Functional analysis of linker insertions and point mutations in the alpha-Amy2/54 GA-regulated promoter

Heme oxygenase-1 is involved in nitric oxide- and cGMP-induced α-Amy2/54 gene expression in GA-treated wheat aleurone layers

Here, α-Amy2/54 gene expression was used as a molecular probe to investigate the interrelationship among nitric oxide (NO), cyclic GMP (cGMP), and heme oxygenase-1 (HO-1) in GA-treated wheat aleurone layers. The inducible expressions of α-Amy2/54 and α-amylase activity were respectively amplified by two NO-releasing compounds, sodium nitroprusside (SNP) and spermine NONOate, in a GA-dependent fashion. Similar responses were observed when an inducer of HO-1, hemin-or one of its catalytic products, carbon monoxide (CO) in aqueous solution-was respectively added. The SNP-induced responses, mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), a cGMP derivative, were NO-dependent. This conclusion was supported by the fact that endogenous NO overproduction was rapidly induced by SNP, and thereafter induction of α-Amy2/54 gene expression and increased α-amylase activity were sensitive to the NO scavenger. We further observed that the above induction triggered by SNP and 8-Br-cGMP was partially prevented by zinc protoporphyrin IX (ZnPPIX), an inhibitor of HO-1. These blocking effects were clearly reversed by CO, confirming that the above response was HO-1-specific. Further analyses showed that both SNP and 8-Br-cGMP rapidly up-regulated HO-1 gene expression and increased HO activity, and SNP responses were sensitive to cPTIO and the guanylate cyclase inhibitor 6-anilino-5,8-quinolinedione (LY83583). Molecular evidence confirmed that GA-induced GAMYB and ABA-triggered PKABA1 transcripts were up-regulated or down-regulated by SNP, 8-Br-cGMP or CO cotreated with GA. Contrasting changes were observed when cPTIO, LY83583, or ZnPPIX was added. Together, our results suggested that HO-1 is involved in NO- and cGMP-induced α-Amy2/54 gene expression in GA-treated aleurone layers.

Molecular mechanism of gibberellin signaling in plants

The hormone gibberellin (GA) plays an important role in modulating diverse processes throughout plant development. In recent years, significant progress has been made in the identification of upstream GA signaling components and trans- and cis-acting factors that regulate downstream GA-responsive genes in higher plants. GA appears to derepress its signaling pathway by inducing proteolysis of GA signaling repressors (the DELLA proteins). Recent evidence indicates that the DELLA proteins are targeted for degradation by an E3 ubiquitin ligase SCF complex through the ubiquitin-26S proteasome pathway.

SAD: a new DOF protein from barley that activates transcription of a cathepsin B-like thiol protease gene in the aleurone of germinating seeds

Functional analysis of hydrolase gene promoters, induced by gibberellin (GA) in aleurone cells following germination, has identified a GA-responsive complex (GARC) as a tripartite element containing a pyrimidine-box motif 5'-CCTTTT-3'. We describe here the characterization of a new barley gene (Sad gene) encoding a transcription factor (SAD) of the DNA binding with One Finger (DOF) class that binds to the pyrimidine box in vitro and activates transcription of a GA-induced protease promoter in bombarded aleurone layers. RT-PCR and in situ hybridization analyses showed that the Sad transcripts accumulated in all tissues analysed, being especially abundant in the scutellum and aleurone cells upon seed germination. The SAD protein, expressed in bacteria, binds in a specific manner to two oligonucleotides containing the sequence 5'-G/CCTTTT/C-3', derived from the promoter region of the Al21 gene encoding a cathepsin B-like cysteine protease. Although the Sad transcript accumulation did not respond to external GA-incubation in aleurone cells, in transient expression experiments in co-bombarded aleurone layers, SAD trans-activated transcription from the Al21 promoter in a similar manner as did GAMYB, a MYB protein previously shown to respond to GA and to activate several hydrolase gene promoters in barley aleurone cells. In vivo interaction between the GAMYB and SAD proteins was shown in the yeast two-hybrid system, where GAMYB potentiates the SAD trans-activation capacity through interaction with its C-terminal domain.

Involvement of multiple cis-elements in the regulation of GA responsive promoters: Definition of a new cis-element in the Amy32b gene promoter of barley (Hordeum vulgare)

The Amy32b gene is a member of the low-pI alpha-amylase gene family of barley, whose expression is tightly regulated by hormones in the aleurone layer. Four cis-elements are known to be important for the GA induction of this gene: GARE, amylase box, pyrimidine box, and O2S. These sequences are located between -101 and -149 relative to the transcription start site. In the present work, we have created a series of Amy32b promoter-GUS reporter constructs introducing mutations in the -79 to -93 region. Using a transient expression system, we have functionally defined an additional region (-81 to -89) essential for the GA activation of the Amy32b promoter. This region is highly conserved among barley, wheat, and wild oat low-pI alpha-amylase promoters. Interestingly, in contrast with the variability in the relative distances among other cis-elements, this region maintains a nearly constant distance to GARE, which suggests that the function of these elements might be coupled. The involvement of this and other sequences in the transactivation of Amy32b by a transcription factor, GAMyb, has also been studied. Our results indicate that the only indispensable element for the GAMyb transactivation of the alpha-amylase promoter is GARE. The present work brings new evidence to the proposed model that considers the GAMyb-GARE interaction as a critical point for the GA induction of alpha-amylase genes, but also strengthens the notion that multiple sequences are required for full regulation of alpha-amylase promoters.

