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

Glaucarubolone, a naturally occurring quassinoid from the root bark of Cast ela polyandra, Simaroubaceae, with a C(8), C(11) hemiacetal bridge, is demo nstrated to be a potent inhibitor of both the auxin-induced component of th e plasma membrane NADH oxidase and of plant cell enlargement in soybean (Gl ycine max), Arabidopsis (Arabidopsis thaliana), tomato (Lycopersicum escule ntum), and Sorghum (Sorghum vulgare). Auxin-stimulated NADH oxidase activit y 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. thalian a seedlings were inhibited half maximally by ca. 10 nM glaucarubolone with significant inhibition of auxin-stimulated growth even at nannomolar glauca rubolone concentrations. Seedlings of A. thaliana and tomato treated with s ublethal concentrations of 1-10 mu M glaucarubolone remained alive but fail ed to elongate for periods of two to several months. Once seedlings recover ed from the effects of the inhibitor, they resumed growth, flowered, and pr oduced fruits. Treated A. thaliana produced viable seeds that germinated an d developed into normal-appearing progeny. In contrast to glaucarubolone, w hich inhibited preferentially auxin-induced growth and plasma membrane NADH oxidase, simalikalactone D, a quassinoid with a C(8), C(13) epoxymethano b ridge, 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 fi ndings provide correlative evidence for a functional role of both the const itutive and the auxin-stimulated plasma membrane NADH oxidase activities in plant cell enlargement.