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

<i>Eucalyptus</i> leaf blight is a globally distributed disease caused by <i>Calonectria</i> fungi, with <i>C. pseudoreteaudii</i> being the dominant pathogen in Fujian, China. The crude toxin produced by <i>C. pseudoreteaudii</i> 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 <i>Arabidopsis thaliana</i> treated with <i>C. pseudoreteaudii</i> crude toxin revealed that a flavin-containing monooxygenase 1 gene (<i>AtFMO1</i>) exhibited the highest differential expression with DMSO control. Compared with <i>Arabidopsis</i> ecotype Col-4 (the wild type, WT), <i>AtFMO1</i> knockout mutant (<i>Δfmo1</i>) 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 <i>Arabidopsis</i>'s primary response pathway for crude toxin. In <i>E. grandis</i>, a total of 38 <i>EgFMO</i>s were identified, with eight <i>EgFMO1</i>s, based on the protein sequence similarity, conserved domain, and motif pattern. qRT-PCR analysis of <i>EgFMO1</i>s revealed two major expression patterns in response to crude toxin treatment: an initial downregulation followed by upregulation, and continuous upregulation. Collectively, these results suggest <i>FMO1</i> plays a positive role in resistance to <i>C. pseudoreteaudii</i> crude toxin in both <i>A. thaliana</i> and <i>E. grandis</i>.