Lecture 31 and 32.ppt
- 1. Virus-Definition, Nature and properties of plant virus, Common symptoms of virus and viroid diseases
- 2. Virus (Latin word) - slimy liquid, poison, venom or infectious matter Luria et al.(1978) defined viruses as ‘entities’ whose genomes are elements of nucleic acid that replicate inside living cells (host ribosomes) Viruses are sub-microscopic entities (which can pass through bacterial filters), which are infectious agents and obligate parasitic pathogens. They are nucleoproteins (nucleic acid and protein) and devoid of enzymes and depend on the hosts protein synthesizing machinery. They have only one type of nucleic acid, either DNA or RNA Viruses are considered as mesobiotic agents .
- 4. Viruses are living because They have ability to assimilate (metabolism) with the release of energy They are able to multiply They exhibit response to environment like temperature, chemicals etc., They have genetic materials like RNA or DNA They have the ability to infect Viruses are non-living because Viruses are crystallized. Viruses can be purified and concentrated from the extract of infected cells and show uniform size, shape and chemical composition even after crystallization by chemical treatments. Viruses do not have cell wall or cell membrane of any type They do not respire or excrete
- 5. Bacteria Viruses Possess cellular organization Do not possess cellular organization Grow on inanimate media Do not grow on inanimate media Multiply by binary fission Do not multiply by binary fission Posses RNA and DNA Do not posses RNA and DNA Posses ribosomes Do not posses ribosomes Sensitive to antibiotics Insensitive to antibiotics Insensitive to interferon Sensitive to interferon Differences between bacteria and viruses
- 6. MORPHOLOGY Shape and size: Plant viruses are smaller than bacteria and are seen only with electron microscope. Generally viruses are smaller than bacterial viruses and are measured in nanometers or millimicrons or millimues (1 nm = 0.001m or 1 x 10-6 mm or 1 x 10-9 m) Plant viruses have different shapes and sizes. They are: 1.Elongated - Tobacco mosaic virus (TMV) a. rod-shaped - Barely stripe mosaic virus b. flexuous threads- Potato virus x, citrus tristeza virus 2. Rhabdovirus - Potato yellow dwarf virus 3. Spherical - Tobacco necrosis satellite virus a. Polyhedral - Wound tumour virus b. Flexible - Tomato spotted wilt virus
- 7. Rod shaped - TMV
- 8. Flexuous rod – Potato virus X and Potato virus Y Bacilliform – Cocoa swollen shoot virus Geminate virus – Maize streak virus
- 9. Isometric particle (spherical) – Tobacco Necrosis Virus Rhabdovirus (bullet shaped / bacilliform) – Potato yellow dwarf virus
- 11. COMPOSITION AND STRUCTURE Two Major components - nucleic acid and protein coat or capsid. Viral nucleic acid: Cellular organism posses both DNA (chromosomal nucleic acid) and RNA (present in cytoplasm). Viruses are not cellular organisms. Nucleic acid is always present in the infective particle (virus) and serves as the genetic apparatus of the virus. It is the infective component of virus. In any virus only one type of nucleic acid and either RNA (Ribovirus) or DNA (Deoxyviruses) is present. A majority of animal and bacterial viruses (bacteriophages) are DNA viruses whereas majority of plant viruses are RNA viruses. About 80 plant viruses containing DNA have been found out. Among the plant viruses cauliflower mosaic virus, Rice Tungro Bacilliform Virus (RTBV), Banana Bunchy top viruses are DNA viruses.
- 12. TMV
- 13. VIRAL PROTEIN Virus proteins like all proteins consist of different amino acids. The proteinaceous sheath consists smaller units called capsomers. Each unit is composed of amino acids. The sequence of amino acids within a protein is dictated by the genome of genetic material, Virion or virus and nucleocapsid A complete, infective form of a virus outside a host cell, with a core of RNA /DNA and a capsid. In membrane bound viruses, the inner nucleoprotein core is called nucleocapsids.
