Evolutionarily conserved DELLA-mediated gibberellin signaling in plants

Gibberellins (GAs) play important roles in many essential plant growth and development processes. A family of nuclear growth-repressing DELLA proteins is the key component in GA signaling. GA perception is mediated by GID1, and the key event of GA signaling is the degradation of DELLA proteins via the 26S proteasome pathway. DELLA proteins integrating other plant hormones signaling and environmental cue modulating plant growth and development have been revealed. GA turning on the de-DELLA-repressing system is conserved, and independently establishes step-by-step recruitment of GA-stimulated GID1-DELLA interaction and DELLA growth-repression functions during land plant evolution. These discoveries open new prospects for the understanding of GA action and DELLA-mediated signaling in plants.     

GID1-mediated gibberellin signaling in plants

Gibberellin (GA) perception is mediated by GID1 (GA-INSENSITIVE DWARF1), a receptor that shows similarity to hormone-sensitive lipases. A key event in GA signaling is the degradation of DELLA proteins, which are negative regulators of GA response that interact with GID1 in a GA-dependent manner. This GID1-DELLA GA-perception system is conserved among vascular plants but is not found in the moss Physcomitrella patens. The identification of factors in GA signaling downstream of DELLA and the development of a new concept of DELLA function beyond its role as a repressor of GA signaling are important advances. DELLA proteins appear to have at least two other distinct roles: maintaining GA homeostasis and regulating cross-talk between GA and other plant hormones.     

Organs of gibberellin synthesis in light-grown sunflower plants

The sites of gibberellin (GA) synthesis in light grown sunflower plants were studied. The results of organ excision and the exogenous application of indole acetic acid and gibberellic acid indicated that gibberellin synthesis occurred in the young leaves of the apical bud. This was substantiated using a combination of diffusion and extraction techniques. Diffusion of sunflower apical buds on agar for 20 hours revealed a level of gibberellin greater than that obtained by solvent extraction of a similar number of apices, indicating that synthesis of gibberellin was occurring in those apices during the diffusion period. The gibberellin level of apices extracted following a 20 hour diffusion period was the same as that obtained from buds extracted immediately following excision from the plant, again suggesting that apical buds are sites of gibberellin synthesis. A similar experiment was conducted with young internodal sections, the results indicating that they were not sites of gibberellin synthesis.     

Reproductive toxicity and infertility effect of Ferula hornonis extracts in mice

The anti-fertility, anti-implantation, and ovarian histological alterations of the ethanolic extract of Ferula hormonis have been investigated in female mice. The intragastric application of 3 mg/kg per day of such extract for 6 weeks resulted in a significant reduction in female mice fertility. Furthermore, it caused a decrease in the number of mated females, the total number of implantations, and the number of viable fetuses. These changes were also associated with ovarian atrophy and a concomitant increase in the connective tissue. The ova showed degeneration while most of the ovarian follicles suffered follicular atresia.     

Auxin promotes gibberellin biosynthesis in decapitated tobacco plants

Excision of the apical bud (decapitation) of tobacco (Nicotiana tabacum L.) plants reduced the endogenous levels of indole-3-acetic acid (IAA), gibberellin A20 (GA20), and GA1 (the bioactive GA), in internode tissue below the excision site. Application of IAA to the stump of decapitated plants dramatically increased GA20 content, to a level 3-fold greater than in intact plants. Gibberellin A1 content was also increased by IAA. Decapitation reduced the conversion of [14C]GA19 to [14C]GA20 and of [14C]GA20 to [14C]GA1, and appeared to promote the deactivation pathway [14C]GA20 to [14C]GA29 to [14C]GA29-catabolite. Application of auxin counteracted these effects, but did not restore the conversion of [14C]GA20 to [14C]GA1 to the level found in intact plants. The results indicate that auxin is necessary for normal GA biosynthesis in stems of tobacco.     

