Cytokinin activation of de novo thiamine biosynthesis in tobacco callus cultures.

The effect of kinetin on the de novo formation of thiamine in tobacco callus cultures was measured by following the isotope dilution of previously introduced (14)C-thiamine. Thiamine was determined by the thiochrome fluorescence assay after chromatographic purification. Morphological effects induced by high kinetin concentrations were visible within a week after tissue transfer, but thiamine synthesis was insignificant for 2 weeks both in cultures with high (1000 mug/l) and low (30 mug/l) kinetin treatments. Thiamine synthesis during the third week was observed at both kinetin levels, the high kinetin treatment supporting 2.5 times the thiamine synthesis of the low kinetin treatment. The kinetin induced increases in thiamine observed earlier by Digby and Skoog apparently resulted from stimulation of thiamine synthesis rather than from sparing its destruction. Thiamine synthesis is initiated when thiamine concentration reaches a minimum in the callus tissue. This suggests that kinetin is required for the synthesis, but that the activation of synthesis is under feedback control sensitive to the level of thiamine in the tissue.