Beneath our feet lies a mysterious, complex world that holds the key to life on Earth—the fascinating realm of plant roots.
400+ Million Years
Complex Networks
Drought Resilience
Cutting-Edge Research
When we admire a majestic tree or a blooming flower, we're only seeing half the picture. The rest—often called "the hidden half"—lies underground, a complex network of roots that not only sustains the plant but shapes our entire ecosystem. Plant roots represent a frontier of science that remains largely unexplored, simply because it's hidden from view. Yet, understanding this hidden half is crucial for our future, helping us address pressing global challenges like food security and climate change.
The term "the hidden half" was popularized by the seminal work of Waisel, Eshel, and Kafkafi in their book Plant Roots: The Hidden Half, which has become a foundational reference for root biologists 4 .
This invisible underground world is now being exposed thanks to cutting-edge technologies that allow us to spy on roots without disturbing their natural environment. As we peel back the soil, we're discovering that roots are far more than simple anchors—they are dynamic, intelligent structures that have evolved over millions of years to master their environment.
Root systems make up approximately 50% of plant biomass but remain largely unexplored due to their underground nature.
Root systems play crucial roles in carbon sequestration, soil formation, and water filtration.
The story of roots begins more than 400 million years ago, when a pioneering plant did something revolutionary: it grew one of its branches downward 5 . This simple act transformed life on Earth forever.
Before this evolutionary innovation, plants had only shallow, hairy runners that provided basic anchorage. But as roots evolved, plants gained remarkable new capabilities: they could forage for water and nutrients deep underground, enabling them to grow to tree-size proportions, form underground alliances with microorganisms, and even reshape the very soil they grew in.
Fossil evidence from the Rhynie chert, a 407-million-year-old deposit found in Scotland, captures this evolutionary transition. Researchers have found Asteroxylon mackiei, a plant with a kind of work-in-progress root—something midway between a branch and a root 5 . It grew downward but lacked the protective root cap that modern roots possess.
What's particularly fascinating is that roots evolved independently on at least two separate occasions 5 . Lycophytes (including the A. mackiei found in the Rhynie chert) gradually developed roots, while seed plants—the group that includes most modern garden plants—later developed them separately. This convergent evolution suggests that roots provided such a significant advantage that nature invented them multiple times.
A plant's root system is far from a tangled mess—it follows an elaborate architectural plan known as Root System Architecture (RSA). This architecture encompasses the spatial configuration of roots in soil, including their depth, distribution, angle, and the density of lateral roots 1 .
Roots explore soil volumes to acquire essential water and mineral nutrients through specialized structures like root hairs.
Root systems provide mechanical support, securing plants against environmental forces like wind and water flow.
Roots store carbohydrates and nutrients and enable vegetative propagation through specialized structures.
The specific architecture of a root system isn't fixed—it's shaped by both genetic factors and environmental conditions 1 . This plasticity allows plants to respond to their specific growing conditions, investing resources where they're most needed.
Determines access to deep water sources
Affects soil exploration efficiency
Influences penetration ability
Controls spatial distribution
Studying plant roots presents unique challenges. As University of Nottingham researchers note, if you dig a plant from the soil in your garden, "you'll quickly discover that it is a destructive process" 1 . Even with extreme care, parts of the roots break off or remain in the soil, and the overall shape collapses as the supporting soil falls away.
To overcome these challenges, scientists have developed increasingly sophisticated tools:
| Technology | How It Works | Applications | Advantages |
|---|---|---|---|
| X-ray Computed Tomography (X-ray CT) | Uses X-ray attenuation to create 3D images of root architecture in soil | Studying root growth patterns, response to soil conditions | Non-invasive, allows 4D imaging (3D over time) 1 |
| MRI | Employs radio-frequency waves and magnetic fields to map hydrogen atoms in water | Monitoring root water content, studying root systems in soil | Non-destructive, good for visualizing water movement 3 |
| Rhizotrons | Growth chambers with transparent observation windows | Direct observation of root growth and development | Allows direct, repeated observation of roots 3 |
| Electrical Resistance Tomography (ERT) | Images sub-surface soil structure from maps of electrical resistivity | Studying large root profiles, particularly trees | Non-invasive, field-deployable 3 |
These technologies are generating massive amounts of data, leading to another breakthrough: the use of deep learning and automated image analysis to process this information efficiently 3 . What was once a painstaking manual process is now becoming automated, dramatically accelerating the pace of root research.
Perhaps nowhere is root research more critical than in understanding how plants cope with drought conditions. As climate change intensifies, understanding these mechanisms becomes increasingly vital for global food security.
Robert Sharp, a plant biologist at the University of Missouri, has dedicated his career to understanding how roots keep plants alive when water is limited. His work, summarized in a recent centennial review, highlights several key discoveries:
When water is scarce, shoot growth typically slows while roots continue to lengthen 2
Root cells can loosen their walls through specialized metabolic functions 2
Root cells accumulate solutes that pull water back into the cells 2
Roots can sense where water is located and grow toward it 2
| Adaptation | Mechanism | Benefit to Plant |
|---|---|---|
| Osmotic Adjustment | Accumulation of solutes in root cells | Maintains cell turgor and hydration under water deficit 2 |
| Continued Root Elongation | Maintained root growth while shoot growth slows | Enhances soil exploration and water foraging capability 2 |
| Hydrotropism | Directional growth toward water sources | Increases efficiency of water acquisition 2 |
| Abscisic Acid (ABA) Accumulation | Hormonal regulation that promotes root growth | Supports root development specifically under drought conditions 2 |
Sharp's research in the 1990s discovered that the accumulation of abscisic acid (ABA), a plant hormone, is essential to maintain root growth in developing corn seedlings under drought conditions 2 . This finding has important implications for developing more drought-resistant crops.
