Transmission of Plant Viruses

Whenever plants are propagated vegetatively by budding or grafting, any viruses present in the mother plant will almost always be transmitted to the progeny
By cuttings, by bulbs, by corms, by rhizomes, by tubers, by runners or through natural root grafts of adjacent plants

Mechanical Transmission of Viruses through Sap

In nature, the direct transfer of sap through contact of one plant with another is uncommon and relatively unimportant
This method of transmission happens when the plants are wounded during cultural practices by tools, hands, or clothes, or by animals feeding on the plants
In the mechanical transmission of viruses, a virus from one kind of plant may be transmitted to dozens of unrelated herbaceous plants
However, several viruses, especially of woody plants, are difficult or impossible to transmit through the sap
Examples of plant viruses transmitted through sap: potato virus X, tobacco mosaic virus and cucumber mosaic virus

Transmitted from one plant to another through the bridge formed between two plants by twining the stem of parasitic plants, dodder (Cuscuta sp.)
Examples: sugar beet curly top virus (BCTV), cucumber mosaic virus

More than 100 viruses are transmitted by seed
The frequency of transmission varies with the host-virus combination and with the stage of growth of the mother plant when it is infected with the viruses
The viruses seem to come primarily from the ovule of infected plants
Examples: Barley stripe mosaic virus, Southern bean mosaic virus

Result in reduced fruit set, may infect the seed and the seedling
In some cases, can spread through the fertilized flower and down into the mother plant
Example: sour cherry with Prunus necrotic ringspot virus

Most common and economically most important means of transmission
Insect vectors of plant viruses:
- Order Homoptera- Aphididae (aphids), Cicadellidae (leafhoppers), Delphacidae (planthopper), Aleyrodidae (whiteflies), Coccoidae (mealybugs), Membracidae (treehoppers)
- Order Hemiptera- true bugs
- Order Thysanoptera- chewing/sucking thrips
- Order Coleoptera- beetles
- Order Orthoptera- grasshoppers
Insects with piercing and sucking mouthparts (stylet): aphids, leafhoppers, whiteflies, thrips, other groups of Homoptera, and true bugs
Insects with chewing mouthparts: beetles and grasshoppers
Examples: Grapevine virus A (transmitted by mealybugs and aphids), Turnip yellow mosaic virus (transmitted by beetles)

Mites of family Eriophyidae is known as gall mites, family Tetranychidae
These mites have piercing and sucking mouthparts
Examples: wheat streak mosaic virus, peach mosaic virus, Ryegrass mosaic virus

One or more species of three genera of soil-inhabiting, ectoparasitic nematodes
Nematodes of genera Longidorus, Paralongidorus, and Xiphinema
- transmit several polyhedral-shaped viruses (nepoviruses), such as grape fanleaf, tobacco ring spot
Nematodes of genera Trichodorus and Paratrichodorus
- transmit at least two rod-shaped tobraviruses, tobacco rattle, and pea early browning
Transmit by feeding on roots of infected plants and moving on to roots of healthy plants

Root-infecting fungal-like organisms, the plasmodiophoromycetes Polymyxa and Spongospora, and the chytridiomycete Olpidium transmit at least 30 plant viruses
Transmitted by zoospores of root-infecting fungus and fungal-like organisms
Examples of plant viruses vectored by zoospores:
- Tobacco necrosis-type viruses
  - single-stranded RNA genome
  - Viruses are carried on the surface of zoospores, not internally
  - Acquired from the soil when the virus particles adhere to the surface of zoospore
- Tobacco stunt type viruses
  - Rod-shaped viruses containing double-stranded RNA
  - Carried internally in the zoospores
  - They also enter the resting spores and survive in the soil
- Barley yellow mosaic group viruses
  - Filamentous, single-stranded RNA viruses
  - These viruses acquired by the fungus within the plant cells and carried internally in both the zoospores and the resting spores
  - With very long persistence time in the soil
- Furovirus group
  - Characteristically vectored by plasmodiophorid zoospores
  - Have a single-stranded RNA genome, which divided into two segments (RNA-1 and RNA-2) that occur in different virus particles
  - Acquired by fungus within the plant cells and carried internally in both the zoospores and resting spores
  - Example: Beet necrotic yellow vein virus

http://www.biologydiscussion.com/viruses/transmission-of-plant-viruses-8-methods-virology/64193
https://www.thoughtco.com/plant-viruses-373892
https://apsjournals.apsnet.org/doi/full/10.1094/PBIOMES-10-18-0045-R
https://www.thoughtco.com/plant-viruses-373892
https://www.annualreviews.org/doi/abs/10.1146/annurev.phyto.34.1.87
https://www.sciencedirect.com/science/article/pii/B9780444887283500160
https://www.ncbi.nlm.nih.gov/pubmed/15012536

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