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Tension wood (TW), characterized by increased cambium cell proliferation and few vessels, is a classical model for the mechanical analysis of wood formation. In this study, we found higher superoxide anion (O2.-) levels in the cambium zone of poplar (Populus alba × P. glandulosa clone "84K") TW than in that of opposite wood during gravistimulation. Treatment with an O2.- activator (methyl viologen) resulted in tension-wood-like xylem tissue formation, and transgenic plants with reduced cambium O2.- levels presented an attenuated gravity response. Time-course detection of O2.- and indole-3-acetic acid (IAA) levels revealed that auxin responses were enhanced following increases in O2.- levels, suggesting that IAA mediates TW induction downstream of O2.-. Rapid division but advanced programmed cell death in cambium cells was detected in both gravistimulated and O2.- activator-treated plants. These findings suggest that high O2.- levels trigger downstream IAA signaling to promote cambium cell proliferation and induce TW formation., (© The Author(s) 2024. Published by Oxford University Press on behalf of American Society of Plant Biologists. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)