Untitled Quiz

Plant Virology - Translocation & Distribution of Viruses

• Viruses must move to new cells and tissues to systemically infect plants.
• Viruses manipulate plant systems to move.
• Virus movement has different types:
  • Cell-to-cell movement
  • Plasmodesmata, the channels between plant cells are key to cell-to-cell transport.
  • Basic strategies of cell-to-cell movement
    • Non-tubule guided movement
    • Tubule guided movement
  • Systemic movement
    • Virus moves through the phloem.
    • Veins are plant's transport system.
    • Food in plants moves through the phloem, which is how plant viruses move systemically.

Plant Virology - Virus Movement Proteins (MPs)

• MPs are non-structural proteins.
• Enable virion movement from one infected cell to neighboring cells.
• Modify plasmodesmata to allow local & systemic spread.
• Multifunctional proteins.
• Functions include:
  • Required for movement.
  • Bind to virus genomes.
  • Interact with plant cytoskeleton.
  • Localize to plasmodesmata.
  • Gate plasmodesmata.

Plant Virology - Steps in Cell-to-Cell Movement

• Viruses leave replication sites.
• Viruses locate plasmodesmata for passage.
• Viral movement proteins (MPs) facilitate the steps.
• MPs allow the virus to overcome plant's plasmodesmata control.

Plant Virology - Genes Encoded by TMV

• TMV has open reading frames encoding proteins
• 126 kDa protein (translation)
• 183 kDa replication-associated protein
• 30 kDa and 17 kDa Movement and coat proteins.

Plant Virology - Replication & Early Steps in Movement

• Specific recognition of viral RNA.
• Viral RNA and Movement Proteins (MPs) assembly.
• Viral RNA transport to plasmodesmata.

Plant Virology - Basic Strategies of Virus Movement

• Non-tubule guided movement
  • Plasmodesmata are gated open at infection and close after, maintaining normal intracellular communication.
• Tubule guided movement
  • Plasmodesmata used for viral transport are permanently modified using Viral Movement Proteins (MPs).

Plant Virology - Non Tubule Guided Movement

• Virus movement proteins associate and coat the virus genome and cause the ribonucleoprotein complexes to move through plasmodesmata.
• TMV forms a complex between its MPs and the viral RNA to move between cells. -Targeting plasmodesmata. -Modifying the size exclusion limit. -Binding single-stranded nucleic acids. -Interacting with the cytoskeleton.

Plant Virology - Tubule Guided Movement

• Movement proteins of many plant viruses form transport tubules within pores of plasmodesmata.
• Transport of mature virus particles is facilitated.
• CPMV and TSWV use this method for movement.

Plant Virology - Systemic Movement (Long Distance)

• Viruses move through plant in systemic movement.
• Movement through sieve elements using the phloem
  • Cells in the sieve elements are connected allowing the transport of virus.
• Passive transport through the sieve elements to reach distant tissues.

Plant Virology - Virus-Host Interactions: Introduction

• Types of responses by plants to inoculation with a virus
  • Immune (non-host)
  • Infectible (host)

Plant Virology - Virus-Host Interactions: Immune Response

• Virus does not replicate in protoplasts or cells of the intact plant
• No progeny viral genomes produced

Plant Virology - Virus-Host Interactions: Infectible Response

• Virus can infect and replicate in protoplasts
• Can be resistant (extreme hypersensitivity or hypersensitivity), or susceptible

Plant Virology - Virus-Host Cell Interactions

• Involved responses such as specific host resistance (R) genes.
• Generalized host defense systems.
• The classical "gene-for-gene" concept of Flor (1971).

Plant Virology - General Steps in the Induction of Disease and Host Response

• Ability of virus to replicate in initial cell
• Ability of virus to move out of first cell
• Ability of virus to spread through various barriers
• Virus symptom development.

Plant Virology - Ability of Virus to Replicate in Initial Cell

• There is no specificity in the uncoating of virus particles in a non-host species.
• Viruses that are in a non-host plant do not support replication.

Plant Virology - Ability of Virus to Move Out of First Cell

• Virus movement from the inoculated to adjacent cells is limited by the ability of the virus to gate plasmodesmata.
• Other factors influencing cell-to-cell movement: hypersensitivity response.

Plant Virology - Systemic Necrosis

• Necrotic response can range from necrosis in a few areas of upper leaves to widespread necrosis leading to death of the plant.
• Host genotype, virus strain, and environmental conditions affect the response.

Plant Virology - Systemic Host Response

• Includes vein clearing, dark green islands, vein bending, vein flecking and color breaking
• Chlorosis, an indicator of virus infection, is a common effect of the infection.
• Symptoms are caused by virus affecting the chloroplasts either directly or indirectly.

Plant Virology - Inherent Host Response: Gene Silencing

• Viral sequence expression interferes with normal target virus functioning.
• Gene silencing is a process of interruption or suppression.
• Gene silencing describes "switching off" of genes.

Plant Virology - Post-Transcriptional Gene Silencing

• mRNA of a particular gene is destroyed or blocked, preventing protein formation.
• Plant PTGS system degrades viral RNA.
• RNAi is a common mechanism of post-transcriptional gene silencing.

Plant Virology - Small RNAs

• Small RNAs (21-24 nt) function in gene silencing.
• Small RNAs contribute to post-transcriptional gene silencing by modifying mRNA translation or stability.

