From Darwin to the ISS Decoding How Plants Grow

                                                                                                                                        Professor R. S. Sengar and Dr. Shalini Gupta

Why roots wiggle, skew, and wave, even when they are floating in space

IN the 1880s, Charles Darwin turned his curiosity to plants. Roots anchor a plant in the soil, absorb nutrients, and I respond to environmental stimuli; they are the foundation of any plant. When a seed germinates, the roots always grow down, and the sprout grows up. He wondered: Why do the roots of many plants not grow straight away from the seed but instead take a slight turn to one side?

Catching Roots in the Act

Darwin devised a clever experiment using smoked glass plates to determine the root growth pattern. He coated two glass plates with soot and pressed seedlings against them. As theroots grew, their tips left delicate scribbles in the soot, tracing graceful waves and unexpected slants. These patterns showed that roots did not just plunge straight down, pulled by gravity. They staggered side-to-side in a waving pattern and drifted at angles skewing from straight lines, and adjusting their path like cautious explorers. However, the patterns were not random; they hinted at an invisible 'script' guiding growth. Darwin hypothesised that gravity and touch, the physical contact with surfaces, shaped the choreography of root tip movement. He meticulously documented these plant movements in his 1880 book, The Power of Movement in Plants.

Understanding the Twist

Later, botanists conducted lab experiments. When plants grew on flat surfaces like agar plates (a jelly-like growth medium used in labs), their roots did not always grow straight down. Instead, researchers observed the same slanted skewing pattern and gentle, repeating bends known as waving. These behaviours fascinated scientists because they revealed how roots respond to their environment, particularly gravity and touch.

Early theories suggested roots naturally twist as they grow, like a rotating drill bit. Researchers noticed that roots usually skew to the left when viewed from above in the typical lab plant Arabidopsis, a small flowering weed often used in genetic studies. When the tip touches the agar surface, it gets pushed slightly off course, and since the twist is usually left-handed, the root ends up veering leftover time. However, later experiments complicated this idea. Some genetically altered mutant variants with right-handed twisting also skewed right, which supported the twist-and-touch theory.

Skewing happens when a root grows mostly downward but with a slight sideways tilt. Different varieties of Arabidopsis show varying degrees of slant; some barely tilt, while others curve sharply.

Making Sense of the Wave

Waving is more dramatic than skewing. Instead of a steady slant, the root makes regular back-and-forth bends as it grows. This happens especially when roots grow on tilted agar plates. One leading explanation is that gravity and touchwork together: the root tries to grow downward, pulled by gravity, but cannot penetrate the hard agar, so it gets deflected sideways. Each time it touches the surface, it changes direction slightly, creating a rhythmic wave.

Further studies suggested that the plant hormone auxin, a key chemical that regulates growth, plays a significant role. Light and moisture gradients might also have minor effects. From these studies, biologists concluded that gravity guides the overall direction, touch steers the bends, and internal signals fine-tune the pattern.

The Gravity Connection

Gravity was seen as a major player in both skewing and waving. On Earth, roots use gravity as their primary guide for downward growth. But when they hit a barrier, like agar, touch briefly takes over, nudging them sideways. Gravity pulls them back and creates a push-and-pull effect that leads to waves or a steady skew. Many experiments where plates were angled differently confirmed this: roots wave more when the surface is slightly tilted (forcing more contact), but grow straight when vertical (minimising touch).

After years of study, several questions remained unanswered: Are skewing and waving truly linked? Why do roots skew left by default? Is this due to ancient evolutionary traits or just a quirk of how Arabidopsis grows?

Help from the Heavens

Scientists believed gravity and touch worked together for years to create gentle curves (waving) and slanted growth (skewing)in plant roots. However, experiments at the International Space Station (ISS) turned this idea on its head.

Gardening in space is a fascinating challenge. Without gravity, plants cannot rely on natural cues like "up" or "down", so astronauts use clever setups to nurture them. Special growth chambers provide artificial light (often LEDs) tuned for photosynthesis. The roots, which would float aimlessly in microgravity (near-weightlessness), are anchored in moisture-rich gels or fibrous mats that deliver water and nutrients. Airflow systems prevent stagnant air around leaves, ensuring they "breathe" properly. Over the years, these experiments have grown lettuce, radishes, peas, sunflowers, and even flowers like zinnias. This proves that plant life can thrive beyond Earth while deepening our understanding of plant physiology.

Surprise in Space

When researchers grew Arabidopsis in microgravity, they found that roots still waved and skewed, even without gravity's pull. Instead of growing downward, the roots grew away from overhead LED lights, a behaviour called negative phototropism, where plants grow away from light. After five days, the roots began slanting sharply to the right, much more than on Earth. Surprisingly, different plant varieties showed distinct patterns, proving that gene expressions, not just gravity, play a significant role.

These results challenged the old theory that gravity dominates skewing and waving. Instead, they showed that when roots press against a surface, touch alone can trigger these patterns by altering auxin distribution. The force of contact also mattered: in microgravity, where roots press more lightly against surfaces, skewing was stronger, suggesting weaker contact exaggerates the slant. Scientists now thinktouch signals involving ATP (a cellular energy molecule) and calcium changes redirect auxin flow, causing uneven growth. This shifts the focus from gravity to mechanical feedback: roots do not need gravity to curve; they just need a surface to interact with.

