Nearly 15 years ago, scientists found that plants could sense their own shape, an ability previously thought to exist only in animals. Now, researchers from the same institution have uncovered how trees use this sense to control their form. Their mechanism works rather like a muscle.
Researchers from France's National Research Institute for Agriculture, Food and Environment (INRAE) and the University Clermont Auvergne have demonstrated that plant growth is more complex than previously realized. Plants are already known to have strategies to optimize light exposure and ways of dealing with salty conditions, nutrient-poor soil, and pathogens. Now, scientists have shown how young poplars use a muscle-like mechanism to shape themselves. This is an active process that requires energy, rather than simply a passive result of growth.
The team focused on proprioception, a plant's ability to sense its own shape. To isolate this sense, the team designed an experiment with young poplars. First, young trees were placed horizontally, causing them to curve upward. The curved trees were then placed on a horizontal rotating platform inside a sphere lit up from every direction. The idea was to prevent the trees from sensing the direction of gravity and light, which normally guide tree growth.
Over time, the young trees gradually straightened as they grew, appearing to correct their curved shape and develop a kind of "posture." This suggested that they were responding to their own form.
On closer inspection, the researchers found that the plants were able to produce a specific kind of wood — called tension wood — on one side of the stem. This wood creates a pulling force that changes the stem's shape. Essentially, it acts like a muscle that helps direct the way the stem grows.
"What we have uncovered is a genuine sensorimotor loop operating in the woody parts of trees," says Bruno Moulia, INRAE research director. "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."
While tension wood on one side of the stem begins the process, it eventually develops on the other side, helping to straighten the poplar's structure. The two sides of the tree's stem work like opposing muscles — the kind we see in many animals. Trees do not have animal muscles, but this comparison helps explain how forces on opposite sides control their shape. This mechanism is distinct from plant cell turgidity, the internal pressure that also plays an important role in shaping plants.
"Revealing the remarkable capabilities of trees requires a great deal of ingenuity," adds Félix Hartmann, INRAE research engineer. "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."
Beyond possible improvements to wood quality, the study offers another reason to look more closely at plants. Understanding their ability to sense and control their shape may help counter "plant blindness" — our tendency to overlook the plants around us.
The research was published in the journal New Phytologist.