Identification of seven polyamine oxidase genes in tomato (Solanum lycopersicum L.) and their expression profiles under physiological and various stress conditions

Polyamines: The valuable bio-stimulants and endogenous signaling molecules for plant development and stress response

Polyamines (PAs) are nitrogenous and polycationic compounds containing more than two amine residues. Numerous investigations have demonstrated that cellular PA homeostasis plays a key role in various developmental and physiological processes. The PA balance, which may be affected by many environmental factors, is finely maintained by the pathways of PA biosynthesis and degradation (catabolism). In this review, the advances in PA transport and distribution and their roles in plants were summarized and discussed. In addition, the interplay between PAs and phytohormones, NO, and H2O2 were detailed during plant growth, senescence, fruit repining, as well as response to biotic and abiotic stresses. Moreover, it was elucidated how environmental signals such as light, temperature, and humidity modulate PA accumulation during plant development. Notably, PA has been shown to exert a potential role in shaping the domestication of rice. The present review comprehensively summarizes these latest advances, highlighting the importance of PAs as endogenous signaling molecules in plants, and as well proposes future perspectives on PA research.

Sequence-based identification of polyamine oxidase genes in Sorghum bicolor L

Polyamine oxidases (PAOs) are enzymes associated with polyamine catabolism and play important roles in growth and development and stress tolerance of plants. In the present study, genome-wide discovery and analysis of the PAO family in sorghum was done utilizing model PAO of Arabidopsis. Six PAO genes were found using publicly available genomic data. Sorghum has the PAO gene representatives distributed throughout four chromosomes (chr1, chr3, chr6, and chr7), and most members have eight to nine exons. The molecular weights of PAO proteins range from 53 to 63 kDa. PAO proteins have a theoretical PI between 5.2 and 8.1. The identification and characterization of PAO gene members in sorghum lay the foundation for further experimental studies elucidating their roles in growth, development, and stress responses, ultimately contributing to our understanding of plant biology, with significant implications for plant breeding by providing valuable insights into potential targets for enhancing stress tolerance and improving crop performance.

Genome-wide analysis and expression profiling of the polyamine oxidase gene family in Solanum tuberosum L

BACKGROUND

Polyamine oxidase (PAO) is a crucial enzyme involved in the breakdown of polyamines (PAs) in plants. It not only regulates the levels of PAs, but also plays a role in the oxidative decomposition of PAs and the release of stress-related signals, contributing to the plant's response and resistance to various adversities. While there have been numerous studies on the response of PAO to stress in other crops, there is a lack of research on this topic in potatoes, a major food crop.

RESULTS

In this study, we aimed to explore the biological function of the StPAO gene in potato growth and development, as well as its expression patterns under stress. Using bioinformatics methods, we identified 14 StPAO genes in the potato genome. Protein sequence comparisons revealed a high similarity between the PAO proteins of potato and Arabidopsis. Chromosomal mapping and gene structure analysis showed that the StPAO genes were not evenly distributed on the chromosome and all contained an amino-oxidase domain. Furthermore, analysis of the promoters of these genes revealed the presence of abiotic and stress-related cis-acting elements, indicating their potential role in responding to different stresses. To investigate the expression patterns of these genes under stress, we used qRT-PCR to study their response to high temperature, drought, and ABA stress. Our results showed that StPAO6 and StPAO10 were significantly up-regulated under high temperature stress, indicating that they were involved in the process of potato resistance to high temperatures. Similarly, StPAO1, StPAO3, and StPAO4 were significantly up-regulated under drought stress, indicating their potential role in potatoes' responses to drought. After ABA treatment, the expression levels of StPAO4, StPAO5, StPAO7, and StPAO14 were significantly up-regulated, suggesting their involvement in chemical defense mechanisms. Interestingly, the expression of StPAO11-13 was inhibited by all three stresses.

CONCLUSIONS

In conclusion, our study highlights the multifunctional nature of the StPAO gene family in potatoes, which plays a crucial role in coping with various stresses. This research deepens our understanding of the potato StPAO gene family and provides a reference for future studies on its function. It also serves as a theoretical basis for breeding stress-resistant potato varieties in the future.

Polyamines: Rising stars against metal and metalloid toxicity

Globally, metal/metalloid(s) soil contamination is a persistent issue that affects the atmosphere, soil, water and plant health in today's industrialised world. However, an overabundance of these transition ions promotes the excessive buildup of reactive oxygen species (ROS) and ion imbalance, which harms agricultural productivity. Plants employ several strategies to overcome their negative effects, including hyperaccumulation, tolerance, exclusion, and chelation with organic molecules. Polyamines (PAs) are the organic compounds that act as chelating agents and modulate various physiological, biochemical, and molecular processes under metal/metalloid(s) stress. Their catabolic products, including H2O2 and gamma amino butyric acid (GABA), are also crucial signalling molecules in abiotic stress situations, particularly under metal/metalloid(s) stress. In this review, we explained how PAs regulate genes and enzymes, particularly under metal/metalloid(s) stress with a specific focus on arsenic (As), boron (B), cadmium (Cd), chromium (Cr), and zinc (Zn). The PAs regulate various plant stress responses by crosstalking with other plant hormones, upregulating phytochelatin, and metallothionein synthesis, modulating stomatal closure and antioxidant capacity. This review presents valuable insights into how PAs use a variety of tactics to reduce the harmful effects of metal/metalloid(s) through multifaceted strategies.

