The PERIANTHIA gene encodes a bZIP protein involved in the determination of floral organ number in Arabidopsis thaliana

Mutations in the PERIANTHIA (PAN) gene of Arabidopsis thaliana specifically transform flowers from tetramerous to largely pentamerous, which is a char acteristic of flowers of ancestral plants. We have cloned the PAN gene and here we show that it encodes a member of the basic region/leucine zipper cl ass of transcription factors. Immunohistochemical analysis shows that the e ncoded protein is present in the apical meristem, the floral meristem, each whorl of organ primordia, and in ovule primordia during wild-type flower d evelopment. PAN expression occurs independently of genes affecting floral m eristem identity, floral meristem size, or floral organ number. The near ab sence of a phenotype in transgenic plants overexpressing PAN and the contra st between the broad expression of PAN and the specificity of its mutant ph enotype suggest that its activity may be regulated post-translationally or by the presence of partner proteins. Based on these results and on data rep orted previously, we propose models for the role of PAN in the evolution of flower pattern in the mustard family.