From witch brooms, plant zombies and ingenious pathogens: Researchers have gained insights into a manipulation mechanism by which parasitic bacteria transform their host plants into optimal victims. The so-called phytoplasms release active substances that ultimately block the maturation and growth processes of the infected plants. As a result, the bacteria adapt their growth and lifespan to their needs. The scientists say that insights into the pathogens’ molecular tricks could benefit crop protection.
Some animals and microbes are notorious for their way of life at the expense of others. Some of these parasites are also known to manipulate their hosts in sometimes drastic ways in order to gain advantages for their nutrition or reproduction. By releasing special active ingredients, they change the tissue or influence the development of other characteristics. Some parasites turn their victims into “zombis” for their own purposes. As the researchers working with Saskia Hogenhout from the John Innes Center in Norwich report, this is also the case with certain bacterial plant parasites, which are of considerable importance for agriculture.
Notorious pathogens in sight
Phytoplasms are bacteria that are transmitted by biting or sucking insects and can cause damage to many plants. The economically important hosts of these pathogens include fruit trees and grapevines. The infestation leads to growth anomalies associated with reduced flowering and fruit formation. In the so-called apple shoot addiction, structures known as witches’ brooms are formed from small, bushy-looking branches. Phytoplasm infestation can also lead to the formation of leaf-like structures instead of flowers. The mechanisms behind these growth anomalies have hardly been investigated so far.
Hogenhout and her colleagues have now devoted themselves to this research topic. They carried out their investigations on the famous model of plant research: the cress wall (Arabidopsis thaliana). An enormous amount of background knowledge has already been accumulated about this inconspicuous plant and many molecular genetic research methods have been established. The researchers have now used this to find out what happens in the plant when it is infected with phytoplasms.
At first they were able to make clear: Under the spell of parasites, Arabidopsis also falls into a kind of “zombie state”, as the researchers call it. The plants develop into witches’ brooms – they form bushy outgrowths, but hardly any more generative organs. They thus serve the parasitic pathogens as an ideal habitat and source of food: Since the phytoplasms only attack vegetative parts of the plant, they keep their hosts in an artificial young state, the scientists explain. As they were able to show through their biochemical and genetic experiments, the pathogens succeed in doing this through a sophisticated manipulation molecule called SAP05.
Cleverly manipulated
This is a bacterial protein that, according to the results, can intervene in the molecular machinery of the infected plants in complex ways. Accordingly, SAP05 cancels the function of certain control molecules that are important for regulating growth and development. The victims are reprogrammed, as it were, whereby the natural maturation process is stopped and the formation of the numerous vegetative drives and tissues is triggered, the scientists explain.
Through molecular genetic experiments, they were able to show that the manipulative function of the bacterial active ingredient SAP05 depends on a certain fine structure in the targeted plant development proteins. If only two specific amino acids in these molecules are changed, SAP05 can no longer develop its effect, experiments have shown: Genetically modified Arabidopsis plants with development proteins that have been modified in this way no longer develop a witch’s broom growth. This is a possible starting point for plant protection, say the scientists: If these two amino acids could be changed in crops, a permanent resistance to phytoplasms could be achieved.
“Our results shed new light on a molecular mechanism that is behind the changes in plants caused by phytoplasm infestation. In addition, they could help to solve a major problem in agricultural production: We are showing a promising starting point for the breeding of useful plants in order to give them permanent resistance to phytoplasms, ”concludes Hogenhout.
Source: John Innes Center, technical article: Cell, doi: 10.1016 / j.cell.2021.08.029