Trees Use Opposing 'Muscle' Wood to Straighten Their Trunks, Study Finds

Young poplar trees were laid on their sides and left to grow. Over about ten days, their stems bent upward, the tops swinging back toward vertical. That part was expected. What came next, inside a purpose-built experimental rig, is what researchers led by Bruno Moulia of INRAE have now reported in Science.

The question behind the work was how plants turn an awareness of their own shape into movement. Plants draw on several kinds of information as they grow: they respond to gravity, they detect the direction of sunlight, and they monitor the shape and position of their own stems. That last sense, a form of proprioception, was already known to exist. What nobody had worked out was the biological process that converts it into physical correction.

To study it on its own, the team had to remove the other signals. They built a system in which young trees sat on a horizontal platform rotating around its axis, inside a sphere lit equally from every direction. The rotation denied the trees a steady gravitational cue, and the even light removed any hint of which way to grow. What remained was the tree's sense of its own form.

The trees went in after their ten days of upward bending, once they had a noticeable curve. Over the following weeks, rather than holding that curve, the stems slowly straightened until they were nearly straight again.

To find out why, the researchers examined the wood that had formed during the straightening. The key player is tension wood, a specialized tissue that, unlike ordinary wood, generates pulling forces as it develops. When it forms along the upper side of a stem, it works like a contracting muscle and draws the stem upward. In the rotating rig, the original tension wood on the upper side stopped forming. Identical tension wood began forming on the opposite side instead, pulling the other way. With the two forces working against each other, the curved stems gradually returned to straight.

That overturns a narrower picture. Scientists had believed tension wood formed only on the upper side of a stem, a pattern most visible near the bases of trees on mountain slopes, where trunks curve to stand upright. The new work shows it can develop on opposing sides and pull in different directions, much like the opposing muscle groups animals use to hold posture. The findings indicate that the tree's internal sense of its shape helps decide where tension wood appears, blending that sense with information about light and gravity.

The practical stakes are laid out plainly. Storms, landslides and other extreme events can bend or displace trees, so the ability to regain an upright posture matters for resilience, and climate change is expected to expose forests to more of those conditions. In forestry, identifying plants with especially effective proprioception could help select varieties that grow straighter and yield better wood. Poorly coordinated tension wood formation can build up excessive internal stress, which can affect wood quality. In agriculture, the same knowledge could help develop cereal crops less prone to lodging, the collapse of plants late in the season that causes significant yield losses.

Moulia called the discovery "a genuine sensorimotor loop operating in the woody parts of trees," and said it reshapes applied research aimed at trees "as straight and as relaxed as possible." His colleague Félix Hartman, an INRAE research engineer, stressed that the result came from bringing together researchers from different disciplines, which takes time and perseverance but, he said, proves worthwhile. The next step is to see whether trees and crops differ in how well they run this loop, since that variation is what breeders would select on.