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Abstract Three members of the Arabidopsis AINTEGUMENTA-LIKE/PLETHORA (AIL/PLT) transcription factor family, AIL5/PLT5, AIL6/PLT3, and AIL7/PLT7, exhibit partially overlapping roles with AINTEGUMENTA (ANT) during flower development. Loss ofANTfunction alone results in smaller floral organs and female sterility indicating that some ANT functions cannot be provided by these related transcription factors. Previously, we showed that expression ofAIL6at the same levels and spatial pattern asANTcould largely rescue the defects ofantmutants. This suggested that the functional differences betweenANTandAIL6were primarily a consequence of expression differences. Here, we investigated the functional differences betweenANTand bothAIL5andAIL7by expressing these twoAILs under the control of theANTpromoter. We found that onlyANT:gAIL5lines with much higher amounts ofAIL5mRNA as compared withANTcould compensate for loss ofANTfunction.ANT:gAIL7lines withAIL7mRNA levels similar to those ofANTwere able to rescue some but not all aspects of theantmutant phenotype. Thus, expression differences alone cannot explain the functional differences between ANT and these two related proteins. Studies in yeast show that AIL5 and AIL7 have lower transcriptional activation activities as compared with ANT and AIL6 when bound to the consensus ANT DNA binding site. Our results suggest that differences in both expression and protein activity contribute to the functional specificity of ANT compared with AIL5 and AIL7.