Light- and sugar-mediated control of direct de novo flower differentiation from tobacco thin cell layers

The nature of organs neoformed from tobacco (Nicotiana tabacum cv Samsun) thin cell layers is influenced by the quantity of light supplied and on the sequence of this supply. It is observed that glucose exhibits similar effects. In the presence of glucose at 167 millimolar, continuous light of 50 watts per square meter is required for optimal flower differentiation in vitro. However, 50 watts per square meter irradiance limited to 6 days is sufficient to trigger flower formation in 80% of the explants provided that light is applied from day 6 to day 11 of culture. This critical phase may correspond to the initiation phase during which soluble sugars are mainly needed as carbon energy source rather than as osmoregulators. Under continuous or precise sequential sugar deprivations, either no organogenesis occurs, or abnormal structures or buds are formed. Therefore, light per se is not sufficient to induce flower differentiation. Conversely, a specific quantitative combination of glucose and sucrose almost substitutes for the light requirement for differentiation of anthers and pistils. These observations suggest that, during the sequence of events leading to flower differentiation, light acts on energy-dependent sugar uptake and metabolism and on the increase of reducing potential of chloroplasts.