Thermospermine catabolism increases Arabidopsis thaliana resistance to Pseudomonas viridiflava

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.

Thermospermine Is an Evolutionarily Ancestral Phytohormone Required for Organ Development and Stress Responses in Marchantia Polymorpha

Thermospermine suppresses auxin-inducible xylem differentiation, whereas its structural isomer, spermine, is involved in stress responses in angiosperms. The thermospermine synthase, ACAULIS5 (ACL5), is conserved from algae to land plants, but its physiological functions remain elusive in non-vascular plants. Here, we focused on MpACL5, a gene in the liverwort Marchantia polymorpha, that rescued the dwarf phenotype of the acl5 mutant in Arabidopsis. In the Mpacl5 mutants generated by genome editing, severe growth retardation was observed in the vegetative organ, thallus, and the sexual reproductive organ, gametangiophore. The mutant gametangiophores exhibited remarkable morphological defects such as short stalks, fasciation and indeterminate growth. Two gametangiophores fused together, and new gametangiophores were often initiated from the old ones. Furthermore, Mpacl5 showed altered responses to heat and salt stresses. Given the absence of spermine in bryophytes, these results suggest that thermospermine has a dual primordial function in organ development and stress responses in M. polymorpha. The stress response function may have eventually been assigned to spermine during land plant evolution.

GUS Reporter-Aided Promoter Deletion Analysis of A. thaliana POLYAMINE OXIDASE 3

Polyamine oxidases (PAOs) have been correlated with numerous physiological and developmental processes, as well as responses to biotic and abiotic stress conditions. Their transcriptional regulation is driven by signals generated by various developmental and environmental cues, including phytohormones. However, the inductive mechanism(s) of the corresponding genes remains elusive. Out of the five previously characterized Arabidopsis PAO genes, none of their regulatory sequences have been analyzed to date. In this study, a GUS reporter-aided promoter deletion approach was used to investigate the transcriptional regulation of AtPAO3 during normal growth and development as well as under various inductive environments. AtPAO3 contains an upstream open reading frame (uORF) and a short inter-cistronic sequence, while the integrity of both appears to be crucial for the proper regulation of gene expression. The full-length promoter contains several cis-acting elements that regulate the tissue-specific expression of AtPAO3 during normal growth and development. Furthermore, a number of TFBS that are involved in gene induction under various abiotic stress conditions display an additive effect on gene expression. Taken together, our data indicate that the transcription of AtPAO3 is regulated by multiple environmental factors, which probably work alongside hormonal signals and shed light on the fine-tuning mechanisms of PAO regulation.

Male Fertility under Environmental Stress: Do Polyamines Act as Pollen Tube Growth Protectants?

Although pollen structure and morphology evolved toward the optimization of stability and fertilization efficiency, its performance is affected by harsh environmental conditions, e.g., heat, cold, drought, pollutants, and other stressors. These phenomena are expected to increase in the coming years in relation to predicted environmental scenarios, contributing to a rapid increase in the interest of the scientific community in understanding the molecular and physiological responses implemented by male gametophyte to accomplish reproduction. Here, after a brief introduction summarizing the main events underlying pollen physiology with a focus on polyamine involvement in its development and germination, we review the main effects that environmental stresses can cause on pollen. We report the most relevant evidence in the literature underlying morphological, cytoskeletal, metabolic and signaling alterations involved in stress perception and response, focusing on the final stage of pollen life, i.e., from when it hydrates, to pollen tube growth and sperm cell transport, with these being the most sensitive to environmental changes. Finally, we hypothesize the molecular mechanisms through which polyamines, well-known molecules involved in plant development, stress response and adaptation, can exert a protective action against environmental stresses in pollen by decoding the essential steps and the intersection between polyamines and pollen tube growth mechanisms.

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.

Pseudomonas fluorescens DN16 Enhances Cucumber Defense Responses Against the Necrotrophic Pathogen Botrytis cinerea by Regulating Thermospermine Catabolism

Plants can naturally interact with beneficial rhizobacteria to mediate defense responses against foliar pathogen infection. However, the mechanisms of rhizobacteria-mediated defense enhancement remain rarely clear. In this study, beneficial rhizobacterial strain Pseudomonas fluorescens DN16 greatly increased the resistance of cucumber plants against Botrytis cinerea infection. RNA-sequencing analyses showed that several polyamine-associated genes including a thermospermine (TSpm) synthase gene (CsACL5) and polyamine catabolic genes (CsPAO1, CsPAO5, and CsCuAO1) were notably induced by DN16. The associations of TSpm metabolic pathways with the DN16-mediated cucumber defense responses were further investigated. The inoculated plants exhibited the increased leaf TSpm levels compared with the controls. Accordantly, overexpression of CsACL5 in cucumber plants markedly increased leaf TSpm levels and enhanced defense against B. cinerea infection. The functions of TSpm catabolism in the DN16-mediated defense responses of cucumber plants to B. cinerea were further investigated by pharmacological approaches. Upon exposure to pathogen infection, the changes of leaf TSpm levels were positively related to the enhanced activities of polyamine catabolic enzymes including polyamine oxidases (PAOs) and copper amine oxidases (CuAOs), which paralleled the transcription of several defense-related genes such as pathogenesis-related protein 1 (CsPR1) and defensin-like protein 1 (CsDLP1). However, the inhibited activities of polyamine catabolic enzymes abolished the DN16-induced cucumber defense against B. cinerea infection. This was in line with the impaired expression of defense-related genes in the inoculated plants challenged by B. cinerea. Collectively, our findings unraveled a pivotal role of TSpm catabolism in the regulation of the rhizobacteria-primed defense states by mediating the immune responses in cucumber plants after B. cinerea infection.

