Stem flexing-induced growth reprogramming in wood involves jasmonate signaling in hybrid aspen trees

Trees react to stem bending by producing flexure wood (FW) but its developmental program and properties are poorly understood in hardwoods. Here we investigated effects of low intensity multidirectional stem flexing simulating wind on growth and wood properties of hybrid aspen, and on transcriptomic and hormonal responses in wood forming tissues. Greenhouse-grown trees were either kept stationary or subjected to several daily shakes causing an average stem deflection of 4 °from vertical for five weeks. Their wood was analyzed by microscopy, SilviScan, saccharification and chemical analyses, and their wood forming tissues were analyzed by hormonomics and RNA sequencing. Stem flexing increased overall primary growth and wood production, and altered wood differentiation by stimulating G-fiber formation, reducing secondary wall thickness, increasing G-lignin content, cell wall porosity, and saccharification yields. Increased levels of jasmonic but decreased abscisic acids were detected, and transcriptomic data indicated increased polyamine and decreased gibberellin levels. Moreover, changes in ethylene, brassinosteroids, and auxins resembled those during tension wood formation. Stem flexing stimulates tree growth, and induces wood anatomical and chemical changes through molecular and hormonal pathways similar to thigmomorphogenesis in herbaceous plants and partially overlapping with the tension wood program of hardwoods.