Thermospermine is required for stem elongation in Arabidopsis thaliana

Specific enhancement of the translation of thermospermine-responsive uORF-containing mRNAs by ribosomal mutations in Arabidopsis thaliana

Auxin-induced xylem formation in angiosperms is negatively regulated by thermospermine, whose biosynthesis is also induced by auxin. In Arabidopsis thaliana, loss-of-function mutants of ACL5, which encodes thermospermine synthase, exhibit a dwarf phenotype accompanied by excessive xylem formation. Studies of suppressor mutants that recover from the acl5 dwarf phenotype suggest that thermospermine alleviates the inhibitory effect of an upstream open-reading frame (uORF) on the main ORF translation of SAC51 mRNA. Many suppressor mutations for acl5 have been mapped to the uORF conserved in the SAC51 family or to ribosomal protein genes, such as RPL10A, RPL4A, and RACK1A. In this study, we identified newly isolated acl5 suppressors, sac501, sac504, and sac506, which are additional alleles of RPL10A and the uORFs of SAC51 family members, SACL1 and SACL3, respectively. To investigate whether acl5-suppressor alleles of ribosomal genes broadly affect translation of uORF-containing mRNAs, we examined GUS activity in several 5'-GUS fusion constructs. Our results showed that these alleles enhanced GUS activity in SAC51 and SACL3 5'-fusion constructs but had no effect on other 5'-fusion constructs unrelated to thermospermine response. This suggests that these ribosomal proteins are specifically involved in the thermospermine-mediated regulation of mRNA translation.

Marine and terrestrial biostimulant elicitors of tolerance to cold stress

The increasing frequency of adverse environmental events, driven by ongoing climate change, has intensified the search for new technological alternatives in crop production and plant protection. Thermal stress can limit plant adaptation and negatively impact metabolism, physiology, morphology, and yield. Cold stress in plants has been extensively studied and can affect various stages of plant's life cycle, from seed formation to development, causing damage to cell membranes, impairing cell division, and disrupting water absorption. Consequently, researchers have focused on mitigating the impacts of abiotic stress by investigating bioactive molecules and biostimulants derived from various organisms, which enhance tolerance mechanisms in plants. In aquatic environments, macro- and microalgae have emerged as key sources of plant elicitors, providing extractable molecules such as polysaccharides, polyamines, polyphenols, and amino acids that enhance plant defense responses. Similarly, certain terrestrial plants have shown potential as sources of biostimulant compounds. Thus, this study aims to highlight advancements in crop systems by emphasizing the potential of algae-based and terrestrial biostimulant elicitors in enhancing tolerance to cold stress. Ultimately, the goal is to improve understanding of promising biological models for food production, fostering innovative developments that can contribute to economically and ecologically sustainable technologies.

Divergent evolution of a thermospermine-dependent regulatory pathway in land plants

Plants adapted to life on land by developing diverse anatomical features across lineages. The molecular basis of these innovations often involves the emergence of new genes or establishing new connections between conserved elements, though evidence for evolutionary genetic circuit rewiring remains scarce. Here, we show that the thermospermine-dependent pathway regulating vascular cell proliferation in Arabidopsis thaliana operates as two distinct modules with different functions in the bryophyte Marchantia polymorpha. One module controls dichotomous branching at meristems, while the other one modulates gemmae and rhizoid production in the thallus. Heterologous assays and comparative expression analyses reveal that the molecular links between these modules, forming a unified circuit in vascular plants, emerged early in tracheophyte evolution. Our results illustrate how the thermospermine-dependent circuit elements followed two divergent evolutionary trajectories in bryophytes and tracheophytes, eventually influencing distinct developmental processes.

RNA-Seq Transcriptomics and iTRAQ Proteomics Analysis Reveal the Dwarfing Mechanism of Blue Fescue (Festuca glauca)

Blue fescue is a widely used ornamental grass because of its strong ecological adaptability. To maintain the optimal ornamental plant shape, blue fescue requires many nutrients and labor. Using dwarf varieties with slow growth is an effective way to fulfill these requirements. In this study, we investigated the dwarfing mechanism of dw-1, a blue fescue dwarfing mutant, using physiological, transcriptomic, and proteomic methods. The peroxidase (POD) enzyme activity and chlorophyll content of dw-1 significantly increased, while the lignin, gibberellin (GA), and indoleacetic acid (IAA) content significantly decreased. A total of 7668 differentially expressed genes (DEGs) were detected using RNA-seq, of which 2543 were upregulated and 5125 were downregulated. A total of 165 differentially expressed proteins (DEPs) were detected using iTRAQ, of which 68 were upregulated and 97 were downregulated. KEGG enrichment analysis showed that the diterpene biosynthesis pathway, tryptophan metabolism pathway, and phenylpropanoid biosynthesis pathway were significantly enriched at both the transcriptional and protein levels. As a result, we can formulate the following hypothesis about the dw-1 dwarfing phenotype: the downregulation of genes and proteins related to IAA and GA biosynthesis is associated with the dwarf phenotype's formation, and metabolic pathways related to lignin synthesis, such as phenylpropanoid biosynthesis, also play an important role. Our work will contribute to a new understanding of the genes and proteins involved in the blue fescue dwarf phenotype.

