Enhancement of spermidine content and antioxidant capacity in transgenic pear shoots overexpressing apple spermidine synthase in response to salinity and hyperosmosis

Foliar application of chitosan-putrescine nanoparticles (CTS-Put NPs) alleviates cadmium toxicity in grapevine (Vitis vinifera L.) cv. Sultana: modulation of antioxidant and photosynthetic status

BACKGROUND

Cadmium (Cd) stress displays critical damage to the plant growth and health. Uptake and accumulation of Cd in plant tissues cause detrimental effects on crop productivity and ultimately impose threats to human beings. For this reason, a quite number of attempts have been made to buffer the adverse effects or to reduce the uptake of Cd. Of those strategies, the application of functionalized nanoparticles has lately attracted increasing attention. Former reports clearly noted that putrescine (Put) displayed promising effects on alleviating different stress conditions like Cd and similarly chitosan (CTS), as well as its nano form, demonstrated parallel properties in this regard besides acting as a carrier for many loads with different applications in the agriculture industry. Herein, we, for the first time, assayed the potential effects of nano-conjugate form of Put and CTS (CTS-Put NP) on grapevine (Vitis vinifera L.) cv. Sultana suffering from Cd stress. We hypothesized that their nano conjugate combination (CTS-Put NPs) could potentially enhance Put proficiency, above all at lower doses under stress conditions via CTS as a carrier for Put. In this regard, Put (50 mg L- 1), CTS (0.5%), Put 50 mg L- 1 + CTS 0.5%" and CTS-Put NPs (0.1 and 0.5%) were applied on grapevines under Cd-stress conditions (0 and 10 mg kg- 1). The interactive effects of CTS-Put NP were investigated through a series of physiological and biochemical assays.

RESULTS

The findings of present study clearly revealed that CTS-Put NPs as optimal treatments alleviated adverse effects of Cd-stress condition by enhancing chlorophyll (chl) a, b, carotenoids, Fv/Fm, Y(II), proline, total phenolic compounds, anthocyanins, antioxidant enzymatic activities and decreasing Y (NO), leaf and root Cd content, EL, MDA and H2O2.

CONCLUSIONS

In conclusion, CTS-Put NPs could be applied as a stress protection treatment on plants under diverse heavy metal toxicity conditions to promote plant health, potentially highlighting new avenues for sustainable crop production in the agricultural sector under the threat of climate change.

A DEAD box helicase Psp68 positively regulates salt stress responses in marker-free transgenic rice plants

Helicases are the motor proteins not only involved in transcriptional and post-transcription process but also provide abiotic stress tolerance in many crops. The p68, belong to the SF2 (DEAD-box helicase) family proteins and overexpression of Psp68 providing enhanced tolerance to transgenic rice plants. In this study, salinity tolerant marker-free transgenic rice has been developed by overexpressing Psp68 gene and phenotypically characterized. The Psp68 overexpressing marker-free transgenic rice plants were initially screened in the rooting medium containing salt stress and 20% polyethylene glycol (PEG). Stable integration and overexpression of Psp68 in marker-free transgenic lines were confirmed by molecular analyses including PCR, southern, western blot, and qRT-PCR analyses. The marker-free transgenic lines showed enhanced tolerance to salinity stress as displayed by early seed germination, higher chlorophyll content, reduced necrosis, more survival rate, improved seedling growth and more grain yield per plant. Furthermore, Psp68 overexpressing marker-free transgenics also accumulated less Na+ and higher K+ ions in the presence of salinity stress. Phenotypic analyses also revealed that marker-free transgenic rice lines efficiently scavenge ROS-mediated damages as displayed by lower H2O2 and malondialdehyde content, delayed electrolyte leakage, higher photosynthetic efficiency, membrane stability, proline content and enhanced activities of antioxidants enzymes. Overall, our results confirmed that Psp68 overexpression confers salinity stress tolerance in marker-free transgenics, hence the technique could be utilized to develop genetically modified crops without any biosafety issues.

Putrescine-functionalized carbon quantum dot (put-CQD) nanoparticle: A promising stress-protecting agent against cadmium stress in grapevine (Vitis vinifera cv. Sultana)

Due to their sessile nature, plant cannot escape from stress factors in their growing environment, in either biotic or abiotic nature. Amid the abiotic stress factors; high levels of soil cadmium (Cd) impose heavy metal stress on plants, resulting in critical injuries and reduced agronomic performance. In order to buffer the adverse effects of Cd stress, novel nanoparticles (NP) have been applied and notable improvements have been reported. According to the literature, the protective roles of polyamines (e.g., Putrescine; Put) and carbon quantum dots (CQD) have been reported with respect to the plant productivity under either stress or non-stress conditions. Those reports led us to hypothesize that the conjugation of Put and CQD (Put-CQD NPs) might lead to further augmented performance of plants under stress and non-stress conditions. In this regard, we successfully synthesized a novel nanomaterial Put-CQD NPs. In this respect, Put (50 mg L^-1), CQD (50 mg L^-1) and Put-CQD NPs (25 and 50 mg L^-1) were sprayed in 'Sultana' grapevines under Cd stress (10 mg kg^-1). As expected, upon stress, Cd content in leaf and root tissues increased by 103.40% and 65.15%, respectively (p < 0.05). The high uptake and accumulation of Cd in plant tissues were manifested in significant alterations of physiological and biochemical attributes of the plant. Concerning stress markers, Cd stress caused increases in content of induced MDA, H₂O₂, and proline as well as electrolyte leakage rate. As expected, Cd stress caused critical reductions in fresh and dry leaf weight by 21.31% and 42.34%, respectively (p < 0.05). On the other hand, both Put-CQD NPs increased fresh and dry leaf weigh up to approximately 30%. The Cd-mediated disturbances in photosynthetic pigments and chlorophyll fluorescence were buffered with Put-CQD NPs. Of the defence system, enzymatic (SOD, APX, GP) as well as anthocyanin and phenolics were induced by both Cd stress and Put-CQD NPs (p < 0.05). On the other hand, Cd stress reduced content of polyamines (putrescine (Put), spermine (Spm) and spermidine (Spd) by 39.28%, 53.36%, and 39.26%, respectively (p < 0.05). However, the reduction levels were buffered by the treatments. Considering the effectiveness of both NP concentrations, the lower dose (25 mg L^-1) could be considered as an optimal concentration. To our knowledge, this is the first report of its kind as a potential agent to reduce the adverse effects of Cd stress in grapevines.

