Exogenous spermidine alleviates fruit granulation in a <Emphasis Type="Italic">Citrus</Emphasis> cultivar (<Emphasis Type="Italic">Huangguogan</Emphasis>) through the antioxidant pathway

The role of exogenous spermidine (Spd) in alleviating fruit granulation in the grafted seedlings of a Citrus cultivar (Huangguogan) was investigated. Granulation resulted in increased electrical conductivity, cell membrane permeability, and total pectin, soluble pectin, cellulose, and lignin contents. However, it decreased the activities of superoxide dismutase, peroxidase, and catalase, as well as the (Spd + Spm):Put ratio. The application of exogenous Spd onto Huangguogan seedlings significantly increased proline and ascorbate contents, but decreased the H₂O₂ and O ₂ ^?· levels, which suggested that exogenous Spd provided some protection from oxidative damage. In addition, exogenous Spd decreased cell membrane permeability and MDA content, and increased the (Spd + Spm):Put ratio. The activities of antioxidant enzymes, such as catalase, peroxidase, and superoxide dismutase, were increased in Spd-treated seedlings affected by fruit granulation, resulting in a decrease in oxidative stress levels. The protective effects of Spd were reflected by a decrease in superoxide levels through osmoregulation, increased proline and ascorbate contents, and increased antioxidant activities. Our observations reveal the importance of exogenous Spd in alleviating citrus fruit granulation.     

External spermine prevents UVA-induced damage of <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 via increased catalase activity and decreased H<Subscript>2</Subscript>O<Subscript>2</Subscript> and malonaldehyde levels

Common polyamines, putrescine (Put), spermidine (Spd), and spermine (Spm), are cationic compounds known as beneficial factors for many cellular processes including cell division, proliferation, differentiation, and stress response in all living organisms. Effects of exogenous Spm on the protective responses of Synechocystis sp. PCC 6803 exposed to UVA were investigated. The presence of 0.5 mM Spm in the culture medium significantly reduced cell growth after 60 min under white light condition but protected the cells after growing for 60 min under UVA. The stress-tolerant response of Synechocystis cells represented by the ratio of putrescine/spermidine (Put/Spd) showed about a 6-fold increase after 60 min UVA in the presence of Spm. In addition, those levels of chlorophyll a, carotenoids, and photosynthetic oxygen evolution were increased by Spm supplementation in UVA-treated cells. Exogenous Spm induced the activity of catalase but not superoxide dismutase in cells under UVA treatment. On the other hand, Spm treatment enabled cells to apparently decrease the intracellular free radical H₂O₂ and malonaldehyde (MDA) levels. Overall results suggested that Spm supplementation could protect Synechocystis sp. PCC 6803 cells via the increase of Put/Spd ratio and the reduction of both H₂O₂ and MDA levels in conjunction with the induction of catalase activity. Interestingly, UVA-treated cells as compared to non-treated cells with exogenous Spm showed a decrease of Spm with an increase of Put and no change in Spd. This suggested the back conversion of Spm to Spd and finally to Put as cellular mechanism in response to UVA.     

Exogenous spermidine enhances Hydrocharis dubia cadmium tolerance

The effects of exogenous spermidine (Spd) on arginine decarboxylase (ADC), ornithine decarboxylase (ODC), polyamine oxidase (PAO), and diamine oxidase (DAO) activities, the rate of superoxide radical (O ₂ ^·? ) generation and polyamine (PA), malondialdehyde (MDA), and H₂O₂ contents in Hydrocharis dubia (Bl.) Backer leaves under cadmium (Cd) toxicity were studied after 6-day treatment. Cd stress increased putrescine (Put) level and lowered spermidine (Spd) and spermine (Spm) levels. In addition, the activities of ADC, DAO, and PAO were increased, while that of ODC was decreased. Exogenous application of Spd markedly reversed these Cd-induced effects. It also significantly reduced the generation of O ₂ ^·? and H₂O₂ and prevented lipid peroxidation. These results suggest that exogenous Spd can enhance the tolerance of H. dubia to Cd. The maintenance of PA homeostasis was necessary for plant metal tolerance.     

