Spatio-temporal control of cell wall propterties and signalling networks in Arabidopsis meristems

Post-embryonic growth and development tailored to the environmental condition is a distinguishing characteristic of plants imposed by their sessile lifestyle. Lifelong growth from seed to plant death is enabled by pluripotent stem cells encompassed in the meristems, which continuously generate new plant material with high proliferation rates in the periphery, and slowly dividing stem cells in the centre. This gradient of varying proliferation rates is tightly controlled to balance cell proliferation and replenishment with differentiation. A plethora of signalling networks consisting of peptides, phytohormones and transcriptional regulators are crucial to control all these processes in a spatio-temporal manner. Particularly in the shoot apical meristem (SAM), these processes have to be tightly monitored, as cells in the SAM acquire cell identities along their trajectory from the centre of the meristem through the periphery irrespective of clonal lineage. In this study, we attempted to achieve three main aims using genetic characterization, live-cell imaging, and transcriptome profiling. We tried to understand how cell wall properties influence cell identity and differentiation by means of pectin modifications in the shoot apical meristem. We could reveal that imbalancing the pectin modifications in the whole SAM leads to disruption of cell size control, cell shapes, and overall meristem size. Second, we attempted to decipher the role of malectin-like containing RLP4 and its RLP4-like subgroup in cell wall signalling. We show that the evolutionarily conserved RLP4 is specifically located in cell edges. In addition, the extracellular domain of RLP4 associates with th cell wall, suggesting RLP4 could be a novel component of cell wall signalling. Last, we wanted to characterize a newly identified mutant, named cle signalling component1 (csc1) and its function in maintenance of the root and shoot apical meristem. csc1 displayed an enlarged SAM, defects in flower development and elevated xylem cells in the vasculature. We identified, that CSC1 determines meristem size, excerting negative control over both cytokinine response and the expression of the stem cell fate inducing transcription factor WUSCHEL (WUS) in the SAM. Together, our findings expand the wiring networks in maintaining stem cells by one essential player and elucidate the importance of cell wall signalling and cell wall properties in the meristems of Arabidopsis