We have identified three Arabidopsis genes with GAMYB-like activity, AtMYB33, AtMYB65, and AtMYB101, which can substitute for barley (Hordeum vulgare) GAMYB in transactivating the barley ␣-amylase promoter. We have investigated the relationships between gibberellins (GAs), these GAMYB-like genes, and petiole elongation and flowering of Arabidopsis. Within 1 to 2 d of transferring plants from short-to long-day photoperiods, growth rate and erectness of petioles increased, and there were morphological changes at the shoot apex associated with the transition to flowering. These responses were accompanied by accumulation of GAs in the petioles (GA 1 by 11-fold and GA 4 by 3-fold), and an increase in expression of AtMYB33 at the shoot apex. Inhibition of GA biosynthesis using paclobutrazol blocked the petiole elongation induced by long days. Causality was suggested by the finding that, with GA treatment, plants flowered in short days, AtMYB33 expression increased at the shoot apex, and the petioles elongated and grew erect. That AtMYB33 may mediate a GA signaling role in flowering was supported by its ability to bind to a specific 8-bp sequence in the promoter of the floral meristem-identity gene, LEAFY, this same sequence being important in the GA response of the LEAFY promoter. One or more of these AtMYB genes may also play a role in the root tip during germination and, later, in stem tissue. These findings extend our earlier studies of GA signaling in the Gramineae to include a dicot species, Arabidopsis, and indicate that GAMYB-like genes may mediate GA signaling in growth and flowering responses.Gibberellins (GAs) regulate many aspects of plant growth and development. In the seed and seedling these include the production of hydrolytic enzymes, germination, and growth. In the adult plant, GAs are important in leaf and stem elongation, flowering, anther development, and fruit set (Pharis and King, 1985).Two classes of mutants have contributed much to an understanding of GA action (Thornton et al., 1999). One class includes dwarf mutants that are defective in GA biosynthesis. The other class includes response mutants such as Arabidopsis spindly (spy), GA-insensitive (gai), repressor of GA1-3 (rga), and the rice (Oryza sativa) d1 mutant. Many of the genes defined by these mutants have been cloned, but their molecular role in GA signaling is not yet fully understood (Jacobsen et al., 1996; Peng et al., 1997; Silverstone et al., 1998; Ashikari et al., 1999).An alternative approach to understanding GA signal transduction has involved functional studies, particularly with aleurone cells of cereals. These studies have identified a number of early GA signaling steps that precede expression of hydrolytic enzymes such as ␣-amylase. These steps involve heterotrimeric G-proteins (Jones et al., 1998; Ueguchi-Tanaka et al., 2000) and cGMP (Penson et al., 1996), which may in turn control the barley (Hordeum vulgare) HvGAMYB gene, whose expression is induced by GAs (Gubler et al., 1995).HvGAMYB encodes a transcriptional act...

We have identified three Arabidopsis genes with GAMYB-like activity, AtMYB33, AtMYB65, and AtMYB101, which can substitute for barley (Hordeum vulgare) GAMYB in transactivating the barley alpha-amylase promoter. We have investigated the relationships between gibberellins (GAs), these GAMYB-like genes, and petiole elongation and flowering of Arabidopsis. Within 1 to 2 d of transferring plants from short- to long-day photoperiods, growth rate and erectness of petioles increased, and there were morphological changes at the shoot apex associated with the transition to flowering. These responses were accompanied by accumulation of GAs in the petioles (GA(1) by 11-fold and GA(4) by 3-fold), and an increase in expression of AtMYB33 at the shoot apex. Inhibition of GA biosynthesis using paclobutrazol blocked the petiole elongation induced by long days. Causality was suggested by the finding that, with GA treatment, plants flowered in short days, AtMYB33 expression increased at the shoot apex, and the petioles elongated and grew erect. That AtMYB33 may mediate a GA signaling role in flowering was supported by its ability to bind to a specific 8-bp sequence in the promoter of the floral meristem-identity gene, LEAFY, this same sequence being important in the GA response of the LEAFY promoter. One or more of these AtMYB genes may also play a role in the root tip during germination and, later, in stem tissue. These findings extend our earlier studies of GA signaling in the Gramineae to include a dicot species, Arabidopsis, and indicate that GAMYB-like genes may mediate GA signaling in growth and flowering responses.