Arginine decarboxylase and polyamines required for embryogenesis in the wild carrot

Polyamines and hydrogen peroxide: Allies in plant resilience against abiotic stress

The increasing prevalence and severity of abiotic stresses on plants due to climate change is among the crucial issues of decreased crop productivity worldwide. These stresses affect crop productivity and pose a challenge to food security. Polyamines (Pas) and hydrogen peroxide (H₂O₂) could play a vital role to minimize the impact of several abiotic stresses on the plants. Pas are small molecules that regulate various physiological and developmental processes in plants and confer stress tolerance and protection against dehydration and cellular damage. Pas also interact with plant growth regulators and participate in various signaling routes that can mediate stress response. H₂O₂ on the other hand, acts as a signaling agent and plays a pivotal part in controlling crop growth and productivity. It can trigger oxidative damage at high levels but acts as a stress transducer and regulator at low concentrations. H₂O₂ is involved in stress defense mechanisms and the activation of genes involved in conferring tolerance. Therefore, the main focus of this paper is to explore roles of Pas and H₂O₂ in plant responses to various abiotic stress, highlighting their involvement in stress retaliation and signaling routes. Emphasis has been placed on understanding how Pas and H₂O₂ function and interact with other signaling molecules. Also, interaction of Pas and H₂O₂ with calcium ions, abscisic acid and nitrogen has been discussed, along with activation of MAPK cascade. This additive understanding could contribute to adopt strategies to improve crop productivity and enhance plant resilience to environmental challenges.

Potential Applications of Polyamines in Agriculture and Plant Biotechnology

The polyamines putrescine, spermidine and spermine have been implicated in a myriad of biological functions in many organisms. Research done during the last decades has accumulated a large body of evidence demonstrating that polyamines are key modulators of plant growth and development. Different experimental approaches have been employed including the measurement of endogenous polyamine levels and the activities of polyamine metabolic enzymes, the study of the effects resulting from exogenous polyamine applications and chemical or genetic manipulation of endogenous polyamine titers. This chapter reviews the role of PAs in seed germination, root development, plant architecture, in vitro plant regeneration, flowering and plant senescence. Evidence presented here indicates that polyamines should be regarded as plant growth regulators with potential applications in agriculture and plant biotechnology.

Transcriptome analysis of creeping bentgrass exposed to drought stress and polyamine treatment

Creeping bentgrass is an important cool-season turfgrass species sensitive to drought. Treatment with polyamines (PAs) has been shown to improve drought tolerance; however, the mechanism is not yet fully understood. Therefore, this study aimed to evaluate transcriptome changes of creeping bentgrass in response to drought and exogenous spermidine (Spd) application using RNA sequencing (RNA-Seq). The high-quality sequences were assembled and 18,682 out of 49,190 (38%) were detected as coding sequences. A total of 22% and 19% of genes were found to be either up- or down-regulated due to drought while 20% and 34% genes were either up- or down- regulated in response to Spd application under drought conditions, respectively. Gene ontology (GO) and enrichment analysis were used to interpret the biological processes of transcripts and relative transcript abundance. Enriched or differentially expressed transcripts due to drought stress and/or Spd application were primarily associated with energy metabolism, transport, antioxidants, photosynthesis, signaling, stress defense, and cellular response to water deprivation. This research is the first to provide transcriptome data for creeping bentgrass under an abiotic stress using RNA-Seq analysis. Differentially expressed transcripts identified here could be further investigated for use as molecular markers or for functional analysis in responses to drought and Spd.

De novo transcriptome analysis reveals insights into dynamic homeostasis regulation of somatic embryogenesis in upland cotton (G. hirsutum L.)

Plant regeneration via somatic embryogenesis (SE) is the key step for genetic improvement of cotton (Gossypium hirsutum L.) through genetic engineering mediated by Agrobacteria, but the molecular mechanisms underlying SE in cotton is still unclear. Here, RNA-Sequencing was used to analyze the genes expressed during SE and their expression dynamics using RNAs isolated from non-embryogenic callus (NEC), embryogenic callus (EC) and somatic embryos (SEs). A total of 101, 670 unigenes were de novo assembled. The genes differentially expressed (DEGs) amongst NEC, EC and SEs were identified, annotated and classified. More DEGs were found between SEs and EC than between EC and NEC. A significant number of DEGs were related to hormone homeostasis, stress and ROS responses, and metabolism of polyamines. To confirm the expression dynamics of selected DEGs involved in various pathways, experiments were set up to investigate the effects of hormones (Indole-3-butytric acid, IBA; Kinetin, KT), polyamines, H2O2 and stresses on SE. Our results showed that exogenous application of IBA and KT positively regulated the development of EC and SEs, and that polyamines and H2O2 promoted the conversion of EC into SEs. Furthermore, we found that low and moderate stress is beneficial for proliferation of EC and SEs formation. Together, our global analysis of transcriptomic dynamics reveals that hormone homeostasis, polyamines, and stress response synergistically regulating SE in cotton.

