Petunia AGAMOUS Enhancer-Derived Chimeric Promoters Specify a Carpel-, Stamen-, and Petal-Specific Expression Pattern Sufficient for Engineering Male and Female Sterility in Tobacco

Previous studies have shown that the AtAGIP promoter derived from the Arabidopsis AGAMOUS (AG) second intron/enhancer specifies a carpel- and stamen-specific expression in its native host species, but not in heterologous species such as tobacco, which restricts its application in the engineering of male and female sterility. These findings also imply that the AG regulatory mechanism that has evolved in Arabidopsis may, to some extent, have diverged from that of tobacco. To test whether a similar chimeric promoter created using the AG second intron/enhancer can overcome this barrier of evolutionary divergence in closely related species, we generated forward- and reverse-oriented chimeric promoters, fPtAGIP and rPtAGIP, from the petunia AG second intron/enhancer (PtAGI) fragment and tested them in tobacco, which, like petunia, belongs to the Solanaceae family. Our results demonstrate that both fPtAGIP and rPtAGIP confer similar carpel- and stamen-specific expression without any leaky activity in vegetative tissues in tobacco as revealed by tissue-specific gene expression and tissue ablation. This pattern resembles that driven by the AtAGIP in Arabidopsis and indicates that the AG regulatory mechanism is more conserved between tobacco and petunia than between tobacco and Arabidopsis. The petunia-derived promoters also exhibited petal-specific activity, and their activities in floral organs were substantially influenced by the orientation of the PtAGI enhancer, with reverse-oriented enhancers displaying approximately double the effectiveness of forward-oriented enhancers. These two properties are novel and have not been observed previously with AtAGIP promoters. Furthermore, we found that PtAGIP promoter-driven tissue ablation is effective for engineering complete sterility in plants, and the resulting sterile trait is stable for at least three mitotic generations at various temperature regimes, which is important for the complete containment of seed-, pollen-, and fruit-mediated gene flow in field conditions.