Dynamics of cytokinins in apical shoot meristems of a day-neutral tobacco during floral transition and flower formation

This study considered cytokinin distribution in tobacco (Nicotiana tabacum L.) shoot apices in distinct phases of development using immunocytochemistr y and quantitative tandem mass spectrometry. In contrast to vegetative apic es and flower buds, we detected no free cytokinin bases (zeatin, dihydrozea tin, or isopentenyladenine) in prefloral transition apices. We also observe d a 3-fold decrease in the content of cytokinin ribosides (zeatin riboside, dihydrozeatin riboside, and isopentenyladenosine) during this transition p hase. The group concluded that organ formation (e.g. leaves and flowers) is characterized by enhanced cytokinin content, in contrast to the very low e ndogenous cytokinin levels found in prefloral transition apices, which show ed no organogenesis. The immunocytochemical analyses revealed a differing i ntracellular localization of the cytokinin bases. Dihydrozeatin and isopent enyladenine were mainly cytoplasmic and perinuclear, whereas zeatin showed a clear-cut nuclear labeling. To our knowledge, this is the first time that this phenomenon has been reported. Cytokinins do not seem to act as positi ve effecters in the prefloral transition phase in tobacco shoot apices. Fur thermore, the differences in distribution at the cellular level may be indi cative of a specific physiological role of zeatin in nuclear processes.