Spermidine, a polyamine, confers resistance to rice blast

Polyamines are involved not only in fundamental cellular processes such as growth, differentiation, and morphogenesis, but also in various environmental stresses. We demonstrated that spermidine, a polyamine, confers resistance to rice blast accompanied by the up-regulation of marker genes for the salicylic acid-mediated signaling pathway PR1b and PBZ1 and of phytoalexin biosynthesis genes CPS4 and NOMT. This is the first report about the involvement of spermidine in rice disease resistance.

Putrescine Alleviates Iron Deficiency via NO-Dependent Reutilization of Root Cell-Wall Fe in Arabidopsis

Plants challenged with abiotic stress show enhanced polyamines levels. Here, we show that the polyamine putrescine (Put) plays an important role to alleviate Fe deficiency. The adc2-1 mutant, which is defective in Put biosynthesis, was hypersensitive to Fe deficiency compared with wild type (Col-1 of Arabidopsis [Arabidopsis thaliana]). Exogenous Put decreased the Fe bound to root cell wall, especially to hemicellulose, and increased root and shoot soluble Fe content, thus alleviating the Fe deficiency-induced chlorosis. Intriguingly, exogenous Put induced the accumulation of nitric oxide (NO) under both Fe-sufficient (+Fe) and Fe-deficient (-Fe) conditions, although the ferric-chelate reductase (FCR) activity and the expression of genes related to Fe uptake were induced only under -Fe treatment. The alleviation of Fe deficiency by Put was diminished in the hemicellulose-level decreased mutant-xth31 and in the noa1 and nia1nia2 mutants, in which the endogenous NO levels are reduced, indicating that both NO and hemicellulose are involved in Put-mediated alleviation of Fe deficiency. However, the FCR activity and the expression of genes related to Fe uptake were still up-regulated under -Fe+Put treatment compared with -Fe treatment in xth31, and Put-induced cell wall Fe remobilization was abolished in noa1 and nia1nia2, indicating that Put-regulated cell wall Fe reutilization is dependent on NO. From our results, we conclude that Put is involved in the remobilization of Fe from root cell wall hemicellulose in a process dependent on NO accumulation under Fe-deficient condition in Arabidopsis.

Polyamine and Its Metabolite H2O2 Play a Key Role in the Conversion of Embryogenic Callus into Somatic Embryos in Upland Cotton (Gossypium hirsutum L.)

The objective of this study was to increase understanding about the mechanism by which polyamines (PAs) promote the conversion of embryogenic calli (EC) into somatic embryos in cotton (Gossypium hirsutum L.). We measured the levels of endogenous PAs and H2O2, quantified the expression levels of genes involved in the PAs pathway at various stages of cotton somatic embryogenesis (SE), and investigated the effects of exogenous PAs and H2O2 on differentiation and development of EC. Putrescine (Put), spermidine (Spd), and spermine (Spm) significantly increased from the EC stage to the early phase of embryo differentiation. The levels of Put then decreased until the somatic embryo stage whereas Spd and Spm remained nearly the same. The expression profiles of GhADC genes were consistent with changes in Put during cotton SE. The H2O2 concentrations began to increase significantly at the EC stage, during which time both GhPAO1 and GhPAO4 expressions were highest and PAO activity was significantly increased. Exogenous Put, Spd, Spm, and H2O2 not only enhanced embryogenic callus growth and embryo formation, but also alleviated the effects of D-arginine and 1, 8-diamino-octane, which are inhibitors of PA synthesis and PAO activity. Overall, the results suggest that both PAs and their metabolic product H2O2 are essential for the conversion of EC into somatic embryos in cotton.

Transcriptome analysis of Sacha Inchi (Plukenetia volubilis L.) seeds at two developmental stages

Background

Sacha Inchi (Plukenetia volubilis L., Euphorbiaceae) is a potential oilseed crop because the seeds of this plant are rich in unsaturated fatty acids (FAs). In particular, the fatty acid composition of its seed oil differs markedly in containing large quantities of α-linolenic acid (18C:3, a kind of ω-3 FAs). However, little is known about the molecular mechanisms responsible for biosynthesis of unsaturated fatty acids in the developing seeds of this species. Transcriptome data are needed to better understand these mechanisms.