Drought-inhibited ribulose-1,5-bisphosphate carboxylase activity is mediated through increased release of ethylene and changes in the ratio of polyamines in pakchoi

To study the mechanisms of drought inhibiting photosynthesis and the role of PAs and ethylene, the photosynthetic rate (P_n), the maximal photochemical efficiency of PSII (F_v/F_m), the intercellular CO₂ concentration (C_i), photorespiratory rate (P_r), the amount of chlorophyll (chl), antioxidant enzyme activity, ethylene levels, RuBPC (ribulose-1,5-bisphosphate carboxylase) activity and endogenous polyamine levels of pakchoi were examined, and an inhibitor of S-adenosylmethionine decarboxylase (SAMDC) and an inhibitor of ethylene synthesis and spermidine (Spd) were used to induce the change of endogenous polyamine levels. The results show that drought induced a decrease in P_n and RuBPC activity, an increase in the intercellular CO₂ concentration (C_i), but no change in the actual photochemical efficiency of PSII (Φ_PSII), and chlorophyll content. In addition, drought caused an increase in the free putrescine (fPut), the ethylene levels, a decrease in the Spd and spermine (Spm) levels, and the PAs/fPut ratio in the leaves. The exogenous application of Spd and amino oxiacetic acid (AOAA, an inhibitor of ethylene synthesis) markedly reversed these drought-induced effects on polyamine, ethylene, P_n, the PAs/fPut ratio and RuBPCase activity in leaves. Methylglyoxal-bis(guanylhydrazone) (MGBG), an inhibitor of SAMDC resulting in the inability of activated cells to synthesize Spd and Spm, exacerbates the negative effects induced by drought. These results suggest that the decrease in P_n is at least partially attributed to the decrease of RuBPC activity under drought stress and that drought inhibits RuBPC activity by decreasing the ratio of PAs/fPut and increasing the release of ethylene.     

Regulation of exogenous spermidine on the reactive oxygen species level and polyamine metabolism in <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> (Mart.) Griseb under copper stress

Effects of exogenous spermidine (Spd) on the reactive oxygen species level and polyamine metabolism against copper (Cu) stress in Alternanthera philoxeroides (Mart.) Griseb leaves were investigated. Cu treatment induced a marked accumulation of Cu and enhanced contents of malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and the generation rate of O₂ ^·−. It also significantly increased putrescine (Put) levels but lowered spermidine (Spd) and spermine (Spm) levels. The activities of arginine decarboxylase (ADC), ornithine decarboxylase (ODC) and polyamine oxidase (PAO) were all elevated with the increase of Cu concentration. However, application of exogenous Spd effectively decreased H₂O₂ content and the generation rate of O₂ ^·−, prevented Cu-induced lipid peroxidation and reduced Cu accumulation. Moreover, it declined level of endogenous Put and increased levels of Spd and Spm. Activities of ADC, ODC and PAO were all inhibited by exogenous Spd. The results indicated that application of exogenous Spd could enhance the tolerance of A. philoxeroides to Cu stress by reducing the reactive oxygen level and balancing polyamine metabolism.     

Putrescine overproduction does not affect the catabolism of spermidine and spermine in poplar and Arabidopsis

The effect of up-regulation of putrescine (Put) production by genetic manipulation on the turnover of spermidine (Spd) and spermine (Spm) was investigated in transgenic cells of poplar (Populus nigra × maximowiczii) and seedlings of Arabidopsis thaliana. Several-fold increase in Put production was achieved by expressing a mouse ornithine decarboxylase cDNA either under the control of a constitutive (in poplar) or an inducible (in Arabidopsis) promoter. The transgenic poplar cells produced and accumulated 8–10 times higher amounts of Put than the non-transgenic cells, whereas the Arabidopsis seedlings accumulated up to 40-fold higher amounts of Put; however, in neither case the cellular Spd or Spm increased consistently. The rate of Spd and Spm catabolism and the half-life of cellular Spd and Spm were measured by pulse-chase experiments using [¹⁴C]Spd or [¹⁴C]Spm. Spermidine half-life was calculated to be about 22–32 h in poplar and 52–56 h in Arabidopsis. The half-life of cellular Spm was calculated to be approximately 24 h in Arabidopsis and 36–48 h in poplar. Both species were able to convert Spd to Spm and Put, and Spm to Spd and Put. The rates of Spd and Spm catabolism in both species were several-fold slower than those of Put, and the overproduction of Put had only a small effect on the overall rates of turnover of Spd or Spm. There was little effect on the rates of Spd to Spm conversion as well as the conversion of Spm into lower polyamines. While Spm was mainly converted back to Spd and not terminally degraded, Spd was removed from the cells largely through terminal catabolism in both species.     

