Regulation of hormone metabolism in Arabidopsis seeds: phytochrome regulation of abscisic acid metabolism and abscisic acid regulation of gibberellin metabolism

In a wide range of plant species, seed germination is regulated antagonistically by two plant hormones, abscisic acid (ABA) and gibberellin (GA). In the present study, we have revealed that ABA metabolism (both biosynthesis and inactivation) was phytochrome-regulated in an opposite fashion to GA metabolism during photoreversible seed germination in Arabidopsis. Endogenous ABA levels were decreased by irradiation with a red (R) light pulse in dark-imbibed seeds pre-treated with a far-red (FR) light pulse, and the reduction in ABA levels in response to R light was inhibited in a phytochrome B (PHYB)-deficient mutant. Expression of an ABA biosynthesis gene, AtNCED6, and the inactivation gene, CYP707A2, was regulated in a photoreversible manner, suggesting a key role for the genes in PHYB-mediated regulation of ABA metabolism. Abscisic acid-deficient mutants such as nced6-1, aba2-2 and aao3-4 exhibited an enhanced ability to germinate relative to wild type when imbibed in the dark after irradiation with an FR light pulse. In addition, the ability to synthesize GA was improved in the aba2-2 mutant compared with wild type during dark-imbibition after an FR light pulse. Activation of GA biosynthesis in the aba2-2 mutant was also observed during seed development. These data indicate that ABA is involved in the suppression of GA biosynthesis in both imbibed and developing seeds. Spatial expression patterns of the AtABA2 and AAO3 genes, responsible for last two steps of ABA biosynthesis, were distinct from that of the GA biosynthesis gene, AtGA3ox2, in both imbibed and developing seeds, suggesting that biosynthesis of ABA and GA in seeds occurs in different cell types.     

Evidence for a rapid effect of abscisic acid on amino acid metabolism in Lemna

Abscisic acid (ABA) causes an increase in free amino acid content of the fronds of Lemna minor, L. paucicostata and L. polyrrhiza within 6–12 h. This increase is followed by a temporary decline and a further, slower rise. Within 3 h of transfer to ABA, a substantial increase in medium amino nitrogen was detected indicating an increased rate of efflux, and possibly implying effects on membrane permeability.     

Dynamic metabolic control: towards precision engineering of metabolism

Advances in metabolic engineering have led to the synthesis of a wide variety of valuable chemicals in microorganisms. The key to commercializing these processes is the improvement of titer, productivity, yield, and robustness. Traditional approaches to enhancing production use the “push–pull-block” strategy that modulates enzyme expression under static control. However, strains are often optimized for specific laboratory set-up and are sensitive to environmental fluctuations. Exposure to sub-optimal growth conditions during large-scale fermentation often reduces their production capacity. Moreover, static control of engineered pathways may imbalance cofactors or cause the accumulation of toxic intermediates, which imposes burden on the host and results in decreased production. To overcome these problems, the last decade has witnessed the emergence of a new technology that uses synthetic regulation to control heterologous pathways dynamically, in ways akin to regulatory networks found in nature. Here, we review natural metabolic control strategies and recent developments in how they inspire the engineering of dynamically regulated pathways. We further discuss the challenges of designing and engineering dynamic control and highlight how model-based design can provide a powerful formalism to engineer dynamic control circuits, which together with the tools of synthetic biology, can work to enhance microbial production.     

Regulation of dormancy in barley by blue light and after-ripening: effects on abscisic acid and gibberellin metabolism

White light strongly promotes dormancy in freshly harvested cereal grains, whereas dark and after-ripening have the opposite effect. We have analyzed the interaction of light and after-ripening on abscisic acid (ABA) and gibberellin (GA) metabolism genes and dormancy in barley (Hordeum vulgare 'Betzes'). Analysis of gene expression in imbibed barley grains shows that different ABA metabolism genes are targeted by white light and after-ripening. Of the genes examined, white light promotes the expression of an ABA biosynthetic gene, HvNCED1, in embryos. Consistent with this result, enzyme-linked immunosorbent assays show that dormant grains imbibed under white light have higher embryo ABA content than grains imbibed in the dark. After-ripening has no effect on expression of ABA biosynthesis genes, but promotes expression of an ABA catabolism gene (HvABA8'OH1), a GA biosynthetic gene (HvGA3ox2), and a GA catabolic gene (HvGA2ox3) following imbibition. Blue light mimics the effects of white light on germination, ABA levels, and expression of GA and ABA metabolism genes. Red and far-red light have no effect on germination, ABA levels, or HvNCED1. RNA interference experiments in transgenic barley plants support a role of HvABA8'OH1 in dormancy release. Reduced HvABA8'OH1 expression in transgenic HvABA8'OH1 RNAi grains results in higher levels of ABA and increased dormancy compared to nontransgenic grains.     

