ECTOPIC EXPRESSION OF A SINGLE HOMEOTIC GENE, THE PETUNIA GENE GREEN PETAL, IS SUFFICIENT TO CONVERT SEPALS TO PETALOID ORGANS

Genetic studies in Arabidopsis and Antirrhinum showed that petal deter mination requires the concomitant expression of two homeotic functions , A and B, whereas the A function alone determines sepal identity. The B function is represented by at least two genes. The Petunia homeotic gene green petal (gp) is essential for petal determination as demonst rated by a Petunia gp mutant that has sepals instead of petals. We hav e used ectopic expression of the gp gene as a tool to study flower dev elopment in Petunia. CaMV 35S-gp expression leads to homeotic conversi on of sepals into petaloid organs when expressed early in development. This demonstrates that a single homeotic gene is sufficient to induce homeotic conversion of sepals to petals, suggesting that other petal determining genes are regulated in part by ectopically expressed gp. I ndeed, two other MADS-box-containing genes, pmads 2 and fbp 1, which s how homology to the Antirrhinum B function gene globosa, are activated in the converted petal tissue. Furthermore, our data provide evidence for autoregulation of gp expression in the petaloid tissue and uncove r the role of gp in fusion of petal tissues.