Citrus Polyamines: Structure, Biosynthesis, and Physiological Functions

Polyamines (PAs) are ubiquitous biogenic amines found in all living organisms from bacteria to Archaea, and Eukaryotes including plants and animals. Since the first description of putrescine conjugate, feruloyl-putrescine (originally called subaphylline), from grapefruit leaves and juice, many research studies have highlighted the importance of PAs in growth, development, and other physiological processes in citrus plants. PAs appear to be involved in a wide range of physiological processes in citrus plants; however, their exact roles are not fully understood. Accordingly, in the present review, we discuss the biosynthesis of PAs in citrus plants, with an emphasis on the recent advances in identifying and characterizing PAs-biosynthetic genes and other upstream regulatory genes involved in transcriptional regulation of PAs metabolism. In addition, we will discuss the recent metabolic, genetic, and molecular evidence illustrating the roles of PAs metabolism in citrus physiology including somatic embryogenesis; root system formation, morphology, and architecture; plant growth and shoot system architecture; inflorescence, flowering, and flowering-associated events; fruit set, development, and quality; stomatal closure and gas-exchange; and chlorophyll fluorescence and photosynthesis. We believe that the molecular and biochemical understanding of PAs metabolism and their physiological roles in citrus plants will help citrus breeding programs to enhance tolerance to biotic and abiotic stresses and provide bases for further research into potential applications.

Thermospermine Synthase (ACL5) and Diamine Oxidase (DAO) Expression Is Needed for Zygotic Embryogenesis and Vascular Development in Scots Pine

Unlike in flowering plants, the detailed roles of the enzymes in the polyamine (PA) pathway in conifers are poorly known. We explored the sequence conservation of the PA biosynthetic genes and diamine oxidase (DAO) in conifers and flowering plants to reveal the potential functional diversification of the enzymes between the plant lineages. The expression of the genes showing different selective constraints was studied in Scots pine zygotic embryogenesis and early seedling development. We found that the arginine decarboxylase pathway is strongly preferred in putrescine production in the Scots pine as well as generally in conifers and that the reduced use of ornithine decarboxylase (ODC) has led to relaxed purifying selection in ODC genes. Thermospermine synthase (ACL5) genes evolve under strong purifying selection in conifers and the DAO gene is also highly conserved in pines. In developing Scots pine seeds, the expression of both ACL5 and DAO increased as embryogenesis proceeded. Strong ACL5 expression was present in the procambial cells of the embryo and in the megagametophyte cells destined to die via morphologically necrotic cell death. Thus, the high sequence conservation of ACL5 genes in conifers may indicate the necessity of ACL5 for both embryogenesis and vascular development. Moreover, the result suggests the involvement of ACL5 in morphologically necrotic cell death and supports the view of the genetic regulation of necrosis in Scots pine embryogenesis and in plant development. DAO transcripts were located close to the cell walls and between the walls of adjacent cells in Scots pine zygotic embryos and in the roots of young seedlings. We propose that DAO, in addition to the role in Put oxidation for providing H2O2 during the cell-wall structural processes, may also participate in cell-to-cell communication at the mRNA level. To conclude, our findings indicate that the PA pathway of Scots pines possesses several special functional characteristics which differ from those of flowering plants.

The eggplant transcription factor MYB44 enhances resistance to bacterial wilt by activating the expression of spermidine synthase

Bacterial wilt (BW) caused by Ralstonia solanacearum is a serious disease affecting the production of Solanaceae species, including eggplant (Solanum melongena). However, few resistance genes have been identified in eggplant, and therefore the underlying mechanism of BW resistance remains unclear. Hence, we investigated a spermidine synthase (SPDS) gene from eggplant and created knock-down lines with virus-induced gene silencing. After eggplant was infected with R. solanacearum, the SmSPDS gene was induced, concurrent with increased spermidine (Spd) content, especially in the resistant line. We speculated that Spd plays a significant role in the defense response of eggplant to BW. Moreover, using the yeast one-hybrid approach and dual luciferase-based transactivation assay, an R2R3-MYB transcription factor, SmMYB44, was identified as directly binding to the SmSPDS promoter, activating its expression. Overexpression of SmMYB44 in eggplant induced the expression of SmSPDS and Spd content, increasing the resistance to BW. In contrast, the SmMYB44-RNAi transgenic plants showed more susceptibility to BW compared with the control plants. Our results provide insight into the SmMYB44-SmSPDS-Spd module involved in the regulation of resistance to R. solanacearum. This research also provides candidates to enhance resistance to BW in eggplant.

