Inhibition of Auxin-induced Deoxyribonucleic Acid Synthesis and Chromatin Activity by 5-Fluorodeoxyuridine in Soybean Hypocotyl

Rootless soybean (Glycine max) seedlings were used as a test system to examine the action of auxin on chromatin-directed RNA synthesis. Chromatin from the basal tissue of rootless seedlings (both control and auxin-treated) had RNA synthetic capacity similar to that of chromatin from comparably treated intact seedlings. When DNA synthesis normally induced in the basal tissue by auxin was blocked in the rootless seedlings by 5-fluorodeoxyuridine, the auxin enhancement of chromatin activity was inhibited 70%. This level was still three times the control level, indicating that auxin influenced the synthetic activity of existing DNA template. Experiments with Escherichia coli RNA polymerase revealed that chromatin from both auxin- and auxin plus 5-fluorodeoxyuridine-treated tissue saturated at higher levels than chromatin from control tissue.     

The Inhibition by Cytokinin of Auxin-Promoted Elongation in Excised Soybean Hypocotyl

The experiments characterize the inhibition by kinetin of auxin-promoted elongation in excised hypocotyl sections of 3-day soybean seedlings (Glycine max cv. Hawkeye 63). It was found that concentrations of kinetin above 4.2 μM did not further inhibit auxin-promoted elongation. Kinetin is as potent an inhibitor of elongation as actinomycin D or cycloheximide. Tissue incubated for 3 or 5 h in the absence of auxin or cytokinin would, upon addition of auxin, exhibit a new growth rate similar to that of tissue grown in auxin for the entire incubation period. Similarly, tissue grown for 3 and 5 h in the presence of auxin would revert to the control rate of elongation upon addition of kinetin. A 10 to 30 min preincubation in kinetin yielded the tissue incapable, for the ensuing 6 h, of increasing its rate of elongation in response to auxin. Zeatin and isopentenyladenine were more potent than kinetin and benzyladenine in the inhibition of elongation. Levels of ethylene produced in the presence of auxin plus cytokinin indicated that it was not involved in this auxin-cytokinin interaction. Kinetin by itself did not promote elongation; nor did it enhance auxin-promoted elongation at low auxin concentrations.     

Fusarium tricinctum T-2 Toxin Inhibits Auxin-promoted Elongation in Soybean Hypocotyl

T-2 toxin, a mycotoxin produced by Fusarium tricinctum, inhibited elongation of excised hypocotyl sections of Glycine max var. Hawkeye 63. Auxin-promoted elongation was inhibited more severely than was control elongation, and a 1 hour preincubation of 5 μm toxin prevented the induction of a faster rate of elongation by auxin. While the inhibition of elongation by cytokinin was similar to that of the toxin, the mode of action of the two compounds appeared to be different, i.e. their effects on elongation were additive, and only kinetin promoted radial enlargement. Toxin treatment did not diminish cytokinin-induced radial enlargement. The properties of the plasma membrane, as measured by electrolyte leakage, were not affected by the toxin.     

被子植物下胚轴细胞伸长的分子机理

作为一种正常的生命现象, 植物下胚轴伸长是长期自然选择的结果, 也是植物进行光合作用、实现自养的必要前提。但下胚轴过度伸长容易造成幼苗徒长, 使植株长势弱, 抗逆能力差, 不利于产量提高和品质改良。该文综述了被子植物下胚轴的发育过程、下胚轴伸长的细胞学机制、植物激素及环境信号调控下胚轴细胞伸长分子机制的最新研究进展, 并展望了未来的研究方向。     

Nucleic Acid Metabolism Involved in Auxin-induced Elongation of Yeast Cells

The following results were obtained using a variant yeast strain, N55, which can respond to the cell-elongating action of auxin. Base analogs of nucleic acids (2-thiouracil, 8-azaguanine, and 5-fluorouracil) inhibited the auxin-induced elongation of yeast cells only when they were added to the preculture prior to auxin treatment. The inhibitory effect of 2-thiouracil and 5-fluorouracil was reversed by uracil and that of 8-azaguanine by guanine. Actino-mycin D inhibited the auxin-induced elongation when given to the culture containing auxin, but not when given to the preculture. The similarity in these respects between yeast and tissues of higher plants is discussed.     