Results

In this study, de novo transcriptome assembly and gene expression analysis were performed using Illumina sequencing technology. A total of 52.6 million 90-bp paired-end reads were generated from two libraries constructed at the initial stage and fast oil accumulation stage of seed development. These reads were assembled into 70,392 unigenes; 22,179 unigenes showed a 2-fold or greater expression difference between the two libraries. Using this data we identified unigenes that may be involved in de novo FA and triacylglycerol biosynthesis. In particular, a number of unigenes encoding desaturase for formation of unsaturated fatty acids with high expression levels in the fast oil accumulation stage compared with the initial stage of seed development were identified.

Conclusions

This study provides the first comprehensive dataset characterizing Sacha Inchi gene expression at the transcriptional level. These data provide the foundation for further studies on molecular mechanisms underlying oil accumulation and PUFA biosynthesis in Sacha Inchi seeds. Our analyses facilitate understanding of the molecular mechanisms responsible for the high unsaturated fatty acids (especially α-linolenic acid) accumulation in Sacha Inchi seeds.

Conversion of protocorm-like bodies of Dendrobium huoshanense to shoots: the role of polyamines in relation to the ratio of total cytokinins and indole-3-acetic acid

In the present paper, a correlation between enhanced conversion of protocorm-like bodies (PLBs) of Dendrobium huoshanense to shoots by free polyamines (PAs) and changes in the levels of endogenous hormones is described. The endogenous levels of free spermidine (Spd) and putrescine (Put) increased during the conversion of PLBs to shoots. The exogenous addition of PAs, mainly Spd or Put at 2.0mM, not only elevated the endogenous levels of PAs but also promoted the frequency of conversion of PLBs to shoots. As compared with control, the enhanced conversion of PLBs to shoots by exogenous PAs was accompanied by an increase in ratio of total cytokinins (CTKs) to indole-3-acetic acid (IAA), which was due to decrease in the endogenous level of IAA and increase in the endogenous level of total CTKs, including the levels of isopentenyladenine+isopentenyladenine 9-riboside and zeatin+zeatin riboside. Analysis of enzyme activities showed that the increased endogenous level of total CTKs by PAs was related to the inhibition of CTK decomposition by CTK oxidase, while the decreased endogenous level of IAA was related to the promotion of IAA decomposition by IAA oxidase. Addition of PA biosynthetic inhibitors, involving alpha-difluoromethylornithine for Put and methylglyoxal(bis)-guanylhydrazone for Spd and Spm, decreased the conversion of PLBs to shoots, the ratio of total CTKs to IAA, and the levels of endogenous Put and Spd. This inhibition could be partly reversed by the application of exogenous Put or Spd.

Polyamines induce rapid biosynthesis of nitric oxide (NO) in Arabidopsis thaliana seedlings

In this study, we examined the regulation by putrescine, spermidine and spermine of nitric oxide (NO) biosynthesis in Arabidopsis thaliana seedlings. Using a fluorimetric method employing the cell-impermeable NO-binding dye diaminorhodamine-4M (DAR-4M), we observed that the polyamines (PAs) spermidine and spermine greatly increased NO release in the seedlings, whereas arginine and putrescine had little or no effect. Spermine, the most active PA, stimulated NO release with no apparent lag phase. The response was quenched by addition of 2-aminoethyl-2-thiopseudourea (AET), an inhibitor of the animal nitric oxide synthase (NOS) and plant NO biosynthesis, and by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-1-oxy-3-oxide (PTIO), an NO scavenger. By fluorescence microscopy, using the cell-permeable NO-binding dye diaminorhodamine-4M acetoxymethyl ester (DAR-4M AM), we observed that PAs induced NO biosynthesis in specific tissues in Arabidopsis seedlings. Spermine and spermidine increased NO biosynthesis in the elongation zone of the Arabidopsis root tip and in primary leaves, especially in the veins and trichomes, while in cotyledons little or no effect of PAs beyond the endogenous levels of NO-induced fluorescence was observed. We conclude that PAs induce NO biosynthesis in plants.

Polyamines and somatic embryogenesis in two Vitis vinifera cultivars

Polyamine content and activities of enzymes of polyamine biosynthesis were assayed during somatic embryogenesis in Vitis vinifera callus cultures of Chardonnay and Brachetto 'a grappolo lungo' (Brachetto g.l.) cultivars. The analyses were carried out on embryogenic callus samples, embryos at different stages and developing plants. Polyamine content, both in the free and PCA-soluble conjugated form, was higher in Brachetto g.l. than in Chardonnay, and putrescine was present at higher concentrations than the other polyamines. In all samples of both cultivars, ornithine decarboxylase activity (ODC, EC 4.1.1.17) was higher than arginine decarboxylase (ADC, EC 4.1.1.19), with a maximum in developing plant roots. S-Adenosylmethionine decarboxylase (SAMDC, EC 4.1.1.50) activity displayed a similar trend. The activities of all three enzymes were detected both in the supernatant and pellet fractions, indicating for the first time the presence of SAMDC activity in the particulate fraction. Particularly in the Chardonnay cultivar, an increase in the mRNAs expression patterns of ODC and SAMDC during morphogenesis from small embryos to plantlets was detected by northern blot, suggesting a direct correlation with enzymatic activities.