Immunomodulation of bioactive gibberellin confers gibberellin-deficient phenotypes in plants

Immunomodulation is a means to modulate an organism's function by antibody production to capture either endogenous or exogenous antigens. This method was applied to plants to repress the function of gibberellins (GAs), a class of phytohormones responsible for plant elongation, by anti-bioactive GA antibodies. Two different antibodies were produced in Arabidopsis as single-chain variable fragment (scFv) fused to green fluorescent protein (GFP) with four different subcellular localizations: endoplasmic reticulum (ER), cytosol, apoplastic space or the outer surface of the plasma membrane. When targeting scFv-GFP to ER, plants showed the highest accumulation of scFv-GFP, with binding activity, strong GFP fluorescence in ER-derived compartments and mild but clear GA-deficient phenotypes, including a smaller leaf size, delayed bolting, shorter inflorescence length and decreased germination. Plants expressing scFv-GFP in ER responded to exogenous GA₄ and contained 15–40 times greater endogenous GA₄ than wild-type plants. They also showed increased gene expression for GA3ox1 , GA20ox1 and GA20ox2 , but decreased expression for GA2ox1 , which are feedback and feedforward regulated by GA signalling, respectively. These results suggest that the level of free functional GA₄ decreased when trapped in the ER with scFv to the extent that mild GA-deficient phenotypes were created. A dramatic increase in the total sum of GA₄ (free plus scFv-GFP bound) was detected as a result of the up-regulation of GA biosynthesis (feedback regulated), and a decrease in GA₄ catabolism as a result of protection by scFv-GFP binding. This study demonstrates that the use of immunomodulation to inhibit the action of bioactive GAs is an effective method of creating GA-deficient plants.     

The role of gibberellin in regulation of dwarf plants development

The effect of exogenously applied gibberellic (GA₃) acid on developmental processes in dwarf pea and dwarf maize seedlings was studied. Plants responded to the phytohormone by accelerated longitudinal growth rate and apparent shortening of developmental phases. Poly(A)-mRNA population isolated from gibberellin-treated pea or maize seedlings exhibited much higher translational activity per mRNA unit in the cell-free wheat germ system when compared with control, untreated plants. Analysis of in vitro translation products made by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS—PAGE) followed by autoradiography and densitometry revealed markedly increased overall intensity of the labelled polypeptide bands in addition to the new protein bands which started to appear in gibberellin-treated pea and maize seedlings while were still not detectable in the control plants of the same age. The banding pattern of translation products programmed by poly(A)-mRNA extracted from 2 days older untreated pea plants resembled that of the gibberellin-treated 2 days younger seedlings. It is concluded that gibberellic acid applied to dwarfs accelerates not exclusively the longitudinal growth of plants but also promotes their transition to the next developmental phases.     

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.     

Silicon nutrition potentiates the antioxidant metabolism of rice plants under iron toxicity

Iron toxicity reduces growth of rice plants in acidic lowlands. Silicon nutrition may alleviate many stresses including heavy metal toxicity in plants. In the present study, the ameliorating effects of silicon nutrition on rice (Oryza sativa L.) plants under toxic Fe levels were investigated. Plants were cultivated in greenhouse in hydroponics under different Fe treatments including 10, 50, 100, and 250 mg L^?1 as Fe-EDTA and silicon nutrition including 0 and 1.5 mM sodium silicate. Iron toxicity imposed significant reduction in plant fresh weight, tiller, and leaf number. The activity of catalase, cell wall, and soluble peroxidases, and polyphenol oxidase in shoots decreased due to moderate Fe toxicity (50 and 100 mg L^?1), but increased at greater Fe concentration. Ascorbate peroxidase activity increased in both roots and shoots of Fe-stressed plants. Iron toxicity led to increased tissue hydrogen peroxide and lipid peroxidation. Silicon nutrition improved plant growth under all Fe treatments and alleviated Fe toxicity symptoms, probably due to lower Fe concentration of Si-treated plants. Silicon application could improve the activity of antioxidant enzymes such as catalase, ascorbate peroxidase, and soluble peroxidase under moderate Fe toxicity, which resulted in greater hydrogen peroxide detoxification and declined lipid peroxidation. Thus, silicon nutrition could ameliorate harmful effects of Fe toxicity possibly through reduction of plant Fe concentration and improvement of antioxidant enzyme activity.     

