Copper oxide nanoparticles boost phenolic compounds, antioxidant activity, and expression of key biosynthetic genes in hairy root cultures of turnip

Turnip (Brassica rapa ssp. rapa) from the Cruciferae family is one of the most valuable leaf and root vegetable crops, with remarkable pharmacological properties and human health-promoting benefits. Turnip roots are rich in phenolic compounds with various biological activities, such as antioxidant properties. Elicitation of transgenic hairy root cultures (HRCs) is one of the successful strategies for enhancing secondary metabolites. Recent research has focused on the capacity of nanoparticles (NPs) to stimulate the production of secondary metabolites. This study aims to evaluate the effect of CuO NPs on the production of phenolic compounds, antioxidant capacity, and expression level of genes involved in the flavonoid biosynthetic pathway in transgenic HRCs of turnip. The biosynthesis of phenolic compounds was affected variably depending on the concentration and exposure time of CuO NPs. Turnip HRCs were treated with 75 mg/L CuO NPs for 48 h, resulting in the largest increase in phenolic component accumulation: caffeic acid (9.04-fold), coumaric acid (6.12-fold), rosmarinic acid (13.45-fold), chlorogenic acid (5.42-fold), apigenin (5.93-fold), and rutin (10-fold). Gallic acid (43.36-fold) and quercetin flavonoid (4.79-fold) levels peaked when HRCs were treated with 50 mg/L CuO NPs for 48 and 72 h, respectively. The highest quantity of cinnamic acid (10.66-fold) was found following a 72-h exposure to 25 mg/L CuO NPs. The highest level of cinnamic acid (10.66-fold) was found after a 72-h exposure to 25 mg/L CuO NPs. The stimulant effect of CuO NPs on phenolic compound production was associated with a raised expression level of some pivotal genes (PAL, CHS, FLS, and MYB3) involved in the biosynthesis pathway. NP-elicited hairy roots had significantly increased antioxidant activity (3.88-fold) than non-elicited roots. The results indicated that CuO NPs could be an effective elicitor for boosting phenolic compound synthesis in turnip HRCs.