Both gravistimulation onset and removal trigger an increase of cytoplasmic free calcium in statocytes of roots grown in microgravity

François Bizet,Joëlle Gérard,Brigitte Eche,Christine Girousse,Veronica Pereda-Loth,Monique Courtade,Valérie Legué,Hugo Chauvet,Gérald Perbal

doi:10.1038/s41598-018-29788-7

Abstract

Gravity is a permanent environmental signal guiding plant growth and development. Gravity sensing in plants starts with the displacement of starch-filled plastids called statoliths, ultimately leading to auxin redistribution and organ curvature. While the involvement in gravity sensing of several actors such as calcium is known, the effect of statolith displacement on calcium changes remains enigmatic. Microgravity is a unique environmental condition offering the opportunity to decipher this link. In this study, roots of Brassica napus were grown aboard the International Space Station (ISS) either in microgravity or in a centrifuge simulating Earth gravity. The impact of short simulated gravity onset and removal was measured on statolith positioning and intracellular free calcium was assessed using pyroantimonate precipitates as cytosolic calcium markers. Our findings show that a ten-minute onset or removal of gravity induces very low statolith displacement, but which is, nevertheless, associated with an increase of the number of pyroantimonate precipitates. These results highlight that a change in the cytosolic calcium distribution is triggered in absence of a significant statolith displacement.

Highlights

Gravity sensing in plants occurs in specialized cells called statocytes which are located in the shoot endodermis and in the central root cap[1]
The ability of plants to orient their growth along the gravity vector, called gravitropism, was hypothesized to be the consequence of the pressure exerted by statoliths on the rootward endoplasmic reticulum (ER) complex
A change in the root apex orientation was not detected after the 10 minutes of simulated gravity onset and removal, suggesting that only early signalling events occurred within this period

Summary

Introduction

Gravity sensing in plants occurs in specialized cells called statocytes which are located in the shoot endodermis and in the central root cap[1]. The ability of plants to orient their growth along the gravity vector, called gravitropism, was hypothesized to be the consequence of the pressure exerted by statoliths on the rootward ER complex This widespread hypothesis was used to explain differences in root sensitivity to gravity observed at various inclination changes[8]. Recent investigations have highlighted that the gravitropism answer is independent of gravity intensity and that the mechanism behind gravity sensing in plants does not rely on a pressure applied by statoliths as a force sensor would do[9] In this framework, the hypothesis of an inclination sensor able to detect statolith position rather than pressure was recently proposed[10]. Space experiments offer the advantage of inducing gravity stimulus without any change in orientation of the seedling

Methods

Results

Conclusion