Plants in zero gravity
Growing plants in space comes with a lot of unique challenges. Space is a completely new environment for plants and forces us to re-invent crop cultivation if we want to provide food for astronauts on long-distance space travels.
Some of these challenges regard the more practical aspects of crop growth in space. Here, scientists deal with questions such as: how can we grow plants under artificial light? How can we guarantee continuous yield of high quality fruits and vegetables for astronauts? How can we germinate plant seeds in space? Or: how can we avoid plant diseases? However, it is equally important to zoom in on plants and investigate how growing in space affects their general development. Scientists explore, for example, how growing in zero gravity changes the composition of plants (for example: "Is there a difference in the amount of sugars of fruits grown on the International Space Station (ISS) compared to plants grown on earth?") or how plants know in which direction to grow without gravity.
On earth, plants know in which direction to grow because of the gravitational field and the direction of light. Shoots grow up, roots down. But how are plants reacting if they are grown in zero gravity? Will roots start to grow in the same direction as shoots? Will this complicate crop cultivation in space? Let’s have a look at how plants are dealing with this new environment. But first:
How are plants sensing gravity in the first place?
Plant roots grow in the direction of the gravitational pull (positive gravitropism), while shoots grow in the opposite direction (negative gravitropism). The gravity “sensor” of the root is located in the root tip. That means, if you cut off the tip of a root, it won’t respond to gravity anymore. The specialized gravity sensing cells in the root tip are called statocytes. Statocytes contain statoliths (yellow and black circles in the image below), which mainly consist of starch and they settle at the lowest part of the root tip in response to the gravitational pull, as you can see in the image. Funfact: Invertebrates such as jellyfish, crabs, octopuses, and squids use a similar system as a balance organ (called statocyst). The statocyst helps these animals to keep their balance or equilibrium.
In plants, the perceived direction of gravity gets translated by plant growth hormones. The hormone, which is responsible for gravity directed growth, is called auxin. Auxin accumulates in high concentrations at the bottom of the root, where it mediates growth and elongation of cells, leading to the downward growth of roots.
If a plant is tipped over on its side, statoliths, and subsequentially auxin, will concentrate on the lower side of the root, making it grow downward instead of horizontally (see the image on the left and on top). Similarly, shoots will continue to grow upwards, even if the plant is tipped on its side, which is due to the combined perception of gravity and the direction of light (see the image below). While we can provide an artificial light source on the ISS, there is no gravitational field that will tell the plant in which direction to grow.
Root growth without gravity
How do you even study root growth in space? On the ISS, plants can’t be grown in soil and watering is problematic if there is no gravity. Because of that, researchers are using an agarose gel that provides nutrients and water for the plants. This way, plants don’t need to be watered externally and can also be transported easily. Plant roots can grow inside or on top of the gel and absorb everything they need. Furthermore, the gel is translucent, which allows us to study root development very closely and in real-time.
In 2010, thale cress (Arabidopsis thaliana), the scientific model plant, was sent into space to examine how their roots grow in zero gravity. At the same time, control plants grew in similar environmental conditions on the ground. As expected, roots of the control plants on earth grew straight downwards (with the direction of gravity). The roots also showed a typical waving pattern while growing. Meaning, the root tip wiggled slightly to the left and right. It has been assumed that plant roots grow in a waving pattern to avoid obstacles such as stones and to enhance the chance of finding optimal nutrition and water in the soil. For decades, we believed that this phenomenon was gravity dependent.
Now: how did our plants do in space? In the image below you can see thale cress plants grown on earth (A, ground control) and in space (B, flight). You can clearly see that the roots of the plants grown in space did not grow straight downwards. However, they somewhat grew in the right direction. Interestingly, also the roots of the plants grown in space showed a waving growth pattern, just a little bit less intense than their counterparts on earth. This suggests that this waving growth phenomenon is not driven by gravity itself. Intriguingly, researchers also noticed that the plants cultivated on the ISS grew slightly smaller compared to the control plants on the ground.
Similar observations have already been made in one of the first experiments in which plants were monitored in space: In 1983, Allan Brown investigated the growth of sunflower seedlings on board the Space Shuttle Columbia (OV-102). The sunflower seedlings showed circumnutation, which is the circular growth of the shoot (you can often see this phenomenon in climbing plants). Also this phenomenon has been thought to be gravity dependent.
It seems like gravity is not necessary to tell the root in which direction to grow and that gravity is not the main influencing factor of the root or shoot growth pattern as we believed for a long while. Plants show an inherent ability to orient themselves without interpreting signals from the environment. This also means that we only understand a small part of plant growth and development and that new environments such as space can help us understand them even better, which will ultimately help us to enable crop cultivation in these extravagant environments.
Researchers of the University of Münster in Germany have already been studying in great detail what is going on inside the plant once it's exposed to zero gravity. They found out that plant roots already react in mere seconds to a change in the gravitational field by studying them on a reduced-gravity aircraft. Here, the aircraft follows a parabolic flight path, which leads to a few seconds (approximately 22 seconds) of weightlessness. Still, we need much more research to fully understand how plants deal with zero gravity and how we can grow crops in space to guarantee food for long-distance space travels in the future. The observation that roots grow away from the shoot and in a more or less expected direction is already a promising result for crop cultivation in space.
NASA is constantly running various experiments with different crops on the ISS. One of the more recent experiments featured growing peppers in zero gravity. We will talk about this in the next blog article, but you can already have a sneak peak :)
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Volkmann, D., Behrens, H. M., & Sievers, A. (1986). Development and gravity sensing of cress roots under microgravity. Naturwissenschaften, 73(7), 438–441. doi:10.1007/bf00367291
Paul, A. L., Amalfitano, C. E., & Ferl, R. J. (2012). Plant growth strategies are remodeled by spaceflight. BMC plant biology, 12 (1), 1-15.
Chamovitz, Daniel (2012). What a plant knows: a field guide to the senses (1st ed.). New York: Scientific American/Farrar, Straus and Giroux. ISBN 978-0-374-28873-0.
Gravitropism image: M. S. Young (c) Baylor College of Medicine