Cytokinin signaling promotes root secondary growth and bud formation in .

Background: , one of the valuable perennial medicinal plants, stores numerous pharmacological substrates in its storage roots. Given its perennial growth habit, organ regeneration occurs each year, and cambium stem cell activity is necessary for secondary growth and storage root formation. Cytokinin (CK) is a phytohormone involved in the maintenance of meristematic cells for the development of storage organs; however, its physiological role in storage-root secondary growth remains unknown.

Methods: Exogenous CK was repeatedly applied to , and morphological and histological changes were observed. RNA-seq analysis was used to elucidate the transcriptional network of CK that regulates growth and development. The () and () genes were cloned in and functionally analyzed in as a two-component system involved in CK signaling.

Results: Phenotypic and histological analyses showed that CK increased cambium activity and dormant axillary bud formation in P. ginseng, thus promoting storage-root secondary growth and bud formation. The evolutionarily conserved two-component signaling pathways in were sufficient to restore CK signaling in the double mutant and rescue its growth defects. Finally, RNA-seq analysis of CK-treated roots revealed that plant-type cell wall biogenesis-related genes are tightly connected with mitotic cell division, cytokinesis, and auxin signaling to regulate CK-mediated development.

Conclusion: Overall, we identified the CK signaling-related two-component systems and their physiological role in . This scientific information has the potential to significantly improve the field-cultivation and biotechnology-based breeding of ginseng.