Physiological leaf spots in barley and wheat

Evidence for oxidative stress involved in physiological leaf spot formation in winter and spring barley

Yue-Xuan Wu* and Andreas von Tiedemann, Phytopathology 92: 145-155, 2002 *present address: Laboratory of Cell Biology, National Heart, Lung and Blood Institute, National Institutes of Health, 9000 Rockville Pike, Bethesda, MD 20892-8017, U.S.A.

Reactive oxygen species (ROS) have been shown in numerous studies to be causally involved in plant stress and plant ageing (Fig. 1). Also, the pathology of various biotic and abiotic diseases is associated with uncontrolled generation and insufficient scavenging of ROS. A leaf spot disease with unknown etiology has become more pronounced in spring and winter barley in Germany in recent years. The symptoms are similar to net blotch and Ramularia-leaf spots, but the causal agents of these diseases were failed to be identified (Fig. 2). The symptom expression varied much on cultivars. Cultivars most affected by the disease of both spring and winter barley showed a significantly higher level of superoxide ( O2.- ) production (Fig. 3) and lipid peroxidation (malondialdehyde, MDA), but a lower level of antioxidant potential expressed as superoxide dismutase (SOD) activity, catalase activity and integral water-soluble antioxidant capacity (ACW), than insensitive cultivars. A high positive correlation between O2.- production and leaf spot development between ear emergence and milk ripeness was established in the most sensitive winter barley cultivar Anoa (r2 = 0.9622) and spring barley cultivar Barke (r2 = 0.9434).

Leaf H2O2 levels were also increased with the severity of leaf spots. The histochemical localization of O2.- and H2O2 in the tissues adjacent to leaf spots indicated that these two active oxygen species (AOS) are involved in the formation of leaf spots (Fig. 4). Reduction of symptom severity by applying strobilurin and azole fungicides was always associated with elevated SOD activity and ACW content and suppressed O2.- production. However, peroxidase activities were significantly higher in sensitive cultivars and in more severely affected tissues and decreased by applying fungicides. Thus, it is assumed that a possible genetic mechanism based on the imbalanced AOS metabolism contributes to formation of physiological leaf spots.

Light-dependent oxidative stress determines physiological leaf spot formation in barley

Physiological leaf spots (PLS) in winter and spring barley are dependent on genotype-related oxidative stress under field conditions (Fig. 5). In the present study, we searched for factors inducing PLS symptoms in the greenhouse similar to those observed in the field, and investigated its relationship to reactive oxygen species (ROS) metabolism. In the greenhouse, spring barley cv. ‘Extract’, which is sensitive to PLS, was induced to express similar symptoms as observed in the field by oxidative stresses.

Leaves severely affected by PLS showed significantly lower activities of key enzymes in the Halliwell-Asada cycle such as ascorbate peroxidase (APX), glutathione reductase (GR), de07-Jul-2008eductase (MDHAR). The sensitive cultivar also showed lower levels of total SOD and Cu, Zn-SOD activity but a higher level of chloroplast specific Fe-SOD activity than the insensitive cultivar. Thus, an imbalanced ROS metabolism in chloroplasts may trigger PLS incidence in sensitive cultivars, which is in agreement with the fact that light was essential for the induction of PLS expression under both field and greenhouse conditions.

Accordingly, under greenhouse conditions, continuous light (7 d) stress, but not light shock treatments, induced PLS similar to the field in the sensitive cultivar ‘Extract’, but not in the resistant cultivar ‘Scarlett’ (Fig. 6). Light with high proportion in the blue wavelength spectrum (350-560 nm) was significantly more PLS-inducive than light with pronounced red (PAR) spectrum (580-650 nm). Exposure to ozone did not produce PLS-like symptoms. Furthermore, similar to earlier observations in the field, PLS symptom expression was closely correlated with the accumulation of superoxide (O2.-) detected by both biochemical and histochemical assays. Taken together, these data suggest that PLS in barley is genotype based but its expression appears to be induced by certain environmental stress factors, among which photosynthetic active radiation plays a major role.

Selected publications:

Grimme E. (2006). Abiotischer Stress in Weizenblättern: Reaktionen im Photosynthese-Apparat in Relation zum Stressmetabolismus. PhD-thesis, University of Göttingen, Germany.
Wu, Y. X., A. v. Tiedemann (2001). Physiological effects of azoxystrobin and epoxiconazole on senescence and the oxidative status of wheat. Pesticide Biochemistry & Physiology 71, 1-10.
Wu, Y. X., A. v. Tiedemann, (2002). Evidence for oxidative stress involved in physiological leaf spot formation in winter and spring barley. Phytopathology 92, 145-155.
Wu, Y. X., A. v. Tiedemann, (2004). Light-dependent oxidative stress determines physiological leaf spot formation in barley. Phytopathology 94, 584-592.

Investigator: Dr. Yue-Xuan Wu

Supervisor: Prof. Andreas von Tiedemann