RNA-Seq analyses reveal the possible molecular bases of resistance against leaf spot infection in olive and candidate genes underlying the resistance

The olive Peacock’s eye infection caused by obligate fungal pathogen Spilocea oleagina is one of the major diseases of cultivated olive which can cause severe yield loss of 20% or higher in susceptible cultivars. To investigate the resistance mechanisms of olive trees against Spilocea oleagina, the transcripts of a low susceptible cultivar ‘Koroneiki’ and a highly susceptible table olive cultivar ‘Nocellara del Belice’ were analysed by using next generation sequencing approaches, to identify genes and putative biomarkers linked to the infection. Many differentially expressed genes (DEGs) were discovered in two different stages (“no sign of disease” and “evident sign of disease”), however very few genes were in common among the two cultivars. In ‘Koroneiki’ the overexpression of many resistance gene analogues or pathogenesis-related (PR) genes was remarkable, as well as genes involved in the production of secondary metabolites, cell wall biosynthesis, and plant hormone signal transduction which are part of a typical defence reaction. It was also noteworthy that transcription factors (TS) involved in induced systemic resistance (ISR) were uniquely expressed in ‘Koroneiki’, while ‘Nocellara del Belice’ lacked an effective defence response. A putative biomarker – the Thaumatine like gene – was found commonly expressed in both cultivars and it was amplified with the portable thermocycler bCUBE® by Hyris to check whether it can be a promising candidate for early detection of the disease. Our results give valuable insights into transcriptional changes related to resistant and susceptible responses to Spilocea oleagina infection, and the candidate genes represent a valuable target to screen and select new resistant/less susceptible genotypes for breeding purposes. We also propose using the putative biomarkers for early detection of the pathogen before the onset of the disease.