Plant viruses Flashcards
What are barriers for plant viruses?
Cellulose cell wall
Fact that plants are seasonal so may not always be around to infect
Plant immune system/resistance
Movement through the plant
How does TMV get into a plant and spread?
Gets across the cell wall due to mechanical damage. It then remains in the plant sap until it moves to another plant. It may reach another plant by plants touching each other or humans handling. Some viruses modify the appearance of the plant to make it more attractive to vectors.
What does the ssRNA for TMV look like?
5’ end is capped. tRNA like structure at the 3’ end. Encodes 4 genes; helicase, replicase, movement protein and capsid. The helicase and replicase is read as a single protein due to the leaky stop codon.
Early genes - movement proteins
Late genes - coat proteins
What happens after TMV enters the cell?
Virus associates with ER, a few capsid proteins fall off spontaneously uncovering the 5’ end. Host ribosome attaches to the RNA and as it moves along the RNA, more capsid proteins are displaced. Called co-translational disassembly.
How does TMV move from one cell to another?
The movement proteins bind to the RNA and the movement protein gates the plasmodesmata. As the complex is too large to pass through, the MP also binds peptin methyl esterase. Complex passes into the next cell and ungates the plasmodesmata behind it by phosphorylation.
30% of plant viruses are not transmitted via biological vectors. What do these methods include?
Mechanical
Seed & pollination
Vegetative Propagation
What do animal vectors include?
Mostly arthropods (insects & mites). Need to have large host range, long life span, over wintering eggs. Coleoptera dominate because they have chewing mouth parts. It's important that the adults retain the chewing mouthparts.
What are viroids and what is the closest virus to it?
Very small, covalently closed, circular RNA molecules which infect plants. They are able to cause disease but do not encode any proteins or polymerase.
Hepatitis D.
What must plant virologists consider when looking at epidemiology?
Host plant (what makes it susceptible), the pathogen/virus (amount of it), the environment (conditions favouring disease) and vector.
What is meant by ecological competence and give some examples of viruses having it
Ecological competence = the ability to hid when the main plant is not in season.
TMV is heat resistant and can survive in cigarettes.
Peanut clump virus survives in the spores of its fungal vector.
Cucumber mosaic virus can survive in lupins, courgettes, tomatoes, peppers - extremely wide host range.
Zucchini yellow mosaic virus - new plants become infected over the whole season so causes epidemics.
What is a viroid?
Small circular ssRNA molecule that infects plants and causes disease by binding to host mRNA and causing its cleavage and loss/gain of function.
Parasite of host transcription machinery.
Give three examples of viroids
Potato spindle tuber viroid was the first viroid identified, it infects tomatoes and avocados also.
Two other economically important viroids include coconut cadang cadang viroid (lethal to coconut palms) and apple scar skin viroid (visually unappealing apples).
Give three differences between viroids and viruses.
Viroids only infect plants - they get in through mechanical damage and move from cell to cell via plasmodesmata and phloem which animals do not have.
Viruses are parasites of the host translation machinery.
No known animal vector to transmit viroids from plant to plant but can be spread mechanically.
What are the two categories of viroids?
Self-cleaving. These have ribozyme activity and replicate in the chloroplasts. The cleavage of the concatamers produced by the rolling circle mechanism is carried out by the ribosome.
Non-self cleaving. These replicate in the nucleus.
Give an example of a protist virus
Mimivirus
