Frequency response analysis of gravitropism in rice roots

Active sensory response behaviors in plants, known as tropisms, are key to a root’s ability to navigate soil. These behaviors, which can alter root growth trajectory in response to light (phototropism), touch (thigmotropism), or gravity (gravitropism) have been widely studied for single stimulus events. However, how these responses are influenced by dynamic stimuli to facilitate root negotiation of complex, changing substrate conditions is not well understood. In the past, dynamic sensory response in animals such as cockroaches, knifefish, and hawk moths have been characterized using concepts from control theory [Cowan et al, ICB, 2014]. Here we extend this analysis framework to plants, using rice (O. sativa) roots. We develop an apparatus that provides programmable, time-dependent gravitational stimuli to roots grown in a transparent gel, and image root growth with two orthogonal cameras. We use this apparatus to track the three-dimensional growth trajectory of rice roots that are rotated between 0° and 90° with respect to gravity at frequencies between 1.5 and 144 cycles/day with square and sine wave stimulus profiles. We present a control model to characterize root sensory response and describe how plant growth trajectory is impacted by periodic stimuli using frequency response techniques such as Bode analysis.