New developments in abscisic acid perception and metabolism

Abscisic acid is a powerful signaling molecule that accumulates in response to abiotic stress. However, no potential receptors that could perceive this increase in abscisic acid had been identified until recent reports of three abscisic acid binding proteins: the nuclear protein Flowering Time Control Locus A, the chloroplast protein Magnesium Protoporphyrin-IX Chelatase H subunit, and the membrane-associated protein G Protein Coupled Receptor 2. Abscisic acid metabolism also has a new and prominent component with the identification of a beta-glucosidase capable of releasing biologically active abscisic acid from inactive abscisic acid-glucose ester in a stress-inducible manner. These observations refocus our attention on the metabolism underlying abscisic acid accumulation, sites of abscisic acid perception, and delivery of abscisic acid to those sites.     

Effects of abscisic acid on nucleic acid metabolism in maize coleoptiles

Summary Following treatment with ABA an inhibition of total RNA synthesis was observed after 30 hours. Total soluble ribonuclease activity did not change during the first 8 hours, after which an increase could be observed.Separation of nucleic acids with polyacrylamide gel electrophoresis indicated that synthesis of soluble RNA was less inhibited by ABA than synthesis of ribosomal RNA.Effects of 5-FU and ABA on ribosomal RNA precursor were investigated. It could be shown that 5-FU did not inhibit ribosomal precursor synthesis, but that ABA did so.     

The metabolism of abscisic acid in flacca,a wilty mutant of tomato

The wilty tomato mutant flacca and the normal variety Rheinlands Ruhm were used in this research. The mutant phenotype was explained mainly by hormonal changes. One of these, the decrease in abscisic acid level, was suggested as the hormonal change closest to the mutated gene. The cause of the lower abscisic acid level in the mutant, which may be enhanced breakdown or inactivation, or inhibited biosynthesis, was investigated. The first possibility was studied by comparing mutant and normal plants treated with t-abscisic acid-2-C¹⁴ for (1) rate of production of labeled methanol-extractable metabolites and (2) radioactivity remaining in the methanol-unextractable fraction. The level of trans, trans-abscisic acid relative to that of cis,trans-abscisic acid was studied in untreated plants. Only two radioactive regions containing metabolites of abscisic acid were detected from either of the plant types, and their rates of production relative to total radioactivity was equal. The radioactivity in the methanolunextractable fraction and the level of trans,trans-abscisic acid were very low in both mutant and normal plants. The second possibility was studied partly by comparing the levels of various xanthophylls in mutant and normal plants and their effect after illumination on cress seed germination. Xanthophylls of both plant types were identical in their absorption spectra, but their levels were higher in the mutant. Of these xanthophylls, illuminated neoxanthin inhibited seed germination in both plant types, but more effectively in the mutant. The most probable explanation for the low level of cis,trans-abscisic acid in flacca is that the conversion of farnesyl PiP to abscisic acid is inhibited in this plant.     

脱落酸调节植物抵御水分胁迫的机制研究

干旱、盐渍、低温等均可导致植物可利用水分的亏缺,表现为水分胁迫。植物感受到水分胁迫,诱导脱落酸（abscisic acid,ABA）生物合成。ABA可通过促使气孔关闭或抑制气孔开放,使作物尽可能地降低蒸腾失水,以抵御水分胁迫。该文就植物激素ABA及其下游信号过氧化氢（hydrogenperoxide,H2O2）、一氧化氮（nitric oxide,NO）以及Ca2＋等在植物气孔运动调节方面的研究进展进行概述,以构建水分胁迫下ABA调节植物气孔运动的可能模式。     

