Cell Wall Layer Induced in Xylem Fibers of Flax Upon Gravistimulation Is Similar to Constitutively Formed Cell Walls of Bast Fibers

Abstract

In the fibers of many plant species after the formation of secondary cell walls, cellulose-enriched cell wall layers (often named G-layers or tertiary cell walls) are deposited which are important in many physiological situations. Flax (Linum usitatissimum L.) phloem fibers constitutively develop tertiary cell walls during normal plant growth. During the gravitropic response after plant inclination, the deposition of a cellulose-enriched cell wall layer is induced in xylem fibers on one side of the stem, providing a system similar to that of tension wood in angiosperm trees. Atomic force microscopy (AFM), immunochemistry, and transcriptomic analyses demonstrated that the G-layer induced in flax xylem fibers was similar to the constitutively formed tertiary cell wall of bast (phloem) fibers but different from the secondary cell wall. The tertiary cell walls, independent of tissue of origin and inducibility, were twice as stiff as the secondary cell walls. In the gravitropic response, the tertiary cell wall deposition rate in xylem was higher than that of the secondary cell wall. Rhamnogalacturonan I (RG-I) with galactan side chains was a prominent component in cellulose-rich layers of both phloem and xylem flax fibers. Transcriptomic events underlying G-layer deposition in phloem and xylem fibers had much in common. At the induction of tertiary cell wall deposition, several genes for rhamnosyltransferases of the GT106 family were activated in xylem samples. The same genes were expressed in the isolated phloem fibers depositing the tertiary cell wall. The comparison of transcriptomes in fibers with both inducible and constitutive tertiary cell wall deposition and xylem tissues that formed the secondary cell walls is an effective system that revealed important molecular players involved in the formation of cellulose-enriched cell walls.

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

“…8b) as this force depends to a great extent on the on the shape of the tip and on the surface roughness of the material, which were not the same (see for example the difference in adhesion forces of the embedding resin in the lumen in the two studies, even though the same resin was used). In conclusion, although the G-layer of tension wood and the G-layer of flax are biochemically, ultrastructurally and mechanically similar (Coste et al ., 2020; Petrova et al ., 2021), here it is clear that they differ in their development and maturation, as summarised in Fig. 9, with a thick, loose, multilayer G n layer in flax that stiffens and densifies abruptly, whereas in poplar, there appears to be a thin, dense immature layer that stiffens gradually.…”

“…9, with a thick, loose, multilayer G n layer in flax that stiffens and densifies abruptly, whereas in poplar, there appears to be a thin, dense immature layer that stiffens gradually. Thus, immunohistochemical and G-layer specific marker gene expression analyses (Decou et al, 2020;Guedes et al, 2017), like those already performed on flax bast and xylem fibres (Petrova et al, 2021), should be performed on the same sample to clarify the origin of these differences and to better understand the mechanisms underlying the maturation and development of poplar tension wood growth stress. Finally, all these results should be used to distinguish between different models of growth stress development in the case of tension wood (Alméras and Clair, 2016), to estimate the internal stress distribution within the G-layer and its consequences for macroscopic growth stress at the tree scale .…”

“…In contrast, flax bast fibres exhibit a strong mechanical gradient with a thick immature, loose and soft G-layer, called G n (Gorshkova and Morvan, 2006;Gorshkova et al, 2010). Evidence for the presence of this thick G n -layer has also been provided in flax xylem tension wood fibres (Petrova et al, 2021). Interestingly, the indentation modulus of flax G-layers is similar to or even a little bit higher than that of mature poplar G-layers and the average indentation modulus of flax G n -layers is comparable to that measured in immature poplar G-layers in fibres close to the cambium and to the inner sub-layers observed in more developed G-fibres.…”

“…In addition, thanks to the nanometric spatial resolution of AFM measurements, we investigated G-layer stiffening during thickening, i.e., the kinetics of stiffening within the G-layer, and fluctuations in the mechanical states of a new freshly deposited sub-layer. Finally, the kinetics and stiffness gradient of the poplar G-layers were compared with data available in the literature on bast (primary phloem) and xylem flax fibres, whose cells walls contain both a thick immature G n -layer and a mature G-layer (Goudenhooft et al ., 2018; Petrova et al ., 2021).…”

“…The same is observed when comparing the transcriptome of developing tension wood fibres and of flax phloem fibres, with multiple, distinct chitinases, bgalactosidases, arabinogalactan proteins and lipid transfer proteins, compare for example Roach et al (2007) and Déjardin et al (2004). Moreover, the thickening of the cell wall of flax fibres involves considerable remodelling of the deposited layers: indeed, the newly formed layers of the secondary cell wall of developing flax fibres, referred to as immature Gn-layer, have a loose structure that will get more and more compact and stiff during their maturation toward a G-layer (Goudenhooft et al, 2018;Petrova et al, 2021).…”