Suppression of Cortical Microtubule Reorientation and Stimulation of Cell Elongation in Arabidopsis Hypocotyls under Microgravity Conditions in Space.

Shiho Kato,Hirofumi Hashimoto,Toru Shimazu,Takayuki Hoson,Takashi Hashimoto,Mana Murakami,Kazuyuki Wakabayashi,Sachiko Yano,Shohei Matsumoto,Motoshi Kamada,Ryo Saika,Haruo Kasahara,Kouichi Soga

doi:10.3390/plants11030465

Abstract

How microgravity in space influences plant cell growth is an important issue for plant cell biology as well as space biology. We investigated the role of cortical microtubules in the stimulation of elongation growth in Arabidopsis (Arabidopsis thaliana) hypocotyls under microgravity conditions with the Resist Tubule space experiment. The epidermal cells in the lower half of the hypocotyls of wild-type Columbia were longer in microgravity than at on-orbit 1 g, which precipitated an increase in the entire hypocotyl length. In the apical region, cortical microtubules adjacent to the outer tangential wall were predominantly transverse to the long axis of the cell, whereas longitudinal microtubules were predominant in the basal region. In the 9th to 12th epidermal cells (1 to 3 mm) from the tip, where the modification of microtubule orientation from transverse to longitudinal directions (reorientation) occurred, cells with transverse microtubules increased, whereas those with longitudinal microtubules decreased in microgravity, and the average angle with respect to the transverse cell axis decreased, indicating that the reorientation was suppressed in microgravity. The expression of tubulin genes was suppressed in microgravity. These results suggest that under microgravity conditions, the expression of genes related to microtubule formation was downregulated, which may cause the suppression of microtubule reorientation from transverse to longitudinal directions, thereby stimulating cell elongation in Arabidopsis hypocotyls.

Highlights

A proper structural form is essential for plant prosperity
Hypocotyls and roots elongated approximately along the gravity vector at on-orbit 1 g, whereas they tended to elongate in various directions under microgravity conditions, as reported previously [23,24]
These results indicate that the reorientation of microtubules from transverse to longitudinal directions is suppressed under microgravity conditions

Summary

Introduction

A proper structural form is essential for plant prosperity. The form of the whole plant reflects the sum of the rate and direction of growth of the constituting organs, and the direction of cell expansion is important for determining the shape of each organ. Hypergravity induced some characteristic changes in the expression of microtubule-associated protein (MAP) genes, such as a transient increase in the expression of γ-tubulin and katanin genes [15,16], and a decrease in the levels of 65 kDa microtubule-associated protein-1 [17] These results suggest that cortical microtubules are involved in the modification of anisotropic cell expansion in response to hypergravity stimuli. We analyzed the relationship between cell elongation and the orientation of cortical microtubules for each cell number from the tip to the base along the epidermal cell file of Arabidopsis hypocotyls, which were cultivated under either artificial 1 g or microgravity conditions on the Kibo Module of the International Space Station, chemically fixed in orbit, and returned to Earth (Resist Tubule space experiment). Combined with information on the modification of the expression of genes related to microtubule formation under microgravity conditions, we discuss the role of cortical microtubules in the regulation of anisotropic cell expansion in response to 1 g gravity

Hypocotyl Growth

Expression of Tubulin and MAPs

Plant Materials and Onboard Experiments

Microscopy

Gene Expression Analysis

Statistical Analysis