Feeling stretched or compressed? The multiple mechanosensitive responses of wood formation to bending

Annals of Botany 121: 1151–1161, 2018 doi: 10.1093/aob/mcx211, available online at www.academic.oup.com/aob Feeling stretched or compressed? The multiple mechanosensitive responses of wood formation to bending Jeanne Roignant1, Éric Badel1, Nathalie Leblanc-Fournier1, Nicole Brunel-Michac1, Julien Ruelle2, Bruno Moulia1 and Mélanie Decourteix1,* Université Clermont Auvergne, INRA, PIAF, F-63000 Clermont-Ferrand, France and 2UMR LERFoB, AgroParisTech, INRA, 54000 Nancy, France * For correspondence. E-mail 1 Returned for revision: 10 November 2017 Editorial decision: 20 December 2017 Accepted: 4 January 2018 Published electronically 24 January 2018 • Background and Aims Trees constantly experience wind, perceive resulting mechanical cues, and modify their growth and development accordingly. Previous studies have demonstrated that multiple bending treatments trigger ovalization of the stem and the formation of flexure wood in gymnosperms, but ovalization and flexure wood have rarely been studied in angiosperms, and none of the experiments conducted so far has used multidirectional bending treatments at controlled intensities. Assuming that bending involves tensile and compressive strain, we hypothesized that different local strains may generate specific growth and wood differentiation responses. • Methods Basal parts of young poplar stems were subjected to multiple transient controlled unidirectional bending treatments during 8 weeks, which enabled a distinction to be made between the wood formed under tensile or compressive flexural strains. This set-up enabled a local analysis of poplar stem responses to multiple stem bending treatments at growth, anatomical, biochemical and molecular levels. • Key Results In response to multiple unidirectional bending treatments, poplar stems developed significant cross-sectional ovalization. At the tissue level, some aspects of wood differentiation were similarly modulated in the compressed and stretched zones (vessel frequency and diameter of fibres without a G-layer), whereas other anatomical traits (vessel diameter, G-layer formation, diameter of fibres with a G-layer and microfibril angle) and the expression of fasciclin-encoding genes were differentially modulated in the two zones. • Conclusions This work leads us to propose new terminologies to distinguish the ‘flexure wood’ produced in response to multiple bidirectional bending treatments from wood produced under transient tensile strain (tensile flexure wood; TFW) or under transient compressive strain (compressive flexure wood; CFW). By highlighting similarities and differences between tension wood and TFW and by demonstrating that plants could have the ability to discriminate positive strains from negative strains, this work provides new insight into the mechanisms of mechanosensitivity in plants. Key words: Populus tremula × alba, mechanical stimuli, flexure wood, reaction wood, secondary growth, tensile/ compressive, strain, mechanosensitivity, wood anatomy, fasciclin, MYB, microtubule-associated protein INTRODUCTION Throughout their life, plants constantly experience various external mechanical stimuli, such as wind, rain, weight of snow, or contacts with other plants or animals. Plants are able to cope with these environmental factors by perceiving mechanical strains and modifying their growth accordingly (Moulia et al., 2015). This acclimation of growth to mechanical perturbations is called thigmomorphogenesis (Jaffe, 1973). In the case of trees growing in windy environments, the main mechanical stimulus in branches and stem is bending. At the tree scale, bending generates a decrease of shoot elongation coupled with an increase of radial expansion and an increasingly developed root anchorage (Telewski and Pruyn, 1998; Coutand et al., 2008; Bonnesoeur et al., 2016). These acclimations of growth are thought to be an adaptive response of plants to improve mechanical safety against breakage, buckling and anchorage failure (Fournier et al., 2006). Apart from thigmomorphogenesis, one of the best-characterized responses to mechanical stimulus in woody angiosperms is the production of ‘tension wood’. This formation of a modified wood is primarily triggered by the perception of change in stem orientation with regard to the gravity field, but also curvature (Coutand et al., 2007; Bastien et al., 2013; Groover, 2016). Its biological function is interpreted as an active motor generating tensile forces that pull the stem back upright (Scurfield, 1973; Alméras and Clair, 2016). In straight inclined axes, the tension wood is produced at the upper side of the leaning organ, while ‘opposite wood’ is formed at the lower side. The term ‘normal wood’ refers to the wood formed in upright trees (Gardiner et al., 2014). Tension wood and opposite wood form through a set of changes at different scales, including asymmetrical radial growth that is higher in the tension wood side and lower in the opposite wood side. Cell differentiation also appears to be affected, since tension wood shows higher ratios © The Author(s) 2018. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For permissions, please e-mail: . Received: 11 September 2017 1152 Roignant et al. — Wood formation under mechanical strain differentiation of a distinct wood, we designed this study with a dual purpose. First, in order to better understand the mechanocontrol of wood formation, we conducted these experiments with the hypothesis that different types of specific local strains (strain amplitude and strain sign) could generate different specific cambium responses. To assess this hypothesis, multiple quantified flexural strains were applied to young Populus stems. This was achieved using unidirectional bending, so that a given cell always experienced strains of the same sign (longitudinal compression or longitudinal tension only), enabling us to distinguish the wood formed under tensile and compressive flexural strains. The different effects of bending were characterized quantitatively by studying radial growth, cell size and cell wall ultrastructure. To gain a first molecular insight into how multiple bending treatments can modulate wood anatomical traits, we used a quantitative PCR (qPCR) approach to investigate the expression of four mechanosensitive target genes known to play a role in wood differentiation, i.e. PtaFLA14-9 (Potri.009G012100), POPFLA6 (Potri.013G151400) and poplar orthologues of the arabidopsis MYB69 (PtaMYB69, Potri.007G106100) and MAP70-5 (PtaMAP70-5, Potri.006G018000) genes (Pomiès et al., 2017). A novel sampling strategy was developed to study zones experiencing different amounts of strain within the same stem cross-section: (1) a zone of wood submitted to controlled longitudinal tensile strains; (2) a zone submitted to controlled longitudinal compressive strains; and (3) a ‘neutral zone’ that experiences a mix of very small longitudinal compre (...truncated)