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

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.

Settling for Less: Do Statoliths Modulate Gravity Perception?

“…A point of interaction between gravitropism and abiotic stresses is the regulation of statolith structure by for example gradients of either moisture [44] or NaCl [45]. The ISS has also been involved in elucidating the role of transient increases in cytosolic calcium ion concentration, where very small changes in statolith positioning are sufficient to trigger measurable changes in concentration [46]. These experiments built on an influential report which used pharmacological treatments to manipulate cytosolic calcium ion concentration to demonstrate the close functional links among cytoplasmic calcium ions, statolith integrity and the gravitropic response [47].…”

Gravity sensing in plant and animal cells

“…Interestingly, most mechanosensitive channels are expressed in the vasculature, where gravity-induced Ca 2+ response is observed 46 . In root statocytes of Brassica grown in ISS, tenminute onset (µg to 1 g) or removal (1 g to µg) of a gravityinduced Ca 2+ response in absence of a significant statolith displacement 59 . Changes in gravity vector and magnitude promote a Ca 2+ response with similar kinetics 46 .…”

Gravity Signaling in Flowering Plant Roots

“…A different Ca 2+ detection method was recently used to measure changes in cytosolic calcium levels in the statocytes of Brassica napus roots exposed to gravistimulation within the microgravity confines of the International Space Station [ 44 ]. Seedlings were grown either under microgravity or exposed to centrifugation simulating Earth’s gravity, then fixed and subsequently used to evaluate both amyloplast sedimentation and cytoplasmic Ca 2+ levels using a pyroantimonate precipitation method.…”

“…Seedlings were grown either under microgravity or exposed to centrifugation simulating Earth’s gravity, then fixed and subsequently used to evaluate both amyloplast sedimentation and cytoplasmic Ca 2+ levels using a pyroantimonate precipitation method. This experiment demonstrated changes in cytosolic Ca 2+ accumulation in the statocytes upon both gravistimulation onset and removal, despite only minor changes in amyloplast location [ 44 ]. While these results are encouraging, it will be very important to rule out the possibility of experimental artifacts with in vivo methods that allow detection of Ca 2+ changes in the statocytes of live tissues as they respond to gravistimulation.…”