Reduced gibberellin response affects ethylene biosynthesis and responsiveness in the Arabidopsis gai eto2-1 double mutant

Ethylene and gibberellins (GAs) control similar developmental processes in plants. The role of ethylene is at least in part to regulate the accumulation of DELLA proteins, key regulators of plant growth, which suppress the GA response. To expand our knowledge of ethylene-GA crosstalk and to reveal how the modulation of the ethylene and GA pathways affects global plant growth, the gibberellin-insensitive (gai), ethylene-overproducing 2-1 (eto2-1) double mutant, which has decreased GA signalling (resulting from gai) and increased ethylene biosynthesis (resulting from eto2-1), was characterized. Both single mutations resulted in reduced elongation growth. The double mutant showed synergistic responses in root and shoot growth, in induction of floral transition, and in inflorescence length, showing that crosstalk between the two pathways occurs in different plant organs throughout development. Furthermore, the altered ethylene-GA interactions affected root-shoot communication, as evidenced by an enhanced shoot:root ratio in the double mutant. When compared with both single mutants and the wild type, double mutants had enhanced content of active GA(4) at both the seedling and the rosette stages, and, unlike the gai mutant, they were sensitive to GA treatment. Finally, it was shown that synergistic responses in the double mutant were not caused by elevated ethylene biosynthesis but that, in the light, enhanced sensitivity to ethylene may, at least in part, be responsible for the observed phenotype.     

Temperature-regulated expression of a glycine-rich RNA-binding protein in the halotolerant alga Dunaliella salina

Acclimation of the halotolerant alga Dunaliella salina to low temperature induced the accumulation of a 12.4 kDa protein (DsGRP-1) and reduction of a 13.1 kDa protein (DsGRP-2). DsGRP-1 and DsGRP-2 are boiling-stable proteins that are localised in the cytoplasm, as revealed by sub-cellular fractionation and by immuno-localisation. The proteins were partially purified and their corresponding genes were cloned. The predicted sequences are homologous to Glycine-Rich RNA-binding Proteins (GRPs) from plants and cyanobacteria. The nucleotide sequences of grp1 and grp2 differ in a short insert encoding 9 amino acids in the glycine-rich domain of DsGRP-2. grp2 contains a single intron at position 179 indicating that DsGRP-1 and DsGRP-2 are not derived from alternative splicing of a common gene. The level of grp mRNA increased at 7 degrees C and was rapidly depressed at 24 degrees C. Analysis of binding to ribonucleotide homopolymers revealed that DsGRP-1 and DsGRP-2 bind preferentially to poly-G and to poly-U indicating that they are RNA-binding proteins. It is proposed that DsGRP-1 and DsGRP-2 are encoded by distinct genes which are differentially regulated by temperature.     

Ectopic expression of pumpkin gibberellin oxidases alters gibberellin biosynthesis and development of transgenic Arabidopsis plants

Immature pumpkin (Cucurbita maxima) seeds contain gibberellin (GA) oxidases with unique catalytic properties resulting in GAs of unknown function for plant growth and development. Overexpression of pumpkin GA 7-oxidase (CmGA7ox) in Arabidopsis (Arabidopsis thaliana) resulted in seedlings with elongated roots, taller plants that flower earlier with only a little increase in bioactive GA4 levels compared to control plants. In the same way, overexpression of the pumpkin GA 3-oxidase1 (CmGA3ox1) resulted in a GA overdose phenotype with increased levels of endogenous GA4. This indicates that, in Arabidopsis, 7-oxidation and 3-oxidation are rate-limiting steps in GA plant hormone biosynthesis that control plant development. With an opposite effect, overexpression of pumpkin seed-specific GA 20-oxidase1 (CmGA20ox1) in Arabidopsis resulted in dwarfed plants that flower late with reduced levels of GA4 and increased levels of physiological inactive GA17 and GA25 and unexpected GA34 levels. Severe dwarfed plants were obtained by overexpression of the pumpkin GA 2-oxidase1 (CmGA2ox1) in Arabidopsis. This dramatic change in phenotype was accompanied by a considerable decrease in the levels of bioactive GA4 and an increase in the corresponding inactivation product GA34 in comparison to control plants. In this study, we demonstrate the potential of four pumpkin GA oxidase-encoding genes to modulate the GA plant hormone pool and alter plant stature and development.     

Glycine-rich RNA-binding protein 7 affects abiotic stress responses by regulating stomata opening and closing in Arabidopsis thaliana

Despite the fact that glycine-rich RNA-binding proteins (GRPs) have been implicated in the responses of plants to environmental stresses, their physiological functions and mechanisms of action in stress responses remain largely unknown. Here, we assessed the functional roles of GRP7, one of the eight GRP family members in Arabidopsis thaliana , on seed germination, seedling growth, and stress tolerance under high salinity, drought, or cold stress conditions. The transgenic Arabidopsis plants overexpressing GRP7 under the control of the cauliflower mosaic virus 35S promoter displayed retarded germination and poorer seedling growth compared with the wild-type plants and T-DNA insertional mutant lines under high salinity or dehydration stress conditions. By contrast, GRP7 overexpression conferred freezing tolerance in Arabidopsis plants. GRP7 is expressed abundantly in the guard cells, and has been shown to influence the opening and closing of the stomata, in accordance with the prevailing stress conditions. GRP7 is localized to both the nucleus and the cytoplasm, and is involved in the export of mRNAs from the nucleus to the cytoplasm under cold stress conditions. Collectively, these results provide compelling evidence that GRP7 affects the growth and stress tolerance of Arabidopsis plants under high salt and dehydration stress conditions, and also confers freezing tolerance, particularly via the regulation of stomatal opening and closing in the guard cells.     

