Differential regulation of meristem size, morphology and organization by the ERECTA, CLAVATA and class III HD-ZIP pathways

The shoot apical meristem (SAM) of angiosperm plants is a highly organized minute structure that gives rise to all above-ground organs. The SAM is divided into three different functional domains. The central zone (CZ) at the SAM tip harbors the self-renewing pluripotent stem cells and the organizing center, providing daughter cells that are continuously displaced into the interior rib zone (RZ) or to the surrounding peripheral zone (PZ), from which organ primordia are initiated. Despite the constant flow of cells from the CZ into the RZ or PZ, and cell recruitment for primordium formation, a stable balance is maintained between the distinct cell populations in the SAM. Here we combined an in depth phenotypic analysis with a comparative RNA-Seq approach to characterize meristems from selected combinations of clavata3 (clv3), jabba-1D (jba1D) and erecta (er) mutants. We demonstrate that CLV3 restricts meristem expansion along the apical basal axis, while class III HD-ZIP and ER pathways restrict meristem expansion laterally, but in distinct and possibly perpendicular orientations. Our k-means analysis reveals that clv3, jba-1D/+ and er lead to meristem enlargement by affecting different aspects of meristem function, e.g., that clv3 displays increase in stem cell population, whereas jba-1D/+ er exhibits increase in mitotic activity and in meristematic cell population. We demonstrate that thecombination of genetic and mRNA-Seq comparative approach provides a precise and sensitive method to identify cell type specific transcriptomes in a small structure such as the SAM.