Bacterial endosymbionts protect beneficial soil fungus from nematode attack

Area of Science:

• Microbiology
• Mycology
• Ecology

Background:

• Fungi, such as Mortierella, are vital soil microorganisms involved in nutrient cycling and plant growth.
• These fungi face threats from fungivorous nematodes, but their defense mechanisms remain poorly understood.
• Endosymbiotic relationships between fungi and bacteria are increasingly recognized for their ecological significance.

Purpose of the Study:

• To investigate the protective mechanisms of Mortierella fungi against nematode predation.
• To identify the microbial agents responsible for defense and their chemical compounds.
• To characterize the genetic basis of the endosymbiont's defensive capabilities.

Main Methods:

• Microscopic examination and 16S ribosomal DNA sequencing to identify bacterial endosymbionts.
• Metabolic profiling and comparative genomics to analyze metabolite production and biosynthetic pathways.
• Anthelmintic assays using Caenorhabditis elegans and Aphelenchus avenae to evaluate compound efficacy.
• Coculture experiments to demonstrate host protection in symbiotic and aposymbiotic fungal strains.

Main Results:

• A novel bacterial endosymbiont, identified as Candidatus Mycoavidus necroximicus, was discovered within Mortierella verticillata hyphae.
• The endosymbiont produces cytotoxic macrolactones (necroximes), previously misattributed to the fungus.
• Comparative genomics revealed extensive natural product biosynthesis pathways in the endosymbiont's genome.
• Necroximes demonstrated potent anthelmintic activity, effectively protecting the host fungus from nematode grazing.

Conclusions:

• Endofungal bacteria play a crucial role in defending fungi against predation by mycophagous nematodes.
• The endosymbiont Mycoavidus necroximicus provides a chemical shield for its fungal host through necroxime production.
• This discovery expands our understanding of symbiotic interactions in soil ecosystems and microbial defense strategies.