23 24 Short title: SUB mediates cell wall stress signaling 25 26 Abstract 29 Plant cells are encased in a semi-rigid cell wall of complex build. As a consequence, 30 cell wall remodeling is essential for the control of growth and development as well as 31 the regulation of abiotic and biotic stress responses. Plant cells actively sense physico-32 chemical changes in the cell wall and initiate corresponding cellular responses. 33 However, the underlying cell wall monitoring mechanisms remain poorly understood.34 In Arabidopsis the atypical receptor kinase STRUBBELIG (SUB) mediates tissue 35 morphogenesis. Here, we show that SUB-mediated signal transduction also regulates 36 the cellular response to a reduction in the biosynthesis of cellulose, a central 37 carbohydrate component of the cell wall. SUB signaling affects early increase of 38 intracellular reactive oxygen species, stress gene induction as well as ectopic lignin 39 and callose accumulation upon exogenous application of the cellulose biosynthesis 40 inhibitor isoxaben. Moreover, our data reveal that SUB signaling is required for 41 maintaining cell size and shape of root epidermal cells and the recovery of root 42 growth after transient exposure to isoxaben. SUB is also required for root growth 43 arrest in mutants with defective cellulose biosynthesis. Genetic data further indicate 44 that SUB controls the isoxaben-induced cell wall stress response independently from 45 other known receptor kinase genes mediating this response, such as THESEUS1 or 46 MIK2. We propose that SUB functions in a least two distinct biological processes: the 47 control of tissue morphogenesis and the response to cell wall damage. Taken together, 48 our results reveal a novel signal transduction pathway that contributes to the 49 molecular framework underlying cell wall integrity signaling. 50 51 52 3 53 Author Summary54 Plant cells are encapsulated by a semi-rigid and biochemically complex cell wall. This 55 particular feature has consequences for multiple biologically important processes, 56 such as cell and organ growth or various stress responses. For a plant cell to grow the 57 cell wall has to be modified to allow cell expansion, which is driven by outward-58 directed turgor pressure generated inside the cell. In return, changes in cell wall 59 architecture need to be monitored by individual cells, and to be coordinated across 60 cells in a growing tissue, for an organ to attain its regular size and shape. Cell wall 61 surveillance also comes also into play in the reaction against certain stresses, 62 including for example infection by plant pathogens, many of which break through the 63 cell wall during infection, thereby generating wall-derived factors that can induce 64 defense responses. There is only limited knowledge regarding the molecular system 65 that monitors the composition and status of the cell wall. Here we provide further 66 insight into the mechanism. We show that the cell surface receptor STRUBBELIG, 67 previously known to control organ development in Ar...

Plant cells are encased in a semi-rigid cell wall of complex build. As a consequence, cell wall remodeling is essential for the control of growth and development as well as the regulation of abiotic and biotic stress responses. Plant cells actively sense physico-chemical changes in the cell wall and initiate corresponding cellular responses. However, the underlying cell wall monitoring mechanisms remain poorly understood. In Arabidopsis the atypical receptor kinase STRUBBELIG (SUB) mediates tissue morphogenesis. Here, we show that SUBmediated signal transduction also regulates the cellular response to a reduction in the biosynthesis of cellulose, a central carbohydrate component of the cell wall. SUB signaling affects early increase of intracellular reactive oxygen species, stress gene induction as well as ectopic lignin and callose accumulation upon exogenous application of the cellulose biosynthesis inhibitor isoxaben. Moreover, our data reveal that SUB signaling is required for maintaining cell size and shape of root epidermal cells and the recovery of root growth after transient exposure to isoxaben. SUB is also required for root growth arrest in mutants with defective cellulose biosynthesis. Genetic data further indicate that SUB controls the isoxaben-induced cell wall stress response independently from other known receptor kinase genes mediating this response, such as THESEUS1 or MIK2. We propose that SUB functions in a least two distinct biological processes: the control of tissue morphogenesis and the response to cell wall damage. Taken together, our results reveal a novel signal transduction pathway that contributes to the molecular framework underlying cell wall integrity signaling.

Cell wall remodeling is essential for the control of growth and development as well as the regulation of stress responses. However, the underlying cell wall monitoring mechanisms remain poorly understood. Regulation of root hair fate and flower development in Arabidopsis thaliana requires signaling mediated by the atypical receptor kinase STRUBBELIG (SUB). Furthermore, SUB is involved in cell wall integrity signaling and regulates the cellular response to reduced levels of cellulose, a central component of the cell wall. Here, we show that continuous exposure to sub-lethal doses of the cellulose biosynthesis inhibitor isoxaben results in altered root hair patterning and floral morphogenesis. Genetically impairing cellulose biosynthesis also results in root hair patterning defects. We further show that isoxaben exerts its developmental effects through the attenuation of SUB signaling. Our evidence indicates that downregulation of SUB is a multi-step process and involves changes in SUB complex architecture at the plasma membrane, enhanced removal of SUB from the cell surface, and downregulation of SUB transcript levels. The results provide molecular insight into how the cell wall regulates cell fate and tissue morphogenesis.

Cell wall remodeling is essential for the control of growth and development as well as the regulation of stress responses. However, the underlying cell wall monitoring mechanisms remain poorly understood. Regulation of root hair fate and flower development in Arabidopsis thaliana requires signaling mediated by the atypical receptor kinase STRUBBELIG (SUB). Furthermore, SUB is involved in cell wall integrity signaling and regulates the cellular response to reduced levels of cellulose, a central component of the cell wall. Here, we show that continuous exposure to sub-lethal doses of the cellulose biosynthesis inhibitor isoxaben results in altered root hair patterning and floral morphogenesis. Genetically impairing cellulose biosynthesis also results in root hair patterning defects. We further show that isoxaben exerts its developmental effects through the attenuation of SUB signaling. Our evidence indicates that down-regulation of SUB is a multi-step process and involves changes in SUB complex architecture at the plasma membrane, enhanced removal of SUB from the cell surface, and downregulation of SUB transcript levels. The results provide molecular insight into how the cell wall regulates cell fate and tissue morphogenesis.

Intercellular communication plays a central role in organogenesis. Tissue morphogenesis in Arabidopsis thaliana requires signaling mediated by a cell surface complex containing the atypical receptor kinase STRUBBELIG (SUB) and the multiple C2 domains and transmembrane region protein QUIRKY (QKY). QKY is required to stabilize SUB at the plasma membrane. However, it is unclear what the in vivo architecture of the QKY/SUB signaling complex is, how it is controlled, and how it relates to the maintenance of SUB at the cell surface. Using a combination of yeast two-hybrid assays and Foerster resonance energy transfer (FRET)/fluorescence lifetime imaging microscopy (FLIM) in epidermal cells of seedling roots we find that QKY promotes the formation of SUB homo-oligomers in vivo, a process that appears to involve an interaction between the extracellular domains of SUB. We also show that QKY and SUB physically interact and form a complex at the cell surface in vivo. In addition, the data show that the N-terminal C2A-B region of QKY interacts with the intracellular domain of SUB. They further reveal that this interaction is essential to maintain SUB levels at the cell surface. Finally, we provide evidence that QKY forms homo-multimers in vivo in a SUB-independent manner. We suggest a model in which the physical interaction of QKY with SUB mediates the oligomerization of SUB and attenuates its internalization, thereby maintaining sufficiently high levels of SUB at the cell surface required for the control of tissue morphogenesis.