The shoot apical meristem (SAM) comprises a group of undifferentiated cells that give rise to all above-ground organs. Recently, it has been well studied that small RNAs and Argonaute (AGO) proteins play essential roles in SAM maintenance and dorsoventral leaf patterning. Though AGO1 and AGO10, phylogenetically close AGOs in Arabidopsis, regulate these events, the underlying mechanism had been obscure. In this lecture, I will focus on a paper that revealed an interesting function of AGO10 in SAM maintenance and organ differentiation, with discussions on cell signaling in these events.