**5. GH 17**

GH17 are encoded by large gene families in plants. In *O. sativa*, GH17 is the largest GH family [17]. In *A. thaliana* and *Populus trichocarpa*, it comprises 50 and 100 members, respectively [54]. The GH3 family includes β-1,3-glucanase (glucan endo-1,3-β-D-glucosidase, E.C 3.2.1.39), glucan 1,3-β-glucosidase (E.C 3.2.1.58), licheninase (EC 3.2.1.73), glucan 1,4-β-glucosidase (EC 3.2.1.74) activities (enzyme.expasy.org/). All the GH17 found in sugarcane and *B. distachyon* cell wall proteomes are predicted to have a glucan endo-1,3-β-D-glucosidase activity (GO:0042973).

β-1,3-glucanases have been shown to be important proteins involved in plant defense reactions against pathogens and are considered as pathogenesis-related proteins of the PR-2 family [55]. Their role is hydrolysis of the β-1,3-glucan bonds, an important structural component of fungal cell walls, resulting in their destabilization and in the release of elicitors that further stimulate defense responses [55]. This antifungal activity was shown both *in vitro* [56] and *in vivo* [57]. In sugarcane, GH17 SCQSRT2031D12 identified in basal internodes was considered similar to the *A. thaliana* At4g16260 β-1,3-endoglucanase that has been associated with increased resistance to pathogen attack [58]. Noteworthy, β-1,3-glucanases can accumulate in vacuoles of root cells or mature leaf cells in response to pathogen infection, whereas others are secreted to the extracellular space, but they can also be secreted in the absence of pathogen infection [55]. They are, thus, also important during plant development, being involved in cell division, pollen development, seed germination, and maturation as well as in signaling [55, 59, 60].

According to phylogenic analyses, the GH17 family is divided into three distinct clades (denoted α, β, and γ) [61, 62], with 10% of its members having cell wall-related functions [61]. GH17 of the α clade are more related to stem elongation, but also responsive to gibberellin, those of the β and γ clades are more related to stress response and defense against pathogens [62–65]. In addition to the GH17 domain *per se*, proteins of the GH17 family comprise other domains as shown by [61] studying the β-1,3-glucanase sequences of *A. thaliana.* They noted that all the sequences had a predicted N-terminal signal peptide linking them to the secretory pathway. Half of them had a C-terminal extension, being first classified as an X8 domain [66]. Previously, the X8 domain was identified as the cellulose binding module 43 (CBM43) responsible for the interaction with β-1,3-glucans [67]. The other GH17 had either a C-terminal glycosylphosphatidylinositol (GPI)-anchor [66, 68] or a vacuolar targeting peptide [55]. The absence or gain of these domains could be related to ancestral traits. All the γ clade members and more than half of the α clade members retained the CBM43 domain, whereas all the members of the β clade lost it through evolution. It is thought that the loss of this domain facilitates the extracellular secretion induced by biotic stresses, thus improving the response to pathogens [61, 62].

Other studies also revealed the antifungal effects of plant extracellular chitinases (GH18 and GH19) in combination with those of GH17 [69]. Indeed, fungi cell walls are composed of chitin and of branched β-(1,3):β-(1,6) glucans [57, 70–73]. Thereby, transgenic plants overexpressing a chitinase and/or a ß-l,3 glucanase became less susceptible to fungal attack [74, 75].
