Systemic reduction of rice blast by means of photosensitizers

Abstract

Systemic disease resistance of plants may be induced by exogenous reactive oxygen species or their sources. Certain compounds (photosensitizers) produce ROS at the expense of light energy. Of them, this study used photodynamic dyes bengal rose and methylene blue, which yield singlet oxygen, and mercaptopyridine-N-oxide, which yields hydroxyl radical. The goals were to find out whether they can systemically protect rice (Oryza sativa L.) from blast caused by the fungus Magnaporthe oryzae Conouch et Kohn and whether ROS is involved in defense mechanisms. The tested compounds were placed on the fourth (uppermost) leaf. When the fifth leaf developed (in approximately 7 days), it was inoculated with virulent fungal strain. We found that almost all the chemical treatments altered the fourth leaf and all of them reduced disease symptoms on the fifth leaf. Antioxidants combined with the tested substances compromised the disease control. Photosensitizers applied to the fourth leaf increased superoxide production in drop diffusates of healthy and, to larger extent, infected fifth leaf. In these diffusates, fungitoxicity also increased, which was diminished by antioxidants added to the diffusates. Besides, treatment with mercaptopyridine-N-oxide systemically weakened the endogenous antioxidative (H2O2-decomposing) activity of a diffusate. It is suggested that the oxidative burst in the treated leaves induced the systemic disease resistance, whose accomplishment might include the secondary oxidative burst in systemic leaves suppressing the pathogen’s development.