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

Polyamines: Rising stars against metal and metalloid toxicity

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

Physiological and biochemical characteristics of milk thistle (Silybum marianum (L.) Gaertn) as affected by some plant growth regulators

Milk thistle (Silybum marianum (L.) Gaertn) is a globally and widely used medicinal plant that contains silymarin. This plant has antioxidant, antimicrobial, anticancer, hepatoprotective, cardiovascular-protective, and neuroprotective effects. Plant quality, yield, and phytochemicals, especially silymarin content, change under various conditions like drought stress. Therefore, this research studied plant growth regulators (PGRs) like salicylic acid, spermidine, and brassinosteroid to increase plant tolerance to drought stress. Experimental treatments include different levels of irrigation (25%, 50%, 75%, and 90% field capacity), and foliar spraying including salicylic acid (75 and 150 mg/L), spermine (70 and 140 mg/L), and brassinosteroid (1 and 1.2 μM), separately, and water as a control and a secondary factor. The results revealed that the highest amount of leaf phenolic compounds was observed in the highest drought stress level (25%) and 75 mg/L salicylic acid application. Furthermore, brassinosteroid at different concentrations and salicylic acid (75 mg/L) increased leaf flavonoid content compared to other treatments. In 50% field capacity, foliar application of salicylic acid (150 mg/L) significantly increased seed yield by approximately 75% compared to control under the same stress level. Brassinosteroid application (1 μM) under 75% field capacity significantly increased the seed's taxifolin amount by 159%. Additionally, salicylic acid noticeably increased the silychristin concentration. The concentration of silydianin in the seed has also been increased under drought stress and foliar spraying with PGRs. Compared to the control, using spermidine below 75% field capacity caused an increase in its concentrations by over seven times. The highest silybin A amount was obtained in 50% field capacity and foliar150 mg/L salicylic acid. Taxifolin, silychristin, silydianin, silybinin B, iso-silybinin A, and iso-silybinin B compounds were identified in the seed extract. Generally, foliar spraying using plant growth regulators increased the number of silymarin compounds under drought stress conditions and field cultivation conditions.

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.

The White Clover TrMYB33-TrSAMS1 Module Contributes to Drought Tolerance by Modulation of Spermidine Biosynthesis via an ABA-Dependent Pathway

Spermidine is well known to accumulate in plants exposed to drought, but the regulatory network associated with its biosynthesis and accumulation and the underlying molecular mechanisms remain unclear. Here, we demonstrated that the Trifolium repens TrMYB33 relayed the ABA signal to modulate drought-induced spermidine production by directly regulating the expression of TrSAMS1, which encodes an S-adenosylmethionine synthase. This gene was identified by transcriptome and expression analysis in T. repens. TrSAMS1 overexpression and its pTRV-VIGS-mediated silencing demonstrated that TrSAMS1 is a positive regulator of spermidine synthesis and drought tolerance. TrMYB33 was identified as an interacting candidate through yeast one-hybrid library screening with the TrSAMS1 promoter region as the bait. TrMYB33 was confirmed to bind directly to the predicted TAACCACTAACCA (the TAACCA MYB binding site is repeated twice in tandem) within the TrSAMS1 promoter and to act as a transcriptional activator. Additionally, TrMYB33 contributed to drought tolerance by regulating TrSAMS1 expression and modulating spermidine synthesis. Additionally, we found that spermidine accumulation under drought stress depended on ABA and that TrMYB33 coordinated ABA-mediated upregulation of TrSAMS1 and spermidine accumulation. This study elucidated the role of a T. repens MYB33 homolog in modulating spermidine biosynthesis. The further exploitation and functional characterization of the TrMYB33-TrSAMS1 regulatory module can enhance our understanding of the molecular mechanisms responsible for spermidine accumulation during drought stress.

Employing Genomic Tools to Explore the Molecular Mechanisms behind the Enhancement of Plant Growth and Stress Resilience Facilitated by a Burkholderia Rhizobacterial Strain

The rhizobacterial strain BJ3 showed 16S rDNA sequence similarity to species within the Burkholderia genus. Its complete genome sequence revealed a 97% match with Burkholderia contaminans and uncovered gene clusters essential for plant-growth-promoting traits (PGPTs). These clusters include genes responsible for producing indole acetic acid (IAA), osmolytes, non-ribosomal peptides (NRPS), volatile organic compounds (VOCs), siderophores, lipopolysaccharides, hydrolytic enzymes, and spermidine. Additionally, the genome contains genes for nitrogen fixation and phosphate solubilization, as well as a gene encoding 1-aminocyclopropane-1-carboxylate (ACC) deaminase. The treatment with BJ3 enhanced root architecture, boosted vegetative growth, and accelerated early flowering in Arabidopsis. Treated seedlings also showed increased lignin production and antioxidant capabilities, as well as notably increased tolerance to water deficit and high salinity. An RNA-seq transcriptome analysis indicated that BJ3 treatment significantly activated genes related to immunity induction, hormone signaling, and vegetative growth. It specifically activated genes involved in the production of auxin, ethylene, and salicylic acid (SA), as well as genes involved in the synthesis of defense compounds like glucosinolates, camalexin, and terpenoids. The expression of AP2/ERF transcription factors was markedly increased. These findings highlight BJ3's potential to produce various bioactive metabolites and its ability to activate auxin, ethylene, and SA signaling in Arabidopsis, positioning it as a new Burkholderia strain that could significantly improve plant growth, stress resilience, and immune function.

Copper-based nanomaterials: Opportunities for sustainable agriculture

The exponential growth of the global population has resulted in a significant surge in the demand for food worldwide. Additionally, the impact of climate change has exacerbated crop losses caused by pests and pathogens. The transportation and utilization of traditional agrochemicals in the soil are highly inefficient, resulting in significant environmental losses and causing severe pollution of both the soil and aquatic ecosystems. Nanotechnology is an emerging field with significant potential for market applications. Among metal-based nanomaterials, copper-based nanomaterials have demonstrated remarkable potential in agriculture, which are anticipated to offer a promising alternative approach for enhancing crop yields and managing diseases, among other benefits. This review firstly performed co-occurrence and clustering analyses of previous studies on copper-based nanomaterials used in agriculture. Then a comprehensive review of the applications of copper-based nanomaterials in agricultural production was summarized. These applications primarily involved in nano-fertilizers, nano-regulators, nano-stimulants, and nano-pesticides for enhancing crop yields, improving crop resistance, promoting crop seed germination, and controlling crop diseases. Besides, the paper concluded the potential impact of copper-based nanomaterials on the soil micro-environment, including soil physicochemical properties, enzyme activities, and microbial communities. Additionally, the potential mechanisms were proposed underlying the interactions between copper-based nanomaterials, pathogenic microorganisms, and crops. Furthermore, the review summarized the factors affecting the application of copper-based nanomaterials, and highlighted the advantages and limitations of employing copper-based nanomaterials in agriculture. Finally, insights into the future research directions of nano-agriculture were put forward. The purpose of this review is to encourage more researches and applications of copper-based nanomaterials in agriculture, offering a novel and sustainable strategy for agricultural development.