The polyamines of Xanthium strumarium and their response to photoperiod

Flowering plants of Xanthium strumarium L., grown in 8 h photoperiods, were analysed for polyamines. Putrescine, spermidine and spermine were found throughout the plant in three forms: (a) as free polyamines; (b) conjugates soluble in 5% trichloracetic acid (TCA); and (c) bound to the TCA-insoluble precipitate. On a fresh weight basis, total polyamines are most abundant in young leaves and buds, especially flower buds. Spermidine predominates in the free polyamine fractions, while spermine is dominant in the conjugated fraction. Transfer of vegetative plants from 16 h photoperiods to 1, 2, 3, or 4 inductive cycles (8 h light + 16 h uninterrupted dark) caused rapid and marked changes in the polyamine titer of the leaves and ultimately, floral initiation. The titer of free putrescine per mg protein declined progressively with induction in all leaf sizes, while the titers of free spermidine and spermine rose during days 2 and 3 in small and expanding leaves. Conjugated putrescine, spermidine and spermine rose sharply after only 1 inductive cycle, especially in small and expanding leaves, and maintained the higher level for at least several cycles. In plants given 4 inductive cycles, buds harvested after 4 additional days had sharply elevated levels of conjugated polyamines, especially spermine, on a protein basis.

Effects of light quality on somatic embryogenesis in Araujia sericifera

The effects of photoperiod, light quality and end-of-day (EOD) phytochrome photoconversion on somatic embryogenesis (SE) of Araujia sericifera petals have been studied. Petals from immature flowers were cultured under 8- and 16-h photoperiods using Gro-lux fluorescent lamps. The photon fluence rate was 90-100 µmol m-2 s-1 and the red (R):far-red (FR) ratio was 98. R, FR, R followed by FR (R-FR) and FR followed by R (FR-R) light treatments were applied for 3 weeks at the end of the photoperiods. In a set of experiments, DL-alpha-difluoromethylarginine (DFMA) or methylglyoxal bis(guanylhydrazone) (MGBG), both inhibitors of polyamine biosynthesis, were added to the culture medium in order to study the involvement of polyamine metabolism. The level of SE was the same in long (LD) and short (SD) days. Thus, the light effect was accomplished after 8 h. All EOD treatments that decreased the Pfr level inhibited SE when applied after SD, but not after LD. The FR-R treatment after LD caused an additional stimulatory effect on SE, even in the presence of polyamine inhibitors. DFMA inhibited SE in both SD and LD, but MGBG did not modify SE in either SD or LD. The R, FR and R-FR treatments did not alter the level of SE when applied after LD in the presence of DFMA or MGBG. However, these treatments decreased SE after SD when the medium contained polyamine inhibitors. Our results suggest that Gro-lux lamps, which produce an extremely high R:FR ratio, promote SE in A. sericifera and a timing response to phytochrome photoconversion during photoperiodic induction. Thus, our data corroborate the involvement of phytochromes and polyamines in SE in A. sericifera, which responded as a light-dominant long-day plant.

Effect of spermine on proliferation of hyphae of Glomus fistulosum, an arbuscular mycorrhizal fungus, in maize roots

Effects of two oligoamines, putrescine and spermine, on proliferation of intraradical hyphae in surface disinfected root segments were studied under axenic conditions in vitro. No significant effects of putrescine were observed. Spermine significantly stimulated hyphal growth at a concentration of about 1.5 mumol/L. High concentration (> 150 mumol/L) caused a strong inhibition of hyphal growth and of the percentage of root segments bearing proliferating hyphae. DL-alpha-difluoromethylornithine, a metabolic inhibitor of polyamine synthesis, caused a significant inhibition of proliferation of the hyphae only in the presence of 2 mumol/L spermine.

Photoinduction of arginine decarboxylase activity in leaves of Pharbitis nil

The activity of arginine decarboxylase (ADC) in leaves of Pharbitis nil was induced by light. The ADC activity increased to a maximum 1 h after illumination, followed by a gradual decrease. This suggested light either induced synthesis of ADC protein de novo or was involved in its activation. Cycloheximide inhibited the photoinduction of ADC activity, and the half life of ADC in leaves was 30-40 min. The temperature and relative humidity in darkness before illumination had no effect on the photoinduction of ADC activity, contrary to the photoresponse of S-adenosylmethionine decarboxylase (SAMDC) activity where the conditions of darkness before lights-on have a marked effect. The light response of the polyamine (PA)-biosynthetic enzyme activity produced transient accumulation of PA. The level of spermidine increased in leaves in which activities of both SAMDC and ADC increased after illumination, while the level of putrescine increased in leaves in which the activity of ADC increased but that of SAMDC did not.

