How powdery mildew knocks down plant's defence system
One of the biggest problems plants contend with is powdery mildew, a disease that is triggered by various fungi species and is really easy to spot. Researchers in Germany have investigated the effects of powdery mildew on barley; not only does powdery mildew manoeuvre itself inside the cereal grain, but it can tap into the nutrients of the plant's cells. Published in the journal The Plant Cell, the findings reveal how the fungus accomplishes this feat on a molecular level, and how barley can stop the fungus in its tracks.
Researchers say plants depend on their immune system to protect them against diseases. Being able to detect pathogens early on, as well as to kick-start the immune system, especially in the cell wall, is key for ensuring the plant's survival. But pathogens are not without their arsenal either. A number of these have the capacity to suppress the natural cell wall reaction in plants.
'One particularly ingenious attacker, powdery mildew, can even reprogram cells in such a way that they adapt their architecture and metabolism to accommodate the fungus,' says Professor Ralph Hückelhoven, Chair of Phytopathology at the Technischen Universität München (TUM) in Germany. 'The plant thus actively fosters the in-growth of the harmful mildew and even supplies it with nutrients.'
Questions, however, remain concerning how the mildew controls this manipulation and which plant components are involved in the process. This is where the TUM team enters the picture.
Professor Hückelhoven and colleagues identified two proteins in barley that powdery mildew takes advantage of during the 'hostile takeover' of living plant cells. According to the researchers, the two protein substances control the plant cell's development process. In the case of barley, the proteins control the growth of root hairs. One of the proteins, RACB, is a molecular switch that reacts to signals from outside to stimulate a structural and metabolic response in the plant cells. It should be noted that it influences the expansion of the plant cell surface during the growth process. The MAGAP1 protein is RACB's counterpart, and can either restrict or prevent these activities in the cell.
In their investigation, the scientists identified how the RACB protein supported the fungus during plant in-growth. One of the roles of the protein is to expand the surface of the plant cell membranes, but this in turn exacerbates the problem, enabling the fungus to proceed with its attack. The end result is that while the plant cell stays intact, the mildew makes good on its plan for damage. Professor Hückelhoven and his team showed how the absence of the protein helps make the plant become less susceptible to powdery mildew.
'That is how the fungus benefits from this barley protein,' explains Professor Hückelhoven. 'RACB makes it easier for powdery mildew to push its haustoria, or feeding organs, into the attacked cell, to then take control of the barley cell.'
They speculate that the fungus is able to remotely control the plant's signal chain and open the door to the plant's nutrients.
But barley does not have to go down without a fight. The TUM researchers discovered that MAGAP1 can put a stop to remote attacks. Found for the most part at the cytoskeleton of the plant cell, the MAGAP1 protein is part of a strong network of protein fibres that strengthen the wall of the plant cell and in turn thwart attacks from outside. The MAGAP1 protein migrates to the cell surface membrane during an attack, and switches off RACB's susceptibility factor. So the cell surface does not expand, effectively blocking the fungus' entrance into the cell.
Commenting on how the findings will impact future work, Professor Hückelhoven says: 'With a better understanding of the cause of diseases we hope, in the mid-term, to find innovative approaches to maintaining the health of crops and grains by enhancing their immunity.'