Salt hypersensitivity is associated with excessive xylem development in a thermospermine-deficient mutant of Arabidopsis thaliana

In Arabidopsis, spermine is produced in most tissues and has been implicated in stress response, while its structural isomer thermospermine is only in xylem precursor cells. Studies on acaulis5 (acl5), a mutant defective in the biosynthesis of thermospermine, have revealed that thermospermine plays a repressive role in xylem development through enhancement of mRNA translation of the SAC51 family. In contrast, the pao5 mutant defective in the degradation of thermospermine has high levels of thermospermine and shows increased salt tolerance, suggesting a role of thermospermine in salt stress response. Here we compared acl5 with a mutant of spermine synthase, spms, in terms of abiotic stress tolerance and found that acl5 was much more sensitive to sodium than the wild-type and spms. A double-mutant of acl5 and sac51-d, which suppresses the excessive xylem phenotype of acl5, recovered normal sensitivity, while a quadruple T-DNA insertion mutant of the SAC51 family, which has an increased thermospermine level but shows excessive xylem development, showed increased salt sensitivity, unlike pao5. Together with the result that the salt tolerance of both wild-type and acl5 seedlings was improved by long-term treatment with thermospermine, we suggest a correlation of the salt tolerance with reduced xylem development rather than with the thermospermine level. We further found that the mutants containing high thermospermine levels showed increased tolerance to drought and heat stress, suggesting another role of thermospermine that may be common with that of spermine and secondary to that in restricting excess xylem development associated with salt hypersensitivity.

Mechanisms of plant protection against two oxalate-producing fungal pathogens by oxalotrophic strains of Stenotrophomonas spp

Oxalotrophic Stenotrophomonas isolated from tomato rhizosphere are able to protect plants against oxalate-producing pathogens by a combination of actions including induction of plant defence signalling callose deposition and the strengthening of plant cell walls and probably the degradation of oxalic acid. Oxalic acid plays a pivotal role in the virulence of the necrotrophic fungi Botrytis cinerea and Sclerotinia sclerotiorum. In this work, we isolated two oxalotrophic strains (OxA and OxB) belonging to the bacterial genus Stenotrophomonas from the rhizosphere of tomato plants. Both strains were capable to colonise endophytically Arabidopsis plants and protect them from the damage caused by high doses of oxalic acid. Furthermore, OxA and OxB protected Arabidopsis from S. sclerotiorum and B. cinerea infections. Bacterial inoculation induced the production of phenolic compounds and the expression of PR-1. Besides, both isolates exerted a protective effect against fungal pathogens in Arabidopsis mutants affected in the synthesis pathway of salicylic acid (sid2-2) and jasmonate perception (coi1). Callose deposition induced by OxA and OxB was required for protection against phytopathogens. Moreover, B. cinerea and S. sclerotiorum mycelial growth was reduced in culture media containing cell wall polysaccharides from leaves inoculated with each bacterial strain. These findings suggest that cell walls from Arabidopsis leaves colonised by these bacteria would be less susceptible to pathogen attack. Our results indicate that these oxalotrophic bacteria can protect plants against oxalate-producing pathogens by a combination of actions and show their potential for use as biological control agents against fungal diseases.

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.

Conservation of Thermospermine Synthase Activity in Vascular and Non-vascular Plants

In plants, the only confirmed function for thermospermine is regulating xylem cells maturation. However, genes putatively encoding thermospermine synthases have been identified in the genomes of both vascular and non-vascular plants. Here, we verify the activity of the thermospermine synthase genes and the presence of thermospermine in vascular and non-vascular land plants as well as in the aquatic plant Chlamydomonas reinhardtii. In addition, we provide information about differential content of thermospermine in diverse organs at different developmental stages in some vascular species that suggest that, although the major role of thermospermine in vascular plants is likely to be xylem development, other potential roles in development and/or responses to stress conditions could be associated to such polyamine. In summary, our results in vascular and non-vascular species indicate that the capacity to synthesize thermospermine is conserved throughout the entire plant kingdom.

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.

Spermine Differentially Refines Plant Defense Responses Against Biotic and Abiotic Stresses

Roles of the major polyamines (mPA), putrescine, spermidine, and spermine (Spm), in various developmental and physiological processes in plants have been well documented. Recently, there has been increasing focus on the link between mPA metabolism and defense response during plant-stress interactions. Empirical evidence is available for a unique role of Spm, distinct from the other mPA, in eliciting an effective defense response to (a)biotic stresses. Our understanding of the precise molecular mechanism(s) by which Spm modulates these defense mechanisms is limited. Further analysis of recent studies indicates that plant Spm functions differently during biotic and abiotic interactions in the regulation of oxidative homeostasis and phytohormone signaling. Here, we summarize and integrate current knowledge about Spm-mediated modulation of plant defense responses to (a)biotic stresses, highlighting the importance of Spm as a potent plant defense activator with broad-spectrum protective effects. A model is proposed to explain how Spm refines defense mechanisms to tailor an optimal resistance response.

