Probing the interaction of forced and endogenous oscillation in rice roots

Rice roots (O. sativa) use various modalities of sensory feedback to navigate their environment. This includes gravitropism, a behavior in which a root alters its growth trajectory to grow in the direction of gravity. Roots also navigate soil via circumnutation (Taylor et al PNAS 2021), or the helical motion of the root tip as it penetrates soil. These mechanisms are both actuated by differential cell elongation, but their interactions are not well understood. To probe interactions of circumnutation and gravitropism, we applied a sinusoidal gravitropic stimulus to rice roots growing in a clear gel model substrate at frequencies sweeping from much lower (0.077 cycles/hr) to much higher (6 cycles/hr) than the typical circumnutation frequency of 0.5 to 1 cycles/hr, tracking root tip angle over time. At stimulus frequencies far from the circumnutation period, circumnutation was superimposed on the gravitropic response, such that each oscillation frequency was distinct in the frequency domain. However, at applied stimulus frequencies near the natural circumnutation period, the relationship between these mechanisms was more complicated, with synchronization of the two responses at some frequencies (0.5 to 0.66 cycles/hr) and cancellation at others (1 cycle/hr). This interaction between circumnutation and gravitropism suggests a shared regulatory mechanism, where signals generated by each behavior can cancel or reinforce one another.