Different reactions of wheat, maize, and rice plants to putrescine treatment

Polyamines play an important role in growth and differentiation by regulating numerous physiological and biochemical processes at the cellular level. In addition to their roborative effect, their essential role in plant stress responses has been also reported. However, the positive effect may depend on the fine-tuning of polyamine metabolism, which influences the production of free radicals and/or signalling molecules. In the present study, 0.3 mM hydroponic putrescine treatment was tested in wheat, maize, and rice in order to reveal differences in their answers and highlight the relation of these with polyamine metabolism. In the case of wheat, the chlorophyll content and the actual quantum yield increased after putrescine treatment, and no remarkable changes were detected in the stress markers, polyamine contents, or polyamine metabolism-related gene expression. Although, in maize, the actual quantum yield decreased, and the root hydrogen peroxide content increased, no other negative effect was observed after putrescine treatment due to activation of polyamine oxidases at enzyme and gene expression levels. The results also demonstrated that after putrescine treatment, rice with a higher initial polyamine content, the balance of polyamine metabolism was disrupted and a significant amount of putrescine was accumulated, accompanied by a detrimental decrease in the level of higher polyamines. These initial differences and the putrescine-induced shift in polyamine metabolism together with the terminal catabolism or back-conversion-induced release of a substantial quantity of hydrogen peroxide could contribute to oxidative stress observed in rice.

Exogenous Spermidine Alleviated Low-Temperature Damage by Affecting Polyamine Metabolism and Antioxidant Levels in Apples

Low-temperature stress significantly limits the growth, development, and geographical distribution of apple cultivation. Spermidine (Spd), a known plant growth regulator, plays a vital role in the plant's response to abiotic stress. Yet, the mechanisms by which exogenous Spd enhances cold resistance in apples remain poorly understood. Therefore, the present study analyzed the effects of exogenous Spd on antioxidant enzyme activity, polyamine metabolism, and related gene expression levels of 1-year-old apple branches under low-temperature stress. Treatment with exogenous Spd was found to stabilize branch tissue biofilms and significantly reduce the levels of reactive oxygen species by elevating proline content and boosting the activity of antioxidants such as superoxide dismutase. It also upregulated the activities of arginine decarboxylase, S-adenosylmethionine decarboxylase, and spermidine synthase and the expression levels of MdADC1, MdSAMDC1, and MdSPDS1 under low-temperature stress and led to the accumulation of large amounts of Spd and spermine. Moreover, compared with the 2 mmol·L-1 Spd treatment, the 1 mmol·L-1 Spd treatment increased the expression levels of cold-responsive genes MdCBF1/2/3, MdCOR47, and MdKIN1, significantly. The findings suggest that exogenous Spd can enhance cold resistance in apple branches significantly. This enhancement is achieved by modulating polyamine metabolism and improving antioxidant defense mechanisms, which could be exploited to improve apple cultivation under cold stress conditions.

The alleviation of salt stress on rice through increasing photosynthetic capacity, maintaining redox homeostasis and regulating soil enzyme activities by Enterobacter sp. JIV1 assisted with putrescine

The detrimental impact of soil salinization on crop productivity and agricultural economy has garnered significant attention. A rhizosphere bacterium with favorable salt tolerance and plant growth-promoting (PGP) functions was isolated in this work. The bacterium was identified as Enterobacter through 16 S rDNA sequencing analysis and designated as Enterobacter sp. JIV1. Interestingly, the presence of putrescine (Put), which had been shown to contribute in reducing abiotic stress damage to plants, significantly promoted strain JIV1 to generate 1-aminocyclopropane-1-carboxylic (ACC) deaminase, dissolve phosphorus and secrete indole-3-acetic acid (IAA). However, the synergy of plant growth promoting rhizobacteria (PGPR) and Put in improving plant salt resistance has not been extensively studied. In this study, strain JIV1 and exogenous Put effectively mitigated the inhibitory impact of salt stress simulated by 200 mM NaCl on rice (Oryza sativa L.) growth. The chlorophyll accumulation, photosynthetic efficiency and antioxidant capacity of rice were also significantly strengthened. Notably, the combined application of strain JIV1 and Put outperformed individual treatments. Moreover, the co-addition of strain JIV1 and Put increased soil protease and urease activities by 451.97% and 51.70% compared to that of salt treatment group. In general, Put-assisted PGPR JIV1 provides a new perspective on alleviating the salt-induced negative impacts on plants.

AnDREB5.1, a A5 group DREB gene from desert shrub Ammopiptanthus nanus, confers osmotic and cold stress tolerances in transgenic tobacco

The Dehydration-Responsive Element Binding (DREB) subfamily of transcription factors plays crucial roles in plant abiotic stress response. Ammopiptanthus nanus (A. nanus) is an eremophyte exhibiting remarkable tolerance to environmental stress and DREB proteins may contribute to its tolerance to water deficit and low-temperature stress. In the present study, an A. nanus DREB A5 group transcription factor gene, AnDREB5.1, was isolated and characterized in terms of structure and function in abiotic stress tolerance. AnDREB5.1 protein is distributed in the nucleus, possesses transactivation capacity, and is capable of binding to DRE core cis-acting element. The transcription of AnDREB5.1 was induced under osmotic and cold stress. Tobacco seedlings overexpressing AnDREB5.1 displayed higher tolerance to cold stress, osmotic stress, and oxidative stress compared to wild-type tobacco (WT). Under osmotic and cold stress, overexpression of AnDREB5.1 increased antioxidant enzyme activity in tobacco leaves, inhibiting excessive elevation of ROS levels. Transcriptome sequencing analysis showed that overexpression of AnDREB5.1 raised the tolerance of transgenic tobacco seedlings to abiotic stress by regulating multiple genes, including antioxidant enzymes, transcription factors, and stress-tolerant related functional genes like NtCOR413 and NtLEA14. This study provides new evidence for understanding the potential roles of the DREB A5 subgroup members in plants.

Nitrogen Nutrition Modulates the Response to Alternaria brassicicola Infection via Metabolic Modifications in Arabidopsis Seedlings

Little is known about the effect of nitrogen nutrition on seedling susceptibility to seed-borne pathogens. We have previously shown that seedlings grown under high nitrate (5 mM) conditions are less susceptible than those grown under low nitrate (0.1 mM) and ammonium (5 mM) in the Arabidopsis-Alternaria brassicicola pathosystem. However, it is not known how seedling metabolism is modulated by nitrogen nutrition, nor what is its response to pathogen infection. Here, we addressed this question using the same pathosystem and nutritive conditions, examining germination kinetics, seedling development, but also shoot ion contents, metabolome, and selected gene expression. Nitrogen nutrition clearly altered the seedling metabolome. A similar metabolomic profile was observed in inoculated seedlings grown at high nitrate levels and in not inoculated-seedlings. High nitrate levels also led to specific gene expression patterns (e.g., polyamine metabolism), while other genes responded to inoculation regardless of nitrogen supply conditions. Furthermore, the metabolites best correlated with high disease symptoms were coumarate, tyrosine, hemicellulose sugars, and polyamines, and those associated with low symptoms were organic acids (tricarboxylic acid pathway, glycerate, shikimate), sugars derivatives and β-alanine. Overall, our results suggest that the beneficial effect of high nitrate nutrition on seedling susceptibility is likely due to nutritive and signaling mechanisms affecting developmental plant processes detrimental to the pathogen. In particular, it may be due to a constitutively high tryptophan metabolism, as well as down regulation of oxidative stress caused by polyamine catabolism.

