Identification and 3D gene expression patterns of WUSCEHEL-related homeobox (WOX) genes from Panax ginseng

Wild ginseng (Panax ginseng) can survive in their natural habitat for hundreds of years, reflecting a remarkable plasticity. Plant stem cells (SCs) play a key role in the regenerative capacity and lifelong activity of these plants. WUSCHEL-RELATED HOMEOBOX (WOX) genes are master regulators of plant SC pluripotency, but their functions in medicinal plants have not been previously reported. To investigate whether these genes define different SC niches in ginseng, we cloned and analysed five WOX genes in ginseng (PgWOXs) and found that they might regulate root reconstruction. Then, the whole-mount RNA in situ hybridization was used to characterize the 3D gene expression pattern of PgWOXs in ginseng seedlings and cultured adventitious roots. PgWOX4 was expressed in vascular cambium SCs; PgWOX5 and PgWOX11 were mainly expressed in the tips of seedling and adventitious roots, which are the energetic centre of the meristem; and PgWOX13a and PgWOX13b were detected in the parenchyma cells of the main root of seedlings and cultured adventitious roots, suggesting that they are important for maintaining the balance between SC differentiation and self-renewal in the phloem and xylem. This is the first report of SC regulation in medical herbs; we expect that P. ginseng can serve as a model herb for investigating the relationship between SCs and their herbal morphological features, which would be a new research direction to improve the yield and quality of the medicinal materials by regulating the herbal SCs.