Emergent behaviors of heterogeneous cell growth patterns in plant root growth

Circumnutation, the endogenous helical motion of a plant root tip, has been recently shown to facilitate exploration, anchoring and penetration in heterogeneous subterranean environments [Taylor et al, PNAS, 2021]. However, the regulatory mechanism that controls root circumnutation at the cellular level is not well understood. To investigate the effects of cell growth on circumnutation, we developed a multiparticle Discrete Element Method (DEM) simulation in which each "particle" represents a single root cell. Previously, we had developed a small scale simulation that could mimic circumnutation. To scale this model , we integrated LAMMPS, a molecular dynamics software, to run efficient parallelized simulations. Our model implements a perimeter of 90 particles to match the azimuthal cell count in rice roots (O. sativa, 0.2 mm diameter) as determined from confocal imaging, and can model up to 15,000 total cells during root growth. Based on our confocal imaging data, the heterogeneity in cell growth is crucial to the emergence of circumnutation. We emulated root meristematic (cell division) and elongation (cell growth) zones in our model, connecting cell level signals and behaviors to the trajectory of the simulated root tip to test the effects of heterogeneous growth. We validated the simulation results through comparison with experimental data from rice root imaging to understand how circumnutation is regulated.