Mercury toxicity in plants

Mercury poisoning has become a problem of current interest as a result of environmental pollution on a global scale. Natural emissions of mercury form two-thirds of the input; manmade releases form about one-third. Considerable amounts of mercury may be added to agricultural land with sludge, fertilizers, lime, and manures. The most important sources of contaminating agricultural soil have been the use of organic mercurials as a seed-coat dressing to prevent fungal diseases in seeds. In general, the effect of treatment on germination is favorable when recommended dosages are used. Injury to the seed increases in direct proportion to increasing rates of application. The availability of soil mercury to plants is low, and there is a tendency for mercury to accumulate in roots, indicating that the roots serve as a barrier to mercury uptake. Mercury concentration in aboveground parts of plants appears to depend largely on foliar uptake of Hg⁰ volatilized from the soil. Uptake of mercury has been found to be plant specific in bryophytes, lichens, wetland plants, woody plants, and crop plants. Factors affecting plant uptake include soil or sediment organic content, carbon exchange capacity, oxide and carbonate content, redox potential, formulation used, and total metal content. In general, mercury uptake in plants could be related to pollution level. With lower levels of mercury pollution, the amounts in crops are below the permissible levels. Aquatic plants have shown to be bioaccumulators of mercury. Mercury concentrations in the plants (stems and leaves) are always greater when the metal is introduced in organic form. In freshwater aquatic vascular plants, differences in uptake rate depend on the species of plant, seasonal growthrate changes, and the metal ion being absorbed. Some of the mercury emitted from the source into the atmosphere is absorbed by plant leaves and migrates to humus through fallen leaves. Mercury-vapor uptake by leaves of the C₃ speciesoats, barley, and wheat is five times greater than that by leaves of the C₄ species corn, sorghum, and crabgrass. Such differential uptake by C₃ and C₄ species is largely attributable to internal resistance to mercury-vapor binding. Airborne mercury thus seems to contribute significantly to the mercury content of crops and thereby to its intake by humans as food. Accumulation, toxicity response, and mercury distribution differ between plants exposed through shoots or through roots, even when internal mercury concentrations in the treated plants are similar. Throughfall and litterfall play a significant role in the cycling and deposition of mercury. The possible causal mechanisms of mercury toxicity are changes in the permeability of the cell membrane, reactions of sulphydryl (-SH) groups with cations, affinity for reacting with phosphate groups and active groups of ADP or ATP, and replacement of essential ions, mainly major cations. In general, inorganic forms are thought to be more available to plants than are organic ones. Plants can be exposed to mercurials either by direct administration as antifungal agents, mainly to crop plants through seed treatment or foliar spray, or by accident. The end points screened are seed germination, seedling growth, relative growth of roots and shoots, and, in some case, studies of leaf-area index, internode development, and other anatomical characters. Accidental exposures occur through soil, water, and air pollution. The level of toxicity is usually tested under laboratory conditions using a wide range of concentrations and different periods of exposure. Additional parameters include biochemical assays and genetical studies. The absorption of organic and inorganic mercury from soil by plants is low, and there is a barrier to mercury translocation from plant roots to tops. Thus, large increases in mercury levels in soil produce only modest increases in mercury levels in plants by direct uptake from soil. Injuries to cereal seeds caused by organic mercurials has been characterized by abnormal germination and hypertrophy of the roots and coleoptile. Mercury affects both light and dark reactions of photosynthesis. Substitution of the central atom of chlorophyll, magnesium, by mercury in vivo prevents photosynthetic light harvesting in the affected chlorophyll molecules, resulting in a breakdown of photosynthesis. The reaction varies with light intensity. A concentration and time-dependent protective effect of GSH seems to be mediated by the restricted uptake of the metal involving cytoplasmic protein