Phenotypic and Genotypic Characterization of Mutant Plants in Polyamine Metabolism Genes During Pathogenic Interactions

Plants respond to pathogen attack by modifying defense gene expression and inducing the production of myriad proteins and metabolites. Among these responses, polyamine (PA) levels suffer remarkable modifications. Evidences demonstrate that plants make use of the polyamine biosynthetic pathway and the oxidative catabolism of these compounds in order to mount adequate defenses against pathogens. In Arabidopsis thaliana, putrescine is synthesized exclusively through the arginine decarboxylase (ADC) pathway, this enzyme exists as two isoforms named ADC1 and ADC2. Even though both isoforms participate in the response to pathogen attack, the mechanisms modulating ADC activity are not completely understood. Therefore, studies to clarify their roles are necessary. In this chapter, we describe the methods that can be applied for the study of plant-pathogen interactions using Arabidopsis adc mutant plants.

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.

Thermospermine: An Evolutionarily Ancient but Functionally New Compound in Plants

Themospermine is a structural isomer of spermine and is present in some bacteria and most of plants. An Arabidopsis mutant, acaulis5 (acl5), that is defective in the biosynthesis of thermospermine displays excessive proliferation of xylem vessels with dwarfed growth. Recent studies using acl5 and its suppressor mutants that recover the growth without thermospermine have revealed that thermospermine plays a key role in the negative control of the proliferation of xylem vessels through enhancing translation of specific mRNAs that contain a conserved upstream open-reading-frame (uORF) in the 5' leader region.

Polyamine Metabolism Responses to Biotic and Abiotic Stress

Plants have developed different strategies to cope with the environmental stresses they face during their life cycle. The responses triggered under these conditions are usually characterized by significant modifications in the metabolism of polyamines such as putrescine, spermidine, and spermine. Several works have demonstrated that a fine-tuned regulation of the enzymes involved in the biosynthesis and catabolism of polyamines leads to the increment in the concentration of these compounds. Polyamines exert different effects that could help plants to deal with stressful conditions. For instance, they interact with negatively charged macromolecules and regulate their functions, they may act as compatible osmolytes, or present antimicrobial activity against plant pathogens. In addition, they have also been proven to act as regulators of gene expression during the elicitation of stress responses. In this chapter, we reviewed the information available till date in relation to the roles played by polyamines in the responses of plants during biotic and abiotic stress.

The Arabidopsis polyamine oxidase/dehydrogenase 5 interferes with cytokinin and auxin signaling pathways to control xylem differentiation

In plants, the polyamines putrescine, spermidine, spermine (Spm), and thermospermine (Therm-Spm) participate in several physiological processes. In particular, Therm-Spm is involved in the control of xylem differentiation, having an auxin antagonizing effect. Polyamine oxidases (PAOs) are FAD-dependent enzymes involved in polyamine catabolism. In Arabidopsis, five PAOs are present, among which AtPAO5 catalyzes the back-conversion of Spm, Therm-Spm, and N1-acetyl-Spm to spermidine. In the present study, it is shown that two loss-of-function atpao5 mutants and a 35S::AtPAO5 Arabidopsis transgenic line present phenotypical differences from the wild-type plants with regard to stem and root elongation, differences that are accompanied by changes in polyamine levels and the number of xylem vessels. It is additionally shown that cytokinin treatment, which up-regulates AtPAO5 expression in roots, differentially affects protoxylem differentiation in 35S::AtPAO5, atpao5, and wild-type roots. Together with these findings, Therm-Spm biosynthetic genes, as well as auxin-, xylem-, and cytokinin-related genes (such as ACL5, SAMDC4, PIN1, PIN6, VND6, VND7, ATHB8, PHB, CNA, PXY, XTH3, XCP1, and AHP6) are shown to be differentially expressed in the various genotypes. These data suggest that AtPAO5, being involved in the control of Therm-Spm homeostasis, participates in the tightly controlled interplay between auxin and cytokinins that is necessary for proper xylem differentiation.

Inhibition of polyamine oxidase activity affects tumor development during the maize-Ustilago maydis interaction

Ustilago maydis is a biotrophic plant pathogenic fungus that leads to tumor development in the aerial tissues of its host, Zea mays. These tumors are the result of cell hypertrophy and hyperplasia, and are accompanied by the reprograming of primary and secondary metabolism of infected plants. Up to now, little is known regarding key plant actors and their role in tumor development during the interaction with U. maydis. Polyamines are small aliphatic amines that regulate plant growth, development and stress responses. In a previous study, we found substantial increases of polyamine levels in tumors. In the present work, we describe the maize polyamine oxidase (PAO) gene family, its contribution to hydrogen peroxide (H2O2) production and its possible role in tumor development induced by U. maydis. Histochemical analysis revealed that chlorotic lesions and maize tumors induced by U. maydis accumulate H2O2 to significant levels. Maize plants inoculated with U. maydis and treated with the PAO inhibitor 1,8-diaminooctane exhibit a notable reduction of H2O2 accumulation in infected tissues and a significant drop in PAO activity. This treatment also reduced disease symptoms in infected plants. Finally, among six maize PAO genes only the ZmPAO1, which encodes an extracellular enzyme, is up-regulated in tumors. Our data suggest that H2O2 produced through PA catabolism by ZmPAO1 plays an important role in tumor development during the maize-U. maydis interaction.

