Tension Wood Forms Bilaterally in Poplar to Drive Proprioceptive Straightening Independent of Gravity

The experiment isolated proprioception by eliminating consistent gravity and light vectors, revealing that trees actively compare current curvature against an internal straight reference and deploy tension wood accordingly. Wood anatomy confirmed that gelatinous fibers on the new lower side generated pulling forces equal in magnitude to the original upper-side tension wood, producing net zero torque as the stem uncoiled. This bilateral control overturns the long-standing assumption that tension wood forms exclusively on the upper side of leaning stems. The finding links plant posture control to cytoskeletal and auxin gradients previously studied only in gravitropism assays. Related work in Moulia et al. (Science 2011) and Bastien et al. (PNAS 2013) established the proprioceptive feedback loop but left the effector mechanism unspecified; the new data close that gap with direct mechanical and anatomical evidence. Future applications include engineering self-straightening timber or bio-inspired actuators that respond to shape error without external sensors.