Editing Plant virus (section)

== Transmission ==

=== Through sap ===

Viruses can be spread by direct transfer of sap by contact of a wounded plant with a healthy one. Such contact may occur during agricultural practices, as by damage caused by tools or hands, or naturally, as by an animal feeding on the plant. Generally TMV, potato viruses and cucumber mosaic viruses are transmitted via sap.

=== By insects ===

[[File:Viruses-08-00303-g001.png|thumb|upright=2|Plant virus transmission strategies in insect vectors]]

Plant viruses need to be transmitted by a [[Vector (epidemiology)|vector]], most often [[insects]] such as [[leafhoppers]]. One class of viruses, the [[Rhabdoviridae]], has been proposed to actually be insect viruses that have evolved to replicate in plants. The chosen insect vector of a plant virus will often be the determining factor in that virus's host range: it can only infect plants that the insect vector feeds upon. This was shown in part when the [[old world]] [[white fly]] made it to the United States, where it transferred many plant viruses into new hosts. Depending on the way they are transmitted, plant viruses are classified as non-persistent, semi-persistent and persistent. In non-persistent transmission, viruses become attached to the distal tip of the [[insect mouthparts|stylet]] of the insect and on the next plant it feeds on, it inoculates it with the virus.<ref>{{cite journal |last1=Gray |first1=Stewart M. |last2=Banerjee |first2=Nanditta |title=Mechanisms of Arthropod Transmission of Plant and Animal Viruses |pmid=10066833 |journal=Microbiology and Molecular Biology Reviews |volume=63 |issue=1|pages=128–148 |pmc=98959 |date=March 1999|doi=10.1128/MMBR.63.1.128-148.1999}}</ref> Semi-persistent viral transmission involves the virus entering the [[foregut]] of the insect. Those viruses that manage to pass through the gut into the [[haemolymph]] and then to the [[salivary glands]] are known as persistent. There are two sub-classes of persistent viruses: propagative and circulative. Propagative viruses are able to replicate in both the plant and the insect (and may have originally been insect viruses), whereas circulative can not. Circulative viruses are protected inside aphids by the chaperone protein [[symbionin]], produced by bacterial [[symbionts]]. Many plant viruses encode within their genome [[polypeptide]]s with domains essential for transmission by insects. In non-persistent and semi-persistent viruses, these domains are in the coat protein and another protein known as the helper component. A bridging [[hypothesis]] has been proposed to explain how these proteins aid in insect-mediated viral transmission. The helper component will bind to the specific domain of the coat protein, and then the insect mouthparts – creating a bridge. In persistent propagative viruses, such as [[tomato spotted wilt virus]] (TSWV), there is often a lipid coat surrounding the proteins that is not seen in other classes of plant viruses. In the case of TSWV, 2 viral proteins are expressed in this lipid envelope. It has been proposed that the viruses bind via these proteins and are then taken into the insect [[Cell (biology)|cell]] by receptor-mediated [[endocytosis]].

=== By nematodes ===

Soil-borne [[nematode]]s have been shown to transmit viruses. They acquire and transmit them by feeding on infected [[root]]s. Viruses can be transmitted both non-persistently and persistently, but there is no evidence of viruses being able to replicate in nematodes. The [[virions]] attach to the stylet (feeding organ) or to the gut when they feed on an infected plant and can then detach during later feeding to infect other plants. Nematodes transmit viruses such as [[tobacco ringspot virus]] and [[tobacco rattle virus]].<ref>{{cite journal |last1=Verchot-Lubicz |first1=Jeanmarie |year=2003 |title=Soilborne viruses: advances in virus movement, virus induced gene silencing, and engineered resistance |journal=Physiological and Molecular Plant Pathology |volume=62 |issue=2 |page=56 |doi=10.1016/S0885-5765(03)00040-7 |bibcode=2003PMPP...62...55V }}</ref>

