Feedback control of gravitropism in rice roots

Tropisms allow plant roots to respond to stimuli such as light, touch, and gravity. They do this via a closed-loop cascade of physical sensing, chemical signaling, and directed growth through differential cell elongation.  These tropisms play a key role in roots’ ability to navigate soil heterogeneities, but quantitative mechanistic links between sensing and behavior remain unclear. To address this gap, we use techniques from control theory to study how rice roots (O. sativa) generate corrective gravitropic feedback during growth. We developed an apparatus to apply sinusoidal gravitational stimuli to roots growing in a transparent gel and recorded their growth trajectories from two orthogonal views. The root tip angle oscillated at two superimposed frequencies: the applied stimulus frequency (1 cycle per 1-30hrs) and a secondary frequency (1 cycle per 1-2 hrs). This secondary oscillation frequency was associated with circumnutation, a helical growth of the root tip [Taylor et al 2021]. The gravitropic response behaves as a time-delayed low-pass filter with unity feedback, attenuating frequencies above 1 cycle per 5 hours and exhibiting a response latency of 11.4-minutes, adding prominent phase lag at high frequencies. We suggest that the delay reflects the time taken for the stimulus to be sensed via statoliths in the root cap and then for the physical auxin signal to be transmitted to the elongation zone, where the growth response occurs.