Plant viruses

Introduction, diseases and types

1. Plant Viruses

2. Introduction Viruses Infectious pathogens Too small to be seen with a light microscope The simplest viruses are composed of A small piece of nucleic acid S urrounded by a protein coat Obligate parasites that depend on the cellular machinery of their hosts Not active outside of their hosts Organisms including animals, plants, fungi, and bacteria are hosts for viruses, but most viruses infect only one type of host

3. Plant viruses Plants have specific viruses Viruses cause many plant diseases They are responsible for losses in crop yield and quality Viroids are infectious RNAs that cause important plant diseases These pathogens are similar to some plant viruses in that they contain an RNA genome T hey differ from RNA plant viruses that they are composed of naked RNAs and lack a protein coat Viroids do not produce any proteins when they infect a plant cell despite the fact that they are made of RNA

4. Plant Viruses Obligate intracellular parasites Do not have the molecular machinery to replicate without a host Pathogenic to higher plants Plant viruses infect plants A virus particle, also known as a virion is an extremely small infectious agent Essentially a nucleic acid (DNA or RNA) enclosed in a protein coat called a capsid

5. Plant Viruses Viral genetic material can be Double-stranded DNA Double-stranded RNA Single-stranded DNA or Single-stranded RNA Most plant viruses are classified as single-stranded RNA or double-stranded RNA virus particles Cause various types of plant diseases These diseases do not typically result in plant death

6. Plant Viruses Plant diseases produce symptoms such as Ring spots Mosaic pattern development Leaf yellowing and distortion Deformed growth Some plant viruses are not limited to one particular plant host May infect different varieties of plants Plants including tomatoes, peppers, cucumbers, and tobacco may all be infected by the tobacco mosaic virus

7. Examples of Plant diseases Tobacco Mosaic Virus The first plant virus discovered Attacks members of the nightshade, or S olanaceae, family including tobacco, pepper, potato, tomato, eggplant, cucumber and petunia Spreads through entry into breaks of cell walls caused by insects or other physical damage

8. Barley Yellow Dwarf Infects several grains and staple crops, including wheat Aphids primarily spread the virus Causes discoloration of leaves and the tips of the plants Reduces photosynthesis, stunts growth and decreases production of seed grains

9. Bud Blight Infects soybeans, a staple crop Causes the stem to bend at the top and the buds to turn brown and drop off the plant Nematodes spread this virus

10. Sugarcane Mosaic Virus Discolors leaves of the sugarcane plant It thus restricts its ability to feed itself through photosynthesis and grow Stunts the growth of young plants. Aphids and infected seed spread the virus

11. Peanut Stunt Virus Causes discoloration and distortion of the leaves of peanuts and some other rhizomes, stunting their growth Aphids and sap spread the virus

12. Maize Mosaic Virus Causes yellow spots and stripes on the leaves of corn, stunting its growth Leafhoppers spread the virus

13. Cauliflower Mosaic Virus The cauliflower mosaic virus infects members of the B rassica family Members includes cabbage, Brussels sprouts, Cauliflower, Broccoli and Rape seed It causes a mosaic or mottle on the leaves which stunts growth Aphids and mechanical exposure spread the virus

14. Lettuce Mosaic Virus Mottles the leaves of almost all types of lettuce, stunting its growth and eliminating its market appeal Aphids and infected seeds spread the virus

15. Cucumber Mosaic Virus Infects cucumber, tomato, peppers, melons, squash, spinach, celery, beet and other plants Aphids spread it, and they cause physical damage to the plant A llows entry of the virus via wind, splashing or dripping sap Causes twisting in young leaves that stunts growth of the entire plant and causes poor fruit or leaf production

16. Top 10 plant virus for Molecular Plant Pathology Tobacco mosaic virus Tomato spotted wilt virus Tomato yellow leaf curl virus Cucumber mosaic virus Potato virus Y Cauliflower mosaic virus African cassava mosaic virus Plum pox virus Brome mosaic virus Potato virus X Including Citrus tristeza virus, Barley yellow dwarf virus, Potato leafroll virus and Tomato bushy stunt virus

17. Transmission Some of the same types of viruses that infect humans can also infect plants Plants and humans do not transmit viruses to each other Humans can spread plant viruses through physical contact Viruses also spread through infected seeds, grafting, wind, splashing, pollination and dripping sap Unlike humans, plants never recover from a virus

18. Transmission Plant cells are eukaryotic cells that are similar to animal cells Plant have a cell wall that is nearly impossible for viruses to breach in order to cause infection Plant viruses are typically spread by two common mechanisms Horizontal transmission Vertical transmission

19. Horizontal Transmission The plant virus is transmitted as a result of an external source In order to invade the plant, the virus must penetrate the plant's outer protective layer Horizontal transmission also occurs by certain artificial methods of vegetative reproduction typically employed by horticulturists and farmers Plant cutting and grafting are common modes by which plant viruses may be transmitted