Spermidine application enhances tomato seedling tolerance to salinity-alkalinity stress by modifying chloroplast antioxidant systems

The purpose of this study was to elucidate whether exogenous spermidine (Spd) protection of tomato (Solanum lycopersicum L.) seedlings under salinity-alkalinity stress is associated with antioxidant enzymes in the chloroplast. The effects of exogenous Spd on antioxidant enzyme activity and antioxidant content in the chloroplast were evaluated in seedlings of salt-sensitive ecotype (Zhongza 9) grown in a 75 mM salinity-alkalinity solution, with or without 0.25 mM Spd foliar spraying. Results showed that salinity-alkalinity stress increased MDA content, superoxide anion O₂^?- generation rate, superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) activities and ratio of AsA/DHA and reduced contents of ascorbate (AsA), dehydroascorbate (DHA), AsA+DHA, glutathione (GSH), oxidized glutathione (GSSG), GSH+GSSG, dehydroascorbate reductase (DHAR) activity and ratio of GSH/GSSG in chloroplasts. The exogenous Spd application combined with salinity-alkalinity stress decreased the O₂^?- generation rate and MDA content compared to salinity-alkalinity stress alone. The exogenous Spd also increased AsA-GSH cycle components and increased all antioxidant enzyme activities in most cases. Therefore, exogenous Spd alleviates salinity-alkalinity stress damage using antioxidant enzymes and non-enzymatic systems in chloroplasts.     

Effects of spermidine and spermine levels on salt tolerance associated with tonoplast H+-ATPase and H+-PPase activities in barley roots

The effects of polyamines (Putrescine— Put; Spermidine—Spd; and Spermine—Spm) on␣salt tolerance of seedlings of two barley (Hordeum vulgare L.) cultivars (J4, salt-tolerant; KP7, salt-sensitive) were investigated. The results showed that, the salt-tolerant cultivar J4 seedlings accumulated much higher levels of Spd and Spm and lower Put than the salt-sensitive cultivar KP7␣under salt stress. At the same time, the dry weight of KP7 decreased significantly than that of␣J4. After methylglyoxal bis(guanylhydrazone) [MGBG, an inhibitor of S-adenosylmethionine decarboxylase (SAMDC)] treatment, Spd and Spm levels together with the dry weight of both cultivars were reduced, but the salt-caused dry weight reduction in two cultivars could be reversed by the concomitant treatment with Spd. MGBG decreased the activities of tonoplast H⁺-ATPase and H⁺-PPase too, but the experiments in vitro indicated that MGBG was not able to affect the above two enzyme activities. However, the polyamines, especially Spd, promoted their activities obviously. These results suggested that the conversion of Put to Spd and Spm and maintenance of higher levels of Spd and Spm were necessary for plant salt tolerance.     

Polyamines and Nitric Oxide Link in Regulation of Dormancy Removal and Germination of Apple (Malus domestica Borkh.) Embryos

Polyamines (PAs) belong to plant growth regulators and in complex with classical phytohormones take part in regulation of seed dormancy and germination. Although the impact of reactive oxygen (ROS) and nitrogen (RNS) species on seed germination is well described, the cross talk of PAs with ROS/RNS has never been analyzed. Due to the close connection of PAs and ethylene biosynthetic pathways to arginine (Arg)-dependent NO biosynthesis we investigated production of nitric oxide (NO), peroxynitrite (ONOO^?) and the level of O ₂ ^?? or H₂O₂ in apple embryos, germination of which was PA regulated. PAs: putrescine (Put) and spermidine (Spd) in contrast to spermine (Spm) stimulated germination of apple embryos. Among amino acids, stimulation of germination was observed in Arg and ornithine (Orn) only. Dormancy removal of embryos by PAs was associated with increased accumulation of H₂O₂ and O ₂ ^?? in embryonic axes. At the same stage of completion of sensu stricto germination the stimulatory effect of PAs (Put and Spd) and amino acids, mainly Arg and Orn, was accompanied by enhanced NO and ONOO^? production in embryonic axis. The beneficial effect of PAs (Put and Spd) and their precursors on germination of apple embryos was removed by NO scavenging, suggesting a crucial role of NO in termination of embryo germination and radicle growth. Moreover, activity of polyamine oxidase in embryo axes was greatly enhanced by embryo fumigation with NO. Our data demonstrate the interplay of RNS/ROS with PAs and point to NO action as an integrator of endogenous signals activating germination.     

