Signaling Modules Regulating Shoot Meristem Function and Organ Development in Arabidopsis

Shoot apical meristems (SAMs), located at the growing tips of land plants, give rise to the primary above-ground organs, relying on the balance of stem cell population and coordinated functional regions in the meristem. Throughout the life cycle of flowering plants, vegetative SAMs produce leaves, whereas inflorescence SAMs give rise to floral meristems that initiate floral organs. The HAIRY MERISTEM (HAM) family of GRAS-domain transcription factors plays crucial and evolutionarily conserved roles in maintaining meristem indeterminacy across species. In Arabidopsis SAMs, HAM genes are directly repressed by microRNA171 (miR171), which is activated by epidermis-specific transcription factors. miR171 specifically mediates the cleavage of HAM1-3 transcripts by targeting conserved miR171-binding sites within their coding sequences, thereby establishing an apical-basal gradient of HAM proteins in SAMs. This epidermis-derived regulatory mechanism ensures that HAM proteins are excluded from the epidermal layer and accumulate predominantly in the inner layers, contributing to stem cell homeostasis. Despite these insights, the broader signaling network mediated by the miR171-HAM module, especially those governing cell proliferation and organ initiation from SAMs, remains unclear. In this study, we identified a signaling pathway downstream of HAM and investigated the regulatory role of this pathway in meristem activity and organ development in both vegetative and reproductive phases. Together, this work dissects the miR171-HAM regulatory module at various stages of plant development and provides new insights into the regulation of cell proliferation and organ growth in multicellular organisms.