A Solanum lycopersicum polyamine oxidase contributes to the control of plant growth, xylem differentiation, and drought stress tolerance

Polyamines are involved in several plant physiological processes. In Arabidopsis thaliana, five FAD-dependent polyamine oxidases (AtPAO1 to AtPAO5) contribute to polyamine homeostasis. AtPAO5 catalyzes the back-conversion of thermospermine (T-Spm) to spermidine and plays a role in plant development, xylem differentiation, and abiotic stress tolerance. In the present study, to verify whether T-Spm metabolism can be exploited as a new route to improve stress tolerance in crops and to investigate the underlying mechanisms, tomato (Solanum lycopersicum) AtPAO5 homologs were identified (SlPAO2, SlPAO3, and SlPAO4) and CRISPR/Cas9-mediated loss-of-function slpao3 mutants were obtained. Morphological, molecular, and physiological analyses showed that slpao3 mutants display increased T-Spm levels and exhibit changes in growth parameters, number and size of xylem elements, and expression levels of auxin- and gibberellin-related genes compared to wild-type plants. The slpao3 mutants are also characterized by improved tolerance to drought stress, which can be attributed to a diminished xylem hydraulic conductivity that limits water loss, as well as to a reduced vulnerability to embolism. Altogether, this study evidences conservation, though with some significant variations, of the T-Spm-mediated regulatory mechanisms controlling plant growth and differentiation across different plant species and highlights the T-Spm role in improving stress tolerance while not constraining growth.

Overexpression of OsACL5 triggers environmentally-dependent leaf rolling and reduces grain size in rice

Major polyamines include putrescine, spermidine, spermine and thermospermine, which play vital roles in growth and adaptation against environmental changes in plants. Thermospermine (T-Spm) is synthetised by ACL5. The function of ACL5 in rice is still unknown. In this study, we used a reverse genetic strategy to investigate the biological function of OsACL5. We generated several knockout mutants by pYLCRISPR/Cas9 system and overexpressing (OE) lines of OsACL5. Interestingly, the OE plants exhibited environmentally-dependent leaf rolling, smaller grains, lighter 1000-grain weight and reduction in yield per plot. The area of metaxylem vessels of roots and leaves of OE plants were significantly smaller than those of WT, which possibly caused reduction in leaf water potential, resulting in leaf rolling with rise in the environmental temperature and light intensity and decrease in humidity. Additionally, the T-Spm contents were markedly increased by over ninefold whereas the ethylene evolution was reduced in OE plants, suggesting that T-Spm signalling pathway interacts with ethylene pathway to regulate multiple agronomic characters. Moreover, the osacl5 exhibited an increase in grain length, 1000-grain weight, and yield per plot. OsACL5 may affect grain size via mediating the expression of OsDEP1, OsGS3 and OsGW2. Furthermore, haplotypes analysis indicated that OsACL5 plays a conserved function on regulating T-Spm levels during the domestication of rice. Our data demonstrated that identification of OsACL5 provides a theoretical basis for understanding the physiological mechanism of T-Spm which may play roles in triggering environmentally dependent leaf rolling; OsACL5 will be an important gene resource for molecular breeding for higher yield.

Molecular mechanism of endophytic bacteria DX120E regulating polyamine metabolism and promoting plant growth in sugarcane

Introduction

Sugarcane endophytic nitrogen-fixing bacterium Klebsiella variícola DX120E displayed broad impact on growth, but the exact biological mechanism, especially polyamines (PAs) role, is still meager.

Methods

To reveal this relationship, the content of polyamine oxidase (PAO), PAs, reactive oxygen species (ROS)-scavenging antioxidative enzymes, phytohormones, 1-aminocyclopropane-1-carboxylic synthase (ACS), chlorophyll content, and biomass were determined in sugarcane incubated with the DX120E strain. In addition, expression levels of the genes associated with polyamine metabolism were measured by transcriptomic analysis.

Results

Genomic analysis of Klebsiella variícola DX120E revealed that 39 genes were involved in polyamine metabolism, transport, and the strain secrete PAs in vitro. Following a 7-day inoculation period, DX120E stimulated an increase in the polyamine oxidase (PAO) enzyme in sugarcane leaves, however, the overall PAs content was reduced. At 15 days, the levels of PAs, ROS-scavenging antioxidative enzymes, and phytohormones showed an upward trend, especially spermidine (Spd), putrescine (Put), catalase (CAT), auxin (IAA), gibberellin (GA), and ACS showed a significant up-regulation. The GO and KEGG enrichment analysis found a total of 73 differentially expressed genes, involving in the cell wall (9), stimulus response (13), peroxidase activity (33), hormone (14) and polyamine metabolism (4).

Discussion

This study demonstrated that endophytic nitrogen-fixing bacteria stimulated polyamine metabolism and phytohormones production in sugarcane plant tissues, resulting in enhanced growth. Dual RNA-seq analyses provided insight into the early-stage interaction between sugarcane seedlings and endophytic bacteria at the transcriptional level. It showed how diverse metabolic processes selectively use distinct molecules to complete the cell functions under present circumstances.

Subfunctionalization of Parental Polyamine Oxidase (PAO) Genes in the Allopolyploid Tobacco Nicotiana tabacum (L.)