Gibbestatin B inhibits the GA-induced expression of alpha-amylase expression in cereal seeds

The expression of alpha-amylase in aleurone layers of barley is known to be induced by gibberellin A3 (GA). In the present study, gibbestatin B (GNB) was isolated from Streptomyces sp. C-39 as an inhibitor of the GA-induced expression of alpha-amylase in barley and rice, with IC50 values of 125 and 70 microM, respectively. GNB suppressed accumulation of GA-induced barley high-pI type B and rice RAmylA alpha-amylase transcripts. However, GNB showed no inhibitory activity on GUS expression in transgenic tobacco harboring the auxin-inducible par B promoter:: GUS fusion gene. The transcription of an abscisic acid (ABA)-inducible gene, HVA1, was unaffected by GNB. In addition, GNB prevented aleurone cells from cell death induced by GA. In tobacco and Arabidopsis plants, GNB suppressed the germination and retarded the growth of seedlings without toxicity. The growth of gai, spy and abi mutants was also retarded by GNB. Normal plants treated with GA-biosynthesis inhibitors and GA-defective and GA-signaling mutants normally have dwarf dark green leaves. However, dwarfed healthy green leaves were observed in normal plants treated with GNB. GA-induced stem elongation of plants was also detected in the presence of GNB. These analyses indicate that GNB inhibits the GA-induced expression of alpha-amylase by regulating one of the steps involved in ABA signaling, but not by acting as a weak ABA analog.

Gibberellin and abscisic acid signalling in aleurone

The plant hormones gibberellin and abscisic acid regulate gene expression, secretion and cell death in aleurone. The emerging picture is of gibberellin perception at the plasma membrane whereas abscisic acid acts at both the plasma membrane and in the cytoplasm - although gibberellin and abscisic acid receptors have yet to be identified. A range of downstream-signalling components and events has been implicated in gibberellin and abscisic acid signalling in aleurone. These include the Galpha subunit of a heterotrimeric G protein, a transient elevation in cGMP, Ca2+-dependent and Ca2+-independent events in the cytoplasm, reversible protein phosphory-lation, and several promoter cis-elements and transcription factors, including GAMYB. In parallel, molecular genetic studies on mutants of Arabidopsis that show defects in responses to these hormones have identified components of gibberellin and abscisic acid signalling. These two approaches are yielding results that raise the possibility that specific gibberellin and abscisic acid signalling components perform similar functions in aleurone and other tissues.

Target genes and regulatory domains of the GAMYB transcriptional activator in cereal aleurone

GAMYB is an MYB transcription factor which is expressed in cereal aleurone cells in response to gibberellin (GA). HvGAMYB binds to the TAACAAA box of a high-pl alpha-amylase gene promoter and transcriptionally activates its expression. In this study, we examined the role of HvGAMYB in activating expression of other GA-regulated genes encoding hydrolytic enzymes. In transient expression experiments, HvGAMYB transactivated expression of reporter genes fused to a low-pl alpha-amylase gene promoter, an EII (1-3, 1-4)-beta-glucanase gene promoter and a cathepsin B-like protease promoter. HvGAMYB DNA binding specificity was determined using a PCR-based random site selection using HvGAMYB fusion protein isolated from E. coli. The deduced consensus closely resembled gibberellin response elements in alpha-amylase promoters. Functional analysis of HvGAMYB by transient expression of C terminal HvGAMYB deletions in barley aleurone cells identified two transcriptional activation domains (TADs) which function in transcriptional regulation of both high- and low-pl alpha-amylase promoters. The same TADs were identified using a heterologous yeast expression system. Together, these results indicate that HvGAMYB has two TADs. These domains are C-terminal to its DNA-binding domain.

Members of a new family of DNA-binding proteins bind to a conserved cis-element in the promoters of alpha-Amy2 genes

The promoters of wheat, barley and wild oat alpha-Amy2 genes contain a number of conserved cis-acting elements that bind nuclear protein, we report here the isolation of two cDNAs encoding proteins (ABF1 and ABF2) that bind specifically to one of these elements, Box 2 (ATTGACTTGACCGTCATCGG). The two proteins are unrelated to each other except for a conserved region of 56-58 amino acids that consists of 25 highly conserved amino acids followed by a putative zinc finger motif, C-X4-5-C-X22-23-H-X1-H. ABF1 contains two such conserved regions, whereas ABF2 possesses only one but also contains a potential leucine zipper motif, suggesting that it could form homo- or heterodimers. ABF1 and ABF2 expressed in Escherichia coli bound specifically to Box 2 probes in gel retardation experiments; this binding was abolished by the transition-metal-chelating agent, 1,10-o-phenanthroline and by EDTA. We propose that ABF1 and ABF2 are representatives of two classes of a new family of plant sequence-specific DNA-binding proteins.