Task-level Dynamics of Rice Root Gravitropism Revealed by Frequency Response Analysis

Plants respond to environmental stimuli via a closed-loop cascade of physical sensing, chemical signaling, and differential cell elongation. These processes, called tropisms, are control mechanisms that serve to regulate stimuli such as light (phototropism), touch (thigmotropism), and gravity (gravitropism).  Tropisms play key roles in roots’ ability to navigate soil heterogeneities during penetration, but quantitative links between sensing and behavior remain unclear. To elucidate mechanisms and algorithms underlying gravitropism during root growth, we employ the “system identification” framework: we apply sinusoidal periodic orientational forcing and monitor how rice roots (O. sativa) generate corrective feedback to attempt to grow downward (in the direction of gravity). Imaging the roots in a transparent gel filled container via co-moving cameras allows tracking of the root tip trajectory versus time. For a fixed maximum angular forcing amplitude of 45 degrees, at all applied stimulus frequencies (periods from 1 to 30 hrs) root tip growth angles relative to the vertical oscillated at the applied frequency. However, as frequency increased, the magnitude of root oscillation decreased by a factor of 3, and the response dynamics led to an almost 180 degrees phase lag at the highest frequencies. Such behavior is reminiscent of animal control systems and indicates an approximately linear control model can capture the dynamics of rice root gravitropism.