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Infectious diseases attack all types of complex creatures, even plants. One of these is the papaya ringspot virus (PRSV), which infects papaya trees and members of the gourd family of plants. It retards their growth and causes the fruits to develop dark rings and become unsaleable. Papaya ringspot virus moves from plant to plant carried by tiny flies called aphids.
Viruses differ in their shapes, type of genetic material and whether they have an outer membrane or not, among other features. Papaya ringspot virus uses ribonucleic acid (RNA) as its genetic material, present in a single strand. Around this RNA is a capsule shaped like a rod, without an outer membrane.
There are two main types of papaya ringspot virus, one of which attacks only gourd plants and not papaya, which is called Papaya Ringspot Virus-W (PRSV-W.) The version that affects both the papaya and the gourd family is PRSV-P. The gourd family includes vegetables and fruits like cucumber, melon and squash.
Aphids, which are little flies, carry PRSV from one plant to another. The virus does not live and grow in the aphid, but rather temporarily resides in the fly after it picks up the virus from one plant during feeding. If the fly then feeds from another plant, which involves breaking the outer skin of the plant and eating the nutrition inside, it can introduce the virus into the new plant. A typical manner of spread for a papaya farm is that the infection moves outward from the plants that are initially infected until the entire crop is affected.
Plants that are infected with PRSV grow more slowly than healthy plants, and produce less fruit. The fruit that does grow shows signs of abnormality like dark rings on the surface of the fruit and develop in odd shapes. Leaves also develop a mosaic-like appearance, and in fact papaya ringspot virus is responsible for infections like papaya mosaic and watermelon mosaic.
As gourd family plants and papaya trees are valuable crop plants, the ability to defend a farm against an infection like PRSV that could decimate the crop is an area that scientists are interested in. Nature sometimes provides options for scientists to exploit when creating new versions of crop plants, and PRSV breeding programs have resulted in strains of papaya that only show minor problems after infection. As these strains are not as profitable as other strains, genetic engineering or a form of vaccination are also options.
Genetically engineered papaya that is resistant to the virus gets its ability to grow properly from a gene that is actually part of the virus. This gene is a protein that makes up part of the coat of the virus, and when a plant contains this gene in its own cells, it is immune to the effects of the virus. Another option for papaya growers is to vaccinate the plants with a strain of PRSV that has only a small effect on the well-being of the plant, and protects from more serious damage from other strains of the virus.