EFFECTS OF LIGHT AND OF FUSARIUM-SOLANI ON SYNTHESIS AND ACTIVITY OF PHENYLALANINE AMMONIA-LYASE IN PEAS

Phenylalanine ammonia-lyase [which catalyzes the conversion of L-phenylalanine to trans-cinnamic acid] was purified from peas, and a specific antiserum against the enzyme was produced in rabbits. The antiserum was used to study the first 8 h of the phenylalanine ammonia-lyase activity response in 2 different organs of the pea from different developmental stages and in response to 2 different stimuli. Etiolated seedlings were pulse-labeled with L-[35S]methionine after either no light exposure or after specific periods of irradiation with blue light. Immature pods were pulse labeled with mixed L-[3H]amino acids after specific time periods following inoculation of the pod endocarp surfaces with macroconidia of F. solani. Immunoprecipitates isolated from extracts of each group were analyzed with sodium dodecyl sulfate disc gel electrophoresis and were found to contain a radioactive protein with an electrophoretic mobility identical to that of the phenylalanine ammonia-lyase subunit (MW 81,000). The radioactivity contained in the subunit band was interpreted as being due to de novo synthesis of the enzyme. The net rate of phenylalanine ammonia-lyase labeling, was initially low in both tissue types and rose dramatically, peaking at approximately a 6 to 10-fold greater level at 4 h after the beginning of the stimulus. Thereafter, the rate of labeling declined slowly. Inoculation with F. solani f. sp. pisi, a true pathogen of peas, caused a 50% greater rate of peak labeling than did inoculation with a nonpathogen, F. solani f. sp. phaseoli. The time profile of the changing rate of labeling correlates with the changing activity level of the enzyme which peaks at 12 h after the onset of the stimulus. The data presented favor a model which explains the changing activity of phenylalanine ammonia-lyase as being due to a changing rate of synthesis or degradation (or both) of the enzyme rather than due to the activation of a preformed zymogen.