Apple mosaic disease (AMD) is a widespread viral disease affecting apple-growing regions around the world. Recent studies have identified a novel ilarvirus, apple necrotic mosaic virus (ApNMV), as the major causal agent of AMD in China. However, the molecular mechanisms underlying AMD pathogenesis and the global gene expression changes during mosaic symptom development remain largely unknown. In this study, we performed transcriptome analysis to investigate apple gene responses to AMD. A total of 815 differentially expressed genes (DEGs) were identified in mosaic leaves compared to healthy controls, while 1050 DEGs were found between symptomless leaves (infected with ApNMV) and mosaic leaves. Functional enrichment analysis revealed that these DEGs were predominantly involved in carbohydrate metabolism, oxidation-reduction processes, secondary metabolite biosynthesis, and plant hormone signal transduction. Further biological assays demonstrated that the manifestation of mosaic symptoms in apple leaves was associated with reactive oxygen species (ROS) accumulation and downregulation of ROS-scavenging genes. Collectively, our findings provide new insights into the molecular basis of ApNMV-induced mosaic symptom development in apple and offer potential targets for the management of AMD.

Plants deploy intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) to detect pathogenic effectors and trigger effector-triggered immunity (ETI). Helper NLRs, such as N required gene 1 (NRG1), act downstream of sensor NLRs to transduce immune signals. NRG1s have been implicated to oligomerize to form Ca2⁺-permeable channels at the plasma membrane, playing a critical role in TIR-NLR (TNL)-mediated ETI. Although its biochemical function is well understood, the transcriptional regulation of NRG1 remains unclear. Here, we show that ETI activation by various TNLs upregulates NbNRG1 expression in Nicotiana benthamiana. We identify NbWRKY57 as a key transcriptional activator that directly binds the W-box motif in the NbNRG1 promoter. Knockdown or knockout of NbWRKY57 compromises TNLRoq1-mediated hypersensitive response (HR) cell death and pathogen resistance, while its overexpression restores mutant HR phenotypes, underscoring its positive regulatory role in TNL-mediated ETI. Moreover, NbWRKY57 is essential not only for TNLRoq1-mediated ETI but also for ETI mediated by other TNLs, including N and RPP1. We further demonstrate that calcium signaling contributes to NbNRG1 activation and TNL-mediated ETI. Together, these findings uncover a previously unknown regulatory mechanism controlling helper NLR transcription and provide insights into the transcriptional activation of NRG1 during ETI.