ENHANCED PHENYLALANINE AMMONIA-LYASE ACTIVITY AND ACCUMULATION OF PHENOLIC COMPOUNDS IN WHEAT RELATED TO INDUCED RESISTANCE BY BENZOTHIADIAZOLE AGAINST BLUMERIA GRAMINIS F.SP. TRITICI
MJ STADNIK and H BUCHENAUER
Institute of Phytomedicine, University of Hohenheim, D-70593 Stuttgart, Germany
Background and objectives
Powdery mildew caused by Blumeria graminis (DC) Speer f.sp. tritici (Bgt) is a major constraint on wheat production worldwide. Several methods have been used to control the disease, such as planting resistant cultivars as well as application of fungicides and more recently, inducers of resistance. BTH (benzo [1,2,3] thiadiazole-7-carbothioic acid S-methylester) is the first commercially applied chemical plant defense activator representing this new technology for disease management in wheat. It has been shown that papilla formation plays an important role in induced resistance against powdery mildew . However, it is still not clear which factors contribute to this defense mechanism. Since de novo synthesis of phenolic compounds is a common response of cereals to the attack by Bgt , experiments were carried out to investigate their synthesis and to determine their significance in the induction of resistance by BTH.
Materials and methods
Experiments were conducted with detached leaves from the wheat cvs. Monopol and Zentos, Wheat plants were sprayed with Bion® (75 µg BTH/ml) at the development stage EC 28 and segments from untreated leaf F-2 were inoculated with Bgt 15 days later. The activity of phenylalanine ammonia lyase (PAL) was determined 0, 4, 8, 12 and 24 h after inoculation (h.a.i.) of un-treated and BTH-treated wheat plants. In a parallel experiment, intensities of autofluorescent responses of epidermal cell walls to Bgt-appressoria were evaluated using the following scale: 0=no visible; I=weak; II=moderate or III=intense autofluorescence.
Results and conclusions
Two peaks of PAL-activity were detected. The first occurred at 4 h.a.i., coinciding with the primary germ tube formation and the second at 12 h.a.i., during the period of appressorium formation. In both cultivars, BTH caused a significant increase of the PAL-activity at 12 h.a.i. However, increases in the PAL-activity caused by BTH were more evident in the susceptible cv. Monopol than in the resistant cv. Zentos.
The intensity of autofluorescence was also modified by BTH. In cv. Monopol, scales II and III were more frequent in comparison to untreated control while in cv. Zentos intensity of autofluorescence in epidermal cell walls of BTH-treated plants was only slightly increased compared to that of cell walls of untreated plants. Increase in PAL-activity was associated with enhanced autofluorescence in BTH-treated plants at the penetration site at 12 h.a.i., suggesting that both phenomena are closely associated.
The results suggest that enhanced PAL-activity and synthesis of phenolic compounds are involved in BTH-induced defence mechanisms of wheat plants against the powdery mildew fungus e.g. by inhibiting the penetration process.
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