Hydroponic cultivation technique in Populus nigra (L.) for the sustainable production of root metabolites with antimicrobial and prebiotic potential for skin microbiota

The cosmetics industry is increasingly seeking natural, safe, and traceable ingredients from sustainable sources. This involves exploring plant extracts as alternatives to traditional preservatives or as agents for preserving the skin microbiota. Among them, those obtained from trees belonging to the genus Populus (poplar) seem to be a promising source of bioactive compounds. While aerial parts of poplars have been studied, their roots, rich in specialized metabolites, are underexplored due to harvesting challenges. To address this gap we established a sustainable hydroponic cultivation system for black poplar (Populus nigra L.) roots by growing cuttings in a growth chamber at 20°C using either rooting gel or ½ MS media containing auxin. As observed, it required approximately three months under hydroponic conditions to achieve maximum root biomass, resulting in about 400 g of fresh biomass from a batch of 10 cuttings. The organic extracts obtained were analyzed by GC-MS and showed several compounds with different antimicrobial and prebiotic properties against four bacterial strains of skin microbiota i.e. Staphylococcus aureus, S. epidermidis, Corynebacterium xerosis and Cutibacterium acnes as well as Malassezia furfur, a fungus causing scalp dermatitis. Ethyl acetate extract showed antibacterial effects against S. aureus, S. epidermidis and C. acnes at concentrations of 0.025 %, 0.025 % and 0.1 % respectively. In addition, three pure metabolites were isolated from the ethyl acetate extract using chromatographic techniques and identified employing spectroscopic, spectrometric, and optical methods. Among them p-hydroxy benzoic acid showed antibacterial activity while methyl p-coumarate showed antifungal activity against M. furfur. These results suggest potential applications of poplar root extracts in cosmetic formulations. Moreover, the use of hydroponic cultivation offers a sustainable and controlled method for producing root biomass, facilitating consistent access to bioactive compounds without destructive harvesting in the natural environment.