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 papillae formed in cereal epidermal cells responding to attack by germ tubes of Blumeria graminis (the powdery mildew fungus). The chemical elements silicon, calcium anc manganese have been reported to accumulate in spatial coincidence with autofluorogens. We tested the hypothesis that chemical element accumulations are dependent upon the presence of autofluorogens. Light and fluorescence microscopy showed that treating seedling leaves of Selma oat (Avena saliva L.) with alpha-amino-beta-phenylpropionic acid (AOPP) to inhibit phenylalanine ammonia lyase (PAL) greatly suppressed the frequency and intensity of localized autofluorescence. 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 inoculation. Leaf specimens were cryopreserved in the frozen-hydrated state allowing elemental analysis of halo and upper papilla regions, and then freeze-dried for analysis from deeper within cells. Although autofluorescence was detected at 12 h, no accumulation of any element was detected until 18 h. Results strongly indicated that Si accumulation is a process independent from autofluorogen accumulation. At attack sites, Mn levels were generally elevated in association with AOPP treatment, while Ca levels deep within epidermal cells decreased. Further comparisons were made between successful and failed B. gr graminis penetration sites and detailed results are presented and discussed. (C) 1998 Academic Press.