Plant GH9 enzymes and their β-1,4–glycan substrates: From in vitro EGase activity to in vivo functions

Cellulases are an increasingly important class of industrial enzymes owing to their use in a wide range of applications and sustainable products. In addition to bacteria and fungi, plants also evolved a variety of cellulases. Plant endo-β-1,4-glucanases (EGases) belong to glycoside hydrolase family 9 (GH9) enzymes, which catalyze the cleavage of internal 1,4-glycosidic bonds in plant cell wall polysaccharides. Plant EGases are divided into three types (GH9A, GH9B, GH9C), which have been implicated in cellulose biosynthesis as well as polysaccharide catabolism to remodel and disassemble cell walls in association with many developmental processes. The recent discoveries of plant GH9 family members facilitating the establishment of cellular interfaces between species encourages to explore plant EGases and their activity properties in more detail. This comprehensive review summarizes the current knowledge of plant EGases concerning their sources, activity properties, substrate specificity and suitable assays to evaluate them, as well as their native roles in plants that provide implications for their potential use in agricultural and industrial applications. The existing knowledge of the functional principles of GH9 activity is compiled and current gaps in knowledge are identified and lastly the potential in using plant EGases in the molecular breeding of GH enzymes is discussed.