HYDROXYBENZOIC ACID METHYLESTERS INHIBIT MAGNESIUM CHELATASE ACTIVITYIN CRESS AND BARLEY SEEDLINGS

The natural compound 3-hydroxybenzoic acid methylester that was found to inhibit light-dependent chlorophyll synthesis in deetiolated cress seedlings, also inhibits protochlorophyllide accumulation in dark-grow n cress seedlings at concentrations >0.1 mM. The isomeric 2-hydroxy an d 4-hydroxy compounds exhibit inhibition to a similar extent. Accumula tion of magnesium protoporphyrin that can be observed after incubation of cress seedlings with 5-aminolevulinic acid, is also inhibited but accumulation of protoporphyrin is nearly not affected. The conclusion that magnesium chelatase activity is the point of attack of hydroxyben zoic acid methylesters was confirmed with barley etioplasts. The enzym e activitiy was inhibited with K-i = 3.0 mM for 2-hydroxybenzoic acid methylester, 7.4 mM for 3-hydroxybenzoic acid methylester and 5.4 mM f or 4-hydroxybenzoic acid methyleter. Chlorophyll accumulation in barle y seedlings was inhibited in the same concentration range with K-i = 6 .0 mM for 2-hydroxybenzoic acid methylester, 11.0 mM for 3-hydroxybenz oic acid methylester and 5.0 mM for 4-hydroxybenzoic acid methylester.