- 14. Envelope virus
- 15. Envelope virus: Some viruses (e.g. HIV and many animal viruses) have viral envelopes covering their protective protein capsids. The envelopes are typically derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins. They may help viruses avoid the host immune system.
- 19. Virus infection Virus enters into the plant cell through wounds made mechanically or by vectors or naturally by deposition into an ovule by an infected pollen grain. Once the virus enters inside the host, it becomes naked by shedding its capsid or coat. Movement of virus inside the plants When a virus infects a plant, it moves from one cell to another and multiples in most, if not all, such cells. Viruses move from cell to cell through the plasmodesmata connecting adjacent cell. Viruses multiply in each parenchyma cell they infect.
- 20. A movement protein is a non structural protein which is encoded by some plant viruses to allow their movement from one infected cell to neighbouring cells.
- 21. There are a group of genetic RNAs, whose size is very small ranging from 240 to 375 ntds long, naked (i.e. not covered or enclosed by any capsid proteins, infectious and pathogenic agents, are called VIROIDS. There are another type of genetic RNAs, incapable of replicating by themselves and reside in other viruses and depend upon them for their replication. They can cause diseases; they are called VIRUSOIDS or SATELLITE RNAs. E.g., Eg: Barley yellow dwarf virus satellite RNA and a helper Luleovirm: Tobacco ring spot virus satellite RNA and a helper Nepovirus
- 22. Prion: There is another class of infectious agents, causing serious diseases, which are neither DNA or nor RNA, but conformationally changed proteins, which can propagate by inducing conformational changes in other proteins of the same kind and they are called PRIONS (proteinaceous infectious particle). Eg. Mad Cow diseases in animals
- 23. A conformational change is a change in the shape of a macromolecule, often induced by environmental factors Normally protein linear strand to three dimensional folding. Prion: Misfolded protein Protein before and after folding Normal Microscopic "holes" are characteristic in prion- affected tissue sections, causing the tissue to develop a "spongy" architecture.
- 24. Satellites are subviral agents that differ from Viroids because they depend on the presence of a helper virus for their propagation. Satellite viruses are particles that contain nucleic acid genomes encoding a structural protein that encapsidates the satellite genome. Eg: Tobacco necrosis satellite virus (sTNV), which has a small piece of ssRNA which codes only for a capsid protein, and depends for its replication on the presence of TNV . Satellite RNAs do not encode capsid protein, but are packaged by a protein encoded in the helper virus genome.
- 26. RNA VIRUSES Single stranded RNA e.g., TMV Lettuce necrotic yellows Tobacco ring spot virus Cucumber mosaic virus Tomato spotted wilt virus Double stranded RNA e.g., Wound tumour virus DNA VIRUSES Single stranded e.g., Maize streak virus Double stranded e.g., Cauliflower mosaic virus
- 27. Transmission of plant viruses The viruses are infectious and capable of being transmitted from diseased to susceptible healthy plants. The viruses may be transmitted in the following methods. • Transmission through seeds or seed materials. • Transmission by budding or grafting • Transmission by sap or mechanical transmission • Transmission by insects • Transmission by mites • Transmission by fungi • Transmission by nematodes • Transmission by phanerogamic parasitic
- 28. Transmission through seeds or seed materials Seed: Urdbean leaf crinkle virus, tobacco ring spot, TMV on tomato, soyabean mosaic on soyabean Seed material: Potato viruses, sugarcane mosaic, Citrus Trizteza Virus, Cassava mosaic and Banana bunchy top. Budding or grafting Most of the viruses have been transmitted either by budding or grafting. Only grafting can transmit viruses like Potato witches’ broom. Sap or mechanical transmission Generally mosaic group of viruses are readily transmitted by sap inoculation. e.g. TMV
