Spermidine stimulates hypocrellin A biosynthesis through nitric oxide signaling in Shiraia sp. S9

Hypocrellin A (HA), a photoactive perylenequinone from the bambusicolous Shiraia fungi, possesses potent photodynamic anticancer and antimicrobial properties. However, the signaling mechanisms governing its biosynthesis remain poorly understood. In this study, we identify spermidine (Spd), a ubiquitous polyamine, as a novel elicitor that significantly enhances HA production in Shiraia sp. S9. Spd activated both nitric oxide synthase (NOS) and nitrate reductase (NR) for nitric oxide (NO) generation, leading to the stimulation of the soluble guanylate cyclase (sGC)-cyclic guanosine monophosphate (cGMP) signaling cascade. Inhibition of NO generation or sGC activity suppressed both cGMP accumulation and HA biosynthesis. Transcriptomic analysis revealed that Spd-induced NO signaling upregulated genes in central carbon metabolism and the hypocrellin biosynthetic gene cluster. The dual elicitation strategy by the combined addition of Spd and the NO donor sodium nitroprusside (SNP) exhibited a strong enhancing effect, increasing HA yield by 4.6-fold compared with control cultures. These results demonstrate that Spd regulates HA biosynthesis through a NO-cGMP-mediated signaling pathway, unveiling polyamines as new metabolic elicitors and providing an efficient dual-elicitation strategy for large-scale hypocrellin production.