Optical microscopy of Arabidopsis seedlings fixed in non-fresh FAA using Kennedy Fixation Tubes

Procedures for chemical fixation in immunohistochemical analyses of PIN proteins regulating polar auxin transport: Relevance to spaceflight experiments

The mechanism by which gravity controls the polar transport of auxin, a plant hormone regulating multiple physiological processes in higher plants, remains unclear, although an important role of PIN proteins as efflux carriers/facilitators in polar auxin transport is suggested. We are going to study the effect of microgravity on the polar transport of auxin, focusing on the cellular localization of its efflux carrier, PsPIN1 in etiolated pea seedlings and ZmPIN1a in etiolated maize seedlings grown under microgravity conditions on the International Space Station (ISS) using immunohistochemical analyses according to space experimental plans (Ueda, 2016). To obtain adequate results regarding the cellular localization of functional proteins, prolonged chemical fixation processes as well as chemical fixatives should be well-matched to the properties of functional proteins as antigens since experimental analyses will be performed on the ground after keeping samples for a long duration on the ISS. As a result of ground verification, clear detection of the cellular localization of PsPIN1 and ZmPIN1a immunohistochemically was successful based on the results of several kinds of chemical fixation tested, even when etiolated pea and maize seedlings were fixed by immersion in chemical fixative for a long duration. The addition of 0.1% (w/v) Nonidet P-40 to chemical fixative composed of 50% (v/v) ethanol and 5% (v/v) acetic acid or that of 50% (v/v) methanol and 5% (v/v) acetic acid has led to a significant improvement in the immunohistochemical detection of PsPIN1 or ZmPIN1a. These chemical fixatives were also shown to be storage-stable for a long time before use. In this study, adequate chemical fixatives and fixation protocols were developed, which can be used to detect localization of PsPIN1 and ZmPIN1a proteins in young etiolated pea and maize seedlings, respectively, using anti PsPIN1 and ZmPIN1a antibodies. These protocols can be used in spaceflight experiments to investigate the effects of the microgravity environment on the ISS on PIN protein localization in pea and maize seedlings.

Spaceflight exploration in plant gravitational biology

Before there was access to space, all experiments on plant tropisms were conducted upon the background of gravity. The gravity vector could be disrupted, such as with clinorotation and random positioning machines, and by manipulating incident angles of root growth with respect to gravity, such as with Darwin's plants on slanted plates, but gravity could not be removed from the experimental equation. Access to microgravity through spaceflight has opened new doors to plant research. Here we provide an overview of some of the methodologies of conducting plant research in the unique spaceflight environment.

Organ-specific remodeling of the Arabidopsis transcriptome in response to spaceflight

BACKGROUND

Spaceflight presents a novel environment that is outside the evolutionary experience of terrestrial organisms. Full activation of the International Space Station as a science platform complete with sophisticated plant growth chambers, laboratory benches, and procedures for effective sample return, has enabled a new level of research capability and hypothesis testing in this unique environment. The opportunity to examine the strategies of environmental sensing in spaceflight, which includes the absence of unit gravity, provides a unique insight into the balance of influence among abiotic cues directing plant growth and development: including gravity, light, and touch. The data presented here correlate morphological and transcriptome data from replicated spaceflight experiments.

RESULTS

The transcriptome of Arabidopsis thaliana demonstrated organ-specific changes in response to spaceflight, with 480 genes showing significant changes in expression in spaceflight plants compared with ground controls by at least 1.9-fold, and 58 by more than 7-fold. Leaves, hypocotyls, and roots each displayed unique patterns of response, yet many gene functions within the responses are related. Particularly represented across the dataset were genes associated with cell architecture and growth hormone signaling; processes that would not be anticipated to be altered in microgravity yet may correlate with morphological changes observed in spaceflight plants. As examples, differential expression of genes involved with touch, cell wall remodeling, root hairs, and cell expansion may correlate with spaceflight-associated root skewing, while differential expression of auxin-related and other gravity-signaling genes seemingly correlates with the microgravity of spaceflight. Although functionally related genes were differentially represented in leaves, hypocotyls, and roots, the expression of individual genes varied substantially across organ types, indicating that there is no single response to spaceflight. Rather, each organ employed its own response tactics within a shared strategy, largely involving cell wall architecture.

CONCLUSIONS

Spaceflight appears to initiate cellular remodeling throughout the plant, yet specific strategies of the response are distinct among specific organs of the plant. Further, these data illustrate that in the absence of gravity plants rely on other environmental cues to initiate the morphological responses essential to successful growth and development, and that the basis for that engagement lies in the differential expression of genes in an organ-specific manner that maximizes the utilization of these signals--such as the up-regulation of genes associated with light-sensing in roots.