- 29. Transmission through insects More than 80 per cent of the viral diseases are spread by different types of insects. The insect, which act as specific carriers in disseminating the diseases, are called insect vector Aphids: Aphids are the most important insect vectors of plant viruses and transmit the great majority of all stylet-borne viruses. Aphis gossypii Bean mosaic virus, cucumber mosaic virus A. maydis Sugarcane mosaic virus Pentalonia nigronervosa Banana bunchy top, Katte disease of cardamom Toxoptera citricides Citrus tristeza virus Myzus persicae PVY
- 30. Leafhoppers: Leafhoppers are phloem feeders and acquire the virus from the phloem region. All leafhopper transmitted viruses are circulatory. Several of these viruses multiply in the vector (propagative) and some persist through the moult and are transmitted through the egg stage of the vector. Rice dwarf – Nephotettix cincticeps RTV – N. virescens, N. nigropictus Nilaparvata lugens - Rice grassy stunt, Rice ragged stunt Cicadulina bipunctata, C. cinai - Maize streak, Ragi mottle streak
- 31. Plant hoppers- Maize mosaic, Sugarcane Fiji White flies Cotton leaf curl Blackgram/greengram yellow mosaic Bhendi yellow vein mosaic Cassava mosaic Chili leaf curl, Tobacco leaf curl Thrips Frankliniella schultzei Tomato spotted wilt virus Scirtothrips dorsalis Thrips tabaci Mealybugs Planococcoides sp. : Cocoa swollen shoot
- 32. Grasshoppers Potato virus X, TMV Lady bird beetle Epilachna sp. Cowpea mosaic Epicanta vitata (blister beetle) Bean pod mottle Transmission by Mites Aceria cajani PPSMV Eriophagus insidiosus Peach mosaic Transmission by fungi Olpidium brassicae – Tobacco necrosis Spongospora subterranean – Potato mob top Synchytrium endobioticum – potato virus Nematode transmission Xiphinema, Longidorus, Trichodorus - Both polyhedral & tubular viruses
- 33. Transmission by parasitic phanerogams When other methods of transmissions like grafting sap inoculation, etc. are not possible this method comes in handy. The parasitic dodder Cuscuta californica, C. subinclusa, C. campestris have been made use of. The virus passes through food material drawn by the parasite and reaches the healthy plants and infects them. e.g., Sugar beet curly top and cucumber mosaic virus.
- 34. Virus -vector relationship Persistent viruses (Circulative) Semi-persistent virues Non-persistent viruses Latent period: The period from acquisition of virus by the vector till the time when the vector becomes capable of infecting healthy plant with the virus. Incubation period: The time between penetration of a host by a pathogen and the first appearance of disease symptoms
- 35. Acquisition feeding period (Actual time) Time for which a virus free vector actually feeds on a virus plant to acquire the virus Acquisition access period (Total time) Time for which a vector is allowed to feed on a source of virus Inoculation feeding period (Actual time) Actual period of feeding to inoculate the virus Inoculation access period (Total time) Time for which vector after acquiring the virus is allowed to feed on healthy plant.
- 36. Non-persistent Viruses Virus is acquired by the insect just after feeding on the infected plant i.e. a few minutes or seconds. The virus is transmitted immediately. Insect transfers the virus from infected to a healthy by inserting its stylet. The insect rapidly (usually within 4 hrs) loses the ability to transmit the virus. Non-persistent type of transmission is only mechanical process of stylet contamination. Most of the non-persistent viruses are transmitted by aphids. No vector specificity and individual viruses are usually transmitted by several aphid species. E.g. The aphids such as Myzus persicae, Aphis craccivora, A. gossypii transmits several viruses.
- 38. S. No. Insect vector Disease 2. Aphis craccivora Bean common mosaic 3. A. gossypii Cucumber mosaic 4. ” Papaya ring spot 5. ” Soybean mosaic 6. Myzus persicae Lettuce mosaic 7. ” Potato virus Y 8. ” Turnip mosaic 9. Rhopalosiphum maidis Sugarcane mosaic Examples of non-persistent virus and vectors.