Polyamines inhibit abscisic acid-induced stomatal closure by scavenging hydrogen peroxide

Stomatal closure is regulated by plant hormones and some small molecules to reduce water loss under stress conditions. Both abscisic acid (ABA) and polyamines alone induce stomatal closure; however, whether the physiological functions of ABA and polyamines are synergistic or antagonistic with respect to inducing stomatal closure is still unknown. Here, stomatal movement in response to ABA and/or polyamines was tested in Vicia faba and Arabidopsis thaliana, and the change in the signaling components under stomatal closure was analyzed. We found that both polyamines and ABA could induce stomatal closure through similar signaling components, including the synthesis of hydrogen peroxide (H₂ O₂ ) and nitric oxide (NO) and the accumulation of Ca²⁺ . However, polyamines partially inhibited ABA-induced stomatal closure both in epidermal peels and in planta by activating antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), to eliminate the ABA-induced increase in H₂ O₂ . These results strongly indicate that polyamines inhibit abscisic acid-induced stomatal closure, suggesting that polyamines could be used as potential plant growth regulators to increase photosynthesis under mild drought stress.

Characterization of Octa-aminopropyl polyhedral oligomeric silsesquioxanes (OA-POSS) nanoparticles and their effect on sweet basil (Ocimum basilicum L.) response to salinity stress

Salt stress is of the most detrimental abiotic stress factors on either crop or non-crop species. Of the strategies employed to boost the performance of the plants against harmful impacts of salt stress; application of novel nano-engineered particles have recently gained great attention as a promising tool. Octa-aminopropyl polyhedral oligomeric silsesquioxanes nanoparticles (OA-POSS NPs) were synthesized and then a foliar-application of OA-POSS NPs were carried out on sweet basil plants subjected to the salt stress. In that context, interactive effects of OA-POSS NPs (25, 50 and 100 mg L^-1) and salinity stress (50 and 100 mM NaCl) were assayed by estimating a series of agronomic, physiological, biochemical and analytical parameters. OA-POSS NPs decreased the harmful effects of salinity by increasing photosynthetic pigment content, adjusting chlorophyll fluorescence, and triggering non-enzymatic (phenolic content) and enzymatic antioxidant components. The findings suggested that 25 mg L^-1 OA-POSS NPs is the optimum concentration for sweet basil grown under salt stress. Considering the essential oil profile, estragole was the predominant compound with a percentage higher than 50% depending on the treatment. In comparison to the control group, 50 mM NaCl did not significantly affect estragole content, whilst 100 mM NaCl caused a substantial increase in estragole content. Regarding OA-POSS NPs treatments, increments by 16.8%, 11.8% and 17.5% were observed following application with 25, 50 and 100 mg L^-1, respectively. Taken together, the current study provides evidence that POSS NPs can be employed as novel, 'green' growth promoting agents in combating salt stress in sweet basil.

Transcriptomic Analysis Elaborates the Resistance Mechanism of Grapevine Rootstocks against Salt Stress

Grapes are subject to a wide range of climatic conditions during their life cycle, but the use of rootstocks can effectively ameliorate the effects of abiotic stress. However, the tolerance mechanism of different grape rootstock varieties varies under various stresses, and systematic research on this aspect is limited. On the basis of previous research, transcriptome sequencing was performed on three tolerant grape rootstock varieties (3309C, 520A, 1103P) and three intolerant grape rootstock varieties (5BB, 101-14, Beta). In total, 56,478,468 clean reads were obtained. One hundred and ten genes only existed in all combinations during P1 with a downregulated trend, and 178 genes existed only in P1 of tolerant grape rootstock varieties. Salt treatment firstly affected the photosynthesis of leaves, and tolerant varieties weakened or even eliminated this effect through their own mechanisms in the later stage. Tolerant varieties mobilized a large number of MFs during the P2 stage, such as hydrolase activity, carboxypeptidase activity, and dioxygenase activity. Carbon metabolism was significantly enriched in P1, while circadian rhythm and flavonoid biosynthesis were only enriched in tolerant varieties. In the intolerant varieties, photosynthesis-related pathways were always the most significantly enriched. There were large differences in the gene expression of the main signal pathways related to salt stress in different varieties. Salt stress affected the expression of genes related to plant abiotic stress, biotic stress, transcription factors, hormones, and secondary metabolism. Tolerant varieties mobilized more bHLH, WRKY, and MYB transcription factors to respond to salt stress than intolerant varieties. In the tolerant rootstocks, SOS was co-expressed. Among these, SOS1 and SOS2 were upregulated, and the SOS3 and SOS5 components were downregulated. The genes of heat shock proteins and the phenylalanine pathway were upregulated in the tolerant varieties. These findings outline a tolerance mechanism model for rootstocks for coping with osmotic stress, providing important information for improving the resistance of grapes under global climate change.