Variations in Proline Content, Polyamine Profiles, and Antioxidant Capacities among Different Provenances of European Beech (Fagus sylvatica L.)

International provenance trials are a hot topic in forestry, and in light of climate change, the search for more resilient beech provenances and their assisted migration is one of the challenges of climate-smart forestry. The main aim of the study was to determine intraspecific variability in European beech (Fagus sylvatica L.) among 11 beech provenances according to total antioxidant capacities estimated by various assays, such as DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic) acid), FRAP (ferric reducing antioxidant power) assay, and radical scavenging capacity against nitric oxide (RSC-NO assays), as well as osmolyte content, primarily individual polyamines (putrescine, spermidine, and spermine), and free proline content. Polyamine amounts were quantified by using HPLC coupled with fluorescent detection after dansylation pretreatment. The highest values for radical scavenger capacity assays (ABTS, DPPH, and FRAP) were measured in the German provenances DE47 and DE49. Also, the highest NO inhibition capacity was found in the provenance DE49, while the highest content of proline (PRO), total phenolic content (TPC), and total flavonoid content (TFC) was recorded in DE47. The Austrian AT56 and German provenance DE49 were most abundant in total polyamines. This research underlines the importance of the application of common antioxidant assays as well as osmolyte quantification as a criterion for the selection of climate-ready beech provenances for sustainable forest management.

Next generation chemical priming: with a little help from our nanocarrier friends

Plants are exposed to multiple threats linked to climate change which can cause critical yield losses. Therefore, designing novel crop management tools is crucial. Chemical priming has recently emerged as an effective technology for improving tolerance to stress factors. Several compounds such as phytohormones, reactive species, and synthetic chimeras have been identified as promising priming agents. Following remarkable developments in nanotechnology, several unique nanocarriers (NCs) have been engineered that can act as smart delivery systems. These provide an eco-friendly, next-generation method for chemical priming, leading to increased efficiency and reduced overall chemical usage. We review novel engineered NCs (NENCs) as vehicles for chemical agents in advanced priming strategies, and address challenges and opportunities to be met towards achieving sustainable agriculture.

Exogenous Spermidine and Amino-Ethoxyvinylglycine Improve Nutritional Quality via Increasing Amino Acids in Rice Grains

Polyamines and ethylene are key regulators of the growth and development, quality formation, and stress response of cereal crops such as rice. However, it remains unclear whether the application of these regulators could improve the nutritional quality via increasing amino acids in rice grains. This study examined the role of exogenous polyamines and ethylene in regulating amino acid levels in the milled rice of earlier-flowered superior grain (SG) and later-flowered inferior grain (IG). Two rice varieties were field grown, and either 1 mmol L-1 spermidine (Spd) or 50 μmol L-1 amino-ethoxyvinylglycine (AVG) was applied to panicles at the early grain-filling stage. The control check (CK) was applied with deionized water. The results showed that the Spd or AVG applications significantly increased polyamine (spermine (Spm) and Spd) contents and decreased ethylene levels in both SG and IG and significantly increased amino acid levels in the milled rice of SG and IG relative to the CK. Collectively, the application of Spd or AVG can increase amino acid-based nutritional quality and grain yield via increasing polyamine (Spm and Spd) contents and reducing ethylene levels in both SG and IG of rice.

Polyamine depletion enhances oil body mobilization through possible regulation of oleosin degradation and aquaporin abundance on its membrane

Oil body (OB) mobilization, a crucial event associated with early seedling growth, is delayed in response to salt stress. Previous reports suggest that careful regulation of polyamine (PA) metabolism is essential for salt stress tolerance in plants. Many aspects of PA-mediated regulation of metabolism have been uncovered. However, their role in the process of OB mobilization remains unexplored. Interestingly, the present investigations reveal a possible influence of PA homeostasis on OB mobilization, while implicating complex regulation of oleosin degradation and aquaporin abundance in OB membranes in the process. Application of PA inhibitors resulted in the accumulation of smaller OBs when compared to control (-NaCl) and the salt-stressed counterparts, suggesting a faster rate of mobilization. PA deficit also resulted in reduced retention of some larger oleosins under controlled conditions but enhanced retention of all oleosins under salt stress. Additionally, with respect to aquaporins, a higher abundance of PIP2 under PA deficit both under control and saline conditions, is correlated with a faster mobilization of OBs. Contrarily, TIP1s, and TIP2s remained almost undetectable in response to PA depletion and were differentially regulated by salt stress. The present work, thus, provides novel insights into PA homeostasis-mediated regulation of OB mobilization, oleosin degradation, and aquaporin abundance on OB membranes.

Enriched endogenous free Spd and Spm in alfalfa (Medicago sativa L.) under drought stress enhance drought tolerance by inhibiting H2O2 production to increase antioxidant enzyme activity

Drought stress is a major factor limiting agricultural development, and exogenous polyamines (PAs) can increase plant drought resistance by enhancing antioxidant activity, but few studies have examined whether endogenous PAs enhance the plant antioxidant system. Here, to investigate the effects of endogenous PAs on the antioxidant system of alfalfa under drought stress and the underlying mechanisms, two alfalfa cultivars, Longzhong (drought resistant) and Gannong No. 3 (drought sensitive), were used as test materials, and their seedlings were treated with polyethylene glycol (PEG-6000) for 8 days at -1.2 MPa to simulate drought stress. The levels of free PAs [putrescine (Put), spermidine (Spd) and spermine (Spm)], hydrogen peroxide (H2O2), malondialdehyde (MDA), key PA metabolism enzyme [arginine decarboxylase (ADC), ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (SAMDC), polyamine oxidase (PAO), and diamine oxidase (DAO)] activities, and antioxidant enzyme [superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)] activities were measured. These physiological indicators were used for correlation analysis to investigate the relationship between PA metabolism and the antioxidant enzyme system. The results showed that PA synthesis in alfalfa under drought stress was dominated by the ADC pathway. Spd and Spm played an important role in improving drought tolerance. The high levels of ADC and SAMDC activities were facilitated by the conversion of Put to Spd and Spm. H2O2 generation by oxidative decomposition of PAs was mainly dependent on the oxidative decomposition of DAO but not PAO. Low DAO activity favored low H2O2 production. Spd, Spm, ADC, ODC and SAMDC were positively correlated with the antioxidant enzymes SOD, CAT and POD in both cultivars under drought. Therefore, we concluded that high ADC and SAMDC activities in alfalfa promoted the conversion of Put to Spd and Spm, leading to high accumulation of Spd and Spm and low Put accumulation. Low Put levels led to low H2O2 production through low DAO activity, and low H2O2 levels induced the expression of antioxidant enzyme-encoding genes to improve antioxidant enzyme activity and reduce MDA accumulation and thereby enhanced drought resistance in alfalfa.

Polyamines and hormesis: Making sense of a dose response dichotomy

The diverse biological effects of polyamines (putrescine, spermidine and spermine) were reviewed in the context of hormesis in an integrative manner for the first time. The findings illustrate that each of these polyamines commonly induces hormetic dose responses in a wide range of biological models and types of cells for multiple endpoints in numerous plant species and animal models. Plant research emphasized preconditioning experimental studies in which the respective polyamines conferred some protection against the damaging effects of a broad range of environmental stressors such as drought, salinity, cold/heat, heavy metals and UV-damage in an hormetic manner. Polyamine-based animal hormesis studies emphasized biomedical endpoints such as longevity and neuroprotection. These findings have important biological and biomedical implications and should guide experimental designs of low dose investigations.