Cotton S-adenosylmethionine decarboxylase-mediated spermine biosynthesis is required for salicylic acid- and leucine-correlated signaling in the defense response to Verticillium dahliae

Cotton S-adenosylmethionine decarboxylase-, rather than spermine synthase-, mediated spermine biosynthesis is required for salicylic acid- and leucine-correlated signaling in the defense response to Verticillium dahliae. Spermine (Spm) signaling is correlated with plant resistance to the fungal pathogen Verticillium dahliae. We identified genes for key rate-limiting enzymes in the biosynthesis of Spm, namely S-adenosylmethionine decarboxylase (GhSAMDC) and Spm synthase (GhSPMS). These were found by screening suppression subtractive hybridization and cDNA libraries of cotton (Gossypium) species tolerant to Verticillium wilt. Both were induced early and strongly by inoculation with V. dahliae and application of plant hormones. Silencing of GhSPMS or GhSAMDC in cotton leaves led to a significant accumulation of upstream substrates and, ultimately, enhanced plant susceptibility to Verticillium infection. Exogenous supplementation of Spm to the silenced cotton plants improved resistance. When compared with the wild type (WT), constitutive expression of GhSAMDC in Arabidopsis thaliana was associated with greater Verticillium wilt resistance and higher accumulations of Spm, salicylic acid, and leucine during the infection period. By contrast, transgenic Arabidopsis plants that over-expressed GhSPMS were unexpectedly more susceptible than the WT to V. dahliae and they also had impaired levels of putrescine (Put) and salicylic acid (SA). The susceptibility exhibited in GhSPMS-overexpressing Arabidopsis plants was partially reversed by the exogenous supply of Put or SA. In addition, the responsiveness of those two transgenic Arabidopsis lines to V. dahliae was associated with an alteration in transcripts of genes involved in plant resistance to epidermal penetrations and amino acid signaling. Together, these results suggest that GhSAMDC-, rather than GhSPMS-, mediated spermine biosynthesis contributes to plant resistance against V. dahliae through SA- and leucine-correlated signaling.

Glutamate, Ornithine, Arginine, Proline, and Polyamine Metabolic Interactions: The Pathway Is Regulated at the Post-Transcriptional Level

The metabolism of glutamate into ornithine, arginine, proline, and polyamines is a major network of nitrogen-metabolizing pathways in plants, which also produces intermediates like nitric oxide, and γ-aminobutyric acid (GABA) that play critical roles in plant development and stress. While the accumulations of intermediates and the products of this network depend primarily on nitrogen assimilation, the overall regulation of the interacting sub-pathways is not well understood. We tested the hypothesis that diversion of ornithine into polyamine biosynthesis (by transgenic approach) not only plays a role in regulating its own biosynthesis from glutamate but also affects arginine and proline biosynthesis. Using two high putrescine producing lines of Arabidopsis thaliana (containing a transgenic mouse ornithine decarboxylase gene), we studied the: (1) effects of exogenous supply of carbon and nitrogen on polyamines and pools of soluble amino acids; and, (2) expression of genes encoding key enzymes in the interactive pathways of arginine, proline and GABA biosynthesis as well as the catabolism of polyamines. Our findings suggest that: (1) the overall conversion of glutamate to arginine and polyamines is enhanced by increased utilization of ornithine for polyamine biosynthesis by the transgene product; (2) proline and arginine biosynthesis are regulated independently of polyamines and GABA biosynthesis; (3) the expression of most genes (28 that were studied) that encode enzymes of the interacting sub-pathways of arginine and GABA biosynthesis does not change even though overall biosynthesis of Orn from glutamate is increased several fold; and (4) increased polyamine biosynthesis results in increased assimilation of both nitrogen and carbon by the cells.

Copper-Containing Amine Oxidases and FAD-Dependent Polyamine Oxidases Are Key Players in Plant Tissue Differentiation and Organ Development

Plant polyamines are catabolized by two classes of amine oxidases, the copper amine oxidases (CuAOs) and the flavin adenine dinucleotide (FAD)-dependent polyamine oxidases (PAOs). These enzymes differ to each other in substrate specificity, catalytic mechanism and subcellular localization. CuAOs and PAOs contribute to several physiological processes both through the control of polyamine homeostasis and as sources of biologically-active reaction products. CuAOs and PAOs have been found at high level in the cell-wall of several species belonging to Fabaceae and Poaceae families, respectively, especially in tissues fated to undertake extensive wall loosening/stiffening events and/or in cells undergoing programmed cell death (PCD). Apoplastic CuAOs and PAOs have been shown to play a key role as a source of H2O2 in light- or developmentally-regulated differentiation events, thus influencing cell-wall architecture and maturation as well as PCD. Moreover, growing evidence suggests a key role of intracellular CuAOs and PAOs in several facets of plant development. Here, we discuss recent advances in understanding the contribution of different CuAOs/PAOs, as well as their cross-talk with different intracellular and apoplastic metabolic pathways, in tissue differentiation and organ development.