20. Vertical Transmission The virus is inherited from a parent This type of transmission occurs in both asexual and sexual reproduction In asexual reproductive methods such as vegetative propagation, the offspring develop from and are genetically identical to a single plant In sexual reproduction, viral transmission occurs as a result of seed infection In most cases it is unable to find cures for plant viruses, so the main focus is on reducing the occurrence and transmission of the viruses

21. Transmission There are a number of routes by which plant viruses may be transmitted: Seeds : These may transmit virus infection either due to external contamination of the seed with virus particles, or due to infection of the living tissues of the embryo. Vegetative propagation/grafting: These techniques are cheap and easy methods of plant propagation, but provide the ideal opportunity for viruses to spread to new plants. Vectors: Many different groups of living organisms can act as vectors and spread viruses from one plant to another

22. Transmission Bacteria (e.g. Agrobacterium tumefaciens - the Ti plasmid of this organism has been used experimentally to transmit virus genomes between plants) Fungi Nematodes Arthropods: Insects - aphids, leafhoppers, plant hoppers, beetles, thrips Arachnids – mites Mechanical: Mechanical transmission of viruses is usually achieved by rubbing virus-containing preparations into the leaves In plant species are particularly susceptible to infection This is also an important natural method of transmission Virus particles may contaminate soil for long periods and may be transmitted to the leaves of new host plants as wind-blown dust or as rain-splashed mud

23. Transmission by insects This method is of particular agricultural importance Extensive areas of monoculture and the inappropriate use of pesticides which kill natural predators can result in massive population booms of insects such as aphids Plant viruses rely on a mechanical breach of the integrity of a cell wall to directly introduce a virus particle into a cell This is achieved either by the vector associated with transmission of the virus or simply by mechanical damage to cells

24. Transmission by insects Transfer by insect vectors is a particularly efficient means of virus transmission Non propagative transmission: Insects which bite or suck plant tissues are the ideal means of transmitting viruses to new hosts. This is known as non-propagative transmission Propagative transmission: The virus may also infect and multiply in the tissues of the insect as well as those of host plants called propagative transmission for example many plant rhabdoviruses

25. MODE OF replication P lant viruses are obligate, biotrophic parasites T heir life cycles start by penetration of the virion into the cell Plant viruses are unable to penetrate the plant cuticle and cell wall T he virion enters the cytoplasm of the cell passively through wounds caused by mechanical damage to the cuticle and cell wall The next phase of virus infection is the partial or complete removal of the coat protein shell of the virion in the cytoplasm

26. MODE OF REPLICATION Next the cell mediates expression of the viral genome B y providing a transcription apparatus (for DNA viruses) and a translation apparatus (for all viruses) The DNA viruses must be transported to the nucleus for transcription T o gain access to the cell proteins required for the production of messenger RNA from viral DNA Translation of viral RNA in the cytoplasm produces viral proteins that are required for completion of the virus life cycle

27. MODE OF Replication All viruses must direct the formation of at least three types of proteins: R eplication proteins that are essential for nucleic acid production S tructural proteins that form the protein shell and other components contained in the virions M ovement proteins that mediate virus transport between plant cells

28. MODE OF Replication The viral replication proteins combine with cellular proteins P roduce a complex of proteins that manufactures multiple copies of the virus genome These newly made genomes interact with the structural proteins to form new virions

29. problems The outer surfaces of plants are composed of protective layers of waxes and pectin M ore significantly each cell is surrounded by a thick wall of cellulose overlying the cytoplasmic membrane

30. problems To date no plant virus is known to use a specific cellular receptor of the type that animal and bacterial viruses use to attach to cells P lant viruses rely on a mechanical breach of the integrity of a cell wall to directly introduce a virus particle into a cell This is achieved either by the vector associated with transmission of the virus or simply by mechanical damage to cells After replication in an initial cell, the lack of receptors poses special problems for plant viruses in recruiting new cells to the infection

31. Multipartite Plant Viruses Segmented virus genomes are those which are divided into two or more physically separate molecules of nucleic acid, all of which are then packaged into a single virus particle Although multipartite genomes also segmented, each genome segment is packaged into a separate virus particle These discrete particles are structurally similar and may contain the same component proteins but often differ in size Genome segmentation reduces the probability of breakages due to shearing, thus increasing the total potential coding capacity of the genome

32. Multipartite Plant Viruses The disadvantage of this is that all the individual genome segments must be packaged into each virus particle, or the virus will be defective as a result of loss of genetic information All the discrete virus particles must be taken up by a single host cell to establish a productive infection. This is perhaps the reason multipartite viruses are only found in plants Many of the sources of infection by plant viruses, such as inoculation by sap-sucking insects or after physical damage to tissues, result in a large inoculum of infectious virus particles Providing opportunities for infection of an initial cell by more than one particle

