Plants amaze many by their rich morphological diversity. At the basis of this diversity is the plant cell wall. At increasing scales, the cell wall itself can adopt many patterns, including the beautiful and functionally relevant patterns in xylem vessels; the cell wall enables complex cell shapes, and, coordinated, complex organ shapes. The relevant cell wall properties --anisotropy, patterned local reinforcements-- are largely derived from the organization states of the plant cortical microtubule array, a collective of dynamic microtubules attached to the cell membrane.
Biophysical models are extremely helpful in understanding how the cortical array can (self-)organize into a diversity of patterns and how mutations in relevant proteins affect the process. We develop increasingly realistic computer simulations with a solid theoretical foundation and in close collaboration with experimentalists.
We have recently discovered that the way new microtubules are nucleated is critical for array versatility. Our models show that realistic nucleation supports a default state that maintains and recovers array homogeneity, as well as a pattern enhancing state within easy reach. It is this pattern enhancing state that is used in xylem patterning. In this case, the cortical array interacts with ROP proteins and their downstream effectors that locally modify microtubule dynamics. If the regional differences are sufficiently large, the nucleation dynamics introduce a local feedback on array density that enhances the nascent pattern. Without such differences, however, they have an equalizing effect, thus promoting cell wall integrity. We also investigate the role of microtubule flexibility in adopting complex patterns.