Abstract text: Cell elongation enables plants to access light, water, and nutrients and is therefore a key driver of plant morphogenesis. Current models propose that cortical microtubules guide cellulose synthesis to deposit cellulose microfibrils in aligned arrays, which mechanically restrict expansion along one axis and promote growth in the perpendicular direction. While this model explains how growth direction can be maintained, it remains unclear how a growth axis initially emerges and signals underlie cellulose fibers orientation.

To investigate this question, we used a single-cell system in which protoplasts regenerate their cell walls and transition from spherical to elongated cells. This approach allowed us to follow individual cellulose fibers in real time. We found that cellulose is not deposited as pre-aligned arrays. Instead, it initially forms a random, reticulated network. As turgor pressure increases, this network ruptures at its weakest point, causing circumferential stretching that reorients existing cellulose fibers to transversal orientation. This is sufficient to propagate further perpendicular growth enforcing cell elongation. Microtubules were not oriented during this transition and mainly supported growth by promoting undirected cellulose synthesis that strengthens the wall. These findings reveal a novel mechanosensitive behavior of the cell wall itself, allowing cellulose reorientation independently of microtubule guidance.