Hairy root induction in hybrid poplar (Populus tremula x Populus alba) for sustainable growth and specialized metabolites production with antioxidant activities

Rhizobium rhizogenes mediated genetic transformation is a promising biotechnological tool to obtain specialized metabolites having valuable biological activities. The present study reports a reliable transformation protocol for hairy root induction with sustainable growth and identification of specialized metabolites with antioxidant activities in hybrid poplar clone 717-1B4 (Populus tremula x Populus alba). Four different wild type agropine R. rhizogenes strains ATCC 15834, A4, MSU 440 and LBA 9402 were employed to check their efficiency on hairy root induction and accumulation of specialized metabolites. Results indicated that the transformation efficiency was highly dependent on strain used and could be increased by adding acetosyringone into the co-cultivation medium. PCR amplification of rolB and rolC genes confirmed the transgenic nature of the hairy roots. Adding polyvinylpyrrolidone into the culture media not only circumvented the problem of browning due to phenolics but also resulted in healthy and fast growth of hairy roots. Colorimetric assays revealed the presence of phenolic compounds, among which flavonoids in ethanolic extracts of different hairy root lines which could be associated with the significant antioxidant activities as depicted by FRAP and DPPH assays. The hairy root lines obtained with different R. rhizogenes strains showed similar phytochemical profiles. UHPLC-HRMS analysis allowed the separation of 136 peaks, out of which catechin, nigracin, salireposide, trichocarpin and tremulacin were identified. In conclusion, poplar hairy roots are a promising raw material for producing specialized metabolites with bioactive properties that could be further used for various pharmaceutical, food and cosmetics applications.