Abstract

Plant shoot gravitropism is a complex phenomenon resulting from gravity sensing, curvature sensing (proprioception), the ability to uphold self-weight and growth. Although recent data analysis and modelling have revealed the detailed morphology of shoot bending, the relative contribution of bending force (derived from the gravi-proprioceptive response) and stretching force (derived from shoot axial growth) behind gravitropism remains poorly understood. To address this gap, we combined morphological data with a theoretical model to analyze shoot bending in wild-type and lazy1-like 1 mutant Arabidopsis thaliana. Using data from actual bending events, we searched for model parameters that minimized discrepancies between the data and mathematical model. The resulting model suggests that both the bending force and the stretching force differ significantly between the wild type and mutant. We discuss the implications of the mechanical forces associated with differential cell growth and present a plausible mechanical explanation of shoot gravitropism.

Highlights

  • IntroductionProcesses in which their body bends according to directional environmental cues

  • To extract the typical bending behaviour of the wild type and the lzy1 mutant, we identified the best-fitted parameters with the averaged values for the wild type and lzy1 mutant. e extracted values of gravitropic sensitivity β in wild type (β = 2.14 ± 1.95) was higher than those in lzy1 mutant (β = 1.38 ± 1.30), reflecting the defects in differential cell growth (DCG) in lzy1 mutant

  • We have clarified that the bending and stretching forces have different effects on shoot morphology during bending. e remaining question is whether the decomposed bending and stretching forces can be translated into biological counterparts. e biological process underlying the change of the curvature is differential cell growth (DCG), which is realized by a combination of the gravi-proprioceptive response and the shoot axial growth as discussed below

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Summary

Introduction

Processes in which their body bends according to directional environmental cues. When shoots or roots bend, the cells within the organ sense their positions and orientations by specific physiological or mechanical means (Haswell, 2003; Morita & Tasaka, 2004; Perbal & DrissEcole, 2003) and induce differential growth between the inner and outer flanks of the organ (Blancaflor & Masson, 2003; Firn & Digby, 1980). In addition to this gravi-sensing mechanism to induce bending, plants have a mechanism to straighten shoots when they bend too much, indicating that the amount of bending depends on the inclination angle of the organ and on its curvature (Bastien et al, 2013; Okamoto et al, 2015). To understand how the molecular mechanisms of gravitropism relate to organ bending, researchers have recently turned to dataand model-approaches (Bastien et al, 2014; 2016; Basu et al, 2007; Chelakkot & Mahadevan, 2017; Coutand et al, 2007; Kutschera, 2001; Moulia et al, 2019; Philippar et al, 1999)

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