Polyamines: pleiotropic molecules regulating plant development and enhancing crop yield and quality

Polyamines (PAs) are pleiotropic bioorganic molecules. Cellular PA contents are determined by a balance between PA synthesis and degradation. PAs have been extensively demonstrated to play vital roles in the modulation of plant developmental processes and adaptation to various environmental stresses. In this review, the latest advances on the diverse roles of PAs in a range of developmental processes, such as morphogenesis, organogenesis, growth and development, and fruit ripening, are summarized and discussed. Besides, the crosstalk between PAs and phytohormones or other signalling molecules, including H2O2 and NO, involved in these processes is dwelled on. In addition, the attempts made to improve the yield and quality of grain and vegetable crops through altering the PA catabolism are enumerated. Finally, several other vital questions that remain unanswered are proposed and discussed. These include the mechanisms underlying the cooperative regulation of developmental processes by PAs and their interplaying partners like phytohormones, H2O2 and NO; PA transport for maintaining homeostasis; and utilization of PA anabolism/catabolism for generating high-yield and good-quality crops. This review aims to gain new insights into the pleiotropic role of PAs in the modulation of plant growth and development, which provides an alternative approach for manipulating and engineering valuable crop varieties that can be used in the future.

Spermine and spermidine inhibit or induce programmed cell death in Arabidopsis thaliana in vitro and in vivo in a dose-dependent manner

Polyamines are ubiquitous biomolecules with a number of established functions in eukaryotic cells. In plant cells, polyamines have previously been linked to abiotic and biotic stress tolerance, as well as to the modulation of programmed cell death (PCD), with contrasting reports on their pro-PCD and pro-survival effects. Here, we used two well-established platforms for the study of plant PCD, Arabidopsis thaliana suspension cultures cells and the root hair assay, to examine the roles of the polyamines spermine and spermidine in the regulation of PCD. Using these systems for precise quantification of cell death rates, we demonstrate that both polyamines can trigger PCD when applied exogenously at higher doses, whereas at lower concentrations they inhibit PCD induced by both biotic and abiotic stimuli. Furthermore, we show that concentrations of polyamines resulting in inhibition of PCD generated a transient ROS burst in our experimental system, and activated the expression of oxidative stress- and pathogen response-associated genes. Finally, we examined PCD responses in existing Arabidopsis polyamine synthesis mutants, and identified a subtle PCD phenotype in Arabidopsis seedlings deficient in thermo-spermine. The presented data show that polyamines can have a role in PCD regulation; however, that role is dose-dependent and consequently they may act as either inhibitors, or inducers, of PCD in Arabidopsis.

Polyamines: Their Role in Plant Development and Stress

This review focuses on the intricate relationship between plant polyamines and the genetic circuits and signaling pathways that regulate various developmental programs and the defense responses of plants when faced with biotic and abiotic aggressions. Particular emphasis is placed on genetic evidence supporting the involvement of polyamines in specific processes, such as the pivotal role of thermospermine in regulating xylem cell differentiation and the significant contribution of polyamine metabolism in enhancing plant resilience to drought. Based on the numerous studies describing effects of the manipulation of plant polyamine levels, two conceptually different mechanisms for polyamine activity are discussed: direct participation of polyamines in translational regulation and the indirect production of hydrogen peroxide as a defensive mechanism against pathogens. By describing the multifaceted functions of polyamines, this review underscores the profound significance of these compounds in enabling plants to adapt and thrive in challenging environments.

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.

ACL5 acquired strict thermospermine synthesis activity during the emergence of vascular plants

Norspermine (Nspm), one of the uncommon polyamines (PAs), was detected in bryophytes and lycophytes; therefore, the aminopropyltransferases involved in the synthesis of Nspm were investigated. The enzymatic activity was evaluated by the transient high expression of various aminopropyltransferase genes in Nicotiana benthamiana, followed by quantification of PA distribution in the leaves using gas chromatography-mass spectrometry. The bryophyte orthologues of ACL5, which is known to synthesise thermospermine (Tspm) in flowering plants, were found to have strong Nspm synthesis activity. In addition, two ACL5 orthologous with different substrate specificities were conserved in Selaginella moellendorffii, one of which was involved in Tspm synthesis and the other in Nspm synthesis. Therefore, further detailed analysis using these two factors revealed that the β-hairpin structural region consisting of β-strands 1 and 2 at the N-terminus of ACL5 is involved in substrate specificity. Through functional analysis of a total of 40 ACL5 genes in 33 organisms, including algae, it was shown that ACL5 has changed its substrate specificity several times during plant evolution and diversification. Furthermore, it was strongly suggested that ACL5 acquired strict Tspm synthesis activity during the emergence of vascular plants, especially through major changes around the β-hairpin structural region.

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.

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.

Quantification of Xylem-Specific Thermospermine-Dependent Translation of SACL Transcripts with Dual Luciferase Reporter System

Thermospermine (Tspm) is a polyamine found to play a crucial role in xylem development in Arabidopsis thaliana. Tspm promotes the translation of the SACL genes by counteracting the activity of a cis element in their 5'-leader region that suppresses the translation of the main ORF. Here we describe a method to test the Tspm-dependent translational regulation of the 5'-leader of the SACL mRNAs in Nicotiana benthamiana leaves and A. thaliana mesophyll protoplasts with a dual luciferase assay. The dual luciferase reporter system is used to assess gene expression and is based on the detection of the Firefly luciferase luminescence driven by a specific promoter. However, it can also be used to evaluate the cis elements found in 5'-leader that influence the translation of the main ORF in a transcript. We have used a modified version of the pGreenII 0800 LUC plasmid carrying a double 35S promoter, followed by a poly-linker sequence in phase with the Firefly luciferase gene (pGreen2x35SLUC) where the full 5'-leader sequence of SACL3 was cloned. This construct was used for Agrobacterium tumefaciens infiltration of N. benthamiana leaves and for transfection of A. thaliana mesophyll protoplasts, followed by mock or Tspm treatments. The resulting translation of the Firefly luciferase in these organisms and conditions was then tested by measuring luminescence with the dual luciferase assay and a luminometer. These experiments have allowed us to quantify the positive effect of Tspm in the translation of SACL3 transcripts.

