A. thaliana XVE_mTCP3 ER 16hr vs. XVE_mTCP3 DMSO 16hr

Cell expansion is crucial for organ morphogenesis in multicellular organisms. Plant cells are surrounded by rigid cell walls and turgor pressure drives cell expansion at a rate that is limited by the mechanical property of cell wall. The regulation of cell wall-loosening underlies the mechanism of cell expansion, but the integration of this regulation with organ morphogenesis is still elusive. In this study, we demonstrate that TEOSINTE BRANCHED1, CYCLOIDEA, PROLIFERATING CELL NUCLEAR ANTIGEN BINDING FACTORs (TCPs) transcription factors integrate cell wall loosening and organ morphogenesis in Arabidopsis thaliana. We observed that TCPs stimulated cell expansion and exaggerated elongation of hypocotyls. The atomic force microscopy analysis demonstrated that TCPs decreased the stiffness of cells, indicating the roles of TCPs to regulate the mechanical properties of cell wall. The gene expression analyses found that TCPs regulated the genes for both cell wall loosening and apoplast acidification such as SMALL AUXIN UP RNAs (SAURs). Because apoplast acidification induces cell wall loosening, we conducted western blot and pharmacological analyses and revealed that TCPs activated a plasma membrane-localized H+-ATPase responsible for apoplast acidification. Monitoring the apoplast pH at cellular resolutions indicated that TCPs stimulated apoplast acidification. Ectopic expression of a SAUR gene in a sextuple tcp mutant substantially recovered the hypocotyl morphology of the tcp mutant, providing the genetic evidence of the TCP-mediated SAUR regulation. Altogether, we demonstrate that TCPs govern cell wall loosening for cell expansion and pave the way to modify plant architectures through the regulation of cell wall loosening. Two-condition experiment, ER treated vs. DMSO treated. Biological replicates: 3 ER-treated replicates, 3 DMSO-treated replicates.