Spermidine enhances the heat tolerance of Ganoderma lucidum by promoting mitochondrial respiration driven by fatty acid β-oxidation.

High temperature is an unavoidable environmental stress that generally exerts detrimental effects on organisms and has widespread effects on metabolism. Spermidine is an important member of the polyamines family and is involved in a range of abiotic stress responses in plants. Mitochondria play an essential role in cellular homeostasis and are key components of the stress response. Our results indicated that mitochondrial respiratory intensity increased by 80% in wild-type (WT) under heat stress, but the activities of key enzymes of the tricarboxylic acid (TCA) cycle and electron transport chain (ETC) were significantly reduced upon the knockdown of the spermidine synthase gene (spdS). Furthermore, the content of mitochondrial pyruvate decreased by 36.1%, whereas the levels of free fatty acid increased by 28.8% under heat stress. Upon spdS knockdown, the content of mitochondrial pyruvate was similar to that in the WT, but the medium-chain fatty acid (C6:0) decreased by 68.6%-84.2%, whereas the long-chain fatty acid (C18:2) marginally increased. Subsequent studies demonstrated that spermidine promoted the translation of long chain acyl-CoA dehydrogenase (LCAD) and mitochondrial trifunctional protein (MTP, also known as HADH), thereby enhancing fatty acid β-oxidation under heat stress. In conclusion, spermidine enhances key TCA cycle and ETC enzyme activities and is involved in heat stress-induced fatty acid β-oxidation by promoting the translation of LCAD and HADH, thereby improving the heat tolerance of Ganoderma lucidum. IMPORTANCE: Polyamines are stress-responsive molecules that enhance the tolerance of plants to multiple abiotic stresses by regulating a variety of biological processes. Our previous research indicated that heat stress induces the the biosynthesis of polyamines and promotes the conversion of putrescine to spermidine in G. lucidum, but the physiological role of elevated spermidine levels is yet to be elucidated. In this study, our findings demonstrated that spermidine enhances the heat tolerance in G. lucidum and that mitochondrial respiration is essential for spermidine-enhanced heat tolerance. This study elucidated a preliminary mechanism by which spermidine enhances heat tolerance of G. lucidum and provided a new insight into the understanding of how microorganisms resist heat stress.