Plant leaves are typically flat structures. To produce this shape, the leaf primordium, as it emerges from the shoot apical meristem, grows perpendicular to its adaxial-abaxial axis - the equivalent of the dorsal-ventral axis in animals. Specialised cells then develop on the two surfaces of the leaf. On , Golz and co-workers report that GRAMINIFOLIA (GRAM) and PROLONGATA (PROL), related YABBY transcription factors, promote the growth and asymmetry of Antirrhinum majus leaves. The researchers show that GRAMexpression in the abaxial margins of leaf primordia promotes lateral growth and abaxial cell fate by excluding adaxial identity. Paradoxically, GRAM (and its paralogue PROL) also promotes adaxial organ identity by acting non cell-autonomously. The researchers draw a parallel with Decapentaplegic signalling, which specifies both dorsal and ventral fates in Drosophila embryos, and propose that the opposing effects of GRAM reinforce and maintain the adaxial-abaxial boundary in leaf primordia.