synthesis. Plant cells contain aquaporins, proteins that facilitate the transport of water, in the vacuolar membrane (tonoplast) and the plasma membrane. Many aquaporins are mercury sensitive, and in AQP1 a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a conserved Asn-Pro-Ala motif. At low concentrations mercury has a toxic effect on the degrading capabilities of microorganisms. Sensitivity to the metal can be enhanced by a reduction in pH, and tolerance of mercury by microorganisms has been found to be in the order: total population > nitrogen fixers > nitrifiers. Numerous experiments have been carried out to study the genetic effects of mercury compounds in experimental test systems using a variety of genetic endpoints. The most noticeable and consistent effect is the induction of c-mitosis through disturbance of the spindle activity, resulting in the formation of polyploid and aneuploid cells and c-tumors. Organomercurials have been reported to be 200 times more potent than inorganic mercury. Exposure to inorganic mercury reduces mitotic index in the root-tip cells and increases the frequency of chromosomal aberrations in degrees directly proportional to the concentrations used and to the duration of exposure. The period of recovery after removal of mercury is inversely related to the concentration and duration of exposure. Bacterial plasmids encode resistance systems for toxic metal ions, including Hg²⁺, functioning by energy-dependent efflux of toxic ions through ATPases and chemiosmotic cationproton antiporters. The inducible mercury resistance (mer) operon encodes both a mercuric ion uptake and detoxification enzymes. In gram-negative bacteria a periplasmic protein,MerP, an inner-membrane transport protein,MerT, and a cytoplasmic enzyme, mercuric reductase, theMerA protein, are responsible for the transport of mercuric ions into cells and their reduction to elemental mercury, Hg(II). InThiobacillus ferrooxidans, an acidophilic chemoautotrophic bacterium sensitive to mercury ions, a group of mercury-resistant strains, which volatilize mercury, has been isolated. The entire coding sequence of the mercury-ion resistance gene has been located in a 2.3 kb fragment of chromosomal DNA (encoding 56,000 and 16,000 molecular-weight proteins) from strain E-l 5 ofEscherichia coli. Higher plants andSchizosaccharomyces pombe respond to heavy-metal stress of mercury by synthesizing phytochelatins (PCs) that act as chelators. The strength of Hg(II) binding to glutathione and phytochelatins follows the order: γGlu-Cys-Gly(γGlu-Cys)₂Gly(γGlu-Cys)₃Gly(γGlu-Cys)₄Gly. Suspension cultures of haploid tobacco,Nicotiana tabacum, cells were subjected to ethyl methane sulfonate to raise mercury-tolerant plantlets. HgCl₂-tolerant variants were selected from nitrosoguanidine (NTG)-treated suspension cell cultures of cow pea,Vigna unguiculata, initiated from hypocotyl callus and incubated with 18 ⧎g/ml HgCl₂. Experiments have been carried out to develop mercury-tolerant plants ofHordeum vulgare through previous exposure to low doses of mercury and subsequent planting of the next generation in mercury-contaminated soil. Phytoremediation involves the use of plants to extract, detoxify, and/or sequester environmental pollutants from soil and water. Transgenic plants cleave mercury ions from methylmercury complexes, reduce mercury ions to the metallic form, take up metallic mercury through their roots, and evolve less toxic elemental mercury. Genetically engineered plants contain modified forms of bacterial genes that break down methyl mercury and reduce mercury ions. The first gene successfully inserted into plants wasmerA, which codes for a mercuric ion reductase enzyme, reducing ionic mercury to the less toxic elemental form.MerB codes for an organomercurial lyase protein that cleaves mercury ions from highly toxic methyl mercury compounds. Plants with themerB gene have been shown to detoxify methyl mercury in soil and water. Both genes have been successfully expressed inArabidopsis thaliana, Brassica (mustard),Nicotiana tabacum (tobacco), andLiriodendron tulipifera (tulip poplar). Plants currently being transformed include cattails, wild rice, andSpartina, another wetland plant. The problem of mercury contamination can be reduced appreciably by combining the standard methods of phytoremediation—removal of mercury from polluted areas through scavenger plants—with raising such plants both by routine mutagenesis and by genetic engineering. The different transgenics raised utilizing the two genesmerA andmerB are very hopeful prospects.     

