Manipulating the expression of small secreted protein 1 (Ssp1) alters patterns of development and metabolism in the white-rot fungus Pleurotus ostreatus

The function of small secreted proteins (SSPs) in saprotrophic fungi is, for the most part, unknown. The white-rot mushroom Pleurotus ostreatus produces considerable amounts of poSSPs at the onset of secondary metabolism, during colony development and in response to chemical compounds such as 5-hydroxymethylfurfural and aryl-alcohols. Genetic manipulation of Ssp1, by knockdown (KD ssp1 ) or overexpression (OE ssp1 ), indicated that they are, in fact, involved in regulation of the ligninolytic system. To elucidate their potential involvement in fungal development, quantitative secretome analyses was performed during the trophophase, idiophase and at a transition point between the two growth phases. The mutations conferred a time shift in the secretion and expression patterns; OE ssp1 preceded the entrance to idiophase and secondary metabolism, while KD ssp1 was delayed. This was also correlated with expression patterns of selected genes. The KD ssp1 colony aged at a slower pace, accompanied by a slower decline in biomass over time. In contrast, OE ssp1 exhibited severe lysis and aging of the colony at the same time point. These phenomena were accompanied by variations in yellow pigment production, characteristic of entrance of the wild-type into idiophase. The pigment was produced earlier and at a higher amount in the OE ssp1 and was absent from KD ssp1. Furthermore, the dikaryon harboring OE ssp1 exhibited a delay in initiation of fruiting body formation as well as earlier aging. It is proposed that Ssp1 might function as a part of the fungal communication network and regulating the pattern of fungal development and metabolism in P. ostreatus . Importance Small secreted proteins (SSPs) are common in fungal saprotrophs, but their roles remain elusive. As such, they comprise part of a gene pool which may be involved in governing fungal lifestyles not limited to symbiosis and pathogenicity, in which they are commonly referred as ‘effectors’. It is proposed that Ssp1 in the white-rot fungus Pleurotus ostreatus regulates the transition from primary to secondary metabolism, development, aging and fruiting body initiation. Our observations uncover a novel regulatory role of effector-like SSPs in a saprotroph, suggesting that they may act in fungal communication as well as in response to environmental cues. The presence of Ssp1 homologues in other fungal species supports a common potential role in environmental sensing and fungal development.