Polyamines play an important role in developmental and environmental stress responses in plants. Polyamine oxidases (PAOs) are flavin-adenine-dinucleotide-dependent enzymes associated with polyamine catabolism. In this study, 14 genes were identified in the tobacco genome that code for PAO proteins being named based on their sequence homology with Arabidopsis PAOs (AtPAO1-5): NtPAO1A-B; NtPAO2A-C, NtPAO4A-D, and NtPAO5A-E. Sequence analysis confirmed that the PAO gene family of the allopolyploid hybrid Nicotiana tabacum is not an exact combination of the PAO genes of the maternal Nicotiana sylvestris and paternal Nicotiana tomentosiformis ones. The loss of the N. sylvestris homeolog of NtPAO5E and the gain of an extra NtPAO2 copy, likely of Nicotiana othophora origin, was revealed. The latter adds to the few pieces of evidence suggesting that the paternal parent of N. tabacum was an introgressed hybrid of N. tomentosiformis and N. othophora. Gene expression analysis indicated that all 14 PAO genes kept their expression following the formation of the hybrid species. The homeologous gene pairs showed similar or opposite regulation depending on the investigated organ, applied stress, or hormone treatment. The data indicate that the expression pattern of the homeologous genes is diversifying in a process of subfunctionalization.

L-Aminoguanidine Induces Imbalance of ROS/RNS Homeostasis and Polyamine Catabolism of Tomato Roots after Short-Term Salt Exposure

Polyamine (PA) catabolism mediated by amine oxidases is an important process involved in fine-tuning PA homeostasis and related mechanisms during salt stress. The significance of these amine oxidases in short-term responses to salt stress is, however, not well understood. In the present study, the effects of L-aminoguanidine (AG) on tomato roots treated with short-term salt stress induced by NaCl were studied. AG is usually used as a copper amine oxidase (CuAO or DAO) inhibitor. In our study, other alterations of PA catabolism, such as reduced polyamine oxidase (PAO), were also observed in AG-treated plants. Salt stress led to an increase in the reactive oxygen and nitrogen species in tomato root apices, evidenced by in situ fluorescent staining and an increase in free PA levels. Such alterations were alleviated by AG treatment, showing the possible antioxidant effect of AG in tomato roots exposed to salt stress. PA catabolic enzyme activities decreased, while the imbalance of hydrogen peroxide (H2O2), nitric oxide (NO), and hydrogen sulfide (H2S) concentrations displayed a dependence on stress intensity. These changes suggest that AG-mediated inhibition could dramatically rearrange PA catabolism and related reactive species backgrounds, especially the NO-related mechanisms. More studies are, however, needed to decipher the precise mode of action of AG in plants exposed to stress treatments.

Interactions of Polyamines and Phytohormones in Plant Response to Abiotic Stress

Numerous environmental conditions negatively affect plant production. Abiotic stresses, such as salinity, drought, temperature, and heavy metals, cause damage at the physiological, biochemical, and molecular level, and limit plant growth, development, and survival. Studies have indicated that small amine compounds, polyamines (PAs), play a key role in plant tolerance to various abiotic stresses. Pharmacological and molecular studies, as well as research using genetic and transgenic approaches, have revealed the favorable effects of PAs on growth, ion homeostasis, water maintenance, photosynthesis, reactive oxygen species (ROS) accumulation, and antioxidant systems in many plant species under abiotic stress. PAs display a multitrack action: regulating the expression of stress response genes and the activity of ion channels; improving the stability of membranes, DNA, and other biomolecules; and interacting with signaling molecules and plant hormones. In recent years the number of reports indicating crosstalk between PAs and phytohormones in plant response to abiotic stresses has increased. Interestingly, some plant hormones, previously known as plant growth regulators, can also participate in plant response to abiotic stresses. Therefore, the main goal of this review is to summarize the most significant results that represent the interactions between PAs and plant hormones, such as abscisic acid, brassinosteroids, ethylene, jasmonates, and gibberellins, in plants under abiotic stress. The future perspectives for research focusing on the crosstalk between PAs and plant hormones were also discussed.

Plant Peroxisomal Polyamine Oxidase: A Ubiquitous Enzyme Involved in Abiotic Stress Tolerance

Polyamines (PAs) are positively charged amines that are present in all organisms. In addition to their functions specific to growth and development, they are involved in responding to various biotic and abiotic stress tolerance functions. The appropriate concentration of PA in the cell is maintained by a delicate balance between the catabolism and anabolism of PAs, which is primarily driven by two enzymes, namely diamine oxidase and polyamine oxidase (PAO). PAOs have been found to be localized in multiple subcellular locations, including peroxisomes. This paper presents a holistic account of peroxisomal PAOs. PAOs are flavin adenine dinucleotide-dependent enzymes with varying degrees of substrate specificity. They are expressed differentially upon various abiotic stress conditions, namely heat, cold, salinity, and dehydration. It has also been observed that in a particular species, the various PAO isoforms are expressed differentially with a spatial and temporal distinction. PAOs are targeted to peroxisome via a peroxisomal targeting signal (PTS) type 1. We conducted an extensive bioinformatics analysis of PTS1s present in various peroxisomal PAOs and present a consensus peroxisome targeting signal present in PAOs. Furthermore, we also propose an evolutionary perspective of peroxisomal PAOs. PAOs localized in plant peroxisomes are of potential importance in abiotic stress tolerance since peroxisomes are one of the nodal centers of reactive oxygen species (ROS) homeostasis and an increase in ROS is a major indicator of the plant being in stress conditions; hence, in the future, PAO enzymes could be used as a key candidate for generating abiotic stress tolerant crops.

