[bioRxiv] Optimal BR signalling is required for adequate cell wall orientation in the Arabidopsis root meristem

Abstract

The plant steroid hormones brassinosteroids (BRs) regulate growth in part through altering the properties of the cell wall, the extracellular matrix of plant cells. Conversely, cell wall signalling connects the state of cell wall homeostasis to the BR receptor complex and modulates BR activity. Here we report that both pectin-triggered cell wall signalling and impaired BR signalling result in altered cell wall orientation in the Arabidopsis root meristem. BR-induced defects in the orientation of newly placed walls are associated with aberrant localization of the cortical division zone but with normal specification of its positioning. Tissue- specific perturbations of BR signalling revealed that the cellular malfunction is unrelated to previously described whole organ growth defects. Thus, tissue type separates the pleiotropic effects of cell wall/BR signals and highlights their importance during cell wall placement.


Tissue-specific expression of PMEIox reveals that root waving is independent from cell wall orientation defects. (A) Ubiquitous trans-activation of PMEI5 recapitulates the PMEIox cell wall phenotype (arrow). (B-D) In lines with tissue-specific PMEI5 expression in epidermis, hair cells, or xylem pole pericycle cells, cell wall orientation in the RAM is normal. 


Cell wall perturbation by PMEIox leads to cell division defects after specification of the cortical division zone.
(A, B) Orientation of pre-prophase bands labelled by see (I) for quantification. (C-F) Metaphase plate orientation (C, D; see J for quantification) and sister chromatid orientation during anaphase (E, F; see K for quantification) deviates from a 90°C angle in both the Col-0 wild type (C, E) and PMEIox (D, F). (G, H) Orientation of the cell plate in the Col-0 wild type stabilized at around 90 °C relative to the cell long axis (G), while PMEIox cell plates showed a wide range of orientations (H, see L for quantification).