The influence of gamma radiation on the biosynthesis of indoles and gibberellins in barley the action of zinc on the restitution of growth substance level in irradiated plants

Investigations were made on the effect of exposing barley seeds to gamma-radiation (5–40 kR), alone and in combination with the application of zinc (soaking the seeds in solutions containing 5.10^?5–5.10^?1% Zn for 12 hours before sowing) on growth and on the content of tryptophan, indole auxins and gibberellin-like substances in seven-day plants. Radiation decreased both growth and the content of tryptophan (e.g. by about 53% at 30 kR), of indole auxins (by about 60% auxin in the zone of IAA on the chromatogram at 30 kR), and also the content of gibberellin-like substances (by about 67% gibberellin content in the zone of GA₃ on the chromatogram) of plants. The irradiation of standard samples of tryptophan, indolyl-acetic acid and gibberellic acid alone with many times greater doses (up to 1000 kR) did not lead to marked radiochemical degradation of these substances. It can be assumed that radiation damages the enzyme systems “synthesizing” natural growth substances in plants. The damaging effect of radiation on auxins is already displayed in the synthesis of tryptophan, which is inhibited. Zinc interacts with the damaging effect of radiation on growth. Optimum concentrations of zinc (5.10^?3% Zn) counteract the effect of radiation, up to doses of about 12 kR, on the growth in height in 7-day plants so that it is equal to the controls. Normal content of tryptophan and auxin in the position of indolecetic acid on chromatograms can only be reached by the addition of zinc when the dose of radiation was not greater than about 8 kR, which is less than the influence exerted by zinc on the restitution of growth. On the other hand, the biosynthesis of gibberellin-like substances at the position of gibberellic acid on chromatograms can be restored by zinc to their original level to doses of up to 30 kR. The increased biosynthesis of auxins and gibberellins caused by zinc in irradiated plants is explained by the activation of the remaining and non—damaged enzyme systems carrying out this biosynthesis. The activation of the biosynthesis of growth substances by zinc will also contribute to the restitution effect of zinc on the growth of plants from irradiated seeds.     

Metodo di Determinazione Dell'acido Beta-Indolacetico Nei Vegetali Per Cromatografia e Diluizione Isotopica (Riassunto)

Abstract

Method of determination of beta-indolacetic acid in plants by chromatography and isotopic dilution. — The quantitative determination of beta-indolacetic acid content in plants is a very complex problem still unsolved because of the difficulty of separation and the scarce specificity and sensitivity of thè analytical methods. The separation of IAA from plant tissues extracts, which can be obtained by chromatography, is not satisfactory; especially for the interfering of substances, the elimination of which is very difficult and which often influence the colorimetric and spectrofotometric determination of the fractions in which we find the IAA after separation. An analysis with biological methods on the same fractions might have perhaps a greater probability to be free from interferences (but that is obviously difficult to control). As far as the specificity of the fluorimetric method in the IAA determination is concerned, the literature data are too scarce to give a judgement about it. According to the method of colorimetric determination it is known that the Salkowsky's reactive and the Herlich's one are aspecific as they both give coloured products with all the indolic compounds; the reaction is altered by the presence of a large number of substances (Nichols, Gordon, Linser). It is clear that all those substances which absorb in the wave length range of the IAA (from 300 to 270 mμ) will influence the spectrophotometric determination.

Considering the uncertainty of the measures carried out by the several mentioned authors and by others we have thought of using both the chromatographic separation technique and the method of isotopic dilution.

So, if we add to the plant tissues extract a known quantity of IAA — C¹⁴ we can make all the operations of separation and identification with greater safety. An attempt to use IAA-C¹⁴ in order to facilitate the measure was made in 1961 by Hamilton and coll. but, perhaps, because of the used technique of separation the recovery of the IAA was not more than the 28%.

With our technique, on the contrary, we can obtain the 98–100% of recovery. The following method has been used: to the plant tissues extracts liofilized we added 0,0126 μc of IAA-C¹⁴ with specificic activity 1,94 mc/mmol. Then we proceeded with the ethereal extraction according to the classic methods (Bennet-Clark and Kefford); the extracts were purified by NaOH (or NaHCO₃) (Bennet-Clark and Kefford, Rakitin-Povo-Lotskaya, Haagen-Smith e coll., Avery e coll.) and were extracted again with ethyl ether after acidification at pH 3 with HCl (or CH₃COOH).

