The Control of Growth Symmetry Breaking in the Arabidopsis Hypocotyl.

Complex shapes in biology depend on the ability of cells to shift from isotropic to anisotropic growth during development. In plants, this growth symmetry breaking reflects changes in the extensibility of the cell walls. The textbook view is that the direction of turgor-driven cell expansion depends on the cortical microtubule (CMT)-mediated orientation of cellulose microfibrils. Here, we show that this view is incomplete at best. We used atomic force microscopy (AFM) to study changes in cell-wall mechanics associated with growth symmetry breaking within the hypocotyl epidermis. We show that, first, growth symmetry breaking is preceded by an asymmetric loosening of longitudinal, as compared to transverse, anticlinal walls, in the absence of a change in CMT orientation. Second, this wall loosening is triggered by the selective de-methylesterification of cell-wall pectin in longitudinal walls, and, third, the resultant mechanical asymmetry is required for the growth symmetry breaking. Indeed, preventing or promoting pectin de-methylesterification, respectively, increased or decreased the stiffness of all the cell walls, but in both cases reduced the growth anisotropy. Finally, we show that the subsequent CMT reorientation contributes to the consolidation of the growth axis but is not required for the growth symmetry breaking. We conclude that growth symmetry breaking is controlled at a cellular scale by bipolar pectin de-methylesterification, rather than by the cellulose-dependent mechanical anisotropy of the cell walls themselves. Such a cell asymmetry-driven mechanism is comparable to that underlying tip growth in plants but also anisotropic cell growth in animal cells.