COST Action FA0807
During the past 5 years, the genomes of 6 phytoplasmas have been completely sequenced (four are already published) and a number of others are partially available. Whilst the majority of these projects have been led by research groups from outside Europe, European research groups have been involved in some of them. Studies are also in progress towards identification of plant genes involved in phytoplasma pathogenicity and in host-pathogen interactions. These studies have provided insights into fundamental aspects of phytoplasma biology, and since the genomes of phytoplasmas are among the smallest known in nature, and are recognized as representing an example of the minimal gene set required for life, the results are providing valuable information both from a plant pathology and from a fundamental biology perspective. From an innovative perspective, these genomics data are providing valuable resources and information for improving diagnostics and classification of phytoplasmas, and as outlined above, these informations are essential prerequisites for improving disease management.
Four working groups (WG) were accepted for this action.
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Working Group 1: Early detection and diagnostics
Early and sensitive detection and diagnosis of phytoplasmas is of paramount importance for effective prevention strategies, particularly because phytoplasmas may have a very long latency period. The main objectives of this WG are to compare diagnostic procedures already available for most phytoplasma pathogens and/or develop novel methods and integrate these into sensitive and simple early detection protocols, suitable for monitoring propagation material and for screening in plant-inspection services. To accomplish the goals in this task marker genes that show sufficient polymorphism will be selected as DNA bar-coding regions, and a database of available collections of phytoplasma strains and/or DNA will be established.
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Working Group 2: Epidemiology and vector ecology
WG2 will study the dispersal of phytoplasma diseases. Phytoplasmas are transmitted by insects belonging to the families Cicadellidae, Cixidae, Psyllidae, Delphacidae, and Derbidae. The vector acquires the phytoplasma by feeding on an infected plant and then transmits the pathogen to a healthy one only after completion of the latent period; factors influencing these periods as well as the efficiency of transmission will be studied. Once a vector becomes infectious, infectivity is retained for life, although some discontinuities in vectoring abilities have been reported for several phytoplasma-vector associations, these will also be investigated. Some factors influencing transmission, such as life stage, gender, presence of associated symbionts, flight behaviour, weed control measures, temperature, phytoplasma strain, source and recipient plant species will be studied for relevant phytoplasmas. Effects on different organs or even reduction of longevity and fecundity will be studied for selected phytoplasma-infected vectors. Although phytoplasmas have been detected in various organs and tissues of the vectors, the existence of two barriers has been suggested: the midgut and the salivary glands. There are reports of phytoplasma multiplication in the midgut of nonvector insects, clearly indicating that there are cases where phytoplasmas colonize the insect but are not transmitted. Moreover in some cases, even the host plant may influence the outcome of transmission. In fact certain plant species may be infected with phytoplasmas by feeding insects, but are unsuitable for further acquisition, at least with some vector species.
Micropropagation together with other agricultural practices such as grafting, cutting, stool bed and other systems to propagate plant germplasm avoiding sexual reproduction are other known ways for transmitting phytoplasma diseases, and recently the possibility of transmission through seed has also been under investigation.
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Working Group 3: Crop systems and control
Control of epidemic outbreak can be carried out in principle either by controlling the vector or by eliminating the pathogen from the infected plants by antibiotics or by other chemicals. However, these protection measures have proved to be quite ineffective under field conditions, firstly because it is impossible to eliminate all vectors from the environment, and secondly because the use of antibiotics is very expensive, not allowed in several countries, and not always effective over the long-term since they do not completely eradicate the phytoplasmas, such that repeated treatments are necessary. Therefore the only effective way to control phytoplasma infection has been to prevent the outbreaks by ensuring that clean planting material is used, or by endeavouring to find and/or breed varieties of crop plants that are resistant or tolerant to the phytoplasma/insect vector. In order to advance this field of research basic knowledge about the epidemiology, the pathogenicity mechanisms of the phytoplasmas, the effects of environmental factors on disease and symptom development, and the nature of resistance/tolerance in host plants is required. The recent sequencing of phytoplasma genomes has provided evidence that small peptides secreted by phytoplasmas are able to enter plant cells and move between cells, and that some of these secreted peptides are likely to be key pathogenicity factors, and may therefore be potential targets for plant defence mechanisms. Identification of alternative control strategies against these diseases, such as the possibility to use biocontrol organisms or phytoplasma mild strains could also provide innovative and promising tools for limiting phytoplasma spread in an environmentally sustainable approach. For example, bacterial symbionts that might be able to reduce phytoplasma transmission by leafhoppers have been identified. Reduced symptom expression in phytoplasma-infected plants treated with arbuscular mycorrhizal fungi, and the capacity of fungal elicitins to prevent symptom expression in tobacco plants infected with stolbur phytoplasmas, were recently reported. In addition, the occurrence of mild strains of phytoplasmas might allow for disease control through cross protection. This WG will coordinate the results from epidemiological and molecular studies to formulate new and improved strategies for the control and management of phytoplasma diseases.
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Working Group 4: Phytoplasma/host interactions
Over the past 5 years, European research teams have been involved in a number of phytoplasma full genome sequencing projects and some of this sequence information is available in public access databases. The genomes themselves encode between 496 and 839 genes, and have very low G+C content. Whilst the main housekeeping genes appear to be conserved among phytoplasmas, there are also other genes that are unique to specific strains. Compared to other organisms, phytoplasmas lack genes encoding components of the pentose phosphate pathway, lack most genes for nucleotide synthesis, and also lack genes for the F0F1-type ATP synthase, which was previously thought to be a component of the minimal gene set required for all living organisms. Studies are currently identifying the various biosynthetic pathways that exist in phytoplasmas, and the transport mechanisms that are involved in importing essential compounds from host plants and insects, and in exporting potential pathogenicity factors into these hosts. In addition, there have been a number of studies to examine the changes in host gene expression that occur in infected plants, and the physiological and metabolic changes that occur in these hosts.
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