Biochemistry of ginseng root tissues affected by rusty root symptoms

Ginseng rusty root, a disorder of unknown cause (s), in which reddish-brown to orange-brown areas develop on the surface of field-grown roots, was studied at the cellular and biochemical levels. Using light microscopy, the affected areas were shown to comprise of the epidermis and underlying 6-8 cell layers of the cortical tissues. Rusty root areas ranged from small clusters of 3-4 cells to larger expanding areas of >80 cells. These cells appeared golden-brown and stained a bluish-green with Toluidine Blue indicating the presence of phenolic compounds. Energy-dispersive X-ray spectroscopy and atomic emission spectrometry of affected epidermal cells revealed a significant accumulation of Fe, Al, Si, Mg and other cations when compared to adjoining healthy cells. The concentrations of the six most common ginsenosides found in ginseng roots (Rg(1), Re, Rb(1), Rc, Rb(2), and Rd) were reduced by 40-50% in rusty root-affected epidermal and cortical tissues when compared to adjacent healthy tissues. Total phenolic compounds were increased by up to threefold in affected tissues and HPLC analysis revealed significantly higher levels of quercetin, cinnamic acid, vanillic acid, p-coumaric acid, benzoic acid, chlorogenic acid and catechin. In vitro phenolic-metal binding assays confirmed that phenolic compounds were able to sequester positively-charged metal ions, in particular Fe, to form a phenolic-metal ion complex. In ginseng callus cultures, accumulation of phenolic compounds was increased threefold within 12 h of treatment with chitosan (1%), and to a lesser extent by wounding. Specific defense enzymes, namely phenylalanine ammonia-lyase (PAL, E.C. 4. 3. 1. 5.), polyphenoloxidase (PPO, E.C. 1. 10. 3. 1.) and peroxidase (POD, E.C. 1. 11. 1. 7.), were also significantly enhanced in treated callus tissues and in rusty root tissues. On field-grown ginseng roots, application of chitosan induced symptoms similar to rusty root, whereas wounding and ethylene treatments did not. Based on these results, rusty root symptoms on ginseng are proposed to result from an induction of host defense responses, especially phenolic production, in epidermal and underlying cortical cells. This induction is likely due to attempted invasion by as-yet uncharacterized chitin-containing soil fungi, which were observed in many of the affected cells. Subsequent oxidation of phenolic compounds and sequestration of metal ions, in particular Fe, appear to be largely responsible for the symptoms observed.