SUPPRESSION OF HOST CINNAMYL ALCOHOL-DEHYDROGENASE AND PHENYLALANINE AMMONIA-LYASE INCREASES OAT EPIDERMAL-CELL SUSCEPTIBILITY TO POWDERY MILDEW PENETRATION

Seedling leaves of oat cvs Maldwyn and Selma have no known major resistance genes to powdery mildew caused by Erysiphe graminis f.sp. avenae, but their susceptibility to infection is quantitative. Thus, only a portion of fungal germlings successfully overcome cell defences to penetrate host epidermis to form haustoria. OH-PAS ([[(2-hydroxyphenyl)amino] sulphinyl]acetic acid, 1,1-dimethylethyl ester) is a potent, specific suicide inhibitor of CAD (cinnamyl alcohol dehydrogenase), an enzyme specifically involved with synthesis oflignin precursors. OH-PAS was shown to inhibit CAD from oat in vitro. For in vivo assays of effects on epidermal cell defences, the cut ends of excised seedling leaves were immersed in OH-PAS solution for 24 h to allow uptake before inoculation with E. graminis conidia. Inoculated leaves were allowed OH-PAS uptake during a further 36 h incubation period. Initial experiments established that OH-PAS at 10-3m decreased the frequency and intensity of localized autofluorescent host epidermal cell responses associated with primary germ-tubes (PGTs) and appressoria. Concurrently, OH-PAS treatment doubled the proportion of appressoria forming haustoria, i.e., it increased quantitative susceptibility by suppressing host cell defences. Similar results were obtained with 10-3m AOPP (α-aminooxy-β-phenyl propionic acid), a competitive inhibitor of PAL (phenylalanine ammonia lyase) which catalyzes the first committed step in phenylpropanoid metabolism. Both inhibitors doubled the proportion of appressoria penetrating epidermal cells and forming haustoria. Both inhibitors reduced the frequency and intensity of localized autofluorescent epidermal host cell responses to PGTs and appressoria, although the effect of AOPP was somewhat greater than that of OH-PAS. Neither OH-PAS nor AOPP had any deleterious effects on fungal development. Results support the idea that host autofluorogens accumulating at sites of fungal germ-tube contact with epidermal cells are phenolic compounds. In addition, the study provides experimental evidence pointing to involvement of products synthesized as part of the lignin biosynthetic pathway in oat epidermal cell defence against attempted penetration by appressoria of E. graminis f.sp. avenae. © 1994 Academic Press, Inc.