Differential Responses to Salt Stress in Four White Clover Genotypes Associated With Root Growth, Endogenous Polyamines Metabolism, and Sodium/Potassium Accumulation and Transport

Selection and utilization of salt-tolerant crops are essential strategies for mitigating salinity damage to crop productivity with increasing soil salinization worldwide. This study was conducted to identify salt-tolerant white clover (Trifolium repens) genotypes among 37 materials based on a comprehensive evaluation of five physiological parameters, namely, chlorophyll (Chl) content, photochemical efficiency of PS II (Fv/Fm), performance index on an absorption basis (PIABS), and leaf relative water content (RWC), and to further analyze the potential mechanism of salt tolerance associated with changes in growth, photosynthetic performance, endogenous polyamine metabolism, and Na⁺/K⁺ uptake and transport. The results showed that significant variations in salt tolerance were identified among 37 genotypes, as PI237292 and Tr005 were the top two genotypes with the highest salt tolerance, and PI251432 and Korla were the most salt-sensitive genotypes compared to other materials. The salt-tolerant PI237292 and Tr005 not only maintained significantly lower EL but also showed significantly better photosynthetic performance, higher leaf RWC, underground dry weight, and the root to shoot ratio than the salt-sensitive PI251432 and Korla under salt stress. Increases in endogenous PAs, putrescine (Put), and spermidine (Spd) contents could be key adaptive responses to salt stress in the PI237292 and the Tr005 through upregulating genes encoding Put and Spd biosynthesis (NCA, ADC, SAMDC, and SPDS2). For Na⁺ and K⁺ accumulation and transport, higher salt tolerance of the PI237292 could be associated with the maintenance of Na⁺ and Ca⁺ homeostasis associated with upregulations of NCLX and BTB/POZ. The K⁺ homeostasis-related genes (KEA2, HAK25, SKOR, POT2/8/11, TPK3/5, and AKT1/5) are differentially expressed among four genotypes under salt stress. However, the K⁺ level and K⁺/Na⁺ ratio were not completely consistent with the salt tolerance of the four genotypes. The regulatory function of these differentially expressed genes (DEGs) on salt tolerance in the white clover and other leguminous plants needs to be investigated further. The current findings also provide basic genotypes for molecular-based breeding for salt tolerance in white clover species.

Differential Association of Free, Conjugated, and Bound Forms of Polyamines and Transcript Abundance of Their Biosynthetic and Catabolic Genes During Drought/Salinity Stress in Tomato (Solanum lycopersicum L.) Leaves

Polyamines have been implicated in ameliorating the detrimental effects of drought and saline conditions on plant growth and development. The independent impact of these two abiotic stresses on polyamine (PA) biosynthesis, catabolism, and homeostasis, as well as on their transcript abundance in tomato leaves, is presented here. We show that the total levels of putrescine (PUT), spermidine (SPD), and spermine (SPM) increase up to 72 h during drought and up to 48 h during salinity stress before their precipitable drop thereafter. Thus, tomato plants maintain survivability to drought as well as salinity stress for up to 3 and 2 days, respectively. Independent multivariant analyses of drought and salinity stress kinetic data separately showed a closer association with levels of free, conjugated, and bound forms of SPD and SPM, but not with free or bound PUT. However, combined multivariant analyses showed a closer association of free SPD, conjugated SPD, and bound SPD with both stresses; SPD-bound and SPM conjugated with drought; and free SPM and conjugated PUT with salinity stress, respectively. PA biosynthesis genes, ARG1, SPDS1, and SAMDc3, segregated with drought and SPDS2 with salinity stress. PA catabolic genes CuAO4-like and PAO4 were associated with drought and salinity stresses, respectively, suggesting differential involvement of PA biosynthesis and catabolic genes in drought and salinity stresses. Pearson correlation indicated mostly positive correlations between the levels of free, conjugated, and bound forms of PUT, SPD, and SPM under drought and salinity stress. However, negative correlations were mostly seen between the levels of various forms of the PAs and their biosynthesis/catabolic genes. Levels of different PA forms had a twofold higher negative correlation during drought as compared to salinity stress (66 vs. 32) and with transcript levels of PA biosynthesis and catabolic genes. Transcripts of light-harvesting chlorophyll a/b-binding genes were generally positively associated with different forms of PAs but negatively to carbon flow genes. Most of the PA biosynthesis genes were coordinately regulated under both stresses. Collectively, these results indicate that PAs are distinctly regulated under drought and salinity stress with different but specific homologs of PA biosynthesis and catabolic genes contributing to the accumulation of free, conjugated, and bound forms of PAs.