Temporal Regulation of Somatic Embryogenesis by Adjusting Cellular Polyamine Content in Eggplant

Four critical stages of embryogenesis, including callus induction, cellular acquisition of morphogenetic competence, expression of embryogenic program, and development and maturation of somatic embryos during somatic embryogenesis from leaf discs of eggplant (Solanum melongena L.), were identified by scanning electron microscopy. Temporal changes in arginine decarboxylase (ADC) activity and polyamines (PAs) during critical stages of embryogenesis revealed that high levels of PAs (especially putrescine [PUT]), due to higher ADC activity in discs from the apical region (with high embryogenic capacity) than from the basal region of the leaf (with poor embryogenic capacity), were correlated with differential embryogenesis response. Kinetic studies of the up- and down-regulation of embryogenesis revealed that PUT and difluoromethylarginine pretreatments were most effective before the onset of embryogenesis. Basal discs pretreated with PUT for 4 to 7 d showed improved embryogenesis that was comparable to apical discs. PA content at various critical steps in embryogenesis from basal discs were found to be comparable to that of apical discs following adjustments of cellular PA content by PUT. In contrast, pretreatment of apical discs with difluoromethylarginine for 3 d significantly reduced ADC activity, cellular PA content, and embryogenesis to levels that were comparable to basal discs. Discs from the basal region of leaves treated with PUT for 3 d during the identified stages of embryogenesis improved their embryogenic potential.

Polyamines as modulators of microcycle conidiation in Aspergillus flavus

Since polyamines (PAs) play a potential role in the regulation of growth and developmental processes in a wide variety of organisms, we have examined the influence of the PAs putrescine (Put) and spermidine (Spd) and the PA biosynthetic inhibitors alpha-difluoromethylornithine (DFMO), alpha-difluoromethylarginine (DFMA), methylglyoxal bis-(guanylhydrazone) (MGBG) and cyclohexylamine (CHA), singly and in combinations on microcycle conidiation (MC) in Aspergillus flavus. The exogenous application of the diamine Put (concentrations ranging from 0.1 to 5 mM) caused a sharp decline of MC in a dose-dependent fashion, but induced vegetative growth. However, the triamine Spd (0.1-5 mM) had a minimal effect on MC and induced a shift from MC to normal condition. PA inhibitors, especially DFMO, MGBG and CHA, produced greater inhibition of MC and complete inhibition of MC was observed at 5 mM of these inhibitors. DFMA even at 5 mM had only a weak inhibitory effect on MC. DFMO also inhibited conidial germination and germ tube growth. MGBG and CHA, while having an inhibitory effect on MC, induced vegetative growth. The inhibitory effect of PA inhibitors was partially reversed by exogenous Put or Spd, with Spd being more effective than Put. The analysis of free PA levels during various phases of MC revealed that undifferentiated spores contained a high Put/Spd ratio and there was a dramatic decrease in Put/Spd ratio before and during microcycle conidiophore maturity. The change in spermine titres could not be detected. These observations imply that Put is essential for vegetative growth, while Spd is involved in MC, and that a low Put/Spd ratio seems to be important for spore differentiation to MC.

Stimulating effect of spermine on bulblet formation in bulb-scale segments of Lilium longiflorum

When bulb-scale segments of Lilium longiflorum were cultured on a medium containing auxin and cytokinin, the proportion of the expiants with newly-formed bulblets was significantly increased by the application of different polyamines. The most effective polyamine was spermine, where more than 90% of segments formed an average of 5 bulblets as compared to controls where less than 50% explants formed an average of 1.5 bulblets. Application of arginine one of the precursors putrescine biosynthesis, slightly promoted bulblet formation. The putrescine-stimulated bulblet formation was strongly inhibited by simultaneous addition of an inhibitor of the spermidine synthase, cyclohexylamine. The spermidine-promoted bulblet formation, however, could not be suppressed by this inhibitor. The promotive effect of spermidine on bulblet formation was reversed by an inhibitor of the spermine synthase, N-(3-aminopropyl)cyclohexylamine, but application of this inhibitor with spermine did not show any apparent effect on the bulblet formation. Endogenous level of spermine increased in common during bulblet formation that were stimulated by exogenous polyamines. Thus, spermine seemed to be the main stimulating chemical on bulblet formation in lily bulb-scale segments.

