Flavin-Containing Monooxygenase 1 Gene Mediates Resistance to Calonectria pseudoreteaudii Crude Toxin in Arabidopsis thaliana and Eucalyptus grandis

Eucalyptus leaf blight is a globally distributed disease caused by Calonectria fungi, with C. pseudoreteaudii being the dominant pathogen in Fujian, China. The crude toxin produced by C. pseudoreteaudii is a key virulent factor. To investigate the resistance mechanism triggered by crude toxin infection, transcriptome sequencing, physiological observations, and qRT-PCR analyses were conducted. Transcriptome analysis of Arabidopsis thaliana treated with C. pseudoreteaudii crude toxin revealed that a flavin-containing monooxygenase 1 gene (AtFMO1) exhibited the highest differential expression with DMSO control. Compared with Arabidopsis ecotype Col-4 (the wild type, WT), AtFMO1 knockout mutant (Δfmo1) plants displayed dose-dependent leaf margin yellowing accompanied by reduced callose deposition and hydrogen peroxide (H2O2) accumulation under crude toxin treatment. qRT-PCR analysis of key genes from two immune pathways showed that the salicylic acid-dependent (SA-dependent) pathway was likely Arabidopsis’s primary response pathway for crude toxin. In E. grandis, a total of 38 EgFMOs were identified, with eight EgFMO1s, based on the protein sequence similarity, conserved domain, and motif pattern. qRT-PCR analysis of EgFMO1s revealed two major expression patterns in response to crude toxin treatment: an initial downregulation followed by upregulation, and continuous upregulation. Collectively, these results suggest FMO1 plays a positive role in resistance to C. pseudoreteaudii crude toxin in both A. thaliana and E. grandis.