Putrescine-functionalized carbon quantum dot (put-CQD) nanoparticles effectively prime grapevine (Vitis vinifera cv. 'Sultana') against salt stress

BACKGROUND

Salinity is an important global problem with destructive impacts on plants leading to different biochemical and metabolic changes in plants through induced oxidative stress that disturbs metabolism, growth, performance and productivity of plants. Given that putrescine (Put) and carbon quantum dots (CQDs), individually, have promising effects in different plant processes, the idea of their combination in a nano-structure "Put-CQD" lead to its synthesis to evaluate the potential exertion of synergistic effects. The current study aimed to investigate the application of newly-synthesized nanoparticles (NPs) consisting of CQDs and Put in grapevine (Vitis vinifera cv. 'Sultana') under salinity stress conditions. For this purpose, Put, CQDs and Put-CQD NPs at 5 and 10 mg L- 1 concentrations were applied as chemical priming agents in 'Sultana' grapevine 48 h prior salinity stress imposition (0 and 100 mM NaCl).

RESULTS

Salinity significantly decreased (P ≤ 0.05) morphological parameters, photosynthetic pigments, chlorophyll fluorescence parameters and membrane stability index. In addition, salinity enhanced MDA, H2O2, proline content and antioxidant enzyme activity. Results revealed that Put-CQD NPs, particularly at 10 mg L- 1 concentration, alleviated the destructive impacts of salinity stress by improving leaf fresh and dry weights, K+ content, photosynthetic pigments, chlorophyll fluorescence and SPAD parameters, proline content, total phenolics and antioxidant enzymatic activities (CAT, APX, GP and SOD), while decreasing Na+ content, EL, MDA and H2O2 levels.

CONCLUSION

To conclude, Put-CQD NPs represent an innovative priming treatment that could be effectively applied on grapevine to improve plant performance under salinity stress conditions.

Polyamines: Small Amines with Large Effects on Plant Abiotic Stress Tolerance

In recent years, climate change has altered many ecosystems due to a combination of frequent droughts, irregular precipitation, increasingly salinized areas and high temperatures. These environmental changes have also caused a decline in crop yield worldwide. Therefore, there is an urgent need to fully understand the plant responses to abiotic stress and to apply the acquired knowledge to improve stress tolerance in crop plants. The accumulation of polyamines (PAs) in response to many abiotic stresses is one of the most remarkable plant metabolic responses. In this review, we provide an update about the most significant achievements improving plant tolerance to drought, salinity, low and high temperature stresses by exogenous application of PAs or genetic manipulation of endogenous PA levels. We also provide some clues about possible mechanisms underlying PA functions, as well as known cross-talks with other stress signaling pathways. Finally, we discuss about the possible use of PAs for seed priming to induce abiotic stress tolerance in agricultural valuable crop plants.

Polyamines and Their Biosynthesis/Catabolism Genes Are Differentially Modulated in Response to Heat Versus Cold Stress in Tomato Leaves (Solanum lycopersicum L.)

Polyamines (PAs) regulate growth in plants and modulate the whole plant life cycle. They have been associated with different abiotic and biotic stresses, but little is known about the molecular regulation involved. We quantified gene expression of PA anabolic and catabolic pathway enzymes in tomato (Solanum lycopersicum cv. Ailsa Craig) leaves under heat versus cold stress. These include arginase1 and 2, arginine decarboxylase 1 and 2, agmatine iminohydrolase/deiminase 1, N-carbamoyl putrescine amidase, two ornithine decarboxylases, three S-adenosylmethionine decarboxylases, two spermidine synthases; spermine synthase; flavin-dependent polyamine oxidases (SlPAO4-like and SlPAO2) and copper dependent amine oxidases (SlCuAO and SlCuAO-like). The spatiotemporal transcript abundances using qRT-PCR revealed presence of their transcripts in all tissues examined, with higher transcript levels observed for SAMDC1, SAMDC2 and ADC2 in most tissues. Cellular levels of free and conjugated forms of putrescine and spermidine were found to decline during heat stress while they increased in response to cold stress, revealing their differential responses. Transcript levels of ARG2, SPDS2, and PAO4-like increased in response to both heat and cold stresses. However, transcript levels of ARG1/2, AIH1, CPA, SPDS1 and CuAO4 increased in response to heat while those of ARG2, ADC1,2, ODC1, SAMDC1,2,3, PAO2 and CuPAO4-like increased in response to cold stress, respectively. Transcripts of ADC1,2, ODC1,2, and SPMS declined in response to heat stress while ODC2 transcripts declined under cold stress. These results show differential expression of PA metabolism genes under heat and cold stresses with more impairment clearly seen under heat stress. We interpret these results to indicate a more pronounced role of PAs in cold stress acclimation compared to that under heat stress in tomato leaves.

