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A mustard plant infected with a certain parasite grows strangely, its development distorted by tiny invaders. Its leaves take on strange shapes, its stems form a bushy structure called a witch’s broom, and it can grow flowers that do not produce seeds. In particular, he lives longer than his uninfected brothers, in a state of perpetual adolescence.
“It looks like it’s staying in a juvenile phase,” said Saskia Hogenhout, a scientist at the John Innes Center in England, who studies the life cycle of the parasite, called Aster Yellows phytoplasma.
The neighbors of the plant age, reproduce and die, but the oddly young host of the phytoplasma persists. It becomes something like a cross between a vampire that never gets old and a zombie host whose body meets the needs of its parasite, namely, tempting the sap-sucking insects to feast on the plant’s bodily fluids for as long as possible. . When the insects ingest the parasite, they spread it to new hosts, and the entire “Night of the Living Dead-Meet-Dracula” cycle repeats itself.
How the parasite exerts such extensive control is a matter of more than casual curiosity among scientists – phytoplasmas can cause destructive diseases in crop plants like carrots. In a September article published in the journal Cell, Dr Hogenhout and his colleagues reveal that some of these frightening alterations are due to the work of a single parasite protein called SAP05, which hinders the maturation of the plant.
SAP05 is not the first substance produced by this phytoplasma that scientists have linked to the symptoms it causes. The team sequenced the parasite’s genome some time ago and identified a handful of proteins it could use to zombify its victims. But in the new document, they explain how SAP05 seems to lead to some of the more surprising effects, like longer lifespan.
It turns out that SAP05 binds to two groups of plant proteins that control the expression of genes used in development. Once he clings to them, he has them broken down by the plant’s own waste disposal machines. As a result, the plants seem frozen in time, unable to progress.
This makes sense, from a parasite’s point of view. If the host plants were to mature normally, they would grow flowers and produce seeds, putting all of their energy into creating the next generation of plants. Soon they would drop their leaves and wither.
“You can imagine that this situation is not a perfect situation for the parasite,” said Dr Hogenhout.
The parasites take advantage of the sterility of the plant, so they can concentrate their energy on the offspring of the microbe. They also take advantage of the fact that the plant remains alive and full of tasty juice for as long as possible, to better facilitate the insects that feed on it.
Oddly, however, scientists have found that SAP05 attaches itself to a very specific piece of the cell elimination machinery to achieve this goal. By tweaking the makeup of this piece, they could drastically reduce the effects of SAP05. Plants – in this case Arabidopsis thaliana, the little mustard which is a common laboratory model – with this modification did not take on the shape of a witches’ broom and did not live longer than uninfected plants.
But that didn’t mean they were better off. Plants designed to escape SAP05 had a significantly shorter lifespan when infected with the parasite. It appears that SAP05 may provide some protection against the stress of an infection, which makes it easier for the host to bear. Without it, the plant may be freer to continue maturing, but it is also more affected by the disease than zombie plants, which are more immune to the other effects of the parasite. Zombies live, protected by the organism that inhabits them.
This control is probably perfectly synchronized with the life cycle of the sap-feeding insects, Dr Hogenhout said. After the insects feed on a plant, infecting it with the parasite, they lay eggs in it. As the parasite takes over, the eggs mature.
When the young insects hatch, perhaps 10 days later, they have just enough time left in the extended lifespan of the plants for them to feast on their juice before taking flight. Along the ride will be their good friend, the phytoplasma.
“The parasite has now proliferated, just in time,” said Dr Hogenhout.
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