Glaucarubolone and simalikalactone D, respectively, preferentially inhibit auxin-induced and constitutive components of plant cell enlargement and the plasma membrane NADH oxidase

Glaucarubolone, a naturally occurring quassinoid from the root bark of Castela polyandra, Simaroubaceae, with a C(8), C(11) hemiacetal bridge, is demonstrated to be a potent inhibitor of both the auxin-induced component of the plasma membrane NADH oxidase and of plant cell enlargement in soybean (Glycine max), Arabidopsis (Arabidopsis thaliana), tomato (Lycopersicum esculentum), and Sorghum (Sorghum vulgare). Auxin-stimulated NADH oxidase activity of isolated vesicles of soybean, A. thaliana, and tomato plasma membranes were inhibited half maximally by ca. 0.1 nM glaucarubolone. Auxin-induced enlargement of stem segments of soybean and elongation growth of A. thaliana seedlings were inhibited half maximally by ca. 10 nM glaucarubolone with significant inhibition of auxin-stimulated growth even at nannomolar glaucarubolone concentrations. Seedlings of A. thaliana and tomato treated with sublethal concentrations of 1-10 mu M glaucarubolone remained alive but failed to elongate for periods of two to several months. Once seedlings recovered from the effects of the inhibitor, they resumed growth, flowered, and produced fruits. Treated A. thaliana produced viable seeds that germinated and developed into normal-appearing progeny. In contrast to glaucarubolone, which inhibited preferentially auxin-induced growth and plasma membrane NADH oxidase, simalikalactone D, a quassinoid with a C(8), C(13) epoxymethano bridge, inhibited the constitutive NADH oxidase activity and growth and had little or no effect on the 2,4-D-responsive NADH oxidase and growth. The findings provide correlative evidence for a functional role of both the constitutive and the auxin-stimulated plasma membrane NADH oxidase activities in plant cell enlargement.