In vitro photodynamic inactivation of conidia of the phytopathogenic fungus Colletotrichum graminicola with cationic porphyrins

Photodynamic inactivation (PDI) is an efficient approach for the elimination of a series of microorganisms; however, PDI involving phytopathogenic filamentous fungi is scarce in the literature. In the present study, we have demonstrated the photoinactivating properties of five cationic meso-(1-methyl-4-pyridinio) porphyrins on conidia of the phytopathogen Colletotrichum graminicola. For this purpose, photophysical properties (photostability and O-1(2) singlet production) of the porphyrins under study were first evaluated. PDI assays were then performed with a fluence of 30, 60, 90 and 120 J cm(-2) and varying the porphyrin concentration from 1 to 25 mu mol L-1. Considering the lowest concentration that enabled the best photoinactivation, with the respective lowest effective irradiation time, the meso-(1-methyl-4-pyridinio) porphyrins herein studied could be ranked as follows: triple-charged 4 (1 mu mol L-1 with a fluence of 30 J cm-2) > double-charged-trans 2 (1 mu mol L-1 with 60 J cm(-2)) > tetra-charged 5 (15 mu mol L-1 with 90 J cm-2) > mono-charged 1 (25 mu mol L-1 with 120 J cm(-2)). Double-charged-cis-porphyrin 3 inactivated C. graminicola conidia in the absence of light. Evaluation of the porphyrin binding to the conidia and fluorescence microscopic analysis were also performed, which were in agreement with the PDI results. In conclusion, the cationic porphyrins herein studied were considered efficient photosensitizers to inactivate C. graminicola conidia. The amount and position of positive charges are related to the compounds' amphiphilicity and therefore to their photodynamic activity.