A team of researchers led by the University of Bonn has examined the ways in which a harmful fungus – Ustilago maydis – reproduces in the aerial parts of the corn plant and renders it defenseless. According to the experts, huge tumor-like tissue growth, known as galls, form at the site of the infection after being triggered by molecules released by the fungus – called effectors – which manipulate the plant’s metabolism and suppress its immune defenses. These effectors are also promoting cell growth and division in the corn by interfering with a plant signaling pathway regulated by the hormone auxin.
“The fungus uses this auxin signaling pathway for its own purposes,” said study senior author Armin Djamei, an expert in Plant Pathology at the University of Bonn. “This is because the huge growth of the tissue devours energy and resources that are then lacking for defense against Ustilago maydis. In addition, the fungus finds an ideal supply of nutrients in the growths and can multiply well there.”
“We therefore wanted to find out how the fungus promotes these proliferation processes. To do this, we searched for genetic material in the fungus that enables it to control the auxin signaling pathway of its host plant and thus its cell growth.”
The investigation revealed a cluster of five genes – called Tip1 to Tip5 – that the fungus uses to manipulate corn’s auxin signaling pathway. “The proteins encoded by the five Tip genes can bind to a protein in the corn plant known to experts as Topless,” explained lead author Janos Bindics, a researcher at the IMBA Institute of Molecular Biotechnology in Vienna.
Topless is a central switch that suppresses various signaling pathways in the plant. The fungal effectors produced by the cluster of Tip genes override this suppression, specifically for signaling pathways that benefit the fungus, such as the auxin one. “Figuratively speaking, the fungus acts with surgical precision. It accomplishes exactly what it needs to accomplish to best infect the corn plant,” said Dr. Djamei.
Since there are various pathogens that interfere with the auxin signaling pathways of the host plants which they infect, further research is needed to better understand the infection process in different plant diseases. By using this study’s findings, “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 the effect of these important plant hormones even more precisely,” as Dr. Djamei put it.
The research is published in the journal New Phytologist.