The phasmid Carausius morosus efficiently digests stachyose, oleuropein, pectin and hemicelluloses, but only weakly...

Great interest centres on novel enzyme sources for the biotechnological exploitation of biomass, especially cellulose. Phytophagous insects, including phasmids, could be such sources. However, 'cellulase' activity has often been assayed with carboxymethylcellulose as an artificial soluble substrate. We tested whether conclusions with carboxymethylcellulose are applicable to phasmids feeding on leaves containing native (semi-crystalline) cellulose and potentially masked cell-wall-matrix polysaccharides. Specifically, we investigated what leaf carbohydrates are utilisable by the 'digestome' of the stick insect Carausius morosus. Alcohol-soluble material and alcohol-insoluble residue (AIR; polysaccharides) from privet (Ligustrum ovalifolium) leaves and from the corresponding insect frass were analysed by thin-layer chromatography (TLC). Before TLC, AIR was hydrolysed in trifluoroacetic acid (releasing pectic and hemicellulosic sugars), H₂SO₄ (releasing cellulosic glucose) or Driselase (distinguishing xylans from xyloglucans). Privet leaves contained 0.5-1.0 μg/mm² of total free sugars (stachyose, raffinose, sucrose, glucose, fructose, galactose) plus the glycoconjugate oleuropein, all of which were efficiently 'utilised' (i.e., not egested) by the insects. Other phytochemicals and γ-aminobutyrate were also utilised. Much more utilisable carbohydrate (∼5 μg/mm²) came from pectin, xylans and xyloglucans. Unexpectedly, leaf cellulose was only ∼6% utilised, contributing ∼0.3 μg/mm² glucose. In conclusion, C. morosus obtains >80% of its utilisable carbohydrate from pectin and hemicelluloses, <15% from free sugars and oleuropein, and only ∼5% from cellulose. Therefore, previous reports of high cellulase activity in phasmids, based on carboxymethylcellulose hydrolysis, cannot be unambiguously applied to native cellulose without further evidence. Utilisation of polysaccharides that are still integrated within leaf-cell-wall architecture is a more realistic indicator of polysaccharide utilisation by herbivores.