- 39. Persistent / circulative viruses A type of virus which is acquired and transmitted by the vector after relatively long feeding times and remains transmissible for a prolonged period while in association with its vector The leafhoppers and plant hoppers transmit the maximum number of circulative or persistent viruses. Circulative viruses usually show a high level of vector specificity Besides hoppers, various vector groups, particularly aphids and whiteflies transmit a large number of circulative viruses. E. g., Pea enation mosaic, lettuce necrotic yellows, bean golden mosaic
- 40. Virus Vector Group Vector species Banana bunchy top Aphid Pentalonia nigronervosa Groundnut rosette Aphid Aphis craccivora Rice grassy stunt Plant hopper Nilaparvata lugens African cassava mosaic Whitefly Bemisia tabaci Bean golden mosaic Whitefly B. tabaci Bhendi yellow vein mosaic Whitefly B. tabaci Tomato spotted wilt Thrips Thrips tabaci Examples of circulative (persistent) viruses and their vectors
- 41. Semi-persistent viruses They are intermediary between non-persistent and persistent viruses. Basically these viruses are non-persistent because they do not circulate within the vector. But their vector retains their ability to transmit the virus for 3-4 days. Like persistent viruses semi-persistent viruses are usually associated with phloem cells. These viruses also show greater vector specificity than non-persistent viruses.
- 42. Vector Virus Myzus persicae (Aphid) Beet yellows Toxoptera citricidus (leafhopper) Citrus tristeza Nephotettix virescens (LH) Rice tungro Bemisia tabaci (whitefly) Lettuce infectious yellow Example of Semi-persistent viruses and vectors.
- 43. S. No Test Non-circulative Circulative Non-persistent Semi-persistent Persistent 1 Pre-acquisition fasting Enhances chances of acquisition No such effect No such effect 2 Tissue of acquisition Epidermis Epidermis, mesophyll Mesophyll, phloem 3 Duration of acquisition access Longer the access, lesser the chance of acquisition Longer the access, better the chance of acquisition Longer the access, better the chance of acquisition 4 Latent period None None Always a latent period 5 Effect of moulting Stops transmission Stops transmission Transmission not affected 6 Retention of infectivity Minutes to hours rarely longer Hours to days Days to life long 7 Vector specificity Generally low Medium Medium to high 8 Examples Sugarcane mosaic transmitted by Rhopalosiphum maidis Citrus tristeza transmitted by Toxoptera citricidus Bunchy top of banana transmitted by Pentalonia nigronervosa
- 44. Common symptoms of virus and viroid diseases Systemic infection: The virus is present throughout the plant and the symptoms produced are called systemic symptoms. Local infection: The virus causes the formation of small, chlorotic or necrotic lesions only at the points of entry and the symptoms are called local lesions. Latent viruses: Viruses infect hosts without causing development of visible symptoms on them and the hosts are called symptomless carriers. Plants that usually develop symptoms on infection with a certain virus may remain temporarily symptomless under certain environmental conditions (e.g. high or low temperature) and such symptoms are called masked.
- 45. Symptoms………
- 46. Mosaic Intermingled patches of pale green or yellow and dark green patches on the leaves is called mosaic. e.g.Tobacco mosaic viruses Cucumber mosaic virus, Cassava mosaic virus, Pigeon pea sterility mosaic virus, Cowpea aphid-borne mosaic virus, Bean yellow mosaic virus In the pale green areas either the chlorophylls are destroyed by increased chlorophyllase activity or the development of chloroplasts is inhibited.
- 47. Tobacco mosaic Tobacco mosaic Virus (TMV) / Nicotiana Virus 1 Symptoms: Development of light and dark green pattern on the lamina. Dark green areas are usually associated with the veins, which later develop into irregular crumpled swellings /blisters due to rapid growth. Dark brown necrotic spots develop under hot weather (mosaic burn). Virus: Rod shaped ss RNA. Transmission: Sap, farm equipments and by contact.
- 48. Mosaic burn in TMV
- 52. Cowpea aphid-borne mosaic virus (CAMV) on cowpea Symptoms: Chlorosis, dark green and light green patches alternated on leaves; distortion of leaves. Vector: Aphids - Aphis gossypii, A.craccivora, Myzus persicae.
