THEME 11: PLANT VIRUSES & VIROIDS

Economic importance of plant viruses

Important only if they spread during the crop’s commercial lifetime

Estimated global damage by plant viruses

Approximately 6 × 10^10 USD per year

Main structural barrier to plant virus infection

Cell wall composed of cellulose

Additional plant surface barriers

Waxes and pectin layers

Use of specific cellular receptors by plant viruses

None known

Requirement for plant virus entry

Mechanical breach of the cell wall

Methods of initial plant virus entry

Vector transmission or mechanical damage

Problem of secondary infection in plants

Lack of receptors for cell-to-cell infection

Types of plant virus genomes

DNA and RNA genomes

Common morphology of plant viruses

Rod-shaped or isometric particles

Seed transmission of plant viruses

External contamination or embryo infection

Effect of seed transmission

Early focal outbreaks in crops

Vegetative propagation as transmission

Grafting spreads viruses efficiently

Biological vectors of plant viruses

Bacteria, fungi, nematodes, arthropods

Important insect vectors

Aphids, leafhoppers, planthoppers, beetles, thrips

Arachnid vectors

Mites

Role of Agrobacterium tumefaciens

Ti plasmid used experimentally to transmit viral genomes

Mechanical transmission

Rubbing virus-containing sap into leaves

Natural mechanical transmission

Soil, wind-blown dust, rain-splashed mud

Agricultural importance of insect transmission

Highly efficient long-distance spread

Propagative transmission

Virus replicates in insect vector

Example of propagative transmission

Plant rhabdoviruses

Non-propagative transmission

Vector acts as passive carrier

Vector of Begomoviruses

Whiteflies

Pathogenesis of plant virus infection

Local replication followed by systemic spread

Initial symptoms of plant virus infection

Necrotic lesions at infection site

Systemic spread routes

Cell-to-cell and vascular system

Plasmodesmata function

Intercellular communication channels

Limitation of plasmodesmata

Too small for virus particles or naked RNA

Viral movement proteins

Modify plasmodesmata to allow spread

TMV movement protein

30k protein

Function of TMV 30k protein

Transports viral RNA between cells

Movement strategy of CPMV

58/48k proteins form tubules for virion transport

Macroscopic effects of plant virus infection

Growth retardation, mosaicism, yellowing, wilting

Necrosis in plant virus infection

Cell death due to viral replication

Hypoplasia

Localized reduced growth causing mosaic patterns

Hyperplasia

Excessive cell division causing distortions

Hypersensitive response in plants

Localized necrosis to limit infection

PR proteins

Pathogenesis-related proteins

Role of phenolics and ROS

Cell wall reinforcement and antiviral defense

Phytoalexins

Antimicrobial compounds produced by plants

Role of salicylic acid

Induces systemic acquired resistance

Airborne plant signaling

Methyl salicylate

Systemic resistance in plants

Long-term resistance after local infection

Tobacco N gene

Encodes protein interfering with TMV replicase

Effect of N gene

Local necrosis instead of systemic infection

Potato RX1 gene

Resistance to potato virus X

Arabidopsis HRT gene

Resistance to turnip crinkle virus

Tomato mosaic virus transmission

Sap, grafting, budwood, seed

Main ToMV transmission route

Handling by personnel

Tomato yellow leaf curl virus vector

Bemisia tabaci

TYLCV transmission mode

Insect vector only

TYLCV preventive strategy

Vector control

Biological control of Bemisia tabaci

Eretmocerus, Orius, Amblyseius swirkii

Cucumber mosaic virus host range

Widest host range of any plant virus

CMV symptoms

Mottling, spotting, stunting, death

CMV transmission vector

Aphids

Definition of viroids

Small circular ssRNA without protein capsid

Discovery of viroids

Potato spindle tuber disease agent

Scientist who named viroids

Theodor Diener

Genome size of viroids

239–401 nucleotides

Requirement of helper virus by viroids

None

Viroid mRNA activity

Absent

Number of known viroid species

29

Viroid families

Pospiviroidae and Avsunviroidae

Replication site of Pospiviroidae

Nucleus and nucleolus

Replication site of Avsunviroidae

Chloroplast

Viroid replication mechanism

Rolling-circle replication

Systemic movement of viroids

Plasmodesmata and vascular transport

Major viroid diseases

PSTVd, CSVd, CEVd, CCCVd, ASBVd

Primary control method for viroids

Exclusion and eradication

Viroid diagnosis methods

Symptoms and molecular detection

New viroid-like entities

Obelisks

Obelisk genome type

Circular ssRNA <1 kb

Obelisk host environment

Human oral and gut microbiome

Obelisk phylogenetic status

Distinct from known biological agents