The Fungal Pathogen Fusarium graminearum Can Hydrolyse Tryptamine-Derived Hydroxy-Cinnamic Acid Amide Defence Compounds, and Generates Highly Increased Auxin Levels in Wheat via Copper Amine-Oxidases.

Abstract

The important cereal pathogen Fusarium graminearum is not only able to produce trichothecene toxins, like deoxynivalenol, but also the plant hormone auxin (indole-3-acetic acid, IAA). Highly elevated levels of IAA and auxin derivatives such as IAA-glucoside or IAA amino-acid conjugates were observed in the susceptible wheat cultivar Apogee infected with F. graminearum. We report here that the fungal pathogen can hydrolyse tryptamine-derived hydroxycinnamic acid amides produced in high amounts during plant defence and is able to convert the released tryptamine in high yield into IAA. We investigated the role of copper amine-oxidase candidate genes, which code for enzymes converting tryptamine into the IAA precursor indole-3-acetaldehyde. After consecutive knock-out of seven copper amine oxidases the resulting septuple-mutant strain (aoxΔ⁷) had strongly reduced ability to produce auxin from tryptamine in vitro. Virulence of the aoxΔ⁷ mutant was significantly impaired while deoxynivalenol production in planta was comparable to the wild type. Our results indicate that F. graminearum, formerly presumed to be a toxin-producing necrotroph, in an early biotrophic phase uses subterfuge by converting abundantly formed plant defence compounds into defence-suppressing auxin.