Monosaccharide analysis of succulent leaf tissue in Aloe

Abstract

The succulent leaf mesophyll in Aloe species supports a burgeoning natural products industry, particularly in Africa. Comparative data necessary to prioritise species with economic potential have been lacking. To survey leaf mesophyll monosaccharide composition in the genus Aloe using a predictive phylogenetic approach. Monosaccharide composition was assessed in 31 species, representing the morphological and taxonomic diversity of Aloe sensu stricto. Leaf mesophyll polysaccharides were partially hydrolysed in a trifluoroacetic acid (TFA)-SilA assay. Oximes and trimethylsilyl ether products were detected by GC-MS. Constituent monosaccharides accounting for the greatest variation among species were identified by principal component analysis. Two plant DNA barcoding regions were sequenced in 28 of the sampled species and the resulting maximum likelihood tree was used to evaluate phylogenetic signal in monosaccharide composition throughout the genus. Nineteen peaks (Rt=16.76-23.67 min) were identified in the GC-MS spectra. All samples were dominated by one constituent; glucose was the major monosaccharide in 19 species, mannose in eight species, and xylose in one species (Aloidendron pillansii). Three monosaccharides therefore account for 90% of the variation in leaf mesophyll in Aloe. Species which do not share this typical monosaccharide profile appear to group outside the core Aloe clade in the phylogeny. Preliminary findings suggest that leaf mesophyll monosaccharide composition is conservative in Aloe. Characterisation of within-species variation and quantitative differences between species will be necessary to authenticate leaf mesophyll products, whereas unusual monosaccharide profiles could be diagnostic in some species. The common glucose-mannose-xylose profile identified in commercially important species is shared by many other Aloe species.