Plants have been
an important part of space biology investigations over the past few decades for two main reasons. First, plants likely will be a key component of bioregenerative life-support systems as generators of oxygen and as a food source for astronauts on long-term space missions (Ferl et al
; Ferl and Paul, 2010
). Second, the spaceflight environment can serve as a unique laboratory for the study of the fundamental processes underlying plant growth and development (Wolverton and Kiss, 2009
; Millar et al
). In the present study, we used the microgravity environment on the space shuttle in low Earth orbit to study the development of seedlings of the model plant Arabidopsis thaliana
Gravity has been a ubiquitous and unidirectional signal throughout the evolution of life on Earth and has provided a directional cue by which plants organize their body plans (Palmieri and Kiss, 2006
). Throughout their entire life cycle, plants use gravity to orient and coordinate their growth in order to maximize access to light, water, and nutrients. In germinating seedlings, gravity is important for orienting the plant so that shoots grow upward and roots grow downward.
Plants sense and respond to gravity through the process of gravitropism, the directed growth in response to this stimulus (Blancaflor and Masson, 2003
). Gravitropism can be divided into three temporal phases: perception, transduction, and response. Gravity perception or sensing occurs in specialized cells (i.e
., statocytes) in the roots and shoots of all flowering plants (Kiss, 2000
). The putative gravity sensors are amyloplasts: dense, starch-filled organelles that are located exclusively in statocytes and move within the cell in response to gravity (Saito et al
During the second phase of gravitropism, signal transduction, the dissipation of the potential energy of statolithic amyloplasts results in the production of chemical signals that ultimately trigger a growth response (Morita, 2010
). Many subcellular structures have been implicated in gravity signal transduction, including the vacuole, endoplasmic reticulum, and the cytoskeleton (Blancaflor and Masson, 2003
). The final (i.e
., response) phase of gravitropism is characterized by directed growth in response to gravity (Perrin et al
). This growth response is elicited by auxin concentration gradients that form across reoriented organs such that more of this hormone is present in the lower portion, as compared to the upper portion of the organs.
Several studies have used the microgravity environment aboard orbiting spacecraft to identify downstream elements in signal transduction (reviewed in Correll and Kiss, 2008
). An experiment with lentil roots was performed to identify the role of the actin cytoskeleton in amyloplast movement in microgravity (Driss-Ecole et al
). Other ground-based studies with drugs that disrupt the actin cytoskeleton have also demonstrated that the cytoskeleton is involved in gravity responses, although results from these studies are often conflicting and depend on the organ, plant species, drug dosage, and experimental conditions (Palmieri and Kiss, 2005
We performed spaceflight experiments to study the effects of microgravity on the structure and organization of the actin cytoskeleton in plants. These studies were performed on space shuttle Discovery during mission STS-131 in April 2010 by utilizing the Biological Research in Canisters (BRIC) hardware system, which was placed in the middeck region of the orbiter (Kern et al
). The specific objectives were to investigate the role of the cytoskeleton in statocytes in microgravity by cytological methods and to study effects of microgravity on actin cytoskeleton–related gene expression by gene profiling. However, in this paper, we will first focus on (1) the implementation of the BRIC system to study the development of Arabidopsis
seedlings during spaceflight and (2) the analysis of the overall morphology of the root system of seedlings that developed in microgravity. Based on our data, we propose that an endogenous response in seedlings causes the roots to skew toward one direction and that this default growth response is largely masked by the normal 1g
conditions on Earth.