The Roles of Functional Amino Acids in Plant Growth and Development

Plants incorporate acquired carbon and nitrogen into amino acid metabolism, whereby the building blocks of proteins and the precursors of various metabolites are produced. This fundamental demand requires tight amino acid metabolism to sustain physiological homeostasis. There is increasing evidence that amino acid metabolism undergoes plastic alteration to orchestrate specific growth and developmental events. Consequently, there has been a gradual exploration of the interface at which amino acid metabolism and plant morphogenesis are mutually affected. This research progress offers an opportunity to explore amino acid metabolism, with the goal to understand how it can be modulated to serve special cellular needs and regulate specific growth and developmental pathways. Continuous improvements in the sensitivity and coverage of metabolomics technology, along with the development of chemoinformatics, have allowed the investigation of these research questions. In this review, we summarize the roles of threonine, serine, arginine and γ-aminobutyric acid as representative examples of amino acids relevant to specific developmental processes in plants ('functional amino acids'). Our objective is to expand perspectives regarding amino acid metabolism beyond the conventional view that it is merely life-supporting machinery.

Flexure wood formation via growth reprogramming in hybrid aspen involves jasmonates and polyamines and transcriptional changes resembling tension wood development

Stem bending in trees induces flexure wood but its properties and development are poorly understood. Here, we investigated the effects of low-intensity multidirectional stem flexing on growth and wood properties of hybrid aspen, and on its transcriptomic and hormonal responses. Glasshouse-grown trees were either kept stationary or subjected to several daily shakes for 5 wk, after which the transcriptomes and hormones were analyzed in the cambial region and developing wood tissues, and the wood properties were analyzed by physical, chemical and microscopy techniques. Shaking increased primary and secondary growth and altered wood differentiation by stimulating gelatinous-fiber formation, reducing secondary wall thickness, changing matrix polysaccharides and increasing cellulose, G- and H-lignin contents, cell wall porosity and saccharification yields. Wood-forming tissues exhibited elevated jasmonate, polyamine, ethylene and brassinosteroids and reduced abscisic acid and gibberellin signaling. Transcriptional responses resembled those during tension wood formation but not opposite wood formation and revealed several thigmomorphogenesis-related genes as well as novel gene networks including FLA and XTH genes encoding plasma membrane-bound proteins. Low-intensity stem flexing stimulates growth and induces wood having improved biorefinery properties through molecular and hormonal pathways similar to thigmomorphogenesis in herbaceous plants and largely overlapping with the tension wood program of hardwoods.

Suppression of the dwarf phenotype of an Arabidopsis mutant defective in thermospermine biosynthesis by a synonymous codon change in the SAC51 uORF

Thermospermine plays a critical role in negatively regulating xylem development in angiosperms. A mutant of Arabidopsis thaliana that is defective in thermospermine biosynthesis, acaulis5 (acl5), exhibits a dwarf phenotype with excessive xylem formation. Mechanistically thermospermine acts in attenuating the inhibitory effect of an evolutionarily conserved upstream open reading frame (uORF) on the main ORF of SAC51, which encodes a basic helix-loop-helix protein involved in xylem repression. Here, we revealed that a semidominant suppressor of acl5, sac503, which partially restores the acl5 phenotype, has a point mutation in the conserved uORF of SAC51 with no amino acid substitution in the deduced peptide sequence. In transgenic lines carrying the β-glucuronidase (GUS) reporter gene fused with the SAC51 5' region containing the uORF, the mutant construct was shown to confer higher GUS activity than does the wild-type SAC51 construct. We confirmed that sac503 mRNA was more stable than SAC51 mRNA in acl5. These results suggest that the single-base change in sac503 positively affects the translation of its main ORF instead of thermospermine. We further found that the uORF-GUS fusion protein could be synthesized in planta from the wild-type and sac503 translational fusion constructs.

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.

SnoRNP is essential for thermospermine-mediated development in Arabidopsis thaliana

Polyamines have been discovered for hundreds of years and once considered as a class of phytohormones. Polyamines play critical roles in a range of developmental processes. However, the molecular mechanisms of polyamine signaling pathways remain poorly understood. Here, we measured the contents of main types of polyamines, and found that endogenous level of thermospermine (T-Spm) in Arabidopsis thaliana is comparable to those of classic phytohormones and is significantly lower than those of putrescine (Put), spermidine (Spd), and spermine (Spm). We further found a nodule-like structure around the junction area connecting the shoot and root of the T-Spm biosynthetic mutant acl5 and obtained more than 50 suppressors of acl5nodule structure (san) through suppressor screening. An in-depth study of two san suppressors revealed that NAP57 and NOP56, core components of box H/ACA and C/D snoRNPs, were essential for T-Spm-mediated nodule-like structure formation and plant height. Furthermore, analyses of rRNA modifications showed that the overall levels of pseudouridylation and 2'-O-methylation were compromised in san1 and san2 respectively. Taken together, these results establish a strong genetic relationship between rRNA modification and T-Spm-mediated growth and development, which was previously undiscovered in all organisms.