Heterologous Expression and Characterization of Tea (Camellia sinensis) Polyamine Oxidase Homologs and Their Involvement in Stresses

Polyamine oxidase (PAO) is a key enzyme maintaining polyamine homeostasis, which affects plant physiological activities. Until now, the gene members and function of PAOs in tea (Camellia sinenesis) have not been fully identified. Here, through the expression in Escherichia coli and Nicotiana benthamiana, we identified six genes annotated as CsPAO in tea genome and transcriptome and determined their enzyme reaction modes and gene expression profiles in tea cultivar 'Yinghong 9'. We found that CsPAO1,2,3 could catalyze spermine, thermospermine, and norspermidine, and CsPAO2,3 could catalyze spermidine in the back-conversion mode, which indicated that the precursor of γ-aminobutyric acid might originate from the oxidation of putrescin but not spermidine. We further investigated the changes of CsPAO activity with temperature and pH and their stability. Kinetic parameters suggested that CsPAO2 was the major PAO modifying polyamine composition in tea, and it could be inactivated by β-hydroxyethylhydrazine and aminoguanidine. Putrescine content and CsPAO2 expression were high in tea flowers. CsPAO2 responded to wound, drought, and salt stress; CsPAO1 might be the main member responding to cold stress; anoxia induced CsPAO3. We conclude that in terms of phylogenetic tree, enzyme characteristics, and expression profile, CsPAO2 might be the dominant CsPAO in the polyamine degradation pathway.

Tomato salt tolerance mechanisms and their potential applications for fighting salinity: A review

One of the most significant environmental factors affecting plant growth, development and productivity is salt stress. The damage caused by salt to plants mainly includes ionic, osmotic and secondary stresses, while the plants adapt to salt stress through multiple biochemical and molecular pathways. Tomato (Solanum lycopersicum L.) is one of the most widely cultivated vegetable crops and a model dicot plant. It is moderately sensitive to salinity throughout the period of growth and development. Biotechnological efforts to improve tomato salt tolerance hinge on a synthesized understanding of the mechanisms underlying salinity tolerance. This review provides a comprehensive review of major advances on the mechanisms controlling salt tolerance of tomato in terms of sensing and signaling, adaptive responses, and epigenetic regulation. Additionally, we discussed the potential application of these mechanisms in improving salt tolerance of tomato, including genetic engineering, marker-assisted selection, and eco-sustainable approaches.

Unraveling the genetics of polyamine metabolism in barley for senescence-related crop improvement

We explored the polyamine (PA) metabolic pathway genes in barley (Hv) to understand plant development and stress adaptation in Gramineae crops with emphasis on leaf senescence. Bioinformatics and functional genomics tools were utilized for genome-wide identification, comprehensive gene features, evolution, development and stress effects on the expression of the polyamine metabolic pathway gene families (PMGs). Three S-adenosylmethionine decarboxylases (HvSAMDCs), two ornithine decarboxylases (HvODCs), one arginine decarboxylase (HvADC), one spermidine synthase (HvSPDS), two spermine synthases (HvSPMSs), five copper amine oxidases (HvCuAOs) and seven polyamine oxidases (HvPAOs) members of PMGs were identified and characterized in barley. All the HvPMG genes were found to be distributed on all chromosomes of barley. The phylogenetic and comparative assessment revealed that PA metabolic pathway is highly conserved in plants and the prediction of nine H. vulgare miRNAs (hvu-miR) target sites, 18 protein-protein interactions and 961 putative CREs in the promoter region were discerned. Gene expression of HvSAMDC3, HvCuAO7, HvPAO4 and HvSPMS1 was apparent at every developmental stage. SPDS/SPMS gene family was found to be the most responsive to induced leaf senescence. This study provides a reference for the functional investigation of the molecular mechanism(s) that regulate polyamine metabolism in plants as a tool for future breeding decision management systems.

Genome-Wide Identification of Polyamine Oxidase (PAO) Family Genes: Roles of CaPAO2 and CaPAO4 in the Cold Tolerance of Pepper (Capsicum annuum L.)

Polyamine oxidases (PAOs), which are flavin adenine dinucleotide-dependent enzymes, catalyze polyamine (PA) catabolism, producing hydrogen peroxide (H₂O₂). Several PAO family members have been identified in plants, but their expression in pepper plants remains unclear. Here, six PAO genes were identified in the ‘Zunla-1’ pepper genome (named CaPAO1–CaPAO6 according to their chromosomal positions). The PAO proteins were divided into four subfamilies according to phylogenetics: CaPAO1 belongs to subfamily I; CaPAO3 and CaPAO5 belong to subfamily III; and CaPAO2, CaPAO4, and CaPAO6 belong to subfamily IV (none belong to subfamily II). CaPAO2, CaPAO4, and CaPAO6 were ubiquitously and highly expressed in all tissues, CaPAO1 was mainly expressed in flowers, whereas CaPAO3 and CaPAO5 were expressed at very low levels in all tissues. RNA-seq analysis revealed that CaPAO2 and CaPAO4 were notably upregulated by cold stress. CaPAO2 and CaPAO4 were localized in the peroxisome, and spermine was the preferred substrate for PA catabolism. CaPAO2 and CaPAO4 overexpression in Arabidopsis thaliana significantly enhanced freezing-stress tolerance by increasing antioxidant enzyme activity and decreasing malondialdehyde, H₂O₂, and superoxide accumulation, accompanied by the upregulation of cold-responsive genes (AtCOR15A, AtRD29A, AtCOR47, and AtKIN1). Thus, we identified candidate PAO genes for breeding cold-stress-tolerant transgenic pepper cultivars.

