Ustilago maydis is a parasitic smut fungus that primarily affects corn plants and causes the disease corn smut. The bumps, so-called galls, are reminiscent of tumor-like tissue growths. The pathogen also rages inside the cells of the host plant: the molecules released by the fungus, so-called effectors, manipulate the metabolism, block the immune system and promote cell growth. Above all, however, they intervene in the plant’s signaling pathway, which is controlled by the plant hormone auxin. As a result, numerous development processes are disrupted.
“The fungus uses this auxin signaling pathway for its own purposes,” explains Armin Djamei, who heads the plant pathology department at the INRES Institute of the University of Bonn. “Because the massive growth of the tissue devours energy and resources, which are then lacking for the defense once morest Ustilago maydis. In addition, the fungus finds an ideal supply of nutrients in the growths and can multiply well there.”
Role of the harmful fungus in the formation of bumps investigated
A team led by Armin Djamei has therefore examined the role of the harmful fungus in the formation of the tumor-like bumps in more detail to find out how Ustilago maydis promotes these processes. “We searched for hereditary factors in the fungus that enable it to control the auxin signaling pathway of its host plant and thus its cell growth,” says Djamei. The phytohormone auxin is on almost involved in all development processes of a plant. The signal substance that is formed in the leaves ensures, among other things, that the plant grows towards the light and forms lateral roots. In addition, it can be found on a wide variety of signaling pathways within the plant and controls the corresponding processes.
Five genes manipulate auxin signaling pathway
As part of the study, the researchers were able to identify five genes that the fungus uses to manipulate the auxin signaling pathway of the host plant. These five genes Type1 bis tip5 according to the study, form a so-called cluster. “The molecules produced by the five Tip genes can bind to a protein in the maize plant known in the art as Topless,” explains Janos Bindics, co-author of the study.
Pilz acts with surgical precision
Topless is a central switching point. It suppresses various signaling pathways in the plant. This suppression is reversed by the fungal effectors produced by the five tip genes. These also include signaling pathways that benefit the fungus, such as the auxin-controlled growth signaling pathway. “The fungus acts, figuratively speaking, with surgical precision,” emphasizes Djamei. “It achieves exactly what it needs to achieve in order to be able to infect the corn plant in the best possible way.”
The researchers were also able to show that the tip effectors of Ustilago maydis also interfere with the auxin signaling pathway of other plant species. The findings might therefore contribute to a better understanding of the infection processes in important plant diseases. This knowledge is particularly relevant for basic research. “They make it possible for the first time to specifically influence certain effects of the auxin signaling pathway and thus to elucidate the effect of these important plant hormones even more precisely,” hopes Armin Djamei.
bb