Effect of Metabolic Inhibitors on the Size and Structure of Halos Induced by the Powdery Mildew Pathogen Blumeria graminis f. sp. tritici on Wheat Leaves

Abstract—A halo is a concentric zone (5–50 µm) of the altered surface of the leaf epidermis of wheat around the penetration site of the causative agent of powdery mildew Blumeria graminis. Halo variability was studied in a control and after treatment with metabolic inhibitors (3-amino-1,2,4-triazole, bisindole-maleimide hydrochloride, diphenyleniodionium chloride, 3-isobutyl-1-methylxanthine, catalase, lanthanum nitrate). After amido black was staining, the epidermis fragments were observed as monochromatic circular blue, or bicolor red and blue structures, or as a system of several purple-red, blue, or slightly colored rings. All treatments contributed to a significant reduction in the rate successful penetration of the pathogen into plant cells with the formation of haustoria, the marker of susceptibility at the tissue level in this pathosystem. Halo sizes were increased by treating with 3-amino-1,2,4-triazole, a peroxidase and catalase inhibitor, and also by treating catalase, an enzyme that degrades hydrogen peroxide. Treatment with lanthanum nitrate, which blocks calcium channels, led to a significant decrease in the halo diameter. Halo associated with haustoria, were 1.5 times smaller compared with those observed in the absence of successful development of the pathogen. The correlation between the sizes of such halos and the occurrence frequency of haustoria is shown for the first time. The average sizes of paired large and small halos for each variant, as well as the average distances between them, reliably correlated with each other and weakly correlated with the number of haustoria. The data indicate the possibility of using some halo parameters in diagnosing the local viability of individual infectious units of B. graminis, as well as in tests for the wheat plant resistance to powdery mildew.