Ustilago maydis attacks and reproduces in the aerial parts of the maize plant. A huge tumor-like tissue growth often forms at the site of infection. These galls can grow to the size of a child’s head. The growths are triggered by molecules released by the fungus, called effectors. They manipulate the plant’s metabolism and suppress its immune system. They also promote cell growth and division in corn. They do this by interfering with a plant signaling pathway regulated by auxin, a plant hormone.
“The fungus uses this auxin signaling pathway for its own purposes,” explains Professor Armin Djamei, who heads the department of plant pathology at the INRES Institute at the University of Bonn. “This is because the enormous tissue growth devours the energy and resources then lacking to defend against Ustilago maydis. In addition, the fungus finds an ideal supply of nutrients in the shoots and can multiply well there. The formation of the characteristic galls is therefore in the interest of the pathogen.
“So we wanted to know how the fungus promotes these proliferation processes,” says Djamei. “To do this, we searched for genetic material in the fungus allowing it to control the auxin signaling pathway of its host plant and therefore its cell growth. The complex research began seven years ago at the Gregor Mendel Institute in Vienna. Later, the culture researcher continued his work at the Leibniz Institute in Gatersleben and later at the University of Bonn.
The pathogen reprograms its host
With success: With his collaborators, he was able to identify five genes that the fungus uses to manipulate the host plant’s auxin signaling pathway. These five genes, called Tip1 to Tip5, form what is called a cluster: if we imagine the entire genome of Ustilago maydis like a thick encyclopedia, these five lies, so to speak, on successive pages.
Genes are construction manuals — the fungus needs them to produce the respective proteins. “Proteins encoded by the five Tip genes can bind to a maize plant protein known to experts as Topless,” says Dr. Janos Bindics. A former employee of the Gregor Mendel Institute, he and his colleague Dr. Mamoona Khan performed many key experiments in the study.
Topless is a central switch that suppresses very different signaling pathways in the plant. Fungal effectors produced by the five Tip genes override this repression — and do so very specifically for signaling pathways that benefit the fungus, such as the auxin-induced growth signaling pathway. On the other hand, the other signaling pathways controlled by Topless are not affected. “In a figurative sense, the fungus works with surgical precision,” Djamei points out. “It does exactly what it needs to do to best infect the corn plant. »
Perspectives for fundamental research
There are a number of pathogens that interfere with the auxin signaling pathway of the hosts they infect. Exactly how is often not fully understood. Topless may also play an important role in this process in other cultures. After all, the protein originated hundreds of millions of years ago and its central role has hardly changed since then. It therefore exists not only in maize, but in a similar form in all other terrestrial plants. For example, the researchers were able to show that the Tip effectors of Ustilago maydis also interfere with the auxin signaling pathway of other plant species.
The results could therefore help to better understand the infection processes in the main plant diseases. The results are of particular interest for basic research: “Thanks to them, it will be possible for the first time to influence specific effects of the auxin signaling pathway in a very targeted manner and thus to elucidate even more precisely the effect of these important plant hormones,” hopes Armin Djamei, who is a member of the “Sustainable Futures” transdisciplinary research space and the PhenoRob center of excellence at the University of Bonn.
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