Dr. Stan Roux and colleagues study how the environmental stimuli of light and gravity alter patterns of growth and development in plants. Germinating spores of Ceratopteris richardii (C-Fern) are used as a single-cell model system to examine the effect of gravity on orientation of nuclear migration and subsequent rhizoid development and growth.

C-Fern was carried on the STS-93 shuttle mission - 23 July 1999.

Gravitational Biology and Ceratopteris

by Dr. Stan Roux

The physiological responses of animals and plants to gravity are complex, involving the interaction of many different cell types. In order to simplify their study of the cellular basis of gravity sensing and responding, biologists have recently begun studying gravity effects in single cells, in which all the gravity sensing and responding occur in the same cell.

Image
Migration of nucleus seen at 1-g. Nucleus starts out (far left frame) in a central position (small circle in center of cell), then, after about 30 hours of random movements near the cell center (middle frame), it migrates downward to the lower part of the cell (far right frame). After its downward migration the nucleus is now positioned for the unequal primary division. By video microscopy scientists will be able to test whether either the random nuclear movements near the center of the cell or the more linear migration to the cell periphery occur in microgravity.

One such model system for gravitational biology studies is the germinating spore cell of the fern Ceratopteris richardii. These spore cells appear to be insensitive to gravity as long as they are kept in darkness, but once induced to germinate by light, they show a characteristic gravity response. Each single-celled spore has a nucleus that starts out in a central position in the cell, where, for the first 30 hours or so after light activation, it moves along a kind of random path restricted to a region near the cell center. Then, under the guidance of 1-g on earth, the nucleus abruptly migrates downward along a relatively straight path to the lower part of the cell. There, about 18 hours later, it divides, producing two cells, a smaller one that develops into a root-like rhizoid, and a larger one that develops into the leafy part of the plant, the prothallus. The gravity-directed migration of the nucleus exactly predicts the direction of the emergence and growth of the rhizoid after the spore germinates. Furthermore, the unequal cell division that results from the asymmetric positioning of the nucleus after its downward migration may be a prerequisite for two different cell types to form (rhizoid and prothallus). Thus, within a limited period following light activation of the spores, gravity determines the polarity of each spore cell: which end will have the rhizoid and which end will have the prothallus.

On STS-93, scientists will take advantage of this simple system for studying gravity effects on the most basic level: that of the single cell. The facilities on the Shuttle will allow them to investigate two sets of questions. One set of experiments will use an on-board video microscopy system called the STL-B to find out whether, in the absence of a strong gravity signal, the nucleus will migrate randomly or not at all, and, if not, whether the failure to migrate will prevent normal development of the rhizoid and prothallus. Another question to be answered by the STL-B concerns the "random walk" of the nucleus about the center of the cell during the first 20 hours after the spore cell is activated. This movement (as well as the later downward movement of the nucleus) is driven by molecular motors. It is possible that these molecular motors need the tension and compression forces that are set up in the cell by gravity in order to be turned on. The STL-B will allow scientists to see whether in microgravity the molecular motors will operate normally, or will fail to turn on, leaving the nucleus motionless in the center of the cell. Thus, the STL-B may give us an insight into how these molecular motors, which are common to all plant and animal cells, can be controlled.

A second set of experiments will investigate whether gravity is turning on or off any specific genes during the period in which it is setting the polarity of the cell. Scientists already know that there are literally hundreds of genes that are turned on (transcribed into messenger RNA) or turned off during this period. They believe that most of these genes are programmed to turn on or turn off at this time whether gravity is present or not. Quite possibly, however, the expression of some of these genes may require the tension and compression forces set up by gravity in the cell. To help scientists investigate the question of whether the stimulus of gravity regulates the expression of specific genes, astronauts on the STS-93 mission will freeze light-activated spores at four different time points, three during the period when gravity fixes the cell polarity on earth, and one after this period should be over (45 h after the spores are light-activated in orbit). After the Shuttle lands, the pattern of gene expression in these space-flown spores at the four time points chosen will be compared to the pattern seen at the same four time points in spores on earth.

The germinating spores represent a relatively uniform population of cells that have all been induced to start their development at the same time (by a light signal). Because of the cells' uniformity and the relative synchrony of their development, the potential to resolve subtle differences in the pattern of gene expression between cells growing in microgravity and cells on earth is high. It would be reasonable to postulate that any genes discovered to be regulated by gravity would be important for the ability of the cells to sense or respond to gravity. If the STS-93 experiments and subsequent tests reveal the identity of genes needed for gravity sensing or responding in single cells, this discovery will greatly illuminate