Effect of Oak Powdery Mildew on Ascorbate-Glutathione Cycle and Other Antioxidants in Plant- Erysiphe alphitoides Interaction

Abstract

Erysiphe alphitoides is a species of powdery mildew responsible for the major foliar disease of oak trees, including Quercus robur. Infection with E. alphitoides leads to a reduction in the growth of the trees and in their ability to survive. This paper reports on the biochemical changes characteristic of defence responses in oak leaves with different infection area sizes, collected in July, August, and September during three growing seasons. The study highlights the effect of E. alphitoides infection on changes in the ascorbate-glutathione cycle, phenolic compound profile, and metal content (mineral distribution). Visible symptoms of pathogen infection appeared gradually in July, but the most intense biochemical plant responses in oak leaves were detected mainly in August and September. These responses included increased ascorbate-glutathione enzyme activities, phenolic compounds, and metal contents. In addition, microscopic analyses revealed a strong fluorescence signal of lignin in the epidermis of pathogen-infected leaves. The involvement of the studied compounds in the basic defence mechanisms of oak against E. alphitoides infection is discussed in the paper.

Keywords: ascorbate–glutathione cycle; lignin; metals; oak powdery mildew; phenolic compounds.

Figure 1

Moisture content of control and E. alphitoides infected oak leaves collected at fixed time points (July, August, and September) of three growing seasons. Values represent the means of 15–18 measurements per variant (C, <5% Inf, 12–15% Inf, 25% Inf) at each time point ±SD. Analysis of variance (ANOVA) for all parameters was followed by the Tukey post hoc test, and statistically significant differences (p ≤ 0.05) are marked using low letters.

Figure 2

Elemental composition (Cr, Cu, Al, Ni, Zn, Fe, Na, Mn, P, Mg, K, Ca) of control and E. alphitoides infected oak leaves collected at fixed time points (July, August, and September) of three growing seasons. Values represent the mean of 9 analyses per variant (C, <5% Inf, 12–15% Inf, 25% Inf) at each time point ±SD. Analysis of variance (ANOVA) for all parameters was followed by the Tukey post hoc test and statistically significant differences (p ≤ 0.05) are marked using low letters.

Figure 3

Changes in the activity of ascorbate-glutathione cycle enzymes (APX, DHAR, MDHAR and GR) in control and E. alphitoides-infected oak leaves collected at fixed time points (July, August and September) of three growing seasons. Values represent the means of 18 measurements per variant (C, <5% Inf, 12–15% Inf, 25% Inf) at each time point ±SD. Analysis of variance (ANOVA) for all parameters was followed by the Tukey post hoc test and statistically significant differences (p ≤ 0.05) are marked using low letters.

Figure 4

Lignin content in the control and E. alphitoides infected oak leaves collected at fixed time points (July, August, and September) of three growing seasons. Values represent the mean of 18 measurements per variant (C, <5% Inf, 12–15% Inf, 25% Inf) at each time point ±SD. Analysis of variance (ANOVA) for all parameters was followed by the Tukey post hoc test, and statistically significant differences (p ≤ 0.05) are marked using low letters.

Figure 5

Detection and visualisation of lignin in control and E. alphitoides-infected oak leaves. A fluorescence confocal microscope (Leica TCS SP8; Leica Microsystems, Mannheim, Germany) was used to detect lignin deposition. White arrows indicate green fluorescence of lignin in the leaf epidermis analysed in August. Representative images were selected after analysing microscopic preparations of 3–4 plants from each experimental variant (C, <5% Inf, 12–15% Inf, 25% Inf).