; Kowitwanich K; Zdziarska AM; Angenent GC; Immink RG; Hogenhout SA (2014). "Phytoplasma effector SAP54 hijacks plant reproduction by degrading MADS-box proteins and promotes insect colonization in a RAD23-dependent manner". PLoS Biology. 12 (4): 831–841. PMC . PMID 21849514. doi:10.1104/pp.111.181586.
- Maejima, K; Kitazawa Y; Tomomitsu T; Yusa A; Neriya Y; Himeno M; Yamaji Y; Oshima K; Namba S (2015). "Degradation of class E MADS-domain transcription factors in Arabidopsis by a phytoplasmal effector, phyllogen". Plant Signaling & Behavior. 10: e1042635. PMC . PMID 26179462. doi:10.1080/15592324.2015.1042635.
- Weintraub, Phyllis G.; Beanland, LeAnn (2006). "Insect vectors of phytoplasmas". Annual Review of Entomology. 51: 91–111. PMID 16332205. doi:10.1146/annurev.ento.51.110104.151039.
- Suzuki, S.; Oshima, K.; Kakizawa, S.; Arashida, R.; Jung, H.-Y.; Yamaji, Y.; Nishigawa, H.; Ugaki, M.; Namba, S. (2006). "Interactions between a membrane protein of a pathogen and insect microfilament complex determines insect vector specificity". Proceedings of the National Academy of Sciences. 103 (11): 4252–4257. PMC . PMID 16537517. doi:10.1073/pnas.0508668103.
- Christensen N, Axelsen K, Nicolaisen M, Schulz A (2005). "Phytoplasmas and their interactions with their hosts". Trends in Plant Science. 10 (11): 526–535. doi:10.1016/j.tplants.2005.09.008.
- Carraro, L.; Loi, N.; Favali, M. A.; Favali, M. A. (1991). "Transmission characteristics of the clover phyllody agent by dodder". J. Phytopathol. 133: 15–22. doi:10.1111/j.1439-0434.1991.tb00132.x.
- Christensen NM, Nicolaisen M, Hansen M, Schulz A (2004). "Distribution of phytoplasmas in infected plants as revealed by real time PCR and bioimaging". Molecular Plant Microbe Interactions. 17 (11): 1175–1184. PMID 15553243. doi:10.1094/MPMI.2004.17.11.1175.
- Chen; et al. (1992). "Detection and identification of plant and insect mollicutes". The Mycoplasmas. 5: 393–424.
- Wang et al. (2007) Effective elimination of sweet potato little lead by cryotherapy of shoot tips. Plant Pathology online early edition.
- Chen, Y. D.; Chen, T. A. (1998). "Expression of engineered antibodies in plants: A possible tool for spiroplasma and phytoplasma disease control". Phytopathology. 88 (12): 1367–1371. PMID 18944841. doi:10.1094/PHYTO.19220.127.116.117.
- Davies, R. E.; Whitcomb, R. F.; Steere, R. L. (1968). "Remission of aster yellows disease by antibiotics". Science. 161 (3843): 793–794. doi:10.1126/science.161.3843.793.
- Drug for Humans Checks Palm Trees Disease. New York Times, July 19 1983
- Oshima K, Kakizawa S, Nishigawa H, Jung HY, Wei W, Suzuki S, Arashida R, Nakata D, et al. (2004). "Reductive evolution suggested from the complete genome sequence of a plant-pathogenic phytoplasma". Nature Genetics. 36 (1): 27–29. PMID 14661021. doi:10.1038/ng1277.
- Bai X, Zhang J, Ewing A, Miller SA, Jancso Radek A, Shevchenko DV, Tsukerman K, Walunas T, et al. (2006). "Living with Genome Instability: the Adaptation of Phytoplasmas to Diverse Environments of Their Insect and Plant Hosts". Journal of Bacteriology. 188 (10): 3682–3696. PMC . PMID 16672622. doi:10.1128/JB.188.10.3682-3696.2006.