High Temperature Sensitivity of Auxin-induced Ethylene Production in Mung Bean Hypocotyl Sections

High temperature sensitivities of IAA-induced and 1-aminocyclopropane-1-carboxylicacid (ACC)-dependent ethylene production in etiolated mung bean(Vigna radiata [L] Wilczek) hypocotyl sections were comparedat 30,40, 42.5°C. When ethylene production at 30°C wastaken as control, IAA-induced production at 40°C was firstenhanced and then suppressed after 3 h, whereas ACC-dependentproduction was enhanced two-fold throughout the 8 h experimentalperiod. However, when hypocotyl sections treated with 1 mM ACCat 30°C for several hours were transferred to 40°C,the ACC-dependent production rate fell below that at 30°C.An initial transient enhancement of IAA-induced ethylene productionat 40°C was supported by increased ACC synthase activityand thus by ACC content. At 42.5°C, both IAA-induced andACC-dependent production were almost completely suppressed.The results indicate that auxin-induced ethylene productionis affected by high temperatures in two different steps: a)at 40°C, the auxin action gradually deteriorates althoughconversion of ACC to ethylene is not affected at all, and at42.5°C, the conversion is nearly completely suppressed. (Received July 8, 1985; Accepted January 24, 1986)     

Growth-limiting Proteins in Relation to Auxin-induced Elongation in Lupin Hypocotyls

The role of protein synthesis in auxin-induced cell elongation in lupin hypocotyl segments was studied using cycloheximide. Cycloheximide inhibited protein synthesis by 9 minutes. Experiments adding cycloheximide at various times before and after indolyl-3-acetic acid are reported. Estimates of the relative amounts of growth-limiting protein(s), and a first order rate constant for the apparent turnover of the growth-limiting protein(s) were made. It was shown that there is a sizeable growth promotion by auxin after protein synthesis has essentially ceased. It is concluded that the initial phases of auxin action do not require protein synthesis but that its action depends on the existing pool of growth-limiting proteins which is rapidly depleted, and protein synthesis is then required for continued elongation.     

Temperature-dependent Inhibition of Hypocotyl Elongation in Some Soybean Cultivars: I. Localization of Ethylene Evolution and Role of Cotyledons

Apical 2-cm hypocotyl segments from seedlings of a "short-hypocotyl"cultivar Amsoy 71, like whole seedlings, evolve about twiceas much ethylene at 25?C as at 30?C. Segments consisting oftwo cotyledons and an attached epicotyl evolve ethylene at lowrates at both 25?C and 30?C. Hypocotyl segments from seedlingsof Cutler 71 also show enhanced ethylene evolution at 25?C.Hypocotyl segments from Corsoy, a "long hypocotyl" cultivar,however, evolve ethylene at low rates at both 25?C and 30?C.Wounding of Amsoy 71 hypocotyl segments does not increase theirethylene evolution. Ethylene evolution at 25?C is reduced andthe short-hypocotyl phenomenon is reversed by partial (50%)removal of Amsoy 71 cotyledons at planting time. ¹ Journal Paper No. J-10412 of the Iowa Agriculture and HomeEconomics Experiment Station, Ames, Iowa, U.S.A. Project 2191. (Received October 5, 1981; Accepted January 28, 1982)     

Requirement of RNA for the Auxin-induced Elongation of Oat Coleoptile

Using etiolated oat coleoptile segments the following results were obtained. Actinomycin D pretreatment for one hour produced about 50 per cent inhibition of RNA synthesis (labeled uracil incorporation), but the elongation caused by IAA was not inhibited in the following 5 hours at least. Actinomycin D pretreatment for three hours produced about 75 per cent inhibition of RNA synthesis and almost complete inhibition of subsequent IAA-induced elongation, which is accompanied by the inhibition of IAA-induced increase in cell wall extensibility. The inhibiting effect of actinomycin D seemed to be reduced when IAA was given within a certain period.     

Reevaluation of the Effect of Calcium Ions on Auxin-induced Elongation

The mechanism by which calcium ions inhibit cell elongation has been reinvestigated. Growth-inhibiting levels of calcium, when applied to isolated walls (in vitro treatment) do not decrease cell wall extensibility as measured by the Instron technique. Thus, the hypothesis that calcium inhibits growth by forming wall-stiffening calcium bridges must be abandoned. Treatment of living auxin-treated sections with calcium (in vivo treatment) does cause a decrease in the subsequently measured wall extensibility, but this decline appears to be simply a consequence of the growth inhibition rather than its cause. Growth-inhibiting levels of calcium do not appreciably reduce the rate of auxin-enhanced H(+) excretion. Pretreatment with calcium does not reduce the capacity of walls to undergo acid-activated wall loosening in the absence of calcium. High concentrations of CaCl(2) (0.02 m) cause an initial elastic shrinkage of Avena sections comparable to that caused by the same osmolarity of mannitol, but the subsequent growth inhibition is too great to be explained by an osmotic inhibition. Calcium ions do inhibit H(+)-induced extension of frozen-thawed sections under tension. The growth-inhibitory effects of calcium, then, may be ascribed to a direct inhibition exerted by calcium ions on the H(+)-induced wall-loosening process.     

