A Biostimulant based on a brown alga (Ecklonia maxima) and yeast extract confers resistance toward Pseudomonas syringae pv. tomato DC3000

Biotic stress is one of the leading causes of the loss of crop productivity. The need to reduce the use of chemical agents in agriculture requires to develop sustainable alternatives. One of the most promising solutions to address this important challenge involves the use of biostimulants, commonly based on seaweed extracts, complex organic materials, hormone-like compounds, amino acids, and humic acids. In this work, a first screening of different plant biostimulants such as triacontanol, chitosan, fulvic acids, and a seaweed/yeast combination extract was performed on tomato seedlings, to evaluate their possible antimicrobial role against the infection with the bacterium Pseudomonas syringae pv. tomato DC3000 (Pst). The results underlined the promising bioprotectant activity of the Ecklonia maxima-based biostimulant. Therefore, its role in mature tomato plants was deeply investigated. After the infection, the biostimulated plants showed a less affected phenotype and significantly reduced Pst replication in leaves. Consistently, callose deposition was promoted in biostimulated plants. Furthermore, ion leakage and oxidative damage induced by the biotic stress were mitigated, while the hormone-mediated response to stress and the expression of biotic stress-related genes were increased. In conclusion, our study demonstrated that biostimulation confers resistance to Pst and proposes a low-cost and eco-friendly strategy as a useful tool to increase plant resistance to pathogens.