=== By plasmodiophorids ===

A number of virus genera are transmitted, both persistently and non-persistently, by soil borne [[zoospore|zoosporic]] [[protozoa]]. These protozoa are not phytopathogenic themselves, but [[parasitic]]. Transmission of the virus takes place when they become associated with the plant roots. Examples include ''[[Polymyxa graminis]]'', which has been shown to transmit plant viral diseases in cereal crops<ref>{{cite journal |last1=Kanyuka |first1=Konstantin |last2=Ward |first2=Elaine |last3=Adams |first3=Michael J. |year=2003|title=Polymyxa graminis and the cereal viruses it transmits: a research challenge |journal=Molecular Plant Pathology |volume=4 |issue=5|pages=393–406 |doi=10.1046/j.1364-3703.2003.00177.x |pmid=20569399 |doi-access=free}}</ref> and ''[[Polymyxa betae]]'' which transmits [[Beet necrotic yellow vein virus]]. Plasmodiophorids also create wounds in the plant's root through which other viruses can enter.

=== On seed and pollen ===
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Plant virus transmission from generation to generation occurs in about 20% of plant viruses. When viruses are transmitted by seeds, the seed is infected in the generative cells and the virus is maintained in the germ cells and sometimes, but less often, in the seed coat.<ref>{{Cite web |date=2020-04-22 |title=4.11.1: Plant DNA Viruses |url=https://bio.libretexts.org/Courses/Northwest_University/MKBN211:_Introductory_Microbiology_(Bezuidenhout)/04:_Viruses/4.11:_9._11-_DNA_Viruses_in_Eukaryotes/4.11.01:_Plant_DNA_Viruses |access-date=2025-04-28 |website=Biology LibreTexts |language=en}}</ref> When the growth and development of plants is delayed because of situations like unfavorable weather, there is an increase in the amount of virus infections in seeds. There does not seem to be a correlation between the location of the seed on the plant and its chances of being infected. Little is known about the mechanisms involved in the transmission of plant viruses via seeds, although it is known that it is environmentally influenced and that seed transmission occurs because of a direct invasion of the embryo via the ovule or by an indirect route with an attack on the embryo mediated by infected gametes. These processes can occur concurrently or separately depending on the host plant. It is unknown how the virus is able to directly invade and cross the embryo and boundary between the parental and progeny generations in the ovule. 
Many plants species can be infected through seeds including but not limited to the families [[Leguminosae]], [[Solanaceae]], [[Compositae]], [[Rosaceae]], [[Cucurbitaceae]], [[Gramineae]]. Bean common mosaic virus is transmitted through seeds.

=== Directly from plant to humans ===

There is tenuous evidence that a virus common to peppers, the [[Pepper mild mottle virus|Pepper Mild Mottle Virus]] (PMMoV) may have moved on to infect humans.<ref>{{cite journal |last1=Aguado-García |first1=Yarenci |last2=Taboada |first2=Blanca |last3=Morán |first3=Patricia |last4=Rivera-Gutiérrez |first4=Xaira |last5=Serrano-Vázquez |first5=Angélica |last6=Iša |first6=Pavel |last7=Rojas-Velázquez |first7=Liliana |last8=Pérez-Juárez |first8=Horacio |last9=López |first9=Susana |last10=Torres |first10=Javier |last11=Ximénez |first11=Cecilia |last12=Arias |first12=Carlos F. |title=Tobamoviruses can be frequently present in the oropharynx and gut of infants during their first year of life |journal=Scientific Reports |date=12 August 2020 |volume=10 |issue=1 |page=13595 |doi=10.1038/s41598-020-70684-w|pmid=32788688 |pmc=7423923 |bibcode=2020NatSR..1013595A }}</ref> This is a rare and unlikely event as, to enter a cell and replicate, a virus must "bind to a receptor on its surface, and a plant virus would be highly unlikely to recognize a receptor on a human cell. One possibility is that the virus does not infect human cells directly. Instead, the naked viral RNA may alter the function of the cells through a mechanism similar to [[RNA interference]], in which the presence of certain RNA sequences can turn genes on and off," according to Virologist Robert Garry.<ref>{{Cite web |title=Evidence of First Virus That Moves from Plants to Humans |date=15 April 2010 |publisher=TechVert |url=http://techvert.com/science/evidence-virus-moves-from-plants-to-humans/ |url-status=dead |archive-url=https://web.archive.org/web/20100422115442/http://techvert.com/science/evidence-virus-moves-from-plants-to-humans/ |archive-date=22 April 2010}}</ref>