Thale cress is, in fact, the darling of science. For centuries, it’s been analyzed and anatomized, sliced and stimulated, tortured and treasured, and cooked up – metaphorically speaking – in every conceivable way. In other words, it’s the plant biologist’s equivalent of the lab rat or fruit fly, even serving as a model organism for genetic research in botany. Its scientific name Arabidopsis thaliana hints at its lineage: it’s part of the mustard family and closely related to cabbage, canola, radish, turnip and arugula.
So what makes it so special? Thale cress stands out for two things: its small genome (it was the first plant genome to be fully sequenced, over 20 years ago) and its reproductive speed (the entire cycle, from seed to plant to seed, takes just two months to complete). It’s also small in stature, produces a large number of offspring, and can reproduce asexually. But that’s not all. Thale cress is particularly hardy: it can withstand temperatures as low as 0°C and as high as 30°C or more. Its entire genome has been mapped. And it’s so easy to manipulate that researchers have created transgenic variants – mutants where target genes can be switched on and off at will.
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- Reaching for the sun
- From gravitropism to microgravity: the plant
that’s destined to explore
new worlds
Scientists are eager to learn everything they can about this unassuming plant: its cells, proteins, molecular interactions, and genetic and genomic features. Why does it flower and in what conditions? Which of its 15,000 genes is activated when its petals wilt and fade? What unique properties does it have? What are its strengths and weaknesses? Can it withstand zero gravity or space radiation? How does it respond to changes in environmental conditions such as light, temperature and nutrient availability?
Thale cress has applications in even the most unexpected areas of research. At EPFL, for instance, a team at the Swiss Plasma Center studied the effect of plasma treatment on seeds to determine whether it could be a viable alternative to pesticides and fungicides. They bombarded thale cress plants with plasma for a few dozen seconds at a time to see how their defenses would respond. The early findings suggested that plasma treatment could be used in agriculture to protect plants against environmental stress and predators without releasing harmful pollutants into the environment.
© wikipedia
The beneficial properties of thale cress don’t end there: it could potentially play a role in making plants more resilient to climate change-induced heat stress; it serves as a source of inspiration for artists; and this tiny weed recently helped a joint research team from the University of Lausanne (UNIL) and EPFL solve a fundamental unanswered question in science.
In a groundbreaking collaborative research project, UNIL biologists and EPFL engineers provided the first-ever demonstration of the correlation between light-scattering patterns and plants’ ability to turn towards the sun. In doing so, they answered a fundamental question: how do plants know where to find light? Their findings were published in the journal Science.
The EPFL team was led by Andreas Schüler, a senior scientist who heads the Nanotechnology for Solar Energy Conversion research group. Working with biologists led by Prof. Christian Fankhauser, a plant development expert at UNIL’s Center for Integrative Genomics, they spent several months conducting interdisciplinary research in an effort to solve this age-old mystery.
Arabidopsis thaliana in the country and in the city. ©Wikipedia
“We knew that the thale cress stem was made up of cells arranged around tubes of air,” says Prof. Fankhauser. “What we didn’t know was how the light scatters inside the plant, or how it transmits information to the cells to activate different responses so the plant can orient itself toward the strongest light source.”
Although phototropism – the ability of plants to turn towards light – is a well-known concept, the process by which the photoreceptors on either side of the plant are activated has always been poorly understood. “The cells that respond to light in plants are called phototropins,” explains Martina Legris, a PhD student at UNIL. “Phytochromes are receptors that respond to red light, while cryptochromes are sensitive to light at the blue and UV-A end of the spectrum – the part that influences plants’ circadian rhythm. Phototropins also play a role in leaf positioning and provide information about the brightness of the light.” However, these established facts didn’t tell scientists enough about what was actually happening inside the cells.
As is often the case in science, the breakthrough came by chance. The UNIL team happened to create a mutant thale cress strain that was waterlogged and translucent. The observation that this variant was incapable of turning towards a light source provided the starting point for their research. But the team hit a stumbling block when it came to calculating variations in brightness – known as the light gradient – at different locations in the plant’s tiny, fragile stem. “We tried several options, such as using pigments and working with the plant’s optical properties, but to no avail,” recalls Prof. Fankhauser. “That’s when we decided to turn to Andreas Schüler for help. His team had the instruments we needed to measure these optical properties much more precisely.”
Phototropins also play a role in leaf positioning and provide information about the brightness of the light.”
Testing the plant like a solar collector
Fankhauser and Schüler had previously met at a conference on natural light – a shared research interest that served as a springboard for this new project. “It’s always exciting to do this kind of interdisciplinary research,” says Schüler. “It took us outside our comfort zones.” For engineers accustomed to working with solar collectors, analyzing plant-stem sections less than 1 centimeter in length and just 250 microns across was no easy task. The EPFL team had to reconfigure some machines and learn how to fix the samples without damaging or altering them. But, as Schüler explains, the overall approach was the same: “In our work on solar collectors, we study interactions between light and structured materials at the micro and nano scales. Likewise, biologists are interested in how light interacts with different parts of plants.”
The team’s first step was to take CT scans of the tiny mutant thale cress and its wild relative. This allowed them to build a detailed picture of their respective structures without destroying the samples. Next, they subjected the plant to a battery of measurements, including placing the sample inside an integrating sphere – an optical device, also known as an Ulbricht sphere, that’s used in photometry to measure surface luminance. This instrument tells researchers how bright a given surface appears when viewed from a particular angle, taking into account factors such as how much light is emitted or reflected by the surface, and how this light is distributed in space. The device works by gathering light from a source, which then bounces off the sphere’s white inner walls to produce multiple scattering reflections.
