Cell wall thickening in developing tension wood of artificially bent poplar trees

International audienceTrees can control their shape and resist gravity thanks to their ability to produce wood under tensile stress. This stress is known to be produced during the maturation of wood fibres but the mechanism of its generation remains unclear. This study focuses on the formation of the secondary wall in tension wood produced in artificially tilted poplar saplings. Thickness of secondary wall layer (SL) and gelatinous layer (GL) were measured from cambium to mature wood in several trees sampled at different times after tilting. Measurements on wood fibres produced before tilting show the progressive increase of secondary wall thickness during the growing season. After the tilting date, SL thickness decreased markedly from normal wood to tension wood while the total thickness increased compared to normal wood, with the development of a thick GL. However, even after GL formation, SL thickness continues to increase during the growing season. GL thickening was observed to be faster than SL thickening. The development of the unlignified GL is proposed to be a low cost, efficient strategy for a fast generation of tensile stress in broadleaved trees

Mechanical characterisation of the developing cell wall layers of tension wood fibres by Atomic Force Microscopy

Abstract Trees can generate large mechanical stresses at the stem periphery to control the orientation of their axes. This key factor in the biomechanical design of trees, named “maturation stress”, occurs in wood fibres during cellular maturation when their secondary cell wall thickens. In this study, the spatial and temporal stiffening kinetics of the different cell wall layers were recorded during fibre maturation on a sample of poplar tension wood using atomic force microscopy. The thickening of the different layers was also recorded. The stiffening of the CML, S 1 and S 2 -layers was initially synchronous with the thickening of the S 2 layer and continued a little after the S 2 -layer reached its final thickness as the G-layer begins to develop. In contrast, the global stiffness of the G-layer, which initially increased with its thickening, was almost stable long before it reached its final maximum thickness. A limited radial gradient of stiffness was observed in the G-layer, but it decreased sharply on the lumen side, where the new sub-layers are deposited during cell wall thickening. Although very similar at the ultrastructural and biochemical levels, the stiffening kinetics of the poplar G-layer appears to be very different from that described in maturing bast fibres. Highlight New insights into the changes in mechanical properties within the cell wall of poplar tension wood fibres during maturation have been obtained using atomic force microscopy

Different routes for conifer- and sinapaldehyde and higher saccharification upon deficiency in the dehydrogenase CAD1

In the search for renewable energy sources, genetic engineering is a promising strategy to improve plant cell wall composition for biofuel and bioproducts generation. Lignin is a major factor determining saccharification efficiency and, therefore, is a prime target to engineer. Here, lignin content and composition were modified in poplar (Populus tremula 3 Populus alba) by specifically down-regulating CINNAMYL ALCOHOL DEHYDROGENASE1 (CAD1) by a hairpin-RNA-mediated silencing approach, which resulted in only 5% residual CAD1 transcript abundance. These transgenic lines showed no biomass penalty despite a 10% reduction in Klason lignin content and severe shifts in lignin composition. Nuclear magnetic resonance spectroscopy and thioacidolysis revealed a strong increase (up to 20-fold) in sinapaldehyde incorporation into lignin, whereas coniferaldehyde was not increased markedly. Accordingly, ultra-high-performance liquid chromatography-mass spectrometry-based phenolic profiling revealed a more than 24,000-fold accumulation of a newly identified compound made from 8-8 coupling of two sinapaldehyde radicals. However, no additional cinnamaldehyde coupling products could be detected in the CAD1-deficient poplars. Instead, the transgenic lines accumulated a range of hydroxycinnamate-derived metabolites, of which the most prominent accumulation (over 8,500-fold) was observed for a compound that was identified by purification and nuclear magnetic resonance as syringyl lactic acid hexoside. Our data suggest that, upon down-regulation of CAD1, coniferaldehyde is converted into ferulic acid and derivatives, whereas sinapaldehyde is either oxidatively coupled into S'(8-8) S' and lignin or converted to sinapic acid and derivatives. The most prominent sink of the increased flux to hydroxycinnamates is syringyl lactic acid hexoside. Furthermore, low-extent saccharification assays, under different pretreatment conditions, showed strongly increased glucose (up to +81%) and xylose (up to +153%) release, suggesting that down-regulating CAD1 is a promising strategy for improving lignocellulosic biomass for the sugar platform industry