Comparative proteomic analysis reveals the positive effect of exogenous spermidine on photosynthesis and salinity tolerance in cucumber seedlings

Key message

Our results based on proteomics data and physiological alterations proposed the putative mechanism of exogenous Spd enhanced salinity tolerance in cucumber seedlings.

Abstract

Current studies showed that exogenous spermidine (Spd) could alleviate harmful effects of salinity. It is important to increase our understanding of the beneficial physiological responses of exogenous Spd treatment, and to determine the molecular responses underlying these responses. Here, we combined a physiological analysis with iTRAQ-based comparative proteomics of cucumber (Cucumis sativus L.) leaves, treated with 0.1 mM exogenous Spd, 75 mM NaCl and/or exogenous Spd. A total of 221 differentially expressed proteins were found and involved in 30 metabolic pathways, such as photosynthesis, carbohydrate metabolism, amino acid metabolism, stress response, signal transduction and antioxidant. Based on functional classification of the differentially expressed proteins and the physiological responses, we found cucumber seedlings treated with Spd under salt stress had higher photosynthesis efficiency, upregulated tetrapyrrole synthesis, stronger ROS scavenging ability and more protein biosynthesis activity than NaCl treatment, suggesting that these pathways may promote salt tolerance under high salinity. This study provided insights into how exogenous Spd protects photosynthesis and enhances salt tolerance in cucumber seedlings.

     

Spermidine pretreatment enhances heat tolerance in rice seedlings through modulating antioxidative and glyoxalase systems

This study was undertaken to investigate the possible involvement of the antioxidant defense and glyoxalase systems in protecting rice seedlings from heat-induced damage in the presence of spermidine (Spd). Hydroponically grown 14-day-old seedlings were subjected to foliar spray with Spd (1 mM, 24 h) prior to heat stress (42 °C, 48 h) followed by subsequent recovery (27 °C, 48 h). Lipoxygenase activity, malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and proline (Pro) content increased significantly whereas fresh weight (FW) and chlorophyll (Chl) content decreased during heat stress and after recovery, indicating unrecoverable damage to rice seedlings. Heat-induced damage was also evident in decreased levels of ascorbate (AsA), glutathione (GSH), and AsA and GSH redox ratios. Superoxide dismutase (SOD) and catalase (CAT) activities increased during heat stress but declined after recovery. Activities of glutathione peroxidase (GPX), ascorbate peroxidase (APX), monodehydroascorbate reductase, dehydroascorbate reductase (DHAR) and glutathione reductase (GR) decreased during heat stress but an opposite trend for most of these enzymes was observed after recovery. Heat stress also resulted in significant increases in the activities of glyoxalase enzymes (Gly I and Gly II). In contrast, exogenous Spd protected rice seedlings from heat-induced damage as marked by lower levels of MDA, H₂O₂, and Pro content coupled with increased levels of AsA, GSH, FW, Chl, and AsA and GSH redox status. After recovery, Spd-pretreated heat-exposed seedlings displayed higher activities of SOD, CAT, GPX, GST APX, DHAR and GR as well as of Gly I and Gly II. In addition, polyamine analysis revealed that exogenously applied Spd significantly elevated the levels of free and soluble conjugated Spd. Therefore, we conclude from our results that heat exposure provoked an oxidative burden while enhancement of the antioxidative and glyoxalase systems by Spd rendered rice seedlings more tolerant to heat stress. Further, co-induction of the antioxidative and glyoxalase systems was closely associated with Spd mediated enhanced level of GSH.     

Dual Application of 24-Epibrassinolide and Spermine Confers Drought Stress Tolerance in Maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) by Modulating Polyamine and Protein Metabolism

