Clarification of Swainsonine Biosynthesis by a Multi-Branched Pathway and Non-Accumulation of Mycotoxin in Plants after Fungal Colonization

Abstract

The indolizidine alkaloid swainsonine (SW) is a deadly mycotoxin to livestock. The genetic mechanism of SW biosynthesis remains obscure for decades. Here we report that SW is produced via a multi-branched pathway by a hybrid nonribosomal peptide/polyketide synthase (NRPS-PKS) gene cluster in the endophytic entomopathogenic fungus Metarhizium robertsii. The precursor pipecolic acid can be redundantly converted from lysine by both clustered and unclustered genes. The hybrid enzyme that begins with a NRPS module produces three intermediates with and without domain skipping. Intriguingly, the biosynthetic process is coupled with the trans to cis non-enzymatic epimerization of C1-OH for both hydroxyl- and dihydroxyl-indolizidine intermediates. We also found that SW production is dispensable for fungal colonization of plants and the infection of insect hosts. Our finding that SW does not accumulate in planta after Metarhizium colonization provides a notable indication of safety when using this fungus to control insect pests.

Keywords: Swainsonine, neurotoxic mycotoxin, biosynthesis, Metarhizium, endophytism