The ChlorISS Project: growing plants in space

The ChlorISS experiment is based on a simple question: how does a plant orient itself when gravity is removed? Plants feed themselves using two organs: leaves and roots. Leaves turn toward the sun to capture light and produce sugars through photosynthesis. Roots draw in essential water and minerals. To survive, they must therefore orient their stems upwards and their roots downwards. On Earth, this orientation depends on two factors: gravity and light. Gravitropism directs the roots downwards and the stems upwards, while phototropism guides organs towards the light. In the International Space Station (ISS), gravity disappears. “In the absence of gravity, the plant's sensory system does not provide any information and it will grow in any direction,” explains the researcher. In this context, light becomes its only compass.

Eugénie Carnero Diaz devised a multi-step protocol to demonstrate this. First, two types of Arabidopsis thaliana are used: a “wild” plant and a mutant variant with a defective gravity sensory system. Comparing their behaviours makes it possible to anticipate what happens when gravity disappears.

To compare results, the experiments were conducted in parallel on Earth and on the ISS. "On Earth, it is easy to verify the effect of gravity: if you lay a plant down, its stems straighten upwards and its roots dive downwards. On the ISS, it's more complex: first, you have to artificially give the young plants an axis by shining light on them from above, then move the light to the side to see if the organs bend in its direction or away from it," Diaz explains.

In the last part of the experiment, three types of light (white, blue, and red) are tested to analyse the plants' sensitivity to each spectrum. Diaz anticipates that “a plant that does not respond to gravity becomes much more sensitive to light and therefore responds much better to it.”
 

An educational dimension

Designed for students from primary school all the way up to high school levels, ChlorISS will enable them to juggle with biological concepts of varying levels of complexity. "There is a twofold objective behind this experiment. First, to stimulate scientific curiosity by showing that a known phenomenon can change or disappear, and to encourage students to ask questions and seek answers. Secondly, to consolidate the biology lessons already in the curriculum (plant growth, interaction with their environment, photosynthesis) by giving them a tangible application," insists the researcher.

In addition to the space experiment, kits are being distributed to nearly 4,000 French schools. These kits contain seeds, instructions, and adapted protocols. For younger children, mizuna (Japanese mustard, from the broccoli family), a plant that is easier to observe, has even been added. These kits allow students, under their teachers’ supervision, to conduct the experiment simultaneously with the astronaut aboard the ISS.

The experiment carried out by Sophie Adenot will only last ten days, in the middle of a busy schedule. “It's a one-off!” emphasises Diaz. “That's why it required a lot of laboratory testing beforehand.” The protocols must be flawless, ensuring sterility in orbit while remaining simple and suitable for classrooms on Earth.
 

A CNES and Sorbonne University collaboration

To set up this project, the researcher was able to count on the support of CNES via Cadmos (Centre d’aide au développement des activités en micropesanteur et des opérations spatiales). Students from four agricultural schools also participated in the propagation of Arabidopsis seeds for the kits.

Sorbonne University has provided its scientific and technical expertise to the preparatory experiments that will be conducted in schools and on board the ISS using microgravity simulators.
 

Challenges for the future of space exploration

ChlorISS opens up much more practical possibilities beyond its clearly educational value.  Although the ISS orbits just 400 kilometres above Earth - “about the distance between Paris and Lyon,” as the researcher puts it - and is regularly resupplied, logistics change completely for missions to the Moon or Mars. Those distances make resupplying rare, if not impossible. A journey to Mars alone takes around six months one way, followed by at least a year on site, making the round trip no less than two years. Under these conditions, it is impossible to carry sufficient reserves of oxygen, water or food.

For these reasons, scientists are working on bio-regulatory life support systems; miniature ecosystems where each element recycles the waste produced by others. Astronauts would produce CO₂ and organic waste, which would be transformed by micro-organisms into nutrients for plants. In return, the plants would provide oxygen, clean water, vitamins and fresh food. “Plants will play a vital role in human survival, while having to survive and remain nutritious in a space environment,” explains Diaz. The challenge, therefore, is to ensure that plants can adapt to the extreme conditions of space without losing their essential qualities.