Cytokinin-Inhibitor Antagonism in the Hormonal Control of α-Amylase Synthesis and Growth in Barley Seed

The effect of cytokinins and gibberellic acid on the inhibition of growth and α-amylase synthesis by germination inhibitors was investigated in intact and embryoless seed halves. The cytokinins, kinetin and benzyladenine, effectively reversed the inhibition of coleoptile growth and α-amylase synthesis by abscisic acid and courmarin in barley seed. An antagonism between cytokinins, kinetin and benzyladenine, effectively reversed the inhibition of coleoptile growth and α-amylase synthesis by abscisic acid and coumarins in barley seed. An antagonism between cytokinins and germination inhibitors was also shown in root growth. Abscisic acid inhibited coleoptile growth to a greater extent than the root growth while the opposite held true in the case of coumarin. The apparent increase in coleoptile growth and α-amylase synthesis by gibberellic acid plus abscisic acid (or coumarins) over abscisic acid (or coumarin) appears to be a result of the overall stimulation of growth and metabolism by exogenous gibberellic acid and probably does not involve an interaction of gibberellic acid with the inhibitors. Gibberellic acid reversed root inhibition to some extent. Abscisic acid inhibition of gibberellic acid induced α-amylase synthesis in the embryoless endosperm was not reversed by excess gibberellic acid or kinetin Cytokinin reversal of inhibition of growth and enzyme synthesis probably depends on some factor(s) in the embryo. Cytokinin reversal of inhibitor action leading to enzymen synthesis and growth may be at the level of genome or at the site protein assembly.     

Effects of abscisic acid on in vitro growth of cotton fiber

Summary Abscisic acid (ABA) inhibits in vitro growth of cotton (Gossypium hirsutum L.) fiber and is effective only when applied during the first four days of culture started on the day of anthesis. Abscisic acid causes a small increase in potassium uptake by the ovules and also enhances leakage of potassium from them. During their period of rapid growth, fibers produced by ABA-treated ovules have a higher potassium content and a lower malate content as compared to fibers on untreated control ovules. Results are discussed in the light of earlier reports on the in vitro growth of cotton fiber and effects of abscisic acid on other plant tissues. It is suggested that ABA inhibits fiber growth, in part, by interfering with malate metabolism.Abbreviations ABA abscisic acid - GA₃ gibberellic acid - IAA indole-3-acetic acid - TFU total fiber units     

脱落酸生物合成研究进展

脱落酸作为一种抑制生长的植物激素,是平衡植物内源激素和调节生长代谢的关键因子。脱落酸具有提高作物抗旱耐盐、减少果实褐变的作用,同时可降低疟疾发病率、刺激胰岛素分泌,因此在农业和医药领域有着广阔的应用前景。相较于传统的植物提取法和化学合成法,利用微生物合成脱落酸是一种经济、可持续的来源方式。目前利用天然微生物如灰葡萄孢霉菌、蔷薇色尾孢菌等合成脱落酸的研究已经取得了诸多进展,而脱落酸的异源微生物合成研究相对较少。酿酒酵母、解脂耶氏酵母、大肠杆菌等工程菌株作为天然产物异源合成的常用宿主,具有遗传背景清晰、易于操作、便于工业化生产等优势,因此利用微生物异源合成脱落酸是一种更具潜力的生产方式。本文着重从底盘细胞的选择、关键酶的筛选与表达强化、辅因子的调节、增强前体供应及促进脱落酸外排5个方面对微生物异源合成脱落酸的研究进行综述。最后,对该领域的未来发展方向进行了展望。     

Deuterium-labeled phaseic acid and dihydrophaseic acids for internal standards

The concentration of abscisic acid in plants is regulated not only by biosynthesis, but also by metabolism. Abscisic acid is metabolized to phaseic acid via 8'-hydroxyabscisic acid, and phaseic acid is then converted to dihydrophaseic acid and its epimer. A quantitative analysis of these metabolites is important as well as that of abscisic acid to understand changes in the concentration of abscisic acid in plants. However, no internal standards of the metabolites suitable for quantitative analysis have been reported. We prepared 7'-deuterium-labeled phaseic acid with a deuterium content of 86%, using the equilibrium reaction between phaseic acid and 8'-hydroxyabscisic acid. 7'-Deuterium-labeled dihydrophaseic acids were obtained by reducing 7'-deuterium-labeled phaseic acid. The levels of the metabolites in plant organs were determined by using the deuterated metabolites as internal standards.     

