The flattening of leaves to form broad blades is an important adaptation that maximizes photosynthesis. However, the underlying molecular mechanism of this process remains unclear. The WUSCHEL-RELATED HOMEOBOX (WOX) genes WOX1 and PRS are expressed in the leaf marginal domain to enable leaf flattening, but the nature of WOX expression establishment remains elusive. Here we report that adaxial-expressed MONOPTEROS (MP) and abaxial-enriched auxin, together, act as positional cues for patterning the WOX domain. MP directly binds to the WOX1 and PRS promoters and activates their expression. Furthermore, redundant abaxial-enriched ARF repressors suppress WOX1 and PRS expression, also through direct binding. In particular, we show ARF2 is redundantly required with ARF3 and 4 to maintain the abaxial identity. Taken together, these findings explain how adaxial-abaxial polarity patterns the mediolateral axis and subsequent lateral expansion of leaves.