Regulation of Benzyladenine-Induced in Vitro Shoot Organogenesis and Endogenous Proline in Melon (Cucumis melo L.) by Exogenous Proline, Ornithine, and Proline Analogues

Abstract

Melon (Cucumis melo L.) is an excellent system to study the role of proline in regulating benzyladenine-induced shoot organogenesis, which is also an important system in our laboratory for investigating flavor and antioxidant metabolite synthesis in differentiated tissues. We have hypothesized that proline stimulated in vitro shoot organogenesis through the activation of the proposed proline-linked pentose phosphate pathway, which may regulate endogenous cytokinin and auxin biosynthesis. To further investigate this hypothesis, it is essential to first confirm whether benzyladenine-induced shoot organogenesis can be stimulated by enhancing proline synthesis through the use of proline analogues and ornithine (proline precursor). In this study, benzyladenine-induced shoot organogenesis was substantially stimulated by exogenous proline and/or ornithine in combination with proline analogues such as azetidine-2-carboxylate (A2C) and hydroxyproline (HP). Among these treatments, proline in combination with A2C or HP showed the highest stimulation. The stimulation of organogenesis correlated closely with increased levels of endogenous proline content, thereby strengthening the hypothesis that proline-linked activation of purine and aromatic metabolism via the stimulation of pentose phosphate pathway may be important for in vitro organogenesis in melon.