Volatile Organic Compounds Emitted by Fungi Have Substantial Effects on Growth and Disease Resistance in <i>Arabidopsis thaliana</i>

ABSTRACT Volatile organic compounds (VOCs) emitted by certain fungi have previously been suggested to modulate plant development and their response to stresses. While some fungal species, such as Trichoderma harzianum , are known to produce VOCs, the production of VOCs in other phylogenetically divergent species, such as Mucor mucedo , was previously unknown. This study aimed to analyse the VOCs profiles emitted by these fungal species and examine their effects on the growth and defensive responses of Arabidopsis thaliana . Volatilome analysis revealed that the most abundant VOCs for both species were alcohols, ketones and esters. However, notable differences were also observed, particularly with regard to terpenes. We then observed that prolonged exposure to VOCs emitted by both fungal species had detrimental effects on A. thaliana growth and photosynthetic performance, whereas shorter exposures enhanced the expression of defense‐related genes and the plant defense against the phytopathogenic bacterium Pseudomonas syringae . This increased resistance does not appear to be mediated by canonical H 2 O 2 ‐induced immunity, but rather by subsequent responses that trigger complex metabolic reprogramming, including glucosinolate biosynthesis. Therefore, our results confirm that VOC‐mediated plant‐fungus interactions are very relevant to plant fitness, highlighting the need to understand them in greater depth.