The ethereal purified extract was vacuum evaporated and the residue extracted with the organic phase of petroleum ether and n-butyl alcohol mixture saturated with formic acid 0,5 M (97:3). Then it was chromatographated on silica gel column (? mm 0,05–0,2). hydrated by H-COOH 0,5 M, in accordance with the method described by Loyd E. Powell in Plant Physiology 1960 and slightly modified by us.

In fact if we perform the separation of the acid indolderivates by petroleum ether containing 0,2 and 3% of butyl alcohol saturated in H-COOH 0,5 M, (as foreseen by the original method) with the second solvent IAA is eluted with an impurity which absorbs in UV in the same wave-length of the IAA. The use of three eluents at 0,2–1,5 and 3% in butyl alcohol permits, on the contrary, to obtain IAA free from impurity interfering with spectrophotometric reading in the fractions eluted by the solvent at 3%.

Recovery values of the 98–100% were obtained by adding known quantities of IAA and IAA-C¹⁴ to the liofilized; moreover the ratio IAA-C¹⁴/IAA remained constant for all the collected fractions. By appling the known formula of the isotopic dilution it is possible in theory to go back from the radiochemical and spectrophotometric measurement of a given fraction to the total value of IAA extracted from plant tissue, knowing the radioactivity of the IAA-C¹⁴ added to the liofilized.

Trials carried out in this way beginning from 25 gr of peas apices or from the same quantity of maize seeds, after 40 hours from germination, did not show presence of IAA. Afterwards, we will try the extraction and the determination beginning from highter quantitatives of plant material.     

Relazioni ormonali nello sviluppo dell'ovario delle orchidee II. Azione delle auxine in segmenti isolati dell'asse fiorale

Abstract

HORMONAL RELATIONS IN THE DEVELOPMENT OF ORCHID OVARY. II. — The effects of auxins on isolated parts of the flower axis. — The changes in fresh weight of segments of column, ovary and peduncle of Cymb[icaron]dium sp., treated in vitro with auxins (IAA, EtIA and NAA) indicate that these parts of the flower react promptly and independently to the three auxins tested. Expansion is very small in the ovary, and is more and more large moving towards the apex of the flower (fig. 1). The apical part of the column reacts somewhat more rapidly to auxin and tolerates higher concentration than the proximal part.

Naphtalenacetic acid proved to be the most active of the three auxins tested. This finding is in agreement with the higher efficiency of this substances as an inducer of parthe-nocarpy and its ability to induce large necroses of the column, when applied to the flower on the intact plant.

The plots of auxin activity against concentration of the auxins suggest thai the lower activity of IAA might be due to its destruction by indoleacetic acid oxidase.     

Hormonal Relations in the Phototropic Response IV. Light-Induced Changes of Endogenous Auxins in the Coleoptile

Beside indoleacetic acid (IAA), 3 auxins were found by chromatographic resolution of acidic fractions of Avena and Zea coleoptile tips. One of these auxins, designated P, occurred at levels of activity approaching those of IAA. The other 2 auxins, termed F and M, occurred at lower levels of activity. When the auxins of the excised coleoptile tips were isolated immediately after equilateral or unilateral irradiation with blue light at first positive energies, the ratio of IAA to the other auxins increases. This rise is the result of a decrease in P and F, and probably an increase in IAA. Light did not affect materially the total auxin content. It is suggested that P and F might be associated with the basipetal transport inequalities of IAA in phototropism.

P has been partially characterized. Its R_F on chromatograms developed in ammoniacal isopropanol is about 0.65. It is converted to IAA in vitro by heat. The ultraviolet absorption spectrum of chromatographically resolved P also suggests an indolyl complex. P is not readily transported basipetally, and the slope of its relative concentration-response curve (Avena section test) is lower than that of IAA. P does not appear to be any of the chemically characterized native auxins.

