Valorization of biowastes from olive oil extraction and related biorefinery as inducers of plant disease resistance and growth biostimulants

Olive oil production generates significant amounts of olive mill wastes, often disposed of in soil with negative environmental and agricultural impacts. Some olive oil mills valorise these wastes by converting two-phase olive pomace into bioenergy via biogas production, but this process generates digestate, a partially degraded fraction used as a soil amendment. While digestate offers agronomic benefits, its application may pose risks, such as ammonia emissions and greenhouse gas release. Additionally, excessive chemical inputs in agriculture threaten human health and promote plant diseases spread. In a circular economy perspective, this project valorises two-phase olive pomace and related digestate as phytovaccines and growth biostimulants to enhance plant defences and productivity. Chemical analysis revealed that the liquid fraction of two-phase olive pomace is enriched in oligosaccharides and phenolic compounds, which were extracted and tested for defence-inducing properties in Arabidopsis thaliana and Solanum lycopersicum. Treatments triggered defence responses and reduced Botrytis cinerea and Pseudomonas syringae infections. Phenolic compounds also exhibited antimicrobial activity against Xylella fastidiosa, P. syringae, Pectobacterium carotovorum, Fusarium graminearum, Colletotrichum graminicola, and B. cinerea. Soil application of raw digestate and its fractions enhanced Arabidopsis growth in controlled conditions and improved tomato productivity in field trials. However, the digestate liquid fraction had a lower biostimulant effect due to its microbial population. To further exploit digestate microbes, they were processed to extract proteins with potential defence-inducing activities. These microbial proteins activated defence responses in Arabidopsis and tomato, enhancing disease resistance. Overall, these findings highlight two-phase olive pomace and related digestate as valuable agro-industrial byproducts, serving as plant defence inducers and growth biostimulants to promote sustainable agriculture.