Regulation of abscisic acid translocation during embryo maturation of Phaseolus vulgaris

When R, S [2-¹⁴C] abscisic acid (ABA) was applied to the leaf of Phaseolus vulgaris , a part of the radioactivity was always found 24 h later in the only pod left on the plant. In early podfill, a large part of the labeled material was found in the maternal tissues while in late podfill, most had migrated to the embryos. During embryogenesis, embryo cells became more and more alkaline with respect to the seed coat cells. These results suggest that the distribution of ABA within the tissue is regulated by the pH differential between the two compartments.
The decrease of endogenous ABA level observed in situ during the second part of the embryo development therefore cannot be explained only in terms of the passive diffusion of the undissociated species (ABA-H). The empty-ovules technique revealed that ABA was only partly affected by an inversion of the pH gradient and that metabolic inhibitors influenced the release of ABA. These results indicate that in addition to a diffusive path, an energy-dependent component was involved.     

Regulation of programmed cell death in maize endosperm by abscisic acid

Cereal endosperm undergoes programmed cell death (PCD) during its development, a process that is controlled, in part, by ethylene. Whether other hormones influence endosperm PCD has not been investigated. Abscisic acid (ABA) plays an essential role during late seed development that enables an embryo to survive desiccation. To examine whether ABA is also involved in regulating the onset of PCD during endosperm development, we have used genetic and biochemical means to disrupt ABA biosynthesis or perception during maize kernel development. The onset and progression of cell death, as determined by viability staining and the appearance of internucleosomal DNA fragmentation, was accelerated in developing endosperm of ABA-insensitive vp1 and ABA-deficient vp9 mutants. Ethylene was synthesized in vp1 and vp9 mutant kernels at levels that were 2–4-fold higher than in wild-type kernels. Moreover, the increase and timing of ethylene production correlated with the premature onset and accelerated progression of internucleosomal fragmentation in these mutants. Treatment of developing wild-type endosperm with fluridone, an inhibitor of ABA biosynthesis, recapitulated the increase in ethylene production and accelerated execution of the PCD program that was observed in the ABA mutant kernels. These data suggest that a balance between ABA and ethylene establishes the appropriate onset and progression of programmed cell death during maize endosperm development.     

A radioimmunoassay for abscisic acid

We have developed a radioimmunoassay (RIA) for abscisic acid (ABA) in the 0.1 ng to 2.5 ng range. Antibodies were obtained from rabbits immunized with ABA bound via its carboxyl group to bovine serum albumin. Cross-reactivity studies indicate that ABA esters are completely cross-reactive with ABA, while trans, trans abscisic acid (t-ABA) phaseic acid (PA) and dihydrophaseic acid (DPA) have much lower but significant cross-reactivities. Purification methods which reduce the levels of cross-reacting substances are described.Abbreviations RIA radioimmunoassay - DPA 4-dihydrophaseic acid - PA phaseic acid - GC gas chromatography - HPLC high performance liquid chromatography - TLC thin-layer chromatography - BSA bovine serum albumin - ABA abscisic acid - t-ABA trans, trans abscisic acid - IAA indoleacetic acid     

Regulation of abscisic acid signaling by the ethylene response pathway in Arabidopsis

Although abscisic acid (ABA) is involved in a variety of plant growth and developmental processes, few genes that actually regulate the transduction of the ABA signal into a cellular response have been identified. In an attempt to determine negative regulators of ABA signaling, we identified mutants, designated enhanced response to ABA3 (era3), that increased the sensitivity of the seed to ABA. Biochemical and molecular analyses demonstrated that era3 mutants overaccumulate ABA, suggesting that era3 is a negative regulator of ABA synthesis. Subsequent genetic analysis of era3 alleles, however, showed that these are new alleles at the ETHYLENE INSENSITIVE2 locus. Other mutants defective in their response to ethylene also showed altered ABA sensitivity; from these results, we conclude that ethylene appears to be a negative regulator of ABA action during germination. In contrast, the ethylene response pathway positively regulates some aspects of ABA action that involve root growth in the absence of ethylene. We discuss the response of plants to ethylene and ABA in the context of how these two hormones could influence the same growth responses.