PHENYLALANINE AMMONIA-LYASE IN BARLEY - ACTIVITY ENHANCEMENT IN RESPONSE TO ERYSIPHE-GRAMINIS F-SP HORDEI (RACE-1), A PATHOGEN, AND ERYSIPHE-PISI, A NONPATHOGEN

Phenylalanine ammonia-lyase (PAL) activity in barley leaves was measur ed at intervals after inoculation with the powdery mildew pathogen Ery siphe graminis Esp, hordei. Measurements were made at times that inclu ded the periods of attempted penetration by the E. graminis primary ge rm tube and appressorium. The results demonstrated that extractable en zyme activity increased at 6 h and between 12-15 h after inoculation, limes consistent with attempted penetration by the primary germ tube a nd the appressorium, respectively. Enzyme activity increased regardles s of the resistance or susceptibility of the barley cultivar to the fu ngus suggesting that the response was non-specific and was not a refle ction of the resistance or susceptibility of the cultivar to the patho gen prior to the time of penetration. When barley was inoculated with the nonpathogen E. pisi, only a single period of elevation in PAL enzy me activity was detected. This was consistent with the fact that, unli ke E. graminis, E. pisi does riot produce a primary germ tube. The enz yme activity increased between 9-15 h after inoculation, consistent wi th the time of the attempted penetration of the leaf by the E. Pisi ap pressorium. 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 th e increase was greatest in response to E. pisi The results are discuss ed with respect to the presumed importance of host phenolic compound m etabolism that occurs as a response to the fungal infection process.