Phytoplasma

Phytoplasma, formerly known as 'Mycoplasma-like organisms' or MLOs, are specialised bacteria that are obligate parasites of plant phloem tissue, and some insects. They were first discovered by scientists in 1967 in plants that were thought to be infected with viruses, but ultrathin sections of the plants phloem revealed the prescence of mycoplasma like organisms. They can't 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 genomes.

Phytoplasmas are pathogens of important crops, including coconuts and sugarcane, causing a wide variety of symptoms that ranges 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.

Morphology
Being mollicutes, phytplasmas lack cell walls and instead are bound by a triple layed unit membrane. Their shape is normally pleiomorphic or filamentous and normally have a diameter of less than 1 micrometer. Like other prokaryotes, DNA is free in the cytoplasm.

Symptoms


A common symptom caused by phytoplasma infection is phyllody, the production of leaf like structures in place of flowers. 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). This causes sepals to form where petals should. 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.. 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 is necessary to produce more axillary shoots that enable to production of pionsettia 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 pathogencity of the phytoplasma. Photosynthesis, espicially 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.

Movement between plants
The phytoplasmas are mainly spread by insects of of the families Cicadellidea (leafhoppers) and Fulgoridea (planthoppers) 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 may overwinter in insect vectors or perrinial plants. Phytoplasmas can have varying affects on their insect hosts, examples of both reduced and increased fitness have been seen.

Phytoplasmas will be found in most of the major organs of an infected insect host once they are established. They will enter the insects body through the stylet and then move through the intestine and bein absorbed into the haemolymph. From here they proceded to colonise the salivary glands, a process that can take up to three weeks. The time between phytoplasmas being taken up by the insect and the phytoplasmas reaching a infectious titre in the salivary gland is called the latency period.

Phytoplasmas can also be spread via vegetative propergation such as the grafting of a piece of infected plant onto a healthy plant.

Movement within plants
Phytoplasmas are able to move within the pholem from source to sink and they are able to pass through sieve tube elements, but 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 due to the fact that 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 prescence. Another diagnostic technique used was to treat infected plants with antibiotics such as tetracycline to see if this cured the plant.

Molecular diagnostic techniques for the detection of phytoplasma began to emerge in the 1980s and included ELISA based methods. In the early 1990's PCR based methods were developed that were far more sensitive than those that used ELISA, and RFLP analysis allowed the accurate indentification of different strains and species of phytoplasma.

There are also techniques that allow the assessement 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 varities of crops (believed to the the most economically viable option) and by the control of the insect vector.

Genetics
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 530Kb, the smallest genome of any known living organism. Larger phytoplasma genomes are around 1350 Kb. 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 2 rRNA operons. 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 sequences. They also contain a unique family of repetative 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.

Taxonomy
Phytoplasmas are mollicutes and within this group belong to the monophyletic order Acholeplasmatales. The genus name Phytoplasma is yet to be formally recognised, and is currently at Candidatus status. Its taxonomy is complicated by the fact that it can not be cultured and thus methods normally used for classification of prokaryotes are not possible. There are currently 15 groups based on RFLP differences in the 16S rRNA gene sequence (16Sr groups). Each group includes at least one Ca. Phytoplasma species, characterised by distinctive biological, phytopathological and genetic properties: 16SrI (Aster yellows group) - Ca. Phytoplasma asteris - Ca. Phytoplasma japonicum 16SrII (Peanut witches'-broom group) - Ca. Phytoplasma aurantifolia 16SrIII (X-disease group) - Ca. Phytoplasma pruni 16SrIV (Coconut lethal yellowing group) - Ca. Phytoplasma palmae - Ca. Phytoplasma castaneae - Ca. Phytoplasma cocosnigeriae 16SrV (Elm yellows group) - Ca. Phytoplasma ziziphi - Ca. Phytoplasma vitis - Ca. Phytoplasma ulmi 16SrVI (Clover proliferation group) - Ca. Phytoplasma trifolii 16SrVII (Ash yellows group)  - Ca. Phytoplasma fraxini 16SrVIII (Luffa witches'-broom group) - Ca. Phytoplasma luffae 16SrIX (Pigeon pea witches'-broom group)  - Ca. Phytoplasma phoenicium 16SrX (Apple proliferation group)  - Ca. Phytopalsma Mali - Ca. Phytoplasma pyri - Ca. Phytoplasma prunorum - Ca. Phytoplasma spartii - Ca. Phytoplasma rhamnii - Ca. Phytoplasma allocasuarinae 16SrXI (Rice yellow dwarf group)  - Ca. Phytopalsma oryzae 16SrXII (Stolbur group)  - Ca. Phytoplasma solani - Ca. Phytoplasma australiense 16SrXIII (Mexican periwinkle virescence group)  - No Ca. Phytoplasma species defined yet 16SrXIV (Bermuda white leaf group)  - Ca. Phytoplasma cynodontis 16SrXV (Hibiscus witches'-broom group) - Ca. Phytoplasma brasiliense

Phytoplasma australiense causes the disease "Sudden Decline" in New Zealand cabbage trees.