Loss of function of an Arabidopsis homologue of JMJD6 suppresses the dwarf phenotype of acl5, a mutant defective in thermospermine biosynthesis

In Arabidopsis thaliana, the ACL5 gene encodes thermospermine synthase and its mutant, acl5, exhibits a dwarf phenotype with excessive xylem formation. Studies of suppressor mutants of acl5 reveal the involvement of thermospermine in enhancing mRNA translation of the SAC51 gene family. We show here that a mutant, sac59, which partially suppresses the acl5 phenotype, has a point mutation in JMJ22 encoding a D6-class Jumonji C protein (JMJD6). A T-DNA insertion allele, jmj22-2, also partially suppressed the acl5 phenotype while mutants of its closest two homologs JMJ21 and JMJ20 had no such effects, suggesting a unique role for JMJ22 in plant development. We found that mRNAs of the SAC51 family are more stabilized in acl5 jmj22-2 than in acl5.

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.

Transcriptome analysis provides insights into the molecular mechanism of GhSAMDC1 involving in rapid vegetative growth and early flowering in tobacco

In previous study, ectopic expression of GhSAMDC₁ improved vegetative growth and early flowering in tobacco, which had been explained through changes of polyamine content, polyamines and flowering relate genes expression. To further disclose the transcript changes of ectopic expression of GhSAMDC₁ in tobacco, the leaves from wild type and two transgenic lines at seedling (30 days old), bolting (60 days old) and flowering (90 days old) stages were performed for transcriptome analysis. Compared to wild type, a total of 938 differentially expressed genes (DEGs) were found to be up- or down-regulated in the two transgenic plants. GO and KEGG analysis revealed that tobacco of wild-type and transgenic lines were controlled by a complex gene network, which regulated multiple metabolic pathways. Phytohormone detection indicate GhSAMDC₁ affect endogenous phytohormone content, ABA and JA content are remarkably increased in transgenic plants. Furthermore, transcript factor analysis indicated 18 transcript factor families, including stress response, development and flowering related transcript factor families, especially AP2-EREBP, WRKY, HSF and Tify are the most over-represented in those transcript factor families. In conclusion, transcriptome analysis provides insights into the molecular mechanism of GhSAMDC₁ involving rapid vegetative growth and early flowering in tobacco.

Cambial meristematic cell culture: a sustainable technology toward in vitro specialized metabolites production

Cambial meristematic cells (CMCs) culture has received a fair share of scientific and industrial attention among the trending topics of plant cell culture, especially their potential toward secondary metabolites production. However, the conventional plant cell culture is often not commercially feasible because of difficulties associated with culture dedifferentiated cells. Several reports have been published to culture CMCs and bypass the dedifferentiation process in plant cell culture. Numerous mitochondria, multiple vacuoles, genetic stability, self-renewal, higher biomass, and stable metabolites accumulation are the characteristics features of CMCs compared with dedifferentiated cells (DDCs) culture. The CMCs culture has a broader application to produce large-scale natural compounds for: pharmaceuticals, food, and cosmetic industries. Cutting-edge progress in plant cellular and molecular biology has allowed unprecedented insights into cambial stem cell culture and its fundamental processes. Therefore, regarding sustainability and natural compound production, cambial cell culture ranks among the most vital biotechnological interventions for industrial and economic perspectives. This review highlights the recent advances in plant stem cell culture and understands the cambial cells induction and culture mechanisms that affect the growth and natural compounds production.

The kinetochore protein NNF1 has a moonlighting role in the vegetative development of Arabidopsis thaliana

The kinetochore is a supramolecular protein complex assembled on the chromosomes, essential for faithful segregation of the genome during cell divisions. More than 100 proteins are known to constitute the eukaryotic kinetochore architecture, primarily identified using non-plant organisms. A majority of them are fast evolving and are under positive selection. Thus, functional characterization of the plant kinetochore proteins is limited as only a few conserved orthologs sharing sequence similarity with their animal counterparts have been examined. Here, we report the functional characterization of the Arabidopsis thaliana homolog of the yeast NNF1/human PMF1 outer kinetochore protein and show that it has both kinetochore and non-kinetochore functions in plant growth and development. Knockout of NNF1 causes embryo lethality implying its essential role in cell division. AtNNF1 interacts with MIS12 in Y2H and co-immunoprecipitation assays, confirming it is one of the constituents of the plant MIS12 complex. GFP-NNF1 localizes to the kinetochore, rescuing the embryo lethal nnf1-1-/- phenotype, but the rescued plants (GFP-NNF1^nnf1-/- ) are dwarf, displaying hypomorphic phenotypes with no evidence of mitotic or meiotic segregation defects. GFP-NNF1^nnf1-/- dwarf plants have reduced levels of endogenous polyamines, which are partially rescued to wild-type levels upon exogenous application of polyamines. Mutations in the putative leucine zipper-like binding motif of NNF1 gave rise to a dominant-negative tall plant phenotype reminiscent of constitutive gibberellic acid (GA) action. These contrasting hypomorphic dwarf and antimorphic tall phenotypes facilitated us to attribute a moonlighting role to Arabidopsis NNF1 affecting polyamine and GA metabolism apart from its primary role in kinetochores.

Function of hydroxycinnamoyl spermidines in seedling growth of Arabidopsis

Hydroxycinnamic acid amides are involved in various developmental processes as well as in biotic and abiotic stress responses. Among them, the presence of spermidine derivatives, such as N1,N8-di(coumaroyl)-spermidine and N1,N8-di(sinapoyl)-spermidine, and their biosynthetic genes have been reported in Arabidopsis, but their functions in plants are still unknown. We chemically synthesized the above-mentioned spermidine derivatives to assess their physiological functions in Arabidopsis. We evaluated the growth and development of chemically treated Arabidopsis and demonstrated that these compounds inhibited seed germination, hypocotyl elongation, and primary root growth, which could be due to modulation of plant hormone homeostasis and signaling. The results suggest that these compounds are regulatory metabolites that modulate plant growth and development.

