In the model plant Arabidopsis thaliana, the establishment of organ polarity leads to the expression of FILAMENTOUS FLOWER(FIL) and YABBY3 (YAB3) on one side of an organ. One important question that has remained unanswered is how does this positional information lead to the correct spatial activation of genes controlling tissue identity? We provide the first functional link between polarity establishment and the regulation of tissue identity by showing that FIL and YAB3 control the non-overlapping expression patterns of FRUITFULL (FUL) and SHATTERPROOF (SHP),genes necessary to form stripes of valve margin tissue that allow the fruit to shatter along two defined borders and disperse the seeds. FIL and YAB3 activate FUL and SHP redundantly with JAGGED (JAG), a gene that also promotes growth in organs,indicating that several pathways converge to regulate these genes. These activities are negatively regulated by REPLUMLESS (RPL),which divides FIL/JAG activity, creating two distinct stripes of valve margin.