Deciphering the protective role of spermidine against saline-alkaline stress at physiological and proteomic levels in tomato

The Effects of Exogenous 2,4-Epibrassinolide on the Germination of Cucumber Seeds under NaHCO3 Stress

This investigation focused on the suppressive impact of varying NaHCO3 concentrations on cucumber seed germination and the ameliorative effects of 2,4-Epibrassinolide (EBR). The findings revealed a negative correlation between NaHCO3 concentration and cucumber seed germination, with increased NaHCO3 concentrations leading to a notable decline in germination. Crucially, the application of exogenous EBR significantly counteracted this inhibition, effectively enhancing germination rates and seed vigor. Exogenous EBR was observed to substantially elevate the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), thereby mitigating oxidative damage triggered under NaHCO3 stress conditions. Additionally, EBR improved enzyme activity under alkaline stress conditions and reduced starch content in the seeds. Pertinently, EBR upregulated genes that were associated with gibberellin (GA) synthesis (GA20ox and GA3ox), and downregulated genes that were linked to abscisic acid (ABA) synthesis (NCED1 and NCED2). This led to an elevation in GA3 concentration and a reduction in ABA concentration within the cucumber seeds. Therefore, this study elucidates that alleviating oxidative stress, promoting starch catabolism, and regulating the GA and ABA balance are key mechanisms through which exogenous EBR mitigates the suppression of cucumber seed germination resulting from alkaline stress.

Exogenous spermine alleviates the negative effects of combined salinity and paraquat in tomato plants by decreasing stress-induced oxidative damage

Plants are frequently exposed to different combinations of soil constraints including salinity and different herbicides. These abiotic conditions negatively affect photosynthesis, growth and plant development resulting in limitations in agriculture production. To respond to these conditions, plants accumulate different metabolites that restore cellular homeostasis and are key for stress acclimation processes. In this work, we analyzed the role of exogenous spermine (Spm), a polyamine involved in plant tolerance to abiotic stress, in tomato responses to the combination of salinity (S) and the herbicide paraquat (PQ). Our findings showed that application of Spm reduced leaf damage and enhanced survival, growth, photosystem II function and photosynthetic rate of tomato plants subjected to the combination of S and PQ. In addition, we revealed that exogenous Spm reduced H₂O₂ and malondialdehyde (MDA) accumulation in plants subjected to S+PQ, suggesting that the role of exogenous Spm in alleviating the negative effects of this stress combination could be attributed to a decrease in stress-induced oxidative damage in tomato plants. Taken together, our results identify a key role for Spm in improving plant tolerance to combined stress.

Proteomics of Reproductive Development, Fruit Ripening, and Stress Responses in Tomato

The fruits of the tomato crop (Solanum lycopersicum L.) are increasingly consumed by humans worldwide. Due to their rich nutritional quality, pharmaceutical properties, and flavor, tomato crops have gained a salient role as standout crops among other plants. Traditional breeding and applied functional research have made progress in varying tomato germplasms to subdue biotic and abiotic stresses. Proteomic investigations within a span of few decades have assisted in consolidating the functional genomics and transcriptomic research. However, due to the volatility and dynamicity of proteins in the regulation of various biosynthetic pathways, there is a need for continuing research in the field of proteomics to establish a network that could enable a more comprehensive understanding of tomato growth and development. With this view, we provide a comprehensive review of proteomic studies conducted on the tomato plant in past years, which will be useful for future breeders and researchers working to improve the tomato crop.

ROS scavenging and ion homeostasis is required for the adaptation of halophyte Karelinia caspia to high salinity

