Biosynthesis of Δ-aminolevulinate in greening barley leaves III: The formation of Δ-aminolevulinate intigrina mutants of barley

Abstract

Thetigrina mutantstig-d 12, tig-b23 andtig-o 34 form increased amounts of protochlorophyllide in the dark relative to the wild type. Levulinate, the inhibitor of δ-aminolevulinate dehydratase was used to show that the high protochlorophyllide levels are due to rates of δ-aminolevulinate formation which are 1.3 to 6-fold higher than in the wild type. Bleached/necrotic bands in thetigrina-phenotype plants were due to photobleaching caused by a pigment accumulated during the dark but not during a light period in the part of the leaf where extension growth occurs. Light inhibited δ-aminolevulinate accumulation in the presence of levulinate in dark growntig-d 12 andtig-b 23. Glutamate-1-14C was incorporated more efficiently than glycine-2-14C by dark grown leaves oftig-d 12 andtig-b 23 into δ-aminolevulinate. It is concluded that these leaves formed δ-aminolevulinate by the 5-carbon pathway with the δ-aminolevulinate synthesising enzyme. Etioplasts isolated fromtig-d 12 incorporated glutamate-1-14C and α-ketoglutarate-1-14C into δ-aminolevulinate at rates 2 to 3 times that of the wild type. In the dark unregulated δ-aminolevulinate formation intig-d 12 and basal δ-aminolevulinate formation in the wild type was maintained by protein synthesis on cytoplasmic ribosomes. Intig-d 12 δ-aminolevulinate formation was also somewhat dependent on protein synthesis on 70S ribosomes. Heme and protoprphyrin at 5 μm inhibited the partially purified δ-aminolevulinate synthesising enzyme from the wild type by 80% and 27% respectively. In disrupted etioplast preparations light stimulated δ-aminolevulinate synthesising activity. It is suggested that light activates the δ-aminolevulinate synthesising enzyme formed in dark-grown barley seedlings through a photosensitive effector and that heme is a feedback inhibitor.