33. Multipartite Plant Viruses Family: Segments: Geminivirus (group III) (single-stranded DNA) Bipartite Comovirus (single-stranded RNA) Bipartite Furovirus (single-stranded RNA) Bipartite Tobravirus (single-stranded RNA) Bipartite Partitiviridae (double-stranded RNA) Bipartite Bromoviridae (single-stranded RNA) Tripartite Hordeivirus (single-stranded RNA) Tripartite

34. Pathogenesis Initially, most plant viruses multiply at the site of infection, giving rise to localized symptoms such as necrotic spots on the leaves The virus may be distributed to all parts of the plant either by direct cell-to-cell spread or by the vascular system, resulting in a systemic infection involving the whole plant Plant cell walls necessarily contain channels called plasmodesmata which allow plant cells to communicate with each other and to pass metabolites between them T hese channels are too small to allow the passage of virus particles or genomic nucleic acids Many plant viruses have evolved specialized movement proteins which modify the plasmodesmata

35. Pathogenesis Typically, virus infections of plants might result in effects such as growth retardation, distortion, mosaic patterning on the leaves, yellowing, wilting, etc. These macroscopic symptoms result from : N ecrosis of cells is caused by direct damage due to virus replication Hypoplasia is localized retarded growth frequently leading to mosaicism (the appearance of thinner, yellow areas on the leaves) Hyperplasia is excessive cell division or the growth of abnormally large cells, resulting in the production of swollen or distorted areas of the plant

36. Plant-Virus Interactions Viruses are capable of infecting virtually all species of cultivated and wild plants. However, host ranges of individual viruses vary from very narrow to very broad For example Citrus tristeza virus infects only a few species in the Citrus genus, whereas Cucumber mosaic virus infects over 1000 species in 85 plant families Susceptibility or resistance of plant species and cultivars to viruses is determined primarily by the plant genotype Plants possess active and passive means of preventing virus infection. Passive defenses are due to the failure of the plant to produce one or more host factors required for virus reproduction and spread within the host

37. Plant-Virus Interactions Active defenses include detection and destruction of the virus-infected cells due to the function of specific resistance genes in the plant In addition, plants possess a general defense system that is analogous to the animal immune system. The major difference between the two is that the immune system in animals targets a pathogen’s proteins, whereas the plant defense system, which is called RNA silencing, detects and degrades viral RNAs Depending on the particular combination of virus and host, and on environmental conditions, a plant’s response to infection may range from a symptomless condition to severe disease and plant death

38. Diagnosis of Plant Virus Diseases Different viruses may elicit similar symptoms T he disease phenotype can provide only limited, although important, information for disease diagnosis More specific and reliable methods of virus identification are based on various properties of the virus These properties and corresponding approaches include the following : 1.Pathogenicity Two major types of responses are : local lesions which are confined to inoculated leaves (local lesion hosts ) systemic infections which produce symptoms on leaves distant from the inoculation site (systemic hosts )

39. Diagnosis of Plant Virus Diseases 2.Transmissibility Due to vector specificity, identification of the organism that transmits the virus may provide important information for virus identification 3.Architecture of virus particles By using electron microscopy the shape and size of virions can be distinguished that is they are rod-shaped, filamentous, icosahedral, or large enveloped particles 4.Presence of virus-specific structures in infected cells Due to their intimate association with components of the cell, viruses often form unusual structures within plant cells as a result of virus infection For example virus-specific inclusions have been characterized for a number of plant virus families and genera, and the detection of these inclusions indicates the presence of a virus within that group

40. Diagnosis of Plant Virus Diseases 5.Properties of the protein coat These tests rely on identification of a virus (the antigen) through its reaction with specific antibodies One of the most widely used diagnostic tests for plant viruses is an antibody-based procedure called the Enzyme-Linked Immunosorbent Assay (ELISA )

41. Management of Plant Virus Diseases Although there are virtually no antiviral compounds available to cure plants with viral diseases, efficient control measures can greatly mitigate or prevent disease from occurring Virus identification is a mandatory first step in the management of a disease caused by a virus The strategy for management will depend on the means by which a particular virus enters a crop, how the virus is transmitted between plants within a crop, and how the virus survives when the crop is not being grown Preventative measures may include use of certified virus-free seed or vegetative stocks, elimination of the virus reservoirs in the surrounding wild vegetation, and modification of planting and harvesting practices If the virus is known to be transmitted by a particular vector, control or avoidance of this vector is important. For instance, insect, nematode or fungal vectors can be controlled by insecticides, nematicides, or fungicides, respectively

42. Conclusion Plant viruses and viroids are diverse and unusual groups of plant pathogens that infect and cause disease in many crop plants T hese pathogens depend on the normal cellular machinery of their plant host for reproduction, it is difficult to eliminate them without damaging the host plant Therefore, most management strategies for diseases caused by plant viruses and viroids are directed at preventing infection of the plant