No information is available concerning the influence of dual application of 24-epibrassinolide (EBL) and spermine (Spm) on the nitrogen metabolism in plants subjected to drought conditions. As a first report, this investigation assesses the role of EBL, Spm, and their dual application on polyamine and protein pools in water-stressed plants. It explores the ameliorative effects of these foliar applications under water deficiency. Two maize hybrids (Giza 10 and Giza 129) were treated with or without EBL and/or Spm foliar applications under well-irrigated and drought-stressed conditions (75 and 50 % of field capacity). Dual application (25 mg l^?1 Spm + 0.1 mg l^?1 EBL) significantly relieved the drought-induced inhibition on the activities of ribulose-1,5-bisphosphate carboxylase and nitrate reductase and the contents of relative water, nitrate, and protein, particularly in hybrid Giza 129. Changes in the content of free polyamines and in the activity of polyamine biosynthetic and catabolic enzymes were detected when water-stressed plants were treated with EBL and/or Spm. Putrescine content and arginine decarboxylase activity were significantly increased in stressed hybrid Giza 10 plants treated by the dual application. However, spermidine and Spm levels as well as ornithine decarboxylase and S-adenosylmethionine decarboxylase activities were significantly increased in stressed hybrid Giza 129 plants treated with the dual application. Diamine oxidase, polyamine oxidase, protease activity, carbonyl content, and ethylene formation were increased in response to water stress and significantly decreased when stressed plants were treated by the dual application. Total free amino acids, phenols, and flavonoids concentration were increased with the increasing water stress level; moreover, they further increased in stressed plants treated with the dual application. Overall, the combined utilization of EBL and Spm serves as complementary tools to confer plant drought tolerance by altering polyamine, ethylene, and protein levels.     

Relationship between Polyamines and Anaerobic Respiration of Wheat Seedling Root under Water-Logging Stress

To elucidate the relationship between polyamines and anaerobic respiration of wheat (Triticum aestivum L.) seedling root under water-logging stress, the contents of polyamines (PAs), lactate and alcohol, and the activities of anaerobic respiration enzymes were investigated in seedling roots of two wheat cultivars, Yumai no. 18 and Yangmai no. 9. On the 5th day after water-logging treatment, spermidine (Spd) and spermine (Spm) contents increased significantly, pyruvate decarboxylase (PDC) activity increased and there was no difference between two cultivars. Alcohol dehydrogenase (ADH) activity and alcohol content in Yangmai no. 9 increased more markedly than Yumai no. 18, while lactate dehydrogenase (LDH) activity and the lactate content in the Yumai no. 18 increased more markedly than Yangmai no. 9. Treatments with exogenous Spd and Spm resulted in enhancing the increases in ADH activity, alcohol content, and the levels of Spd and Spm. This concomitantly inhibited the increases in LDH activity and lactate content in Yumai no. 18 under water-logging stress, alleviating stress-induced injury to the seedlings. Treatment with exogenous inhibitor methylglyoxyl-bis-guanylhydrazone (MGBG), resulted in reducing the increases in ADH activity, alcohol content, and Spd and Spm levels, promoting the increases in LDH activity and lactate content in Yangmai no. 9 under water-logging stress, and aggravating the stress-induced injury to the seedlings. The results suggested that under water-logging stress, increased Spd and Spm could facilitate the tolerance of wheat seedling to the stress by enhancing the increases in ADH activity and alcohol content, and inhibiting the increases in LDH activity and lactate content.     

Effects of exogenous polyamines on nitrate tolerance in cucumber

Putrescine (Put), spermidine (Spd), and spermine (Spm) are the major polyamines (PAs) in plant, which are not only involved in the regulation of plant developmental and physiological processes, but also play key roles in modulating the defense response of plants to diverse environmental stresses. In this study, Cucumis sativus L. seedlings were cultivated in nutrient solution and sprayed with three kinds of PAs (Put, Spd, and Spm). The effects of PAs were investigated on excess nitrate stress tolerance of C. sativus by measuring growth and nitrogen (N) metabolism parameters. The contents of NO_3-^?N, NH_4-⁺N, proline and soluble protein in leaves were increased; while plant height, leaf area, shoot fresh and dry weight, root fresh weight were decreased under 140 mM NO₃^? treatment for 7 d. In addition, the activities of nitrate reductase (NR), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) were significantly inhibited under 140 mM NO₃^? treatment for 7 d. With foliar treatment by 1 mM Spd or Spm under stress treatment, the contents of Spm, Put, and Spd in leaves increased significantly, except that Spm content decreased under Spd treatment. The activities of NR, glutamine synthetase (GS), GOGAT and GDH and plant height, leaf area, shoot fresh and dry weights were significantly increased. The contents of proline and soluble protein in leaves were significantly enhanced. In contrast, the accumulation of NO_3-^?N and NH_4-⁺N were significantly decreased. However, there were minor differences in activities of N metabolism enzymes and the content of osmotic adjustment substances under 1 mM Put treatment. These findings suggest that 1 mM exogenous Spm or Spd could enhance the capacity of N metabolism, promote growth and increase resistance to high concentrations of NO₃^?. The ameliorating effect of Spd was the best, and that of Put the worst.     

