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Summary The stereospecific 5β‐reduction of progesterone is a required step for cardiac glycoside biosynthesis in foxglove plants. Recently, we have isolated the gene P5βR, and here we investigate the function and regulation of P5βR2, a new progesterone 5β‐reductase gene from Digitalis purpurea. P5βR2 cDNA was isolated from a D. purpurea cDNA library and further characterized at the biochemical, structural and physiological levels. Like P5βR, P5βR2 catalyzes the 5β‐reduction of the Δ4 double bond of several steroids and is present in all plant organs. Under stress conditions or on treatment with chemical elicitors, P5βR expression does not vary, whereas P5βR2 is highly responsive. P5βR2 expression is regulated by ethylene and hydrogen peroxide. The correlation between P5βR2 expression and cardenolide formation demonstrates the key role of this gene in cardenolide biosynthesis, and therefore in the chemical defense of foxglove plants.

The aldo-keto reductase (AKR) superfamily comprises proteins that catalyse mainly the reduction of carbonyl groups or carbon-carbon double bonds of a wide variety of substrates including steroids. Such types of reactions have been proposed to occur in the biosynthetic pathway of the cardiac glycosides produced by Digitalis plants. Two cDNAs encoding leaf-specific AKR proteins (DpAR1 and DpAR2) were isolated from a D. purpurea cDNA library using the rat D 4-3-ketosteroid 5b-reductase clone. Both cDNAs encode 315 amino acid proteins showing 98.4% identity. DpAR proteins present high identities (68-80%) with four Arabidopsis clones and a 67% identity with the aldose/aldehyde reductase from Medicago sativa. A molecular phylogenetic tree suggests that these seven proteins belong to a new subfamily of the AKR superfamily. Southern analysis indicated that DpARs are encoded by a family of at most five genes.RNA-blot analyses demonstrated that the expression of DpAR genes is developmentally regulated and is restricted to leaves. The expression of DpAR genes has also been induced by wounding, elevated salt concentrations, drought stress and heat-shock treatment. The isolated cDNAs were expressed in Escherichia coli and the recombinant proteins purified. The expressed enzymes present reductase activity not only for various sugars but also for steroids, preferring NADH as a cofactor. These studies indicate the presence of plant AKR proteins with ketosteroid reductase activity. The function of the enzymes in cardenolide biosynthesis is discussed.