Ethylene signaling is required for fully functional tension wood in hybrid aspen

Tension wood (TW) in hybrid aspen trees forms upon stem displacement and is characterized by asymmetric stem growth, reduction of vessel element formation and vessel diameter and by the presence of gelatinous (G-)fibers. G-fibers contain an inner cell wall layer (G-layer) rich in crystalline cellulose oriented parallel to the fiber cell axis. The G-layer is thought to contribute to generating the mechanical force needed to correct the growth position of the displaced tree stem. Biosynthesis of the phytohormone ethylene is stimulated upon TW formation and exogenous ethylene application to upright trees triggers G-fiber formation and reduction of vessel number and vessel diameter. In this study, we show using wild type and ethylene insensitive (ETI) hybrid aspen trees together with time-lapse imaging that functional ethylene signaling is required for full uplifting of inclined stems. Investigation of TW in ETI trees by X-ray diffraction and Raman microspectroscopy indicated that, although G-fibers are formed, the cellulose microfibril angle in the S-layer of their cell wall was decreased and the chemical composition of S- and G-layer was altered as compared to wild type TW. Also, the characteristic asymmetric growth and reduction of vessel density was suppressed in the TW of ETI trees. A genome-wide transcriptome profiling revealed ethylene dependent genes in TW related to cell-division, cell wall composition, vessel differentiation and microtubule orientation. Our results evoke that ethylene is a key regulator of transcriptional responses and modifications in xylem anatomy, chemistry and physical properties during TW formation, which ultimately generates correction of stem bending in hybrid aspen.