Spilocea oleagina is a dangerous obligate fungal pathogen of olive, feared in the Mediterranean countries, causing Peacock's eye or leaf spot
infection, which can lead to a serious yield loss of approximately 20% or higher depending on climatic conditions. Coping with this disease is much more problematic for organic farms. To date, knowledge on the genetic control of possible mechanisms of resistance/low susceptibility is quite limited. In this work, comparative transcriptomic analysis (
RNA-seq) was conducted in leaf tissues of a low susceptible cultivar Koroneiki and a high susceptible cultivar Nocellara del Belice, both tested in the field using the NaOH test, considering two stages-"zero sign of disease" and "evident sign of
infection". Cultivars showed a very large number of differentially expressed genes (DEGs) in both stages. 'Koroneiki' showed an extensive hormonal crosstalk, involving
Abscisic acid (ABA) and
ethylene synergistically acting with
Jasmonate, with early signaling of the disease and remarkable defense responses against Spilocea through the over-expression of many resistance gene analogs or pathogenesis-related (PR) genes: non-specific
lipid-transfer genes (nsLTPs), LRR receptor-like
serine/threonine-protein kinase genes, GDSL
esterase lipase,
defensin Ec-AMP-D2-like, pathogenesis-related leaf
protein 6-like, Thaumatin-like gene, Mildew resistance Locus O (MLO) gene,
glycine-rich
protein (GRP), MADS-box genes, STH-21-like, endochitinases,
glucan endo-1,3-beta-glucosidases, and finally, many
proteinases. Numerous genes involved in cell wall biogenesis, remodeling, and cell wall-based defense, including
lignin synthesis, were also upregulated in the resistant cultivar, indicating the possible role of wall composition in
disease resistance. It was remarkable that many
transcription factors (TS), some of which involved in Induced Systemic Resistance (ISR), as well as some also involved in abiotic stress response, were found to be uniquely expressed in 'Koroneiki', while 'Nocellara del Belice' was lacking an effective system of defense, expressing genes that overlap with wounding responses, and, to a minor extent, genes related to phenylpropanoid and
terpenoid pathways. Only a Thaumatin-like gene was found in both cultivars showing a similar expression. In this work, the genetic factors and mechanism underlying the putative resistance trait against this fungal pathogen were unraveled for the first time and possible target genes for breeding resistant olive genotypes were found.