Phenolics metabolism and lignin synthesis in root suspension cultures of Panax ginseng in response to copper stress

Abstract

In this study of tissue cultured Panax ginseng roots in bioreactor, growth responses, carbonyl content, phenolics, flavonoids, diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity, cysteine, non-protein thiol (NPSH) content, phenolics related enzymes and lignin content were analysed after exposure to copper (Cu 2+) sulfate at various concentrations. Cu 2+ significantly inhibited root growth and resulted in an increase in protein oxidation (21%) (measured as carbonyl content) at 50 μM. Cu 2+ stress resulted in increased activities of glucose 6 phosphate dehydrogenase (G6PDH), shikimate dehydrogenase (SKDH), phenylalanine ammonia lyase (PAL) and cinnamyl alcohol dehydrogenase (CAD). The ensuing effect was an accumulation of phenolics and lignin. The phenolics, flavonoids, cysteine and NPSH contents as well as the DPPH activity, increased by 26%, 83%, 30%, 40% and 76%, respectively, at 50 μM Cu 2+ treatment compared to the control after 40 days. The induced activities of substrate specific peroxidases (caffeic acid peroxidase, CA-POD), chlorogenic acid peroxidase (CH-POD), polyphenol oxidase (PPO) and β-glucosidase (β-GS) indicated that these enzymes played an important role in the synthesis of phenolics compounds. The results also provided evidence that the application of Cu 2+ at 25 and 50 μM was accompanied by a substantial increase in oxidative stress as indicated by protein oxidation and reduced root growth. The increase in phenolic compounds, related enzymes and lignin clearly reflects the protective response to cellular damage induced by higher levels of Cu 2+.