Stem bending in trees induces flexure wood but its properties and development are poorly understood. Here, we investigated the effects of low-intensity multidirectional stem flexing on growth and wood properties of hybrid aspen, and on its transcriptomic and hormonal responses. Glasshouse-grown trees were either kept stationary or subjected to several daily shakes for 5 wk, after which the transcriptomes and hormones were analyzed in the cambial region and developing wood tissues, and the wood properties were analyzed by physical, chemical and microscopy techniques. Shaking increased primary and secondary growth and altered wood differentiation by stimulating gelatinous-fiber formation, reducing secondary wall thickness, changing matrix polysaccharides and increasing cellulose, G- and H-lignin contents, cell wall porosity and saccharification yields. Wood-forming tissues exhibited elevated jasmonate, polyamine, ethylene and brassinosteroids and reduced abscisic acid and gibberellin signaling. Transcriptional responses resembled those during tension wood formation but not opposite wood formation and revealed several thigmomorphogenesis-related genes as well as novel gene networks including FLA and XTH genes encoding plasma membrane-bound proteins. Low-intensity stem flexing stimulates growth and induces wood having improved biorefinery properties through molecular and hormonal pathways similar to thigmomorphogenesis in herbaceous plants and largely overlapping with the tension wood program of hardwoods.

Department of Chemistry Umeå University 90187 Umeå Sweden

Laboratory of Growth Regulators Institute of Experimental Botany of the Czech Academy of Sciences and Faculty of Science of Palacký University 78371 Olomouc Czech Republic

RISE Research Institutes of Sweden Drottning Kristinas väg 61 11428 Stockholm Sweden

SciLifeLab Umeå University 90187 Umeå Sweden

Umeå Plant Science Centre Department of Forest Genetics and Plant Physiology Swedish University of Agricultural Sciences 90183 Umeå Sweden

Umeå Plant Science Centre Department of Plant Physiology Umeå University 90187 Umeå Sweden

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Modification of xylan in secondary walls alters cell wall biosynthesis and wood formation programs and improves saccharification