     

Effetto dell'Irraggiamento X sulla Biosintesi degli Amminoacidi in Piante di Orzo

Abstract

The effects of irradiation on aminoacids biosynthesis in barley. — The effect of radiations on the biosynthesis of aminoacids in barley has been studied. Seeds have been irradiated with X-tays of 7.5 and seedlings with 7.5, 15 and 30 Kr. After 6–7 days of growth the control and irradiated seedlings, and the seedlings from control and irradiated seeds, have been supplied with 10–100 μC of C¹⁴O₂ in a closed chamber for 44–42 hours. The material was extracted with hot ethanol (85%–80% and 40%). Total radioactivity and the radioactivity of the basic fraction, eluted with HCl from Dowex 50, X-8′ (200–400 mesh, H^?) have been measured. It was observed that radiations increase the radioactivity of the aminoacid fraction, both in leaves and roots. A correlation between the decrease of fresh weight and increase of aminoacids radioactivity was shown. A possible explanation might be visualized in terms of the inhibition of growth and protein synthesis by radiations.     

Regulation of organogenesis and somatic embryogenesis by auxin in melon,Cucumis melo L.

Various tissues of seeds and seedlings of melon were cultured in vitro to study the effects of auxin concentration on organogenesis and embryogenesis. Adventitious shoots and somatic embryos were formed from explants of cotyledons of mature seeds, hypocotyls of seedlings, and leaves and petioles of young plantlets. Expanded cotyledons of seedlings formed only adventitious shoots. All tissues responded similarly to the 2,4-D concentration in the media, that is, adventitious shoots were formed at low concentration, callus proliferated without differentiation at intermediate concentration and somatic embryos were induced at high concentration. Cotyledons of mature seeds formed both adventitious shoots and somatic embryos more efficiently than any other tissues cultured.Effects of three auxins, 2,4-D, NAA and IAA, on organogenesis and embryogenesis were compared using cotyledons of mature seeds. Adventitious shoots were formed at low level of auxins (0 to 0.01 mg/l 2,4-D; 0 to 0.1 mg/l NAA; 0 to 1.0 mg/l IAA), and embryos were formed at high level of auxins (1.0 to 2.0 mg/l 2,4-D; 3.0 to 10.0 mg/l NAA; 20.0 to 100.0 mg/l IAA). IAA gave more efficient shoot formation and embryogenesis than the other auxins.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - NAA -naphthaleneacetic acid - IAA 3indoleacetic acid - BA 6-benzylaminopurine - MS Murashige and Skoog     

Physiological characteristics of two auxin-resistant mutants of Arabidopsis thaliana,aux-2 and Dwf

Two auxin-resistant mutants of Arabidopsis thaliana L. have been characterized physiologically: aux-2 is a recessive mutation and is unlinked to a dominant mutation, Dwf, which is apparently lethal when homozygous. The progeny of selfed Dwf plants segregate into Dwf (agravitropic) and dwf ⁺ (normal) phenotypes. aux-2 phenotype was indistinguishable from the wild-type on criteria other than resistance to exogenous auxins: 3-fold to 2,4-D and 2-fold to IAA. On the other hand, Dwf plants had a typical dwarf phenotype with single unbranched roots which lacked hairs. Compared to the wild-type, Dwf seedling roots were highly resistant to exogenous auxins: 2000-fold to 2,4-D and 360-fold to IAA. Both aux-2 and Dwf were normal in their response to exogenous ABA. The dwarf phenotype was insensitive to gibberellins but root hair formation was restored by application of auxins.The results indicate that altered auxin phsysiology can lead to agravitropism and dwarfism.Abbrevations ABA Abscisic acid - GA₃ Gibberellic acid - IAA indole-3-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid     

Different Behaviour of Indole-3-Acetic Acid and Indole-3-Butyric Acid in Stimulating Lateral Root Development in Rice (Oryza sativa L.)

The plant hormone auxin has been shown to be involved in lateral root development and application of auxins, indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA), increases the number of lateral roots in several plants. We found that the effects of two auxins on lateral root development in the indica rice (Oryza sativa L. cv. IR8) were totally different from each other depending on the application method. When the roots were incubated with an auxin solution, IAA inhibited lateral root development, while IBA was stimulatory. In contrast, when auxin was applied to the shoot, IAA promoted lateral root formation, while IBA did not. The transport of [³H]IAA from shoot to root occurred efficiently (% transported compared to supplied) but that of [³H]IBA did not, which is consistent with the stimulatory effect of IAA on lateral root production when applied to the shoot. The auxin action of IBA has been suggested to be due to its conversion to IAA. However, in rice IAA competitively inhibited the stimulatory effect of IBA on lateral root formation when they were applied to the incubation solution, suggesting that the stimulatory effect of IBA on lateral root development is not through its conversion to IAA.     