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.

Cadaverine regulates biotin synthesis to modulate primary root growth in Arabidopsis

Cadaverine, a polyamine, has been linked to modification of root growth architecture and response to environmental stresses in plants. However, the molecular mechanisms that govern the regulation of root growth by cadaverine are largely unexplored. Here we conducted a forward genetic screen and isolated a mutation, cadaverine hypersensitive 3 (cdh3), which resulted in increased root-growth sensitivity to cadaverine, but not other polyamines. This mutation affects the BIO3-BIO1 biotin biosynthesis gene. Exogenous supply of biotin and a pathway intermediate downstream of BIO1, 7,8-diaminopelargonic acid, suppressed this cadaverine sensitivity phenotype. An in vitro enzyme assay showed cadaverine inhibits the BIO3-BIO1 activity. Furthermore, cadaverine-treated seedlings displayed reduced biotinylation of Biotin Carboxyl Carrier Protein 1 of the acetyl-coenzyme A carboxylase complex involved in de novo fatty acid biosynthesis, resulting in decreased accumulation of triacylglycerides. Taken together, these results revealed an unexpected role of cadaverine in the regulation of biotin biosynthesis, which leads to modulation of primary root growth of plants.

Metabolic Control of Gametophore Shoot Formation through Arginine in the Moss Physcomitrium patens

Shoot formation is accompanied by active cell proliferation and expansion, requiring that metabolic state adapts to developmental control. Despite the importance of such metabolic reprogramming, it remains unclear how development and metabolism are integrated. Here, we show that disruption of ANGUSTIFOLIA3 orthologs (PpAN3s) compromises gametophore shoot formation in the moss Physcomitrium patens due to defective cell proliferation and expansion. Trans-omics analysis reveals that the downstream activity of PpAN3 is linked to arginine metabolism. Elevating arginine level by chemical treatment leads to stunted gametophores and causes Ppan3 mutant-like transcriptional changes in the wild-type plant. Furthermore, ectopic expression of AtAN3 from Arabidopsis thaliana ameliorates the defective arginine metabolism and promotes gametophore formation in Ppan3 mutants. Together, these findings indicate that arginine metabolism is a key pathway associated with gametophore formation and provide evolutionary insights into the establishment of the shoot system in land plants through the integration of developmental and metabolic processes.

Putrescine Depletion Affects Arabidopsis Root Meristem Size by Modulating Auxin and Cytokinin Signaling and ROS Accumulation

Polyamines (PAs) dramatically affect root architecture and development, mainly by unknown mechanisms; however, accumulating evidence points to hormone signaling and reactive oxygen species (ROS) as candidate mechanisms. To test this hypothesis, PA levels were modified by progressively reducing ADC1/2 activity and Put levels, and then changes in root meristematic zone (MZ) size, ROS, and auxin and cytokinin (CK) signaling were investigated. Decreasing putrescine resulted in an interesting inverted-U-trend in primary root growth and a similar trend in MZ size, and differential changes in putrescine (Put), spermidine (Spd), and combined spermine (Spm) plus thermospermine (Tspm) levels. At low Put concentrations, ROS accumulation increased coincidently with decreasing MZ size, and treatment with ROS scavenger KI partially rescued this phenotype. Analysis of double AtrbohD/F loss-of-function mutants indicated that NADPH oxidases were not involved in H2O2 accumulation and that elevated ROS levels were due to changes in PA back-conversion, terminal catabolism, PA ROS scavenging, or another pathway. Decreasing Put resulted in a non-linear trend in auxin signaling, whereas CK signaling decreased, re-balancing auxin and CK signaling. Different levels of Put modulated the expression of PIN1 and PIN2 auxin transporters, indicating changes to auxin distribution. These data strongly suggest that PAs modulate MZ size through both hormone signaling and ROS accumulation in Arabidopsis.

Effects of Structural Isomers of Spermine on the Higher-Order Structure of DNA and Gene Expression

Polyamines are involved in various biological functions, including cell proliferation, differentiation, gene regulation, etc. Recently, it was found that polyamines exhibit biphasic effects on gene expression: promotion and inhibition at low and high concentrations, respectively. Here, we compared the effects of three naturally occurring tetravalent polyamines, spermine (SPM), thermospermine (TSPM), and N⁴-aminopropylspermidine (BSPD). Based on the single DNA observation with fluorescence microscopy together with measurements by atomic force microscopy revealed that these polyamines induce shrinkage and then compaction of DNA molecules, at low and high concentrations, respectively. We also performed the observation to evaluate the effects of these polyamine isomers on the activity of gene expression by adapting a cell-free luciferase assay. Interestingly, the potency of their effects on the DNA conformation and also on the inhibition of gene expression activity indicates the highest for TSPM among spermine isomers. A numerical evaluation of the strength of the interaction of these polyamines with negatively charged double-strand DNA revealed that this ordering of the potency corresponds to the order of the strength of the attractive interaction between phosphate groups of DNA and positively charged amino groups of the polyamines.

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.

