Cell wall modification contributed to high aluminum retention in root epidermis for red skin root syndrome in Panax ginseng

Panax ginseng (ginseng) is an important herbal medicine plant; however, successive ginseng cultivation causes intense acidification and active aluminum (Al) dissolution in ginseng bed soil. High Al retention in the root epidermis has been well documented to associate with the occurrence of red skin root syndrome of ginseng. The present study aimed to clarify the origins and effects of this Al accumulation patterns. Healthy and red skin ginseng were collected to compare Al concentrations in whole plants and root epidermis cell wall modification from physiological parameters, transcriptional expression, and the sugar-targeted-metabolome perspectives. In healthy ginseng plants, around 50 % of Al content was distributed in the aboveground parts, while red skin ginseng of comparable ages had markedly higher Al concentrations in the main root epidermis and fibrous roots than those in healthy plants. Isolated red skin ginseng root epidermis showed strong Al-morin fluorescence staining, lower cell wall Al adsorption ability, lower hemicellulose and pectin content, and altered monosaccharide concentrations, relative to those from healthy plants. Fibrous roots from red skin ginseng had higher transcriptional abundance of several cell wall degradation-related genes. Overall, comparable Al concentrations were transported to discarded organs, such as leaves and stems, of healthy ginseng plants. Conversely, in red skin ginseng, higher Al concentrations were detained in the root epidermis, resulting from epidermal cell wall degradation.