Identification of 4-Chloroindole-3-acetic Acid and Its Methyl Ester in Immature Seeds of Vicia amurensis (the Tribe Vicieae), and Their Absence from Three Species of Phaseoleae

The natural chlorinated auxins 4-chloroindole-3-acetic acid(4-Cl-IAA) and its methyl ester (4-Cl-IAA Me ester) were found,in addition to IAA and its Me ester, by gas chromatography-massspectrometry in immature seeds of Vicia amurensis, a Vicieaespecies. In contrast, only non-chlorinated, IAA and IAA Me esterwere present in immature seeds of three Phaseoleae species.These results are further evidence of the wide distributionof 4-Cl-IAA and its Me ester in various Vicieae. (Received October 3, 1986; Accepted December 22, 1986)     

Radioresistance of kohlrabi (Brassica oleracea L,var.gongylodes L.) seeds in relation to the metabolism of indoles,auxins and gibberellins

Irradiation of the seeds of radioresistant kohlrabi with gamma-rays causes at doses SO to 300 kR a decrease in growth and a drop in tryptophan (Try) level in seven-day-old plants. The level of glucobrassicin (GLUBR), 3-indolylacetonitrile (IAN) and gibberellic acid (GA3) in these plants increases up to a maximum with a dose of about 150–200 kR, afterwards it decreases. We assume that the specific system of auxin synthesis inBrassicaceae plants takes an important part in the reparation processes and thus in the radioresistance of these plants as well. In contrast to the divergent systems of auxin biosynthesis in other plants which are damaged by lower doses of irradiation, the specific system of auxin formation via GLUBR synthesis, under formation of IAN intermediate, is widely not. attacked by radiation. Thus, these irradiated plants are supplied with auxins and with gibberellins too, the both hormones having a radioprotective effect.     

Auxin production by the Klebsiella planticola strain TSKhA-91 and its effect on development of cucumber (Cucumis sativus L.) seeds

Capacity of Klebsiella planticola strain TSKhA-91 for synthesis of indole-3-acetic acid (IAA) and other auxins was studied. The qualitative and quantitative composition of these compounds depends on the presence of exogenous tryptophan and on the nitrogen source. The highest IAA yield was obtained at the stationary phase of growth. Addition of L-tryptophan to the medium resulted in a significant increase (up to 85.5 μg/mL) of auxin biosynthesis, especially in the presence of nitrates. Thin-layer chromatography revealed that the indole-3-acetamide pathway was not active in this strain. The biological activity of auxins was confirmed by assay with kidney bean cuttings; the height of root formation and root number increased 16- and 6-fold, respectively. Under conditions of low-temperature stress, protective effect of K. planticola TSKhA-91 on development of cucumber (Cucumis sativus L.) seeds and stimulation of germination and root formation by its seeds were shown.     

Indole-3-acetic acid homeostasis in transgenic tobacco plants expressing the Agrobacterium rhizogenes rolB gene