Metabolomics and lipidomics insight into the effect of different polyamines on tomato plants under non-stress and salinity conditions

Polyamines (PAs) are key signaling molecules involved in plant growth and stress acclimation processes. This work investigated the effect of spermidine, spermine, and putrescine (alone and in a mixture) in tomato plants using a combined metabolomics and lipidomics approach. The experiments were carried out under non-stress and 100 mM NaCl salinity conditions. Shoot and root biomass, as well as SPAD values, were increased by the application of exogenous PAs but with differences across treatments. Similarly, root length density (F: 34, p < 0.001), average root diameter (F: 14, p < 0.001), and very fine roots (0.0-0.5 mm) increased in PA-treated plants, compared to control. Metabolomics and lipidomics indicated that, despite being salinity the hierarchically prevalent factor, the different PA treatments imposed distinct remodeling at the molecular level. Plants treated with putrescine showed the broader modulation of metabolite profile, whereas spermidine and spermine induced a comparatively milder effect. The pathway analysis from differential metabolites indicated a broad and multi-level intricate modulation of several signaling molecules together with stress-related compounds like flavonoids and alkaloids. Concerning signaling processes, the complex crosstalk between phytohormones (mainly abscisic acid, cytokinins, the ethylene precursor, and jasmonates), and the membrane lipids signaling cascade (in particular, sphingolipids as well as ceramides and other glycerophospholipids), was involved in such complex response of tomato to PAs. Interestingly, PA-specific processes could be observed, with peculiar responses under either control or salinity conditions.

Genome-Wide Identification and Functional Analysis of Polyamine Oxidase Genes in Maize Reveal Essential Roles in Abiotic Stress Tolerance

Polyamines (PAs) play a critical role in growth and developmental processes and stress responses in plants. Polyamine oxidase (PAO) is a flavin adenine dinucleotide (FAD)-dependent enzyme that plays a major role in PA catabolism. Here, for the first time, PAO genes in maize were screened for the whole genome-wide and nine ZmPAO genes were identified in this study, named as ZmPAO1-9. Based on structural characteristics and a comparison of phylogenetic relationships of PAO gene families from seven representative species, all nine PAO proteins in maize were categorized into three distinct subfamilies. Further, chromosome location and schematic structure revealed an unevenly distribution on chromosomes and evolutionarily conserved structure features of ZmPAO genes in maize, respectively. Furthermore, transcriptome analysis demonstrated that ZmPAO genes showed differential expression patterns at diverse developmental stages of maize, suggesting that these genes may play functional developmental roles in multiple tissues. Further, through qRT-PCR validation, these genes were confirmed to be responsive to heat, drought and salinity stress treatments in three various tissues, indicating their potential roles in abiotic stress responses. Eventually, to verify the biological function of ZmPAO genes, the transgenic Arabidopsis plants overexpressing ZmPAO6 gene were constructed as a typical representative to explore functional roles in plants. The results demonstrated that overexpression of ZmPAO6 can confer enhanced heat tolerance through mediating polyamine catabolism in transgenic Arabidopsis, which might result in reduced H₂O₂ and MDA accumulation and alleviated chlorophyll degradation under heat stress treatment, indicating that ZmPAO6 may play a crucial role in enhancing heat tolerance of transgenic Arabidopsis through the involvement in various physiological processes. Further, the expression analysis of related genes of antioxidant enzymes including glutathione peroxidase (GPX) and ascorbate peroxidase (APX) demonstrated that ZmPAO6 can enhance heat resistance in transgenic Arabidopsis through modulating heat-induced H₂O₂ accumulation in polyamine catabolism. Taken together, our results are the first to report the ZmPAO6 gene response to heat stress in plants and will serve to present an important theoretical basis for further unraveling the function and regulatory mechanism of ZmPAO genes in growth, development and adaptation to abiotic stresses in maize.

Polyamine Oxidase Triggers H2O2-Mediated Spermidine Improved Oxidative Stress Tolerance of Tomato Seedlings Subjected to Saline-Alkaline Stress

Saline-alkaline stress is one of several major abiotic stresses in crop production. Exogenous spermidine (Spd) can effectively increase tomato saline-alkaline stress resistance by relieving membrane lipid peroxidation damage. However, the mechanism through which exogenous Spd pre-treatment triggers the tomato antioxidant system to resist saline-alkaline stress remains unclear. Whether H2O2 and polyamine oxidase (PAO) are involved in Spd-induced tomato saline-alkaline stress tolerance needs to be determined. Here, we investigated the role of PAO and H2O2 in exogenous Spd-induced tolerance of tomato to saline-alkaline stress. Results showed that Spd application increased the expression and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and the ratio of reduced ascorbate (AsA) and glutathione (GSH) contents under saline-alkaline stress condition. Exogenous Spd treatment triggered endogenous H2O2 levels, SlPAO4 gene expression, as well as PAO activity under normal conditions. Inhibiting endogenous PAO activity by 1,8-diaminooctane (1,8-DO, an inhibitor of polyamine oxidase) significantly reduced H2O2 levels in the later stage. Moreover, inhibiting endogenous PAO or silencing the SlPAO4 gene increased the peroxidation damage of tomato leaves under saline-alkaline stress. These findings indicated that exogenous Spd treatment stimulated SlPAO4 gene expression and increased PAO activity, which mediated the elevation of H2O2 level under normal conditions. Consequently, the downstream antioxidant system was activated to eliminate excessive ROS accumulation and relieve membrane lipid peroxidation damage and growth inhibition under saline-alkaline stress. In conclusion, PAO triggered H2O2-mediated Spd-induced increase in the tolerance of tomato to saline-alkaline stress.

