Background: GREEN PETAL (GP) is thought to be a petunia class B floral homeotic gene, because the gp mutant flower displays a severe homeotic conversion of petals into sepals in the second whorl. However, since the third whorl stamens remain unaffected in the gp null mutant, gp is different from class B mutants in Arabidopsis and Antirrhinum, which also show a conversion of the third whorl stamens into the carpelloid tissue. BLIND (BL) is thought to be a petunia class A floral homeotic gene, because the bl mutant flower displays homeotic conversions of sepals into the stigmatoid tissue in the first whorl and of the corolla limb into antheroid structures in the second whorl.
Results: A double mutant line homozygous for both bl and gp mutations was constructed. The bl gp double mutant flower displays homeotic conversions of sepals into the stigmatoid tissue in the first whorl and of the corolla limb into antheroid structures with stigmatoid tips in the second whorl. In the third and fourth whorls of the mutant flower, organs remained unchanged. In the gp flower, a petunia B-type gene FBP1 is expressed strongly in the third whorl organs, but much more weakly in the second whorl organs. In the bl gp flower, FBP1 was found to be expressed strongly in the second whorl organs as well as in the third whorl organs.
Conclusions: Petunia has a class B gene other than GP that determines organ identities, both in the second and third whorls of the double mutant flower, and the action of the postulated class B gene (here called PhBX) is prevented by the BL gene in the second whorl of the gp flower. PhBX appears to be a gene that specifically interacts with the FBP1 gene, and is involved in the up-regulation of FBP1.