Involvement of the Accumulation of Glutamine in the Initiation of Adventitious Buds in Stem Segments of Torenia

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Abstract
Active meristematic divisions in stem segments of Torenia cultured in vitro can be induced in the epidermis by application of cytokinin or the calcium ionophore A23187, resulting in the differentiation of adventitious buds. Endogenous free glutamine accumulated at a high concentration in the epidermal tissues during the early stages of such cultures. The accumulation of glutamine was caused by an increase in glutamine synthetase (GS) activity, and the increase of GS activity was suppressed by the application of some inhibitors of GS activity, mRNA synthesis, protein synthesis, or calmodulin. Incorporation of these inhibitors into the culture medium also inhibited initiation of adventitious buds. The inhibitory effect of an inhibitor of GS, methionine sulfoximine (MSX), was apparent only at the very begining of the culture, and the effect could be overcome by the simultaneous addition of glutamine. The inhibitory action of MSX on initiation of buds seemed to be caused by an accumulation of ammonium ions. Reduction in levels of NH4NO3 in or its elimination from the culture medium stimulated the initiation of adventitious buds. Therefore, both the accumulation of glutamine and the reduction in levels of ammonium ions seem to play a role in the initiation of adventitious buds in stem., segments of Torenia.