Non-cellulosic polysaccharide distribution during G-layer formation in poplar tension wood fibers: abundance of rhamnogalacturonan I and arabinogalactan proteins but no evidence of xyloglucan

Abstract

RG-I and AGP, but not XG, are associated to the building of the peculiar mechanical properties of tension wood. Hardwood trees produce tension wood (TW) with specific mechanical properties to cope with environmental cues. Poplar TW fibers have an additional cell wall layer, the G-layer responsible for TW mechanical properties. We investigated, in two poplar hybrid species, the molecules potentially involved in the building of TW mechanical properties. First, we evaluated the distribution of the different classes of non-cellulosic polysaccharides during xylem fiber differentiation, using immunolocalization. In parallel, G-layers were isolated and their polysaccharide composition determined. These complementary approaches provided information on the occurrence of non-cellulosic polysaccharides during G-fiber differentiation. We found no evidence of the presence of xyloglucan (XG) in poplar G-layers, whereas arabinogalactan proteins (AGP) and rhamnogalacturonan type I pectins (RG-I) were abundant, with an apparent progressive loss of RG-I side chains during G-layer maturation. Similarly, the intensity of immunolabeling signals specific for glucomannans and glucuronoxylans varies during G-layer maturation. RG-I and AGP are best candidate matrix components to be responsible for TW mechanical properties.

Plant ‘muscles’: fibers with a tertiary cell wall

“…In some plant species, lignifications of the G‐layer may occur at later stages of fiber development (Ghislain & Clair, ). The cell wall polymer that occurs specifically in fibers at the stage of tertiary cell wall deposition is rhamnogalacturonan‐I (RG‐I) with long β‐(1→4)‐galactan side chains (Gorshkova et al ., , ; Guedes et al ., ). The galactose content in cell wall polymers has even been suggested to be an indicator of the degree of G‐layer development in fibers of tension wood (Ruel & Barnoud, ).…”

“…Other cell wall components present in the G‐layers are glucomannan, the only hemicellulosic polymer found in both secondary and tertiary cell walls (Gorshkova et al ., ), and the arabinogalactan proteins (Morvan et al ., ; Gorshkova et al ., ). Xyloglucan, known to be involved in restoring the vertical position of inclined poplar trees (Mellerowicz et al ., ), is located at the external boundary of the G‐layer, where it becomes stapled to the previously deposited secondary cell wall (Baba et al ., ); xyloglucan was recently reported to be absent in the G‐layer itself (Guedes et al ., ). The biochemical differences between secondary and tertiary cell walls are reflected in corresponding gene expression, as revealed by comparison of the transcriptomes in flax fibers isolated at the stage of tertiary cell wall deposition and in tissues forming secondary cell walls (Gorshkov et al ., ).…”

“…The presence of RG‐I with side chains of β‐(1→4)‐galactan was detected in the tertiary cell walls of nonflax plants by biochemical characterization (Gorshkova et al ., ; Guedes et al ., ) and by immunocytochemistry (Bowling & Vaughn, ; Gritsch et al ., ; Guedes et al ., ). Immunolabeling with an antibody recognizing β‐1,4‐galactans is not always successful in the tertiary cell wall, as reported for phloem fibers of hemp (Blake et al ., ; Behr et al ., ).…”

“…However, antibodies specific for the backbone of RG‐I and for β‐(1→4)‐galactosidase heavily bind tertiary cell walls of primary and secondary hemp fibers (Chernova et al ., ). In addition, galactosidase activity in the G‐layers of tension wood fibers was recently detected (Gorshkova et al ., ) and decreased labeling by β‐(1→4)‐galactan‐specific antibody with G‐layer maturation (Guedes et al ., ), which may be due to the in muro action of galactosidase. Together with the reports on the axial orientation of cellulose microfibrils in G‐fibers of many plant species (e.g.…”

Flax rhamnogalacturonan lyases: phylogeny, differential expression and modeling of protein structure

“…Thus, based on RGL expression data, the presence of functionally distinct RGLs could be assumed: the RGLs that use RG‐I of the primary cell walls (Yapo ) as potential substrate, and the RGLs that modify RG‐I present in the gelatinous (tertiary) cell walls of plant fibers (Gorshkova et al , , Guedes et al ). The subfunctionalization of RGL genes probably occurred on the basis of RGL genes from group II of eudicots (Fig.…”