Plant Virology - Core of RNA Silencing

• RNA silencing utilizes Dicers and Argonautes.
• Double-stranded RNA (dsRNA) is processed into short RNA duplexes.
• Small RNAs associate with the Argonaute family of proteins to produce silencing.

Plant Virology - Plant Recovery from Viral Infection

• Younger leaves of an infected plant can recover in symptom-free state due to resistance from the host.
• Viral resistance involves siRNA-mediated gene silencing.

Plant Virology - Transmission of Plant Viruses: Introduction

• Plant viruses must spread from one host to another in a living cell and survive.
• Viruses most often enter plants through wounds, not natural openings.
• Insect vectors are crucial to natural spread.
• Seed transmission is relatively common.

Plant Virology - Transmission of Plant Viruses: Mode of Transmission

• Vegetative propagation transmission: Viruses are passed to new plants during reproduction via tubers, corms, rhizomes etc.
• Mechanical Transmission: Introducing virus into the plant through wounds (e.g., rubbing leaves together).
• Transmission via seed, pollen, and dodder: Transmission involving seeds, pollen, and dodder.

Plant Virology - Transmission of Plant Viruses: Insect Transmission

• Insects are the primary means of virus transmission in plants;
• Insects acquire virus during feeding.
• They transmit the virus through mouthparts while feeding on plants.

Plant Virology - Transmission of Plant Viruses: Mite Transmission

• Mites transmit plant viroses.
• Mites transmit 6 plant viruses including Wheat Streak Mosaic viruses.

Plant Virology - Transmission of Plant Viruses: Nematode Transmission

• Nematodes transmit plant viruses.
• Three genera of soil inhabiting ectoparasitic nematodes transmit plant viroses: Longidorus, Paralongidorus & Xiphinema.

Plant Virology - Transmission of Plant Viruses: Fungus Transmission

• Fungal-like organisms transmit viroses.
• Examples of some fungal-like organisms include: Polymyxa sp., Spongospora sp., and Olpidium sp.
• It is estimated that these organisms transmit 30 plant viruses.

Plant Virology - Virus-Vector Relationships

• Based on virus retention time by the vector (Non-persistent, Semi-persistent, Persistent)
• Site of virus retention (Stylet-borne, Circulative, Propagative, Transovarial)

Plant Virology - Symptoms of Plant Viruses: Introduction

• Symptoms (e.g., color, leaf shape, location) can identify virus and plant.
• Symptoms indicate plant response.
• Viruses present different symptoms in different plants.

Plant Virology - Symptoms of Plant Viruses: Important Terms

• Local symptom/primary symptoms
• Systemic or secondary symptoms
• Incubation period
• Latent infection
• Latent virus
• Latency period
• Cryptic virus
• Masking

Plant Virology - Symptoms of Plant Viruses: Classification of Symptoms

• Internal symptoms: Structural and histological differences, inclusion bodies, physiological changes
• External Symptoms: Morphological characteristics of the plant itself; e.g., changes to leaf shapes, color, size

Plant Virology - Symptoms of Plant Viruses: Cytological and Histological Effects

• Hypotrophy (decreased size of cell)
• Hypoplasia (decrease in number of cells)
• Hypertrophy (increased size of cell)
• Hyperplasia (increased number of cells)
• Degeneration
• Necrosis (cell death)
• Inclusion bodies

Plant Virology - Symptoms of Plant Viruses: Inclusion Bodies

• Intracellular structures due to virus infection.
• Occur in the cell's cytoplasm, commonly found in the epidermis and leaf hairs.
• Contain virus particles, virus-related material, or degenerate conditions.
• May be amorphous, crystalline, or pinwheel.

Plant Virology - Symptoms of Plant Viruses: Physiological and Biochemical Changes

• Decreased photosynthesis
• Increased respiration
• Delayed starch translocation in infected tissues
• Accumulation of starch and sugars in leaves leading to leaf brittleness

Plant Virology - Symptoms of Plant Viruses: Types of External Symptoms

• Stunting and dwarfing
• Color deviation in leaves (Mosaic, mottling, variegation)
• Color deviation in stem, flower, fruit
• Malformations (e.g. leaf rolling, curling, distortions, narrowing, roughness, rosetting, stem pitting)

Plant Virology - Symptoms of Plant Viruses: Specific Symptom Descriptions

• Mosaic (various shades of green & yellow; irregularly angular; sharply delimited)
• Yellow mosaic (complete yellowing of leaves)
• Vein clearing (veins become light and distinct)
• Vein banding (greener bands along veins)
• Vein necrosis (veins become necrotic, no longer functional)
• Vein flecking
• Papaya ring spot (ring spot can be chlorotic or necrotic)
• Color breaking (change in color)
• Leaf roll (describing folding leaf along their mid-axis)
• Bunchy top (petioles arise from one point at the tip)
• Enation (abnormal outgrowth of vascular tissue in leaves or on the stem)
• Rugose (rough leaves)
• Leaf distortion (deviations from normal leaf shape)
• Leaf curling (irregular or non-tube-like leaf structures)
• Stem pitting (pits and grooves below the bark)
• Shoe-string of papaya (or fern leaf symptom)
• Local lesion (small necrotic areas limited to entry site)

Plant Virology - Virus Disease Symptoms in Plants

• Determined by host, virus, environment.
• Often confused with symptoms from deficiency, toxicity, herbicide injury, and other abiotic stress.
• Not dependable for correct diagnosis.