Transcriptome Analysis Reveals Involvement of Oxidative Stress Response in a Copper-Tolerant Fusarium oxysporum Strain

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High amount of copper is toxic to most organisms, but endophytic fungi can develop survival strategies to tolerate and respond to environmental stressors such as heavy metal contaminants. While high copper induces oxidative stress, it is still unclear which genes are associated with copper tolerance. Here, we performed a metatranscriptome analysis of endophytic fungi isolated from a black nightshade plant Solanum nigrum L. growing on mine tailings of a gold processing area. Initial screening revealed the presence of a copper-tolerant strain of Fusarium oxysporum, designated as IB-SN1W, which tolerated up to 1000 ppm and 300 ppm copper in solid and liquid media, respectively. Differential gene expression analysis by RNA sequencing showed that 23% of contigs are uniquely expressed in the copper-treated fungus. These genes are involved in copper ion import, polyamine transport, oxidoreductase activity, and oxidative stress response. Catalase transcripts were also highly upregulated in IB-SN1W compared to a non-tolerant F. oxysporum strain. Catalase inhibition decreased copper-tolerance in IB-SN1W, while the addition of antioxidants prevented the copper-dependent growth inhibition in the non-tolerant strain. Overall, these results suggest that oxidative stress response contributes to copper tolerance in F. oxysporum.