Plant cell walls represent the first line of defence against biotic stresses, including phytopathogens and herbivores. The mustard leaf beetle Phaedon cochleariae feeds on brassicaceous plants and possesses various plant cell wall degrading enzymes for the digestion of plant cell wall polysaccharides. Amongst those, polygalacturonases (PGs) of the glycoside hydrolase family 28 (GH28) facilitate the breakdown of pectin. Several PG-inhibiting proteins (PGIPs) – plant-derived, cell wall-associated proteins containing multiple leucine-rich repeat (LRR) motifs – have been shown to inhibit microbial PGs and thus contribute to the defence against phytopathogens. This thesis takes a biochemical approach to elucidate the interaction between the P. cochleariae pectinolytic system and plant-derived inhibitors. It provides the first direct evidence that beetle PGs are inhibited by PGIPs and suggests that PGIPs play a role in the plant defence against herbivorous beetles. Beyond the “classical” PG-PGIP interaction, my studies link new players – PG pseudoenzymes and PGIP-like proteins – to the complex interplay between herbivore and plant. These discoveries form the basis for future in-depth studies on how pectin digestion in herbivorous insects and the PGIP-mediated plant defence system co-evolved.