Molecular Effects of Xylella fastidiosa and Drought Combined Stress in Olive Trees

Due to global climate change, complex combinations of stresses are expected to occur, among which the interaction between pathogens and drought stress may have a significant effect on growth and yield. In this study, the Xylella fastidiosa (Xf)-resistant cultivar Leccino and the susceptible one Cellina di Nardò were subjected to (a) individual drought stress, (b) Xf infection and (c) combination of both stress conditions. Here we report the physiological response to stresses in water content in leaves and the modulation in the expression level of seven genes responsive to plant water status and pathogen infection. In Xf-resistant plants, higher expression levels are reported for genes belonging to ROS-scavenging systems and for genes involved in pathogen stress (pathogenesis-related, PR, and leucine-rich repeat genes, LRR-RLK). However, PR and LRR-RLK were not further induced by water deficit. Interestingly, the genes related to drought response (aquaporin, PIP2.1, dehydration responsive element binding, DREB, and dehydrin, DHN), which induction was higher in Cellina di Nardò compared to Leccino during drought stress, was poorly induced in Xf-susceptible plants when Xf occur. Conversely, DHN was induced by Xf presence in Leccino. These results were consistent with observations on water content. Indeed, response was similar in Leccino regardless kind of stress or combination, whereas a strong reduction was observed in Xf-susceptible plants infected by Xf or in presence of combined stresses. Thus, the reported findings indicate that resistance of Leccino to Xf could be linked to its lower resistance to water stress, probably leading to the activation of alternative defense pathways that support the plant in Xf response.