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This study investigated the infection process of Mycosphaerella graminicola and enzyme activities related to reactive oxygen species (ROS) or oxylipin biosynthesis in four French wheat cultivars with variable resistance to M. graminicola infection. At field level, cultivars Caphorn, Maxyl and Gen11 were susceptible, whereas Capnor showed high levels of quantitative resistance. Moreover, Capnor and Gen11 were tolerant, i.e., their yield was less affected by infection compared to non-tolerant Maxyl and Caphorn. These four cultivars were inoculated under laboratory conditions with the M. graminicola IPO323 reference strain. Cytological and biochemical responses were studied on collected first plantlet leaves and several features discriminated between cultivars. However, resistance and tolerance had no impact on the fungal infection process. Levels of lipoxygenase (LOX), peroxidase (PO) and glutathione-S-transferase (GST) activities were also compared with regard to cultivar resistance or tolerance to M. graminicola. LOX, PO and GST activities did not discriminate resistance and tolerance profiles, although a low level of PO in inoculated and non-inoculated plants could be associated with tolerance. In addition, cell necrosis correlated positively with LOX in non-tolerant cultivars, while mycelia surrounding stomata were positively correlated with PO in the resistant cultivar. GST activity presented correlations between cytological and biochemical parameters only for susceptible cultivars. Stomatal and direct penetration were positively correlated with GST activity in the susceptible non-tolerant cultivars, while these correlations were negative in the tolerant cultivar. When combining cytological and biochemical observations with resistance and tolerance profiles, for each cultivar and at each time point, cultivars could be classified in tight accordance with their previous field characterisation. Moreover, tolerance allowed us to distinguish susceptible cultivars when both biochemical and cytological parameters were considered together. [ABSTRACT FROM AUTHOR]

In wheat, little is known about disease resistance inducers and, more specifically, about the biological activities from those derived from endogenous elicitors, such as oligogalacturonides (OGAs). Therefore, we tested the ability of two fractions of OGAs, with polymerization degrees (DPs) of 2-25, to induce resistance to Blumeria graminis f. sp. tritici and defense responses in wheat. One fraction was unacetylated (OGAs-Ac) whereas the second one was 30% chemically acetylated (OGAs+Ac). Infection level was reduced to 57 and 58% relative to controls when OGAs-Ac and OGAs+Ac, respectively, were sprayed 48 h before inoculation. Activities of various defense-related enzymes were then assayed in noninoculated wheat leaves infiltrated with OGAs. Oxalate oxidase, peroxidase, and lipoxygenase were responsive to both OGAs-Ac and OGAs+Ac, which suggests involvement of reactive oxygen species and oxilipins in OGAs-mediated responses in wheat. In inoculated leaves, both fractions induced a similar increase in H₂O₂ accumulation at the site of fungal penetration. However, only OGAs+Ac led to an increase in papilla-associated fluorescence and to a reduction of formed fungal haustoria. Our work provides the first evidence for elicitation and protection effects of preventive treatments with OGAs in wheat and for new properties of acetylated OGAs.

Biological activities, priming and protective effects of two oligogalacturonides fractions (OGAs) were assayed during a compatible wheat/Blumeria graminis f. sp. tritici interaction. These fractions were obtained from commercial polygalacturonic acid. They both consisted of oligogalacturonides with polymerisation degrees (DP) ranging from 2 to 25, and one of them was a 30% chemically acetylated fraction. A 5 g x L(-1) solution of each fraction was infiltrated in the first leave of ten-days-old plantlets, and activities of defence-related enzymes were measured 48H post-treatment. Among them, oxalate oxidase and peroxidase activities increased, suggesting an elicitation due to both fractions of oligogalacturonides. Some of the pre-treated plantlets were subsequently submitted to powdery mildew infection. As revealed by 3,3'-diaminobenzidine (DAB) staining, the accumulation of hydrogen peroxide (H2O2) at the penetration site of the fungus increased 21H after inoculation to the same extent in areas of plantlets infiltrated by both fractions. On the other hand, the intensity of fluorescence associated with papillae was higher when plantlets were pre-infiltrated with the acetylated fraction, whereas no difference was observed between control plantlets and those treated with the non-acetylated fraction. Moreover, microscopic assessment of the number of haustoria occurring 40H post-inoculation showed it was only reduced when acetylated fraction was used. Despite different modes of action of these molecules, a similar 45% protective effect occurred in both cases when the oligogalacturonides fractions were sprayed on ten-days-old plantlets.

The aim of this study was to investigate the infection process of M. graminicola and the defence mechanisms related to active oxygen species (AOS) in five French wheat cultivars. These cultivars exhibited various resistant levels to M. graminicola infection: Maxyl, Caphorn and Gen11 are susceptible cultivars, whereas Capnor and Gen23 show high levels of quantitative resistances. In addition, Capnor, Gen23 and Gen11 are tolerant cultivars, i.e., their yield performance was less affected by infection compared to non-tolerant cultivars. Cultivars were inoculated with the IPO323 reference M. graminicola strain. First wheat leaves were collected 3, 5, 7, 9, 11, 13, 15, 17, 19, and 21 days after inoculation. The cytological and antioxidant response of the cultivars were both studied over the whole time course. Although infection occurred mainly through stomata, direct penetration attempts were also scored. Moreover, papilla formation turned out to be very rare. Assays for changes in peroxydase (PO), glutathione-S-transferase (GST) and lipoxygenase (LOX) activities allowed us to compare their levels in the five French wheat cultivars regarding to their resistance and/or tolerance towards M. graminicola infection. PO and GST were correlated to necrosis probably as a consequence of detoxification and LOX was related to some of the germination process steps. We also showed that significant differences for several biochemical parameters exist between the studied cultivars in non inoculated conditions but these differences were less important in the presence of the fungus.

Prophylactic efficacies of Iodus 40 and salicylic acid (SA) against wheat powdery mildew caused by Blumeria graminis f. sp. tritici have been shown and compared with those of heptanoyl salicylic acid (HSA) and trehalose. Plantlets treated once exhibited 55%, 50%, 95%, and 38% protection levels, respectively. Two sprayings increased these levels up to 60%, 65%, 100%, and 60%, respectively. Biological effects of these resistance inducers on reactive oxygen species (ROS) metabolism and lipid peroxidation were also investigated. We found clear differences in the extent and the type of induced responses, with HSA exhibiting both the most numerous and the highest effects. HSA and SA induced a 5.5-fold increase of whole cell DAB staining due to hydrogen peroxide accumulation, whereas Iodus 40 and trehalose increased staining intensity at the penetration sites only. However, these effects were not correlated with any modification of catalase (CAT), oxalate oxidase (OXO) or lipoxygenase (LOX) activities, except for HSA which decreased CAT in non-inoculated conditions and increased LOX in infectious conditions. HSA also induced an increase in the rate of lipid peroxidation, whereas Iodus 40 induced a decrease. The effects of the inducers on germinating conidia and wheat epidermal cells responding to fungal penetration were also investigated. Papilla-linked autofluorescence was affected by SA and Iodus 40 whereas germination was slightly altered by Iodus 40. The newly described protective efficacies and the partial, distinct and non-overlapping activities of these inducers on the wheat/powdery mildew interaction are discussed.