Two Sugarcane Expansin Protein-Coding Genes Contribute to Stomatal Aperture Associated with Structural Resistance to Sugarcane Smut

<i>Sporisorium scitamineum</i> is a biotrophic fungus responsible for inducing sugarcane smut disease that results in significant reductions in sugarcane yield. Resistance mechanisms against sugarcane smut can be categorized into structural, biochemical, and physiological resistance. However, structural resistance has been relatively understudied. This study found that sugarcane variety ZZ9 displayed structural resistance compared to variety GT42 when subjected to different inoculation methods for assessing resistance to smut disease. Furthermore, the stomatal aperture and density of smut-susceptible varieties (ROC22 and GT42) were significantly higher than those of smut-resistant varieties (ZZ1, ZZ6, and ZZ9). Notably, <i>S. scitamineum</i> was found to be capable of entering sugarcane through the stomata on buds. According to the RNA sequencing of the buds of GT42 and ZZ9, seven <i>Expansin</i> protein-encoding genes were identified, of which six were significantly upregulated in GT42. The two genes c111037.graph_c0 and c113583.graph_c0, belonging to the α-<i>Expansin</i> and β-<i>Expansin</i> families, respectively, were functionally characterized, revealing their role in increasing the stomatal aperture. Therefore, these two sugarcane <i>Expansin</i> protein-coding genes contribute to the stomatal aperture, implying their potential roles in structural resistance to sugarcane smut. Our findings deepen the understanding of the role of the stomata in structural resistance to sugarcane smut and highlight their potential in sugarcane breeding for disease resistance.