Gravity has shaped the evolution of life since its origin. However, experiments in the absence of this overriding force, necessary to precisely analyse its role, e.g. for growth, development, and orientation of plants and single cells, only became possible with the advent of spaceflight. Consequently, this research has been supported especially by space agencies around the world for decades, mainly for two reasons: first, to enable fundamental research on gravity perception and transduction during growth and development of plants; and second, to successfully grow plants under microgravity conditions with the goal of establishing a bioregenerative life support system providing oxygen and food for astronauts in long-term exploratory missions. For the second time, the International Space Life Sciences Working Group (ISLSWG), comprised of space agencies with substantial life sciences programmes in the world, organised a workshop on plant biology research in space. The present contribution summarises the outcome of this workshop. In the first part, an analysis is undertaken, if and how the recommendations of the first workshop held in Bad Honnef, Germany, in 1996 have been implemented. A chapter summarising major scientific breakthroughs obtained in the last 15 years from plant research in space concludes this first part. In the second part, recommendations for future research in plant biology in space are put together that have been elaborated in the various discussion sessions during the workshop, as well as provided in written statements from the session chairs. The present paper clearly shows that plant biology in space has contributed significantly to progress in plant gravity perception, transduction and responses - processes also relevant for general plant biology, including agricultural aspects. In addition, the interplay between light and gravity effects has increasingly received attention. It also became evident that plants will play a major role as components of bioregenerative life support and energy systems that are necessary to complement physico-chemical systems in upcoming long-term exploratory missions. In order to achieve major progress in the future, however, standardised experimental conditions and more advanced analytical tools, such as state-of-the-art onboard analysis, are required.

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Crude extracts of dark-kept resting cells of a chlorophyll-free, carotenoid-containing mutant of Chlorella vulgaris Beijerinck (211-11h/20) were found to convert 14.44±0.77 nmol PEP per min and mg protein into pyruvate by the action of pyruvate kinase (=PK; EC 2.7.1.40). When such cells were exposed to blue light (λ<550 nm, ∼300 μW cm(-2)) for 3 hrs the PK-activity/protein of their crude extracts rose to 21.47±1.30, i.e., it was enhanced by 43%. Poisoning with 10(-3) mol cycloheximide or with 150 μg actinomycin D/ml prevented the effect of blue light by 80-90% (Table 1). This result points to an induction of enzyme synthesis in blue light. Addition of 1% glucose in the dark resulted in an increase in PK-activity, too. Three hrs after application of glucose the PK-activity was 28.05±1.88 nmol/min and mg protein, which was 94% greater than in the control. The effect of glucose was also largely preventable by cycloheximide (10(-3) mol) or by actinomycin D (150 μg/ml) (Table 2). These results lead to the conclusion that blue light may induce the synthesis of PK by supplying free sugars at the site of enzyme synthesis. The assumption is supported by the observation that in hot water extracts of blue illuminated cells in which glucose oxidation had been poisoned by. 10(-2) mol monoiodoacetic acid there was 60% more glucose, glucose-6-phosphate, fructose-6-phosphate and sucrose detectable than in extracts of equally poisoned algae from darkness (Table 3). It is suggested that blue light activates a system for the transport of sugar out of the chloroplast, which results in the induction of respiratory enzyme synthesis and thus in enhanced respiration.

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Starch breakdown and respiratory O2 uptake in the green algaDunaliella tertiolecta (Butcher) are stimulated not only by blue, but also by red light. In the present study, attempts are described to identify the photoreceptor(s) involved. Fluence rate-response curves with different slopes in the ultraviolet (UV)/blue and in the red spectral region as well as differences in the kinetics and in the unfluence of dark pre-incubation on the stimulation of respiratory O2 uptake by blue and red light strongly indicate the action of two photoreceptors. Since the effect of red light shows some far-red reversibility, and since simultaneous irradiation with red and far-red light decreases the effectiveness of red light, the involvement of phytochrome - in addition to the UV/blue photoreceptor(s) - is suggested in the light-stimulated respiration inDunaliella.