The intricate and elegant architecture of a flower requires the activity of a plethora of proteins. Many belong to a group of gene regulators known as MADS-box proteins. These form multiprotein complexes that direct the formation of concentric whorls of sepals, petals, stamens and carpels. Gary Ditta et al. now show that without the concerted encouragement of four such proteins, all that is produced are clusters of leaves (Curr. Biol. 14, 1935–1940; 2004).
The model plant Arabidopsis thaliana has more than a hundred genes for MADS-box proteins. Mutations in some of them produce dramatic effects whereas others have redundant functions requiring double, triple or even more mutants to uncover their roles. The SEPALLATA (SEP) family fall into this latter category, and earned their name when triple mutants — in which three members of the family were all disrupted — produced flowers made entirely of sepals. With this new work, a fourth sibling, SEP4, emerges.
Ditta et al. found that mutating SEP4 alone had no obvious effect, but quadruple mutants lacking all four SEP proteins formed ‘flowers’ with no recognizable floral organs (as shown here). Closer inspection revealed epidermal cells with the same shape and arrangement as in normal leaves. The presence of branched, hair-like structures called trichomes, a feature of Arabidopis leaves, put this identification beyond doubt.
In his treatise of 1790, Die Metamorphose der Pflanzen (The Metamorphosis of Plants), the poet and philosopher Johann Wolfgang von Goethe proposed that flowers were modified leaves. Two hundred years later, stripping away the influence of the SEP proteins has returned them to their foliar state.
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Surridge, C. A bunch of leaves. Nature 432, 161 (2004). https://doi.org/10.1038/432161a