GRAS salts and eastern hemlock extract: a dual approach to sustainable plant disease management

Several studies assessed the potential of salts and plant extracts as sustainable, eco-friendly phytosanitary products in plant protection. Here, we examined the efficacy of Generally Recognized as Safe (GRAS) salts sodium benzoate (SBE) and sodium bicarbonate (SBI) used alone or in combination with a twig extract from eastern hemlock (EH) against the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 ( Pst DC3000). The antimicrobial activity of the three products, and their ability to bolster host plant natural defenses, were assessed using the Arabidopsis– Pst DC3000 pathosystem as a model. RT-qPCR and leaf staining assays on a transgenic Arabidopsis line engineered to express reporter protein ß-glucuronidase under the control of pathogen-inducible promoter PR1 showed both GRAS salts and the EH extract to trigger the expression of salicylic acid (SA)-inducible genes. Salt- and EH extract-treated plants exhibited callose deposits in leaf tissue, further pointing to the induction of the SA signaling pathway and suggesting the establishment of a physical barrier in the leaf apoplast upon treatment. Direct toxicity measurements and a bacterial infection assay in growth chamber showed both GRAS salts and the EH extract to inhibit Pst DC3000 growth and Arabidopsis leaf colonization. Toxicity assays also revealed complementary, additive effects between the two salts and the EH extract in their detrimental effect against the pathogen, as evidenced by reduced minimum inhibitory concentrations when used in combination. Interestingly, SBE and SBI strongly delayed leaf senescence following infection, either alone or combined with the EH extract. Together, these findings confirm the potential of SBE, SBI and the EH twig extract as both toxic compounds against Pst DC3000 and natural defense stimulators in Arabidopsis. They also highlight the potential of GRAS salt and plant extract combinations as a sustainable alternative to chemical pesticides in plant pathogen management.