Proline, Cysteine and Branched-Chain Amino Acids in Abiotic Stress Response of Land Plants and Microalgae

Proteinogenic amino acids are the building blocks of protein, and plants synthesize all of them. In addition to their importance in plant growth and development, growing evidence underlines the central role played by amino acids and their derivatives in regulating several pathways involved in biotic and abiotic stress responses. In the present review, we illustrate (i) the role of amino acids as an energy source capable of replacing sugars as electron donors to the mitochondrial electron transport chain and (ii) the role of amino acids as precursors of osmolytes as well as (iii) precursors of secondary metabolites. Among the amino acids involved in drought stress response, proline and cysteine play a special role. Besides the large proline accumulation occurring in response to drought stress, proline can export reducing equivalents to sink tissues and organs, and the production of H2S deriving from the metabolism of cysteine can mediate post-translational modifications that target protein cysteines themselves. Although our general understanding of microalgae stress physiology is still fragmentary, a general overview of how unicellular photosynthetic organisms deal with salt stress is also provided because of the growing interest in microalgae in applied sciences.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

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

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

Polyamine, 1,3-diaminopropane, regulates defence responses on growth, gas exchange, PSII photochemistry and antioxidant system in wheat under arsenic toxicity

The metalloid arsenic (As) is extremely hazardous to all living organisms, including plants. Pollution with As is very detrimental to the photosynthetic machinery, cell division, energy generation, and redox status. In order to cope with stress, the use of growth regulators such as polyamines (PA), which strengthen the antioxidant system of plants, has become widespread in recent years. PAs can modulate the plant growth through basic mechanisms common to all living organisms, such as membrane stabilization, free radical scavenging, deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and protein synthesis, enzyme activities and second messengers. However, the effect of 1,3- diaminopropane (Dap), which is a product of PA catabolism, is not clear enough in plants exposed to As toxicity. In the current study, the different concentrations of 1,3-diaminopropane (0.1, 0.5 and 1 mM Dap) were hydroponically treated to wheat (Triticum aestivum) under arsenic stress (100 μM As) and then relative growth rate (RGR), relative water content (RWC), proline content (Pro), gas exchange parameters, PSII photochemistry, chlorophyll fluorescence kinetics, antioxidant activity and lipid peroxidation were assessed. RGR, RWC, osmotic potential and Pro content decreased in As-applied plants. The inhibition of these parameters could be reversed by Dap treatments. Besides, Dap applications mitigated the As toxicity-induced suppression on chlorophyll fluorescence (Fv/Fm, Fv/Fo and Fo/Fm) and the performance of PSII photochemistry. As impaired the balance on antioxidant capacity by decreased activities of catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and the contents of ascorbate (AsA) and glutathione (GSH) and then lipid peroxidation (TBARS content) increased. In the presence of Dap under As stress, the plants exhibited an increase in superoxide dismutase (SOD), POX, and GPX. Dap treatments contributed to the maintenance of cellular redox state (AsA/DHA and GSH/GSSG) by regulating the activities/contents of enzyme/non-enzyme involved in the AsA-GSH cycle. After Dap applications against stress, ROS accumulation (H2O2 content) and lipid peroxidation (TBARS) were effectively reduced. The findings showed that by eliminating As-induced oxidative damage and protecting the biochemical processes of photosynthesis, Dap treatments have a substantial potential to give resistance to wheat.

Monofloral Corn Poppy Bee-Collected Pollen-A Detailed Insight into Its Phytochemical Composition and Antioxidant Properties

The aim of this study was to compile a detailed phytochemical profile and assess the antioxidant properties of bee-collected pollen (PBP) obtained from corn poppy (Papaver rhoeas L.) plants. To achieve this, a lipid fraction was prepared for quantifying fatty acids using GC-FID. Extractable and alkaline-hydrolysable PBP fractions (obtained from a defatted sample) were used to determine the qualitative and quantitative profiles of phenolic compounds, phenylamides and alkaloids using UHPLC/Q-ToF-MS. Additionally, various spectrophotometric assays (TAC, FRP, CUPRAC, DPPH^⦁) were conducted to evaluate the antioxidant properties. Phenolic compounds were more present in the extractable fraction than in the alkaline-hydrolysable fraction. Luteolin was the predominant compound in the extractable fraction, followed by tricetin and various derivatives of kaempferol. This study presents one of the first reports on the quantification of tricetin aglycone outside the Myrtaceae plant family. The alkaline-hydrolysable fraction exhibited a different phenolic profile, with a significantly lower amount of phenolics. Kaempferol/derivatives, specific compounds like ferulic and 5-carboxyvanillic acids, and (epi)catechin 3-O-gallate were the predominant compounds in this fraction. Regarding phenylamides, the extractable fraction demonstrated a diverse range of these bioactive compounds, with a notable abundance of different spermine derivatives. In contrast, the hydrolysable fraction contained six spermine derivatives and one spermidine derivative. The examined fractions also revealed the presence of seventeen different alkaloids, belonging to the benzylisoquinoline, berberine and isoquinoline classes. The fatty-acid profile confirmed the prevalence of unsaturated fatty acids. Furthermore, both fractions exhibited significant antioxidant activity, with the extractable fraction showing particularly high activity. Among the assays conducted, the CUPRAC assay highlighted the exceptional ability of PBP's bioactive compounds to reduce cupric ions.

Dynamics of Polyamines, Proline, and Ethylene Metabolism under Increasing Cold in Winter Oilseed Rape

Cold stress is among the most important environmental factors reducing the yield of crops. The present study aimed to investigate the impact of increasing cold stress conditions on winter oilseed rape polyamines, proline, and ethylene metabolism in acclimated and non-acclimated winter oilseed rape. This study was carried out under controlled conditions in the laboratory. The winter oilseed rape hybrid 'Visby' was used in the experiment. Acclimated and non-acclimated plants were subjected to a two-day-long increasing cold (from -1 °C to -3 °C) treatment. HPTLC, RT-qPCR, spectral analysis, and gas chromatography methods were used to analyse the levels of polyamines, gene expression, proline, and ethylene, respectively. This study showed a decrease in putrescine, spermidine, and spermine content during cold acclimation and a decrease in putrescine and spermidine levels at sub-zero temperatures. There were intensive changes in ADC2 gene expression, proline, and ethylene levels in non-acclimated plants: a substantial increase after exposure to -1 °C temperature and a sharp decrease after exposure to -3 °C temperature. The changes in these parameters were lower or absent in acclimated plants. The phenomena observed in this study add new insights to the knowledge about the plant stress response and suggest questions to be answered in the future.