Polyamine Metabolism Is Involved in the Direct Regeneration of Shoots from Arabidopsis Lateral Root Primordia

Plants can be regenerated from various explants/tissues via de novo shoot meristem formation. Most of these regeneration pathways are indirect and involve callus formation. Besides plant hormones, the role of polyamines (PAs) has been implicated in these processes. Interestingly, the lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems by exogenous cytokinin application. In this system, no callus formation takes place. We report that the level of PAs, especially that of spermidine (Spd), increased during meristem conversion and the application of exogenous Spd improved its efficiency. The high endogenous Spd level could be due to enhanced synthesis as indicated by the augmented relative expression of PA synthesis genes (AtADC1,2, AtSAMDC2,4, AtSPDS1,2) during the process. However, the effect of PAs on shoot meristem formation might also be dependent on their catabolism. The expression of Arabidopsis POLYAMINE OXIDASE 5 (AtPAO5) was shown to be specifically high during the process and its ectopic overexpression increased the LRP-to-shoot conversion efficiency. This was correlated with Spd accumulation in the roots and ROS accumulation in the converting LRPs. The potential ways how PAO5 may influence direct shoot organogenesis from Arabidopsis LRPs are discussed.

Transcriptional networks regulating root vascular development

Vascular development involves multiple processes, including the establishment of vascular stem cells (e.g. procambium/cambium cells), stem cell divisions, and cell specification. A number of key transcription factors regulating vascular development have been identified, and the molecular mechanisms underlying these regulators have been keenly investigated. These studies uncovered that transcriptional regulation and phytohormone signaling have central roles in proceeding vascular developmental processes. Recent research approaches contributed to identify key transcription factors and their downstream genes, which enhanced our understanding of vascular development. This review discusses some research approaches and emerging molecular mechanisms that mediate the activation of transcriptional networks regulating root vascular development.

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.

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.

Polyamines alleviate the inhibitory effect of the DNA cross-linking agent mitomycin C on root growth

Polyamines (putrescine, spermidine and spermine) are ubiquitously present in various types of cells of living organisms. They are involved in a variety of cellular processes, including cell proliferation and cell differentiation, and are required for abiotic stress tolerances in plants. However, it is still not understood whether polyamines are involved in the plant growth inhibition caused by DNA-damaging agents. In this study, we examined the effects of polyamines on the inhibition of plant root growth and gene expression in Arabidopsis thaliana treated with mitomycin C (MMC), a genotoxic agent that induces DNA interstrand crosslinks. We found that polyamines alleviated the inhibitory effect caused by MMC on root growth. In addition, we also found that polyamines alleviated the increased expression of AtBRCA1 and AtRAD51 genes induced by MMC treatment. Our study provides the first evidence that polyamines contribute to tolerance against plant-growth inhibition caused by a DNA-damaging chemical.

Effect of Thermospermine on the Growth and Expression of Polyamine-Related Genes in Rice Seedlings

A mutant defective in the biosynthesis of thermospermine, acaulis5 (acl5), shows a dwarf phenotype with excess xylem vessels in Arabidopsis thaliana. Exogenous supply of thermospermine remarkably represses xylem differentiation in the root of seedlings, indicating the role of thermospermine in proper repression of xylem differentiation. However, the effect of thermospermine has rarely been investigated in other plant species. In this paper, we examined its effect on the growth and gene expression in rice seedlings. When grown with thermospermine, rice seedlings had no clearly enlarged metaxylem vessels in the root. Expression of OsACL5 was reduced in response to thermospermine, suggesting a negative feedback control of thermospermine biosynthesis like in Arabidopsis. Unlike Arabidopsis, however, rice showed up-regulation of phloem-expressed genes, OsHB5 and OsYSL16, by one-day treatment with thermospermine. Furthermore, expression of OsPAO2 and OsPAO6, encoding extracellular polyamine oxidase whose orthologs are not present in Arabidopsis, was induced by both thermospermine and spermine. These results suggest that thermospermine affects the expression of a subset of genes in rice different from those affected in Arabidopsis.

Cell wall composition and transcriptomics in stem tissues of stinging nettle (Urtica dioica L.): Spotlight on a neglected fibre crop

Stinging nettle (Urtica dioica L.) produces silky cellulosic fibres, as well as bioactive molecules. To improve the knowledge on nettle and enhance its opportunities of exploitation, a draft transcriptome of the "clone 13" (a fibre clone) is here presented. The transcriptome of whole internodes sampled at the top and middle of the stem is then compared with the core and cortical tissues sampled at the bottom. Young internodes show an enrichment in genes involved in the biosynthesis of phytohormones (auxins and jasmonic acid) and secondary metabolites (flavonoids). The core of internodes collected at the bottom of the stem is enriched in genes partaking in different aspects of secondary cell wall formation (cellulose, hemicellulose, lignin biosynthesis), while the cortical tissues reveal the presence of a C starvation signal probably due to the UDP-glucose demand necessary for the thickening phase of bast fibres. Cell wall analysis indicates a difference in rhamnogalacturonan structure/composition of mature bast fibres, as evidenced by the higher levels of galactose measured, as well as the occurrence of more water-soluble pectins in elongating internodes. The targeted quantification of phenolics shows that the middle internode and the cortical tissues at the bottom have higher contents than top internodes. Ultrastructural analyses reveal the presence of a gelatinous layer in bast fibres with a lamellar structure. The data presented will be an important resource and reference for future molecular studies on a neglected fibre crop.

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.

Polyamines as Quality Control Metabolites Operating at the Post-Transcriptional Level

Plant polyamines (PAs) have been assigned a large number of physiological functions with unknown molecular mechanisms in many cases. Among the most abundant and studied polyamines, two of them, namely spermidine (Spd) and thermospermine (Tspm), share some molecular functions related to quality control pathways for tightly regulated mRNAs at the level of translation. In this review, we focus on the roles of Tspm and Spd to facilitate the translation of mRNAs containing upstream ORFs (uORFs), premature stop codons, and ribosome stalling sequences that may block translation, thus preventing their degradation by quality control mechanisms such as the nonsense-mediated decay pathway and possible interactions with other mRNA quality surveillance pathways.