Fungal auxin overproduction affects the anatomy of Hebeloma cylindrosporum-pinus pinaster ectomycorrhizas

International audienceWe studied the effect of fungal auxin overproduction on the growth polarity of cortical cells in pine mycorrhizas by comparing the anatomy of Pinus pinaster (Ait.) Sol. mycorrhizas formed by an IAA-overproducing mutant of Hebeloma cylindrosporumRomagnesi or by the corresponding wild type with non-mycorrhizal short roots. Both wild- type and mutant strains induced an increase in root diameter that was mostly a result of the influence of the fungus on root cortical development. Both strains affected growth polarity of P. pinaster cortical cells and induced a change in their shape. The main modifications were a large reduction in axial diameter and an increase in the radial diameter of the cortical cells. The modifications were more marked with the mutant than with the wild type. The mutant induced a 43% reduction in cortical cell elongation and a 35% increase in radial diameter, whereas the corresponding changes induced by the wild type were 30 and 10%, respectively. The volume of cortical cells in mature mycorrhizas was generally lower than in uninoculated short roots indicating that wild-type and mutant strains induced a reorientation of cortical cell growth but did not induce an increase in turgor pressure of the cells. Immunolocalization allowed visualization of α-tubulin in root cortical cells, but no obvious modification in α-tubulin distribution was detected as a consequence of symbiosis establishment. Likewise, cytochemical localization of polysaccharides in cortical cell walls did not show significant modification following symbiosis establishment and Hartig net formation. The only noticeable modification was a reduction in cortical cell wall thickness in mycorrhizas compared with uninoculated short roots. The possible involvement of fungal auxin in the observed modifications is discussed

11es Journées du réseau français des parois

11es Journées du réseau français des parois. 11. Journées du Réseau Français des Paroi

Caractérisation mécanique de la paroi cellulaire du bois de tension en cours de maturation par microscopie à force atomique

International audienc

Physiologie de la formation des parois de fibres de bois

National audienceThe heteroxylated wood of hardwood species is constituted of fibres, vessels and ray cells. Fibres play a prominent role in the mechanical support of the tree and in the ability of trees to reach a big size. Fibres result from cambium activity that takes place just under the bark, between phloem and xylem. The fibre cell wall is made of several layers of a cellulose microfibril network glued into a matrix of lignins and hemicelluloses. These layers widely differ according to their thickness, lignin content and cellulose microfibril orientation. The fibre cell wall can be compared to a composite material, whose properties depend on matrix composition and cellulose microfibril orientation. The wood properties largely result from variations in cell wall composition and arrangement. In this paper, we will review the state of knowledge on wood fibre formation and we will introduce functional genomics strategy currently developed on a model species, poplar, in order to increase our knowledge on wood fibre biogenesis.Les feuillus possèdent un bois hétéroxylé, de structure complexe, constitué principalement de fibres, de vaisseaux et de rayons. Les fibres jouent un rôle primordial dans le soutien mécanique de l'arbre et dans sa capacité à développer des axes de grande taille. Les fibres résultent du fonctionnement d'une assise génératrice appelée cambium et située sous l'écorce de l'arbre entre le bois et le liber. La paroi des fibres est composée de plusieurs couches, formées d'un réseau de microfibrilles de cellulose cimenté dans une matrice d'hémicelluloses et de lignines. Les couches diffèrent par leur épaisseur, leur degré de lignification et l'orientation des microfibrilles de cellulose. La paroi des fibres peut être comparée à un matériau composite dont les propriétés mécaniques varient selon la composition de la matrice et l'angle des microfibrilles de cellulose par rapport à l'axe de la fibre. Les variations de ces paramètres au niveau de la paroi sont en grande partie responsables des propriétés mécaniques du bois. Dans cette revue, nous présentons l'état des connaissances sur la formation des fibres de bois et nous introduisons les approches de génomique fonctionnelle menées chez une espèce modèle, le peuplier, afin d'approfondir les connaissances sur le sujet