Tissue-wide cues are sensed at the cellular level to coordinate microtubule orientations in plants

Plant morphogenesis depends on tissue-wide coordination of cortical microtubule orientations. This coordination is thought to depend on microtubules responding to mechanical stress orientations. Here we test this stress-sensing hypothesis by quantifying microtubule behaviours on different faces and edges of growing leaf cells. We show that microtubules orientations exhibit both cell-geometric and tissue biases. Stress sensing can account for tissue biases but not microtubule trajectories and densities. An alternative hypothesis is suggested by two edge behaviours: generation of microtubules within cell edges and a filter that prevents microtubules entering side faces at shallow angles. Incorporating these features into a combinatorial model in which cell face identities modulate microtubule behaviours, accounts for tissue coordination and observed dynamics. Thus, instead of orientations being sensed at the microtubule level, they are sensed at the cellular level through changes in statistical behaviours at cell faces and edges, coordinated across a tissue through cell polarity.