Use of the APETALA1 promoter to assay the in vivo function of chimeric MADS box genes

The MADS domain proteins AP1, AP3, PI, and AG are required to specify the f our classes of organs in an Arabidopsis flower. Each of these proteins is i nvolved in specifying the identity of two different organs in two adjacent floral whorls. They all share a 56-amino acid MADS domain required for DNA binding and dimerization, a region (I or L) involved in dimerization specif icity, the K domain named for its sequence similarity to the coiled-coil of keratin, and a variable carboxy terminal sequence. The abilities of these four related proteins to specify distinct organs presumably result from dif ferential effects on transcriptional regulation. We have previously used ch imeric MADS box genes, expressed under the constitutive 35S promoter, to ma p the regions of these proteins that are responsible for their different or gan identity activities. In this paper, we extend these studies by characte rizing the phenotypes of plants ectopically expressing chimeric genes under the control of the endogenous API promoter. Similar results are obtained w ith the 35S and AP1 promoters, although the endogenous promoter does provid e a more rigorous test of function. We also describe results from new chime ric gene constructs that show new in vivo functions for the K domain and th e amino-terminal portion of the MADS domain.