© Wikipedia
Plants are smart
“We combined this approach with our spectrometer to break up the light beam into its constituent wavelengths,” explains Schüler. “The sample was so small and the amount of light was so low that we had to find a more powerful source. Nevertheless, we successfully demonstrated a correlation between light-scattering patterns in the micrometer-scale air tubes and the plant’s ability to move toward the light source.”
“Our model shows that the air tubes play a key role in enabling the plant to precisely perceive a light gradient and sense its direction. This mechanism wasn’t previously known to science,” says Fankhauser. When saturated with water, the plant loses its phototropic ability because the light is absorbed and scattered. For Prof. Fankhauser, this discovery is yet another example of plants’ limitless capacity for adaptation and survival: “Plants are smart. They can also adjust their direction of growth by sensing gravity, in a process known as gravitropism. That’s why, if you take a stem and lay it down flat in a dark room, with no light information, it’ll still end up growing vertically.”
8-day-old Arabidopsis root. Brown is the epidermis, red is the axial cylinder, blue is the endodermis and green is the pericycle. ©Wikipedia
In the late 1990s, Arabidopsis thaliana took its first trip into space when seedlings were grown on board the Mir space station. A series of similar experiments followed on the International Space Station (ISS) in the early 2000s, when a team of scientists from the Wisconsin Center for Space Automation and Robotics created an orbital laboratory to observe how the plant would behave in microgravity conditions. The thale cress successfully completed its life cycle, proving that its growth and development were not gravity-dependent. These pioneering space farming experiments paved the way to longer missions aimed at finding a fresh alternative to the freeze-dried and dehydrated meals that had long been staples of astronauts’ diets. The idea continued to gain traction, not least when the plant returned to Earth virtually unscathed, its seeds still viable, following more than a year exposed to solar radiation and extreme temperatures in the emptiness of space. These findings confirmed that thale cress could grow and develop normally in the absence of gravity.
The Arabidopsis thaliana study built on previous research conducted on board the ISS to determine how hypobaric environments affect the growth and development of plants in microgravity for long-term space missions. ©NASA
Can thale cress thrive in moon dust?
During the Apollo missions, the astronauts brought back samples of lunar soil for analysis. This material is known as regolith: a layer of loose rock and dust that covers the solid surface of the Moon and other terrestrial bodies. In 2021, a NASA-funded research project led by the Horticultural Sciences Department at the University of Florida demonstrated that plants could grow in this nutrient-scarce material. It should come as no surprise that the plant in question was none other than the intrepid Arabidopsis thaliana – or that it successfully germinated in this hostile substrate. However, when the seedlings were harvested after around 20 days, an RNA analysis showed weaknesses similar to those found in plants grown in inhospitable environments here on Earth. The fact that the scientists had managed to grow thale cress in regolith made headline news. But the plants struggled to survive and produced no offspring, suggesting that the idea of growing potatoes, wheat and other food crops on the Moon is, for the time being at least, little more than a pipe dream.
© UF/IFAS Tyler Jones
Temperature-sensitive mutants
Scientists at EPFL’s Laboratory of Statistical Biophysics and UNIL ran a study looking at how thale cress responds to high temperatures. Their goal was to develop a method for assessing plants’ ability to cope with thermal stress induced by climate change. To help them observe the underlying mechanisms, the researchers created a genetically modified strain called HIBAT with a marker that reacts to the presence of D-valine (a particular form of valine, an amino acid). They found that the seedlings remained unaffected by D-valine at 22°C, but that 98% of specimens were unable to survive in the presence of this compound at 38°C. Their discovery suggests that HIBAT could be a valuable candidate tool for identifying Arabidopsis thaliana mutants with a defective response to high temperature stress.
A landmine-detecting strain
In the early 2000s, Danish scientists published a paper in Nature detailing their discovery of a mutant thale cress strain that could detect antipersonnel landmines. The team of biologists spent several years manipulating the genes that produce anthocyanins, the pigments that cause leaves and fruits to turn red in the fall. They added a gene that switches on this pigmentation process in the presence of nitrogen dioxide, a gas given off by underground mines when they explode. Sowing seeds of this mutant strain on potentially contaminated land could support clearance efforts, since the leaves would turn red if mines were present. However, this promising approach has yet to prove effective in the real world.
Plants that change color in the presence of underground explosives. © Wikipedia
Could steroids produce a plant-human hybrid?
Špela Petrič, an artist and biologist specializing in phytoteratology, the study of plant growth abnormalities, challenged the notion of inter-species boundaries by adding some of her own genetic material to a Petri dish containing a thale cress embryo. Back in 2016, Petrič began examining the impact of steroidal hormones – a type of endocrine disruptor – on plants and marine organisms.
A later project entitled Ectogeneses took this research to the next level: Petrič explored the concept of plant-human hybridization by lacing thale cress cells with steroidal hormones extracted from her own urine. These sex hormones boosted the process of somatic embryogenesis, leading to the development of a seedless plant embryo. The resulting seedlings, nourished by material from the artist’s own body, were part plant, part human: hybrids that Petrič described as “monsters.”
A love story with thale cress. © Špela Petrič
A tiny plant with big potential
Arabidopsis thaliana – the humble thale cress – is and will remain a key enabler of scientific progress, driving major leaps forward in plant genetics, physiology and biology more generally. As a model organism, it’s deepened our understanding of the fundamental biological processes at play in plants, paving the way to the development of improved crop varieties that can withstand harsher environmental conditions. And because research projects involving thale cress have little economic value, the findings are readily accessible to researchers the world over. With its inimitable talent for thriving in even the most challenging environments, this unassuming little weed will remain a pillar of plant research for many years to come.
© Alain Herzog/EPFL