Deuterium-labeled Phaseic Acid and Dihydrophaseic Acids for Internal Standards

The concentration of abscisic acid in plants is regulated not only by biosynthesis, but also by metabolism. Abscisic acid is metabolized to phaseic acid via 8′-hydroxyabscisic acid, and phaseic acid is then converted to dihydrophaseic acid and its epimer. A quantitative analysis of these metabolites is important as well as that of abscisic acid to understand changes in the concentration of abscisic acid in plants. However, no internal standards of the metabolites suitable for quantitative analysis have been reported. We prepared 7′-deuterium-labeled phaseic acid with a deuterium content of 86%, using the equilibrium reaction between phaseic acid and 8′-hydroxyabscisic acid. 7′-Deuterium-labeled dihydrophaseic acids were obtained by reducing 7′-deuterium-labeled phaseic acid. The levels of the metabolites in plant organs were determined by using the deuterated metabolites as internal standards.     

Water Stress and Protein Synthesis: II. Interaction between Water Stress, Hydrostatic Pressure, and Abscisic Acid on the Pattern of Protein Synthesis in Avena Coleoptiles

Water stress causes a reduction in hydrostatic pressure and can cause an increase in abscisic acid in plant tissues. To assess the possible role of abscisic acid and hydrostatic pressure in water stress effects, we have compared the effects of water stress, abscisic acid, and an imposed hydrostatic pressure on the rate and pattern of protein synthesis in Avena coleoptiles. Water stress reduces the rate and changes the pattern of protein synthesis as judged by a double labeling ratio technique, Abscisic acid reduces the rate but does not alter the pattern of protein synthesis. Gibberellic acid reverses the abscisic acid-induced but not the stress-induced inhibition of protein synthesis. The effect of hydrostatic pressure depends on the gas used. With a 19: 1 N₂-air mixture, the rate of protein synthesis is increased in stressed but not in turgid tissues. An imposed hydrostatic pressure alters the pattern of synthesis in stressed tissues, but does not restore the pattern to that found in turgid tissues. Because of the differences in response, we conclude that water stress does not affect protein synthesis via abscisic acid or reduced hydrostatic pressure.     

The role of abscisic acid in the ripening of grapes

Ripening in grapes ( Vitis vinifera L. cv. Thompson seedless) was accompanied by an increase in the levels of sucrose, glucose and fructose and a decrease in the levels of acids. The activity of glucose-6-phosphatase and fructose-l–6-bisphospbatase was lower in sweet grapes as compared to sour ones. Abscisic acid (10⁻⁶ M) stimulated the gluconeogenic process in sour grapes. The levels of some gluconeogenic enzymes were also elevated in its presence. Cyclohexitnide (0.036–1.8 mM) nullified the abscisic acid effect, suggesting that this effect involves de novo protein synthesis. The incorporation of [¹⁴C]-leucine into proteins was enhanced about 80% by abscisic acid, confirming that abscisic acid promoted protein synthesis. Again, cycloheximide blocked the hormone mediated increase in the incorporation of radioactivity into proteins. The results indicate that one of the factors for sourness in certain mature ripe grapes may be that abscisic acid is not available.     

Influence of exogenous abscisic acid on germination and plantlet conversion frequencies of hybrid larch somatic embryos (Larix × leptoeuropaea)

The effect of exogenous abscisic acid, provided to somatic embryos during the maturation step, on endogenous abscisic acid and its main conjugated form (abscisic acid glucose ester), germination and conversion frequencies is presented in this paper. Abscisic acid measurements were obtained after a methanolic extraction, a fractionation through high performance liquid chromatography, quantitation with an immunoassay and identification of the quantitated compound using gas chromatography-mass spectrometry. Results show that endogenous abscisic acid and abscisic acid glucose ester levels are clearly correlated with the exogenous abscisic acid concentration provided to the embryos. Maturation was clearly enhanced by exogenous abscisic acid, but no correlation was found between abscisic acid concentration and germination frequency. Conversely, development of the aerial part of the germinated somatic embryos was dependent upon the abscisic acid concentration in the culture medium and results suggest that this dependence could be related to the endogenous abscisic acid content.     

A Rapid and Simple Radioassay for Abscisic Acid using 14C-Diazomethane

A simple physical method for measuring abscisic acid concentrationin plant material is described. Abscisic acid in partially purifiedextracts was radiolabelled by reaction with ¹⁴C-diazomethaneto give ¹⁴C-methyl abscisate, which was purified from otherradiolabelled products by thin layer chromatography. Abscisicacid concentration was measured by comparison of the ¹⁴C radioactivityincorporated into plant abscisic acid with that in standard¹⁴-C-methyl abscisate prepared under the same conditions fromknown amounts of pure abscisic acid. Losses of abscisic acidwhich occurred during purification were corrected by measuringthe recovery of ³H-abscisic acid added to initial extracts. Abscisic acid concentration was measured by radioassay and byconventional electron capture-gas chromatography in oat, bean,and turgid or wilted tobacco leaves. Results from the two methodswere closely comparable. Radioassay is as rapid and sensitiveas existing procedures for measuring abscisic acid, but requiresonly simple and inexpensive chromatographic equipment.     

