Comparative transcriptomics unravels new genes imparting scab resistance in apple (Malus x domestica Borkh.)

Apple scab is caused by an ascomycete fungus, Venturia inaequalis (Cke.) Wint., which is one of the most severe disease of apple (Malus × Domestica Borkh.) worldwide. The disease results in 30-40% fruit loss annually and even complete loss in some places. Owing to the evolving susceptibility of resistant apple genotypes harboring R-genes to new variants of V. inaequalis, a comparative transcriptome analysis using Illumina (HiSeq) platform of three scab-resistant (Florina, Prima, and White Dotted Red) and three susceptible (Ambri, Vista Bella, and Red Delicious) apple genotypes was carried out to mine new scab resistance genes. The study led to the identification of 822 differentially expressed genes in the tested scab-resistant and scab-susceptible apple genotypes. The most upregulated genes uniformly expressed in resistant varieties compared to susceptible ones were those coding for 17.3 kDa class II heat shock protein-like, chaperone protein ClpB1, glutathione S-transferase L3-like protein, B3 domain-containing protein At3g18960-like, transcription factor bHLH7, zinc finger MYM-type protein 1-like, and nine uncharacterized proteins, besides three lncRNAs. The genes that were downregulated in susceptible and upregulated in resistant cultivars were those coding for non-specific lipid transfer protein GPI-anchored 1, rust resistance kinase Lr10-like, disease resistance protein RPS6-like, and many uncharacterized proteins. DESeq2 analysis too revealed 20 DEGs that were upregulated in scab-resistant cultivars. Furthermore, a total of 361 genes were significantly upregulated in scab-susceptible variety, while 461 were found downregulated (P value < 0.05 and Log2 (FC) > 1). The differentially expressed genes (DEGs) were related to various pathways, i.e., metabolic, protein processing, biosynthesis of secondary metabolites, plant hormone signal transduction, autophagy, ubiquitin-mediated proteolysis, plant-pathogen interaction, lipid metabolism, and protein modification pathways. Real-time expression of a set of selected twelve DEGs further validated the results obtained from RNA-seq. Overall, these findings lay the foundation for investigating the genetic basis of apple scab resistance and defense pathways that might have a plausible role in governing scab resistance in apple against V. inaequalis.