The soil bacterial community regulates germination of Plasmodiophora brassicae resting spores rather than root exudates

Abstract

Clubroot, caused by Plasmodiophora brassicae, is a severe soil-borne disease that restricts the production of cruciferous crops worldwide. A better understanding of biotic and abiotic factors regulating germination of P. brassicae resting spores in the soil is significant for developing novel control methods. Previous studies reported that root exudates can trigger P. brassicae resting spore germination, thus enabling a targeted attack of P. brassicae on host plant roots. However, we found that native root exudates collected under sterile conditions from host or non-host plants cannot stimulate the germination of sterile spores, indicating that root exudates may not be direct stimulation factors. Instead, our studies demonstrate that soil bacteria are essential for triggering germination. Through 16s rRNA amplicon sequencing analysis, we found that certain carbon sources and nitrate can reshape the initial microbial community to an inducing community leading to the germination of P. brassicae resting spores. The stimulating communities significantly differed in composition and abundance of bacterial taxa compared to the non-stimulating ones. Several enriched bacterial taxa in stimulating community were significantly correlated with spore germination rates and may be involved as stimulation factors. Based on our findings, a multi-factorial ‘pathobiome’ model comprising abiotic and biotic factors is proposed to represent the putative plant-microbiome-pathogen interactions associated with breaking spore dormancy of P. brassicae in soil. This study presents novel views on P. brassicae pathogenicity and lays the foundation for novel sustainable control strategies of clubroot. Author summary Cruciferous crops are crucial in our daily lives as vegetables, sources of oil, green manure and animal feed. Clubroot caused by Plasmodiophora brassicae is a severe soil-borne disease that significantly threatens the production of cruciferous crops. The disease is difficult to control and therefore effective sustainable control methods are urgently required. An important step in the life cycle of P. brassicae is the germination of resting spores which is required for root infection because dormant spores cannot infect plants. Previous studies believed that plant root exudates are the key factors to trigger spore germination. However, our study shows contrasting results and suggests that soil bacteria play an important role in stimulating germination of resting spores. A model is proposed to elucidate the relationship between the plant, the pathogen and biotic and abiotic factors in the soil. These findings provide novel insights into the regulation of dormancy and germination of resting spores of P. brassicae as key processes preceding infection, which may enable the development of innovative integrated sustainable control strategies. For example, the microbial community could be artificially manipulated (synthetic community) to stimulate spore germination in the absence of host plants in order to reduce soil inoculum. This could significantly enhance the soil sanitation effects of particular crop rotation cycles and lead to durable disease control.