Glutamate, Ornithine, Arginine, Proline, and Polyamine Metabolic Interactions: The Pathway Is Regulated at the Post-Transcriptional Level

The metabolism of glutamate into ornithine, arginine, proline, and polyamines is a major network of nitrogen-metabolizing pathways in plants, which also produces intermediates like nitric oxide, and γ-aminobutyric acid (GABA) that play critical roles in plant development and stress. While the accumulations of intermediates and the products of this network depend primarily on nitrogen assimilation, the overall regulation of the interacting sub-pathways is not well understood. We tested the hypothesis that diversion of ornithine into polyamine biosynthesis (by transgenic approach) not only plays a role in regulating its own biosynthesis from glutamate but also affects arginine and proline biosynthesis. Using two high putrescine producing lines of Arabidopsis thaliana (containing a transgenic mouse ornithine decarboxylase gene), we studied the: (1) effects of exogenous supply of carbon and nitrogen on polyamines and pools of soluble amino acids; and, (2) expression of genes encoding key enzymes in the interactive pathways of arginine, proline and GABA biosynthesis as well as the catabolism of polyamines. Our findings suggest that: (1) the overall conversion of glutamate to arginine and polyamines is enhanced by increased utilization of ornithine for polyamine biosynthesis by the transgene product; (2) proline and arginine biosynthesis are regulated independently of polyamines and GABA biosynthesis; (3) the expression of most genes (28 that were studied) that encode enzymes of the interacting sub-pathways of arginine and GABA biosynthesis does not change even though overall biosynthesis of Orn from glutamate is increased several fold; and (4) increased polyamine biosynthesis results in increased assimilation of both nitrogen and carbon by the cells.

Cotton ACAULIS5 is involved in stem elongation and the plant defense response to Verticillium dahliae through thermospermine alteration

Overexpression of GhACL5 , an ACAULIS5 from cotton, in Arabidopsis increased plant height and T-Spm level. Silencing of GhACL5 in cotton exhibited a dwarf phenotype and reduced resistance to Verticillium dahliae. The Arabidopsis thaliana gene ACAULIS5 (ACL5), for which inactivation causes a defect in stem elongation, encodes thermospermine (T-Spm) synthase. However, limited information is available about improvement in plant height by the overexpression of ACL5 gene, and the biological functions of ACL5 genes in response to biotic stress. Here, this study reports that constitutive expression of the cotton ACL5 gene (GhACL5) in Arabidopsis thaliana significantly increased plant height and elevated the level of T-Spm. Silencing of that gene in cotton reduced the amount of T-Spm and led to a severe dwarf phenotype. Expression of GhACL5 was induced upon treatment with the fungal pathogen Verticillium dahliae and plant hormones salicylic acid, jasmonic acid, and ethylene in resistant cotton plants, but gene silencing in cotton enhanced their susceptibility to V. dahliae infection. Furthermore, T-Spm exposure effectively inhibited V. dahliae growth in vitro. In summary, GhACL5 expression is related to in planta levels of T-Spm and is involved in stem elongation and defense responses against V. dahliae.

Cotton polyamine oxidase is required for spermine and camalexin signalling in the defence response to Verticillium dahliae

Verticillium dahliae is a destructive, soil-borne fungal pathogen that causes vascular wilt disease in many economically important crops worldwide. A polyamine oxidase (PAO) gene was identified and cloned by screening suppression subtractive hybridisation and cDNA libraries of cotton genotypes tolerant to Verticillium wilt and was induced early and strongly by inoculation with V. dahliae and application of plant hormone. Recombinant cotton polyamine oxidase (GhPAO) was found to catalyse the conversion of spermine (Spm) to spermidine (Spd) in vitro. Constitutive expression of GhPAO in Arabidopsis thaliana produced improved resistance to V. dahliae and maintained putrescine, Spd and Spm at high levels. Hydrogen peroxide (H2 O2 ), salicylic acid and camalexin (a phytoalexin) levels were distinctly increased in GhPAO-overexpressing Arabidopsis plants during V. dahliae infection when compared with wild-type plants, and Spm and camalexin efficiently inhibited growth of V. dahliae in vitro. Spermine promoted the accumulation of camalexin by inducing the expression of mitogen-activated protein kinases and cytochrome P450 proteins in Arabidopsis and cotton plants. The three polyamines all showed higher accumulation in tolerant cotton cultivars than in susceptible cotton cultivars after inoculation with V. dahliae. GhPAO silencing in cotton significantly reduced the Spd level and increased the Spm level, leading to enhanced susceptibility to infection by V. dahliae, and the levels of H2 O2 and camalexin were distinctly lower in GhPAO-silenced cotton plants after V. dahliae infection. Together, these results suggest that GhPAO contributes to resistance of the plant against V. dahliae through the mediation of Spm and camalexin signalling.

Role of Arginine decarboxylase (ADC) in Arabidopsis thaliana defence against the pathogenic bacterium Pseudomonas viridiflava

Polyamine biosynthesis starts with putrescine production through the decarboxylation of arginine or ornithine. In Arabidopsis thaliana, putrescine is synthesised exclusively by arginine decarboxylase (ADC), which exists as two isoforms (ADC1 and 2) that are differentially regulated by abiotic stimuli, but their role in defence against pathogens has not been studied in depth. This work analysed the participation of ADC in Arabidopsis defence against Pseudomonas viridiflava. ADC activity and expression, polyamine levels and bacterial resistance were analysed in null mutants of each ADC isoform. In non-infected wild-type (WT) plants, ADC2 expression was much higher than ADC1. Analysis of adc mutants demonstrated that ADC2 contributes to a much higher extent than ADC1 to basal ADC activity and putrescine biosynthesis. In addition, adc2 mutants showed increased basal expression of salicylic acid- and jasmonic acid-dependent PR genes. Bacterial infection induced putrescine accumulation and ADC1 expression in WT plants, but pathogen-induced putrescine accumulation was blocked in adc1 mutants. Results suggest a specific participation of ADC1 in defence, although basal resistance was not decreased by dysfunction of either of the two ADC genes. In addition, and as opposed to WT plants, bacterial infection increased ADC2 expression and ADC activity in adc1 mutants, which could counterbalance the lack of ADC1. Results demonstrate a major contribution of ADC2 to total ADC activity and the specific induction of ADC1 in response to infection. A certain degree of functional redundancy between the two isoforms in relation to their contribution to basal resistance is also evident.