- 53. Cassava mosaic virus on cassava (Tapioca) Symptoms: Mosaic mottling, chlorosis, distortion, twisting of leaves; Stunting of plants and tuber splitting. Vector: White fly - Bemisia tabaci
- 54. Yellow mosaic of greengram and blackgram caused by Mungbean Yellow Mosaic Virus (MYMV) Symptoms: Small yellow patches or spots intermingled with green patches on the leaves initially, later entire leaf changes yellow in colour. In severe infections discolouration of pods and seeds to yellow. Vector: Whitefly - Bemisia tabaci
- 55. Sterility mosaic of pigeonpea - Pigeonpea sterility mosaic virus Symptoms: Intermingling of light green and dark green patches in the leaves, reduction in leaf size; Clustering of small leaves near the tip of the plants, shortening of internodes, stimulation of auxillary buds giving a bushy appearance. No flower and pod formation leading to sterility of affected plant. Plants remain green till harvest. Vector: Eriophyid mite - Aceria cajani
- 56. Chlorosis Chlorosis is yellowing of normally green tissue due to destruction of chlorophyll or due to failure of chlorophyll formation. e.g., Banana infectious chlorosis caused by cucumber mosaic virus. Vector: Aphids - Aphis gossypii and A. maidis
- 57. Vein clearing : e.g. Vein clearing or yellow vein mosaic of bhendi - Bhendi Yellow vein mosaic virus (BYVMV) Symptoms: Light yellow streaks along with the smaller veins, later all the veins become yellow giving yellow network of veins. Chlorosis of interveinal areas, reduction in size of leaves and small and fibrous fruits. Virus: ssDNA Vector: Whitefly - Bemisia tabaci
- 58. Vein banding Bands of green tissues seen along the veins because of chlorosis of interveinal tissues. e.g., Potato vein banding virus on tobacco. Virus: ss RNA Vector: Myzus persicae (Aphid). Yellow vein banding of Grapevine caused by Grapevine fan leaf virus
- 59. Colour breaking The colour of the petals may be altered and such flowers present shades of colour on virus infection. e.g. Tulip breaking virus. Diseased plants produce variegated flowers.
- 60. Leaf curl: Curling of leaves is prominently seen. The smooth margin of the leaves may become dentate. e.g., Tomato and tobacco leaf curl - Tobacco leaf curl virus. (TLCV) Symptoms: Leaves curled, twisted and puckered, leafy outgrowth called enations can be seen on the under surface of leaves, thickening and greening of veins in the leaf and calyx, mottling and vein clearing, stunted plant growth, inflorescence greatly condensed and complete or partial sterility. Vector: Whitefly - Bemisia tabaci
- 61. Blackgram leaf curl- Tomato Spotted Wilt Virus (TSWV) Symptoms: Lateral veins show chlorosis near the leaf margin and the lamina curl downwards slowly. Infected leaves are brittle and sometimes vein necrosis present on the under surface of the leaves, which extends up to the petiole. Virus: ss RNA Vector: Thrips tabaci, Frankliniella schultzii
- 62. Leaf roll: Leaves of virus-infected plants are folded upward giving a boat shape. e.g., Potato leaf roll virus.
- 63. Leaf crinkle: In the crinkle, the surface of leaf is not uniform and is with undulations, brittle and dark green. e.g., Urdbean leaf crinkle virus. Leaf crinkle of blackgram - urdbean leaf crinkle virus (ULCV) Symptoms: Crinkling and curling of leaves, stunted and bushy plants and malformed inflorescence with sterile flowers. Virus: ssDNA Vector: Whitefly - Bemisia tabaci
- 64. Shoe string and rat- tail : Partial or complete suppression of leaf blade development leads to shoe string or rat tail symptoms. e.g., Strains of Tobacco mosaic virus and Cucumber mosaic virus.
- 65. Enations: Small leaf like outgrowths usually seen on the veins on the lower surface of leaves. They may be cup or boat shaped also. e.g., Tomato leaf curl - Tobacco leaf curl virus, Pea enation mosaic virus.
- 66. Streak It is characterized by elongate lesions or areas of discolouration, usually of limited length on leaves with parallel venations or on stems. e.g., Banana streak virus, Maize streak virus, wheat streak mosaic virus, Tobacco streak virus. Banana streak virus
- 67. Stripe It is characterized by elongate lesions or areas of discolouration of indefinite length on leaves with parallel venations or on stems. e.g., Barley stripe mosaic virus.
