Molecular and biological characterization of phytoplasmas from coconut palms affected by the lethal yellowing disease in Africa

Plants and Phytoplasmas: When Bacteria Modify Plants

Plant pathogen presence is very dangerous for agricultural ecosystems and causes huge economic losses. Phytoplasmas are insect-transmitted wall-less bacteria living in plants, only in the phloem tissues and in the emolymph of their insect vectors. They are able to manipulate several metabolic pathways of their hosts, very often without impairing their life. The molecular diversity described (49 'Candidatus Phytoplasma' species and about 300 ribosomal subgroups) is only in some cases related to their associated symptomatology. As for the other plant pathogens, it is necessary to verify their identity and recognize the symptoms associated with their presence to appropriately manage the diseases. However, the never-ending mechanism of patho-adaptation and the copresence of other pathogens makes this management difficult. Reducing the huge impact of phytoplasma-associated diseases in all the main crops and wild species is, however, relevant, in order to reduce their effects that are jeopardizing plant biodiversity.

Containment of Phytoplasma-Associated Plant Diseases by Antibiotics and Other Antimicrobial Molecules

Phytoplasmas are plant-pathogenic bacteria that infect many important crops and environmentally relevant plant species, causing serious economic and environmental losses worldwide. These bacteria, lacking a cell wall, are sensitive to antibiotics such as tetracyclines that affect protein synthesis mechanisms. Phytoplasma cultivation in axenic media has not been achieved for many strains; thus, the screening of antimicrobials must be performed using mainly in vivo materials. Some studies have investigated using in vitro phytoplasma-infected shoots, and several antimicrobials, including tetracyclines, have been tested. The screening of phytoplasma antimicrobials is important for the sustainable control of phytoplasma-associated diseases. The use of molecules with different modes of action such as ribosome inactivating proteins, plant hormones, and resistance inducers such as plasma-activated water, is advised, to avoid the use of antibiotics in agriculture and the possible emergence of resistant microbial strains.

Phytoplasma diseases of plants: molecular diagnostics and way forward

Phytoplasmas are obligate phytopathogenic bacteria associated with devastating diseases in hundreds of crops across the world. They have been responsible for huge economic losses in many crop plants for decades now. Isolation and establishment of axenic culture of phytoplasma in complex media is a recent progress in phytoplasma research. Earlier methods for phytoplasma disease detection included symptom profiling, microscopy, serology and dodder transmission studies. With advancement in the field of molecular biology, phytoplasma diagnostics and characterisation witnessed radical improvement. Starting from PCR amplification which often necessities a nested PCR on account of low titre of phytoplasmas, to the closed tube quantitative PCR assays and then the ddPCR, an array of diagnostics have been developed for phytoplasma. The isothermal diagnostic platforms are the latest addition to this and the Loop Mediated Isothermal Amplification (LAMP) assay has been applied for the detection of phytoplasma from several hosts. The futuristic approach in phytoplasma detection will be very likely provided by an integration of nanotechnology and molecular diagnostics. Phytoplasma disease management majorly relies on early detection, vector control, use of disease free planting materials and cultivation of resistant varieties. Hence understanding the molecular mechanism of phytoplasma-host interaction is as important as timely and accurate detection, in the management of phytoplasma diseases. Further, the changing climatic scenario and global warming may lead to an upsurge in the phytoplasma diseases spread and severity across the world, making disease management even more challenging.

Global Status of Phytoplasma Diseases in Vegetable Crops

The presence of phytoplasmas and their associated diseases is an emerging threat to vegetable production which leads to severe yield losses worldwide. Phytoplasmas are phloem-limited pleomorphic bacteria lacking the cell wall, mainly transmitted through leafhoppers but also by plant propagation materials and seeds. Phytoplasma diseases of vegetable crops are characterized by symptoms such as little leaves, phyllody, flower virescence, big buds, and witches' brooms. Phytoplasmas enclosed in at least sixteen different ribosomal groups infecting vegetable crops have been reported thus far across the world. The aster yellows phytoplasma group (16SrI) is presently the prevalent, followed by the peanut witches' broom (16SrII). Wide and overlapping crop and non-crop host ranges of phytoplasmas, polyphagous insect vectors, limited availability of resistance sources and unavailability of environmentally safe chemical control measures lead to an arduous effort in the management of these diseases. The most feasible control of vegetable phytoplasma diseases is a consequence of the development and implementation of integrated disease management programs. The availability of molecular tools for phytoplasma identification at the strain level greatly facilitated this kind of approach. It is moreover essential to understand the molecular basis of phytoplasma-vector interaction, epidemiology and other factors involved in disease development in order to reduce the disease outbreaks. Information on the knowledge about the most widespread phytoplasma diseases in vegetable crops is reviewed here in a comprehensive manner.