American ginseng (Panax quinquefolius L., Araliaceae) is a perennial herb that is native to the eastern deciduous woodlands of North America (Catling et al., 1994). For many years it has been harvested from the wild as well as cultivated in several regions in the United States and Canada for its highly valued root (Pritts, 1995). Ginseng root is renowned for its medicinal properties and is often used in herbal therapy to reduce stress, lower blood pressure, and boost the body's immunity (Vuksan et al., 2001). The pharmacological properties of ginseng are attributed to a group of secondary metabolites called ginsenosides (Attele et al., 1999). Besides their use as therapeutic agents, ginsenosides are also marketed as dietary supplements and are often included in health foods and energy drinks (Qu et al., 2009; Shen et al., 2003).
Most of the P. quinquefolius roots supplied to the market are harvested from the wild. Recent demographic studies, however, have reported that abundance of wild populations is declining as a result of overharvesting, habitat fragmentation, and degradation (Nantel et al., 1996; Robbins, 1998). On the other hand, cultivation of P. quinquefolius is on the rise in response to declining wild populations and increasing demand for ginseng from nutraceutical and beverage industries. Field cultivation of P. quinquefolius is labor-intensive and time-consuming as a result of the slow-growing nature of this species (Proctor, 1996). It takes between 5 and 7 years from seedling to final marketable root size, and during that time, caring for plants against various pathogens and pests is needed (Proctor, 1996). In addition, there are no improved cultivars of P. quinquefolius, although this species is reported to be genetically diverse (Bai et al., 1997; Boehm et al., 1999; Cruse-Sanders and Hamrick, 2004; Schluter and Punja, 2002).
To circumvent the lengthy duration and difficulties associated with traditional cultivation, and to meet the increasing demand for ginseng products, researchers have attempted to produce ginsenosides using in vitro cultures (Wu and Zhong, 1999). With in vitro culture, ginsenoside production is more controlled with regard to quality and quantity because desirable cell lines can be selected (Wu and Zhong, 1999). Since the first report on in vitro culture of P. quinquefolius by Jhang et al. (1974), there has been few reports on its vitro culture compared with numerous reports on other species of the genus Panax. The few reports on in vitro culture of P. quinquefolius have basically focused on optimizing conditions either for cell growth and ginsenoside production using suspension cultures or plantlet regeneration through somatic embryogenesis (Mathur et al., 1994; Punja et al., 2004; Wang, 1990; Wang et al., 1999; Zhong et al., 1996; Zhou and Brown, 2005).
Although in vitro production of ginsenosides has been successful in the genus Panax, fluctuation of ginsenoside content among in vitro cultures is often reported (Wu and Zhong, 1999). This has been attributed to use of different plant growth hormone combinations and concentrations (Zhong et al., 1996); however, the influence of genotype has not been examined in P. quinquefolius. Naturally, P. quinquefolius plants grow roots of different morphotypes (Roy et al., 2003), and such variation of root morphology could be the result of underlying genetic differences among plants. Most common root morphotypes of P. quinquefolius are BLB, a compact, round, or bullet-shaped root; ML, a branched root resembling a human shape; and STK, a slender elongated taproot without lateral roots (Fig. 1). The roots of different morphotypes are often valued and preferred differently by ginseng consumers because they are purported to have different medicinal potencies.
From our previous comparative study on ginsenoside content in P. quinquefolius roots of different morphotypes, we found that roots of the ML morphotype had higher ginsenoside content compared with those of BLB and STK morphotypes (Obae and West, 2008). In the current study, we sought to: 1) evaluate how explants from the three root morphotypes (ML, BLB, and STK, hereafter referred to as ginseng lines) responded to in vitro callus induction and growth; 2) compare ginsenoside profiles and content among stock roots and their callus tissues; and 3) assess genetic variation among stock roots.
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