PHENOLIC BIOSYNTHESIS INHIBITORS SUPPRESS ADULT-PLANT RESISTANCE TO ERYSIPHE-GRAMINIS IN OAT AT 20-DEGREES-C AND 10-DEGREES-C

Seedling and adult plant leaves of three oat genotypes, Selma, Maldwyn and OM1387 (a hybrid derived from a Selma x Maldwyn cross), were exci sed and treated with water or with alpha-amino-beta-phenylpropionic ac id (AOPP) to inhibit phenylalanine ammonia lyase activity, or with [[( 2-hydroxyphenly)amino] sulfinyl]acid 1,1-dimethylethyl ester (OH-PAS) to inhibit cinnamyl alcohol dehydrogenase activity. None of these geno types possess any known major gene resistance to the isolate of Erysip he graminis f.sp. avenae with which they were inoculated, although Mal dwyn is known to possess quantitative adult plant resistance that has proved durable since 1947. All genotypes showed some adult plant resis tance which limited the proportion of fungal appressoria that formed h austoria in water-treated leaves. However, this resistance was express ed more strongly in Maldwyn and OM1387 than in Selma. Treatment with A OPP or OH-PAS increased substantially the proportion of fungal appress oria that formed haustoria in adult as well as seedling leaves, indica ting that phenolic compounds, probably products of the lignin biosynth etic pathway, contributed to resistance. However, AOPP-treated adult l eaves remained more resistant to infection than AOPP-treated seedling leaves, suggesting that additional unknown factor(s) contributed to ad ult plant resistance. In all genotypes and in seedling and adult leave s, failure of appressoria to infect was correlated to the localized ac cumulation of autofluorescent components in host cell papillae and cel l wall regions subtending appressoria. This localized autofluorescence was suppressed by AOPP treatment, and to a lesser extent by OH-PAS tr eatment, indicating that the fluorogens were phenolic compounds. In Ma ldwyn and OM1387 a proportion of host cells showed whole-cell autofluo rescence, probably indicative of cell death, following attack by appre ssoria. Whole-cell autofluorescence was almost totally suppressed by A OPP or OH-PAS treatment, suggesting that this response was also depend ent upon active synthesis of phenolic compounds. The contribution of p henolic compound synthesis to disease resistance appeared to be indepe ndent of temperature; AOPP treatment had similar effects in suppressin g resistance in plants acclimated and incubated at 10 degrees C and at 20 degrees C. (C) 1996 Academic Press Limited