AbstractIn Arabidopsis thaliana and many other plant species, ovules arise from carpel tissue as new meristematic formations. Cell fate in proliferating ovule primordia is specified by particular ovule identity factors, such as the homeodomain factor BELL1 (BEL1) and MADS box family members SEEDSTICK (STK), SHATTERPROOF1 (SHP1), SHP2, and AGAMOUS. Both in the bel1 mutant and the stk shp1 shp2 triple mutant, integuments are transformed into carpelloid structures. Combining these mutants in a bel1 stk shp1 shp2 quadruple mutant, we showed that the bel1 phenotype is significantly enhanced. We also demonstrate that ovule differentiation requires the regulation of the stem cell maintenance gene WUSCHEL, repression of which is predominantly maintained by BEL1 during ovule development. Based on yeast three-hybrid assays and genetic data, we show that BEL1 interacts with the ovule identity MADS box factors when they dimerize with SEPALLATA proteins. We propose a model for ovule development that explains how the balance between carpel identity activity and ovule identity activity is established by a MADS box homeodomain protein complex.