Conversion of leaves into petals in Arabidopsis

More than 200 years ago, Goethe proposed that each of the distinct flower o rgans represents a modified leaf [1]. Support for this hypothesis has come from genetic studies, which have identified genes required for flower organ identity. These genes have been incorporated into the widely accepted ABC model of flower organ identity, a model that appears generally applicable t o distantly related eudicots as well as monocot plants. Strikingly, triple mutants lacking the ABC activities produce leaves in place of flower organs , and this finding demonstrates that these genes are required for floral or gan identity [2], However, the ABC genes are not sufficient for floral orga n identity since ectopic expression of these genes failed to convert vegeta tive leaves into flower organs. This finding suggests that one or more addi tional factors are required [3, 4], We have recently shown that SEPALLATA ( SEP) represents a new class of floral organ identity genes since the loss o f SEP activity results in all flower organs developing as sepals [5], Here we show that the combined action of the SEP genes, together with the A and B genes, is sufficient to convert leaves into petals.