PHENYLALANINE AMMONIA-LYASE INHIBITION, AUTOFLUORESCENCE, AND LOCALIZED ACCUMULATION OF SILICON, CALCIUM AND MANGANESE IN OAT EPIDERMIS ATTACKED BY THE POWDERY MILDEW FUNGUS BLUMERIA-GRAMINIS (DC) SPEER

Phenolic-based autofluorogens accumulate locally in haloes and papilla e formed in cereal epidermal cells responding to attack by germ tubes of Blumeria graminis (the powdery mildew fungus). The chemical element s silicon, calcium anc manganese have been reported to accumulate in s patial coincidence with autofluorogens. We tested the hypothesis that chemical element accumulations are dependent upon the presence of auto fluorogens. Light and fluorescence microscopy showed that treating see dling leaves of Selma oat (Avena saliva L.) with alpha-amino-beta-phen ylpropionic acid (AOPP) to inhibit phenylalanine ammonia lyase (PAL) g reatly suppressed the frequency and intensity of localized autofluores cence. Energy dispersive X-ray microanalysis was used to identify and quantify levels of Si, Ca and Mn at 12, 16, 18, 24 and 30 h after inoc ulation. Leaf specimens were cryopreserved in the frozen-hydrated stat e allowing elemental analysis of halo and upper papilla regions, and t hen freeze-dried for analysis from deeper within cells. Although autof luorescence was detected at 12 h, no accumulation of any element was d etected until 18 h. Results strongly indicated that Si accumulation is a process independent from autofluorogen accumulation. At attack site s, Mn levels were generally elevated in association with AOPP treatmen t, while Ca levels deep within epidermal cells decreased. Further comp arisons were made between successful and failed B. gr graminis penetra tion sites and detailed results are presented and discussed. (C) 1998 Academic Press.