Induction of Hypoxic Root Metabolism Results from Physical Limitations in O2 Bioavailability in Microgravity

Abstract

Numerous spaceflight experiments have noted changes in the roots that are consistent with hypoxia in the rootzone. These observations include general ultrastructure analysis and biochemical measurements to direct measurements of stress specific enzymes. in experiments that have monitored alcohol dehydrogenase (ADH), the data shows this hypoxically responsive gene is induced and is associated with increased ADH activity in microgravity. These changes in ADH could be induced either by spaceflight hypoxia resulting from inhibition of gravity mediated O 2 transport, or by a non-specific stress response due to inhibition of gravisensing. We tested these hypotheses in a series of two experiments. the objective of the first experiment was to determine if physical changes in gravity-mediated O 2 transport can be directly measured, while the second series of experiments tested whether disruption of gravisensing can induce a non-specific ADH response. to directly measure O 2 bioavailability as a function of gravity, we designed a sensor that mimics metabolic oxygen consumption in the rhizosphere. Because of these criteria, the sensor is sensitive to any changes in root O 2 bioavailability that may occur in microgravity. in a KC-135 experiment, the sensor was implanted in a moist granular clay media and exposed to microgravity during parabolic flight. the resulting data indicated that root O 2 bioavailability decreased in phase with gravity. in experiments that tested for non-specific induction of ADH, we compared the response of transgenic Arabidopsis plants (ADH promoted GUS marker gene) exposed to clinostat, control, and waterlogged conditions. the plants were grown on agar slats in a growth chamber before being exposed to the experimental treatments. the plants were stained for GUS activity localization, and subjected to biochemical tests for ADH, and GUS enzyme activity. These tests showed that the waterlogging treatment induced significant increases in GUS and ADH enzyme activities, while the control and clinostat treatments showed no response. This work demonstrates: (1) the inhibition of gravity-driven convective transport can reduce the O 2 bioavailability to the root tip, and (2) the perturbation of gravisensing by clinostat rotation does not induce a non-specific stress response involving ADH. Together these experiments support the microgravity convection inhibition model for explaining changes in root metabolism during spaceflight. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Department(s)

Civil, Architectural and Environmental Engineering

Second Department

Biological Sciences

Keywords and Phrases

Hypoxia; Oxygen; Plant; Root; Spaceflight

International Standard Serial Number (ISSN)

0273-1177

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2024 Elsevier, All rights reserved.

Publication Date

01 Jan 2004

PubMed ID

15880895

Link to Full Text

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