Aphids Transmit Plant Viruses
Viruses, but not Bacteria or Molds, are Transmitted by Plant Lice
Most disease organisms spread through the air or are carried from one host to another by vectors. Animals that bite or suck body fluids often pick up parasitic organisms, many of which cause diseases in their hosts. As consumers of plant body fluids (sap), aphids often pick up viruses from one host and spread them throughout a field of plants. Almost fifty percent of viruses are transferred by aphids, and there are many mechanisms of transfer.
Modes of Disease Transfer
Plant scientists are just now understanding some of the interections between aphids, plants, and viruses. Unlike the symbiotic bacteria that live inside aphid cells, many viruses attach to the surfaces of the aphid's cells and, in general, aphids only become infected when they feed on plants that already have the viruses. Once inside the aphid, the viruses may all be injected into the next plant (nonpersistent infection), may be injected into the next several plants (semipersistent infection), or may leave the gut of the aphid and enter the hemolymph (similar to our blood) and be able to infect new plants for several days or weeks (persistent infection).
In most types of infections, plant viruses do not reproduce inside aphids and once all the virus particles have been transmitted, the aphid is no longer able to infect new plants. But a select group of viruses reproduce both inside the aphid and the plant and can even be transferred from one aphid to its offspring. These viruses are discussed on page 2.
Although the specifics differ, the general mechanisms of transfer are varied but similar within each type of infection.
Nonpersistent Infections
Viruses that infect the epidermal cells on the surface of leaves and stems have very little time to be picked up because the aphid's stylus only remains in these cells a few seconds as it passes through on the way to deeeper phloem cells. Nonpersistent viruses attach to specific proteins on the inner surface of the aphid stylus within four to eight seconds after the stylus enters the cell. Once the aphid begins to draw sap up through its stylus, these viruses then move upward in the stylus and reattach to proteins at the base of the pump that pushes saliva out of the stylus.
When the aphid begins to feed from a new plant, it pumps saliva into the cells to prevent the plant from mobilizing its defenses against injury. Any viruses attached to the proteins in the aphid's stylus immediately release and are injected into the new epidermal cell of the same or a different plant. All of the viruses leave the aphid and the aphid can no longer infect other cells until it passes through another infected cell. These infections remain inside the aphid body for the few minutes the aphid remains feeding on the infected cell.
Some infections require two viruses to be transmitted at or near the same time in order to become active. This mechanism is more fully described on page 2.
Semipersistent Infections
Viruses that infect the deeper phloem cells of the plant are drawn into the aphid's stylus as it drinks the sap. As they feed, these viruses attach to proteins in the foregut of the aphid where they remain until the aphid moves to another location. Upon penetrating into a new phloem cell, the aphid begins to manufacture and egest (regurgitate) chemicals that prevent the new cell from blocking the aphid's stylus. At this time, some of the viruses are egested into the new cell and infect it. The virus then reproduces inside the phloem cell and these new viruses reinfect the aphid.
This type of infection of the aphid ends when the aphid molts, usually a few hours after becoming infected. Although the viruses are attached to the wall of the foregut, the lining of the foregut is shed when the aphid molts, the virus is eliminated with the molted lining, and the aphid normally moves to a new site after molting. In order to the aphid can infective again, the aphid has to feed on another infected cell.
For at least one virus, the active virus cannot become infective unless it is transmitted by an aphid. Cauliflower mosaic virus is found inside plant cells as distinctly separate particles: P2 and P3. When an aphid ingests these particles, the P2 particles coat the inner wall of the stylet, but the P3 particles are lost. Once the P2 particles coat the stylet, new P3 particles from the same or another infected cell conjugate with (attach to) the P2 particles to produce inactive units called virions. When the conjugated virions are injected into a cell, they attach to the DNA of the plant cell and become active viruses.
Persistent Infections
Some viruses can stay in aphids for several weeks. These viruses produce persistent infections that the aphid can transmit for longer periods than the nonpersistent and semipersistent infections. Click here for descriptions of the cycles of these infections.
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