Characterization of secondary wall polysaccharides and FLA function in Cunninghamia lanceolata (Chinese fir)

Cunninghamia lanceolata (Chinese fir) is one of the most important fast-growing timber tree species in China. Knowledge of secondary wall composition in wood of Chinese fir and the key genes involved in biosynthesis and remodelling processes is largely unknown, information necessary for efficient genetic breeding. In this study, we performed monosaccharide linkage analysis of Chinese fir wood cell walls and used monoclonal antibodies to analyse the cellular distributions of polysaccharide structures in stems. Cellulose, heteroxylan, and mannan were the most enriched polysaccharides that accounted for 71.13 %, 10.71 % and 8.04 % of total sugars. Xyloglucan and type II arabinogalactan contributed less than 5 % in wood tissues. Immunolabelling of stems shows that both xylan and mannan backbones were evenly distributed in tracheid cell walls, with α-1,3-Ara and methyl-esterified α-1,2-GlcA substituted xylans accumulated at developing tracheid walls compared to mature tracheid walls. In addition, Type II arabinogalactans that are mostly attached to arabinogalactan-proteins (AGPs) showed accumulation at vascular bundles. Fasciclin-like AGPs (FLAs) belong to AGPs subfamily that play important functions in regulating vascular development. Genome blast shows 16 fasciclin-like arabinogalactan-proteins in Chinese fir (ClFLA1-ClFLA16). RNA-sequencing, qRT-PCR, and immuno-histology analysis show that ClFLA2 and ClFLA13 were highly expressed in xylem tissues compared to other cell types. Overexpression of ClFLA2 and ClFLA13 in Arabidopsis altered plant stem development, with reduced xylem vessel numbers, increased lignin content and decreased crystalline cellulose compared to wild-type plants, suggesting important functions of ClFLA2 and ClFLA13 in regulating secondary wall development. • Xylan branches decreased from developing to matured tracheid cell walls. • Type II arabinogalactans accumulated at vascular bundles of stems. • ClFLA2 and ClFLA13 located at xylem and regulate secondary wall development.