The mechanics of cell fate determination in petals.

The epidermal cells of petals of many species are specialized, having a pronounced conical shape. A transcription factor, MIXTA, is required for the formation of conical cells in Antirrhinum majus; in shoot epidermal cells of several species, expression of this gene is necessary and sufficient to promote conical cell formation. Ectopic expression has also shown MIXTA to be able to promote the formation of multicellular trichomes, indicating that conical cells and multicellular trichomes share elements of a common developmental pathway. Formation of conical cells or trichomes is also mutually exclusive with stomatal formation. In Antirrhinum, MIXTA normally only promotes conical cell formation on the inner epidermal layer of the petals. Its restricted action in cell fate determination results from its specific expression pattern. Expression of MIXTA, in turn, requires the activity of B-function genes, and biochemical evidence suggests that the products of DEFICIENS, GLOBOSA and SEPALLATA-related genes directly activate MIXTA expression late in petal development, after the completion of cell division in the petal epidermis. A MIXTA-like gene, AmMYBML1, is also expressed in petals. AmMYBML1 expression is high early in petal development. This gene may direct the formation of trichomes in petals. In specifying the fates of different cell types in petals, regulatory genes like MIXTA may have been duplicated. Changes in the timing and spatial localization of expression then provides similar regulatory genes which specify different cell fates.