Exogenous application of free polyamines enhance salt tolerance of pistachio (Pistacia vera L.) seedlings

The protective effects of free polyamines (PAs) against salinity stress were investigated for pistachio seedlings (Pistacia vera cv. Badami-Zarand) in a controlled greenhouse. Seedlings were treated with 25, 50, 100 and 150 mM of salts including NaCl, CaCl₂ and MgCl₂. Foliar treatments of putrescine, spermidine (Spd) and spermine (Spm) (0.1 and 1 mM) were applied during the salinity period. Results showed that growth characteristics of pistachio seedlings decreased under salinity stress and the application of PAs efficiently reduced the adverse effects of salt stress. PAs reduced the severe effects of salt stress in pistachio seedlings neither by increasing the activities of peroxidase and ascorbate peroxidase nor by increasing the proline content but by increasing the activities of superoxide dismutase and catalase and decreasing the hydrogen peroxide (H₂O₂) activity. PAs treated seedlings showed a lower Na⁺:K⁺ ratio and Cl^? in leaves suggesting the role of PAs in balancing the ion exchange and better Na⁺:K⁺ discrimination under salt stress condition. These results showed the promising potential use of PAs especially Spm and Spd for reducing the negative effects of salinity stress and improving the growth of pistachio seedlings.     

H2O2 Involvement in Polyamine-Induced Cell Death in Tobacco Leaf Discs

The response of tobacco (Nicotiana tabacum L.) wild-type SR1 leaf discs in terms of reactive oxygen species (ROS) formation and cell death occurrence was evaluated after exposure to the polyamines (PAs) putrescine (Put), spermidine (Spd), and spermine (Spm). Although NADPH oxidase-like enzyme activity was inhibited by all PAs at 3 or 21 h of treatment, H₂O₂ content increased significantly in a time- and concentration-dependent manner, suggesting that H₂O₂ accumulation was linked to the activity of other ROS-generating enzymes. Polyamine oxidase (PAO) activity, which increased markedly upon application of Spd or Spm, is a prime candidate for the increased H₂O₂ accumulation. Except for 0.1 mM Put, which maintained guaiacol peroxidase (GPOX) and catalase (CAT) activities at the same level as the control, the other PA treatments decreased CAT, ascorbate peroxidase, and GPOX activities at 21 h, contributing to the H₂O₂ increase. Esterase activity and Evans blue staining, two cell death parameters, were negatively affected at 3 h of treatment with 1 mM Spd and with both concentrations of Spm, whereas at 21 h there was an increase in cell death with both concentrations of the three PAs, except for 0.1 mM Put, which did not alter those parameters. The expression of the senescence-associated cysteine protease gene CP1 was measured to monitor senescence, a physiological cell death process. Application of all PAs increased the expression of the gene, except for 0.1 mM Put, which decreased its expression at 21 h. This result was in agreement with the prevention of cell death exerted by Put and evidenced by Evans blue staining, esterase activity, and electrolyte release.     

Spermidine biosynthesis as affected by osmotic stress in oat leaves

A new assay for the evaluation of spermidine (Spd) synthase activity was developed. It involves a coupled reaction and avoids the use of decarboxylated S-adenosylmethionine, which is unstable and not easily available. This assay was applied to assess changes in enzyme activity in oat leaves subjected to osmotic stress in the dark. The results indicate that osmotically-induced putrescine (Put) accumulation in cereals results not only from the activation of the arginine decarboxylase pathway, but also from the inhibition of the activity of Spd synthase, the enzyme which catalyzes the transformation of Put to Spd. Other possibilities which could contribute to the decline of Spd and spermine levels under osmotic stress are also discussed.Abbreviations ADC arginine decarboxylase - Dap diaminopropane - DFMA -difluoromethylarginine - MGBG methylglyoxal-bis-guanylhydrazone - MTA 5-deoxy-5-methylthioadenosine - ODC ornithine decarboxylase - PA polyamines - PAO polyamine oxidase - PCA perchloric acid - PLP pyridoxal phosphate - Put putrescine - SAM S-adenosylmethionine - dSAM decarboxylated S-adenosylmethionine - SAMDC S-adenosylmethionine decarboxylase - Spd spermidine - Spm spermine     

