Trends in Microbiome and Digital One Health

Microorganisms produce a wide array of volatile organic compounds (VOCs), which are used as signalling molecules and mediators of many biological processes and biotic interactions. For instance, VOCs may inhibit growth of other organisms and have been proposed as tools to control plant fungal pathogens by inhibiting their growth. However, as for all growth inhibitors the application of VOCs for killing pathogens should trigger the process of selecting resistant variants, breaking down biocontrol effectiveness. The reversion of the growth inhibition paradigm by hindering the virulence rather than the growth, should transform a pathogen in a saprophyte minimizing the raising of resistant variants. We have examined this hypothesis with Magnaporthe oryzae a melanin-producing fungal phytopathogen and the most dangerous threat to rice worldwide. We have screened three bacterial strains from the rhizosphere of rice, whose VOCs do not inhibit M. oryzae growth, but strongly affect the black pigmentation. The bacterial VOCs strongly decreased M. oryzae sporulation and conidia germination, and completely abolished appressoria formation. Trials of identification of the active VOC(s) by head space solid-phase micro-extraction gas chromatography mass spectrometry, identified a group of nine VOCs shared by the three strains. Among these, 1-butanol-3-methyl reduced fungal pigmentation of more than 60% and completely inhibited conidia germination in vitro, effects that were further enhanced when used in mixture with other VOCs. Fungal transcriptomic analysis upon exposure to the bacterial VOCs indicated transcriptional downregulation of key enzymes of the melanin biosynthesis pathway. The inhibition effects were maintained in vivo on rice leaves, with less than 6% lesions incidence in VOCs-exposed leaves, respect to 98% in leaves not exposed to VOCs. Optical and scanning electron microscopy of rice leaves inoculated with M. oryzae conidia revealed the abolishment of conidia germination and of the tissue invasion by the pathogen demonstrating that VOCs of rice associated-bacteria can change the lifestyle of M. oryzae from pathogenic to saprophytic, potentially minimizing the selection of resistant forms.