Double-stranded RNA-binding protein DRB3 negatively regulates anthocyanin biosynthesis by modulating PAP1 expression in Arabidopsis thaliana

Journal of Plant Research - The model plant Arabidopsis thaliana has five double-stranded RNA-binding proteins (DRB1-DRB5), two of which, DRB1 and DRB4, are well characterized. In contrast, the...     

The Arabidopsis GA1 locus encodes the cyclase ent-kaurene synthetase A of gibberellin biosynthesis.

The first committed step in the gibberellin (GA) biosynthetic pathway is the conversion of geranylgeranyl pyrophosphate (GGPP) through copalyl pyrophosphate (CPP) to ent-kaurene catalyzed by ent-kaurene synthetases A and B. The ga1 mutants of Arabidopsis are gibberellin-responsive male-sterile dwarfs. Biochemical studies indicate that biosynthesis of GAs in the ga1 mutants is blocked prior to the synthesis of ent-kaurene. The GA1 locus was cloned previously using the technique of genomic subtraction. Here, we report the isolation of a nearly full-length GA1 cDNA clone from wild-type Arabidopsis. This cDNA clone encodes an active protein and is able to complement the dwarf phenotype in ga1-3 mutants by Agrobacterium-mediated transformation. In Escherichia coli cells that express both the Arabidopsis GA1 gene and the Erwinia uredovora gene encoding GGPP synthase, CPP was accumulated. This result indicates that the GA1 gene encodes the enzyme ent-kaurene synthetase A, which catalyzes the conversion of GGPP to CPP. Subcellular localization of the GA1 protein was studied using 35S-labeled GA1 protein and isolated pea chloroplasts. The results showed that the GA1 protein is imported into and processed in pea chloroplasts in vitro.     

Evidence that the Arabidopsis nuclear gibberellin signalling protein GAI is not destabilised by gibberellin

Plant growth is regulated by bioactive gibberellin (GA), although there is an unexplained diversity in the magnitude of the GA responses exhibited by different plant species. GA acts via a group of orthologous proteins known as the DELLA proteins. The Arabidopsis genome contains genes encoding five different DELLA proteins, the best known of which are GAI and RGA. The DELLA proteins are thought to act as repressors of GA-regulated processes, whilst GA is thought to act as a negative regulator of DELLA protein function. Recent experiments have shown that GA induces rapid disappearance of nuclear RGA, SLR1 and SLN1 (DELLA proteins from rice and barley), suggesting that GA signalling and degradation of DELLA proteins are coupled. However, RGL1, another Arabidopsis DELLA protein, does not disappear from the nucleus in response to GA treatment. Here, we present evidence suggesting that GAI, like RGL1, is stable in response to GA treatment, and show that transgenic Arabidopsis plants containing constructs that enable high-level expression of GAI exhibit a dwarf, GA non-responsive phenotype. Thus, GAI appears to be less affected by GA than RGA, SLR1 or SLN1. We also show that neither of the two putative nuclear localisation signals contained in DELLA proteins are individually necessary for nuclear localisation of GAI. The various DELLA proteins have different properties, and we suggest that this functional diversity may explain, at least in part, why plant species differ widely in their GA response magnitudes.     

A glycine-rich RNA-binding protein gene is differentially expressed during acute hypersensitive response following Tobacco Mosaic Virus infection in tobacco

During efforts for cloning disease resistance-responsive genes, a cDNA encoding a putative Nicotiana glutinosa glycine-rich RNA binding protein (ngRBP) was isolated from TMV induced cDNA library. Northern blot hybridization revealed that ngRBP gene is negatively regulated during early hours of TMV induced acute hypersensitive response (HR). Under greenhouse conditions induced expression of ngRBP gene was observed after 24 h following TMV infection. Salicylic acid and copper also induced ngRBP mRNA expression. Our findings are suggestive of some possible role for ngRBP in plant-pathogen interaction.     

cDNA encoding a wheat (Triticum aestivum cv. Chinese Spring) glycine-rich RNA-binding protein

A wheat cDNA encoding a glycine-rich RNA-binding protein, whGRP-1, was isolated. WhGRP-1 contains two conserved domains, the RNA-binding motif (RNP motif) combined with a series of glycine-rich imperfect repeats, characteristic of a conserved family of plant RNA-binding proteins. Northern analysis revealed that whGRP-1 mRNA accumulates to high levels in roots and to lower levels in leaves of wheat seedlings. whGRP-1 mRNA accumulation is not enhanced by exogenous abscisic acid in seedlings and accumulates to very high levels during wheat embryo development, showing a pattern different from that of the ABA-inducible wheat Em gene.     

Independent control of gibberellin biosynthesis and flowering time by the circadian clock in Arabidopsis

Flowering of the facultative long-day plant Arabidopsis is controlled by several endogenous and environmental factors, among them gibberellins (GAs) and day length. The promotion of flowering by long days involves an endogenous clock that interacts with light cues provided by the environment. Light, and specifically photoperiod, is also known to regulate the biosynthesis of GAs, but the effects of GAs and photoperiod on flowering are at least partially separable. Here, we have used a short-period mutant, toc1, to investigate the role of the circadian clock in the control of flowering time by GAs and photoperiod. We show that toc1 affects expression of several floral regulators and a GA biosynthetic gene, but that these effects are independent.