Endogenous Hormonal Levels in <i>Scutellaria baicalensis</i> Calli Induced by Thidiazuron

Callusing was induced from three kinds of explants of Chinese medical plant Scutellaria baicalensis with thidiazuron (TDZ) as an exogenous growth regulator. Multiple shoots were formed from these calli on hormone-free media. Calli induced by 0.3 mg/l TDZ produced shoots directly on the induction medium after culturing for 30–40 days without any interval, while at a higher concentrations of TDZ (1.0, 2.0, and 5.0 mg/l), regeneration occurred only after transfer to hormone-free medium. Endogenous hormones were detected by reverse-phase HPLC-MS and ELISA. The higher TDZ concentrations decreased endogenous benzyladenine (BA) and IAA levels, while iPA and ZR levels and their ratios to IAA were not correlated with TDZ concentrations, growth of calli, or plant regeneration. BA was the dominant cytokinin during calli growth and bud formation, 10 000-fold more abundant than iPA and ZR. Moreover, low IAA/BA ratio, rather than low IAA or high cytokinin levels alone, is the most important factor for shoot formation by calli. TDZ might act indirectly by changing endogenous hormonal levels, most likely the IAA/BA ratio which controls the calli growth and bud formation.