Brassinosteroids Render Cell Walls Softer but Less Extensible in Growing Arabidopsis Hypocotyls

Cell wall extensibility is a key biophysical characteristic that defines the rate of plant cell growth. It depends on the wall structure and is controlled by numerous proteins that cut and/or (re)form links between the wall constituents. Cell wall extensibility is currently estimated by different in vitro biomechanical tests. We used the creep method, in which isolated cell walls are extended under a constant load and their time-dependent deformation (creep) is recorded to reveal the biophysical basis of growth inhibition of Arabidopsis thaliana hypocotyls in the presence of 24-epibrassinolide (EBL), one of the most active natural brassinosteroids. We found that EBL rendered the walls of hypocotyl cells softer, i.e., more deformable under mechanical force, which was revealed using heat-inactivated cell walls to eliminate endogenous activities of cell-wall-loosening/tightening proteins. This effect was caused by the altered arrangement of cellulose microfibrils. At the same time, EBL made the walls less extensible, which was detected with native walls under conditions optimized for activities of endogenous cell-wall-loosening proteins. These apparently conflicting changes in the wall mechanics can be an adaptation by which EBL enables plant cells to grow under stress conditions.