Phenylalanine ammonia-lyase, phenolic acids and ethylene in alfalfa (Medicago sativa L.) cell cultures in relation to their embryogenic ability

Phenylalanine ammonia-lyase (PAL) activity, contents of phenolic acids and ethylene production during the lag-phase, and contents of phenolic acids at the late exponential phase, showed significant differences in embryogenic (EC) and non-embryogenic (NEC) suspension cultures of Medicago sativa L. Maximum PAL activity at 6 h after inoculation was followed by an increase in the level of phenolic acids from 9.6 μg g(-1) fresh mass to 21 μg g(-1) fresh mass in NEC at 12 h. Thereafter the level of phenolic acids decreased to 5.2 μg g(-1) fresh mass at 72 h. The decline was caused predominantly by the decrease of ester-bound cinnamic acid derivatives, the decrease ranging from 83 to 20% of total phenolics. Two maxima of ethylene production were observed in NEC: the first one immediately after inoculation and the second at 6 h, coinciding with the peak of PAL activity. In NEC, most of the phenolic acids occurred in esterified form. Ability to form somatic embryos (EC) was associated with the absence of the second peak of ethylene production as well as of the peak of PAL activity at 6 h. The level of phenolic acids during the lag-phase remained low (7.2 μg g(-1) FM) and did not change. The proportion of cinnamic acid derivatives was very low (18% of total phenolics), mostly due to the extremely low level of ferulic acid. In EC, phenolic acids bound to methanol insoluble material formed the major fraction. Loss of embryogenic potential of the embryogenic culture (ECL) was associated with qualitative and quantitative changes in the contents of phenolic acids insignificantly increased PAL activity after inoculation was followed by a moderate increase in the contents of phenolic acids from 9.35 μg g(-1) fresh mass to 12.42 μg g fresh mass. A high rate of ethylene production was observed only immediately after the transfer of the culture to fresh medium. The loss of embryogenicity correlated also with changes in the relative amounts of the investigated fractions of phenolic acids. A distinct increase in the level of methoxy-substituted phenolic acids is a characteristic feature of the ECL culture.

Analysis of a cDNA encoding arginine decarboxylase from oat reveals similarity to the Escherichia coli arginine decarboxylase and evidence of protein processing

Arginine decarboxylase is the first enzyme in one of the two pathways of putrescine synthesis in plants. We purified arginine decarboxylase from oat leaves, obtained N-terminal amino acid sequence, and then used this information to isolate a cDNA encoding oat arginine decarboxylase. Comparison of the derived amino acid sequence with that of the arginine decarboxylase gene from Escherichia coli reveals several regions of sequence similarity which may play a role in enzyme function. The open reading frame (ORF) in the oat cDNA encodes a 66 kDa protein, but the arginine decarboxylase polypeptide that we purified has an apparent molecular weight of 24 kDa and is encoded in the carboxyl-terminal region of the ORF. A portion of the cDNA encoding this region was expressed in E. coli, and a polyclonal antibody was developed against the expressed polypeptide. The antibody detects 34 kDa and 24 kDa polypeptides on Western blots of oat leaf samples. Maturation of arginine decarboxylase in oats appears to include processing of a precursor protein.

Further experiments on spermidine-mediated floral-bud formation in thin-layer explants of Wisconsin 38 tobacco

We studied the effects of various polyamines on bud regeneration in thin-layer tissue explants of vegetative and floweringNicotiana tabacum L. cv. Wisconsin 38, in which application of exogenous spermidine (Spd) to vegetative cultures causes the initiation and development of some flower buds (Kaur-Sawhney et al. 1988 Planta173, 282). We now show that this effect is dependent on the time and duration of application, Spd being required from the start of the cultures for about three weeks. Neither putrescine nor spermine is effective in the concentration range tested. Spermidine cannot replace kinetin (N(6)-furfurylaminopurine) in cultures at the time of floral bud formation, but once the buds are initiated in the presence of kinetin, addition of Spd to the medium greatly increases the number of floral buds that develop into normal flowers. Addition of Spd to similar cultures derived from young, non-flowering plants did not cause the appearance of floral buds but rather induced a profusion of vegetative buds. These results indicate a morphogenetic role of Spd in bud differentiation.

Stimulation ofDaucus carota somatic embryogenesis by inhibitors of ethylene synthesis: cobalt and nickel

The effects of Co(2+) and Ni(2+) on ethylene production and somatic embryogenesis by carrot (Daucus carota L.) cell cultures were studied. At concentrations of 10 μM to 50 μM, CoCl2 effectively inhibited ethylene production by embryogenic cultures and significantly stimulated somatic embryogenesis. The observed increase of embryo number was proportional to the inhibition level of ethylene production. However, CoCl2 had no effect when Ethephon was supplied. Nickel also reduced ethylene production, but to a slightly lesser extent than CoCl2, bringing about a lower increase in the number of somatic embryos. The role of ethylene on somatic embryogenesis is discussed.