Exogenous plant growth regulator and foliar fertilizers for phytoextraction of cadmium with Boehmeria nivea [L.] Gaudich from contaminated field soil

As a enrichment plant, ramie can be used for the phytoremediation of cadmium (Cd)-contaminated soil. However, it is worth exploring the role of plant growth regulators and foliar fertilizers in the process of plant growth and development and Cd adsorption. By measuring the agronomic traits, Cd content of aboveground and underground ramie, calculating the Cd transfer coefficient (TF) and Cd bioconcentration factors (BCF), and the correlation between various indicators. This study examined the effects of plant growth regulators and foliar fertilizers on ramie's capacity for Cd accumulation and transportation. Plant growth regulators and foliar fertilizers increased the Cd content of the aboveground ramie, reduced the Cd content of the underground ramie, and increased the TF. Among them, GA-1 increased the Cd content of the aboveground ramie to 3 times more than that of the control and reduced the Cd content of the underground ramie by 54.76%. Salicylic acid (SA) increased the Cd content of the aboveground ramie to three times more than that of the control. The combination of GA and foliar fertilizer reduced the Cd content of the aboveground and underground ramie and the TF and BCF of the underground ramie. After the hormones were sprayed, the TF of ramie had a significant positive correlation with the Cd content of the aboveground ramie; the BCF of the aboveground ramie had a significant positive correlation with the Cd content and TF of the aboveground ramie. The results indicate that Brassinolide (BR), gibberellin (GA), ethephon (ETH), polyamines (PAs), and salicylic acid (SA) have different effects on the enrichment and transport of Cd in ramie. This study provided an effective method to improve the capacity for ramie to adsorb heavy metals during cultivation.

Comparative metabolomic analysis between tomato and halophyte plants under intercropping conditions

Halophyte-based intercropping appears nowadays as a valuable approach in soil remediation and agriculture. In this work, intercropping between the halophyte Arthrocaulon macrostachyum and tomato (Solanum lycopersicum var. Sargento) was studied in both plant species using comparative mass spectrometry-based metabolomics coupled to metabolic pathway predictions. A significant number of changes in metabolites was observed in the halophyte. In terms of alteration of specific metabolic pathways, intercropping conditions stimulated sugar and starch metabolisms in tomato, whereas in the halophyte, intercropping mainly altered amino acid-related pathways. In addition, arginine and proline metabolism were commonly affected in both tomato and halophyte plants. Moreover, metabolomic changes were associated with physiological alterations in tomato. In this sense, mild oxidative stress was induced in intercropped tomato plants, which, in turn, could trigger signaling events leading to plant adjustment to intercropping conditions. This study represents the first approach toward understanding intercropping interactions at the metabolome level and its effect on plant physiology, opening up prospects for further characterization of this crop cultivation strategy.

Salt and drought stress-mitigating approaches in sugar beet (Beta vulgaris L.) to improve its performance and yield

MAIN CONCLUSION

Although sugar beet is a salt- and drought-tolerant crop, high salinity, and water deprivation significantly reduce its yield and growth. Several reports have demonstrated stress tolerance enhancement through stress-mitigating strategies including the exogenous application of osmolytes or metabolites, nanoparticles, seed treatments, breeding salt/drought-tolerant varieties. These approaches would assist in achieving sustainable yields despite global climatic changes. Sugar beet (Beta vulgaris L.) is an economically vital crop for ~ 30% of world sugar production. They also provide essential raw materials for bioethanol, animal fodder, pulp, pectin, and functional food-related industries. Due to fewer irrigation water requirements and shorter regeneration time than sugarcane, beet cultivation is spreading to subtropical climates from temperate climates. However, beet varieties from different geographical locations display different stress tolerance levels. Although sugar beet can endure moderate exposure to various abiotic stresses, including high salinity and drought, prolonged exposure to salt and drought stress causes a significant decrease in crop yield and production. Hence, plant biologists and agronomists have devised several strategies to mitigate the stress-induced damage to sugar beet cultivation. Recently, several studies substantiated that the exogenous application of osmolytes or metabolite substances can help plants overcome injuries induced by salt or drought stress. Furthermore, these compounds likely elicit different physio-biochemical impacts, including improving nutrient/ionic homeostasis, photosynthetic efficiency, strengthening defense response, and water status improvement under various abiotic stress conditions. In the current review, we compiled different stress-mitigating agricultural strategies, prospects, and future experiments that can secure sustainable yields for sugar beets despite high saline or drought conditions.

Role of Polyamines in the Response to Salt Stress of Tomato

Plants irrigated with saline solutions undergo osmotic and oxidative stresses, which affect their growth, photosynthetic activity and yield. Therefore, the use of saline water for irrigation, in addition to the increasing soil salinity, is one of the major threats to crop productivity worldwide. Plant tolerance to stressful conditions can be improved using different strategies, i.e., seed priming and acclimation, which elicit morphological and biochemical responses to overcome stress. In this work, we evaluated the combined effect of priming and acclimation on salt stress response of a tomato cultivar (Solanum lycopersicum L.), very sensitive to salinity. Chemical priming of seeds was performed by treating seeds with polyamines (PAs): 2.5 mM putrescine (PUT), 2.5 mM spermine (SPM) and 2.5 mM spermidine (SPD). Germinated seeds of primed and non-primed (controls) were sown in non-saline soil. The acclimation consisted of irrigating the seedlings for 2 weeks with tap water, followed by irrigation with saline and non-saline water for 4 weeks. At the end of the growth period, morphological, physiological and biochemical parameters were determined. The positive effects of combined treatments were evident, when primed plants were compared to non-primed, grown under the same conditions. Priming with PAs improved tolerance to salt stress, reduced the negative effects of salinity on growth, improved membrane integrity, and increased photosynthetic pigments, proline and enzymatic and non-enzymatic antioxidant responses in all salt-exposed plants. These results may open new perspectives and strategies to increase tolerance to salt stress in sensitive species, such as tomato.

A Fine-Tuning of the Plant Hormones, Polyamines and Osmolytes by Ectomycorrhizal Fungi Enhances Drought Tolerance in Pedunculate Oak

The drought sensitivity of the pedunculate oak (Quercus robur L.) poses a threat to its survival in light of climate change. Mycorrhizal fungi, which orchestrate biogeochemical cycles and particularly have an impact on the plant's defense mechanisms and metabolism of carbon, nitrogen, and phosphorus, are among the microbes that play a significant role in the mitigation of the effects of climate change on trees. The study's main objectives were to determine whether ectomycorrhizal (ECM) fungi alleviate the effects of drought stress in pedunculate oak and to investigate their priming properties. The effects of two levels of drought (mild and severe, corresponding to 60% and 30% of field capacity, respectively) on the biochemical response of pedunculate oak were examined in the presence and absence of ectomycorrhizal fungi. To examine whether the ectomycorrhizal fungi modulate the drought tolerance of pedunculate oak, levels of plant hormones and polyamines were quantified using UPLC-TQS and HPLC-FD techniques in addition to gas exchange measurements and the main osmolyte amounts (glycine betaine-GB and proline-PRO) which were determined spectrophotometrically. Droughts increased the accumulation of osmolytes, such as proline and glycine betaine, as well as higher polyamines (spermidine and spermine) levels and decreased putrescine levels in both, mycorrhized and non-mycorrhized oak seedlings. In addition to amplifying the response of oak to severe drought in terms of inducible proline and abscisic acid (ABA) levels, inoculation with ECM fungi significantly increased the constitutive levels of glycine betaine, spermine, and spermidine regardless of drought stress. This study found that compared to non-mycorrhized oak seedlings, unstressed ECM-inoculated oak seedlings had higher levels of salicylic (SA) and abscisic acid (ABA) but not jasmonic acid (JA), indicating a priming mechanism of ECM is conveyed via these plant hormones. According to a PCA analysis, the effect of drought was linked to the variability of parameters along the PC1 axe, such as osmolytes PRO, GB, polyamines, and plant hormones such as JA, JA-Ile, SAG, and SGE, whereas mycorrhization was more closely associated with the parameters gathered around the PC2 axe (SA, ODPA, ABA, and E). These findings highlight the beneficial function of the ectomycorrhizal fungi, in particular Scleroderma citrinum, in reducing the effects of drought stress in pedunculate oak.

