Modeling plant root circumnutation using cellular simulation

Circumnutation, the helical motion of a growing plant root tip, is hypothesized to benefit plants by allowing them to penetrate heterogeneities in soil [Taylor and Lehner et al, PNAS 2021]. We develop a numerical simulation using a DEM (Discrete Element Modeling) approach to model plant roots to study circumnutation as an emergent property of cell level behaviors. The simulated root is treated as a growing array of cells, which divide close to the tip and enlarge in the elongation zone, implementing the meristematic and elongation zones found in real roots. Each cell is represented as a particle with variable length to allow for elongation during osmotic expansion. Circumnutation is generated by a 'nearest neighbor' coupling model that reproduces the delay in the onset of circumnutation observed in biological experiments. The simulation was tested for a range of parameters intended to capture laboratory experiments of rice roots grown in a gel-based media [Taylor and Lehner et al, PNAS 2021]. We compare the simulation to both wild-type circumnutating and mutant non-circumnutating biological roots and analyze their curvatures over time. We also note that the frequency of circumnutation decreases at 24 hours in rice roots and discuss hypotheses for why this might occur.