Genome-wide identification of genes involved in polyamine biosynthesis and the role of exogenous polyamines in Malus hupehensis Rehd. under alkaline stress

Polyamines (PAs) in plants are growth substrates with functions similar to phytohormones. Although they contribute to diverse processes, little is known about their role in stress responses, especially for perennial woody plants. We conducted a genome-wide investigation of 18 sequences involved in PA biosynthesis in the genome of apple (Malus domestica). Further analysis was performed to construct a phylogenetic tree, analyze their protein motifs and gene structures. In addition, we developed their expression profiles in response to stressed conditions. Both MDP0000171041 (MdSAMDC1) and MDP0000198590 (MdSPDS1) were induced by alkaline, salt, ABA, cold, and dehydration stress treatments, suggesting that these genes are the main contributors to activities of S-adenosylmethionine decarboxylase (EC 4.1.1.50) and spermidine synthase (EC 2.5.1.16) in apple. Changes in PA biosynthesis under stress conditions indicated that spermidine and spermine are more essential than putrescine for apple, especially when responding to alkaline or salt stress. When seedlings of M. hupehensis Rehd. were supplied with exogenous PAs, their leaves showed less chlorosis under alkaline stress when compared with untreated plants. This application also inhibited the decline in SPAD levels and reduced relative electrolyte leakage in those stressed seedlings, while increasing their concentration of active iron. These results suggest that the alteration in PA biosynthesis confers enhanced tolerance to alkaline stress in M. hupehensis Rehd.

Polyamines: Bio-Molecules with Diverse Functions in Plant and Human Health and Disease

Biogenic amines-polyamines (PAs), particularly putrescine, spermidine and spermine are ubiquitous in all living cells. Their indispensable roles in many biochemical and physiological processes are becoming commonly known, including promoters of plant life and differential roles in human health and disease. PAs positively impact cellular functions in plants-exemplified by increasing longevity, reviving physiological memory, enhancing carbon and nitrogen resource allocation/signaling, as well as in plant development and responses to extreme environments. Thus, one or more PAs are commonly found in genomic and metabolomics studies using plants, particulary during different abiotic stresses. In humans, a general decline in PA levels with aging occurs parallel with some human health disorders. Also, high PA dose is detrimental to patients suffering from cancer, aging, innate immunity and cognitive impairment during Alzheimer and Parkinson diseases. A dichotomy exists in that while PAs may increase longevity and reduce some age-associated cardiovascular diseases, in disease conditions involving higher cellular proliferation, their intake has negative consequences. Thus, it is essential that PA levels be rigorously quantified in edible plant sources as well as in dietary meats. Such a database can be a guide for medical experts in order to recommend which foods/meats a patient may consume and which ones to avoid. Accordingly, designing both high and low polyamine diets for human consumption are in vogue, particularly in medical conditions where PA intake may be detrimental, for instance, cancer patients. In this review, literature data has been collated for the levels of the three main PAs, putrescine, spermidine and spermine, in different edible sources-vegetables, fruits, cereals, nuts, meat, sea food, cheese, milk, and eggs. Based on our analysis of vast literature, the effects of PAs in human/animal health fall into two broad, Yang and Yin, categories: beneficial for the physiological processes in healthy cells and detrimental under pathological conditions.

Transgenic Centipedegrass (Eremochloa ophiuroides [Munro] Hack.) Overexpressing S-Adenosylmethionine Decarboxylase (SAMDC) Gene for Improved Cold Tolerance Through Involvement of H2O2 and NO Signaling

Centipedegrass (Eremochloa ophiuroides [Munro] Hack.) is an important warm-season turfgrass species. Transgenic centipedgrass plants overexpressing S-adenosylmethionine decarboxylase from bermudagrass (CdSAMDC1) that was induced in response to cold were generated in this study. Higher levels of CdSAMDC1 transcript and sperimidine (Spd) and spermin (Spm) concentrations and enhanced freezing and chilling tolerance were observed in transgenic plants as compared with the wild type (WT). Transgenic plants had higher levels of polyamine oxidase (PAO) activity and H₂O₂ than WT, which were blocked by pretreatment with methylglyoxal bis (guanylhydrazone) or MGBG, inhibitor of SAMDC, indicating that the increased PAO and H₂O₂ were a result of expression of CdSAMDC1. In addition, transgenic plants had higher levels of nitrate reductase (NR) activity and nitric oxide (NO) concentration. The increased NR activity were blocked by pretreatment with MGBG and ascorbic acid (AsA), scavenger of H₂O₂, while the increased NO level was blocked by MGBG, AsA, and inhibitors of NR, indicating that the enhanced NR-derived NO was dependent upon H₂O₂, as a result of expression CdSAMDC1. Elevated superoxide dismutase (SOD) and catalase (CAT) activities were observed in transgenic plants than in WT, which were blocked by pretreatment with MGBG, AsA, inhibitors of NR and scavenger of NO, indicating that the increased activities of SOD and CAT depends on expression of CdSAMDC1, H₂O₂, and NR-derived NO. Our results suggest that the elevated cold tolerance was associated with PAO catalyzed production of H₂O₂, which in turn led to NR-derived NO production and induced antioxidant enzyme activities in transgenic plants.

