The orientation of the cortical microtubule array, a unique acentrosomal cytoskeletal structure in plant cells, plays an important role in controlling the direction of anisotropic cell expansion and hence in directing plant morphogenesis. Strikingly, exposing dark-grown hypocotyl cells to blue light can reorient the array from its default direction transverse to the axis of cell expansion to a longitudinal one. This reorientation is driven by a katanin-mediated mechanism in which microtubules at microtubule crossovers are selectively severed, effectively creating a new population of microtubules orthogonal to the initial array [1, 2, 3]. We propose a stochastic model for this phenomenon, where the competition between the stochastic waiting time for the severing event at a crossover between two differently oriented microtubules, and the disappearance of the crossover through shrinkage of either of them due to microtubule dynamic instability, defines a threshold of values in the dynamic parameters that needs to be overcome in order to create the new orthogonal population. We show how this critical threshold can be determined using a combination of computer simulations and a novel analytical technique to calculate the first passage time distribution for a microtubule to reach a target.

[1] Lindeboom et al., Science (2103)

[2] Nakamura, Lindeboom, MS, Mulder, Ehrhardt, J Cell Biol (2018)

[3] Lindeboom, Nakamura, MS et al., J Cell Biol (2018)