V.P. Maier

Biosynthesis of limonoids in Citrus seedlings

Abstract Radioactive tracer work showed that nomilinoate A-ring lactone was the predominant, if not the only, limonoid biosynthesized and accumulated in seedlings of lemon, Valencia orange, grapefruit and tangerine. Lemon seedlings were excellent tools for biosynthetic preparation of [14C]nomilin.

Phenylalanine ammonia-lyase activity in citrus fruit tissue cultured in vitro

Abstract The presence of l -phenylalanine ammonia-lyase in citrus fruit tissue cultured in vitro is reported. Changes in the level of activity of this enzyme with time were determined in callus derived from the grape-fruit (Citrus paradisi, Macf. and other citrus species). The influence of light, the substrate and end-products of the enzyme reaction, growth regulators, and cycloheximide on the activity of the enzyme were determined.

Cinnamate hydroxylation and the enzymes leading from phenylpyruvate to p-coumarate synthesis in grapefruit tissues

Abstract Cinnamic acid was converted to p -coumaric acid in grapefruit tissue slices. The rate of the conversion appeared to be much faster in young fruit than in mature fruit. It was shown also that tyrosine is synthesized mainly from p -hydroxyphenylpyruvate by tyrosine : 2 ketoglutarate aminotransferase. The absence of phenylalanine hydroxylase and tyrosine ammonia-lyase suggested strongly that tyrosine is not an effective precursor for the biosynthesis of flavonoids in grapefruit.

Cytokinins Inhibit Abscisic Acid Biosynthesis in Cercospora Rosicola

Summary The fungus Cercospora rosicola produces the plant hormone, abscisic acid (ABA), as a secondary metabolite. We developed C. rosicola into a convenient system to determine the effects of compounds on the biosynthesis of ABA. Another important group of plant hormones, the cytokinins, inhibited ABA biosynthesis in C. rosicola. The inhibition of ABA biosynthesis was concentration dependent and, in studies using farnesyl-[1-14C]pyrophosphate, appeared to be specific rather than resulting from a general blocking of terpenoid biosynthesis. Inhibitors of ABA biosynthesis have not previously been reported. Knowledge that cytokinins have the ability to inhibit ABA biosynthesis in this fungal system indicates the need for further studies to determine if cytokinin inhibition of ABA biosynthesis occurs in plants and is an element of the hormonal action of cytokinins.

Biosynthesis of abscisic acid from [1,2-13C2]acetate in Cercospora rosicola

Abstract [1,2- 13 C 2 ]Sodium acetate was converted to abscisic acid (ABA) by Cerospora rosicola . The labelling pattern, determined by NMR spectroscopy,

Chalcone cyclase and flavonoid biosynthesis in grapefruit

Abstract A chalcone cyclase (CC), which acts unidirectionally upon the chalcone-flavanone equilibrium reaction, was isolated from immature grapefruit. The enzyme required neohesperidose at C-4′ of the chalcone A-ring and a free, unhindered hydroxyl group at C-4 of the B-ring for activity. The CC bound, but did not cyclize, prunin chalcone (Ki= 2.5 × 10−5 M). The results suggest that the intermediates that form the B-ring of chalcones are hydroxylated prior to chalcone formation, that chalcones are glycosylated during their formation, and that methylation occurs after cyclization of the chalcones to flavanones.

Metabolism of abscisic acid: Bacterial conversion to dehydrovomifoliol and vomifoliol dehydrogenase activity

Abstract A species of Corynebacterium , capable of metabolizing abscisic acid (ABA), was isolated from soil. The organism converted ABA to dehydrovomifoliol [(±)-1′-hydroxy-4′-keto-α-ionone] as the major metabolise. A cell-free extract exhibited vomifoliol dehydrogenase activity. This suggests that vomifoliol is most likely the immediate precursor of dehydrovomifoliol.

Detection of limonoate dehydrogenase activity in albedo tissues of Citrus sinensis

Abstract 19-Deoxylimonoic acid 3-methyl- 14 C ester was converted to 17-dehydro-19-deoxylimonoic acid 3-methyl- 14 C ester in albedo tissues of navel oranges. This conversion is direct evidence of limonoate dehydrogenase activity in the tissues.

New bioregulators of gibberellin biosynthesis in Gibberella fujikuroi

Abstract A number of new inhibitors of gibberellin (GA) biosynthesis in Gibberella fujikuroi are reported, including secondary, tertiary and quaternary amines. Octyltrimethylammonium iodide and 3-chloropropyltrimethylammonium iodide were equally as effective as 2-chloroethyltrimethylammonium chloride (CCC). At least two of the other inhibitors reported, diethyloctylamine hydrochloride and octyltriethylammonium iodide, showed a different pattern of accumulation of GAs and kaurenoic acids than did CCC.

17-dehydrolimonoate A-ring lactone: A possible metabolite of limonoate A-ring lactone in Citrus fruits☆

Abstract A new limonoid, 17-dehydrolimonoate A-ring lactone (III), was isolated from orange peel, juice and lemon seeds and seedlings. This compound which is non-bitter appears to be an initial product of limonoate A-ring lactone (II) metabolism. As such it may be the initial intermediate in at least one debittering pathway in citrus fruits.