Phenylalanine ammonia-lyase in barley: activity enhancement in response to Erysiphe graminis f.sp. hordei (race 1) a pathogen, and Erysiphe pisi, a nonpathogen

Abstract

Phenylalanine ammonia-lyase (PAL) activity in barley leaves was measured at intervals after inoculation with the powdery mildew pathogen Erysiphe graminis f.sp. hordei. Measurements were made at times that included the periods of attempted penetration by the E. graminis primary germ tube and appressorium. The results demonstrated that extractable enzyme activity increased at 6 h and between 12-15 h after inoculation, times consistent with attempted penetration by the primary germ tube and the appressorium, respectively. Enzyme activity increased regardless of the resistance or susceptibility of the barley cultivar to the fungus suggesting that the response was non-specific and was not a reflection of the resistance or susceptibility of the cultivar to the pathogen prior to the time of penetration. When barley was inoculated with the nonpathogen E. pisi, only a single period of elevation in PAL enzyme activity was detected. This was consistent with the fact that, unlike E. graminis, E. pisi does not produce a primary germ tube. The enzyme activity increased between 9-15 h after inoculation, consistent with the time of the attempted penetration of the leaf by the E. pisi appressorium. Northern blot analyses to detect the time of appearance of PAL mRNA indicated that the level of the message began to increase at 0·5 h after inoculation with both fungi, and that the intensity of the increase was greatest in response to E. pisi. The results are discussed with respect to the presumed importance of host phenolic compound metabolism that occurs as a response to the fungal infection process.