The roles of polyamines during the lifespan of plants: from development to stress

Compelling evidence indicates that free polyamines (PAs) (mainly putrescine, spermidine, spermine, and its isomer thermospermine), some PA conjugates to hydroxycinnamic acids, and the products of PA oxidation (hydrogen peroxide and γ-aminobutyric acid) are required for different processes in plant development and participate in abiotic and biotic stress responses. A tight regulation of PA homeostasis is required, since depletion or overaccumulation of PAs can be detrimental for cell viability in many organisms. In plants, homeostasis is achieved by modulation of PA biosynthesis, conjugation, catabolism, and transport. However, recent data indicate that such mechanisms are not mere modulators of PA pools but actively participate in PA functions. Examples are found in the spermidine-dependent eiF5A hypusination required for cell division, PA hydroxycinnamic acid conjugates required for pollen development, and the involvement of thermospermine in cell specification. Recent advances also point to implications of PA transport in stress tolerance, PA-dependent transcriptional and translational modulation of genes and transcripts, and posttranslational modifications of proteins. Overall, the molecular mechanisms identified suggest that PAs are intricately coordinated and/or mediate different stress and developmental pathways during the lifespan of plants.

A plant spermine oxidase/dehydrogenase regulated by the proteasome and polyamines

Polyamine oxidases (PAOs) are flavin-dependent enzymes involved in polyamine catabolism. In Arabidopsis five PAO genes (AtPAO1-AtPAO5) have been identified which present some common characteristics, but also important differences in primary structure, substrate specificity, subcellular localization, and tissue-specific expression pattern, differences which may suggest distinct physiological roles. In the present work, AtPAO5, the only so far uncharacterized AtPAO which is specifically expressed in the vascular system, was partially purified from 35S::AtPAO5-6His Arabidopsis transgenic plants and biochemically characterized. Data presented here allow AtPAO5 to be classified as a spermine dehydrogenase. It is also shown that AtPAO5 oxidizes the polyamines spermine, thermospermine, and N(1)-acetylspermine, the latter being the best in vitro substrate of the recombinant enzyme. AtPAO5 also oxidizes these polyamines in vivo, as was evidenced by analysis of polyamine levels in the 35S::AtPAO5-6His Arabidopsis transgenic plants, as well as in a loss-of-function atpao5 mutant. Furthermore, subcellular localization studies indicate that AtPAO5 is a cytosolic protein undergoing proteasomal control. Positive regulation of AtPAO5 expression by polyamines at the transcriptional and post-transcriptional level is also shown. These data provide new insights into the catalytic properties of the PAO gene family and the complex regulatory network controlling polyamine metabolism.

Physiological and molecular implications of plant polyamine metabolism during biotic interactions

During ontogeny, plants interact with a wide variety of microorganisms. The association with mutualistic microbes results in benefits for the plant. By contrast, pathogens may cause a remarkable impairment of plant growth and development. Both types of plant-microbe interactions provoke notable changes in the polyamine (PA) metabolism of the host and/or the microbe, being each interaction a complex and dynamic process. It has been well documented that the levels of free and conjugated PAs undergo profound changes in plant tissues during the interaction with microorganisms. In general, this is correlated with a precise and coordinated regulation of PA biosynthetic and catabolic enzymes. Interestingly, some evidence suggests that the relative importance of these metabolic pathways may depend on the nature of the microorganism, a concept that stems from the fact that these amines mediate the activation of plant defense mechanisms. This effect is mediated mostly through PA oxidation, even though part of the response is activated by non-oxidized PAs. In the last years, a great deal of effort has been devoted to profile plant gene expression following microorganism recognition. In addition, the phenotypes of transgenic and mutant plants in PA metabolism genes have been assessed. In this review, we integrate the current knowledge on this field and analyze the possible roles of these amines during the interaction of plants with microbes.

Thermospermine modulates expression of auxin-related genes in Arabidopsis

Thermospermine, a structural isomer of spermine, is widely distributed in the plant kingdom and has been shown to play a role in repressing xylem differentiation by studies of its deficient mutant, acaulis5 (acl5), in Arabidopsis. Our results of microarray and real-time PCR analyses revealed that, in addition to a number of genes involved in xylem differentiation, genes related to auxin signaling were up-regulated in acl5 seedlings. These genes include MONOPTEROS, an auxin response factor gene, which acts as a master switch for auxin-dependent procambium formation, and its target genes. Their expression was reduced by exogenous treatment with thermospermine or by transgenic induction of the ACL5 gene. We examined the effect of synthetic polyamines on the expression of these auxin-related genes and on the vascular phenotype of acl5, and found that tetramines containing the NC3NC3N chain could mimic the effect of thermospermine but longer polyamines containing the same chain had little or no such effect. We also found that thermospermine had an inhibitory effect on lateral root formation in wild-type seedlings and it was mimicked by synthetic tetramines with the NC3NC3N chain. These results suggest the importance of the NC3NC3N chain of thermospermine in its action in modulating auxin signaling.

