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Inside the Grim World of the Phytoplasmas 'Zombie' Plant Bacteria

The budding dead.
Image: University of Georgia/USDA/Forest Service

Zombie-ism for humans isn't even really science-fiction so much as it is the domain of supernatural horror. (And woe to those who dare to try and think otherwise.) But in the natural world, it's just a thing that happens, with usually pretty horrible results, e.g. fungal spores erupting from an ant's head or twitching wasp-gutted ladybugs..

A study accepted for publication in the journal Trends in Plant Science offers new insight into the zombie-enlisting mechanisms wielded by a genus of bacteria known as phytoplasmas. Insects, as they go about their usually beneficial business of gorging on plant pollen and juices, sometimes offer plants a little something extra in the form of this not-so-beneficial pathogen.

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Once infected, the plants begin forming leaf-like structures instead of the expected blossoms. The effect is that the plants lose their ability to reproduce, spawning only more bacteria. Sort of like a virus hijacks a healthy cell to do its reproductive bidding, the bacteria hijacks the entire plant.

The process is known more properly as a "homeotic mutation." "In these types of mutants certain structures are 'changed into the likeness of something else,'" the study explains. "For example, the Antennapedia mutant from Drosophila [fruit flies] develops legs instead of antennae on its head. Likewise, the sepallata (sep) mutants from Arabidopsis [a sort of cabbage] develop a bunch of sepals or leaves instead of different floral organs as parts of flowers."

In both of the above examples the changes are effected through genetic reprogramming. Here things are a bit different. The zombie-making bacteria interferes with the developmental processes of the target host. This is accomplished through the secretion of what are known as effector proteins, molecules capable of doing all kinds of stuff in different situations: regulating enzyme activity, gene expression, and-or cell signalling. It's a reasonable way of taking command of an organism.

The phytoplasmas bacteria's ability in this case to force the plant to stop producing flowers and instead start producing weird leaves—a symptom called phyllody (below)—is among the more striking and consequential examples of effector reprogramming.

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"The alterations in floral structure are so dramatic that infected plants are often sterile," the study continues. "Several phytoplasma strains possess a wide plant host range and are transmitted by leafhoppers, planthoppers, and related insects that suck phloem sap of infected plants. An infection can thus spread rapidly in a plant population, with devastating consequences for some economically important crops."

As the authors note, there's still a lot to learn about the phytoplasmas bacteria and its effects on a wide range of plants. Phyllody is only a part of the zombie-ism picture, and other effects include dwarfism and witch's broom, a plant disease symptom in which the affected plant, usually a tree, starts sending up new shoots in dense bunches. The result tends to look like the namesake broom or a bird's nest.

Researchers have also yet to fully characterize how the bacteria influences the insects it uses to spread among hosts.

"Clearly, we are only beginning to explore the many facets of phytoplasma biology," the paper concludes. "The most exciting years in 'zombie research' might be still ahead of us."