GAMYB-like Genes, Flowering, and Gibberellin Signaling in Arabidopsis

Abstract

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...

A GAMYB-like gene in tomato and its expression during seed germination

“…The derived amino acid sequence (Fig. 1) also shows that it contains three conserved regions (BOX1, BOX2 and BOX3), which are speciWc characteristics of these GAMYB and GAMYB-like proteins (Gocal et al 2001). A putative miRNA 159 target motif responding to the GA signal is present in the corresponding cDNA between BOX1 and BOX2.…”

“…Tomato GAMYB-like gene Based on the known conserved features of barley, Arabidopsis and rice GAMYB genes (Gubler et al 1995;Gocal et al 2001;Tsuji et al 2006), we used genomic DNA as a template for PCR, with degenerate primers to isolate a partial DNA fragment of the GAMYB-like gene from tomato. A »560-bp DNA fragment was obtained; then the complete sequence of this gene was determined by primer extension based on the sequence of this partial DNA; it was named LeGAMYB-like1 (LeGAMYBL1, GenBank accession no.…”

Regulation of flowering time: all roads lead to Rome

“…In particular, GAMYBs, a family of R2R3 type MYB transcription factors, have been shown to play an important role during germination in cereals. In A. thaliana, AtMYB33, a potential homolog of GAMYBs, was found to be expressed in the shoot apex as a response to endogenous GAs or application of exogenous GAs [150]. In addition, MYB33 protein was shown by EMSA studies to bind to the predicted GA-responsive element in the LFY promoter.…”

Roles for auxin during morphogenesis of the compound leaves of pea ( Pisum sativum )

“…The environmental pathways include long days and vernalization, and others include the autonomous, GA and sucrose pathways. Gibberellin enhances Lfy expression via Myb proteins, which bind to a specific 8-bp sequence in the Lfy promoter and activate it (Bla´zquez et al 1998;Gocal et al 2001). An important characteristic of Lfy's function in directing development is that it does not act alone, but interacts with other elements to regulate downstream targets (Busch et al 1999;Lohmann et al 2001;Parcy et al 2002).…”

“…Only Vernoux et al (2000) have postulated that Lfy is regulated by auxin, since pin1 mutants have reduced Lfy expression and expression of its downstream targets is also reduced; however, they provided no specific mechanism for this. A mechanism for auxin regulation of Lfy/Uni expression might be through GA since both IAA and 4-Cl-IAA regulate GA biosynthesis in pea and a mechanism for GA regulation of Lfy has been established (Bla´zquez et al 1998;Gocal et al 2001). On the other hand, auxin might act downstream of Uni.…”