ABA-dependent amine oxidases-derived H2O2 affects stomata conductance

Recently we showed that ABA is at least partly responsible for the induction of the polyamine exodus pathway in Vitis vinifera plants. Both sensitive and tolerant plants employ this pathway to orchestrate stress responses, differing between stress adaptation and programmed cell death. Herein we show that ABA is an upstream signal for the induction of the polyamine catabolic pathway in Vitis vinifera. Thus, amine oxidases are producing H₂O₂ which signals stomata closure. Moreover, the previously proposed model for the polyamine catabolic pathway is updated and discussed.Key words: plant growth, abscissic acid, polyamines, amine oxidases, signaling, oxidative stress, programmed cell deathWe have shown that tobacco salinity induces an exodus of the polyamine (PA) spermidine (Spd) into the apoplast where it is oxidized by polyamine oxidase (PAO) generating hydrogen peroxide (H₂O₂). Depending on the size of H₂O₂, it signals either tolerance-effector genes or the programmed cell death syndrome¹ (PCD). PAs are ubiquitous and biologically active molecules. In the recent years remarkable progress has been accomplished regarding the regulation of PAs biosynthesis and catalysis, not only under normal physiological but also under stress conditions.¹ The most studied PAs are the diamine Putrescine (Put) and its derivatives the triamine Spd and the tetramine spermine (Spm). They are present in the cells in soluble form (S), or conjugated either to low molecular weight compounds (soluble hydrolyzed form, SH) or to “macro” molecules or cell walls (pellet hydrolyzed form, PH). In higher plants, Put is synthesized either directly from ornithine via ornithine decarboxylase (ODC; EC 4.1.1.17) or indirectly from arginine via arginine decarboxylase (ADC; EC 4.1.1.19). Spd and Spm are synthesized via Spd synthase (EC 2.5.1.16, SPDS) and Spm synthase (EC 2.5.1.22, SPMS), respectively, by sequential addition of aminopropyl groups to Put, catalyzed by S-adenosyl-L-methionine decarboxylase (SAMDC; EC 4.1.1.50).^2,3 In plants, PAs are present in the cytoplasm, as well as in cellular organelles.⁴ Recently it was shown that during stress, they are secreted into the apoplast where they are oxidized by amine oxidases (AOs), such as diamine oxidase for Put (DAO, E.C. 1.4.3.6) and polyamine oxidase (PAO, E.C. 1.4.3.4) for Spd and Spm.^1,5,6 Oxidation of PAs generates, amongst other products, H₂O₂^1,7,8 which is involved in cell signaling processes coordinated by abscissic acid (ABA),⁹ but also acts as efficient oxidant and, at high concentration, orchestrates the PCD syndrome.^6,10 Two types of PA catabolism by PAO are known in plants: the terminal and the back-conversion pathways. The terminal one takes place in the apoplast, produces except H₂O₂, 1,3-diaminopropane and an aldehyde depending on the species. On the other hand, the back-conversion pathway is intracellular (cytoplasm and peroxisomes) resulting to the production of H₂O₂ and the sequential production of Put by Spm via Spd.^1,7 Now we have shown that PA exodus also occurs in Vitis vinifera and this phenomenon is at least partially induced by abscissic acid (ABA).¹¹ Thus, exogenous application of ABA results to PA exodus into the apoplast of grapevine. PA is oxidized by an AO resulting to production of H₂O₂. When the titer of H₂O₂ is below a threshold, expression of tolerance-effector genes is induced, while when it exceeds this threshold the programmed cell death (PCD) syndrome is induced.     

Polyamine titer and biosynthetic enzymes during tuber formation ofHelianthus tuberosus

During the formation ofHelianthus tuberosus tubers the activities of arginine decarboxylase (ADC) and S-adenosylmethionine decarboxylase (SAMDC), examined in medullary parenchyma cells, increase with the increase in weight of the tuber. The ornithine decarboxylase (ODC) activity is about 100-fold less with respect to ADC activity, and it was detected only during the deceleration phase of the growth curve. Spermidine and spermine content are strictly related to the SAMDC activity and tuber growth. The increase of ADC and SAMDC activity is directly related to cell extension and increase in weight. The limited area of cell division in parenchyma tissue found during the first stage of tuber formation could justify the low ODC activity. The data suggest that ADC affects mainly growth processes, while ODC seems to be preferentially related to cell division.