The Effect of Glycine Betaine on Nitrogen and Polyamine Metabolisms, Expression of Glycoside-Related Biosynthetic Enzymes, and K/Na Balance of Stevia under Salt Stress

The beneficial role of glycine betaine (GB) in the adaptation of plants to abiotic stresses is well known; therefore, the study of physiological and molecular responses induced by exogenous GB under NaCl stress can provide a suitable reference for the application of this compound to enhance the adaptation of plants to salinity. The present study was conducted under in vitro conditions to evaluate the effect of GB (25 and 50 mM) on the growth, physiological, and molecular traits of Stevia rebaudiana during NaCl toxicity (50 mM). The results showed that applying NaCl treatment increased Na accumulation, induced oxidative stress, and disrupted N metabolism and K/Na homeostasis, which, as a result, decreased the stevia plant's growth and biomass. However, application of GB improved the adaptation of NaCl-stressed plants by improving N metabolism and modulating the metabolism of polyamines. By increasing the activity of antioxidant enzymes, GB diminished oxidative stress, protected the plasma membrane, and restored photosynthetic pigments under NaCl toxicity. By reducing Na accumulation and increasing K accumulation, GB maintained the K/Na balance and reduced the effects of toxicity caused by the high Na concentration in stevia leaves. GB increased the leaf accumulation of rebaudioside A in NaCl-stressed plants by modulating the expression of genes (KAH, UGT74G1, UGT76G1, and UGT85C2) involved in the sugar compounds of the stevia plants. Our results provide a broad understanding of GB-induced responses in NaCl-stressed plants, which can help increase our knowledge of the role of GB in the defense mechanisms of plants under abiotic stresses.

Putrescine transformation to other forms of polyamines in filling grain embryos functioned in enhancing the resistance of maize plants to drought stress

Polyamines (PAs), one of plant growth regulators, play an important role in the plant resistance to drought stress. However, the precise function of putrescine (Put) transformation to other forms of PAs is not clear in filling maize grain embryos. In this study, two maize (Zea mays L.) cultivars, Yedan No. 13 (drought-resistant) and Xundan No. 22 (drought-sensitive), were used as experimental materials. Maize was planted in big plastic basins during whole growth period, and from the 25th day after fertilization, the plants were treated with drought (-1.0 MPa), PAs and inhibitors for 12 d. The experiments were performed during three consecutive years. The changes in the levels of three main free PAs, Put, spermidine (Spd) and spermine (Spm), covalently conjugated PAs (perchloric acid-soluble), covalently bound PAs (perchloric acid-insoluble), the activities of arginine decarboxylase, S-adenosylmethionine decarboxylase, and transglutaminase were investigated in embryos of filling grains. During drought stress, free Put increased from 109 to 367 nmol g^-1 FW and from 107 to 142 nmol g^-1 FW in Xundan 22 and in Yedan 13, respectively. Meanwhile, free Spd, free Spm and bound Put increased 2.7, 3.0 and 4.2 times in Yedan 13, respectively, and they merely increased about 1.5 times in Xundan 22. These results suggested that free Spd/Spm and bound Put, which were transformed from free Put, were possibly involved in drought resistance. Exogenous Spd treatment enhanced the drought-induced increase in endogenous free Spd/Spm content in drought-sensitive Xundan 22, coupled with the increase in drought resistance, as judged by the decrease in ear leaf relative plasma membrane permeability and increases in ear leaf relative water content, 1000-grain weight and grain number per ear. The suggestion was further testified with methylglyoxal-bis guanylhydrazone and o-phenanthrolin treatments. Collectively, it could be inferred that transformation of free Put to free Spd/Spm and bound Put in filling grain embryos functioned in enhancing the resistance of maize plants to soil drought.

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

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

Combined Effect of Salt Stress and Nitrogen Level on the Primary Metabolism of Two Contrasting Hydroponically Grown Cichorium spinosum L. Ecotypes

Stamnagathi (Cichorium spinosum L.) is an indigenous plant species well-known for its health-promoting properties. Salinity is a long-term issue with devastating consequences on land and farmers. Nitrogen (N) constitutes a crucial element for plant growth and development (chlorophyll, primary metabolites, etc.). Thus, it is of paramount importance to investigate the impact of salinity and N supply on plants’ metabolism. Within this context, a study was conducted aiming to assess the impact of salinity and N stress on the primary metabolism of two contrasting ecotypes of stamnagathi (montane and seaside). Both ecotypes were exposed to three different salinity levels (0.3 mM—non-saline treatment, 20 mM—medium, and 40 mM—high salinity level) combined with two different total-N supply levels: a low-N at 4 mM and a high-N at 16 mM, respectively. The differences between the two ecotypes revealed the variable responses of the plant under the applied treatments. Fluctuations were observed at the level of TCA cycle intermediates (fumarate, malate, and succinate) of the montane ecotype, while the seaside ecotype was not affected. In addition, the results showed that proline (Pro) levels increased in both ecotypes grown under a low N-supply and high salt stress, while other osmoprotectant metabolites such as γ-aminobutyric acid (GABA) exhibited variable responses under the different N supply levels. Fatty acids such as α-linolenate and linoleate also displayed variable fluctuations following plant treatments. The carbohydrate content of the plants, as indicated by the levels of glucose, fructose, α,α-trehalose, and myo-inositol, was significantly affected by the applied treatments. These findings suggest that the different adaptation mechanisms among the two contrasting ecotypes could be strongly correlated with the observed changes in their primary metabolism. This study also suggests that the seaside ecotype may have developed unique adaptation mechanisms to cope with high N supply and salinity stress, making it a promising candidate for future breeding programs aimed at developing stress tolerant varieties of C. spinosum L.

Plant Metabolomics: An Overview of the Role of Primary and Secondary Metabolites against Different Environmental Stress Factors

Several environmental stresses, including biotic and abiotic factors, adversely affect the growth and development of crops, thereby lowering their yield. However, abiotic factors, e.g., drought, salinity, cold, heat, ultraviolet radiations (UVr), reactive oxygen species (ROS), trace metals (TM), and soil pH, are extremely destructive and decrease crop yield worldwide. It is expected that more than 50% of crop production losses are due to abiotic stresses. Moreover, these factors are responsible for physiological and biochemical changes in plants. The response of different plant species to such stresses is a complex phenomenon with individual features for several species. In addition, it has been shown that abiotic factors stimulate multi-gene responses by making modifications in the accumulation of the primary and secondary metabolites. Metabolomics is a promising way to interpret biotic and abiotic stress tolerance in plants. The study of metabolic profiling revealed different types of metabolites, e.g., amino acids, carbohydrates, phenols, polyamines, terpenes, etc, which are accumulated in plants. Among all, primary metabolites, such as amino acids, carbohydrates, lipids polyamines, and glycine betaine, are considered the major contributing factors that work as osmolytes and osmoprotectants for plants from various environmental stress factors. In contrast, plant-derived secondary metabolites, e.g., phenolics, terpenoids, and nitrogen-containing compounds (alkaloids), have no direct role in the growth and development of plants. Nevertheless, such metabolites could play a significant role as a defense by protecting plants from biotic factors such as herbivores, insects, and pathogens. In addition, they can enhance the resistance against abiotic factors. Therefore, metabolomics practices are becoming essential and influential in plants by identifying different phytochemicals that are part of the acclimation responses to various stimuli. Hence, an accurate metabolome analysis is important to understand the basics of stress physiology and biochemistry. This review provides insight into the current information related to the impact of biotic and abiotic factors on variations of various sets of metabolite levels and explores how primary and secondary metabolites help plants in response to these stresses.