Influence of Nitrogen Source, Thiamine, and Light on Biosynthesis of Abscisic Acid by Cercospora rosicola Passerini

Abscisic acid production by Cercospora rosicola Passerini in liquid shake culture was measured with different amino acids in combination and singly as nitrogen sources and with different amounts of thiamine in the media. Production of abscisic acid was highest with aspartic acid-glutamic acid and aspartic acid-glutamic acid-serine mixtures as nitrogen sources. Single amino acids that supported the highest production of abscisic acid were asparagine and monosodium glutamate. Thiamine was important for abscisic acid production. Leucine inhibited abscisic acid production. C. rosicola produced abscisic acid in the dark, but production more than doubled in the presence of light.     

Clumping and dispersal of chloroplasts in succulent plants

Plants have evolved various photoprotective mechanisms to mitigate photodamage. Here we report the diurnal movement of chloroplasts in the leaves of succulent crassulacean acid metabolism (CAM) plants under combined light and water stress. In leaves of water-stressed plants, the chloroplasts became densely clumped in one or sometimes two areas in the cytoplasm under light and dispersed during darkness. The chloroplast clumping resulted in leaf optical changes, with a decrease in absorptance and an increase in transmittance. The plant stress hormone abscisic acid induced chloroplast clumping in the leaf cells under light. We suggest that the marked chloroplast movement in these CAM plants is a photoprotective strategy used by the plants subjected to severe water stress.Abbreviations ABA Abscisic acid - CAM Crassulacean acid metabolism     

Cytokinin Reversal of Abscisic Acid Inhibition of Growth and α-Amylase Synthesis in Barley Seed

The effect of cytokinin, kinetin, on abscisic acid (dormin) inhibition of α-amylase synthesis and growth in intact barley seed was investigated. Abscisic acid at 5 × 10^?5M nearly completely inhibited growth response and α-amylase synthesis in barley seed. Kinetin reversed to a large extent abscisic acid inhibition of α-aniylase synthesis and coleoptile growth. The response curves of α-amylase synthesis and coleoptile growth in presence of a fixed amount of abscisic acid (6 × l0^?6M) and increasing concentrations of kinetin (from 5 × l0^?7M to 5 × 10^?5 M) showed remarkable similarity. Kinetin and abscisic acid caused synergistic inhibition of root growth. Gibberellic acid was far less effective than kinetin in reversing abscisic acid inhibition of α-amylase synthesis and coleoptile growth. A combination of kinetin and gibberellic acid caused nearly complete reversal of abscisic acid inhibition of α-amylase synthesis but not the abscisic acid inhibition of growth. The results suggest that factors controlling α-amylase synthesis may not have a dominant role in all growth responses of the seed. Kinetin possibly acts by removing the abscisic acid inhibition of enzyme specific sites thereby allowing gibberellic acid to function to produce α-amylase.     

Some Structural Changes During Ripening of Mangoes (Mangifera indica var. Alphonso) by Abscisic Acid Treatment

Abscisic acid at 10^–6 M concentration enhances ripeningof mangoes. The cells in the pulp of the fruit are large andparenchymatous and lose their integrity due to cell wall hydrolysisat the ripe stage. They contain abundant polysaccharides, consistingmainly of starch, which is degraded during ripening. The chloroplastsare transformed to chromoplasts containing red or yellow carotenoidpigment. Abscisic acid treatment enhances all of these processes.Mitochondria, on the other hand, retain their structural integritythroughout the ripening process in untreated and abscisic acid-treatedmangoes. Mangoes, ripening, abscisic acid, structure     

The Barley Magnesium Chelatase 150-kD Subunit Is Not an Abscisic Acid Receptor

Magnesium chelatase is the first unique enzyme of the chlorophyll biosynthetic pathway. It is composed of three gene products of which the largest is 150 kD. This protein was recently identified as an abscisic acid receptor in Arabidopsis (Arabidopsis thaliana). We have evaluated whether the barley (Hordeum vulgare) magnesium chelatase large subunit, XanF, could be a receptor for the phytohormone. The study involved analysis of recombinant magnesium chelatase protein as well as several induced chlorophyll-deficient magnesium chelatase mutants with defects identified at the gene and protein levels. Abscisic acid had no effect on magnesium chelatase activity and binding to the barley 150-kD protein could not be shown. Magnesium chelatase mutants showed a wild-type response in respect to postgermination growth and stomatal aperture. Our results question the function of the large magnesium chelatase subunit as an abscisic acid receptor.     