The halophyte Karelinia caspia has not only fodder and medical value but also can remediate saline-alkali soils. Our previous study showed that salt-secreting by salt glands is one of main adaptive strategies of K. caspia under high salinity. However, ROS scavenging, ion homeostasis, and photosynthetic characteristics responses to high salinity remain unclear in K. caspia. Here, physio-biochemical responses and gene expression associated with ROS scavenging and ions transport were tested in K. caspia subjected to 100-400 mM NaCl for 7 days. Results showed that both antioxidant enzymes (SOD, APX) activities and non-enzymatic antioxidants (chlorogenic acid, α-tocopherol, flavonoids, polyamines) contents were significantly enhanced, accompanied by up-regulating the related enzyme and non-enzymatic antioxidant synthesis gene (KcCu/Zn-SOD, KcAPX6, KcHCT, KcHPT1, Kcγ-TMT, KcF3H, KcSAMS and KcSMS) expression with increasing concentrations of NaCl. These responses are beneficial for removing excess ROS to maintain a stable level of H2O2 and O2 - without lipid peroxidation in the K. caspia response to high salt. Meanwhile, up-regulating expression of KcSOS1/2/3, KcNHX1, and KcAVP was linked to Na+ compartmentalization into vacuoles or excretion through salt glands in K. caspia. Notably, salt can improve the function of PSII that facilitate net photosynthetic rates, which is helpful to growing normally in high saline. Overall, the findings suggested that ROS scavenging systems and Na+/K+ transport synergistically contributed to redox equilibrium, ion homeostasis, and the enhancement of PSII function, thereby conferring high salt tolerance.

Gene-coexpression network analysis identifies specific modules and hub genes related to cold stress in rice

Background

When plants are subjected to cold stress, they undergo a series of molecular and physiological changes to protect themselves from injury. Indica cultivars can usually withstand only mild cold stress in a relatively short period. Hormone-mediated defence response plays an important role in cold stress. Weighted gene co-expression network analysis (WGCNA) is a very useful tool for studying the correlation between genes, identifying modules with high phenotype correlation, and identifying Hub genes in different modules. Many studies have elucidated the molecular mechanisms of cold tolerance in different plants, but little information about the recovery process after cold stress is available.

Results

To understand the molecular mechanism of cold tolerance in rice, we performed comprehensive transcriptome analyses during cold treatment and recovery stage in two cultivars of near-isogenic lines (9311 and DC907). Twelve transcriptomes in two rice cultivars were determined. A total of 2509 new genes were predicted by fragment splicing and assembly, and 7506 differentially expressed genes were identified by pairwise comparison. A total of 26 modules were obtained by expression-network analysis, 12 of which were highly correlated with cold stress or recovery treatment. We further identified candidate Hub genes associated with specific modules and analysed their regulatory relationships based on coexpression data. Results showed that various plant-hormone regulatory genes acted together to protect plants from physiological damage under short-term low-temperature stress. We speculated that this may be common in rice. Under long-term cold stress, rice improved the tolerance to low-temperature stress by promoting autophagy, sugar synthesis, and metabolism.

Conclusion

Through WGCNA analysis at the transcriptome level, we provided a potential regulatory mechanism for the cold stress and recovery of rice cultivars and identified candidate central genes. Our findings provided an important reference for the future cultivation of rice strains with good tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08438-3.

Effect of cold stress on polyamine metabolism and antioxidant responses in chickpea