Space biology offers a unique way to separate how plants behave on Earth from how they could behave under different conditions. On Earth, plants rely heavily on gravity to orient themselves; roots grow downward (gravitropism), and shoots grow upward, guided by specialised gravity-sensing cells. Light also plays a role, but gravity is the dominant cue. In microgravity, where Earth's pull is absent, plants must rely on alternative cues, like light gradients, in ways they would not normally.

Even more fascinating, researchers found that specific genes involved in light sensing, usually active only in leaves on Earth, became active in roots in space. This suggests plants have latent genetic pathways that can be "switched on" in new environments, which reveals a level of biological plasticity (adaptability) that would have been hard to detect in Earth-based studies.

The Naturalist Fallacy

A fundamental mistake in science is confusing observation ("what is") with moral or natural necessity ("what ought to be"). For example, when we see the sun "rise" and "set".it appears to move around Earth, leading ancient thinkers to "rationally" believe Earth was the universe's centre. But science reveals the more profound truth: Earth rotates, creating this illusion.

This same confusion arises in discussions about human behaviour, such as gender roles. Because women are often seen as caregivers and men as leaders, some assume these rolesare biologically fixed. However, science shows that while minor average differences may exist, most gender norms are shaped by culture, not nature. Just as the sun doesn't actually revolve around Earth, social patterns are not inevitable; they result from history, power, and societal structures.

In fact, all science would be superfluous if appearances and reality directly matched. True scientific thinking requires separating facts from assumptions and recognising that reality often lies beneath surface perceptions. As an Indian mathematician and historian, DD Kosambi said, "Science is cognition of necessity." Appearances deceive, science reveals.

Insights from Heaven

Another discovery from space biology involves how plants manage various stresses. On Earth, plants experience mechanical stress from wind, rain, and their own weight, influencing growth and cell wall strength. These forces are absent in microgravity, leading to weaker stems and altered cell walls. Such studies help scientists understand which plant structures are indispensable and which are mere adaptations to Earth's environment.

For example, when researchers grew wheat in space, the plants developed thinner cell walls but were otherwise healthy. This challenges the assumption that thick, rigid stems are the only way wheat can grow. Instead, it suggests plants can thrive with different structural strategies depending on their environment, aflexibility crucial for growing crops on the Moon or Mars, where gravity is weaker.

Space biology has also reshaped our understanding of plants' circadian rhythms (internal biological clocks). On Earth, the day-night cycle and gravity regulate a plant's internal clock, influencing when genes related to photosynthesis or growth activate. However, in space, where the sun rises and sets every 90 minutes (due to the ISS's orbit), and gravity is nearly absent, organisms must adjust differently.

Experiments showed that some organisms, like Chlamydomonasreinhardtii (single-celled green algae), maintain a roughly 24-hour cycle even without Earth's cues for over six days in space, suggesting an innate biological clock. Others struggle to synchronise, revealing which aspects of circadian regulation are hardwired and which are flexible. This knowledge is vital for future space agriculture. If we want to grow food on Mars (where days are 40 minutes longer than Earth's), we must know whether plants can adapt or if we will need artificial light cycles.

Rethinking Plant Biology

These insights force us to reconsider long-held assumptions. For centuries, plant physiology studies on Earth interpreted findings as universal truths when, in reality, they were observing how plants behave under Earth-specific conditions (gravity, atmosphere, light cycles). Space experiments reveal that many "rules" of plant biology are just adaptations to Earth, not fixed laws.

This has profound implications for future space habitation. If we want sustainable habitats on the Moon or Mars, we can't rely solely on Earth-based knowledge. We must understand how plants behave in those settings, not just how we expect them to act.

Moreover, space biology does not just benefit space travel; it improves our understanding of plants on Earth. Scientists can identify genetic pathways that are functional for engineering crops to withstand drought, flooding, or other stresses by seeing how plants adapt to extreme environments. For example, discovering that plants use light as a directional cue in microgravity might inspire new ways to manipulate root growth in arid soils where water is scarce.

The snowball effect of past CO2 emissions is warming the planet, profoundly impacting the climate. At one level, knowledge from space experiments has a ripple effect-enhancing extraterrestrial agriculture and terrestrial food security.

Ultimately, space biology acts as a "scientific control experiment" for life on Earth. By removing gravity, researchers isolate and study biological processes in ways impossible on Earth. This helps distinguish what is fundamental to plant biology from what is merely an Earth-specific adaptation. Just as the heliocentric model revealed the sun doesn't revolve around Earth, space biology shows many "natural" plant behaviours are context-dependent, not inherent truths.

India's Role in Space Biology Two exciting Indian experiments aboard

the Axiom Space Mission 4- featuring India's astronaut crew on the ISS -are helping unlock some of these mysteries. The first, "Sprouting Salad Seeds in Space," led by the University of Agricultural Sciences, Dharwad, and IIT Dharwad, compares methi (fenugreek) and green gram sprouts grown in microgravity with Earth-grown ones, testing if space sprouts are safe to eat (free from toxins or harmful microbes). The second, "Impact of Microgravity on Food Crop Seeds," acollaboration between the Indian Institute of Space Science and Technology and Kerala Agricultural University, exposes seeds of six crops to space conditions. After returning to Earth, researchers will track their growth over generations to see if space alters their yield or resilience, a critical step for future space farming.

Writer: Professor R. S. Sengar, Director Training and Placement, Sardar Vallabhbhai Patel University of Agriculture and Technology, Modipuram, Meerut.