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Questions and Answers

Which term describes a virus that does not induce a reaction in its main host but may cause severe symptoms in others?

Cryptic virus

Latent virus (correct)

Active virus

Necrotic virus

What is the direction of food flow in systemic movement?

From the flowers to the stems

From younger leaves to mature leaves

From the roots to the leaves

From mature leaves to younger leaves (correct)

What is the term for the time between the acquisition of a virus by a vector and when the vector becomes capable of transmitting it?

Transmission period

Incubation period

Vector period

Latency period (correct)

Which of the following is considered an external symptom of viral infection in plants?

Color deviation in leaves Signup and view all the answers

How do viruses initially move within a leaf?

From cell to cell via plasmodesmata Signup and view all the answers

What term describes a virus that does not cause any visible abnormalities in the host?

Cryptic virus Signup and view all the answers

What type of movement allows viruses to spread once in systemic leaves?

Unloading from veins Signup and view all the answers

What is the primary structure that viruses utilize for long distance movement in plants?

Sieve elements Signup and view all the answers

Which type of symptom is characterized by morphological deviations that are more obvious?

External symptoms Signup and view all the answers

What is meant by hypertrophy in relation to viral symptoms?

Increase in cell size Signup and view all the answers

What cellular barriers must viruses pass through for long distance movement?

Bundle sheath, vascular parenchyma, & companion cells Signup and view all the answers

Which term refers to when symptoms are temporarily absent due to environmental factors?

Masking Signup and view all the answers

What is the main advantage of systemic movement for viruses in plants?

It allows for faster spread than cell-to-cell movement. Signup and view all the answers

What is a common inclusion body type observed in plant viruses?

Amorphous Signup and view all the answers

Which type of movement is NOT associated with how viruses spread within plants?

Surface wetting movement Signup and view all the answers

What is a characteristic of systemic movement concerning food flow?

Food flows from older tissues to younger tissues. Signup and view all the answers

Which of the following is NOT an insect vector of plant viruses?

Mites Signup and view all the answers

What is the acquisition access period in insect transmission?

Duration a virus-free vector remains in contact with a virus source Signup and view all the answers

What does the inoculation feeding period refer to?

The period a vector feeds on a virus-free plant to transmit the virus Signup and view all the answers

Which transmission method involves the direct transfer of viruses without insect involvement?

Mechanical transmission Signup and view all the answers

Which of the following is a true statement about the acquisition feed period?

It is the time needed for a vector to successfully acquire a virus Signup and view all the answers

What are inclusion bodies in the context of plant viruses?

Intracellular structures formed due to virus infection Signup and view all the answers

Which of the following is NOT a common type of color deviation observed in leaves infected by plant viruses?

Brittleness Signup and view all the answers

What kind of inclusion bodies may contain viral particles?

All of the above Signup and view all the answers

What is a common physiological change in virus-infected plants?

Delayed translocation of starch Signup and view all the answers

Which of the following external symptoms indicates viral infection in plants?

Stunting and dwarfing Signup and view all the answers

Which of the following terms describes the accumulation of starch in leaves due to viral infection?

Leaf brittleness Signup and view all the answers

What type of external symptom includes patterns like vein banding and yellow mosaic?

Color deviation in leaves Signup and view all the answers

What is one of the effects of viral infection on plant physiology?

Increased respiration Signup and view all the answers

What is a common external symptom of plant viruses?

Color deviation on stem Signup and view all the answers

Which symptom describes leaves that cannot perform photosynthesis effectively?

Yellow mosaic Signup and view all the answers

What happens to the veins in the yellow vein mosaic symptom?

The veins become yellow while the leaf remains green Signup and view all the answers

What is one characteristic of vein banding?

Presence of greener bands along the veins Signup and view all the answers

Which of the following is NOT a symptom caused by plant viruses?

Nutrient deficiency Signup and view all the answers

Which symptom indicates internal problems due to plant viruses?

Stunted growth Signup and view all the answers

What kind of color changes might be observed in fruit due to plant viruses?

Color deviation Signup and view all the answers

Which condition is associated with abnormal growth patterns in plants infected by viruses?

Malformations Signup and view all the answers

What characteristic defines persistent viruses in plant virus transmission?

They require long feeding times for transmission. Signup and view all the answers

What is the primary mode of transmission for viruses that are transmitted by insects?

Circulative transmission, which involves a long acquisition period. Signup and view all the answers

What does mechanical transmission of plant viruses imply?

Viruses are transmitted via feeding probes to phloem. Signup and view all the answers

Which type of transmission allows for viruses to multiply within their vectors?

Propagative transmission. Signup and view all the answers

In circulative transmission, what happens after the virus is acquired by the vector?

There is a latent period before it becomes transmissible. Signup and view all the answers

What is meant by the term 'transovarial transmission'?

Viruses are transferred from adults to their eggs or nymphs. Signup and view all the answers

What is a key feature of vegetative propagation in relation to plant virus transmission?

It can facilitate the spread of viruses through plant cuttings. Signup and view all the answers

What type of organisms can transmit plant viruses apart from insects?

Mites, nematodes, and fungi. Signup and view all the answers

Study Notes

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.

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