The rolB gene of the plant pathogen Agrobacterium rhizogenes has an important role in the establishment of hairy root disease in infected plant tissues. When expressed as a single gene in transgenic plants the RolB protein gives rise to effects indicative of increased auxin activity. It has been reported that the RolB product is a β-glucosidase and proposed that the physiological and developmental alterations in transgenic plants expressing the rolB gene are the result of this enzyme hydrolysing bound auxins, in particular (indole-3-acetyl)-β-D-glucoside (IAGluc), and thereby bringing about an increase in the intracellular concentration of indole-3-acetic acid (IAA). Using tobacco plants as a test system, this proposal has been investigated in detail. Comparisons have been made between the RolB phenotype and that of IaaM/iaaH transformed plants overproducing IAA. In addition, the levels of IAA and IAA amide and IAA ester conjugates were determined in wild-type and transgenic 35S-rolB tobacco plants and metabolic studies were carried out with [¹³C₆]IAA [2′-¹⁴C]IAA, [¹⁴C]IAGluc, [5-³H]-2-o-(indole-3-acetyl)-myo-inositol and [¹⁴C]indole-3-acetylaspartic acid. The data obtained demonstrate that expression of the rolB encoded protein in transgenic tobacco does not produce a phenotype that resembles that of IAA over producing plants, does not alter the size of the free IAA pool, has no significant effect on the rate of IAA metabolism, and, by implication, appears not to influence the overall rate of IAA biosynthesis. Furthermore, the in vivo hydrolysis of IAGluc, and that of the other IAA conjugates that were tested, is not affected. On the basis of these findings, it is concluded that the RolB phenotype is not the consequence of an increase in the size of the free IAA pool mediated by an enhanced rate of hydrolysis of IAA conjugates.     

Induced parthenocarpy—a way of changing the levels of endogenous hormones in tomato fruits (Lycopersicon esculentum Mill.) 1. Extractable hormones

Ethylene, indol-3-acetic acid (IAA), gibberellin-like substances (GAs) and abscisic acid (ABA) were analysed in extracts from normal, seed-containing and parthenocarpic tomato fruits throughout fruit development. Parthenocarpic fruit growth was induced with an auxin (4-CPA), morphactin (CME) or gibberellic acid (GA₃) and compared with that of pollinated control fruits. Fruit growth was only affected by the treatment with GA₃, decreasing size and fresh weight by 60%. The peak sequence of hormones during fruit development was ethylene-GAs-IAA-ABA. Seeded fruits contained the highest levels of IAA and ABA but the lowest levels of GAs. Also, in seeded fruits, a high proportion of IAA and ABA was found in the seeds whereas this was not the case for GAs.Hormone levels of tomato fruits may be successfully, easily and reproducibly altered by inducing parthenocarpic fruit growth and thus eliminating development of seeds which are a major source of hormone synthesis. In spite of markedly changed hormone levels, there was no obvious relationship between fruit growth and extractable hormones per se. However, the results indicate that a high ratio of GAs: auxins is unfavourable for growth of tomato fruits.     

Effects of Natural and Synthetic Auxins on the Gravitropic Growth Habit of Roots in Two Auxin-Resistant Mutants of Arabidopsis, axr1 and axr4: Evidence for Defects in the Auxin Influx Mechanism of axr4

Arabidopsis thaliana, axr4 , was restored by the addition of 30–300 nM 1-naphthaleneacetic acid (NAA) to the growth medium. Neither indole-3-acetic acid (IAA) nor 2,4-dichlorophenoxyacetic acid (2,4-D) showed such an effect. Growth of axr4 roots was resistant to IAA and 2,4-D, but not at all to NAA. The differential effects of the three auxins suggest that the defects of axr4 result from a lower auxin influx into its cells. The partially agravitropic growth habit of axr1 roots, which was less severe than that of axr4 roots, was only slightly affected by the three auxins in the growth medium at concentrations up to 300 nM; growth of axr1 roots was resistant to all three of the auxins. These results suggest that the lesion of axr1 mutants is different from that of axr4. Received 9 June 1999/ Accepted in revised form 16 August 1999     

Studio Cito-Istologico sulle Curvature Prodotte dalle Radiazioni Ultraviolette in Apici Radicali di Scilla Obtusifolia Poir

Summary

The Author studies the cyto-histological effects produced by ultraviolet radiations on root tips of Scilla obtusifolia Poir. and finds that the most frequent effects are negative curvatures, i. e., the ones causing the withdrawal of the root tip from the irradiating source.

The cyto-histological effects produced by ultraviolet radiations are as follows: 1) The root tip is normal; 2) The zone of elongation is normal too; only the dermatogen, which is limited by the piliferous layer on the irradiated side, is replaced by a necrotic tissue on the non irradiated side; 3) In the piliferous layer, on the irradiated part the epidermis produces a lot of root hairs; the cells of the cortex are very large, elongated and greatly vacuolised; the stele, at the level of the piliferous layers looks normal; on the contrary the cells of the cortex on the opposite side of the irradiation-source are degenerating and the epidermis seems to be necrotic.