NO is involved in spermidine-induced drought tolerance in white clover via activation of antioxidant enzymes and genes

Nitric oxide (NO), a key signaling molecule, can be induced by polyamines (PAs), which play an important role in improving drought tolerance in plants. This study was to further investigate the role of NO in spermidine (Spd)-induced drought tolerance associated with antioxidant defense in leaves of white clover (Trifolium repens) under drought stress induced by -0.3 MPa polyethylene glycol (PEG-6000) solution. A hydroponic growth method was used for cultivating plants in a controlled growth chamber for 30-33 days until the second leaves were fully expanded. Two relative independent experiments were carried out in our study. One is that exogenous application of Spd or an NO donor (sodium nitroprusside (SNP)) significantly improved drought tolerance in whole plants, as demonstrated by better phenotypic appearance, increased relative water content (RWC), and decreased electrolyte leakage (EL) and malondialdehyde (MDA) content in leaves as compared to untreated plants. For another detached leaf experiment, PEG induced an increase in the generation of NO in cells and significantly improved activities of nitrate reductase (NR) and nitric oxide synthase (NOS). These responses could be blocked by pre-treatment with a Spd biosynthetic inhibitor, dicyclohexyl amine (DCHA), and then reversed by application of exogenous Spd. Meanwhile, PEG induced up-regulation of activities and gene transcript levels of corresponding antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) to varying degrees, while these effects were partially blocked by pre-treatment with DCHA, the scavenger of NO, the inhibitors of NR or NOS. In addition, Spd-induced antioxidant enzyme activities and gene expression also could be effectively inhibited by an NO scavenger as well as inhibitors of NR and NOS. These findings suggest that both Spd and NO can enhance drought tolerance. Spd was involved in drought stress-activated NR and NOS pathways associated with NO release, which mediated antioxidant defense and thus contributed to drought tolerance in white clover.

Spermidine Synthase is Required for Growth of Synechococcus sp. PCC 7942 Under Osmotic Stress

The Synechococcus sp. PCC 7942 spermidine synthase encoded by spds gene (Synpcc7942_0628) is responsible for spermidine biosynthesis. Two Synechococcus strains, the overexpressing spds (OX-spds) and the spds knockout (Δspds), were constructed and characterized for their growth and photosynthetic efficiency under osmotic stress imposed by sorbitol. The growth of Δspds was completely inhibited when cells were grown in the presence of 400 mM sorbitol. Under the same condition, the OX-spds showed a slightly higher growth than the wild type. The OX-spds under osmotic stress also had a significant increase of spermidine level in conjunction with the up-regulation of the genes involved in spermidine biosynthesis. A higher ratio of spermidine to putrescine, an index for stress tolerance, under osmotic stress was found in the OX-spds strain than in the wild type. Overall results indicated that the spermidine synthase enzyme plays an essential role in the survival of Synechococcus sp. PCC 7942 under osmotic stress.

Accumulation of free polyamines enhances the antioxidant response in fruits of grafted tomato plants under water stress

Polyamines, small aliphatic polycations, have been suggested to play key roles in a number of biological processes. In this paper, attempts were made to investigate the possibility of improving antioxidant response of tomato fruits in relation with endogenous free polyamines content. We studied the reactive oxygen species and polyamines content, and antioxidant and polyamine-biosynthesis enzyme activities in fruits of ungrafted and grafted tomato plants under moderate water stress. We used a drought-tolerant cultivar (Zarina) and drought-sensitive cultivar (Josefina) to obtain reciprocal graft, selfgraft and ungraft plants. Fruits contained higher endogenous polyamine content during the course of the experiment relative to the control, coupled with higher arginine decarboxylase and spermine synthase activities in Zarina ungrafted and ZarxJos. In these cultivars, tomato fruits showed a lower reactive oxygen species generation and higher catalase and superoxide dismutase activities, suggesting that a higher content in polyamines (especially spermine) exerted a positive effect on antioxidant systems. All of these data suggest that spermine leads to more effective reactive oxygen species scavenging (less tissue damage) in tomato fruits, which may function collectively to enhance dehydration tolerance.

Fast responses of metabolites in Vicia faba L. to moderate NaCl stress

Salt stress impairs global agricultural crop production by reducing vegetative growth and yield. Despite this importance, a number of gaps exist in our knowledge about very early metabolic responses that ensue minutes after plants experience salt stress. Surprisingly, this early phase remains almost as a black box. Therefore, systematic studies focussing on very early plant physiological responses to salt stress (in this case NaCl) may enhance our understanding on strategies to develop crop plants with a better performance under saline conditions. In the present study, hydroponically grown Vicia faba L. plants were exposed to 90 min of NaCl stress, whereby every 15 min samples were taken for analyzing short-term physiologic responses. Gas chromatography-mass spectrometry-based metabolite profiles were analysed by calculating a principal component analysis followed by multiple contrast tests. Follow-up experiments were run to analyze downstream effects of the metabolic changes on the physiological level. The novelty of this study is the demonstration of complex stress-induced metabolic changes at the very beginning of a moderate salt stress in V. faba, information that are very scant for this early stage. This study reports for the first that the proline analogue trans-4-hydroxy-L-proline, known to inhibit cell elongation, was increasingly synthesized after NaCl-stress initiation. Leaf metabolites associated with the generation or scavenging of reactive oxygen species (ROS) were affected in leaves that showed a synchronized increase in ROS formation. A reduced glutamine synthetase activity indicated that disturbances in the nitrogen assimilation occur earlier than it was previously thought under salt stress.

