Viroids and Satellites

What are viroids?

Smaller than viruses

Circular ssRNA

Can replicate without help of virus

Does not encode proteins

Has extensive internal base-pairing—makes it similar to dsRNA

Some have catalytic activity

What is the name of the structure causing catalytic activity?

Hammerhead self-cleaving structure

How can viroids be transmitted?

Mechanical

Pollen/seeds

Vegetative propagation

Insect vectors

How can viroids maintain reservoirs?

By infecting non-susceptible plants: asymptomatic infection

What kind of symptoms can viroids cause?

Dwarfing

Deformations

Discolouration

What are the two families of viroids? What are their main differences, and what is an example disease?

Pospiviroidae: replicate in nucleus, RNA does not self cleave/ Potato Spindle Tuber Viroid

Avsunviroidae: replicates in chloroplasts, self-cleaving RNA, Avocado Sunblotch Viroid

What are the two ways viroids can replicate?

Asymmetric rolling circle replication in nucleus

Symmetric rolling circle replication in chloroplast

Steps of asymmetric pathway

Circular replication: get long negative strand

Strand copied to make positive strand

Ligase used to make circle

How does the symmetric pathway work?

Get negative strand made but with Rz (hammerhead ribozyme)

tRNA ligase makes circular negative RNA

Circular replication again to get positive strand

tRNA ligase again used to make circular positive

How can viroids cause disease?

Interaction between viroid RNA and host RNA

siRNA derived from viroid likely nb as binds to and cleaves host mRNA

Difference between viroid and satellite?

Satellites depend on a virus

Features of satellite viruses

Cannot replicate/move without helper virus

Not necessary for helper virus

Not related genetically to helper virus

Difference between messenger and non-messenger satellite RNAs

Non-messenger don’t encode any proteins

Non-messenger may have self-cleaving RNA

Features of tombusvirus-like associated RNAs (tlRNAs)

+ssRNA co-infecting with Polerovirus

Encodes own RdRp

Depends on helper virus for capsid protein

What transmits Begomoviruses?

Whitefly

What satellites do begomoviruses have? What do they encode?

Alphasatellite (ssDNA). Encodes replication-associated protein

Betasatellite (ssDNA) Encodes suppressor of RNA silencing

Deltasatellite (ssDNA) Does not encode proteins

Clinical signs of cotton leaf curl disease and causative agent

Vein darkening

Vein swelling

Leaf curling

Stunting

Enations

Agent: infection of begomovirus and betasatellite

Why do betasatellites cause increased pathogenicity?

Encode a protein (betaC1) that suppressesa plant defences