Abstract text: How plant cells acquire the capacity to expand directionally remains a central unresolved question in cell wall biology. While cortical microtubules and cellulose organisation are closely associated with anisotropic growth, the developmental wall states that enable cells to establish a growth axis remain poorly defined.

Our observations indicate that proliferating cells accumulate walls enriched in low-methyl-esterified pectin organised in Ca²⁺-mediated egg-box structures. We propose that in this state, walls impose mechanically uniform constraints that restrict expansion and favour largely isotropic growth. Developmental progression, therefore, requires a transition from this restrictive baseline towards methyl-esterified, tuneable pectin, generating mechanical heterogeneity within the wall.

Building on our identification of Golgi S-adenosyl methionine transporters as key regulators of pectin methylation, we developed an inducible CRISPR-based system enabling near-complete suppression of methyl-esterification. Under these conditions, cells show strongly reduced expansion even when exposed to cues that normally promote elongation, together with increased wall stiffness measured by atomic force microscopy. In parallel, cortical microtubules fail to adopt the characteristic cortical organisation observed in elongating cells, as does cellulose orientation.

Together, these findings support a model in which regulated pectin methylation defines the developmental acquisition of mechanical competence required for cytoskeletal reorganisation and directional growth.