FUNCTIONAL-ANALYSIS OF PETUNIA FLORAL HOMEOTIC MADS BOX GENE PMADS1

The petunia mutant green petal (gp, line PLV) shows a homeotic effect in one floral whorl, that is, the conversion of petal to sepal. We dem onstrate that this mutant contains a chromosomal deletion, including t he petunia MADS box gene pMADS1. Second whorl petal development in thi s null mutant can be restored with a CaMV 35S-pMADS1 transgene, demons trating the essential role of pMADS1 in this process. Because gp (PLV) shows only a minor effect on stamen development, the homeotic effects of pMADS1 are different from those of B-type genes in Antirrhinum and Arabidopsis. Two other MADS box genes, pMADS2 and fbp1 (Angenent et a l. 1992), require pMADS1 to maintain expression in the second whorl. H owever, in the absence of pMADS1 these two genes continue to be expres sed in the third whorl. The functions assigned to pMADS1 are further s upported by experiments in which we phenocopy gp by cosuppression of p MADS1 gene expression. The flowers, obtained through cosuppression and phenotype restoration, display different degrees of sepal to petal co nversion. Analysis of these flowers indicate that pMADS1 controls grow th under the zone of petal and stamen initiation, which causes the cor olla tube and stamen filaments to emerge as a congenitally fused struc ture.