Metabolic and genomic characteristics of polyamines (PAs) may be associated with the induction of cold tolerance (CT) responses in plants. Characteristics of PAs encoding genes in chickpea (Cicer arietinum L.) and their function under cold stress (CS) are currently unknown. In this study, the potential role of PAs along with the antioxidative defense systems were assessed in two chickpea genotypes (Sel96th11439, cold-tolerant and ILC533, cold-sensitive) under CS conditions. Six days after exposure to CS, the leaf H₂O₂ content and electrolyte leakage index increased in the sensitive genotype by 47.7 and 59 %, respectively, while these values decreased or remained unchanged, respectively, in the tolerant genotype. In tolerant genotype, the enhanced activity of superoxide dismutase (SOD) (by 50 %) was accompanied by unchanged activities of ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and catalase (CAT) as well as the accumulation of glutathione (GSH) (by 43 %) on the sixth day of CS. Higher levels of putrescine (Put) (322 %), spermidine (Spd) (45 %), spermine (Spm) (69 %) and the highest ratio of Put/(Spd + Spm) were observed in tolerant genotype compared to the sensitive one on the sixth day of CS. Gamma-aminobutyric acid (GABA) accumulation was 74 % higher in tolerant genotype compared to the sensitive one on the sixth day of CS. During CS, the activity of diamine oxidase (DAO) and polyamine oxidase (PAO) increased in tolerant (by 3.02- and 2.46-fold) and sensitive (by 2.51- and 2.8-fold) genotypes, respectively, in comparison with the respective non-stressed plants (normal conditions). The highest activity of DAO and PAO in the tolerant genotype was accompanied by PAs decomposition and a peak in GABA content on the sixth day of CS. The analysis of chickpea genome revealed the presence of five PAs biosynthetic genes, their chromosomal locations, and cis-regulatory elements. A significant increase in transcript levels of arginine decarboxylase (ADC) (24.26- and 7.96-fold), spermidine synthase 1 (SPDS1) (3.03- and 1.53-fold), SPDS2 (5.5- and 1.62-fold) and spermine synthase (SPMS) (3.92- and 1.65-fold) genes was detected in tolerant and sensitive genotypes, respectively, whereas the expression of ornithine decarboxylase (ODC) genes decreased significantly under CS conditions in both genotypes. Leaf chlorophyll and carotenoid contents exhibited declining trends in the sensitive genotype, while these photosynthetic pigments were stable in the tolerant genotype due to the superior performance of defensive processes under CS conditions. Overall, these results suggested the specific roles of putative PAs genes and PAs metabolism in development of effective CT responses in chickpea.

Physiological and transcriptomic analyses of yellow horn (Xanthoceras sorbifolia) provide important insights into salt and saline-alkali stress tolerance

Yellow horn (Xanthoceras sorbifolia) is an oil-rich woody plant cultivated for bio-energy production in China. Soil saline-alkalization is a prominent agricultural-related environmental problem limiting plant growth and productivity. In this study, we performed comparative physiological and transcriptomic analyses to examine the mechanisms of X. sorbifolia seedling responding to salt and alkaline-salt stress. With the exception of chlorophyll content, physiological experiments revealed significant increases in all assessed indices in response to salt and saline-alkali treatments. Notably, compared with salt stress, we observed more pronounced changes in electrolyte leakage (EL) and malondialdehyde (MDA) levels in response to saline-alkali stress, which may contribute to the greater toxicity of saline-alkali soils. In total, 3,087 and 2,715 genes were differentially expressed in response to salt and saline-alkali treatments, respectively, among which carbon metabolism, biosynthesis of amino acids, starch and sucrose metabolism, and reactive oxygen species signaling networks were extensively enriched, and transcription factor families of bHLH, C2H2, bZIP, NAC, and ERF were transcriptionally activated. Moreover, relative to salt stress, saline-alkali stress activated more significant upregulation of genes related to H+ transport, indicating that regulation of intracellular pH may play an important role in coping with saline-alkali stress. These findings provide new insights for investigating the physiological changes and molecular mechanisms underlying the responses of X. sorbifolia to salt and saline-alkali stress.

Spermidine enhanced resistance of Chlorella to high levels of CO2 and light intensity for improving photosynthetic growth rate

In order to promote the photosynthetic growth rate of Chlorella in the presence of flue gas CO₂ from coal-fired power plants, spermidine was first used to enhance cellular resistance to a high CO₂ concentration (15%) and high light intensity (30 000 lux). It was found that low concentrations (100–300 μM) of spermidine significantly enhanced the photosynthetic growth rate of Chlorella. The accelerated cell division decreased the cell diameter from 3.64 μm to 2.71 μm and the fractal dimension from 1.60 to 1.49, and the activity of total superoxide dismutase (T-SOD) increased from 0.48 U mL⁻¹ to 5.33 U mL⁻¹. Expression levels of key enzymes of photosystems I and II, ATP synthase and transportase markedly increased, thereby enhancing the electron transport and energy supply that reduced oxidative damage. Finally, an enhanced cellular resistance to the high CO₂ concentration and high light intensity increased the biomass yield from 0.11 g L⁻¹ to 1.71 g L⁻¹ (300 μM).

Spermidine enhanced resistance of Chlorella to high levels of CO₂ and light intensity.

