Exploring 2,6-DibromoBODIPYs as Potential Photosensitizers for Photodynamic Inactivation of Phytopathogenic Fungi

Antimicrobial photodynamic therapy (aPDT) is a promising alternative strategy to overcoming fungal resistance. It is based on the generation of reactive oxygen species (ROS) upon irradiation of a suitable photosensitizer (PS) with abundant natural sunlight. Currently, further application of aPDT in controlling plant diseases has been hampered by the limited access to suitable PSs. In this work, we designed and synthesized a set of 2,6-dibromoBODIPYs and investigated their performance as novel PSs for aPDT against eight highly destructive plant pathogenic fungi. Most of these target compounds showed excellent and broad-spectrum antifungal photodynamic activities, as demonstrated in the bioassay results, by entering into the interior of fungal hyphal cells and in situ generating ROS to destroy subcellular organelles. Among those, compound 17a exhibits the highest singlet oxygen (1O2) generation efficiency, with an excellent EC50 value against each of the five common plant pathogens (0.955, 0.883, 0.281, 1.09, and 1.05 mg/L for Valsa mali, Sclerotinia sclerotiorum, Rhizoctonia solani, Fusarium graminearum, and Trichoderma viride, respectively). These values are comparable to or even slightly better than those achieved with the commercial PS Rose Bengal (RB) and the fungicide thifluzamide. It is capable of inhibiting the growth of R. solani on the detached leaves of rice and provides effective protection and curative activities at 50 mg/L. In addition, compound 17a also features excellent plant compatibility and low toxicity to nontargeted organisms including Apis mellifera L. These results indicate that 2,6-dibromoBODIPYs may be used as potential PSs for the photodynamic killing of phytopathogenic fungi.