Metabolic links between the biosynthesis of pyrrolizidine alkaloids and polyamines in root cultures of Senecio vulgaris

Isotope feeding and inhibitor experiments were performed in order to elucidate the pathway common to polyamine and alkaloid biosynthesis in root cultures of Senecio vulgaris L. α-Difluoromethylarginine, a specific inhibitor of arginine decarboxylase, prevented completely the incorporation of radioactivity from [(14)C]arginine and [(14)C]ornithine into spermidine and the pyrrolizidine alkaloid senecionine N-oxide. In contrast, α-difluoromethylornithine, a specific ornithine-decarboxylase inhibitor, had no effect on the flow of radioactivity from labelled ornithine and arginine into polyamines and alkaloids. Thus, putrescine, the common precursor of polyamines and pyrrolizidine alkaloids, is exclusively derived via the arginine-agmatine route. Ornithine is rapidly transformed into arginine. Recycling of the guanido moiety of agmatine back to ornithine can be excluded. Putrescine and spermidine were found to be reversibly interconvertable and to excist in a highly dynamic state. In contrast, senecionine N-oxide did not show any turnover but accumulated as a stable metabolic product. In-vivo evidence is presented that the carbon flow from arginine into the polyamine/alkaloid pathway may be controlled by spermidine. The possible importance of the metabolic coupling of pyrrolizidine-alkaloid biosynthesis to polyamine metabolism is discussed.

Effect of l-aminocyclopropane-l-carboxylic acid, silver nitrate, and norbornadiene on plant regeneration from maize callus cultures

The effect of the ethylene antagonists norbornadiene and silver nitrate and the ethylene precursor l-aminocyclopropane-l-carboxylic acid (ACC) on Zea mays plant regeneration was studied. A 12-fold increase in plant regeneration, as measured by number of plants obtained per gram fresh weight from callus cultures of maize inbreds Pa91 and H99, was obtained by 250 μM norbornadiene and 100 μM silver nitrate treatments. An increase in amout of nonregenerable tissue and a 68% decrease in plant regeneration were associated with callus treated with 1 mM ACC. Ethylene emanation from 1 mM ACC treated callus reached a maximum of 170 nl g(-1) h(-1) after 3 days compared to 7 nl g(-1) h(-1) for the control. The free proline content was up to 80% lower in 1 mM ACC treated callus grown for 30 days on medium with or without 12 mM proline, respectively, as compared to each control. These studies indicate that ethylene action inhibitors such as norbornadiene and silver nitrate can be used to increase plant regeneration efficiency from maize callus cultures.

Spermidine and flower-bud differentiation in thin-layer explants of tobacco

Three lines of evidence indicate a connection between high spermidine levels and floral initiation in thin-layer tissue cultures of Wisconsin-38 tobacco (Nicotiana tabacum L.). (1) Spermidine levels are much higher in floral buds than in vegetative buds. (2) Inhibition of spermidine synthesis by cyclohexylamine prevents the rise in spermidine titer, inhibits floral initiation and promotes the formation of vegetative buds instead. (3) Application of exogenous spermidine causes floral initiation in cultures which would otherwise form vegetative buds.

Catalytic irreversible inhibition of bacterial and plant arginine decarboxylase activities by novel substrate and product analogues

Arginine decarboxylase (ADC) activity from Escherichia coli and two plant species (oats and barley) was inhibited by five new substrate (arginine) and product (agmatine) analogues. The five compounds, (E)-alpha-monofluoromethyldehydroarginine (delta-MFMA), alpha-monofluoromethylarginine (MFMA), alpha-monofluoromethylagatine (FMA), alpha-ethynylagmatine (EA) and alpha-allenylagmatine (AA), were all more potent inhibitors of ADC activity than was alpha-difluoromethylarginine (DFMA), the only irreversible inhibitor of this enzyme described previously. The inhibition caused by the five compounds was apparently enzyme-activated and irreversible, since the loss of enzyme activity followed pseudo-first-order kinetics, was time-dependent, the natural substrate of ADC (arginine) blocked the effects of the inhibitors, and the inhibition remained after chromatography of inhibited ADC on Sephadex G-25 or on overnight dialysis of the enzyme. DFMA, FMA, delta-MFMA and MFMA were effective at very low concentrations (10 nM-10 microM) at inhibiting ADC activity in growing E. coli. FMA was also shown to deplete putrescine effectively in E. coli, particularly when combined with an inhibitor of ornithine decarboxylase, alpha-monofluoromethyl-putrescine. The potential uses of the compounds for the study of the role of polyamine biosynthesis in bacteria and plants is discussed.

