Morphoregulatory role of thidiazuron: Evidence of the involvement of endogenous auxin in thidiazuron-induced somatic embryogenesis of geranium (Pelargonium x hortorum Bailey)

Thidiazuron-induced somatic embryogenesis in hypocotyl cultures of geranium (Pelargonium x hortorum Bailey cv. Scarlet Orbit Improved) consists of an induction phase on 10 or 20 mu mol . L(-1) thidiazuron (TDZ) followed by an expression phase on a basal medium lacking growth regulators. The induction and development of somatic embryos were found to be closely related to levels of endogenous and exogenous auxin. Inclusion of the auxin-action inhibitor, PCIB, and the auxin-transport inhibitor, TIBA, in a TDZ-induction medium decreased embryogenic response of the cultures by different mechanisms. The decrease of the embryogenic response in the presence of PCIB was accompanied by a corresponding decrease in endogenous levels of auxins, cytokinins, and ABA. These changes in the profiles of endogenous plant growth regulators were nor evident when TIBA was used. High concentrations of exogenous IAA in the expression medium suppressed the development of somatic embryos. Although the precise mode of action of TDZ is unknown, the results of this study provide evidence that TDZ modulates endogenous auxin metabolism during somatic embryo development in geranium hypocotyl cultures.