Microtubule-based nucleation results in a large sensitivity to cell geometry of the plant cortical array.

Many plant cell functions, including cell morphogenesis and anisotropic growth, rely on the self-organisation of cortical microtubules into aligned arrays with the correct orientation. An important ongoing debate is how cell geometry, wall mechanical stresses, and other internal and external cues are integrated to determine the orientation of the cortical array. Here, we demonstrate that microtubule-based nucleation can markedly shift the balance between these often competing directional cues. For this, we developed a novel, more realistic model for microtubule-based nucleation in the simulation platform CorticalSim, which avoids the longstanding inhomogeneity problem stemming from previous, less realistic models for microtubule-based nucleation. We show that microtubule-based nucleation increases the sensitivity of the array to cell geometry, and extends the regime of spontaneous alignment compared to isotropic nucleation. In the case of cylindrical cell shapes, we show that this translates into a strong tendency to align in the transverse direction rather than along the vertical axis, and this is robust against small directional cues favouring the longitudinal direction. Comparing various cylinders and boxes, we show that different nucleation mechanisms result in different preferred array orientations, with the largest differences on cylinders. Our model provides a powerful tool for investigating how plant cells integrate multiple biases to orient their cortical arrays, offering new insights into the biophysical mechanisms underlying cell shape and growth.