Looking at a tree, it can be hard to visualise the sheer volume of water being drawn up from the roots to the canopy. That volume of was is massive, and puts cells under a lot of pressure, so lignin, the substance plants use to strengthen cell walls, is an important product. But what happens to lignin if you take gravity away? Growth and Lignification in Seedlings Exposed to Eight Days of Microgravity by Cowles et al. is a study that aims to find out.
The experiment on STS-3 was growing pine seedlings with mung beans and oat seeds. There were a couple of targets. One was to examine how gravity affected the production of lignin. The other was to test the PGU, the plant growth unit, that would be used in following missions.
To see the effect of gravity a PGU with similar plants was kept on Earth, so the development of the plants could be compared.
Germination of the orbiting plants was much like the 1g plants. However, Cowles et al. point out that the seeds have to be prepared before launch, which gave them twelve hours on Earth to germinate. They found that the flying plants grew less, and in the case of the seeds, roots were growing ‘up’ as well as ‘down’. Some of the plants that grew in orbit also contained less lignin.
There have been plenty of papers that went on to cite this research, most recently Expression of stress-related genes in zebrawood (Astronium fraxinifolium, Anacardiaceae) seedlings following germination in microgravity by Inglis et al. in Genetics and Molecular Biology from this year.
Recently in Annals of Botany there’s been Xylem Development and Cell Wall Changes of Soybean Seedlings Grown in Space and in the opposite directon Hypergravity Stimulus Enhances Primary Xylem Development and Decreases Mechanical Properties of Secondary Cell Walls in Inflorescence Stems of Arabidopsis thaliana by Nakabayashi et al.
This is interesting that it still gets cited because the results weren’t all significant. While the mung beans had less lignin, the oat and pine seedlings didn’t have significantly less and the experiment was relatively small. However, this flight wasn’t just about the results, it also worked to establish a method. By laying out the experimental technique used to analyse the plant Cowles et al laid down a baseline for other researchers to compare and improve their techniques.
The basic question they studied remains important. Understanding the processes that produce lignin could help with technology on Earth. For example, it would be helpful in producing biofuel if there were less lignin in it to start with. Launching plants and growing them in space would be a spectacularly inefficient way to do that. However for small samples, it can be a useful way to isolate one variable and help figure out the mechanics of lignin production.
You can read more posts on papers from our spaceflight supplement by clicking the STS-3 tag.
Chapple C. & Rick Meilan (2007). Loosening lignin’s grip on biofuel production, Nature Biotechnology, 25 (7) 746-748. DOI: http://dx.doi.org/10.1038/nbt0707-746
de Micco V., J.-P. Joseleau & K. Ruel (2008). Xylem Development and Cell Wall Changes of Soybean Seedlings Grown in Space, Annals of Botany, 101 (5) 661-669. DOI: http://dx.doi.org/10.1093/aob/mcn001
Inglis P.W., Ciampi A.Y., Salomão A.N., Costa T.D.S.A. & Azevedo V.C.R. (2013). Expression of stress-related genes in zebrawood (Astronium fraxinifolium, Anacardiaceae) seedlings following germination in microgravity., Genetics and molecular biology, PMID: http://www.ncbi.nlm.nih.gov/pubmed/24688295
NAKABAYASHI I. (2006). Hypergravity Stimulus Enhances Primary Xylem Development and Decreases Mechanical Properties of Secondary Cell Walls in Inflorescence Stems of Arabidopsis thaliana, Annals of Botany, 97 (6) 1083-1090. DOI: http://dx.doi.org/10.1093/aob/mcl055