Growth is a major factor in plant organ morphogenesis and is influenced by exogenous and endogenous signals including hormones. Although recent studies have identified regulatory pathways for the control of growth during vegetative development, there is little mechanistic understanding of how growth is controlled during the reproductive phase. Using Arabidopsis fruit morphogenesis as a platform for our studies, we show that the microRNA miR172 is critical for fruit growth, as the growth of fruit is blocked when miR172 activity is compromised. Furthermore, our data are consistent with the FRUITFULL (FUL) MADS-domain protein and Auxin Response Factors (ARFs) directly activating the expression of a miR172-encoding gene to promote fruit valve growth. We have also revealed that MADS-domain (such as FUL) and ARF proteins directly associate in planta. This study defines a novel and conserved microRNA-dependent regulatory module integrating developmental and hormone signalling pathways in the control of plant growth.