If irradiation time is very long and fixation is made 14–24 days after the treatment, the piliferous layer on the irradiated side, has only a few root hairs, since cells, even though with meristemoid characters don't develop into root hairs.

From these data the Author thinks that the stimulating action, on the epidermis directly exposed to ultraviolet radiations, is balanced by the inhibiting one on the opposite side not directly influenced by ultraviolet radiations.

The ultraviolet rays probably break the balance of the growth and inhibition substances acting as auxines, which show a different action according to their concentration.

The different penetration-power of ultraviolet rays on the various levels of the meristematic radical tissues and of the piliferous layer obviously causes more or less intense action.

It is clear that on the side directly irradiated the stimulating action is very evident, but it produces only a phenomenon of hypertrophy, since there is no cell-multiplication.

In all the treatment ultraviolet radiations have caused no hyperplasy-phenomenon.     

Growth and anthocyanin synthesis in excised Sorghum internodes

Summary Ligh-induced anthocyanin synthesis in excised dark-grown internodes of Sorghum was depressed by the addition of auxin to the incubating medium at physiological concentrations. Both IAA and the synthetic auxin, 2,4-D, reduced anthocyanin yield. Similar results were obtained with isolated internode segments and in internodes incubated with coleoptiles (the major source of endogenous auxins) attached. Auxin increased the duration of the lag phase before anthocyanin synthesis began and reduced the rate during the subsequent linear phase. Elongation continued longer with IAA than without it and anthocyanin formation did not begin until extension growth had ceased or was slowing down in both cases; the rate of anthocyanin synthesis in the IAA solution remained depressed compared with that in buffer even after extension growth had ceased in both.At low concentrations IAA stimulated elongation growth without reducing anthocyanin yield and it is unlikely that the effect of IAA on anthocyanin synthesis results from the increased utilisation in growth of substrates needed for anthocyanin formation. The results of reciprocal transfer experiments from dark to light, and vice versa, showed that the action of IAA was associated with its presence in the incubating medium during the irradiation period. If present only in darkness, before or after transfer to light, IAA did not reduce anthocyanin formation; in the former case total yield was increased by IAA as a result of the stimulation of elongation growth, the concentration of anthocyanin remaining unchanged.GA₃ also decreased anthocyanin content; the effect was greater in sections incubated with coleoptiles attached and it is possible that GA₃ acts by increasing the concentration of endogenous auxins. However, CCC, which has been reported to decrease endogenous auxin levels, also reduced anthocyanin yield.The effect of IAA was not influenced by the presence of ascorbate in the incubating medium, nor did added ascorbate result in the formation of any acylated cyanidin derivative in internodes maintained in darkness.Possible relationships between light-induced anthocyanin formation and the photo-inhibition of elongation are discussed.     

Untersuchungen über da Auxin der Maiskoleoptile

Summary By means of theAvena section test in conjunction with paper chromatography very efficient growth inhibiting substances of acid nature, but no native auxins, could be demonstrated in ether extracts of Zea mays coleoptile tips. However it was possible to show that in the same extracts as much as 10^–5 g of added IAA was no longer demonstrable by either method. The significance of these results for the evaluation of rerent findings on the nature of auxins in Zea mays seedlings is discussed.

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Indole-3-butyric acid in plants: occurrence, synthesis, metabolism and transport