Biochemical differences between embryogenic and nonembryogenic callus of Picea abies (L.) Karst

Both embryogenic and nonembryogenic calli of Picea abies (L.) Karst. were initiated from the hypocotyl region of immature embryos. The two callus phenotypes were manually separated and subsequently maintained independently, but under identical culture conditions. Biochemical analysis of the two phenotypes revealed significant differences in ethylene evolution rate and in concentrations of glutathione and total reductants. Due to the constancy of the genetic background, age and growth conditions of the two callus types, differences in the measured quantities are not likely to be traceable to the genetic origin of the callus and serve to highlight biochemical changes associated with somatic embryogenesis in Norway spruce.

Effect of polyamine biosynthetic inhibitors on alkaloids and organogenesis in tobacco callus cultures

We studied the effects of inhibitors of ornithine decarboxylase (ODC), arginine decarboxylase (ADC) and spermidine synthase (Spd synthase) on organogenesis and the titers of polyamines (PA) and alkaloids in tobacco calli. DL-alpha-diffluromethylarginine (DFMA) and D-arginine (D-Arg), both inhibitors of ADC activity, were more effective than DL-alpha-difluromethylorinithine (DFMO), an inhibitor of ODC, in reducing titers of PA and the putrescine (Put)-derived alkaloids (nornicotine and nicotine). Dicyclohexylammonium sulfate (DCHA), an inhibitor of Spd synthase, was also more efficient than DFMO in reducing PA and alkaloid levels. Root organogenesis is inversely related to the titers of Put and alkaloids. Thus, DFMA and D-Arg, which strongly inhibit Put and alkaloid biosynthesis, markedly promote root organogenesis, while control callus with high Put and alkaloid content showed poor root organization. These results suggest that morphological differentiation is not required for activation of secondary metabolic pathways and support the view that ADC has a major role in the generation of Put going to the pyrrolidine ring of tobacco alkaloids.

Exogenous polyamines alter membrane fluidity in bean leaves - a basis for potential misinterpretation of their true physiological role

Changes in the rotational motion of paramagnetic and fluorescent lipid-soluble probes were used to assess the effects of putrescine, spermidine and spermine on the fluidity of microsomal membranes from primary leaves of bean (Phaseolus vulgaris L.). Surface probes were more strongly immobilized by physiological concentrations of the polyamines than probes that partitioned deep into the bilayer interior. Spermidine and spermine were more effective than putrescine at reducing membrane fluidity, and at equimolar concentrations, the polyamines and calcium had similar effects on the mobility of the membrane probes. Spermine had essentially equivalent effects on the fluidity of native membranes, heat-denatured membranes and liposomes prepared from the total lipid extract of the membranes, indicating that polyamines associate with membrane lipid. These results raise the possibility that some of the physiological effects previously attributed to exogenously added polyamines could reflect membrane rigidification rather than a true physiological response.

Polyamines and crown gall tumor growth

The effect of the polyamine spermidine on the growth of crown gall tumors was determined using the potato disc bioassay. Addition of lmM spermidine resulted in a 30-50% increase in tumor growth. The spermidine effect was found to be biphasic, with lmM being optimal. Closely related polyamines including spermine, as well as other nitrogen containing compounds such as arginine and alanine, failed to promote tumor growth or inhibited the growth of these tumors. Endogenous levels of spermidine in crown gall tumor tissue were consistently greater than those of corresponding normal potato tissue. Rapidly dividing normal potato tissue derived from buds also contained elevated spermidine levels.

Polyamine levels as related to growth, differentiation and senescence in protoplast-derived cultures of Vigna aconitifolia and Avena sativa

We have previously reported that aseptically cultured mesophyll protoplasts of Vigna divide rapidly and regenerate into complete plants, while mesophyll protoplasts of Avena divide only sporadically and senesce rapidly after isolation. We measured polyamine titers in such cultures of Vigna and Avena, to study possible correlations between polyamines and cellular behavior. We also deliberately altered polyamine titer by the use of selective inhibitors of polyamine biosynthesis, noting the effects on internal polyamine titer, cell division activity and regenerative events. In Vigna cultures, levels of free and bound putrescine and spermidine increased dramatically as cell division and differentiation progressed. The increase in bound polyamines was largest in embryoid-forming callus tissue while free polyamine titer was highest in root-forming callus. In Avena cultures, the levels of total polyamines decreased as the protoplast senesced. The presence of the inhibitors alpha-difluoromethyl-arginine (specific inhibitor of arginine decarboxylase), alpha-difluoromethylornithine (specific inhibitor of ornithine decarboxylase) and dicyclohexylamine (inhibitor of spermidine synthase) reduced cell division and organogenesis in Vigna cultures. Addition of low concentration of polyamines to such cultures containing inhibitors or removal of inhibitors from the culture medium restored the progress of growth and differentiation with concomitant increase in polyamine levels.