Homoterpene Biosynthesis in Fungi

Homoterpenes carrying an additional methyl group in their carbon backbones are an emerging class of natural products that challenge the biogenic isoprene rule, stating that terpenes are composed of integer multiples of C₅ units. We and others have recently shown that biosynthetic pathways to homoterpenes are widespread in bacteria, leading either to specialized scaffolds such as the "Greek philosophers homoterpenes" in Pseudomonadota or to simple methyl analogs of central eudesmanes and germacranes ("humanists homoterpenes") in Actinomycetota. Here we report the discovery of the first homoterpene biosynthetic pathway in the fungal kingdom using targeted genomic data mining in combination with in vitro pathway reconstitution. Functional analyses of a fungal methyltransferase (NdiMT) and terpene cyclase (NdiTC) pair from the plant-pathogenic fungus Neonectria ditissima, the causative agent of apple canker, led to the discovery of a novel homosesquiterpene featuring an intriguing heptamethylbicyclo[3.3.1]nonane scaffold. Phylogenetic analyses indicate that the fungus acquired the key methyltransferase via horizontal gene transfer from bacteria, whereas the terpene cyclase appears to have evolved from a fungal ancestor. The discovery raises fundamental questions about the evolutionary rationale and functional consequences of terpene methylation in nature.