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The RNA base analogues, 2-thiouracil, 6-azauracil and 8-azaguanine incorporated singly into the medium, increased the number of floral buds in excised stem segments of Nicotiana tabacum variety Wisconsin No. 38 cultured in vitro. Combined treatments with 2 and 3 base analogues were even more effective. The effects were prevented by the corresponding natural counterparts, uracil, uridine, and guanosine respectively. These nucleic acid constituents added to cultures without base analogues did not affect the number of floral buds formed. In stem segments from the lower internodes treatments with the analogues effected a transition from vegetative to floral bud formation, thus in a sense removing the floral gradient as defined by Chouard and Aghion.The inhibition of flower bud formation ordinarily achieved even with low concentrations of indole-3-acetic acid (IAA) was counteracted by low concentrations of 2-thiouracil, 6-azauracil and 8-azaguanine. Reversal to vegetative bud formation was not achieved even with high IAA concentrations in the presence of the base analogues.Stem segments excised from young, vegetative tobacco plants never formed floral buds in vitro. Thus, the inhibiting effect of IAA and the promoting effect of base analogues seem to be on the expression and not on the induction of flowering. The regulatory action of the IAA and base analogues in flower bud formation is tentatively ascribed to their differential effects on RNA synthesis.

Flower Formation in Excised Tobacco Stem Segments; II. Reversible Removal of IAA Inhibition by RNA Base Analogues