Spermidine application alleviates salinity damage to antioxidant enzyme activity and gene expression in alfalfa

We investigated whether spermidine (Spd) application alleviates salinity-induced damage in alfalfa (Medicago sativa L), and explored defence mechanisms associated with stress-related ion balance, antioxidant metabolism, and gene expression. We examined the response of 30-day-old alfalfa maintained in hydroponic culture tests for 7 days and subjected to one of six treatments: half-strength Hoagland solution (control); 1% NaCl; 10 μM Spd + 1% NaCl; 20 μM Spd + 1% NaCl; 40 μM Spd + 1% NaCl; and 60 μM Spd + 1% NaCl. In salinity-stressed plants, chlorophyll b, chlorophyll a + b, and total protein showed significant decreases, while marked increases were detected in relative electrolyte leakage, H₂O₂ content, glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), glutathione reductase (GR) activity, the Na⁺/K⁺ ratio, and APX1, APX2, GR, and SOD gene expression levels. Chlorophyll a and total protein content markedly increased under exogenous application of 20 μM Spd, while H₂O₂ content, GSH, SOD, CAT, POD, GR activity, the Na⁺/K⁺ ratio, and APX2, GR, and SOD expression levels all decreased. These results indicated that exogenous application of 20 μM spermidine effectively alleviates salinity-induced damage in alfalfa. These findings could benefit alfalfa cultivation and promote the development and utilization of saline-alkali soil.

Changes in photosynthetic pigments, osmolytes and antioxidants of Indian mustard by drought and exogenous polyamines

A pot experiment was performed as factorial based on randomized complete block design with three replications, to assess the effects of 1 mM spermidine (SPD) and 1 mM putrescine (PUT) on Indian mustard (Brassica Juncea L.) under different levels of watering (100, 75, 50 and 25% of field capacity). Chlorophyll a and b contents decreased, but the ratio of Chl a/b and carotenoid content increased with decreasing water supply. Foliar sprays of polyamines improved chlorophylls a and b and carotenoid contents, while the ratio of Chl a/b was reduced by these growth regulators. Relative water content, glycine betaine, proteins and soluble sugars contents were increased, but proline content was decreased by exogenous polyamines under limited water supply. Antioxidant enzyme (POX, CAT, SOD and APX) activities were enhanced by drought stress and polyamine treatments. This resulted in lower electrolyte leakage and lipid peroxidation (less MDA) under stressful conditions. The present results indicate that exogenous polyamines such as putrescine and spermidine can alleviate some of the deleterious impacts of water limitation on Indian mustard.

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.

Crystal structure of thermospermine synthase from Medicago truncatula and substrate discriminatory features of plant aminopropyltransferases

Polyamines are linear polycationic compounds that play a crucial role in the growth and development of higher plants. One triamine (spermidine, SPD) and two tetraamine isomers (spermine, SPM, and thermospermine, TSPM) are obtained by the transfer of the aminopropyl group from decarboxylated S-adenosylmethionine to putrescine and SPD. These reactions are catalyzed by the specialized aminopropyltransferases. In that respect, plants are unique eukaryotes that have independently evolved two enzymes, thermospermine synthase (TSPS), encoded by the gene ACAULIS5, and spermine synthase, which produce TSPM and SPM, respectively. In this work, we structurally characterize the ACAULIS5 gene product, TSPS, from the model legume plant Medicago truncatula (Mt). Six crystal structures of MtTSPS - one without ligands and five in complexes with either reaction substrate (SPD), reaction product (TSPM), or one of three cofactor analogs (5'-methylthioadenosine, S-adenosylthiopropylamine, and adenosine) - give detailed insights into the biosynthesis of TSPM. Combined with small-angle X-ray scattering data, the crystal structures show that MtTSPS is a symmetric homotetramer with an interdomain eight-stranded β-barrel. Such an assembly and the presence of a hinge-like feature between N-terminal and C-terminal domains give the protein additional flexibility which potentially improves loading substrates and discarding products after the catalytic event. We also discuss the sequence and structural features around the active site of the plant aminopropyltransferases that distinguish them from each other and determine their characteristic substrate discrimination.

