Linking Brassinosteroid Signaling to Stem Cell Homeostasis in the Shoot Apical Meristem

Cel

Brassinosteroids (BRs) are steroidal phytohormones that regulate nearly all aspects of plant growth and development. The shoot and root apical meristems (SAM and RAM, respectively) each contain a small group of pluripotent stem cells that continuously divide to generate aerial and underground tissues. Recent work from the host lab revealed fluctuations in BR activity during the cell cycle in the Arabidopsis RAM, where polarity-driven asymmetric BR signaling and biosynthesis between daughter cells ensure optimal root growth. Whether this mechanism also operates in the SAM remains unknown. While auxin and cytokinin are well established in maintaining SAM homeostasis, the contribution of BRs is poorly understood. Preliminary findings from the host lab show distinct expression patterns of BR-related proteins within the SAM, along with abnormal SAM phenotypes in BR mutants of Arabidopsis and tomato, suggesting an essential but unexplored role for BRs in stem cell regulation. Based on these, I hypothesize that BRs determine SAM size by modulating stem cell homeostasis. Within BRIDGE, I will pursue four objectives: (1) apply live-cell imaging in Arabidopsis and multiplexed RNA imaging in tomato to generate high-resolution maps of BR metabolism and signaling; (2) employ MorphoGraphX-based 3D reconstruction to quantify cellular parameters in wild-type and BR mutants; (3) perform snRNA-seq of tomato SAMs to identify cell type–specific transcriptional responses to BR deficiency; and (4) functionally validate at least two candidate regulators uncovered by snRNA-seq that mediate BR-dependent SAM control. Together, these approaches will reveal how BRs orchestrate cell division and state transitions in the SAM. This work will advance fundamental understanding of stem cell regulation in plants and provide translational strategies to enhance crop productivity by fine-tuning BR-mediated growth control.