Responses of Polyamine-Metabolic Genes to Polyamines and Plant Stress Hormones in Arabidopsis Seedlings

In plants, many of the enzymes in polyamine metabolism are encoded by multiple genes, whose expressions are differentially regulated under different physiological conditions. For comprehensive understanding of their regulation during the seedling growth stage, we examined the expression of polyamine metabolic genes in response to polyamines and stress-related plant hormones in Arabidopsis thaliana. While confirming previous findings such as induction of many of the genes by abscisic acid, induction of arginase genes and a copper amine oxidase gene, CuAOα3, by methyl jasmonate, that of an arginine decarboxylase gene, ADC2, and a spermine synthase gene, SPMS, by salicylic acid, and negative feedback regulation of thermospermine biosynthetic genes by thermospermine, our results showed that expressions of most of the genes are not responsive to exogenous polyamines. We thus examined expression of OsPAO6, which encodes an apoplastic polyamine oxidase and is strongly induced by polyamines in rice, by using the promoter-GUS fusion in transgenic Arabidopsis seedlings. The GUS activity was increased by treatment with methyl jasmonate but neither by polyamines nor by other plant hormones, suggesting a difference in the response to polyamines between Arabidopsis and rice. Our results provide a framework to study regulatory modules directing expression of each polyamine metabolic gene.

Expression profile of seven polyamine oxidase genes in rice (Oryza sativa) in response to abiotic stresses, phytohormones and polyamines

Polyamine levels are controlled by biosynthesis, intra- and inter-cellular flux by the respective transporters, and catabolism. The catabolism is catalyzed by two groups of enzymes. One is copper-containing amine oxidases and the other is polyamine oxidases (PAOs). In Oryza sativa, seven PAO genes exist and they are termed as OsPAO1 to OsPAO7. However, their physiological function has not been elucidated yet. Here, we examined the expressional changes of seven OsPAO genes upon abiotic and oxidative stress, phytohormone, and exogenous polyamines application. The transcript of extracellular polyamine oxidase OsPAO2 and OsPAO6 are strongly induced upon wounding, drought, salinity, oxidative stress (H₂O₂), and exogenous application of jasmonic acid, spermidine, spermine, thermospermine and negatively regulated upon indole acetic acid, isopentenyl adenine (iPT), gibberellic acid (GA), abscisic acid; OsPAO7 is to iPT, GA and all polyamines; OsPAO4 and OsPAO5 are mildly responsive to heat, cold, oxidative stress. These results suggest that polyamine oxidase encoding extracellular enzyme may play a pivotal role during exogenous stimulus to protect the plant cell.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01006-1.

Update on the Roles of Polyamines in Fleshy Fruit Ripening, Senescence, and Quality

Ripening of fleshy fruits involves complex physiological, biochemical, and molecular processes that coincide with various changes of the fruit, including texture, color, flavor, and aroma. The processes of ripening are controlled by ethylene in climacteric fruits and abscisic acid (ABA) in non-climacteric fruits. Increasing evidence is also uncovering an essential role for polyamines (PAs) in fruit ripening, especially in climacteric fruits. However, until recently breakthroughs have been made in understanding PA roles in the ripening of non-climacteric fruits. In this review, we compare the mechanisms underlying PA biosynthesis, metabolism, and action during ripening in climacteric and non-climacteric fruits at the physiological and molecular levels. The PA putrescine (Put) has a role opposite to that of spermidine/spermine (Spd/Spm) in cellular metabolism. Arginine decarboxylase (ADC) is crucial to Put biosynthesis in both climacteric and non-climacteric fruits. S-adenosylmethionine decarboxylase (SAMDC) catalyzes the conversion of Put to Spd/Spm, which marks a metabolic transition that is concomitant with the onset of fruit ripening, induced by Spd in climacteric fruits and by Spm in non-climacteric fruits. Once PA catabolism is activated by polyamine oxidase (PAO), fruit ripening and senescence are facilitated by the coordination of mechanisms that involve PAs, hydrogen peroxide (H₂O₂), ABA, ethylene, nitric oxide (NO), and calcium ions (Ca²⁺). Notably, a signal derived from PAO5-mediated PA metabolism has recently been identified in strawberry, a model system for non-climacteric fruits, providing a deeper understanding of the regulatory roles played by PAs in fleshy fruit ripening.

