Wilhelm Rademacher

Inhibition of gibberellin 2β-hydroxylases by acylcyclohexanedione derivatives

Two acyleyclohexanedione growth retardants, 3,5-dioxo-4-butyryl-cyclohexane carboxylic acid ethyl ester and 5-(3,5,-dioxo-4-propionylcyclohexane)-pentanoic acid, were shown to be effective inhibitors of partially purified gibberellin 2β-hydroxylases from imbibed Phaseolus vulgaris seeds. The former, which was the more potent inhibitor, acted competitively with respect to 2-oxoglutarate, a cofactor for the enzymes, when the inhibitor was present at low (< lO μM) concentrations. Both compounds were much more effective than 2,4-, 2,5- and 2,6-dicarboxypyridines, which are known to inhibit prolyl 4-hydroxylase, a mechanistically related enzyme. The free acid was more active than the ethyl ester; the acyl side chain was essential for activity. This free acid was shown by computer modelling to have close structural similarity to 2-oxoglutarate. At superoptimal concentrations of 2-oxoglutarate, low concentrations of the inhibitors stimulated enzyme activity, perhaps by preventing its inactivation by 2-oxoglutarate.

Luteoforol, a flavan 4-ol, is induced in pome fruits by prohexadione-calcium and shows phytoalexin-like properties against Erwinia amylovora and other plant pathogens

Treatments with prohexadione-calcium led to lowered incidence of fire blight, scab and other diseases in pome fruit trees and other crop plants. In addition to acting as a growth regulator, prohexadione-calcium interferes with flavonoid metabolism and induces the accumulation of the 3-deoxycatechin luteoliflavan in shoots of pome fruit trees. Luteoliflavan does not possess any remarkable antimicrobial activity. Therefore luteoforol, its unstable and highly reactive precursor, has been tested in vitro for its bactericidal and fungicidal activities. Luteoforol was found to be highly active against different strains of Erwinia amylovora, the causal agent of fire blight, and all other bacterial and fungal organisms tested. Phytotoxic effects were also observed in pear plantlets. The results obtained indicate that prohexadione-calcium induces luteoforol as an active principle with non-specific biocidal properties. It is proposed that luteoforol is released upon pathogen attack from its cellular compartment and inhibits further disease development by destroying pathogen cells as well as by inducing a hypersensitive-like reaction in the host plant tissue. This mechanism would be closely analogous to the one known for structurally related phytoalexins in sorghum.

The mode of action of acylcyclohexanediones — a new type of growth retardant

Acylcyclohexanediones (also named cyclohexanetriones) represent a new class of plant growth retardants, which include prohexadione (Nakayama et al. 1990), cimectacarb (Kerber et al. 1989) and LAB 198 999 (Rademacher, 1991 a). When applied to plants, these compounds inhibit shoot elongation without affecting the pattern of development and, therefore, act similarly to GA-biosynthesis inhibitors such as chlormequat chloride, ancymidol and paclobutrazol. Consequently, the major potential for practical application would be in control of lodging control in cereals, although they may also be of use in reduction of turf growth and inhibition of excessive vegetative growth in fruit trees.

Induction of antimicrobial 3-deoxyflavonoids in pome fruit trees controls fire blight.

Abstract Fire blight, a devastating bacterial disease in pome fruits, causes severe economic losses worldwide. Hitherto, an effective control could only be achieved by using antibiotics, but this implies potential risks for human health, livestock and environment. A new approach allows transient inhibition of a step in the flavonoid pathway, thereby inducing the formation of a novel antimicrobial 3-deoxyflavonoid controlling fire blight in apple and pear leaves. This compound is closely related to natural phytoalexins in sorghum. The approach does not only provide a safe method to control fire blight: Resistance against different pathogens is also induced in other crop plants.

Occurrence of gibberellins in different species of the fungal genera Sphaceloma and Elsinoe

Abstract Twenty-eight species of the fungal genera Sphaceloma (imperfect stage) and Elsinoe (perfect stage) were investigated for their ability to produce gibberellins. In addition to Sphaceloma manihoticola , considerable amounts of GAs were found in the culture liquids of S. bidentis, S. menthae, S. persae and S. rhois . GA 4 was the predominating GA with concentrations of up to 20 mg l −1 of culture filtrate. GA 13 , GA 14 , GA 24 , GA 9 , GA 15 , GA 25 , GA 36 and GA 37 were also detected at concentrations decreasing in this sequence. No indications could be found of the presence of GA 1 , GA 3 , or GA 7 , indicating that Sphaceloma species, in contrast to Gibberella fujikuroi , are unable to synthesize 1,2-dehydro or 13-hydroxy-GAs. Species of the genus Elsinoe were not found to produce any GA-like activity.