Roots are far from isolated—they participate in complex social networks underground. Through sophisticated alliances with microorganisms, they've developed relationships that benefit both plant and microbe.
Legumes—including beans, peas, peanuts, and clover—form a special symbiotic relationship with bacteria that can fix nitrogen from the air and convert it into ammonia 5 . The plant then converts this into proteins to support its growth.
In return, the plant houses the bacteria in root nodules—small, greyish-pinkish balls on the roots—and provides them with sugars and oxygen 5 .
This friendship is approximately 60 million years old and plays a crucial role in global nutrition. "They're a really important crop for feeding the world," explains Hélèna Dove, head kitchen gardener at the Royal Botanic Gardens at Kew. "Peas, beans, chickpeas, they get more protein because they have this relationship, so they're really good if you're having a plant-based diet." 5
Even more widespread is the mycorrhizal symbiosis—the association between plants and fungi that is formed by more than 85% of plant species 5 .
Laura Suz, a mycologist at the Royal Botanic Gardens, explains that "these fungi, compared to the roots, they are thinner and they are longer so they can get further from the roots, they can fuse, and they can get into very small pores, and reach nutrients that are not accessible for the plants." 5
Perhaps most remarkably, plants use these fungal networks to communicate with each other. Research has shown that broad bean plants use mycorrhizal networks for warfare, alerting each other about aphid attacks so unaffected plants can prepare their defenses 5 . Some plants even use such webs to poison the ground of competitors.
Roots release chemical compounds to communicate with nearby plants and microbes
Fungal hyphae connect multiple plants, enabling resource sharing and warning signals
Complex interactions between roots, bacteria, fungi, and other soil organisms
At the University of Nottingham's Hounsfield Facility, researchers have undertaken groundbreaking work to make the hidden half visible. Their experiment uses X-ray Computed Tomography to create detailed 3D images of root systems growing undisturbed in soil 1 .
The experiment has yielded unprecedented views of root architecture in its natural state. Researchers can now identify specific features of crop roots—such as root depth, thickness, angle, or number of lateral roots—that could be improved to allow more efficient production with limited water or nutrient supply 1 .
The Hidden Half website at the University of Nottingham serves as a directory of plant root systems that have been imaged using this technique, showcasing for the first time "the beauty, diversity and complexity of plant root systems in their undisturbed soil environment." 1
| Trait Category | Specific Measurable Traits | Importance for Crop Improvement |
|---|---|---|
| Overall Architecture | Root depth, distribution pattern, branching density | Determines soil volume explored and resources accessed 1 |
| Individual Root Features | Root thickness, angle, lateral root number | Affects penetration ability and exploration efficiency 1 |
| Dynamic Traits | Growth rate, response to nutrients, temporal development | Indicates plasticity and environmental responsiveness 1 |
As root research advances, scientists are working to translate findings from controlled laboratory settings to real-world field conditions. This transition presents significant challenges but is essential for practical applications.
Samuele Ceolin, a plant biologist at the Luxembourg Institute of Science and Technology, is studying how roots adapt to sudden changes in water availability. "One of the main challenges remaining in root science is to understand how dynamically roots adapt when exposed to quick changes in water availability around them, for example after a summer thunderstorm," Ceolin explains 6 .
His research uses MRI technology to monitor root systems repeatedly without destroying them, every 48 hours. While still analyzing the data, Ceolin notes that preliminary results "seemingly indicat[e] that roots do adapt fast and dynamically whenever the conditions in soil moisture quickly vary along the soil profile." 6
The latest edition of Plant Roots: The Hidden Half reflects progress in the field, featuring chapters on modern topics including single-cell genomics and computational root biology 8 . These advances promise to further illuminate the complex world beneath our feet.
Identifying genes responsible for beneficial root traits for crop improvement
Automated analysis of root imaging data and predictive modeling
Developing crops with root systems adapted to changing climate conditions
Roots may be the hidden half of plants, but their impact is visible everywhere—from the food we eat to the air we breathe. As we face global challenges like climate change, population growth, and soil degradation, understanding roots becomes not just a scientific curiosity but a necessity.
The silent, unseen work of roots reminds us that the most fundamental processes often happen out of sight. As research continues to expose this hidden half, we gain not only knowledge but new tools to cultivate a more sustainable and food-secure future. The next time you admire a plant, remember that half its story—and potentially half the solutions to our agricultural and environmental challenges—lies beneath the surface, waiting to be discovered.
This article was inspired by the seminal reference work "Plant Roots: The Hidden Half," edited by Waisel, Eshel, and Kafkafi, which has guided root biologists for decades 4 .