Molecular changes promoted by long-term water deficit in tomato can be largely mitigated by a fungal biostimulant

• Fungal culture filtrates improve tomato performance and fruit yield under water deficit • The biostimulant attenuates a large portion of drought-induced molecular responses • The biostimulant reduces stress perception, making plants less sensitive to drought • Plant responses to the biostimulant depend on water status • The mode of action of this biostimulant differs from that reported for others The use of cell-free microbial culture filtrates (CF) as biostimulants is emerging as a safe and ecologically sound approach to improve crop performance while reducing anthropogenic pressure. However, the mechanisms underlying their biological activity remain poorly understood. We previously identified volatile organic compounds as major bioactive constituents of fungal CF. Here, we show that foliar application of cell-free CF derived from Trichoderma harzianum enhanced fruit yield, root growth, photosynthetic performance and agronomic water use efficiency in a commercial tomato cultivar grown in Mediterranean greenhouses under long-term water deficit conditions. To elucidate the biochemical and molecular bases of this phenomenon, we adopted an integrative approach to characterize plants grown under optimal and suboptimal irrigation conditions (OIC and SOIC, respectively) with or without the fungal CF treatment. Water deficit induced extensive changes in drought stress-related signaling molecules and in the leaf transcriptome, which accounted for many of the physiochemical differences recorded between OIC- and SOIC-grown plants. Notably, many of these changes were largely alleviated by foliar application of fungal CF to SOIC-grown plants, including those affecting the expression of approximately 50% of water deficit-responsive genes. These genes did not respond to CF in OIC-grown plants, indicating that the transcriptomic response to CF is strongly dependent on plant water status. Taken together, our results indicate that foliar application of fungal CF enhances tomato tolerance to long-term water deficit primarily by attenuating a substantial fraction of drought-induced metabolic and transcriptional responses rather than by inducing new ones, thereby reducing plant sensitivity to water stress.