Low-Speed Clinorotation of Brachypodium distachyon and Arabidopsis thaliana Seedlings Triggers Root Tip Curvatures That Are Reminiscent of Gravitropism

Clinostats are instruments that continuously rotate biological specimens along an axis, thereby averaging their orientation relative to gravity over time. Our previous experiments indicated that low-speed clinorotation may itself trigger directional root tip curvature. In this project, we have investigated the root curvature response to low-speed clinorotation using Arabidopsis thaliana and Brachypodium distachyon seedlings as models. We show that low-speed clinorotation triggers root tip curvature in which direction is dictated by gravitropism during the first half-turn of clinorotation. We also show that the angle of root tip curvature is modulated by the speed of clinorotation. Arabidopsis mutations affecting gravity susception (pgm) or gravity signal transduction (arg1, toc132) are shown to affect the root tip curvature response to low-speed clinorotation. Furthermore, low-speed vertical clinorotation triggers relocalization of the PIN3 auxin efflux facilitator to the lateral membrane of Arabidopsis root cap statocytes, and creates a lateral gradient of auxin across the root tip. Together, these observations support a role for gravitropism in modulating root curvature responses to clinorotation. Interestingly, distinct Brachypodium distachyon accessions display different abilities to develop root tip curvature responses to low-speed vertical clinorotation, suggesting the possibility of using genome-wide association studies to further investigate this process.

Automated summary

Clinostats are used to rotate biological specimens, triggering root tip curvature in response to low-speed clinorotation. The direction of root tip curvature is dictated by gravitropism during the first half-turn of clinorotation, and the angle is modulated by the speed of rotation. Mutations affecting gravity susception or signal transduction in Arabidopsis impact the root tip curvature response. Low-speed vertical clinorotation leads to relocalization of auxin efflux facilitator, creating a lateral gradient of auxin across the root tip. Different accessions of Brachypodium distachyon display varying abilities to develop root tip curvature in response to clinorotation, suggesting the potential use of genome-wide association studies in further investigating this process.