Plant Growth Regulator Effects on In Vitro Propagation and Stevioside Production in Stevia rebaudiana Bertoni
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Stevia rebaudiana is of great importance due to its steviol glycosides (SGs) which are natural sweeteners used by the food industry as well as having medicinal purposes. In the present study, the effect of plant growth regulators (PGRs) and explant types on in vitro propagation and shoot growth of S. rebaudiana were studied, the effect of PGRs on SGs production was determined. For this purpose, nodal explants and shoot tip explants were cultured on woody plant medium (WPM) supplemented with cytokinins [6-benzyladenine (BA), kinetin (Kn), or thidiazuron (TDZ)] or cytokinins + auxins combinations [BA + indoleacetic acid (IAA); BA + naphthaleneacetic acid (NAA); Kn + IAA; Kn + NAA]. Although, the best shoot proliferationwas obtained onWPMsupplementedwith BA+ NAA combinations, shoots grown on PGR-containing media produced callus at the base of the shoots and showed chlorosis and necrosis. Additionally, shoots showed at all concentrations of TDZ, and at higher concentrations of BA, morphological changes such as malformed leaves and poor shoot growth. In contrast to PGR-containing media, on the PGR-free control medium, the development of shoots and roots occurred simultaneously and healthy and well-developed plantlets were obtained. Thus, we developed an economical viable means of in vitro propagation byminimizing themicropropagation steps and removing the requirement of PGRs. According to the high-performance liquid chromatography (HPLC) results, PGR-free control medium (WPM) led to considerably higher stevioside content in the leaves compared with the PGR(s)-containing media and the highest stevioside [34 mg.g(-1) dry weight (DW)] and rebaudioside A content was only detected on the control medium without PGRs. Steviolbioside, rubusoside, and dulcoside A were detected qualitatively in the leaves of shoots grown on WPM supplemented with 2.27 mu M TDZ, 4.54 mu M TDZ, 2.22 mu M BA + 2.69 mu M NAA, 2.22 mu M BA + 5.37 mu M NAA, 2.32 mu M Kn + 5.71 mu M IAA, or 2.32 mu M Kn + 2.69 mu M NAA.