Effects of spaceflight (STS-87) on tropisms and plastid positioning in protonemata of the moss Ceratodon purpureus.

Apical cells of moss protonemata represent a single-celled system that perceives and reacts to light (positive and negative phototropism) and to gravity (negative gravitropism). Phototropism completely overrides gravitropism when apical cells are laterally irradiated with relatively high red light intensities, but below a defined light intensity threshold gravitropism competes with the phototropic reaction. A 16 day-long exposure to microgravity conditions demonstrated that gravitropism is allowed when protonemata are laterally illuminated with light intensities below 140 nmol m-2s-1. Protonemata that were grown in darkness in microgravity expressed an endogenous tendency to grow in arcs so that the overall culture morphology resembled a clockwise spiral. However this phenomenon only was observed in cultures that had reached a critical age and/or size. Organelle positioning in dark-grown apical cells was significantly altered in microgravity. Gravisensing most likely involves the sedimentation of starch-filled amyloplasts in a well-defined area of the tip cell. Amyloplasts that at 1-g are sedimented were clustered at the apical part of the sedimentation zone in microgravity. Clustering observed in microgravity or during clino-rotation significantly differs from sedimentation-induced plastid aggregations after inversion of tip cells at 1-g.     

Cell biology of plant gravity sensing.

The debate about whether gravity sensing relies upon statoliths (amyloplasts that sediment) has intensified with recent findings of gravitropism in starchless mutants and of claims of hydrostatic gravity sensing. Starch and significant plastid sedimentation are not necessary for reduced sensing in mutant roots, but plastids might function here if there were a specialized receptor for plastid mass e.g. in the ER. Alternatively, components in addition to amyloplasts might provide mass for sensing. The nucleus is dense and its position is regulated, but no direct data exist for its role in sensing. If the weight of the protoplast functioned in sensing, why would there be specific cytological specializations favoring sedimentation rather than cell mass? Gravity has multiple effects on plants in addition to gravitropism. There may be more than one mechanism of gravity sensing.     

Autonomic straightening of gravitropically curved cress roots in microgravity.

The typical response of plant organs to gravistimulation is differential growth that leads to organ bending. If the gravitropic stimulus is withdrawn, endogenous compensation of the graviresponse and subsequent straightening occur in some plants. For instance, autonomic straightening of Lepidium roots occurs when gravitropically-curved rootsare rotated on a clinostat (Stankovi et al., 1998a). To determine whether endogenous compensation of the graviresponse also occurs in space, microgravity-grown cress roots were laterally centrifuged in-flight and then returned to microgravity using Biorack hardware on a shuttle mission (STS-81). The cress roots were centrifuged at 4 different g-doses (0.1 x g and 1 x g for 15 or 75 min). All four treatments yielded varying degrees of root curvature. Upon removal from the centrifuge, roots in all four treatments underwent subsequent straightening in microgravity. This straightening resulted from a loss of gravitropic curvature in older regions of the root and the coordinated alignment of new growth. These results show that both microgravity and clinostat rotation on Earth are equivalent in stimulus withdrawal with respect to the induction of endogenous compensation of the curvature. Cress roots are the only plant organ shown to undergo compensation of the curvature in both microgravity and on a clinostat. The compensation of graviresponse in space rules out the hypothesis that the endogenous root straightening ("autotropism") represents a commitment to a pre-stimulus orientation with respect to gravity and instead suggests that there is a default tendency towards axiality following a withdrawal of a g-stimulus.     

Gravity sensing in moss protonemata.

Moss protonemata are a valuable system for studying gravitropism because both sensing and upward curvature (oriented tip growth) take place in the same cell. We review existing evidence, especially for Ceratodon purpureus, that addresses whether the mass that functions in sensing is that of amyloplasts that sediment. Recent experiments show that gravitropism can take place in media that are denser than the apical cell. This indicates that gravity sensing relies on an intracellular mass rather than that of the entire cell and provides further support for the starch-statolith hypothesis of sensing. Possible mechanisms for how amyloplast mass functions in sensing and transduction are discussed.     

Red light-induced suppression of gravitropism in moss protonemata.

Moss protonemata are among the few cell types known that both sense and respond to gravity and light. Apical cells of Ceratodon protonemata grow by oriented tip growth which is negatively gravitropic in the dark or positively phototropic in unilateral red light. Phototropism is phytochrome-mediated. To determine whether any gravitropism persists during irradiation, cultures were turned at various angles with respect to gravity and illuminated so that the light and gravity vectors acted either in the same or in different directions. Red light for 24h (> or = l40nmol m-2 s-1) caused the protonemata to be oriented directly towards the light. Similarly, protonemata grew directly towards the light regardless of light position with respect to gravity indicating that all growth is oriented strictly by phototropism, not gravitropism. At light intensities < or = l00nmol m-2 s-1, no phototropism occurs and the mean protonemal tip angle remains above the horizontal, which is the criterion for negative gravitropism. But those protonemata are not as uniformly upright as they would be in the dark indicating that low intensity red light permits gravitropism but also modulates the response. Protonemata of the aphototropic mutant ptr1 that lacks a functional Pfr chromophore, exhibit gravitropism regardless of red light intensity. This indicates that red light acts via Pfr to modulate gravitropism at low intensities and to suppress gravitropism at intensities < or = 140nmol m-2 s-1.     

Spaceflight hardware allowing unilateral irradiation and chemical fixation in petri dishes.

To accommodate a spaceflight experiment with moss (SPM), experiment-unique equipment (EUE) was developed by engineers at Kennedy Space Center. The hardware allows sterile culture for an extended period of time in commercial petri dishes, lateral illumination of each culture with light of a specific wavelength (660 nm; other wavelengths are possible) and a range of intensities (0.05-5 micromoles photons m-2 s-1), incubation in complete darkness, and chemical fixation to terminate the experiment under conditions of microgravity. The use of a fixative required triple containment to protect the astronaut crew. An external panel on the experiment container allowed the timing of illumination and fixation to be controlled by the crew. Light quality is provided by light emitting diodes (LEDs) that are located in the lid of the outer container, the BRIC (Biological Research In Canisters)-LED. Each canister accommodates 6 Petri Dish Fixation Units (PDFUs), and each PDFU holds one 6 cm petri dish. All components are autoclavable. LED illumination is piped through a transparent glass rod. Each PDFU contains fixative in a reservoir that is released by the depression of an actuator. This hardware performed well during its first flight, the 16-day STS-87 mission in Nov./Dec., 1997 as part of the Collaborative USA and Ukrainian Experiment (CUE). It supported vigorous and sterile moss growth, cells were maintained in position and were well-fixed, and there was a vigorous and consistent response to light. Although here used for moss, in future flight experiments this unique new hardware can be used for many types of organisms normally grown in petri dishes, with or without a requirement for illumination.     

Segmented battery charger for high energy 28 V lithium ion battery

Since they were first introduced in the early 1990s, lithium ion batteries have enjoyed unprecedented growth and success in the consumer marketplace. Combining excellent performance with affordability, they have become the product of choice for portable computers and cellular phones. Building on the same energy and life cycle attributes, which marked their consumer market success, but adding new high power storage capability, lithium ion technology is now poised to play a similar role in the transportation, military, and space sectors. With major program in various aspects of electric and hybrid electric vehicles, Saft has developed a family of battery products that address the power and energy storage where lightweight, long life, and excellent energy or power storage capabilities are needed. Significant progress in the packaging and control of high power, yet compact, batteries has been accomplished for a variety of vehicle applications. This paper discusses the charger and balancing strategies of one of this family of products