EDWARD SIKORA/AUBURN UNIVERSITY, BUGWOOD.ORG
One common virus takes a sneaky route to success. It doesn’t kill its leafy hosts. Instead, it makes infected plants smell more attractive to bees. That ensures this germ will have a new generation of the plants to host it in the future.
Pathogens are disease-causing germs and other organisms. The strategy used by the virus might keep plants from developing a way to fight off this pathogen.
“It looks like the pathogen is cheating a little bit. but in a way that helps its host,” says John Carr. He’s a biologist at the University of Cambridge in England. He and his colleagues shared their data August 11 in PLOS Pathogens.
Plants normally give off a mix of scented chemicals that can waft through the air. These signal a plant’s whereabouts to pollinators, predators and other plants. Carr and his team studied tomato plants infected with the cucumber mosaic virus. These plants gave off a different scent than non-infected plants. And bumblebees preferred the sick plants’ perfume.
So the infected plants will attract more bees, which will help pollinate their flowers. That’s the good news. What’s not good: Tomatoes infected with the virus become misshapen runts.
Tomato plants don’t always need insects to pollinate them; they can pollinate themselves. But when infected tomato plants tried to fertilize themselves, this way, they made fewer seeds than normal. If bumblebees helped out with the pollination, infected plants made a healthy number of seeds.
So while the virus sickens plants, it also ensures they can still make plenty of seeds. And this helps the virus, Carr points out. The sick plants that reproduce will pass on the genes that had left them vulnerable to the virus. So their offspring will be vulnerable too. Even if some plants are resistant to the virus, they won’t take over the population, since sick plants can still multiply well.
2b or not 2b
The team found another way that cucumber mosaic virus changes plant scents. It turns off their natural defenses against disease.
For a virus to reproduce, it inserts its genes into a host’s cells. It then hijacks those cells, forcing them to copy — reproduce — the germ. Normally, plants can tell when bits of foreign genetic material get inside them. Specialized plant enzymes should snap into action and chop up the foreign invaders. But the cucumber mosaic virus doesn’t let this happen, Carr and his colleagues found. The virus makes a protein called 2b. That protein then locks onto the plant enzymes. Now they can’t do their job.
The virus now can infect a plant more easily. It also changes the way the plant turns its own genes on and off. And that’s how the virus makes plants give off a different chemical mix. When the researchers infected plants with a virus lacking the 2b protein, those plants now made the same scent as a normal, healthy tomato plant again.
The link between the virus’s 2b protein and the plants’ chemical scents is a major finding, says Andrew Stephenson. He’s a biologist at Pennsylvania State University, in Philadelphia, and was not involved in the new work. Its discovery, he says, could help scientists better understand how viruses trick their hosts.
But it will take more research to prove that the virus’s role in increased pollination really helps a plant, Stephenson argues. Even though infected plants produce a healthy number of seeds, those seeds might be smaller than normal, he says. Or they might be less likely to sprout. And the shift in fragrance might not only invite friendly bumblebees. It may also lure aphids, a sap-sucking pest that also can spread the virus to its leafy neighbors.
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aphid Small, insects with a soft, pear-shaped body. Their long slender mouthparts can pierce into stems, leaves or some other tender part of a plant. The insect then uses that "straw" to suck out sap or other nutritious fluid.
chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H 2 O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.
colleague Someone who works with another; a co-worker or team member.
defense (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)
enzymes Molecules made by living things to speed up chemical reactions.
fertilize (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
generation A group of individuals born about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet -are referred to as belonging to a particular generation of humans. The term is sometimes extended to inanimate objects, such as electronics or automobiles.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics . People who work in this field are geneticists .
germ Any one-celled microorganism, such as a bacterium, fungal species or virus particle. Some germs cause disease. Others can promote the health of higher-order organisms, including birds and mammals. The health effects of most germs, however, remain unknown.
host (in biology and medicine) The organism in which another lives. Humans may be a temporary host for food-poisoning germs or other infective agents.
infect To spread a disease from one organism to another. This usually involves introducing some sort of disease-causing germ to an individual.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
pathogen An organism that causes disease.
pollinate To transport male reproductive cells — pollen — to female parts of a flower. This allows fertilization, the first step in plant reproduction.
pollinator Something that carries pollen, a plant’s male reproductive cells, to the female parts of a flower, allowing fertilization. Many pollinators are insects such as bees.
population (in biology) A group of individuals from the same species that lives in the same area.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
resistance (as in drug resistance) The reduction in the effectiveness of a drug to cure a disease, usually a microbial infection. (as in disease resistance) The ability of an organism to fight off disease. (as in exercise) A type of rather sedentary exercise that relies on the contraction of muscles to build strength in localized tissues.
runt An organism that starts its life much smaller than usual, and may stay smaller than normal into adulthood.
virus Tiny infectious particles consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.