Interactions of Polyamines and Phytohormones in Plant Response to Abiotic Stress

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

An Undefined Interaction between Polyamines and Heat Shock Proteins Leads to Cellular Protection in Plasmodium falciparum and Proliferating Cells in Various Organisms

Environmental stimuli can distress the internal reaction of cells and their normal function. To react promptly to sudden environmental changes, a cascade of heat shock proteins (Hsps) functions to protect and act as housekeepers inside the cells. In parallel to the heat shock response, the metabolic polyamine (PA) status changes. Here, we discuss possible ways of putative interactions between Hsps and polyamines in a wide lineage of eukaryotic model organisms with a particular focus on parasitic protozoa such as Plasmodium falciparum (P. falciparum). The supposed interaction between polyamines and Hsps may protect the parasite from the sudden change in temperature during transmission from the female Anopheles mosquito to a human host. Recent experiments performed with the spermidine mimetic inhibitor 15-deoxyspergualine in Plasmodium in vitro cultures show that the drug binds to the C-terminal EEVD motif of Hsp70. This leads to inhibition of protein biosynthesis caused by prevention of eIF5A2 phosphorylation and eukaryotic initiation factor 5A (eIF5A) modification. These observations provide further evidence that PAs are involved in the regulation of protein biosynthesis of Hsps to achieve a protective effect for the parasite during transmission.

Polyamines effectively mitigate senescence in persistent leaves of Berginia ciliata - a novel model system

Plant leaves provide a unique insight into the changes that occur in organs, tissues and cells as they approach senescence. As part of the parental outlay, plants instigate leaf senescence to reallocate resources from older tissues to new organs towards the termination of the growing season. The aim of crop breeding initiatives is to optimize senescence for specific species. Considering hormonal regulation and their crosstalk during leaf senescence through integration of developmental signals, this work examines the efficacy of polyamines (PAs) in modulating several biochemical and physiological aspects with an ultimate aim to delay leaf senescence in leaf discs of Berginia ciliata (Haw.) sternb. Leaf discs were treated with putrescine (Put), spermidine (Spd) and spermine (Spm) at 20μM, 20μM and 15μM concentration, respectively. A set of leaf discs kept in distilled water served as the control. Leaf discs treated with PAs were green and fresh by about 4 days compared to the control, thus exhibited delayed senescence. This delayed leaf senescence corroborated with the maintenance of high activity of reactive oxygen species (ROS) scavenging antioxidant enzymes viz , superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and higher content of chlorophylls. A marked increase was also observed in membrane stability and soluble proteins in leaf discs treated with PAs. Exogenous PAs reduced oxidative stress in the leaf discs, as revealed by lower malondialdehyde (MDA) level, which is manifested as reduced lipid peroxidation (LPO). Improved membrane stability was proportional to lower LPO, as measured by the membrane stability index (MSI).

Polyamine Oxidase-Generated Reactive Oxygen Species in Plant Development and Adaptation: The Polyamine Oxidase-NADPH Oxidase Nexus

Metabolism and regulation of cellular polyamine levels are crucial for living cells to maintain their homeostasis and function. Polyamine oxidases (PAOs) terminally catabolize polyamines or catalyse the back-conversion reactions when spermine is converted to spermidine and Spd to putrescine. Hydrogen peroxide (H₂O₂) is a by-product of both the catabolic and back-conversion processes. Pharmacological and genetic approaches have started to uncover the roles of PAO-generated H₂O₂ in various plant developmental and adaptation processes such as cell differentiation, senescence, programmed cell death, and abiotic and biotic stress responses. Many of these studies have revealed that the superoxide-generating Respiratory Burst Oxidase Homolog (RBOH) NADPH oxidases control the same processes either upstream or downstream of PAO action. Therefore, it is reasonable to suppose that the two enzymes co-ordinately control the cellular homeostasis of reactive oxygen species. The intricate relationship between PAOs and RBOHs is also discussed, posing the hypothesis that these enzymes indirectly control each other's abundance/function via H₂O₂.

Ectomycorrhizal Fungi Modulate Pedunculate Oak's Heat Stress Responses through the Alternation of Polyamines, Phenolics, and Osmotica Content

The physiological and biochemical responses of pedunculate oaks (Quercus robur L.) to heat stress (HS) and mycorrhization (individually as well in combination) were estimated. One-year-old Q. robur seedlings were grown under controlled conditions in a pot experiment, inoculated with a commercial inoculum of ectomycorrhizal (ECM) fungi, and subjected to 72 h of heat stress (40 °C/30 °C day/night temperature, relative humidity 80%, photoperiod 16/8 h) in a climate chamber, and they were compared with seedlings that were grown at room temperature (RT). An in-depth analysis of certain well-known stress-related metrics such as proline, total phenolics, FRAP, ABTS, non-protein thiols, and lipid peroxidation revealed that mycorrhized oak seedlings were more resistant to heat stress (HS) than non-mycorrhized oaks. Additionally, levels of specific polyamines, total phenolics, flavonoids, and condensed tannins as well as osmotica (proline and glycine betaine) content were measured and compared between four treatments: plants inoculated with ectomycorrhizal fungi exposed to heat stress (ECM-HS) and those grown only at RT (ECM-RT) versus non-mycorrhized controls exposed to heat stress (NM-HS) and those grown only at room temperature (NM-RT). In ectomycorrhiza inoculated oak seedlings, heat stress led to not only a rise in proline, total phenols, FRAP, ABTS, non-protein thiols, and lipid peroxidation but a notable decrease in glycine betaine and flavonoids. Amounts of three main polyamines (putrescine, spermine, and spermidine) were quantified by using high-performance liquid chromatography coupled with fluorescent detection (HPLC/FLD) after derivatization with dansyl-chloride. Heat stress significantly increased putrescine levels in non-mycorrhized oak seedlings but had no effect on spermidine or spermine levels, whereas heat stress significantly increased all inspected polyamine levels in oak seedlings inoculated with ectomycorrhizal inoculum. Spermidine (SPD) and spermine (SPM) contents were significantly higher in ECM-inoculated plants during heat stress (approximately 940 and 630 nmol g^-1 DW, respectively), whereas these compounds were present in smaller amounts in non-mycorrhized oak seedlings (between 510 and 550 nmol g^-1 DW for Spd and between 350 and 450 nmol g^-1 DW for Spm). These findings supported the priming and biofertilizer roles of ectomycorrhizal fungi in the mitigation of heat stress in pedunculate oaks by modification of polyamines, phenolics, and osmotica content.

