Trees have a remarkable ability to sense their own shape and adjust their growth to stay upright. This awareness of body position, known as proprioception, was once thought to exist only in animals. However, in 2012, researchers working with INRAE demonstrated that plants also possess this ability (Bastien R. et al., PNAS).
This internal sense allows plants to monitor their posture and make adjustments that help them grow as straight as possible (Moulia et al., Science). Although scientists had established that plants could detect their own shape, the biological processes responsible for turning that information into physical movement remained a mystery.
How Scientists Isolated Trees’ Hidden Sense
Plants use several kinds of information to guide their growth. They respond to gravity, detect the direction of sunlight, and monitor the shape and position of their own stems. To understand how this internal sense works independently, researchers needed to eliminate the directional signals provided by gravity and light.
They developed an unusual experimental system designed to isolate proprioception. Young trees were placed on a horizontal platform that rotated around its axis inside a sphere illuminated equally from every direction. The rotating platform prevented the trees from receiving a consistent directional signal from gravity, while the surrounding light eliminated cues about which way they should grow.
This arrangement allowed the researchers to investigate how trees respond to their own shape without relying on the usual environmental signals that influence their growth.
The Muscle-Like Wood That Helps Trees Straighten Themselves
The experiment began with young poplar trees positioned horizontally. As the trees grew, their stems gradually curved upward, bringing their tops back toward a vertical position.
This response depends on a specialized type of wood called tension wood. Unlike ordinary wood, tension wood can generate pulling forces as it develops. When it forms along the upper side of a stem, it acts much like a contracting muscle, pulling the stem upward and causing it to bend.
After approximately ten days, once the trees had developed a noticeable curve, researchers moved them into the experimental apparatus. Over the following weeks, something remarkable happened. Instead of maintaining their curved shapes, the stems slowly straightened until they became nearly straight again.
To determine what caused this change, the team examined the structure of the wood that developed during the straightening process.
They found that the original tension wood, which had formed along the upper side of the stems and pulled them upward, stopped developing once the experimental system was activated. At the same time, identical tension wood began forming on the opposite side.
This newly formed wood appeared to act like an opposing muscle, producing a pulling force in the other direction. As the forces worked against each other, the curved stems gradually returned to a straight shape.
The production of tension wood involves a complex sequence of biological events controlled at the cellular level. The findings indicate that a tree’s internal sense of its own shape helps regulate this process, determining where tension wood develops to correct its posture.
A Surprising Discovery About How Trees Control Their Shape
Previously, scientists believed that tension wood formed only on the upper side of stems, where it helped pull them upward. This is particularly noticeable near the bases of trees growing on mountain slopes, where trunks often need to curve to maintain an upright position.
The new research reveals that tension wood has a more versatile role. It can develop on opposing sides of a stem and generate forces in different directions, functioning much like the opposing muscle groups animals use to maintain posture and control movement.
This discovery suggests that trees possess a sophisticated system for coordinating their growth. They combine information about their surroundings (such as light and gravity) with an internal awareness of their own shape. By processing these signals, they can regulate where tension wood develops and adjust their posture accordingly.
Why This Discovery Matters for Forests and Agriculture
The ability to sense and correct their own shape is especially important when trees encounter environmental disturbances. Powerful storms, landslides, and other extreme events can bend or displace trees, making the capacity to regain an appropriate posture essential for their resilience.
Understanding this mechanism could become increasingly valuable as climate change exposes forests to more extreme conditions.
The findings also have practical implications for forestry and agriculture. By identifying plants with particularly effective proprioception, scientists may be able to select varieties that grow straighter, maintain better posture, and produce higher-quality wood.
In agriculture, the same knowledge could help researchers develop cereal crops that are less likely to fall over before harvest. This problem, known as lodging, can cause significant yield losses when crops collapse toward the end of the growing season.
The discovery could also improve timber quality. Poorly coordinated tension wood formation can create excessive internal stresses in trees, potentially affecting the properties of the wood they produce.
Bruno Moulia, INRAE research director, explained:
“What we have uncovered is a genuine sensorimotor loop operating in the woody parts of trees! Poor coordination in the successive activation of tension wood results in excessive internal tension, which can affect wood quality. These findings therefore reshape more applied research aimed at improving wood quality… and at obtaining trees that are as straight and as relaxed as possible, whatever life throws at them!”
The research also highlights the importance of combining expertise from different scientific fields to understand the complex ways plants interact with their environment.
Félix Hartman, INRAE research engineer, emphasized the collaborative effort behind the discovery:
“Revealing the remarkable capabilities of trees requires a great deal of ingenuity. In this project, we achieved it by bringing together researchers from different disciplines, with complementary skills and perspectives. This requires time and perseverance, but these interdisciplinary discoveries show that the effort is worthwhile.”