Photosensitizer to the Rescue: In Planta and Field Application of Photodynamic Inactivation Against Plant-Pathogenic Bacteria

Control of plant pathogens using chemical and synthetic pesticides raises a major safety concern for humans and the environment. Despite the ongoing exploration of sustainable alternative methods, management practices for pathogens, especially bacteria, have remained almost unchanged over decades, whereby long-term uses of copper and antibiotics has led to widespread bacterial resistance in the field. Antimicrobial photodynamic inactivation (aPDI) of bacteria is emerging as an alternative strategy to combat resistant plant pathogens. aPDI utilizes light-sensitive molecules (photosensitizers) that, upon illumination, produce reactive oxygen species able to kill pathogens. Here, we explore the potential of an anionic semisynthetic watersoluble derivative of chlorophyl (sodium magnesium chlorophyllin [Mg-chl]) as an antibacterial agent in planta, by simulating processes naturally occurring in the field. Mg-chl in combination with Na2EDTA (cell-wall-permeabilizing agent) was able to effectively inhibit Pseudomonas syringae pv. tomato DC3000 in vitro and in planta in both tomato and Nicotiana benthamiana. Notably, Mg-chl in combination with Na2EDTA and the common surfactant Morwet D-400 significantly reduced Xanthomonas hortorum pv. gardneri and X. fragarie, respectively, in a commercial greenhouse trial against bacterial spot disease in tomato and in field experiments against angular leaf spot disease in strawberry.