Plant cell wall degradation in Coleoptera: investigation of three glycoside hydrolase families implicated in cellulose and hemicellulose digestion in Phytophaga beetles

Abstract

The first line of defense against biotic and abiotic stresses in plants consists of a diverse set of sugar-based compounds forming the plant cell wall. The major component of the plant cell wall is cellulose, a polysaccharide consisting of β-1,4-linked glucose moieties. Any organism being able to degrade cellulose would benefit from a huge source of energy as well as gaining access to nutrient-rich cell contents. Cellulose-degrading enzymes (cellulases) are well described in a wide range of microbes but were thought to be absent in animals. Recently, it became clear that some animals encode endogenous plant cell wall degrading enzymes (PCWDEs) belonging to several glycoside hydrolase families (GH), including putative cellulases of family 45 (GH45) and putative mannanases of family 5 subfamily 10 (GH5_10). In Arthropoda, GH45s and GH5_10s are most prominently encoded by insects including the Phytophaga clade of beetles (leaf beetles, longhorned beetles, bark beetles and weevils). Nonetheless, the distribution of both GH families in insects is erratic and it is assumed that they are not of ancestral origin but were acquired separately, likely through horizontal gene transfer (HGT) events from microbe to animal. Despite the intricate evolution of GH45s and GH5_10s and an emerging role of PCWDEs in biofuel industries both GH families in Phytophaga beetles remain largely unexplored. Therefore, the major aim of this thesis was to investigate beetle-derived members of GH5_10 and GH45 with focus on their enzymatic activity, physiological importance and evolutionary history. In conclusion, this thesis has greatly contributed to our understanding of PCWDEs encoded by Phytophaga beetles. In particular, our knowledge on GH45 and GH5_10 members encoded by Chrysomelidae and Curculionidae has greatly increased, demonstrating that not a vertical but a horizontal gene transfer was likely responsible for GH45 (and possibly for GH5_10) inheritance in these beetles. The following species-specific, independent gene duplications allowed for functional diversification and likely adaptation to their food source. These results provide fundamental insights into the evolution of PCWDEs and the molecular mechanisms of acquiring novel enzymatic functions. Furthermore, based on a variety of industrial applications of PCWDEs, beetle-derived GH45 and GH5_10 enzymes may contribute greatly to Society by being introduced into several industrial applications (e.g. biofuel production) and ultimately reducing a progressing greenhouse effect.