Root Circumnutation Facilitates Effective Subterranean Surface Exploration

Circumnutation is the oscillatory movement first described by Darwin of a variety of plant organs including roots. A number of root traits have been suggested to improve tip penetration in environments where soil mechanical impedance is a limiting factor in growth, though little is known about the roles of circumnutation. After observing a root coiling phenotype on flat surfaces in non-circumnutating mutant rice roots, we hypothesized that root tip circumnutation facilitates effective root-surface exploration. To model a surface environment of a compact soil horizon with biopores we used plates with 2mm holes equally spaced at different densities. Mutant and wild-type (WT) rice were grown in a clear gel-based media and an automated high-throughput system acquired images to visualize the root growth. As hole density decreased mutants showed reduced success in finding a hole. WT roots had higher success indicating WT roots are more effective in flat surface exploration and less affected by sparse hole density, providing a plausible mechanism to buffer against environmental uncertainty inherent in soil exploration. We propose circumnutation provides a mechanism to break the intrinsic root coiling pattern seen in mutants, and that this movement consequently promotes root exploration.