Effect of growth regulators on maturation of geranium (Pelargonium × hortorum) somatic embryos

Interactions of growth regulators and polyethylene glycol on maturation of geranium somatic embryos were investigated. Somatic embryos were induced on medium with 20 M thidiazuron for 3 days. The growth regulators used were 1 µM abscisic acid, jasmonic acid, napthaleneacetic acid and benzylaminopurine at 21 days from the start of induction. Benzylaminopurine and napthaleneacetic acid did not enhance abscisic acid effects on maturation frequency but only improved maturation frequency in the presence of polyethylene glycol. Abscisic acid significantly improved protein content in the presence of polyethylene glycol. Benzylaminopurine and napthalene acetic acid in combination with abscisic acid and jasmonic acid improved protein types in somatic embryos only in the absence of polyethylene glycol. Osmoticum effected by polyethylene glycol seems the main component required for protein synthesis. This study showed significant improvement of somatic embryo quality for artificial seed production.     

Abscisic acid antagonizes the effect of cytokinin on DNA-replication origins

In previous studies (Houssa et al., 1990, 1994) we observedthat cytokinins stimulate the cell division process in vegetativeand reproductive shoot meristems of monocotyledonous and dicotyledonousspecies by activating latent DNA-replication origins. Here wereport that abscisic acid antagonizes this effect in the shootmeristem of Sinapis alba L. Abscisic acid reduces DNA synthesisby inactivating some DNA-replication origins resulting in alengthening of the replicon size. It is hypothesized that thebalance between abscisic acid and cytokinin levels is one ofthe major factors controlling the rate of DNA replication, andultimately the rate of cell division, in shoot meristems. Key words: Abscisic acid, cell division, cytokinin, DNA replication, replicon, shoot meristem, Sinapis alba     

Regulation of Senescence in Carnation (Dianthus caryophyllus): Effect of Abscisic Acid and Carbon Dioxide on Ethylene Production

Abscisic acid hastened senescence of carnation flowers and this was preceded by stimulation of accelerated ethylene production. Carbon dioxide delayed the onset of autocatalytic ethylene production in flowers regardless of treatment with abscisic acid. Flowers exhibited a low and transient climacteric of ethylene production without wilting while in 4% carbon dioxide and underwent accelerated ethylene production culminating in wilting when removed from carbon dioxide. Hypobaric ventilation, which lowers ethylene to hyponormal levels within tissues, extended flower longevity and largely negated enhancement of senescence by abscisic acid. Supplementing hypobarically ventilated flowers with ethylene hastened senescence irrespective of abscisic acid treatment. Collectively, the data indicate that abscisic acid hastens senescence of carnations largely as a result of advancing the onset of autocatalytic ethylene production.     

脱落酸信号转导研究进展

脱落酸(ABA)对植物生长发育和适应环境胁迫等多方面有重要的调节作用,其信号转导机制非常引人注目,近年来这方面研究进展很快。本文利用现有文献,对脱落酸不敏感和超敏感性突变体、脱落酸的结合位点与受体、ABA信号转导涉及的细胞第二信使（Ca2+、磷酸肌醇、cADPR、阴离子通道与H+）、蛋白质可逆磷酸化以及ABA诱导基因表达所必需的顺式作用元件(cis-acting element) 和反式作用因子trans-acting factor）等几方面的最新研究进展作了介绍。     

脱落酸信号转导研究进展

脱落酸（ABA）对植物生长发育和适应环境胁迫等多方面有重要的调节作用,其信号转导机制非常引人注目,近年来这方面研究进展很快,本文利用现有文献,对脱落酶不敏感和超敏感性突变体,脱落酸的结合位点与受体,ABA信号转涉及的细胞第二信使（Ca^2 ,磷酸肌醇,cADPR,阴离子通道与H^ ), 蛋白质可逆磷酸化以及ABA诱导基因表达所必需的顺序作用元件（cis-acting elenment)和反式作用因子（trans-acting factor)等几方面的最新研究进展作了介绍。     

A comparative study of the effect of abscisic acid and cAMP on protein synthesis in wheat caryopses under drought conditions

The effect of exogenous abscisic acid and cAMP on synthesis of soluble proteins in wheat caryopses in drought has been studied. Both compounds affected the formation of the polypeptides whose synthesis was stimulated by dehydration: they increased the incorporation of the label into polypeptides of 13, 15, and 26 kD and decreased the incorporation of the label into polypeptides of 14, 64, and 77 kD. Abscisic acid and cAMP increased the level of the incorporation of [¹⁴C]leucine into the low-molecular-weight polypeptides of 12, 17, and 19 kD whose synthesis was suppressed by drought. These data suggest that the cyclic adenylate signal system is probably involved in the effect of abscisic acid on protein synthesis in drought.