Phytoplasmas are small wall-less bacteria, they have a single cell membrane and a small genome averaging 750kb 1. They cause disease in plants and some insects, typical symptoms include witches' broom (clustering of branches) of developing tissues, phyllody (metamorphosis of the floral organs into leaf like structures), virescence (green colouration of non-green flower parts), bolting (growth of elongated stalks), reddening of leaves and stems, general yellowing and phloem necrosis 4.
All phytoplasmas are transmitted from plant to plant via insect vectors, they must traverse the insect gut cells, replicate in the insects tissue cells and then traverse the salivary gland cells so that it can enter the saliva and then can be passed into the phloem of other plants during feeding.
The methods by which these symptoms are induced are not completely understood. One reason for this is because phytoplasmas cannot be cultured in cell-free artificial culture media. However in 2004 and 2006 the genomes of two phytoplasmas were published 4 and since then so have two more. The first two sequenced were onion yellow strain M (OY-M) and aster yellow strain witches broom (AY-WB), this allowed for the opportunity to study the role of the secretory systems of the phytoplasmas in their pathogenicity.
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There is no evidence to show that phytoplasmas secret proteins via type III secretory systems. Type IV secretory systems are widely used by bacteria however neither OY-M nor AY-WB contain similar genes to those which encode for the secretory proteins. It has been suggested that phytoplasmas instead use the Sec protein-translocation pathway. This theory was been strengthened when in 2004 the genes encoding SecA, SecY and SecE were identified in OY phytoplasmas and SecA expression was shown to be present in infected plants 4. The three genes were also found in the AY-WB genome and SecY genes have been found in other phytoplasmas. Through this they can introduce effector proteins into the host cells. It has been shown that the phytoplasmas produce a series of virulence proteins that can disrupt the plants defence mechanisms and alter other functions of the plant including growth. They also produce proteins which mean that the phytoplasma can depend of the metabolic compounds produced by the plant for survival. This was seen in ‘Bois Noir', where it was shown that plant genes involved in primary and secondary metabolic pathways were altered during infection including down-regulation of photosynthetic genes and carbohydrate metabolism 3.
In 2009 the AY-WB genome was found to contain coding for membrane-targeting proteins. It was found that fifty six secreted AY-WB proteins (SAPs) are released via secretion 6, the function of these proteins is discussed below.
The phytoplasma genome contains areas of repeated gene sequences, these are known as putative mobile units (PMUs). These PMU regions have been shown to contain the genes to encode for virulence proteins, SAP11 and SAP30. These proteins contain nuclear localization signals (NLSs). SAP11 and SAP30 are able to effect the plants transcription inside the nucleus, this was shown by fluorescent protein fusions with the SAPs which only showed in the nucleiof plant cells. However SAPs without the NLSs gathered randomly about the cell 6. Interestingly all PMUs contain the genes to encode for specialized transcription factors and membrane-targeted proteins, this would suggest that PMUs are part of the method by which phytoplasmas alter their plant hosts.
Immunoflourescnce microscopy has found SAP11 in cells away from the phloem, this suggests that SAP11 can move out of the phloem. The size exclusion limits (SELs) of developing tissue plasmodesmata are between 10 and 40 kDa, most of the AY-WB proteins produced (including SAP11) are smaller than 40 kDa 6, this again shows movement of proteins from the phloem and into the developing tissues of the host which is where the hosts features are primarily altered.
YidC is a bacterial membrane protein and is required in the membrane integration process of synthesised proteins. It was originally believed that YidC worked with Sec translocase to insert Sec-dependant substrate proteins into a membrane, however in 2004 it was shown that YidC can function separately from the Sec translocase in protein membrane integration. OY-M and AY-WB both contain the genomic code of YidC 5 and therefore likely use this integration system for their synthesised virulence factors.
In 2009 a virulence protein was found in the OY phytoplasmas called tengu-su (TENGU). TENGU is small peptide which is able to travel through the phloem tissue to the apical meristem, where auxin is synthesized, microarray analyses shows a down-regulation in auxin-responsive genes during infection 2 suggesting that TENGU may suppress the auxin biosynthesis and signalling pathways resulting in the growth inhibition of certain plant structures. This causes induction of witches' broom and dwarfism.
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In summation phytoplasmas are able to infect plants through a series of complex and unique membrane-bound and secreted proteins (virulence proteins). The detection and prevention of phytoplasmas is becoming more necessary as its prevalence is expected to increase in the following years due to tighter regulation of pesticide control, global warming and the increase in use of organic farming. By sequencing more genomes and gaining a better understanding into the secretion systems of phytoplasmas we will be able to better combat the bacteria in days to come.
1. Dickinson, M. (2003). Molecular Plant Pathology. BIOS Sci. Pub.
2. Hoshi, A. et al (2009). PNAS. 106, 6416-6421.
3. Hren, M. et al (2009). BMC Gen. 10, 460.
4. Hogenhout, S.A. et al (2008). Mol Plant Path. 9, 403-423.
5. Samuelson, J.C. et al (2000). Nature. 406, 637-641.
6. Bai, X.D. et al (2009). Mol. Plant Intera.22, 18-30.