Genome-Wide Identification of Seven Polyamine Oxidase Genes in Camellia sinensis (L.) and Their Expression Patterns Under Various Abiotic Stresses

Polyamines (PAs) in plant play a critical role in growth and development and in response to environmental stress. Polyamine oxidase (PAO) is a flavin adenine dinucleotide dependent enzyme that plays a major role in PA catabolism. For the first time, PAO genes in tea plant were screened for the whole genome-wide and seven CsPAO genes were identified, which were named CsPAO1-7. Phylogenetic tree analysis revealed seven CsPAO protein sequences classed into three groups, including clade I, III, and IV. Compared with other plants, the tea plant lacked clade II members. Genetic structure and tissue specific expression analysis showed that there were significant differences among members of the CsPAO gene family. Among members of the CsPAOs family, CsPAO4 and CsPAO5 contain more introns and are highly expressed in various organizations. CsPAO1, CsPAO4, and CsPAO5 genes were cloned and expressed heterologously to verify theirs function. Heat map showed high response of CsPAO5 to drought stress, while CsPAO1 and CsPAO2 were sensitive to changes in nitrogen nutrition. Furthermore, exogenous abscisic acid (ABA) treatment indicated that the expression of most CsPAO genes in roots and leaves was significantly induced. In the root, Spm content increased significantly, while Put and Spd content decreased, suggesting that ABA has great influence on the biosynthesis of PAs. Anaerobic treatment of picked tea leaves showed that the decomposition of PAs was promoted to a certain extent. The above data help to clarify the role of CsPAO in response abiotic and nitrogen nutritional stresses in tea plants, and provide a reference perspective for the potential influence of PAs on the tea processing quality.

Genome-wide analysis of the polyamine oxidase gene family in wheat (Triticum aestivum L.) reveals involvement in temperature stress response

Amine oxidases (AOs) including copper containing amine oxidases (CuAOs) and FAD-dependent polyamine oxidases (PAOs) are associated with polyamine catabolism in the peroxisome, apoplast and cytoplasm and play an essential role in growth and developmental processes and response to biotic and abiotic stresses. Here, we identified PAO genes in common wheat (Triticum aestivum), T. urartu and Aegilops tauschii and reported the genome organization, evolutionary features and expression profiles of the wheat PAO genes (TaPAO). Expression analysis using publicly available RNASeq data showed that TaPAO genes are expressed redundantly in various tissues and developmental stages. A large percentage of TaPAOs respond significantly to abiotic stresses, especially temperature (i.e. heat and cold stress). Some TaPAOs were also involved in response to other stresses such as powdery mildew, stripe rust and Fusarium infection. Overall, TaPAOs may have various functions in stress tolerances responses, and play vital roles in different tissues and developmental stages. Our results provided a reference for further functional investigation of TaPAO proteins.

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.

FaPAO5 regulates Spm/Spd levels as a signaling during strawberry fruit ripening

Polyamines are important for non-climacteric fruit ripening according to an analysis of the model plant strawberry. However, the molecular mechanism underlying the polyamine accumulation during ripening has not been fully elucidated. In this study, an examination of our proteome data related to strawberry fruit ripening revealed a putative polyamine oxidase 5, FaPAO5, which was localized in the cytoplasm and nucleus. Additionally, FaPAO5 expression levels as well as the abundance of the encoded protein continually decreased during ripening. Inhibiting FaPAO5 expression by RNAi promoted Spd, Spm, and ABA accumulation while inhibited H2O2 production, which ultimately enhanced ripening as evidenced by the ripening-related events and corresponding gene expression changes. The opposite effects were observed in FaPAO5-overexpressing transgenic fruits. Analyses of the binding affinity and enzymatic activity of FaPAO5 with Spm, Spd, and Put uncovered a special role for FaPAO5 in the terminal catabolism of Spm and Spd, with a K d of 0.21 and 0.29 µM, respectively. Moreover, FaPAO5 expression was inhibited by ABA and promoted by Spd and Spm. Furthermore, the RNA-seq analysis of RNAi and control fruits via differentially expressed genes (DEGs) indicated the six most enriched pathways among the differentially expressed genes were related to sugar, abscisic acid, ethylene, auxin, gibberellin, and Ca2+. Among four putative PAO genes in the strawberry genome, only FaPAO5 was confirmed to influence fruit ripening. In conclusion, FaPAO5 is a negative regulator of strawberry fruit ripening and modulates Spm/Spd levels as a signaling event, in which ABA plays a central role.

Polyamine homeostasis in tomato biotic/abiotic stress cross-tolerance

Adverse conditions and biotic strain can lead to significant losses and impose limitations on plant yield. Polyamines (PAs) serve as regulatory molecules for both abiotic/biotic stress responses and cell protection in unfavourable environments. In this work, the transcription pattern of 24 genes orchestrating PA metabolism was investigated in Cucumber Mosaic Virus or Potato Virus Y infected and cold stressed tomato plants. Expression analysis revealed a differential/pleiotropic pattern of gene regulation in PA homeostasis upon biotic, abiotic or combined stress stimuli, thus revealing a discrete response specific to diverse stimuli: (i) biotic stress-influenced genes, (ii) abiotic stress-influenced genes, and (iii) concurrent biotic/abiotic stress-regulated genes. The results support different roles for PAs against abiotic and biotic stress. The expression of several genes, significantly induced under cold stress conditions, is mitigated by a previous viral infection, indicating a possible priming-like mechanism in tomato plants pointing to crosstalk among stress signalling. Several genes and resulting enzymes of PA catabolism were stimulated upon viral infection. Hence, we suggest that PA catabolism resulting in elevated H₂O₂ levels could mediate defence against viral infection. However, after chilling, the activities of enzymes implicated in PA catabolism remained relatively stable or slightly reduced. This correlates to an increase in free PA content, designating a per se protective role of these compounds against abiotic stress.

