A single protein from the parasite called SAP05 is partly responsible for the alterations, which hinder the plant's growth.
Infected with a particular parasite, mustard grows strangely, with tiny invaders warping it. Its leaves have unusual shapes, its stems form a bushy structure known as a witches' broom and its flowers may not set seed. Uniquely, it lives longer than the uninfected plants, in a state of perpetual adolescence.
Saskia Hogenhout, a scientist at the John Innes Centre in England who studies Aster Yellows phytoplasma, said the parasite appears to be in a juvenile phase.
Although the plant's neighbours grow old, reproduce, and die, the phytoplasma's host remains eerily youthful. Like a vampire that never ages, it becomes a zombie whose body serves the needs of its parasite by luring sap-sucking insects to ingest as much of its fluid as possible. When the insects ingest the parasite, they spread it to new hosts, and the whole cycle repeats, leaving the hosts carriers of the living parasites.
Scientists are intrigued by the parasite's broad control - phytoplasmas can cause destructive diseases in crops like carrots. As reported in the September issue of Cell, Hogenhout and her colleagues explain how some of these creepy alterations are caused by the parasite's protein SAP05, which hinders the growth of the plant.
SAP05 was not the first substance made by this phytoplasma that was linked to symptoms it causes. Recently, the parasite's genome was sequenced, and a handful of proteins were identified that may contribute to its zombifying abilities. However, in the new paper, they explain how SAP05 may be responsible for a few of the surprising results, such as prolonged lifespan.
Interestingly, SAP05 binds to two groups of plant proteins that control the expression of genes required for plant development. Upon latching onto them, they are broken down by the plant's garbage disposal system. Thus, the plants are frozen in time, incapable of progress.
Normal maturation of the host plants would result in flowers and seeds, keeping all their energy focused on producing new plants. Eventually, their leaves would fall off and they would wither. This isn't conducive to parasite growth.
Parasites benefit from the plant being sterile, as it can focus its energy on reproducing the microbe. The plant must be kept alive and bursting with juices as long as possible so that it may facilitate the feeding of insects on it.
Interestingly, however, SAP05 is found to attach to only a specific part of the cell disposal machinery in order to accomplish this objective. SAP05's effects could be radically curtailed by tweaking the composition of that piece. As a result of the modified plant, Arabidopsis thaliana, a common plant model in labs and gardens, did not grow into witches' broom forms and lasted the same amount of time as uninfected plants.
That doesn't mean they had a better life. The survival of SAP05-resistant plants was significantly reduced when they were infected by the parasite. It appears that SAP05 may ease the host's stress during an infection, making it easier for the host to survive. A plant without that protection may have more time to mature, but it is also more susceptible to the disease than zombie plants, which are impervious to the parasites' other effects. Zombie plants live on, protected by the organism that lives inside them.
According to Hogenhout, this control is likely to be exactly timed with the life cycle of the sap-feeding insects. The insects lay eggs after infecting the plant with the parasite after feeding on it.
There is just enough time left in the plants' extended life span for young insects to feast heartily on their juices before they take flight a few days after hatching, perhaps a week or two later making it easier for the parasites to multiply.
This article originally appeared in The New York Times.