Indole-3-butyric acid (IBA) was recently identified by GC/MS analysis as an endogenous constituent of various plants. Plant tissues contained 9 ng g^?1 fresh weight of free IBA and 37 ng g^?1 fresh weight of total IBA, compared to 26 ng g^?1 and 52 ng g^?1 fresh weight of free and total indole-3-acetic acid (IAA), respectively. IBA level was found to increase during plant development, but never reached the level of IAA. It is generally assumed that the greater ability of IBA as compared with IAA to promote rooting is due to its relatively higher stability. Indeed, the concentrations of IAA and IBA in autoclaved medium were reduced by 40% and 20%, respectively, compared with filter sterilized controls. In liquid medium, IAA was more sensitive than IBA to non-biological degradation. However, in all plant tissues tested, both auxins were found to be metabolized rapidly and conjugated at the same rate with amino acids or sugar. Studies of auxin transport showed that IAA was transported faster than IBA. The velocities of some of the auxins tested were 7. 5 mm h^?1 for IAA, 6. 7 mm h^?1 for naphthaleneacetic acid (NAA) and only 3. 2 mm h^?1 for IBA. Like IAA, IBA was transported predominantly in a basipetal direction (polar transport). After application of ³H-IBA to cuttings of various plants, most of the label remained in the bases of the cuttings. Easy-to-root cultivars were found to absorb more of the auxin and transport more of it to the leaves. It has been postulated that easy-to-root, as opposed to the difficult-to-root cultivars, have the ability to hydrolyze auxin conjugates at the appropriate time to release free auxin which may promote root initiation. This theory is supported by reports on increased levels of free auxin in the bases of cuttings prior to rooting. The auxin conjugate probably acts as a ‘slow-release’ hormone in the tissues. Easy-to-root cultivars were also able to convert IBA to IAA which accumulated in the cutting bases prior to rooting. IAA conjugates, but not IBA conjugates, were subject to oxidation, and thus deactivation. The efficiency of the two auxins in root induction therefore seems to depend on the stability of their conjugates. The higher rooting promotion of IBA was also ascribed to the fact that its level remained elevated longer than that of IAA, even though IBA was metabolized in the tissue. IAA was converted to IBA by seedlings of corn and Arabidopsis. The K_m value for IBA formation was low (approximately 20 μM), indicating high affinity for the substrate. That means that small amounts of IAA (only a fraction of the total IAA in the plant tissues) can be converted to IBA. It was suggested that IBA is formed by the acetylation of IAA with acetyl-CoA in the carboxyl position via a biosynthetic pathway analogous to the primary steps of fatty acid biosynthesis, where acetyl moieties are transferred to an acceptor molecule. Incubation of the soluble enzyme fraction from Arabidopsis with ³H-IBA, IBA and UDP-glucose resulted in a product that was identified tentatively as IBA glucose (IBGIc). IBGIc was detected only during the first 30 min of incubation, showing that it might be converted rapidly to another conjugate.     

Does Seed Priming Induce Changes in the Levels of Some Endogenous Plant Hormones in Hexaploid Wheat Plants Under Salt Stress？

In order to assess whether salt tolerance could be Improved In spring wheat （Triticum aestivum L.）, the present study was performed by soaking the seeds of two cultlvars, namely MH-97 （salt sensitive） and Inqlab-91 （salt tolerant）, for 12 h In distilled water or 100 mol/m^3 CaCl2, KCI, or NaCI. Primed seeds from each treatment group and non-primed seeds were sown In a field In which NaCI salinity of 15 dS/m was developed. Priming of seeds with CaCl2, followed by priming with KCI and NaCI, was found to be effective In alleviating the adverse effects of salt stress on both wheat cultivars In terms of shoot fresh and dry weights and grain yield. Priming with CaCl2 alleviated the adverse effects of salt stress on hormonal balance In plants of both cultlvars. In MH-97 plants, CaCl2 pretreatment considerably reduced leaf absclslc acid （ABA） concentrations and Increased leaf free salicylic acid （SA） concentrations under both saline and non-saline conditions. In contrast, In the Inqlab-91 plant, CaCl2 Increased free Indoleacetic acid （IAA） and indolebutyrlc acid （IBA） content. However, priming of seeds with CaCl2 did not alter free polyamlne levels in either cultlvar, although spermldlne levels were considerably lower In plants raised from seeds treated with CaCl2 for both cultlvars under saline conditions. Priming with KCI Increased growth In Inqlab-91 plants, but not In MH-97 plants, under saline conditions. The salinity Induced reducUon In auxins （IAA and IBA） was alleviated by NaCI priming In both cultlvars under saline conditions. However, NaCI Increased leaf free ABA content and lowered leaf SA and putresclne levels In Inqlab-91 plants under saline conditions. In conclusion, although all three priming agents （I.e. CaCl2, KCI, and NaCI） were effective In alleviating the adverse effects of salt stress on wheat plants, their effects on altering the levels of different plant hormones were different In the two cuItlvars.