Witch’s brooms are deformities that affect some trees in which a very dense mass of shoots arises from a single point, and the resulting structure resembles a broom, hence its name. This is a classic example of how some parasitic organisms are able to manipulate host organisms at will to satisfy their needs. When under the spell of a parasite, some plants undergo changes so extensive that they are described as ‘zombies’, that they stop reproducing and serve only as habitats and hosts for parasitic pathogens.
Until now, little information was available to understand the molecular and genetic mechanisms that regulate these spells. Now, an article published in the scientific journal Cell has identified a molecule produced by the bacterium Phytoplasma , which causes the witch’s broom disease, which is involved in hijacking the development of the host plant. This protein causes key growth regulators to break down, leading to these deformities.
The bushy outgrowths seen on the trees indicate that the plant is trapped in a vegetative zombie state, and is unable to reproduce, thus returning to a state of ‘eternal youth’. “Phytoplasmas are a spectacular example of how the reach of genes can extend beyond organisms to impact surrounding environments,” explains Saskia Hogenhout, researcher at the John Innes Center and author of the work.
The new findings show how the bacterial protein known as SAP05 manipulates plants by taking advantage of part of the host’s own molecular machinery.
This machinery, called a proteasome, generally breaks down proteins that are no longer needed within plant cells. SAP05 hijacks this process, causing plant proteins that are important for regulating growth and development to be disposed of to a molecular recycling center. Without these proteins, the development of the plant is reprogrammed to favor bacteria, which triggers the growth of multiple shoots and vegetative tissues and stops the aging of the plant.
Through genetic and biochemical experiments on the model plant Arabidopsis thaliana, the team discovered in detail the role of SAP05. Interestingly, SAP05 binds directly to both plant growth proteins and the proteasome. Typically, proteins that are degraded by the proteasome are tagged with a molecule called ubiquitin beforehand, but this would not be the case.
The plant developmental proteins that SAP05 targets are similar to proteins also found in animals. The team was curious to see if SAP05 therefore also affects insects that carry the bacteria from one plant to another. They found that the structure of these host proteins in animals differs enough that they do not interact with SAP05 and therefore do not affect insects.
However, this research allowed the team to identify the only two amino acids in the proteasome unit that are needed to interact with SAP05. Their research showed that if plant proteins are modified to have the two amino acids found in insect protein, SAP05 no longer breaks them down, preventing abnormal growth of the “witch’s broom.”
