Development of plant protoplasts during the IML-1 mission

The Utilization of Plant Facilities on the International Space Station-The Composition, Growth, and Development of Plant Cell Walls under Microgravity Conditions

In the preparation for missions to Mars, basic knowledge of the mechanisms of growth and development of living plants under microgravity (micro-g) conditions is essential. Focus has centered on the g-effects on rigidity, including mechanisms of signal perception, transduction, and response in gravity resistance. These components of gravity resistance are linked to the evolution and acquisition of responses to various mechanical stresses. An overview is given both on the basic effect of hypergravity as well as of micro-g conditions in the cell wall changes. The review includes plant experiments in the US Space Shuttle and the effect of short space stays (8-14 days) on single cells (plant protoplasts). Regeneration of protoplasts is dependent on cortical microtubules to orient the nascent cellulose microfibrils in the cell wall. The space protoplast experiments demonstrated that the regeneration capacity of protoplasts was retarded. Two critical factors are the basis for longer space experiments: a. the effects of gravity on the molecular mechanisms for cell wall development, b. the availability of facilities and hardware for performing cell wall experiments in space and return of RNA/DNA back to the Earth. Linked to these aspects is a description of existing hardware functioning on the International Space Station.

Fundamental plant biology enabled by the space shuttle

The relationship between fundamental plant biology and space biology was especially synergistic in the era of the Space Shuttle. While all terrestrial organisms are influenced by gravity, the impact of gravity as a tropic stimulus in plants has been a topic of formal study for more than a century. And while plants were parts of early space biology payloads, it was not until the advent of the Space Shuttle that the science of plant space biology enjoyed expansion that truly enabled controlled, fundamental experiments that removed gravity from the equation. The Space Shuttle presented a science platform that provided regular science flights with dedicated plant growth hardware and crew trained in inflight plant manipulations. Part of the impetus for plant biology experiments in space was the realization that plants could be important parts of bioregenerative life support on long missions, recycling water, air, and nutrients for the human crew. However, a large part of the impetus was that the Space Shuttle enabled fundamental plant science essentially in a microgravity environment. Experiments during the Space Shuttle era produced key science insights on biological adaptation to spaceflight and especially plant growth and tropisms. In this review, we present an overview of plant science in the Space Shuttle era with an emphasis on experiments dealing with fundamental plant growth in microgravity. This review discusses general conclusions from the study of plant spaceflight biology enabled by the Space Shuttle by providing historical context and reviews of select experiments that exemplify plant space biology science.

Xylem development and cell wall changes of soybean seedlings grown in space

BACKGROUND AND AIMS

Plants growing in altered gravity conditions encounter changes in vascular development and cell wall deposition. The aim of this study was to investigate xylem anatomy and arrangement of cellulose microfibrils in vessel walls of different organs of soybean seedlings grown in Space.

METHODS

Seeds germinated and seedlings grew for 5 d in Space during the Foton-M2 mission. The environmental conditions, other than gravity, of the ground control repeated those experienced in orbit. The seedlings developed in space were compared with those of the control test on the basis of numerous anatomical and ultrastructural parameters such as number of veins, size and shape of vessel lumens, thickness of cell walls and deposition of cellulose microfibrils.

KEY RESULTS

Observations made with light, fluorescence and transmission electron microscopy, together with the quantification of the structural features through digital image analysis, showed that the alterations due to microgravity do not occur at the same level in the various organs of soybean seedlings. The modifications induced by microgravity or by the indirect effect of space-flight conditions, became conspicuous only in developing vessels at the ultrastructural level. The results suggested that the orientation of microfibrils and their assembly in developing vessels are perturbed by microgravity at the beginning of wall deposition, while they are still able to orient and arrange in thicker and ordered structures at later stages of secondary wall deposition.

CONCLUSIONS

The process of proper cell-wall building, although not prevented, is perturbed in Space at the early stage of development. This would explain the almost unaltered anatomy of mature structures, accompanied by a slower growth observed in seedlings grown in Space than on Earth.

Plants in space

Virtually all scenarios for the long-term habitation of spacecraft and other extraterrestrial structures involve plants as important parts of the contained environment that would support humans. Recent experiments have identified several effects of spaceflight on plants that will need to be more fully understood before plant-based life support can become a reality. The International Space Station (ISS) is the focus for the newest phase of space-based research, which should solve some of the mysteries of how spaceflight affects plant growth. Research carried out on the ISS and in the proposed terrestrial facility for Advanced Life Support testing will bring the requirements for establishing extraterrestrial plant-based life support systems into clearer focus.

Simulated weightlessness and hyper-g results in opposite effects on the regeneration of the cortical microtubule array in protoplasts from Brassica napus hypocotyls

Enzymatic digestion of the cell wall of Brassica napus hypocotyls gave a heterogeneous suspension of protoplasts with the cortical microtubules (CMTs) randomly organised or CMTs organised in parallel. The effect of variable g-influences has been tested on CMT organisation. In contrast to the 1 g-protoplasts, which reorganised the CMTs into parallel arrays during the 96 h test period, the frequency of randomly-oriented CMTs in the protoplasts exposed to simulated weightlessness (0 g) on a 2-D clinostat increased significantly during the same period. The opposite effect was obtained when the protoplasts were exposed to hyper-g (7 or 10 g), where the reorganisation of the CMTs into parallel arrays was accelerated compared to the 1 and 0 g-protoplasts. These results indicate that a unidirectional gravity force is a necessity for the reorganisation of CMTs in protoplasts to parallel arrays and that CMTs act as responding elements that are able to sense different levels of gravity. Besides the inability of the protoplasts to reorganise the CMTs into parallel arrays, the quantity of CMTs in the individual protoplast decreased during 4 days of simulated weightlessness, both compared to the CMTs quantity in the protoplasts immediately after isolation and compared to the 1 g- and hyper-g-protoplasts after 24 and 48 h of g-exposure. The size of the protoplasts was also affected by the g-exposure. Protoplasts exposed to simulated 0 g increased significantly after 24 and 48 h, whereas the 1 g- and 10 g-protoplasts maintained the same size during the 48 h test period.