Auxin, photoperiod and putrescine affect flower neoformation in normal and rolB-transformed tobacco thin cell layers

Auxin, photoperiod and putrescine are factors involved in the control of the reproductive phase of the flowering process in planta. The aim of this study was to investigate whether auxin is able, in the absence of other exogenous hormones, to stimulate flower neoformation in Nicotiana tabacum L. thin cell layers excised from flowering plants, whether an interaction exists between the hormone and the length of the light treatment per day, and whether exogenous putrescine interferes with auxin and light in the realization of the flowering programme. Since tissues transformed with rolB gene show an increased binding capacity of auxin, the response of rolB-explants was compared with that of the wild type. Thin cell layers from floral branches were cultured in the presence of a wide range (0–10 μM) of indoleacetic acid alone, under various phototreatments (continuous darkness, 8 h light/day, and 16 h light/day). Floral, vegetative and rooting responses were favoured in the transgenic explants. Flower neoformation was stimulated by long days and by exogenous auxin (1 μM). In rolB-thin cell layers flowering also occurred in the most unfavourable culture conditions (i.e. continuous darkness combined with either hormone-free medium or 10 μM auxin). Long days and 1 μM auxin also favoured caulogenesis, though to a lesser extent than flower neoformation. Rhizogenesis occurred in rolB-explants only and was induced by exogenous auxin and favoured by continuous darkness. In both wild type and rolB-explants, exogenous putrescine (1 mM) reduced flower neoformation in the presence of 1 μM auxin and long days.