Characterization of Endoglucanase (GH9) Gene Family in Tomato and Its Expression in Response to Rhizophagus irregularis and Sclerotinia sclerotiorum

In this study, we report bioinformatics analysis of the endoglucanase GH9 gene family in tomato (Solanum lycopersicum L.) using the SL5.0 genome, confirming the presence of 19 members that clustered into classes A, B, and C. To explore their potential role in plant–microbe interactions, we determined the transcriptional regulation of 10 SlGH9 gene members in tomato leaves and roots during interactions with the mutualistic root mycorrhizal fungus Rhizophagus irregularis and the foliar pathogen Sclerotinia sclerotiorum. The upregulation of several SlGH9 genes in the leaves of mycorrhizal plants suggests that they are involved in cellulose remodeling and biosynthesis rather than its degradation. This would be consistent with the observed increase in foliar area. On the other hand, downregulation of some SlGH9 genes in leaves of pathogen-infected mycorrhizal plants suggests that these genes may play a role in the enhanced resistance observed by reducing cellulose degradation, thereby maintaining cell wall integrity. The potential involvement of endoglucanase genes in expansive growth (foliar area) and in defense in mycorrhizal and pathogen-infected plants may reflect a growth–defense trade-off.