- Tran-Nguyen, LT; Kube, M; Schneider, B; Reinhardt, R; Gibb, KS (2008). "Comparative Genome Analysis of "Candidatus Phytoplasma australiense" (Subgroup tuf-Australia I; rp-A) and "Ca. Phytoplasma asteris" Strains OY-M and AY-WB". Journal of Bacteriology. 190 (11): 3979–91. PMC . PMID 18359806. doi:10.1128/JB.01301-07.
- Kube, M; Schneider, B; Kuhl, H; Dandekar, T; Heitmann, K; Migdoll, AM; Reinhardt, R; Seemüller, E (2008). "The linear chromosome of the plant-pathogenic mycoplasma 'Candidatus Phytoplasma mali'". Bmc Genomics. 9 (1): 306. PMC . PMID 18582369. doi:10.1186/1471-2164-9-306.
- Dikinson, M. Molecular Plant Pathology (2003) BIOS Scientific Publishers
- Marcone, C; Neimark H; Ragozzino A; Lauer U; Seemüller E (1999). "Chromosome Sizes of Phytoplasmas Composing Major Phylogenetic Groups and Subgroups". Phytopathology. APS Press. 89 (9): 805–810. PMID 18944709. doi:10.1094/PHYTO.1918.104.22.1685.
- Nishigawa; et al. (2003). "Complete set of extrachromosomal DNAs from three pathogenic lines of onion yellows phytoplasma and use of PCR to differentiate each line". Journal of General Plant Pathology. 69: 194–198.
- Razin S, Yogev D, Naot Y (1998). "Molecular Biology and Pathogenicity of Mycoplasmas". Microbiology Molecular Biology Review. 62 (4): 1094–1156. PMC . PMID 9841667.
- Jomantiene R, Davis RE (2006). "Clusters of diverse genes existing as multiple, sequence variable mosaics in a phytoplasma genomes". FEMS Microbiology Letters. 255 (1): 59–65. PMID 16436062. doi:10.1111/j.1574-6968.2005.00057.x.
- 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 (2004). "Candidatus Phytoplasma, a taxon for the wall-less, non-helical prokaryotes that colonize plant phloem and insects". Int. J. Syst. Evol. Microbiol. 54: 1243–1255. PMID 15280299. doi:10.1099/ijs.0.02854-0.
- Murray, R. G. E.; Stackebrandt, E. (1995). "Taxonomic Note: Implementation of the Provisional Status Candidatus for Incompletely Described Procaryotes". International Journal of Systematic Bacteriology. 45 (1): 186–187. ISSN 0020-7713. PMID 7857801. doi:10.1099/00207713-45-1-186.
- Hodgetts, J.; Ball, T.; Boonham, N.; Mumford, R.; Dickinson, M. (2007). "Taxonomic groupings based on the analysis on the 16s/23s spacer regions which shows greater variation than the normally used 16srRNA gene results in classification similar to that derived from 16s rRNA data but with more detailed subdivisions". Plant Pathology. 56: 357–365. doi:10.1111/j.1365-3059.2006.01561.x.
- Wei W, Davis RE, Lee IM, Zhao Y (2007). "Computer-simulated RFLP analysis of 16S rRNA genes: identification of ten new phytoplasma groups". International Journal of Systematic and Evolutionary Microbiology. 57 (Pt 8): 1855–1867. PMID 17684271. doi:10.1099/ijs.0.65000-0.
- Yadav, A.; Bhale, U.; Thorat, V.; Shouche, Y. (2014). "First Report of new subgroup 16SrII- M ‘Candidatus Phytoplasma aurantifolia’ associated with ‘Witches Broom’ disease of Tephrosia purpurea in India". Plant Disease. 98: 990. doi:10.1094/PDIS-11-13-1183-PDN.
- Nasare, K.; Yadav, Amit; Singh, A. K.; Shivasharanappa, K. B.; Nerkar, Y. S.; Reddy, V. S. (2007). "Molecular and symptom analysis reveal the presence of new phytoplasmas associated with sugarcane grassy shoot disease in India". Plant Disease. 91 (11): 1413–1418. doi:10.1094/PDIS-91-11-1413.
- Phytoplasma Classification Iphyclassifier
- 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
- Phytoplasma universal detection kit NIPPON GENE CO., LTD.