Structural organisation and compositions of the cell walls of selected gymnosperms

Abstract

The polysaccharide components of unlignified primary cell walls from the cambium and of lignified secondary cell walls from the wood of the same Pinus radiata tree were examined. The major non-cellulosic polysaccharides in the primary walls were pectic polysaccharides, predominantly arabinans, homogalacturonans, and galactans and/or Type I arabinogalactans, with small proportions of rhamnogalacturonans (I and II) and Type II arabinogalactans. Smaller proportions of xyloglucans, and minor proportions of heteroxylans and glucomannans (and/or galactoglucomannans) were also present. This composition is similar to that of other primary gymnosperm walls that have been studied and also to that of primary dicotyledon walls. The predominant non-cellulosic polysaccharides in the lignified secondary walls (mostly of tracheids) were glucomannans and/or galactoglucomannans; smaller proportions of heteroxylans, and minor proportions of xyloglucans and pectic polysaccharides were also present. These walls contained more cellulose than the primary walls. Using solid-state l3C NMR spectroscopy, the molecular ordering of the polysaccharides in both wall preparations was investigated. The cellulose in both wall preparations was entirely crystalline (cellulose I). The triclinic form (Iα) was more abundant than the monoclinic form (Iß). The cross-sectional dimensions of the cellulose crystallites were calculated to be the same in both wall preparations. Pectic polysaccharides in the primary walls of the cambium had a range of mobilities: some were restricted in their mobility, whereas others, especially the arabinans, were freely mobile. Most non-cellulosic polysaccharides and lignin polymers in the secondary walls had restricted mobilities; small regions of these polymers were more mobile. Unlignified primary walls of 41 species of gymnosperms representing all 17 gymnosperm families were examined by UV fluorescence for the presence of ester-linked ferulic acid which was detected in all the species examined. Wall-preparations containing>95% primary walls were obtained from nine species in nine families and the monomeric phenolic acid components quantified. Ferulic acid (mostly trans) was found in all the preparations, and p-coumaric acid (mostly trans) was also found in all except one. From these results I postulated that the extant classes of gymnosperms are monophyletic. Preliminary results suggested that in primary walls of Agathis australis the ferulic acid is ester-linked to pectic polysaccharides.