- 68. Necrosis Necrosis is death of tissues, organs or whole plant. Vein necrosis: e.g., Tomato spotted wilt virus (TSWV) in Tobacco, Muskmelon, vein necrosis virus. Phloem necrosis: Phloem tissues are killed leading to accumulation of starch in the leaves. e.g., Potato leaf roll virus Top necrosis: Sudden death of top leaves. e.g., Strains of Potato virus X and Y on chillies. Bud necrosis or Bud blight: Necrosis of tissues in the terminal leaf buds. e.g., TSWV on groundnut. Stem necrosis: Necrosis in the form of streaks on the tender stems of tomato plants leading to drying. e.g., TSWV on tomato.
- 69. Bud necrosis of groundnut - Tomato spotted wilt virus Symptoms: Chlorotic spots on leaves or mottling and necrosis of terminal buds; reduction in leaflet size; distortion of lamina and shoe- string formation stunting and bushy appearance of plants. Virus: ss RNA; Vector: Thrips - Frankliniella schultzei, Thrips tabaci
- 70. Rosette Severe reduction of internode growth in a vertical axis without comparable reduction in the sides of the leaves. e.g., Groundnut rosette virus Vector: Aphids- Aphis craccivora
- 71. Ring spots Appearance of chlorotic or necrotic rings mostly on the leaves. Sometimes they appear on the fruits and stems. e.g., Tobacco ring spot virus on tomato. Papaya Ring Spot Virus
- 72. Wilt Wilting of plants due to shortage of water or due to excessive transpiration or due to impeded supply of plant nutrients. e.g., Tomato spotted wilt virus.
- 73. Bunchy top Extreme stunting of the plant with shortened internodes bunching of small, erect, brittle leaves. e.g., Bunchy top of banana - Banana bunchy top virus (BBTV) Virus: ss DNA Vector: Banana aphid- Pentalonia nigronervosa var. typica
- 74. Scorching It is found in the margins of leaves or in the entire leaves and gives burnt up appearance. e.g., Rice Tungro viruses (RTBV and RTSV)
- 75. Swelling Swelling mostly in the stem of woody plants. e.g., Cocoa swollen shoot virus.
- 76. Tumour Tumour may be found on leaves, stem or fruits. E.g. Wound tumour virus on sweet clover.
- 77. Stem pitting Shallow pits or depressions of different size on the wood of stem. e.g., Citrus tristeza virus, Apple stem pitting virus
- 78. VIROIDS Viroids are small naked, circular low molecular weight ssRNA without coat protein capable of causing diseases in plants. They are pathogenic RNAs. These 'mini viruses' are the smallest known agents of infectious diseases. They are subviral in size. The term 'viroid' was first introduced by Diener in 1971. Symptoms: Viroids produce stunting, mottling, leaf distortion and necrosis. Degenerative abnormalities have been found in the chloroplasts of viroid-infected cells. Viroid infection caused marked change in the amounts of host proteins. Transmission: Viroids are transmitted by mechanical means in most of their hosts. Transmission in the field may be by contaminated tools and by vegetative means.
- 79. Coconut Cadang-cadang It was first described in Philippines Cadang-cadang is a lethal disease of coconut characterized by yellow- bronze colouration of the lower two- third of fronds in the crown, cessation of nut production, crown diminution and death. Leaflets become brittle. The crown of the palm dies. Generally diseased palms die between 8 and 16 years. Natural transmission by mechanical damage and pollen is suspected. No vector has been identified.
- 80. Potato spindle tuber Potato spindle tuber viroid (PSTVd) is the first recognized viroid disease in plants. Symptoms: Diseased plants are less vigorous, erect, spindly and dwarf. Leaves are small, erect, and dark green. Leaflets sometimes show rolling and twisting. The tubers are elongated with a cylindrical middle and tapering ends. Transmission: PSTVd is mechanically transmitted by knives
- 81. Thank you