The alterations of the synthetic pathway and metabolic flux of auxin indole-3-acetic acid govern thermotolerance in <i>Lentinula edodes</i> mycelia subjected to heat stress

Heat stress poses a significant constraint on the annual production of <i>Lentinula edodes (L. edodes</i>), a challenge that has been intensified by global warming. Previous studies have established a close relationship between intracellular indole-3-acetic acid (IAA) content and heat tolerance in <i>L. edodes</i>. However, the specific changes in the IAA synthesis pathway and the target genes regulated by IAA under heat stress remain unclear. We employed targeted metabolomics and transcriptomics analyses to investigate the alterations in IAA synthesis pathways and gene expression in both heat-tolerant and heat-sensitive strains at various time points during heat stress. Our findings revealed that IAA is primarily synthesized via the tryptamine (TAM) and indole-3-pyruvic acid (IPA) pathways in <i>L. edodes</i>. Heat-sensitive strain YS3357 exhibited excessive accumulation of TAM after heat stress. Silencing of the key genes of IAA synthesis, including tryptophan decarboxylase and tryptophan transaminase in strain S606, could reduce the thermotolerance of <i>L. edodes</i> mycelia. Transcriptome analysis revealed that heat-tolerant strain S606 had an earlier response to protein folding and mitochondrial gene expression compared to the heat-sensitive strain YS3357. Additionally, most genes in the mitogen-activated protein kinase signaling pathway were upregulated after 10-24 h of heat stress, with auxin response elements identified in their promoters. These results suggest that the excessive TAM accumulation is the newly discovered limiting factor for thermotolerance, and the expression levels of key genes in the IAA synthesis pathway could directly influence hyphal thermotolerance. This study provides a new perspective on the mechanism by which IAA and the synthesis precursors affect thermotolerance of <i>L. edodes</i>.IMPORTANCEAs an important plant hormone, the potential role of indole-3-acetic acid (IAA) in enhancing the heat resistance of <i>L. edodes</i> strains has garnered significant attention. This study systematically investigated the intracellular IAA biosynthesis pathway and its metabolic flows in <i>L. edodes</i> under varying durations of thermal stress, with particular emphasis on temporal gene expression patterns. Research has demonstrated that excessive accumulation of tryptamine may impair the heat stress recovery capability of <i>L. edodes</i>. In contrast, IAA can improve its thermotolerance by modulating the expression of genes associated with the mitogen-activated protein kinase signaling pathway.