Rapeseed and the enemy in the soil

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Growths on rapeseed roots due to infestation with Plasmodiophora brassicae Photo
Growths on rapeseed roots due to infestation with Plasmodiophora brassicae Photo: Andreas von Tiedemann
Research group led by Göttingen University identifies importance of bacteria for germination of disease-causing fungus

Crops have a high nutritional value, which makes them essential to people. However, this also makes them attractive to harmful microorganisms, such as bacteria or fungi. In fact, most pathogens can only attack specific plants and there has been extensive research to try to discover how fungi locate their host plants. Until now, it was thought that host plants stimulate fungal spores in soil to germinate by releasing specific sugars, amino acids and other compounds from their roots - known as -root exudates- - thereby unwittingly triggering their own infestation. Researchers at the University of Göttingen have now shown that this is not the primary mechanism for triggering spore germination that leads to root infection. Instead, the activity of the spores is influenced by a combination of different abiotic factors and particular soil bacteria communities. The results were published in PLOS Pathogens.

Researchers from the Plant Pathology and Crop Protection Division investigated whether root exudates from host and non-host plants stimulate dormant spores of the common fungus Plasmodiophora brassicae to germinate under sterile conditions. "In nature, the fungus infects the roots of oilseed rape plants making them swell and become misshapen," explains Yao Wang, first author of the study. "The disease is known as clubroot and it infects cabbage, broccoli and other brassica crops. It occurs in almost all rapeseed-growing areas around the world, causes significant damage to crops and cannot be controlled directly."

The researchers found that sterile root exudates did not stimulate clubroot spores to germinate, and there was no difference between host and non-host plants. Experiments in the laboratory showed that the presence of soil bacteria as well as nitrate and certain sugars and amino acids is essential for triggering germination. The composition of the bacterial microbiome changed when nitrate and a suitable carbon source were available at a certain soil moisture: at this point, the proportion of stimulating bacterial groups increased and this triggered spore germination. It is still unclear which bacterial groups are required and the exact mechanism which ultimately breaks spore dormancy.

"Understanding the factors and mechanisms that control dormancy and germination of such resilient spores in the soil is an important step towards innovative strategies in the control of root diseases such as clubroot," emphasises Professor Andreas von Tiedemann, Head of Göttingen University’s Division of Plant Pathology and Crop Protection. "With this knowledge, the infection potential of arable soil could be reduced. For instance, if farmers were able to trigger the germination of these spores in a year in which the host plant is absent from the fields, the pathogen will not be able to find a host and will not reproduce."

Original publication: Wang, Y.; Zheng, X.; Sarenqimuge, S.; von Tiedemann, A. The soil bacterial community regulates germination of Plasmodiophora brassicae resting spores rather than root exudates. PLoS Pathogens 19(3) 2023. DOI: 10.1371/journal.ppat.1011175 .



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