Hormonal Regulation of Cell Elongation in the Hypocotyl of Rootless Soybean: An Evaluation of the Role of DNA Synthesis

A method was developed where soybean seedlings were grown without roots to study the influence of hormones of root origin on shoot growth. Excision of the root resulted in inhibition of apical section growth and DNA synthesis and inhibited elongating section growth. A synthetic cytokinin restored DNA synthesis in the apical section, but did not influence growth in either the apical or elongating sections. Low concentrations of gibberellin with the cytokinin restored growth in the apical section. Gibberellin alone was sufficient to restore growth in the elongating section.An inhibitor of DNA synthesis, 5-fluorodeoxyuridine, inhibited the increase in apical section DNA without inhibiting control or gibberellin-induced growth in the elongating section. Experiments with (14)C-thymidine resulted in no DNA labeling differences in the elongating section under conditions where gibberellin-induced elongation varied from 50% to 73% above controls. It was concluded that gibberellin-induced elongation in soybean hypocotyl occurred in the absence of DNA synthesis. Gibberellin does stimulate DNA synthesis in the apical tissue apart from its effect on cell elongation.Excised soybean hypocotyl elongated maximally at 10(-6)m auxin. At higher auxin concentrations, fresh weight and ethylene production increased, but elongation was reduced. Addition of GA to the higher auxin concentrations resulted in a 50% inhibition in auxin-induced ethylene production and resumption in maximal elongation. Added ethylene inhibited elongation 30% at 2 mul/l. Addition of up to 100 mul/l ethylene did not inhibit elongation with GA present in the incubation medium. Thus GA may counteract ehtylene inhibition of cell elongation in addition to inhibiting ethylene production in auxin-treated tissues.     

Promotion of Hypocotyl Elongation in Watermelc Seedlings by 6-Benzyladenine

Hypocotyl elongation under white fluorescent light was aboutdoubled in dwarf watermelon (Citrullus lanatus0 (Thunb.) Matsu.and Nakai) seedlings treated with 0.1 to 0.3 µg apicaland 3 x 10–6 to 10.3 M root applications of 6-benzyladenine(BA). BA-enhancement of growth occurred primarily during thefirst 48 h after treatment. Increased hypocotyl length in BA-treatedseedlings was attributed more to longer cells than to an increasein cell number. Early hypocotyl growth of normal seedlings wasalso significantly enhanced by BA although final hypocotyl lengthwas not substantially affected. Benzyladenine caused expansion of cotyledons and, at higherdoses, lateral expansion of hypocotyls. BA-induced increasesin fresh weight of cotyledons and hypocotyls were accompaniedby an increase in dry weight of hypocotyls at the expense ofroots which had less dry matter than untreated seedlings.     

Arabidopsis Mutants Lacking Blue Light-Dependent Inhibition of Hypocotyl Elongation

We have isolated a new class of photomorphogenic mutants in Arabidopsis. Hypocotyl elongation is not inhibited in the mutant seedlings by continuous blue light but is inhibited by far red light, indicating that these mutations are phenotypically different from the previously isolated long hypocotyl (hy) mutants. Complementation analysis indicated that recessive nuclear mutations at three genetic loci, designated blu1, blu2, and blu3, can result in the blu mutant phenotype and that these mutants are genetically distinct from other long hypocotyl mutants. The BLU genes appear to be important only during seedling development because the blu mutations have little effect on mature plants, whereas hypocotyl elongation and cotyledon expansion are altered in seedlings. The genetic separation of the blue and far red sensitivities of light-induced hypocotyl inhibition in the blu and hy mutants demonstrates that two photosensory systems function in this response.     

The Role of DNA Synthesis During Hypocotyl Elongation in Light and Dark

Fluorodeoxyuridine, an inhibitor of DNA synthesis, did not affectgermination and post-germinative growth in the aerial part oflettuce and Haplopappus gracilis seedlings when grown in thelight. In the dark, however, elongation of the hypocotyl wasinhibited by fluorodeoxyuridine, strikingly in lettuce and onlyslightly in Haplopappus gracilis. This could imply that thecontrolling mechanism of hypocotyl elongation is in some casesrelated to DNA synthesis, either because mitotic processes (oftenlittle taken into account in considering hypocotyl growth) areinvolved in the elongation of hypocotyls only when they aregrown in the dark, or because DNA synthesis affects cell elongationdirectly, or through the production of a greater number of endopolyploidcells in the dark. Using mainly autoradiographic and cytofluorimetricmethods, these possibilities were tested. Besides lettuce (Lactucasativa L. var. Great Lakes) and H. gracilis (Nutt.) Gray, radish(Raphanus sativus L. var. Tondo rosso quarantino) and soybean(Soya hyspida Sieb. and Zucc. var. Tokyo) seedlings were alsostudied. Fluorodeoxyuridine drastically inhibits cell elongation onlywhen it is preceded or accompanied by mitotic or endomitoticevents. Need for DNA synthesis during hypocotyl elongation,as well as during early post-germinative growth, seems to beof particular importance when endomitotic processes are involved. DNA synthesis, elongation, endoreduplication, fluorodeoxyuridine, Haplopappus gracilis (Nutt.) Gray, Lactuca sativa L., Raphanus sativus L., Soya hyspida Sieb and Zucc     

