Adaxial and abaxial layers are molecularly and morphologically distinct. (A, B) Adaxial (AD) and abaxial (AB) domains are morphologically distinct. Arabidopsis leaves usually possess two layers of adaxial cells and four layers of abaxial cells. (C) The adaxial–abaxial axis is patterned by a complex and redundant network of transcription factors and small RNAs (see main text). T-bars denote repressive relationships. Direct interactions (solid lines) and indirect interactions (dotted lines) are noted. Barbell between ARF and KAN denotes protein–protein interaction. (D) Cell layers (C₁ through C₆) of the adaxial–abaxial axis constituting the fixed domain of the model. Shown are sites of accumulation for mobile small RNAs and cell-autonomous transcription factors. Triangles extending from miR166 and tasiARF transcription domains illustrate small RNA diffusion gradients.