EFFECTS OF 2 PAL INHIBITORS ON THE SUSCEPTIBILITY AND LOCALIZED AUTOFLUORESCENT HOST-CELL RESPONSES OF OAT LEAVES ATTACKED BY ERYSIPHE-GRAMINIS DC

Seedling leaves of oat (Avena sativa L.) cv. Maldwyn, with adult plant resistance to Erysiphe graminis DC f.sp. avenae ex Merat., were detached and treated with competitive inhibitors of phenylalanine ammonia-lyase (PAL), the enzyme catalysing the first committed step of phenylpropanoid metabolism. The two inhibitors used were α-aminooxy-β-phenylpropionic acid (AOPP) and α-aminooxy acetic acid (AOA). Both chemicals inhibited PAL extracted from seedling leaves, but AOPP was more effective than AOA. To examine effects on host response and pathogen development, cut ends of leaves were immersed in various concentrations of inhibitor for 24 h before inoculation with fungal conidia, and then incubated for a further 36 h. AOPP had no deleterious effects on host tissues or pathogen development, but high (10-3 m) concentrations of AOA caused loss of leaf turgor, and prevented fungal haustoria from maturing. At high concentrations (≥ 10-4 m) both inhibitors increased leaf susceptibility to fungal penetration. Both inhibitors reduced the intensity and frequency of localized autofluorescent host cell responses associated with contact by primary germ tubes and appressorial germ tubes of the fungus. AOPP significantly reduced diameters of autofluorescent host cell responses. The results support the view that the autofluorescent materials are phenolic and that they play a role in the resistance of epidermal cells to penetration. Attempts to reverse the inhibitory effects of AOPP by simultaneous application of cinnamate, the product of PAL activity, were largely unsuccessful. © 1991.