Involvement of abscisic acid in microbe-induced saline-alkaline resistance in plants

Soil salinity-alkalinity is one of abiotic stresses that lead to plant growth inhibition and yield loss. It has recently been indicated that plant growth promoting rhizobacteria (PGPR) can enhance the capacity of plants to counteract negative effects caused by adverse environments. However, whether PGPR confers increased saline-alkaline resistance of plants and the underlying mechanisms remain unclear. We thus investigated the effects of Bacillus licheniformis (strain SA03) on Chrysanthemum plants grown under saline-alkaline conditions. Soil inoculation with SA03 significantly mitigated saline-alkaline stress in plants with augmented photosynthesis, biomass and survival rates. Moreover, the inoculated plants accumulated more Fe and less Na⁺ content than the non-inoculated plants under the stress. However, the inoculation with SA03 failed to trigger a series of saline-alkaline stress responses in abscisic acid (ABA)- and nitric oxide (NO)-deficient plants. Furthermore, NO acted as a secondary messenger of ABA to regulate the stress responses and tolerance in Chrysanthemum plants. Therefore, these findings indicated that B. licheniformis SA03 could be employed to improve saline-alkaline tolerance of plants by mediating cellular ABA levels.

Proteomics survey of Solanaceae family: Current status and challenges ahead

Solanaceae is one of the major economically important families of higher plants and has played a central role in human nutrition since the dawn of human civilization. Therefore, researchers have always been interested in understanding the complex behavior of Solanaceae members to identify key transcripts, proteins or metabolites, which are potentially associated with major traits. Proteomics studies have contributed significantly to understanding the physiology of Solanaceae members. A compilation of all the published reports showed that both gel-based (75%) and gel-free (25%) proteomic technologies have been utilized to establish the proteomes of different tissues, organs, and organelles under normal and adverse environmental conditions. Among the Solanaceae members, most of the research has been focused on tomato (42%) followed by potato (28%) and tobacco (20%), owing to their economic importance. This review comprehensively covers the progress made so far in the field of Solanaceae proteomics including novel methods developed to isolate the proteins from different tissues. Moreover, key proteins presented in this review can serve as a resource to select potential targets for crop improvement. We envisage that information presented in this review would enable us to design the stress tolerant plants with enhanced yields.

Bacillus licheniformis SA03 Confers Increased Saline-Alkaline Tolerance in Chrysanthemum Plants by Induction of Abscisic Acid Accumulation

Soil saline-alkalization is a major abiotic stress that leads to low iron (Fe) availability and high toxicity of sodium ions (Na+) for plants. It has recently been shown that plant growth promoting rhizobacteria (PGPR) can enhance the ability of plants to tolerate multiple abiotic stresses such as drought, salinity, and nutrient deficiency. However, the possible involvement of PGPR in improving saline-alkaline tolerance of plants and the underlying mechanisms remain largely unknown. In this study, we investigated the effects of Bacillus licheniformis (strain SA03) on the growth of Chrysanthemum plants under saline-alkaline conditions. Our results revealed that inoculation with SA03 alleviated saline-alkaline stress in plants with increased survival rates, photosynthesis and biomass. The inoculated plants accumulated more Fe and lower Na+ concentrations under saline-alkaline stress compared with the non-inoculated plants. RNA-Sequencing analyses further revealed that SA03 significantly activated abiotic stress- and Fe acquisition-related pathways in the stress-treated plants. However, SA03 failed to increase saline-alkaline tolerance in plants when cellular abscisic acid (ABA) and nitric oxide (NO) synthesis were inhibited by treatment with fluridone (FLU) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), respectively. Importantly, we also found that NO acted downstream of SA03-induced ABA to activate a series of adaptive responses in host plants under saline-alkaline stress. These findings demonstrated the potential roles of B. licheniformis SA03 in enhancing saline-alkaline tolerance of plants and highlighted the intricate integration of microbial signaling in regulating cellular Fe and Na+ accumulation.

