Despite its central role in organ formation throughout a plant's life,little is known of how the shoot meristem forms in the Arabidopsisembryo. Now, on p. 2839, Tucker and colleagues reveal that the ARGONAUTE (AGO) family member ZWILLE (ZLL) signals from the embryo's vascular primordium to potentiate the function of WUSCHEL (WUS), a transcription factor that maintains stem cells in an undifferentiated state. In a feedback loop that controls stem cells numbers, shoot meristem cells receive WUS signals and express the signal peptide CLAVATA3 (CLV3), which in turn restricts WUS expression. However, in zll mutant embryos,the authors report, WUS expression expands abnormally and CVL3 expression decreases, probably because WUS function is impaired and cannot maintain CVL3 expression. The authors also show that embryos with a partial loss of ZLL function develop more severe defects when AGO1 is also mutated. Together, these findings indicate that, in a process that potentially involves small RNAs, ZLL promotes a signal from the vasculature that maintains the stem cell niche.