Rapid Inhibition of Auxin-induced Elongation of Avena Coleoptile Segments by Cordycepin

The effects of cordycepin (3'-deoxyadenosine), an RNA synthesis inhibitor, on auxin-induced elongation in Avena coleoptile segments were studied with a position-sensing transducer. Cordycepin rapidly inhibited auxin-stimulated growth in the coleoptile segments whether added before, at the same time as, or after, the 2 mum auxin treatment. Midcourse additions of 100, 50, and 25 mug/ml cordycepin inhibited auxin-promoted elongation in an average of 18, 22, and 35 minutes, respectively. Additions of cordycepin before or at the same time as the auxin treatment partially inhibited the magnitude of the subsequent auxin-promoted growth but did not appreciably alter the latent period of the auxin response. It was concluded that if cordycepin is inhibiting the synthesis of RNA required for growth, the decay time for this RNA may be considerably shorter than that suggested in the literature from actinomycin D experiments. Preliminary kinetic evidence indicated that cordycepin does not inhibit auxin-induced elongation by acting as a respiratory inhibitor. Studies in mung bean shoot mitochondria demonstrated that cordycepin has no effect on respiration, respiratory control, or ADP/oxygen ratios.     

Hydroxyproline Formation and Its Relation to Auxin-induced Cell Elongation in the Avena Coleoptile

A study has been made of the effects on hydroxyproline formation of 4 factors that influence the rate of cell elongation in the Avena coleoptile; auxin, sugars, an external osmoticum, and actinomycin D. Hydroxyproline formation is increased by a combination of auxin and sucrose, but is affected to a much lesser extent by either factor alone. Its formation is inhibited by an external osmoticum but is scarcely affected by actinomycin D. The lack of correlation between the amount of hydroxyproline synthesis and the growth rate suggests that hydroxyproline formation is not involved in the actual process of wall loosening. It is suggested, instead, that if the wall is to retain its capacity for rapid extension, those hemicelluloses which are incorporated into it by intussusception rather than by apposition must be attached to a hydroxyproline-protein.     

蕨藻红素促进大豆插条不定根的形成

对蕨藻红素影响大豆下胚轴插条不定根形成的研究表明：蕨藻红素促进大豆插条不定根的形成,其最适浓度为0.5μmol·L^-1,最适处理时间为2d。0.5μmol·L^-1蕨藻红素处理大豆插条后,不定根诱导阶段（0～24h）的POD、CAT、IAAox活性较低;而不定根形成生长阶段（24～72h）的CAT和IAAox活性较高,POD活性低于未作处理的。试验推测蕨藻红素促进大豆插条不定根形成的生理基础可能与其影响POD、CAT和IAAox活性有关。     

Role of Cation and Anion Uptake in Salt-stimulated Elongation of Lettuce Hypocotyl Sections

The role of cation and anion uptake in salt-stimulated growth of light-grown, GA₃-treated lettuce (Lactuca sativa L.) hypocotyl sections was investigated. Potassium chloride (10 mm) causes a 2-fold increase in the growth rate of GA₃-treated hypocotyl sections without affecting the growth rate of sections incubated in the absence of GA₃. Salt uptake is the same in both treatments, and furthermore the uptake of cation and anion is stoichiometric during the first 24 hours under all incubation conditions. The importance of the anion for cation uptake is demonstrated in experiments with benzenesulfonate⁻ and iminodiacetate²⁻. When K⁺ and Na⁺ are supplied only as the benzenesulfonate and iminodiacetate salts, growth and cation uptake are markedly reduced compared to KCl and NaCl. Calculation of the osmotic potential of salt-treated sections based on measurement of K⁺ and Cl⁻ uptake suggests that the observed increase in tissue osmolality is a result of salt uptake. Similarly, uptake of ions can account for the shift in water potential when sections are incubated in 10 mm KCl. We conclude that the change in growth rate of light-grown, GA₃-treated sections caused by the addition of KCl or NaCl to the incubation medium results solely from decreased water potential of the tissue due to ion uptake.