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Wily bacteria create ‘zombie’ plants

Plant parasites thrive by transforming hosts into ‘living dead’

By
8:16am, April 23, 2014

Not all zombies look like the walking dead, at least when it comes to plants. Normally, this Arabidopsis plant would sport a wealth of white flowers. But after being infected with bacteria, it makes only leaves and lives to do nothing but help its bacterial master.

JIC Photography

Here’s a case where the little guys win big. Some plant-infecting bacteria convert their host’s flowers into leaflike structures. Those leaves attract hungry sap-sucking insects that spread the bacteria. And the infected plant? Unable to make flowers, it cannot reproduce. It becomes a ‘zombie,’ living only to sustain its masters.

Those bacteria are parasites. They live off of another species — the plant — while offering it no help. Through its tinkering with the plant to attract insect predators, “the parasite wins,” notes biologist Saskia Hogenhout. She studies plants and insect pests at the John Innes Centre in Norwich, England. Her team published a new study about the plant parasites April 8 in the journal PLOS Biology.

The bacteria hijack the plant’s reproductive organs, Hogenhout’s team has discovered. Through chemical tampering, the germs make the flowers greener and greener. Eventually, nothing but leaves remain.

From the bacteria’s point of view, that’s a good thing. Those leaves attract the bugs needed to shuttle them to a new host. There, the parasites start a new infection.

By probing this foul play, the researchers uncovered clues about how the parasites outsmart their hosts. Using what they’ve learned, the scientists might even figure out how to make plants immune.

So why might the parasites need such devilish tactics? They can’t spread in the same way as germs that infect people. With each sneeze, we prolong a germ’s life by spewing it, perhaps into a classmate’s nose or mouth. But plants don’t sneeze. Their parasites have to leap to a new host in other ways. Some, like the bacteria in the new study, rely on help from insects.

As those bugs nibble on the plant, they also slurp up some of the parasites. Later, the bug’s saliva dribbles some of those bacteria onto new plants. The germs get to infect that new host. That’s why making plants appealing to insects — in this case, turning plants into leafy green zombies — works to boost the survival of the bacteria.

How the bacteria become hijackers

Several years ago, Hogenhout and her co-workers identified a specific protein in the bacteria that lets them transform flowering plants into ones that make only leaves. In their new study, these researchers reveal how that protein, called SAP54, pulls off the germ’s self-serving heroics.

Plants have a set of molecules that help remove cellular waste. When bacteria infect a plant, SAP54 comes in and takes over this “garbage disposal.” Instead of removing trash, the molecules degrade the plant proteins needed to make flowers, Hogenhout explains.

“It is really quite fascinating that this interaction between a bacterium, plant and insect takes place. The only apparent loser in this relationship is the plant,” Russell Groves told Science News for Students. As an entomologist, Groves studies insects at the University of Wisconsin-Madison. He was not involved with the current research. (He does, however, work with Hogenhout’s group on other projects involving plant parasites.)

Another scientist found it amazing that a single bacterial protein could control two completely different organisms. SAP54 is “multi-tasking,” says David Hughes. The protein alters the plant’s appearance. And it affects the insects’ behavior by luring them to land on a plant they would otherwise ignore. Hughes is a behavioral ecologist at Pennsylvania State University in State College. He says the protein works like “one key for two fundamentally different locks.”

This isn’t the first example of an infection changing its host’s behavior. Ants infected with some fungi, for instance, will crawl to the top of their host plants and then die. This scatters the fungus. In that instance, the insect becomes the “zombie” that is guided by its parasite.

In the current work, insects and bacteria cooperate to take advantage of the plant. Understanding how parasites mess with flower development may help researchers find new strategies that allow plants to fight these takeovers.

Power Words

bacterium (pluralbacteria)  A single-celled organism forming one of the three domains of life. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

behavior    The way a person or other organism acts towards others, or conducts itself.

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

entomologist    A scientist who specializes in studying insects.

fungus  (plural: fungi) Any of a group of unicellular or multicellular, spore-producing organisms that feed on organic matter, both living and decaying. Molds, yeast and mushrooms are all types of fungi.

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.

organ    (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary makes eggs, the brain interprets nerve signals and a plant’s roots take in nutrients and moisture.

molecule  An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

parasite     An organism that gets benefits from another species, called a host, but doesn’t provide it any benefits. Classic examples of parasites include ticks, fleas and tapeworms.

pathogen   An organism that causes disease.

proteins   Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. 

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