Control of rice root circumnutation revealed by direct cell-scale simulation

Roots dynamically adjust their growth patterns to navigate complicated soil environments. One strategy, circumnutation (the helical tip movement of roots), is crucial for plant seedling anchoring and obstacle avoidance. How organ-level circumnutation emerges from cell growth remains unclear. To examine this process, we grew O sativa rice roots in clear gels of varying stiffness (0.15% to 0.6% Gelzan). For increasing stiffness, circumnutation amplitude and period increased from 0.1 to 0.7 mm and 0.7 to 2 hours, respectively. We also observed systematic changes at the cell level; mature cell length decreased from 70 to 40 μm with increasing stiffness. To investigate how these trends emerge from cellular behaviors, we constructed a Discrete Element Method (DEM) simulation of the epidermal layer of a growing root (~20,000 cells/cm), representing cells as particles that elongate using bonds with time-dependent equilibrium lengths. We implemented circumnutation via differential elongation - cells on one side of the root grow faster than the other, causing a bend. To investigate coordination, we created an agent-based model: neighboring cells send a growth inhibition signal upon reaching a critical concentration. Varying this threshold to control mature cell lengths reproduced the amplitude, frequency, and cell length in circumnutation experiments across stiffnesses. The integration of root imaging and computational modeling connects cell behaviors to emergent organismal growth patterns.