A biological agent modulates the physiology of barley infected with Drechslera teres

Abstract

Recognized as the causal agent of net blotch, Drechslera teres is responsible for major losses of barley crop yield. The consequences of this leaf disease are due to the impact of the infection on the photosynthetic performance of barley leaves. To limit the symptoms of this ascomycete, the use of beneficial bacteria known as "Plant Growth Promoting Rhizobacteria" constitutes an innovative and environmentally friendly strategy. A bacterium named as strain B25 belonging to the genus Burkholderia showed a strong antifungal activity against D. teres. The bacterium was able to limit the development of the fungus by 95% in detached leaves of bacterized plants compared to the non-bacterized control. In this study, in-depth analyses of the photosynthetic performance of young barley leaves infected with D. teres and/or in the presence of the strain B25 were carried out both in and close to the necrotic area. In addition, gas exchange measurements were performed only near the necrotic area. Our results showed that the presence of the beneficial bacterium reduced the negative impact of the fungus on the photosynthetic performance and modified only the net carbon assimilation rate close to the necrotic area. Indeed, the presence of the strain B25 decreased the quantum yield of regulated non-photochemical energy loss in PSII noted as Y(NPQ) and allowed to maintain the values stable of maximum quantum yield of PSII photochemistry known as F_v/F_m and close to those of the control in the presence of D. teres. To the best of our knowledge, these data constitute the first study focusing on the impact of net blotch fungus and a beneficial bacterium on photosynthesis and respiratory parameters in barley leaves.

Figure 1

Parameters of chlorophyll a fluorescence (F₀), maximal fluorescence (F_m), maximal photosystem II quantum yield (F_v/F_m), the quantum yield of regulated non-photochemical energy loss Y(NPQ) and quantum yield of non-regulated energy dissipation Y(NO) determined on the leaflets of barley plants non inoculated (NI) or inoculated with D. teres at 3, 4, 5, 7 and 10 dpi. The red arrows indicate the measurement zones. Bar 0.5 cm.

Figure 2

Monitoring of the Y(II) (a), ETR (b) and PAR (c) parameters as a function of time for different experimental conditions: control barley (blue), barley infected with D. teres (red), barley bacterized with strain B25 (green) and barley infected and bacterized (purple). The data correspond to the mean of three independent experimental replicates, each with three or four plants per treatments (n = 9 or 12). Data were obtained from 2 to 9 dpi noted in this figure T2 to T9. Measurements were taken every 20 min. Asterisks (*) show the significant differences between the experimental conditions during a period of day or night for a time-point (Student’s test, p value < 0.05).

Figure 3

Variations of PSI parameters with PSI acceptor side limitation Y(NA), PSI donor side limitation Y(ND) and efficient quantum yield of PSI Y(I) in barley leaf accompanied by changes of PSII parameters with quantum yield of regulated energy dissipation Y(NPQ), quantum yield of non-regulated energy dissipation Y(NO) and efficient quantum yield of PSII Y(II). The measurements were performed with several experimental conditions: control barley, barley infected with D. teres, barley bacterized with strain B25 and barley infected with D. teres in combination with strain B25. These data were obtained 4 days before inoculation with D. teres (T-4), 0 days before (T0 BP) or after pulverization of the pathogen (T0 AP), 2, 4, 7 and 9 dpi with D. teres noted T2, T4, T7 and T9, respectively. The mean was calculated from three independent experiments for each experimental condition and for each time-point, n = 15. Different letters indicate statistically different means (Student’s test; p value < 0.05) between times points.

Figure 4

Changes in (a) quantum yield of cyclic electron flow (Y_CEF), (b) quantum yield of linear electron flow or maximum efficiency of PSII photochemistry (F_v/F_m), (c) electron transport rate at PSI reaction centers (ETRI) and (d) electron transport rate at PSII reaction centers (ETRII) in the first barley leaf at 4 day before inoculation (T-4) with D. teres and 0, 2, 4, 7 and 9 dpi noted T2, T4, T7 and T9, respectively. The mean ± SE was calculated from three independent experiments for each experimental condition and for each time-point, n = 15. Different letters indicate statistically different means (Student’s test; p value < 0.05) between times points.

Figure 5

Changes in net carbon assimilation rate (A) in day condition (a), dark respiration (Rd) (b), internal CO₂ concentration (Ci) in day condition (c) and in night condition (d), transpiration rate (E) in day condition (e) and in night condition (f) and stomatal conductance (gs) in day condition (g) and in night condition (h) determined on the leaflets of barley plants non-inoculated (control) or inoculated (D. teres) or bacterized with strain B25 or inoculated and bacterized (D. teres + B25). The measurements were carried out in the first barley leaf 4 days before inoculation (T-4) with D. teres and 0, 2, 4, 7 and 9 dpi noted T2, T4, T7 and T9, respectively. Bars represent the standard error of the means, calculated from three independent experiments for each experimental condition and for each time-point, n = 15. Different letters indicate statistically different means (Student’s test; p value < 0.05) between times points.