Regulation and manipulation of flavonoid gene expression in anthers of petunia: the molecular basis of the Po mutation.

Molecular mechanisms governing development of the male reproductive organs of flowers, the anthers, are largely unknown. In this article, we report on the investigation of the molecular basis of a mutation involving the expression of a gene encoding the flavonoid biosynthesis enzyme chalcone flavanone isomerase (CHI) in anthers of petunia. In petunia, the gene Po regulates the expression of CHI in anthers: PoPo petunia lines contain CHI enzyme activity in petals and anthers, whereas popo lines contain the CHI enzyme only in petals but not in anthers. As a result of the Po mutation, the substrate of CHI accumulates and therefore the pollen of a popo line are yellow or greenish. The genome of petunia contains two chi genes, chiA and chiB. In a restriction fragment length polymorphism analysis, a 100% linkage was observed between Po and chiA. This result suggested that Po is identical to chiA and that Po is not a regulatory gene of chiA. Introduction of a chiA gene isolated from a PoPo line into a popo line resulted in a complementation of the mutation that was directly visible because the pollen color shifted from yellow to white. This proved that chiA and Po are identical. Because chiA encodes a functional CHI enzyme in flower petals of a popo line, we propose that the Po mutation is a mutation in the regulatory region of chiA abolishing chiA promoter activity in anthers but not in corollas. This change in anther color is a fine illustration of how floral pigmentation can be manipulated in a predictable way and suggests the use of CHI as a visible marker.