Effect of Localized Promotion of Cytokinin Biosynthesis on Flower Morphology in Flower Buds of Torenia fournieri Lind.

To analyze the relationship between flower morphology and organ-specific promotion of cytokinin biosynthesis within flower buds, we introduced Arabidopsis isopentenyltransferase 4 (AtIPT4) into torenia (Torenia fournieri L.) under the control of APETALA1 (AP1) or APETALA3 (AP3) promoter. AP1::AtIPT4 plants had an increased number of petals, whereas AP3::AtIPT4 plants had an expanded corolla, a paracorolla, and serrated petal margins along with an increased number of petals. In AP3::AtIPT4 plants, marked receptacle enlargement was observed when the flower buds were in the early corolla development stage in which the paracorolla primordia differentiate. As expected, AtIPT4 was expressed in the sepals and petals of AP1::AtIPT4 plants, and in the petals and stamens of AP3::AtIPT4 plants. Furthermore, the type-A response regulator (TfRR1) and cytokinin oxidase (TfCKX5) genes, which were used as indices of cytokinin signal, showed the same expression patterns as the transgene. These findings indicate that expansion of the corolla and development of the paracorolla and serrated petal margins after receptacle enlargement in AP3::AtIPT4 plants are induced by localized elevated cytokinin signal in the petals and stamens. In contrast, localized elevated cytokinin signal in the sepals and petals only induced an increase in the number of petals. Therefore, an elevated cytokinin signal in the stamen may be important for inducing corolla expansion and for developing a paracorolla and serrated petal margins.