HISTOLOGY OF THE MORPHOGENIC RESPONSE IN THIN CELL LAYER EXPLANTS FROM VEGETATIVE TOBACCO PLANTS

The morphogenic response of thin cell layers (TCLs) from vegetative tobacco (Nicotiana tabacutn L.) plants can be directed very precisely by varying the concentrations of benzyladenine (BA) and α-naphthalene acetic acid (NAA) in the culture medium. Medium containing 1-6 µm BA and 0-5 µm NAA was optimal for shoot formation, concentrations of 0-5 µm BA and 1-6 µmNAA were optimal for the induction of shoots and roots on the same explant, whereas concentrations of NAA. higher than 16 µm resulted in callus proliferation only. Polarity in the distribution of the shoot buds was observed, i.e. a switch from basal to apical shoot formation occurred with increasing NAA concentrations, suggesting basipetal transport of NAA. Histological examination of TCLs on shoot induction medium revealed that first cell divisions occurred within 2 d in cortical cells which were directly in contact with the medium along the longitudinal cut surface, and after 2 d in subepidermal cells along the lateral edges of the explants. Individual lateral buds originated from one subepidermal and one or more epidermal cells, while apical buds originated from single subepidermal or cortical cells located directly at the apical end of the explant. After culture of TCLs for 2-3 d on root/shoot induction medium cells in the regeneration-competent subepidermis elongated, while on callus induction medium subepidermal cells elongated and dedifferentiated. The regeneration system as described in this study will be used to identify cells competent for regeneration as well as for transformation. © 1994 Annals of Botany Company.