A Polyamine Oxidase from Selaginella lepidophylla (SelPAO5) can Replace AtPAO5 in Arabidopsis through Converting Thermospermine to Norspermidine instead to Spermidine

Of the five polyamine oxidases in Arabidopsis thaliana, AtPAO5 has a substrate preference for the tetraamine thermospermine (T-Spm) which is converted to triamine spermidine (Spd) in a back-conversion reaction in vitro. A homologue of AtPAO5 from the lycophyte Selaginella lepidophylla (SelPAO5) back-converts T-Spm to the uncommon polyamine norspermidine (NorSpd) instead of Spd. An Atpao5 loss-of-function mutant shows a strong reduced growth phenotype when growing on a T-Spm containing medium. When SelPAO5 was expressed in the Atpao5 mutant, T-Spm level decreased to almost normal values of wild type plants, and NorSpd was produced. Furthermore the reduced growth phenotype was cured by the expression of SelPAO5. Thus, a NorSpd synthesis pathway by PAO reaction and T-Spm as substrate was demonstrated in planta and the assumption that a balanced T-Spm homeostasis is needed for normal growth was strengthened.

How plants grow up

A plant's lateral structures, such as leaves, branches and flowers, literally hinge on the shoot axis, making its integrity and growth fundamental to plant form. In all plants, subapical proliferation within the shoot tip displaces cells downward to extrude the cylindrical stem. Following the transition to flowering, many plants show extensive axial elongation associated with increased subapical proliferation and expansion. However, the cereal grasses also elongate their stems, called culms, due to activity within detached intercalary meristems which displaces cells upward, elevating the grain-bearing inflorescence. Variation in culm length within species is especially relevant to cereal crops, as demonstrated by the high-yielding semi-dwarfed cereals of the Green Revolution. Although previously understudied, recent renewed interest the regulation of subapical and intercalary growth suggests that control of cell division planes, boundary formation and temporal dynamics of differentiation, are likely critical mechanisms coordinating axial growth and development in plants.

Extracellular Spermine Triggers a Rapid Intracellular Phosphatidic Acid Response in Arabidopsis, Involving PLDδ Activation and Stimulating Ion Flux

Polyamines, such as putrescine (Put), spermidine (Spd), and spermine (Spm), are low-molecular-weight polycationic molecules found in all living organisms. Despite the fact that they have been implicated in various important developmental and adaptative processes, their mode of action is still largely unclear. Here, we report that Put, Spd, and Spm trigger a rapid increase in the signaling lipid, phosphatidic acid (PA) in Arabidopsis seedlings but also mature leaves. Using time-course and dose-response experiments, Spm was found to be the most effective; promoting PA responses at physiological (low μM) concentrations. In seedlings, the increase of PA occurred mainly in the root and partly involved the plasma membrane polyamine-uptake transporter (PUT), RMV1. Using a differential ³²P_i-labeling strategy combined with transphosphatidylation assays and T-DNA insertion mutants, we found that phospholipase D (PLD), and in particular PLDδ was the main contributor of the increase in PA. Measuring non-invasive ion fluxes (MIFE) across the root plasma membrane of wild type and pldδ-mutant seedlings, revealed that the formation of PA is linked to a gradual- and transient efflux of K⁺. Potential mechanisms of how PLDδ and the increase of PA are involved in polyamine function is discussed.

Complexity and Conservation of Thermospermine-Responsive uORFs of SAC51 Family Genes in Angiosperms

ACAULIS5 (ACL5) encodes thermospermine synthase in Arabidopsis and its loss-of-function mutant acl5 shows excess xylem differentiation and severe dwarfism. SAC51 encodes a basic helix-loop-helix (bHLH) protein and was identified from sac51-d, a dominant suppressor mutant of acl5, which restores the wild-type phenotype without thermospermine. The 5' leader of the SAC51 mRNA contains multiple upstream open-reading frames (uORFs) and sac51-d has a premature stop codon in the fourth uORF. This uORF is conserved among SAC51 family genes in vascular plants. According to the GUS reporter assay, the SAC51 promoter was not responsive to thermospermine but the SAC51 5' leader fused to the constitutive 35S promoter enhanced the GUS activity in response to thermospermine. Disruption experiments of each start codon of the SAC51 uORFs revealed that uORF4 and uORF6 whose start codon corresponds to the second methionine codon of uORF4 had an inhibitory effect on the main ORF translation while the other four uORFs rather had a stimulatory effect. The response of the 5' leader to thermospermine was retained after disruption of each one of six start codons of these uORFs but abolished by mutating both uORF4 and uORF6 start codons, suggesting the importance of the C-terminal sequence shared by these uORFs in the action of thermospermine. We introduced GUS fusions with 5' leaders of SAC51 family genes from other angiosperm species into Arabidopsis and found that all 5' leaders responsive to thermospermine, so far examined, contained these two conserved, and overlapping uORFs.

