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To establish an infection, most plant viruses move from cell to cell in the plant. Virus-encoded movement proteins mediate this process and appear to use two mechanisms for transport. Both mechanisms involve interaction with and potential modification of plant intercellular connections, the plasmodesmata. Thus, although viral movement proteins are a diverse group, they share an ability to interact with specific plant components.
An appreciation of the risks caused by emergent plant viruses is critical in tropical areas that rely heavily on agriculture for subsistence and rural livelihood. Molecular ecology, within 10 years, has unraveled the factors responsible for the emergence of several of the economically most important tropical plant viruses: Rice yellow mottle virus (RYMV), Cassava mosaic geminiviruses (CMGs), Maize streak virus (MSV), and Banana streak virus (BSV). A large range of mechanisms--most unsuspected until recently--were involved: recombination and synergism between virus species, new vector biotypes, genome integration of the virus, host adaptation, and long-distance dispersal. A complex chain of molecular and ecological events resulted in novel virus-vector-plant-environment interactions that led to virus emergence. It invariably involved a major agricultural change: crop introduction, cultural intensification, germplasm movement, and new genotypes. A current challenge is now to complement the analysis of the causes by an assessment of the risks of emergence. Recent attempts to assess the risks of emergence of virulent virus strains are described.
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Explore the source record for details and available documents.
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Explore the source record for details and available documents.
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An Egyptian isolate of Streptomyces afghanensis was examined for the production of antiviral metabolites. Concentrated broth exudates of this microorganism were tested against tobacco mosaic virus (TMV), tomato mosaic virus (ToMV) and potato virus X (PVX) infecting Nicotiana tabacum L. cv. White Burley. Both concentrated metabolites and their acetone extract inhibited local lesion development of the tested viruses. In all cases, maximum antiviral effect was observed 2 hr after infection. The ultrafiltrate of broth culture reduced the number of local PVX lesions produced on the challenged half leaves in the case of later applications.
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The bioelectric recognition assay (BERA) is a novel biosensory method based on a unique combination of a group of cells, their immobilization in a matrix that preserves their physiological functions and the expression of the cell interaction with viruses as a change in electrical properties. A BERA sensor consists of an electroconductive, tube-like probe containing components of immobilized cells in a gel matrix. Cells are selected to specifically interact with the virus under detection. In this way, when a positive sample is added to the probe, a characteristic, 'signature-like' change in electrical potential occurs upon contact between the virus and the gel matrix. In the present study, we demonstrate that BERA can be used for the detection of viruses in humans (hepatitis C virus) and plants (tobacco and cucumber viruses) in a remarkably specific, rapid (1-2 min), reproducible and cost-efficient fashion. The sensitivity of the virus detection with BERA (0.1 ng) is equal or even better than with advanced immunological, cytological and molecular techniques, such as the reverse transcription polymerase chain reaction. Moreover, a good storability of the sensors can be achieved without affecting their performance. The potential use of portable BERA biosensors in medicine, for mass screening purposes, as well as for the detection of biological warfare agents without prior knowledge of a specific receptor-molecule interaction is discussed.
Transmission of non-persistent plant viruses is related to aphid behaviour during superficial brief probes. A widely accepted hypothesis postulates that virus acquisition occurs during ingestion of plant cell contents, and inoculation during egestion or regurgitation of previously ingested sap. Although conceptually attractive, this ingestion-egestion hypothesis has not been clearly demonstrated. Furthermore, it overlooks the anatomy of the tips of the stylets (mouthparts) and, consequently, the potential role of salivation in the inoculation process. Here, we used the electrical penetration graph (EPG) technique to investigate aphid-stylet activities associated with uptake (acquisition) and release (inoculation) of two non-persistently transmitted viruses. Our results show that acquisition occurs primarily during the last sub-phase (II-3) of intracellular stylet punctures, whereas inoculation is achieved during the first sub-phase (II-1). An alternative mechanism to the ingestion-egestion hypothesis is proposed on the basis of our findings.
The three fundamental types of precipitin patterns obtained with plant viruses by double radial immunodiffusion (DRID) tests can also be obtained by single radial immunodiffusion (SRID) tests. Using virus antigens that were serologically identical or unrelated, the reaction patterns were readily recognized. However, the precipitin reaction corresponding to the spur in DRID tests using virus antigens that were partially identical serologically was difficult to distinguish. It seems, therefore, that when virus isolates are serologically closely related, DRID should be preferred to SRID. Different viruses in a mixed infection can be successfully distinguished by SRID and also by 'rocket' immunoelectrophoresis.
The actinomycin-D(AMD)-sensitive step of replication of three plant viruses - cowpea chlorotic mottle virus (CCMV), cowpea mosaic virus, and the cowpea strain of tobacco mosaic virus - in cowpea was specific for each virus. Different combinations of the viruses were examined in which leaves were inoculated with one virus and incubated for 48 h until multiplication was almost resistant to AMD. At that time, the same leaves were inoculated with a different virus and the ability of AMD to inhibit multiplication of the second virus was determined at intervals afterwards. In all combinations tested, the first virus had no effect on the sensitivity of the second virus to AMD. Also, resistance to AMD, which appeared to spread from cell to cell in advance of the virus in mechanically inoculated leaves, did not spread appreciably faster than CCMV into systemically infected upper leaves.
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Plant viruses transmitted by invertebrate vectors either reversibly bind to vector mouthparts or are internalized by the vector and later secreted. Viral proteins mediate the binding of plant viruses to vector mouthparts and the transport of virus across vector-cell membranes. Both mechanisms probably involve conformational changes of virus proteins during their association with the vector.