Hypericin-Mediated Photodynamic Inactivation Against the Plant Pathogen Clavibacter michiganensis: Preventative Seed Decontamination Enhanced by Potassium Iodide

<i>Clavibacter michiganensis</i> causes significant crop losses in tomatoes, and the disease may be transferred by plant seeds. This study evaluates the efficacy of Photodynamic Inactivation (PDI) with a water-soluble hypericin derivative, developed as a complex with polyvinylpyrrolidone (high hypericin-loaded PVP, HHL-PVP), as a decontamination strategy for tomato seeds. HHL-PVP was chosen for its overall stability, as the complex remains stable in solution for over 950 days, maintains its absorption capacity after illumination with 200 J·cm^-2, and produces reactive oxygen species (ROS) even at concentrations as low as 1 µM. PDI against <i>C. michiganensis</i> with 5 μM HHL-PVP, 10 min drug to light interval (DLI), and illumination with red light (600-700 nm, 100 J·cm^-2) exceeded the antimicrobial effect of a 99.9% reduction in liquid culture. Increasing the DLI to 24 h did not alter the photokilling effect. A 14 h light/10 h dark cycle in white light (118 J·cm^-2) with 0.3 µM HHL-PVP inhibited the growth of <i>C. michiganensis</i> by more than 6 log₁₀ steps, indicating that HHL-PVP has a stable and long-lasting photokilling effect. The combination of HHL-PVP with potassium iodide (KI, 100 mM) completely eradicated <i>C. michiganensis</i> in liquid culture with red and white light, indicating KI's role in enhancing phototoxicity. Tomato seed photodynamic decontamination using 1.0 µM HHL-PVP activated by 200 J·cm^-2 white light inactivated >5 log₁₀ of <i>C. michiganensis</i>, without diminishing sprouting. An addition of 100 mM KI increased the percentage of sprouted seedlings and inactivated 100% of bacteria. These results demonstrate that HHL-PVP-mediated PDI combined with KI could be highly effective as a preventative strategy in tomato protection against <i>C. michiganensis</i>.