Polyamines in Microalgae: Something Borrowed, Something New

Microalgae of different evolutionary origins are typically found in rivers, lakes, and oceans, providing more than 45% of global primary production. They provide not only a food source for animals, but also affect microbial ecosystems through symbioses with microorganisms or secretion of some metabolites. Derived from amino acids, polyamines are present in almost all types of organisms, where they play important roles in maintaining physiological functions or against stress. Microalgae can produce a variety of distinct polyamines, and the polyamine content is important to meet the physiological needs of microalgae and may also affect other species in the environment. In addition, some polyamines produced by microalgae have medical or nanotechnological applications. Previous studies on several types of microalgae have indicated that the putative polyamine metabolic pathways may be as complicated as the genomes of these organisms, which contain genes originating from plants, animals, and even bacteria. There are also several novel polyamine synthetic routes in microalgae. Understanding the nature of polyamines in microalgae will not only improve our knowledge of microalgal physiology and ecological function, but also provide valuable information for biotechnological applications.

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

Polyamines (PAs) are implicated in developmental processes and stress responses of plants. Polyamine oxidases (PAOs), flavin adenine dinucleotide-dependent enzymes that function in PA catabolism, play a critical role. Even though PAO gene families of Arabidopsis and rice have been intensely characterized and their expression in response to developmental and environmental changes has been investigated, little is known about PAOs in tomato (Solanum lycopersicum). We found seven PAO genes in S. lycopersicum and named them SlPAO1∼7. Plant PAOs form four clades in phylogenetic analysis, of which SlPAO1 belongs to clade-I, SlPAO6 and SlPAO7 to clade-III, and the residual four (SlPAO2∼5) to clade-IV, while none belongs to clade-II. All the clade-IV members in tomato also retain the putative peroxisomal-targeting signals in their carboxy termini, suggesting their peroxisome localization. SlPAO1 to SlPAO5 genes consist of 10 exons and 9 introns, while SlPAO6 and SlPAO7 are intronless genes. To address the individual roles of SlPAOs, we analyzed their expression in various tissues and during flowering and fruit development. The expression of SlPAO2∼4 was constitutively high, while that of the other SlPAO members was relatively lower. We further analyzed the expressional changes of SlPAOs upon abiotic stresses, oxidative stresses, phytohormone application, and PA application. Based on the data obtained, we discuss the distinctive roles of SlPAOs.

Detection of Thermospermine and Spermine by HPLC in Plants

Thermospermine, a structural isomer of spermine, is widely spread in the plant kingdom and has recently been shown to play a key role in the repression of xylem differentiation in vascular plants. However, a standard high-performance liquid chromatography (HPLC) protocol for detecting polyamines as their dansyl derivative cannot distinguish themospermine from spermine. These isomers become separated from each other after benzoylation. In this chapter, we describe a simple protocol for extraction, benzoylation, and HPLC detection of thermospermine and spermine with other polyamines from plant material.

Scots pine aminopropyltransferases shed new light on evolution of the polyamine biosynthesis pathway in seed plants

Background and Aims

Polyamines are small metabolites present in all living cells and play fundamental roles in numerous physiological events in plants. The aminopropyltransferases (APTs), spermidine synthase (SPDS), spermine synthase (SPMS) and thermospermine synthase (ACL5), are essential enzymes in the polyamine biosynthesis pathway. In angiosperms, SPMS has evolved from SPDS via gene duplication, whereas in gymnosperms APTs are mostly unexplored and no SPMS gene has been reported. The present study aimed to investigate the functional properties of the SPDS and ACL5 proteins of Scots pine (Pinus sylvestris L.) in order to elucidate the role and evolution of APTs in higher plants.

Methods

Germinating Scots pine seeds and seedlings were analysed for polyamines by high-performance liquid chromatography (HPLC) and the expression of PsSPDS and PsACL5 genes by in situ hybridization. Recombinant proteins of PsSPDS and PsACL5 were produced and investigated for functional properties. Also gene structures, promoter regions and phylogenetic relationships of PsSPDS and PsACL5 genes were analysed.

Key Results

Scots pine tissues were found to contain spermidine, spermine and thermospermine. PsSPDS enzyme catalysed synthesis of both spermidine and spermine. PsACL5 was found to produce thermospermine, and PsACL5 gene expression was localized in the developing procambium in embryos and tracheary elements in seedlings.

Conclusions

Contrary to previous views, our results demonstrate that SPMS activity is not a novel feature developed solely in the angiosperm lineage of seed plants but also exists as a secondary property in the Scots pine SPDS enzyme. The discovery of bifunctional SPDS from an evolutionarily old conifer reveals the missing link in the evolution of the polyamine biosynthesis pathway. The finding emphasizes the importance of pre-existing secondary functions in the evolution of new enzyme activities via gene duplication. Our results also associate PsACL5 with the development of vascular structures in Scots pine.

Diminution of arsenic accumulation in rice seedlings co-cultured with Anabaena sp.: Modulation in the expression of lower silicon transporters, two nitrogen dependent genes and lowering of antioxidants activity

The present study was intended to investigate the role of algae, Anabaena sp. in the amelioration of As toxicity, when co-cultured with rice seedlings. The reduction of growth in rice seedlings against As(III) and As(V) was recovered with Anabaena sp. The Anabaena sp. also reduced the accumulation of As, where it was more efficient against 60µM As(III) (49%) than As(V) (23%) in rice shoot. Similarly, with reduction of As accumulation, lower silicon transporters (Lsi-1 and Lsi-2) was found to be suppressed against As treatments. However, the expression of two nitrogen dependent genes i.e., NR and SAMT were found to be enhanced with the Anabaena sp. Likewise, the activity of antioxidant enzyme, GST, was enhanced, whereas, the activity of other enzymes such as SOD, APX, GPX, GR and DHAR were decreased with As+Algae combinations. Overall, the result suggested that the Anabaena sp. reduces As accumulation, modulates gene expressions and antioxidants to ameliorate the As toxicity in Oryza sativa L.