A Novel DCL2-Dependent Micro-Like RNA Vm-PC-3p-92107_6 Affects Pathogenicity by Regulating the Expression of Vm-VPS10 in Valsa mali

Dicer proteins are mainly responsible for generating small RNAs (sRNAs), which are involved in gene silencing in most eukaryotes. In previous research, two DCL proteins in Valsa mali, the pathogenic fungus causing apple tree Valsa canker, were found associated with both the pathogenicity and generation of sRNAs. In this study, the differential expression of small interfering RNAs (siRNAs) and miRNA-like RNAs (milRNAs) was analyzed based on the deep sequencing of the wild type and Vm-DCL2 mutant, respectively. Overall, the generation of 40 siRNAs and 18 milRNAs was evidently associated with Vm-DCL2. The target genes of milRNAs were then identified using degradome sequencing; according to the prediction results, most candidate targets are related to pathogenicity. Further, expression of Vm-PC-3p-92107_6 was confirmed in the wild type but not in the Vm-DCL2 mutant. Moreover, the pathogenicity of Vm-PC-3p-92107_6 deletion mutants (ΔVm-PC-3p-92107_6) and the over-expression transformants (Vm-PC-3p-92107_6-OE) was significantly increased and decreased, respectively. Based on those degradome results, vacuolar protein sorting 10 (Vm-VPS10) was identified as the target of Vm-PC-3p-92107_6. Co-expression analysis in tobacco leaves further confirmed that Vm-PC-3p-92107_6 could suppress the expression of Vm-VPS10. Meanwhile, the expression levels of Vm-PC-3p-92107_6 and Vm-VPS10 displayed divergent trends in ΔVm-PC-3p-92107_6 and Vm-PC-3p-92107_6-OE, respectively. Perhaps most importantly, ΔVm-VPS10 featured a significant reduction in pathogenicity. Taken together, our results indicate that a DCL2-dependent milRNA Vm-PC-3p-92107_6 plays roles in pathogenicity by regulating the expression of Vm-VPS10. This study lays a foundation for the comprehensive analysis of pathogenic mechanisms of V. mali and deepens our understanding of the generation and function of fungal sRNA.