THE UP INNOVATION. “Deriving from the apical vegetative meristem flank, the abaxial surface is as old as land plants themselves, so genes specifying adaxial identity constitute a key innovation in leaf evolution. Plants appear to have evolved a complex hierarchy of transcription factor activation and depression, with the HD-ZIP (homeodomain–leucine zipper) gene family promoting adaxial leaf surfaces and others promoting abaxial differentiation.”
http://aob.oxfordjournals.org/content/96/3/345.full.pdf