Plant peroxisomes at the crossroad of NO and H2 O2 metabolism

Plant peroxisomes are subcellular compartments involved in many biochemical pathways during the life cycle of a plant but also in the mechanism of response against adverse environmental conditions. These organelles have an active nitro-oxidative metabolism under physiological conditions but this could be exacerbated under stress situations. Furthermore, peroxisomes have the capacity to proliferate and also undergo biochemical adaptations depending on the surrounding cellular status. An important characteristic of peroxisomes is that they have a dynamic metabolism of reactive nitrogen and oxygen species (RNS and ROS) which generates two key molecules, nitric oxide (NO) and hydrogen peroxide (H₂ O₂ ). These molecules can exert signaling functions by means of post-translational modifications that affect the functionality of target molecules like proteins, peptides or fatty acids. This review provides an overview of the endogenous metabolism of ROS and RNS in peroxisomes with special emphasis on polyamine and uric acid metabolism as well as the possibility that these organelles could be a source of signal molecules involved in the functional interconnection with other subcellular compartments.

Polyamine Oxidases Play Various Roles in Plant Development and Abiotic Stress Tolerance

Polyamines not only play roles in plant growth and development, but also adapt to environmental stresses. Polyamines can be oxidized by copper-containing diamine oxidases (CuAOs) and flavin-containing polyamine oxidases (PAOs). Two types of PAOs exist in the plant kingdom; one type catalyzes the back conversion (BC-type) pathway and the other catalyzes the terminal catabolism (TC-type) pathway. The catabolic features and biological functions of plant PAOs have been investigated in various plants in the past years. In this review, we focus on the advance of PAO studies in rice, Arabidopsis, and tomato, and other plant species.

Polyamine Catabolism in Plants: A Universal Process With Diverse Functions

Polyamine (PA) catabolic processes are performed by copper-containing amine oxidases (CuAOs) and flavin-containing PA oxidases (PAOs). So far, several CuAOs and PAOs have been identified in many plant species. These enzymes exhibit different subcellular localization, substrate specificity, and functional diversity. Since PAs are involved in numerous physiological processes, considerable efforts have been made to explore the functions of plant CuAOs and PAOs during the recent decades. The stress signal transduction pathways usually lead to increase of the intracellular PA levels, which are apoplastically secreted and oxidized by CuAOs and PAOs, with parallel production of hydrogen peroxide (H2O2). Depending on the levels of the generated H2O2, high or low, respectively, either programmed cell death (PCD) occurs or H2O2 is efficiently scavenged by enzymatic/nonenzymatic antioxidant factors that help plants coping with abiotic stress, recruiting different defense mechanisms, as compared to biotic stress. Amine and PA oxidases act further as PA back-converters in peroxisomes, also generating H2O2, possibly by activating Ca2+ permeable channels. Here, the new research data are discussed on the interconnection of PA catabolism with the derived H2O2, together with their signaling roles in developmental processes, such as fruit ripening, senescence, and biotic/abiotic stress reactions, in an effort to elucidate the mechanisms involved in crop adaptation/survival to adverse environmental conditions and to pathogenic infections.

Polyamine Function in Plants: Metabolism, Regulation on Development, and Roles in Abiotic Stress Responses

Polyamines (PAs) are low molecular weight aliphatic nitrogenous bases containing two or more amino groups. They are produced by organisms during metabolism and are present in almost all cells. Because they play important roles in diverse plant growth and developmental processes and in environmental stress responses, they are considered as a new kind of plant biostimulant. With the development of molecular biotechnology techniques, there is increasing evidence that PAs, whether applied exogenously or produced endogenously via genetic engineering, can positively affect plant growth, productivity, and stress tolerance. However, it is still not fully understood how PAs regulate plant growth and stress responses. In this review, we attempt to cover these information gaps and provide a comprehensive and critical assessment of the published literature on the relationships between PAs and plant flowering, embryo development, senescence, and responses to several (mainly abiotic) stresses. The aim of this review is to summarize how PAs improve plants' productivity, and to provide a basis for future research on the mechanism of action of PAs in plant growth and development. Future perspectives for PA research are also suggested.

PtrA/NINV, an alkaline/neutral invertase gene of Poncirus trifoliata, confers enhanced tolerance to multiple abiotic stresses by modulating ROS levels and maintaining photosynthetic efficiency

BACKGROUND

Alkaline/neutral invertase (A/N-INV), an enzyme that hydrolyzes sucrose irreversibly into glucose and fructose, is essential for normal plant growth,development, and stress tolerance. However, the physiological and/or molecular mechanism underpinning the role of A/N-INV in abiotic stress tolerance is poorly understood.

RESULTS

In this report, an A/N-INV gene (PtrA/NINV) was isolated from Poncirus trifoliata, a cold-hardy relative of citrus, and functionally characterized. PtrA/NINV expression levels were induced by cold, salt, dehydration, sucrose, and ABA, but decreased by glucose. PtrA/NINV was found to localize in both chloroplasts and mitochondria. Overexpression of PtrA/NINV conferred enhanced tolerance to multiple stresses, including cold, high salinity, and drought, as supported by lower levels of reactive oxygen species (ROS), reduced oxidative damages, decreased water loss rate, and increased photosynthesis efficiency, relative to wild-type (WT). The transgenic plants exhibited higher A/N-INV activity and greater reducing sugar content under normal and stress conditions.

CONCLUSIONS

PtrA/NINV is an important gene implicated in sucrose decomposition, and plays a positive role in abiotic stress tolerance by promoting osmotic adjustment, ROS detoxification and photosynthesis efficiency. Thus, PtrA/NINV has great potential to be used in transgenic breeding for improvement of stress tolerance.