Sterol complex visualisation in Phytophthora cinnamomi and expression analysis of genes involved in sterol sensing, recruitment and conversion

Sterols are a fundamental component of the biology of all eukaryotes, including the sterol auxotrophic plant pathogen Phytophthora cinnamomi. This study identified the requirement of sterols by P. cinnamomi for the formation of sporangia. Using fluorescence microscopy, we also observed the localisation of filipin-complexed free sterols within P. cinnamomi hyphae, upon exogenous sterol supplementation. We have identified four genes that encode the sterol-sensing domain (SSD)-containing proteins (SCPs) and two genes that encode the β-cinnamomin protein, all of which exhibited reduced transcript levels following exogenous sterol supplementation. Despite lacking de novo sterol biosynthesis, P. cinnamomi retains the mevalonate pathway and demonstrates independence from regulation by exogenous sterols. We show that conserved enzyme, 7-dehydrocholesterol reductase facilitated the transformation of sterols to obtain Δ5-sterols, which are preferred by P. cinnamomi. This study provides new insights into the influence of exogenous sterols on the life cycle of P. cinnamomi and gene regulation, related to the acquisition of sterols, which highlight this pathogen's multifaceted, adaptive strategy to identify, recruit and utilize sterols for growth and reproduction.