Phasmid insect digests cell-wall pectin and hemicelluloses, but not cellulose: misleading conclusions from conventional ‘cellulase’ assays
Stephen C. Fry (UK)1; Lenka Franková (UK)1; Luke Yuan (UK)1; E. Alanie Lapina (UK)1;
1 - The University of Edinburgh;
Keywords: Biomass; Cellulase; Phasmids;
Abstract Topics: Theme 12: Cell Walls in Crop Quality, Biomass Utilisation and Sustainability
Type of Presentation: Poster

Abstract text: Great interest centres on novel enzyme sources for biomass valorisation. Phasmid insects are potentially such sources,* harbouring genes for cellulases, hemicellulases and pectinases. However, their ‘cellulase’ activity had been mainly assayed on carboxymethylcellulose (CMC), an artificial soluble substrate, which differs fundamentally from native semicrystalline cellulose. We tested whether conclusions based on CMC assays are applicable to insects feeding on leaves containing native cellulose and wall-bound matrix polysaccharides.

We quantified cellulose, pectin and hemicelluloses in alcohol-insoluble residues from privet (Ligustrum ovalifolium) leaves and from the corresponding frass (faeces) egested by the stick-insect Carausius morosus.

Approximately 65% of leaf wall-matrix material (homogalacturonan, rhamnogalacturonan-I, heteroxylans and xyloglucan) was ‘utilised’ (i.e., not egested) by live stick insects. However, in contrast to earlier claims, the phasmids utilised only 6% of the leaf cellulose.

In conclusion, Carausius morosus obtains >80% of its utilisable carbohydrate from pectin and hemicelluloses, ~15% from free sugars, and only ~5% from cellulose. Therefore, previous reports of high ‘cellulase’ activity in phasmids, based on genomics and CMC assays [1], cannot be directly applied to native cell-wall cellulose. Utilisation of wall-bound polysaccharides is the only realistic indicator of biomass valorisation.

*Reference: Shelomi et al., 2016. Insect Biochem Mol Biol 71: 1-11.


Overview of ‘what goes in and what comes out’. Left (green bars): carbohydrates in privet leaves. Right (brown bars): carbohydrates in stick-insect frass.The fate of Ligustrum cellulose in the stick-insect gut. The alpha-cellulose fraction of leaves and frass was hydrolysed in H2SO4 and the products (mainly glucose) were analysed by TLC. Loadings were normalised per mm² of foliage and the frass produced therefrom. Note that the cellulosic Glc (and the Xyl + Man entrapped therein) are scarcely less in frass than in leaves. The stick insects’ “cellulase” does not successfully digest native leaf cellulose.