New insights into the regulation of plant immunity by amino acid metabolic pathways

Besides defence pathways regulated by classical stress hormones, distinct amino acid metabolic pathways constitute integral parts of the plant immune system. Mutations in several genes involved in Asp-derived amino acid biosynthetic pathways can have profound impact on plant resistance to specific pathogen types. For instance, amino acid imbalances associated with homoserine or threonine accumulation elevate plant immunity to oomycete pathogens but not to pathogenic fungi or bacteria. The catabolism of Lys produces the immune signal pipecolic acid (Pip), a cyclic, non-protein amino acid. Pip amplifies plant defence responses and acts as a critical regulator of plant systemic acquired resistance, defence priming and local resistance to bacterial pathogens. Asp-derived pyridine nucleotides influence both pre- and post-invasion immunity, and the catabolism of branched chain amino acids appears to affect plant resistance to distinct pathogen classes by modulating crosstalk of salicylic acid- and jasmonic acid-regulated defence pathways. It also emerges that, besides polyamine oxidation and NADPH oxidase, Pro metabolism is involved in the oxidative burst and the hypersensitive response associated with avirulent pathogen recognition. Moreover, the acylation of amino acids can control plant resistance to pathogens and pests by the formation of protective plant metabolites or by the modulation of plant hormone activity.

Longer uncommon polyamines have a stronger defense gene-induction activity and a higher suppressing activity of Cucumber mosaic virus multiplication compared to that of spermine in Arabidopsis thaliana

KEY MESSAGE

Our work suggests that long chain polyamines and their derivatives are potential chemicals to control viral pathogens for crop production. Previously we showed that two tetraamines, spermine (Spm) and thermospermine (T-Spm), induce the expression of a subset of defense-related genes and repress proliferation of Cucumber mosaic virus (CMV) in Arabidopsis. Here we tested whether the longer uncommon polyamines (LUPAs) such as caldopentamine, caldohexamine, homocaldopentamine and homocaldohexamine have such the activity. LUPAs had higher gene induction activity than Spm and T-Spm. Interestingly the genes induced by LUPAs could be classified into two groups: the one group was most responsive to caldohexamine while the other one was most responsive to homocaldopentamine. In both the cases, the inducing activity was dose-dependent. LUPAs caused local cell death and repressed CMV multiplication more efficiently as compared to Spm. LUPAs inhibited the viral multiplication of not only avirulent CMV but also of virulent CMV in a dose-dependent manner. Furthermore, LUPAs can activate the systemic acquired resistance against CMV more efficiently as compared to Spm. When Arabidopsis leaves were incubated with LUPAs, the putative polyamine oxidase (PAO)-mediated catabolites were detected even though the conversion rate was very low. In addition, we found that LUPAs induced the expression of three NADPH oxidase genes (rbohC, rbohE and rbohH) among ten isoforms. Taken together, we propose that LUPAs activate two alternative reactive oxygen species evoked pathways, a PAO-mediated one and an NADPH-oxidase-mediated one, which lead to induce defense-related genes and restrict CMV multiplication.

Thermospermine levels are controlled by an auxin-dependent feedback loop mechanism in Populus xylem

Polyamines are small polycationic amines that are widespread in living organisms. Thermospermine, synthesized by thermospermine synthase ACAULIS5 (ACL5), was recently shown to be an endogenous plant polyamine. Thermospermine is critical for proper vascular development and xylem cell specification, but it is not known how thermospermine homeostasis is controlled in the xylem. We present data in the Populus model system supporting the existence of a negative feedback control of thermospermine levels in stem xylem tissues, the main site of thermospermine biosynthesis. While over-expression of the ACL5 homologue in Populus, POPACAULIS5, resulted in strong up-regulation of ACL5 expression and thermospermine accumulation in leaves, the corresponding levels in the secondary xylem tissues of the stem were similar or lower than those in the wild-type. POPACAULIS5 over-expression had a negative effect on accumulation of indole-3-acetic acid, while exogenous auxin had a positive effect on POPACAULIS5 expression, thus promoting thermospermine accumulation. Further, over-expression of POPACAULIS5 negatively affected expression of the class III homeodomain leucine zipper (HD-Zip III) transcription factor gene PttHB8, a homologue of AtHB8, while up-regulation of PttHB8 positively affected POPACAULIS5 expression. These results indicate that excessive accumulation of thermospermine is prevented by a negative feedback control of POPACAULIS5 transcript levels through suppression of indole-3-acetic acid levels, and that PttHB8 is involved in the control of POPACAULIS5 expression. We propose that this negative feedback loop functions to maintain steady-state levels of thermospermine, which is required for proper xylem development, and that it is dependent on the presence of high concentrations of endogenous indole-3-acetic acid, such as those present in the secondary xylem tissues.