Net-Immobilization of β-glucosidase on Nonwoven Fabrics to Lower the Cost of "Cellulosic Ethanol" and Increase Cellulose Conversions

The main limitation preventing the use of enzymatic cellulosic ethanol in industrial production is its higher cost which is mainly due to the elevated price of β-glucosidase (BG). Herein, we report on a simple strategy for the in-situ encapsulation of BG for repeated cellulosic ethanol production. In this strategy, BG was net-immobilized into a poly(ethylene glycol) (PEG) net-cloth layer on a PP nonwoven fabric by way of the visible light-induced surface controlled/living graft cross-linking polymerization. The visible light and mild reaction conditions could ensure the activity retention of BG during immobilization, while the non-swelling uniform net-mesh formed by living cross-linking polymerization could prevent the leakage of BG effectively (at the immobilization rate of more than 98.6% and the leakage rate of only 0.4%). When the BG-loaded fabric was used in combination with free cellulase (CEL), the results of the catalytic reaction demonstrated that these BG-loaded fabrics could not only give a 40% increase in cellulose conversions but also be reused for more than fifteen batches without losing the activity. These BG-loaded fabrics with characteristics including easy separation, excellent operation stability, a low cost of the polymeric matrix and a simple fabrication process are particularly interesting for a future bio-fuel production strategy.

Structural Investigations of N-carbamoylputrescine Amidohydrolase from Medicago truncatula: Insights into the Ultimate Step of Putrescine Biosynthesis in Plants

Putrescine, 1,4-diaminobutane, is an intermediate in the biosynthesis of more complexed polyamines, spermidine and spermine. Unlike other eukaryotes, plants have evolved a multistep pathway for putrescine biosynthesis that utilizes arginine. In the final reaction, N-carbamoylputrescine is hydrolyzed to putrescine by N-carbamoylputrescine amidohydrolase (CPA, EC 3.5.1.53). During the hydrolysis, consecutive nucleophilic attacks on the substrate by Cys158 and water lead to formation of putrescine and two by-products, ammonia and carbon dioxide. CPA from the model legume plant, Medicago truncatula (MtCPA), was investigated in this work. Four crystal structures were determined: the wild-type MtCPA in complex with the reaction intermediate, N-(dihydroxymethyl)putrescine as well as with cadaverine, which is a longer analog of putrescine; and also structures of MtCPA-C158S mutant unliganded and with putrescine. MtCPA assembles into octamers, which resemble an incomplete left-handed helical twist. The active site of MtCPA is funnel-like shaped, and its entrance is walled with a contribution of the neighboring protein subunits. Deep inside the catalytic cavity, Glu48, Lys121, and Cys158 form the catalytic triad. In this studies, we have highlighted the key residues, highly conserved among the plant kingdom, responsible for the activity and selectivity of MtCPA toward N-carbamoylputrescine. Moreover, since, according to previous reports, a close MtCPA relative from Arabidopsis thaliana, along with several other nitrilase-like proteins, are subjected to allosteric regulation by substrates, we have used the structural information to indicate a putative secondary binding site. Based on the docking experiment, we postulate that this site is adjacent to the entrance to the catalytic pocket.

Polyamines function in stress tolerance: from synthesis to regulation

Plants are challenged by a variety of biotic or abiotic stresses, which can affect their growth and development, productivity, and geographic distribution. In order to survive adverse environmental conditions, plants have evolved various adaptive strategies, among which is the accumulation of metabolites that play protective roles. A well-established example of the metabolites that are involved in stress responses, or stress tolerance, is the low-molecular-weight aliphatic polyamines, including putrescine, spermidine, and spermine. The critical role of polyamines in stress tolerance is suggested by several lines of evidence: firstly, the transcript levels of polyamine biosynthetic genes, as well as the activities of the corresponding enzymes, are induced by stresses; secondly, elevation of endogenous polyamine levels by exogenous supply of polyamines, or overexpression of polyamine biosynthetic genes, results in enhanced stress tolerance; and thirdly, a reduction of endogenous polyamines is accompanied by compromised stress tolerance. A number of studies have demonstrated that polyamines function in stress tolerance largely by modulating the homeostasis of reactive oxygen species (ROS) due to their direct, or indirect, roles in regulating antioxidant systems or suppressing ROS production. The transcriptional regulation of polyamine synthesis by transcription factors is also reviewed here. Meanwhile, future perspectives on polyamine research are also suggested.