Induction of conidial traps in the nematode-trapping fungus Drechslerella dactyloides by soil microbes.

Abstract

Nematode-trapping fungi, renowned for their specialized predatory structures that ensnare nematodes, offer a promising biological approach to managing plant-parasitic nematodes. However, the efficacy of these fungi is frequently hampered by biotic and abiotic factors within the soil, which can significantly impede fungal germination (fungistasis). To counteract these environmental challenges, certain nematode-trapping fungi have evolved to produce traps from their conidia, referred to as conidial traps. This adaptation allows them to bypass the inhibitory effects of their surroundings, enhancing their predatory capabilities. In this study, we explored how soil affects conidial trap formation in Drechslerella dactyloides. Our findings revealed that Acinetobacter spp. and Pantoea spp. present in soil extracts play pivotal roles in triggering the development of these traps. Using metagenomic sequencing, we mapped the shifts in bacterial communities and their relative abundances before and after incubation for up to 24 hours to optimize soil induction effects. This analysis highlighted the enrichment of specific functional genes in soil microbes and provided insights into the mechanisms driving conidial trap formation, based on changes in soil characteristics. Furthermore, through bacterial isolation procedures, we successfully cultured and characterized the bacteria responsible for this phenomenon, confirming their potent ability to stimulate the production of conidial traps in nematode-trapping fungi. This study not only underscores the critical role of bacterial diversity in modulating the life cycle transitions of nematode-trapping fungi but also sets the stage for the development of more effective and sustainable strategies to harness these fungi in the battle against pathogenic nematodes.

Importance: Predatory nematode-trapping fungi are important microbial antagonists of nematodes and can be developed into biocontrol agents. However, microbial biocontrol agents often suffer from inconsistent efficacy, primarily due to biotic and abiotic stresses in the rhizosphere soil. Drechslerella dactyloides, a nematode-trapping fungus, produces conidial traps in soil, serving as a survival strategy to overcome these stresses. In this study, we optimized soil suspensions to efficiently induce the formation of conidial traps. We found that bacteria in the soil directly trigger this formation. Metagenomic sequencing revealed bacterial enrichment during optimization, and we isolated and purified these bacteria with inducible activity. Our research deepens the understanding of this survival strategy of nematode-trapping fungi in nature, laying the foundation for enhancing the effectiveness of nematode biocontrol using this mechanism.