Exogenous spermidine improves seed germination of white clover under water stress via involvement in starch metabolism, antioxidant defenses and relevant gene expression

This study was designed to determine the effect of exogenous spermidine (Spd) (30 μM) on white clover seed germination under water stress induced by polyethylene glycol 6000. Use of seed priming with Spd improved seed germination percentage, germination vigor, germination index, root viability and length, and shortened mean germination time under different water stress conditions. Seedling fresh weight and dry weight also increased significantly in Spd-treated seeds compared with control (seeds primed with distilled water). Improved starch metabolism was considered a possible reason for this seed invigoration, since seeds primed with Spd had significantly increased α-amylase/β-amylase activities, reducing sugar, fructose and glucose content and transcript level of β-amylase gene but not transcript level of α-amylase gene. In addition, the physiological effects of exogenous Spd on improving seeds’ tolerance to water deficit during germination were reflected by lower lipid peroxidation levels, better cell membrane stability and significant higher seed vigour index in seedlings. Enhanced antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase and ascorbate peroxidase), ascorbate-glutathione cycle (ASC-GSH cycle) and transcript level of genes encoding antioxidant enzymes induced by exogenous Spd may be one of the critical reasons behind acquired drought tolerance through scavenging of reactive oxygen species (ROS) in water-stressed white clover seeds. The results indicate that Spd plays an important function as a stress-protective compound or physiological activator.

Salinity-induced accumulation of organic osmolytes in barley and wheat leaves correlates with increased oxidative stress tolerance: in planta evidence for cross-tolerance

Salinity tolerance in plants is dependent on their abilities to adjust osmotically to reduced soil water potential and to keep intracellular ROS levels under control. Both these processes are believed to rely on de novo synthesis of organic osmolytes (traditionally defined as compatible solutes). However direct in planta evidence for anti-oxidant roles of compatible solutes are scarce. In this work, we induced changes in the level of endogenous organic osmolytes by exposing plants to various levels of NaCl (salinity stress; 50-300 mM range) and then studying sensitivity of leaves to oxidative (UV-B) stress. Increase in the external NaCl concentrations was accompanied by the progressive accumulation in leaf Na(+). This accumulation was much higher in old leaves compared with young ones. In old leaves, three major inorganic ions (Na(+), Cl(-) and K(+)) have made 67.7% and 70.4% of leaf osmotic potential (in wheat and barley, respectively) when exposed to 200 mM NaCl treatment, while in young leaves their contribution was only 43.9% and 46.8%, respectively. Here, organic osmolytes played a substantial role in leaf osmotic adjustment. Increased accumulation of organic osmolytes correlated strongly with activity of PSII in leaves exposed to oxidation inducing UV-B treatment in both species (R(2) = 0.50 for wheat and 0.71 for barley). We conclude that salinity-induced accumulation of organic osmolytes in barley and wheat leaves correlates with increased oxidative stress tolerance and provides the evidence for a mechanism of cross-tolerance between these two stresses.

Overexpression of PtADC confers enhanced dehydration and drought tolerance in transgenic tobacco and tomato: effect on ROS elimination

Drought is a major environmental factor that limits plant growth and productivity. Polyamines have been shown to act as stress molecules that accumulate in plant adaptation to abiotic stresses. In this study, an arginine decarboxylase gene isolated from Poncirus trifoliata, PtADC, was introduced into tobacco and tomato to investigate its function in drought tolerance. We demonstrate that the transgenic plants showed an improvement in dehydration and drought tolerance. Under dehydration stress conditions, the accumulation of reactive oxygen species (ROS) was remarkably decreased in the transgenic lines as compared with the wild type. Moreover, the transcript levels of three stress-responsive genes were increased in the transgenic tobacco lines. Taken together, our results suggest that PtADC plays a key role in drought tolerance, which is, at least partially, attributed to its role in ROS detoxification.

Profiling the aminopropyltransferases in plants: their structure, expression and manipulation

Polyamines are organic polycations that are involved in a wide range of cellular activities related to growth, development, and stress response in plants. Higher polyamines spermidine and spermine are synthesized in plants and animals by a class of enzymes called aminopropyltransferases that transfer aminopropyl moieties (derived from decarboxylated S-adenosylmethionine) to putrescine and spermidine to produce spermidine and spermine, respectively. The higher polyamines show a much tighter homeostatic regulation of their metabolism than the diamine putrescine in most plants; therefore, the aminopropyltransferases are of high significance. We present here a comprehensive summary of the current literature on plant aminopropyltransferases including their distribution, biochemical properties, genomic organization, pattern of expression during development, and their responses to abiotic stresses, and manipulation of their cellular activity through chemical inhibitors, mutations, and genetic engineering. This minireview complements several recent reviews on the overall biosynthetic pathway of polyamines and their physiological roles in plants and animals. It is concluded that (1) plants often have two copies of the common aminopropyltransferase genes which exhibit redundancy of function, (2) their genomic organization is highly conserved, (3) direct enzyme activity data on biochemical properties of these enzymes are scant, (4) often there is a poor correlation among transcripts, enzyme activity and cellular contents of the respective polyamine, and (5) transgenic work mostly confirms the tight regulation of cellular contents of spermidine and spermine. An understanding of expression and regulation of aminopropyltransferases at the metabolic level will help us in effective use of genetic engineering approaches for the improvement in nutritional value and stress responses of plants.

