Plants are built from repeating units called phytomers, each containing a leaf, a lateral (axillary) meristem (which can produce a flower or branch), and an internode (the stem between adjacent leaves). Boundaries within the phytomer separate these three components but what establishes these boundaries? On p. 1243, George Chuck and co-workers propose that tasselsheath4 (tsh4), a microRNA-targeted SBP-box transcription factor, establishes the boundaries within the maize inflorescence (tassels and ears). They report that tsh4 mutants have fewer lateral meristems than normal and that ectopic leaves replace these missing meristems. Other genetic experiments indicate that tsh4 is required for branch meristem initiation and maintenance. Interestingly, however, TSH4 protein is localised adjacent to the lateral meristems rather than within them. Finally, double-labelling experiments suggest that the meristem-specific pattern of miR156, a microRNA that regulates tsh4, is complementary to that of TSH4. Together, these results suggest that tsh4 downregulation by microRNAs plays a major role in defining organ boundaries within phytomers.