A novel miRNA negatively regulates resistance to Glomerella leaf spot by suppressing expression of an NBS gene in apple

Abstract

Glomerella leaf spot (GLS) of apple (<i>Malus</i>×<i>domestica</i> Borkh.), caused by <i>Glomerella cingulata</i>, is an emerging fungal epidemic threatening the apple industry. Little is known about the molecular mechanism underlying resistance to this devastating fungus. In this study, high-throughput sequencing technology was used to identify microRNAs (miRNAs) involved in GLS resistance in apple. We focused on miRNAs that target genes related to disease and found that expression of a novel miRNA, <i>Md-miRln20</i>, was higher in susceptible apple varieties than in resistant ones. Furthermore, its target gene <i>Md-TN1-GLS</i> exhibited the opposite expression pattern, which suggested that the expression levels of <i>Md-miRln20</i> and its target gene are closely related to apple resistance to GLS. Furthermore, downregulation of <i>Md-miRln20</i> in susceptible apple leaves resulted in upregulation of <i>Md-TN1-GLS</i> and reduced the disease incidence. Conversely, overexpression of <i>Md-miRln20</i> in resistant apple leaves suppressed <i>Md-TN1-GLS</i> expression, with increased disease incidence. We demonstrated that <i>Md-miRln20</i> negatively regulates resistance to GLS by suppressing <i>Md-TN1-GLS</i> expression and showed, for the first time, a crucial role for miRNA in response to GLS in apple.