Enhancing Drought Resistance in Olive Trees: Understanding the Synergistic Effects of the Combination of PGPR and Biochar

Abstract The erratic rainfall typical of the Mediterranean basin and the increasing frequency of droughts periods affect olive tree crops. Given the economic importance of such a crop and the large productive area, solutions are needed to maintain crop yield under decreasing water availability. In this work, we combine biochar and plant growth-promoting rhizobacteria (PGPR) as biostimulants for olive tree crops with the aim of increasing crop tolerance to water stress. Although biochar on the one hand and PGPR on the other have been shown to be good plant biostimulants under abiotic stress conditions, there is a lack of information on the application of this combined treatment in olive trees under water stress and the underlying mechanism involved during drought stress. This work analyzes the individual and combined effect of a Bacillus siamensis strain and biochar from olive tree pruning on the improvement of water stress tolerance in olive plants and the mechanisms involved, considering biochemical (proline, ABA, H 2 O 2 , and MDA levels) and transcriptomic (expression rates of water stress-related genes) aspects. Olive trees treated with the biochar-bacteria combination showed increased aerial biomass compared to those treated separately or untreated under both water-stressed and optimal conditions. Biochemical and genetic analyses revealed reduced stress indicators, including ABA and oxidative damage markers (H 2 O 2 and MDA), along with downregulation of dehydrin (DHN) genes, which counteract lipid peroxidation. Additionally, aquaporin (AQP) genes downregulation suggested improved water status by reducing water transport across cell walls, thus aiding cell hydration. This study highlights a promising approach for enhancing olive tree resilience to water stress, crucial for sustainable crop production in the Mediterranean region.