Regulation of chalcone flavanone isomerase (CHI) gene expression inPetunia hybrida: the use of alternative promoters in corolla, anthers and pollen

In this paper we report on the organization and expression of the two chalcone flavanone isomerase (CHI) genes A and B from thePetunia hybrida inbred line V30. From a combination of sequence data, primer extension and RNAse protection experiments we infer the presence of two promoters PA1 and PA2 upstream of the CHI gene A coding region. It is shown that both promoters are used differentially in various flower tissues: the PA1 promoter is active in corolla and tube tissue whereas the PA2 promoter, which gives rise to a 437 bp longer transcript, is only active in late stages of anther development and more specifically in pollen grains. The CHI-B gene, on the other hand, has only one promoter (PB) which is active only in immature anther tissue. Thus, in addition to the use of two alternative promoters in front of the same CHI coding region (CHI-A), the promoters in front of the two distinct CHI gene copies are also used differentially as a mechanism to regulate their expression. Comparison of PB with other flavonoid gene promoters active in immature anther tissue revealed a highly conserved region which was designated as 'anther box'. We hypothesize that it plays a regulatory role in anther-specific gene expression. Finally, a model describing the evolutionary relationship between both CHI genes is presented.