How do you water a plant in zero gravity?

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Scientists shoot plants into space in an attempt to unearth the mysteries of root growth

Gel. Gel is the answer. By growing experimental plants on plates filled with nutrient gel instead of pots of soil, researchers have been able to capture on camera the growth of roots in space, without having to worry about watering them on their journey into orbit.

And you can see exactly what happens to them here:

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In fact, the plants in these experimental chambers grow along the surface of the gel, not through the substrate as they would in soil. This allows visualisation of growth behaviour in two dimensions, captured in this time-lapse video that was run over the course of 15 days whilst in orbit on the International Space Station (ISS).

Enlisting the help of astronauts aboard the shuttle STS-130, which launched in February 2010, the cargo of Arabidopsis plants – the workhorse of the plant science world—was unloaded onto ISS, and their growth compared with identical control plants on the ground, at Ground Control in the Kennedy Space Centre.

But why send plants into space in the first place?

Hundreds of experiments are conducted on the ISS , on microbes to muscles cells, to investigate what happens when you remove the effects of gravity from normal biological systems. In the case of plant root growth, it is well know that a complex interaction of environmental stimuli acts to determine the direction in which they grow. In general, two phenomena occur during this growth: “waving” of the root tips as they grow, and “skewing” of the roots on an angle relative to the direction of above-ground shoot growth.

These patterns are thought to be involved in a plant’s avoidance of obstacles when growing in a substrate like soil, where they exhibit a corkscrew-like motion. If this growth occurs on top of a vertical surface, contact with this surface causes the roots to skew away from the vertical axis. However, the extent to which gravity affects this phenomenon is not clear. Unless gravity is removed completely.

And this is just what Anna-Lisa Paul and her colleagues did. Their study, published today in BMC Plant Biology, analyses how two different ecotypes of the same species of thale cress (Arabidopsis thaliana) reacted to weightless growth. They found that although both ecotypes exhibited characteristic growth patterns which were exaggerated in the absence of gravity, neither were consistent with the hypothesis that gravity itself was driving their orientation. Interestingly, they also found that the plants themselves were uniformly smaller than their counterparts on earth.

Commenting on what the findings of this mission mean for our understanding of the mechanisms of plant root growth, Professor of Molecular Cell & Developmental Biology Stan Roux feels that attention now needs to shift to investigating how mechanical stimuli can influence these effects, saying:

The data of Paul et al. provide novel and valuable documentation that the force of gravity is not needed for the waving and skewing patterns of root growth on solid surfaces… They thereby focus more attention on the role of touch in these patterns, and especially on how the touch stimulus is linked to altered auxin transport, which is likely to be a key controller of waving and skewing in roots.”

Although both mechanical and gravitational influences on plant growth have been studied for well over a century—Charles Darwin wrote about these, and other phenomena, in his book “The Power of Movement in Plants”, which he co-wrote with his son Francis—the opportunity to launch experiments into orbit represents a unique way in which to investigate how forces that we take for granted can influence everyday biological processes.

Darwin was never able to remove the effects of gravity from his own home-grown experiments on plants, but would he ever have envisaged that we would boldly send them where no plant scientist has gone before?