Mechanical constraint causes lower turgor, thicker walls, and faster growth in Arabidopsis root hairs.

Root hairs absorb water and mineral nutrients while anchoring growing root tips. They must navigate through soils of varying mechanical properties. Mechanical changes in their microenvironment affect root hair shape and growth rate, but what underlies these responses has remained elusive. To uncover these mechanisms, we grew seedlings of Arabidopsis thaliana (Col-0) in media with increasing mechanical stiffness. Col-0 seedlings showed both shorter and fewer root hairs with increasing mechanical resistance. We used incipient plasmolysis to estimate turgor pressure in trichoblasts and found that it also decreased with increasing media stiffness. Because cellulose is the major load-bearing polymer in the cell wall and influences cell expansion, we quantified cellulose orientation in root hairs. We found that cellulose fibrils were oriented at steeper angles relative to the growth axis in root hairs grown in stiffer media, suggesting a response to mechanical stress. Cell wall thickness also increased with increasing media stiffness. Microtubule orientations followed patterns that were similar to those of cellulose fibrils, but at a smaller angle relative to the growth axis, whereas microtubule density decreased with increasing media stiffness. Unexpectedly, we observed that root hairs grew faster in stiffer media, implying that their growth is misregulated, potentially triggering wall integrity signaling that causes early growth arrest. Finite element modeling of root hairs predicted decreased surface stress, explaining these growth phenotypes. These findings help establish mechanistic links among mechanotransduction, cytoskeletal dynamics, and cell wall assembly during root hair growth.