Exploring the polysaccharide composition of plant cell walls in succulent aloes

Abstract

Societal Impact Statement: Aloes are iconic succulent plants native to Africa, Madagascar and the Arabian Peninsula. The succulent leaf mesophyll of aloes has been used extensively as a herbal product for centuries, contributing to their overexploitation. Health benefits are attributed to their polysaccharide content. We present a comprehensive comparison of the polysaccharide composition of succulent tissues from 93 Aloe species. We found polysaccharide composition primarily related to leaf morphology in alignment with the broad range of Aloe species used medicinally. All aloes except Aloe ferox and Aloe vera are endangered raising concern about over-harvesting of wild species. Summary: Aloes are iconic succulent plants native to Africa, Madagascar and the Arabian Peninsula. All aloes except the commercially grown Aloe ferox and Aloe vera are protected according to the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Major factors contributing to their over-exploitation are their ornamental value and medicinal use with more than 25% of Aloe species being utilised. The succulent inner leaf mesophyll of aloes is used in traditional medicine, with the healing effect ascribed to the properties of their structural polysaccharides. To explore the correlation between Aloe polysaccharide profiles and other biologically relevant traits across the genus, we (1) extracted polysaccharides and created profiles for nearly 100 representative species using carbohydrate microarrays and molecular probes. We targeted six major plant cell wall polysaccharide groups using 27 different molecular probes. We (2) tested for phylogenetic signal in the polysaccharide profiles and (3) assembled an exhaustive database from literature on the geographic region, level of endemism, altitude, habitat, habit, medicinal use and leaf morphology of the individual species of Aloe. In the absence of phylogenetic signal of polysaccharide profiles, multivariate linear modelling without phylogenetic correction was used and showed that polysaccharide composition primarily correlated with leaf morphology, highlighting the fundamental role of polysaccharides as the building blocks of plants. No correlations between polysaccharide composition of commercial and non-commercial species were found. We found polysaccharide composition to primarily relate to leaf morphology emphasising the fundamental and structural role of polysaccharides.