Plant Pathology - Viruses

Obligate parasite

Need a living host - don’t want to kill their host

What actually are viruses?

Acellular, infectious, non-organism entities

Are viruses alive?

No

Can viruses reproduce on their own?

No, they require host cell machinery

What types of DNA do viruses have?

DNA or RNA

What DNA type do most plant pathogenic viruses have?

RNA

Virion

Individual particle inside of its own nucleic package, outside of a cell

What are virus strains?

Variants of the same species

About how many plant pathogenic virus species are there?

~1,000

Can plants create antibodies to defend themselves against viruses?

No (unlike animals), they lack specific defense systems

What is the best management strategy for viruses?

Prevention. There are no cures once infection happens.

How many types of genomic variations are there in viruses?

3

• Monopartite: genome on 1 single piece of nucleic acid

• Multipartite: genome divided amongst several pieces of nucleic acid

• Multiparticulate: genome segments are packaged in several different particles

How does a muliparticulate virus function?

All particles containing each genome segment need to be physically together to survive.

What is the first stage of a plant virus life cycle?

Entering the plant through a vector

How does a virus enter a plant?

Through a vector - a wound, entry, opening, insect, etc…

Uncoating

The 2nd stage of life cycle. The particle sheds its “coat” to expose the genome for expression.

What is the 3rd stage?

After uncoating, the viral genome instructions are translated and decoded.

Genome replication

The 4th stage after translation - when the virus makes copies of itself. The instructions for replication are within the viral genome.

How do viruses move?

Cell to cell, either local or systemic throughout the plant

How are viruses transmitted to new hosts?

The viral load becomes systemic, increases, and is present in plant tissue for a vector to pick up/move around.

Virion

The “seed”

An extracellular form of the virus

It is a fully formed virus particle that exists outside of the host and is ready to infect a new cell

How is the virion different from the virus?

The virus is the “living plant” - what “grows” from the virion

This refers to the entire life cycle of the virus itself

Including its extracellular (virion) stage, and the intracellular stage

What are the 3 physical components that make up a virion?

1. Nucleic acid: holds DNA or RNA

2. Capsid: coat protein that protects genomic nucleic acid

3. Envelope: sometimes present to enclose entire particle

What are the types of DNA held within the nucleic acid?

DNA - can be single or double stranded

RNA - most common in plant viruses

• Single stranded: positive OR negative sense

• Double stranded

What are the 2 RNA senses in viral genomes?

Positive Sense:

• Shorthand: +strand

• Same orientation as mRNA (messenger RNA)

• Ready to be translated immediately

• Instructions for proteins available

• Does not need RNA polymerase to be replicated

• “Ready-to-read book”

Negative Sense:

• Shorthand: -strand

• Complementary to mRNA

• Must be copied into a +strand by RNA-dependent RNA polymerase (RdRp)

• An extra step before instructions for proteins are available

• “Book written in code”

How are +strand RNA translated?

After uncoating, the +strand RNA is released in the host cell cytoplasm, where ribosomes translate RNA into viral proteins.

What can newly produced +RNA do?

• Be translated to produce additional viral proteins

• Be enclosed in coat protein (capsid) to produce new virion particles

Sources of Genetic variability in RNA viruses

• Mutations that happen in translation process

• Lacks proofreading

How does genetic variability increase virulence?

Enables rapid adaptation to conditions or management strategies

Can shift in host range significantly, allowing greater spread

Resistance breakdown

Escapes plant defenses

What are the 2 types of viral morphology?

Helical

• rod shape

• most common

• ~2,130 identical coat protein subunits merge

• Creates structure around single-stranded +RNA (+strand)

Icosahedral

• Cubic in shape, symmetrical

• 180 coat protein subunits (smaller number)

• Single-stranded +RNA (+strand)

What is an example of a helical and icosahedral plant virus?

Helical: Tobacco Mosaic Virus (TMV)

Icosahedral: Tomato Bushy Stunt Virus (TBSV)

Are plant virus symptoms usually localized or systemic?

Systemic - throughout the whole plant

What are some symptoms of viruses in plants?

Stunting, dwarfing, discolorations, mosaics, streaks/stripes, malformations

Why do plant pathogenic viruses result in abnormal growth?

Because viruses use up the host plant’s resources, and interfere with the plant’s normal gene expression. This can impact the plant’s hormone production too.

Do viruses live mostly in the xylem or phloem of plants?

The phloem, because they are obligate parasites and need living plant cells to survive. The phloem transports sugar and contains living cells.

What is phloem necrosis?

Cell death in the phloem occurs as viruses use plants cells for reproduction. This inhibits the flow of photosynthates throughout the plant and causes necrosis.

How do viruses move systemically throughout a plant?

Through infecting the phloem and being transported by the plant’s vascular system.

What are the 2 forms viruses can move as?

Complete virions, with RNA fully encapsulated

Ribonucleoprotein complexes (RNPs)

What are Ribonucleoprotein complexes (RNPs)?

RNPs are incomplete virions. They are viral RNA bound to a protein without forming a particle. Nucleic acid + protein complex.

What does a coat protein do?

Enables movement of virions

Stabilizes viral RNA

Movement through sieve elements within a host

Needed for systemic movement of virions

What are movement proteins?

Proteins that bind to viral genome and enable cell-to-cell movement of virions. They help viruses pass through the plasmodesmata of a plant by increasing the size threshold, surpassing plant defenses.

Cell to cell movement is dependent on what type of protein?

Movement protein

Systemic movement is dependent on what type of protein?

Coat protein

What is programmed cell death?

A plant defense, where localized cells are killed off to prevent the movement of pathogens throughout the rest of the plant.

What are the 3 types of vector transmission relationships?

1. Non-persistent transmission

2. Persistent, circulative transmission

3. Persistent, propagative transmission

Explain non-persistent transmission

The vector holds viral load on infested/contaminated mouthparts. When probing a leaf with their stylet, they quickly deposit viral load into plant. Happens in minutes. The virus is not retained in the body of the vector.

Explain persistent, circulative transmission

Happens in a longer period of feeding - hours. The vector feeds on an infected plant for a long time, and retains the virus inside of its body. The virus does not replicated in the body, just is stored, to be deposited in other plants. Replication happens in the phloem of a plant host, not the animal.

Explain persistent, propagative transmission

Virus replication does occur in the vector host cells, and is retained for life. This only happens when a vector is feeding on the infected plant for a sustained amount of time. There is a longer latent period between transmission and infection.

What are the 5 tests to diagnose virus disease?

1. Koch’s Postulates

2. Host range tests using an indicator plant

3. Leaf dip electron microscopy

4. ELISA

5. PCR

What is a viroid, and how is it different from a virus or virion?

Viroids are simply single stranded RNA that lack coat proteins. Naked RNA! Replicate inside plant cells.

How and by whom were viruses first able to be seen under a microscope?

1935 - Wendell Stanley - crystallize viral particles through purification. Put them together as “legos” to be seen under a microscope