Antimutagenic effect of Eleutherococcus extracts on plants treated with ethylmethane sulfonate

It is established that Eleutherococcus extracts used in weak concentrations decrease the frequency of chromosome mutations induced by ethylmethanesulphonate in onion and barley. As a result the survival rate of plants under conditions of the field experiment increases. It is shown that the fixation of chromosome mutations takes place in the dividing cells of onion roots in the first four hours after treatment with the mutagen. One of the possible mechanisms of defensive Eleutherococcus action (low concentrations) and of mutagenic one (high concentrations) is discussed.     

Shoot growth in Ilex paraguariensis plants grown under varying photosynthetically active radiation is affected through gibberellin levels

The aim of this work was to analyse the response ofNH₄ ⁺ assimilation in leaves of tobacco plants(Nicotiana tabacum L. cv. Tennessee 86), to different Bapplications (B1, 5 M H₃BO₃; B2, 10M H₃BO₃; B3, 20 MH₃BO₃). The plants were grown under controlledenvironmental conditions and received a complete nutrient solution. In thisexperiment, we analysed the foliar concentrations of B andNH₄ ⁺, as well as the corresponding enzymaticactivities: GS,GOGAT, GDH, PEPC; the end products of this assimilation, aminoacids and proteins; and finally the concentration of non-structural sugars. Ourresults indicated that the different B treatments influenced the utilization ofNH₄ ⁺ by tobacco leaves. The B3 treatment registeredthe lowest NH₄ ⁺ concentration, and B1 the highest,due probably to the higher GS, GOGAT and GDH activities registered at B3.Conversely, a decline in the concentration of non-structural sugars wasrecordedat B3. In addition, the high assimilation rate caused a progressiveaccumulationof amino acids as well as proteins, and boosted biomass production in theleaves.     

Overexpression of a novel class of gibberellin 2-oxidases decreases gibberellin levels and creates dwarf plants

Degradation of active C(19)-gibberellins (GAs) by dioxygenases through 2beta-hydroxylation yields inactive GA products. We identified two genes in Arabidopsis (AtGA2ox7 and AtGA2ox8), using an activation-tagging mutant screen, that encode 2beta-hydroxylases. GA levels in both activation-tagged lines were reduced significantly, and the lines displayed dwarf phenotypes typical of mutants with a GA deficiency. Increased expression of either AtGA2ox7 or AtGA2ox8 also caused a dwarf phenotype in tobacco, indicating that the substrates for these enzymes are conserved. AtGA2ox7 and AtGA2ox8 are more similar to each other than to other proteins encoded in the Arabidopsis genome, indicating that they may constitute a separate class of GA-modifying enzymes. Indeed, enzymatic assays demonstrated that AtGA2ox7 and AtGA2ox8 both perform the same GA modification: 2beta-hydroxylation of C(20)-GAs but not of C(19)-GAs. Lines containing increased expression of AtGA2ox8 exhibited a GA dose-response curve for stem elongation similar to that of the biosynthetic mutant ga1-11. Double loss-of-function Atga2ox7 Atga2ox8 mutants had twofold to fourfold higher levels of active GAs and displayed phenotypes associated with excess GAs, such as early bolting in short days, resistance to the GA biosynthesis inhibitor ancymidol, and decreased mRNA levels of AtGA20ox1, a gene in the GA biosynthetic pathway.     

Adventitious staminate flower formation in gibberellin treated gynoecious cucumber plants

Single gibberellin (A₄₊₇) treatments induced the appearanceof staminate floral buds in several consecutive nodes on themain stem of genetically female cucumber (Cucumis sativus L.).The staminate buds appeared next to pistillate buds which showedvarious degrees of degeneration. Similarly, repeated GA treatmentsinduced the appearance of staminate flowers in otherwise strictlyhermaphrodite plants, next to bisexual flowers. However, thebisexual buds, unlike the pistillate ones, did not show anydeleterious effects of the GA treatment. Therefore, it is inferredthat the hormonally induced staminate buds did not develop bysexual reversion of would-be pistillate or bisexual buds, butrather, represent adventitious buds which, in normally grownfemale or hermaphrodite plants, never develop. It thus seemsthat predetermined pistillate or bisexual buds do not changeinto staminate ones, while change in the reverse direction hasbeen demonstrated in the past (at least for the gynoecious ones). The effectiveness of the GA treatment in the gynoecious plantsshowed an acropetal gradient both within the affected region,as well as along the main stem. Autoradiographic histologicalexaminations showed that the course of development of the inducedstaminate floral bud did not differ from that of normally developingbuds. (Received June 16, 1977; )