Biotic and abiotic stress tolerance in transgenic tomatoes by constitutive expression of S-adenosylmethionine decarboxylase gene

Recent findings have implicated the role of polyamines (putrescine, spermidine and spermine) in stress tolerance. Therefore, the present work was carried out with the goal of generating transgenic tomato plants with human S-adenosylmethionine decarboxylase (samdc) gene, a key gene involved in biosynthesis of polyamines, viz. spermidine and spermine and evaluating the transgenic plants for tolerance to both biotic and abiotic stresses. Several putative transgenic tomato plants with normal phenotype were obtained, and the transgene integration and expression was validated by PCR, Southern blot analysis and RT-PCR analysis, respectively. The transgenic plants exhibited high levels of polyamines as compared to the untransformed control plants. They also showed increased resistance against two important fungal pathogens of tomato, the wilt causing Fusarium oxysporum and the early blight causing Alternaria solani and tolerance to multiple abiotic stresses such as salinity, drought, cold and high temperature. These results suggest that engineering polyamine accumulation can confer tolerance to both biotic and abiotic stresses in plants.

Ectopic expression of MdSPDS1 in sweet orange (Citrus sinensis Osbeck) reduces canker susceptibility: involvement of H₂O₂ production and transcriptional alteration

BACKGROUND

Enormous work has shown that polyamines are involved in a variety of physiological processes, but information is scarce on the potential of modifying disease response through genetic transformation of a polyamine biosynthetic gene.

RESULTS

In the present work, an apple spermidine synthase gene (MdSPDS1) was introduced into sweet orange (Citrus sinensis Osbeck 'Anliucheng') via Agrobacterium-mediated transformation of embryogenic calluses. Two transgenic lines (TG4 and TG9) varied in the transgene expression and cellular endogenous polyamine contents. Pinprick inoculation demonstrated that the transgenic lines were less susceptible to Xanthomonas axonopodis pv. citri (Xac), the causal agent of citrus canker, than the wild type plants (WT). In addition, our data showed that upon Xac attack TG9 had significantly higher free spermine (Spm) and polyamine oxidase (PAO) activity when compared with the WT, concurrent with an apparent hypersensitive response and the accumulation of more H₂O₂. Pretreatment of TG9 leaves with guazatine acetate, an inhibitor of PAO, repressed PAO activity and reduced H₂O₂ accumulation, leading to more conspicuous disease symptoms than the controls when both were challenged with Xac. Moreover, mRNA levels of most of the defense-related genes involved in synthesis of pathogenesis-related protein and jasmonic acid were upregulated in TG9 than in the WT regardless of Xac infection.

CONCLUSION

Our results demonstrated that overexpression of the MdSPDS1 gene prominently lowered the sensitivity of the transgenic plants to canker. This may be, at least partially, correlated with the generation of more H₂O₂ due to increased production of polyamines and enhanced PAO-mediated catabolism, triggering hypersensitive response or activation of defense-related genes.

Polyamines: natural and engineered abiotic and biotic stress tolerance in plants

Polyamines (PAs) are ubiquitous biogenic amines that have been implicated in diverse cellular functions in widely distributed organisms. In plants, mutant and transgenic plants with altered activity pointed to their involvement with different abiotic and biotic stresses. Furthermore, microarray, transcriptomic and proteomic approaches have elucidated key functions of different PAs in signaling networks in plants subjected to abiotic and biotic stresses, however the exact molecular mechanism remains enigmatic. Here, we argue that PAs should not be taken only as a protective molecule but rather like a double-faced molecule that likely serves as a major area for further research efforts. This review summarizes recent advances in plant polyamine research ranging from transgenic and mutant characterization to potential mechanisms of action during environmental stresses and diseases.

Polyamines and abiotic stress tolerance in plants

Environmental stresses including climate change, especially global warming, are severely affecting plant growth and productivity worldwide. It has been estimated that two-thirds of the yield potential of major crops are routinely lost due to the unfavorable environmental factors. On the other hand, the world population is estimated to reach about 10 billion by 2050, which will witness serious food shortages. Therefore, crops with enhanced vigour and high tolerance to various environmental factors should be developed to feed the increasing world population. Maintaining crop yields under adverse environmental stresses is probably the major challenge facing modern agriculture where polyamines can play important role. Polyamines (PAs)(putrescine, spermidine and spermine) are group of phytohormone-like aliphatic amine natural compounds with aliphatic nitrogen structure and present in almost all living organisms including plants. Evidences showed that polyamines are involved in many physiological processes, such as cell growth and development and respond to stress tolerance to various environmental factors. In many cases the relationship of plant stress tolerance was noted with the production of conjugated and bound polyamines as well as stimulation of polyamine oxidation. Therefore, genetic manipulation of crop plants with genes encoding enzymes of polyamine biosynthetic pathways may provide better stress tolerance to crop plants. Furthermore, the exogenous application of PAs is also another option for increasing the stress tolerance potential in plants. Here, we have described the synthesis and role of various polyamines in abiotic stress tolerance in plants.

Plants as bioreactors: Recent developments and emerging opportunities

In recent years, the use of plants as bioreactors has emerged as an exciting area of research and significant advances have created new opportunities. The driving forces behind the rapid growth of plant bioreactors include low production cost, product safety and easy scale up. As the yield and concentration of a product is crucial for commercial viability, several strategies have been developed to boost up protein expression in transgenic plants. Augmenting tissue-specific transcription, elevating transcript stability, tissue-specific targeting, translation optimization and sub-cellular accumulation are some of the strategies employed. Various kinds of products that are currently being produced in plants include vaccine antigens, medical diagnostics proteins, industrial and pharmaceutical proteins, nutritional supplements like minerals, vitamins, carbohydrates and biopolymers. A large number of plant-derived recombinant proteins have reached advanced clinical trials. A few of these products have already been introduced in the market.