Evaluation of Carob and Pomegranate Extracts as Inducers of Resistance against Verticillium Wilt in Olive Tree

Pomegranate and carob extracts have been characterized as potential bioprotectors against Verticillium wilt of olive caused by Verticillium dahliae, suggesting they may act as host resistance inducers. This study aimed to elucidate whether these extracts activate metabolic pathways involved in olive defense against V. dahliae infection through marker gene expression analysis. The relative expression of seven resistance-related marker genes was evaluated by RT-qPCR in noninoculated and treated olive plants. Subsequently, the effect of the extracts as inducers of resistance against Verticillium wilt was evaluated by comparing the overexpression of the marker genes selected from the previous experiment in inoculated and treated plants. Both extracts significantly modulated the relative expression of the marker genes related to salicylic [OeSAMT (salicylic acid carboxyl methyltransferase)], jasmonic [OeTIFY (TIFY transcription factor)] and abscisic acid [OeNCED (9-cis-epoxy carotenoid dioxygenase)], as well as the enzyme polyphenol oxidase (OePPO1 and OePPO2) in noninoculated plants. In inoculated plants, OeSAMT expression increased in plants treated with both extracts at one (T53) and three months (T119) after the third treatment. While the jasmonic acid pathway was not activated post-inoculation, the abscisic acid pathway showed increased activity, particularly one month after the third application, with an antagonistic pattern to OeSAMT expression. OePPO1 and OePPO2 genes expression was notably higher in stems of inoculated plants treated with either extract, than in the inoculated and untreated plants. Fungal biomass quantification revealed an increase in inoculated plants treated with water and a decrease or stabilization in those treated with pomegranate and carob extracts, respectively.