INHIBITING PHENYLALANINE AMMONIA-LYASE AND CINNAMYL-ALCOHOL DEHYDROGENASE SUPPRESSES MLA1 (HR) BUT NOT MLO5 (NON-HR) BARLEY POWDERY MILDEW RESISTANCES

The effects of AOPP, an inhibitor of phenylalanine ammonia lyase (PAL) and of OH-PAS, an inhibitor of cinnamyl-alcohol dehydrogenase (CAD) o n the barley powdery mildew resistances conditioned by the dominant Ml a1 (hypersensitive cell death based) and the recessive mlo5 (condition ing penetration resistance without cell death) major gene alleles were determined in leaf and coleoptile tissues. The inhibitors were applie d either by uptake through cut ends of detached leaves or directly to monolayers of epidermal cells dissected from coleoptiles. In leaves, b oth inhibitors decreased localized autofluorescence in epidermal cell walls directly beneath primary germ tubes and appressoria. With the Ml a1 resistance allele both inhibitors strongly suppressed race-specific hypersensitive epidermal cell death (as indicated by whole cell autof luorescence in leaves or by cell collapse in the coleoptile epidermis) . AOPP was effective at 1 mM in tissues of both leaves and coleoptiles . OH-PAS was effective at 1 mM in leaves and at 1-11 mu M in coleoptil e tissue. Concomitant with the reduction in hypersensitive cell death in the coleoptile epidermis was an increase in average haustorium size and percentage of germlings producing hyphae. OH-PAS was effective wh en applied to the coleoptile epidermis as late as 16 h after inoculati on, approximately 2 h before the onset of hypersensitive cell death. R esults strongly suggest that the products of CAD activity play a direc t role in stimulating events (like peroxidase activity and free radica l formation) leading to programmed hypersensitive epidermal cell death conditioned by Mla1. In contrast, the recessive mlo5 resistance, whic h is expressed by failure of the fungus to fully penetrate barley epid ermal cells and form haustoria, was unaffected in either leaves or mon olayers of coleoptile epidermis by OH-PAS or AOPP. With mlo5, localize d autofluorescence was reduced at attack sites by inhibitor treatment, but treatment had no effect on rates of penetration based on observat ions of haustorium formation. These results indicate that mlo5 resista nce does not require PAL or CAD activity for its effectiveness. Theref ore barley resistances due to the major Mendelian resistance alleles M la1 (HR resistance) and mlo5 (non-HR) differ in their respective mecha nisms and in the physiological bases that underlie these mechanisms. ( C) 1995 Academic Press Limited