Lighting Up Agricultural Sustainability in the New Era through Nanozymology: An Overview of Classifications and Their Agricultural Applications

With the concept of sustainable agriculture receiving increasing attention from humankind, nanozymes, nanomaterials with enzyme-like activity but higher environmental endurance and longer-term stability than natural enzymes, have enabled agricultural technologies to be reformative, economic, and portable. Benefiting from their multiple catalytic activities and renewable nanocharacteristics, nanozymes can shine in agricultural scenarios using enzyme engineering and nanoscience, acting as sustainable toolboxes to improve agricultural production and reduce the risk to agricultural systems. Herein, we comprehensively discuss the classifications of nanozymes applied in current agriculture, including peroxidase-like, oxidase-like, catalase-like, superoxide dismutase-like, and laccase-like nanozymes, as well as their biocatalytic mechanisms. Especially, different applications of nanozymes in agriculture are deeply reviewed, covering crop protection and nutrition, agroenvironmental remediation and monitoring, and agroproduct quality monitoring. Finally, the challenges faced by nanozymes in agricultural applications are proposed, and we expect that our review can further enhance agricultural sustainability through nanozymology.

Reduced stem nonstructural carbohydrates caused by plant growth retardant had adverse effects on maize yield under low density

Enhancing maize lodging resistance with plant growth retardants (PGRs) is common in maize production. However, the underlying mechanisms of yield formation as affected by PGRs are still poorly understood. A field experiment contained PGR application (a mixture of ethephon and cycocel, EC) with normal (T1) and double (T2) doses and water control (CK) was conducted at four maize plant densities (4.5, 6.0, 7.5, and 9.0 plants m^-2) in 2020 and 2021. In this two-year study, the grain yield and kernel number per ear (KNE) of EC treatments were reduced by 4.8-9.0% and 3.3-12.2%, respectively, compared with CK under densities of 4.5, 6.0, and 7.5 plants m^-2 without lodging. However, under the density of 9.0 plants m^-2, EC treatments had no pronounced effects on grain yield and yield components. Across all densities, EC significantly decreased the leaf area index (LAI), and the lowest LAI was recorded in T2. The concentrations of nonstructural carbohydrates (NSCs; starch and soluble sugar) in the stem were significantly decreased by 9.9-10.2% in T2 averaged all densities. The sucrose and starch concentrations in grains also declined in the EC treatments. The key enzymes (cell wall acid invertase, sucrose synthase, and adenosine diphosphate pyrophosphorylase) and grain polyamine concentrations showed a slight downward trend under EC treatments compared to CK. NSCs in stems and grains, kernel enzyme activities, and polyamines in grains presented significant positive correlations with KNE. Additionally, structural carbohydrate (SC; including cellulose, hemicellulose, and lignin) concentrations in stems were improved with enhanced lodging resistance by spraying EC. Significant negative relationships were observed between SC with kernel number m^-2 (KNM) and yield, suggesting that improved SC in stems might affect the availability of NSCs for kernel set. Although the lowest kernel weight and KNE were obtained at 9.0 plant m^-2, relatively high LAI still ensured high KNM and high yield. Collectively, EC treatment increased SC in stems, enhanced lodging resistance of maize and reduced NSC availability for kernels, ultimately presenting adverse effects on maize kernel number and yield under relative low density.

Versatile roles of polyamines in improving abiotic stress tolerance of plants

In recent years, extreme environmental cues such as abiotic stresses, including frequent droughts with irregular precipitation, salinity, metal contamination, and temperature fluctuations, have been escalating the damage to plants' optimal productivity worldwide. Therefore, yield maintenance under extreme events needs improvement in multiple mechanisms that can minimize the influence of abiotic stresses. Polyamines (PAs) are pivotally necessary for a defensive purpose under adverse abiotic conditions, but their molecular interplay in this remains speculative. The PAs' accretion is one of the most notable metabolic responses of plants under stress challenges. Recent studies reported the beneficial roles of PAs in plant development, including metabolic and physiological processes, unveiling their potential for inducing tolerance against adverse conditions. This review presents an overview of research about the most illustrious and remarkable achievements in strengthening plant tolerance to drought, salt, and temperature stresses by the exogenous application of PAs. The knowledge of underlying processes associated with stress tolerance and PA signaling pathways was also summarized, focusing on up-to-date evidence regarding the metabolic and physiological role of PAs with exogenous applications that protect plants under unfavorable climatic conditions. Conclusively, the literature proposes that PAs impart an imperative role in abiotic stress tolerance in plants. This implies potentially important feedback on PAs and plants' stress tolerance under unfavorable cues.

The role of ethylene signalling in the regulation of salt stress response in mature tomato fruits: Metabolism of antioxidants and polyamines

Salt stress-induced ethylene (ET) can influence the defence responses of plants that can be dependent on plant organs. In this work, the effects of salt stress evoked by 75 mM NaCl treatment were measured in fruits of wild-type (WT) and ET receptor-mutant Never ripe (Nr) tomato. Salt stress reduced the weight and size of fruits both in WT and Nr, which proved to be more pronounced in mutants. In addition, significantly higher H₂O₂ levels and lipid peroxidation were measured after the salt treatment in Nr as compared to the untreated control than in WT. ET regulated the key antioxidant enzymes, especially ascorbate peroxidase (APX), in WT but in the mutant fruits the activity of APX did not change and the superoxide dismutase and catalase activities were downregulated compared to untreated controls after salt treatment contributing to a higher degree of oxidative stress in Nr fruits. The dependency of PA metabolism on the active ET signalling was investigated for the first time in fruits of Nr mutants under salt stress. 75 mM NaCl enhanced the accumulation of spermine in WT fruits, which was not observed in Nr, but levels of putrescine and spermidine were elevated by salt stress in these tissues. Moreover, the catabolism of PAs was much stronger under high salinity in Nr fruits contributing to higher oxidative stress, which was only partially alleviated by the increased total and reduced ascorbate and glutathione pool. We can conclude that ET-mediated signalling plays a crucial role in the regulation of salt-induced oxidative stress and PA levels in tomato fruits at the mature stage.

Exogenous spermidine modulates polyamine metabolism and improves stress responsive mechanisms to protect tomato seedlings against salt stress

Salt stress negatively affects plant growth, development, and crop productivity causing serious economic loss to agricultural production. Here, we investigated the exogenous application of spermidine (Spd) on tomato seedlings grown under salt stress. Salt stress reduced plant growth, biomass accumulation and chlorophyll contents, thus negatively affecting photosynthesis. Alternatively, Spd application effectively reduced the salinity-induced adverse effects in tomato seedlings by activating the H2O2 mediated signaling involving the enhanced expression of RBOH1 and salt stress-responsive genes SlMYB102, SlHKT1, SlWRKY1 and SlDREB2, and improving detoxification through higher antioxidative activity and osmolyte (proline) accumulation under salt stress. It was further confirmed by significantly lower amount of H2O2, malondialdehyde and electrolyte leakage, and better ion homeostasis (Na+/K+ ratio) and photosynthetic performance of Spd-treated seedlings under salt stress. Furthermore, Spd application modulated endogenous polyamines and enhanced the biosynthesis of endogenous Spd and spermine from putrescine. Altogether, these results confirm the important role of Spd against salt stress and suggest that the increased endogenous Spd content in plants could regulate a number of stress-responsive mechanisms to protect tomato seedlings against salt stress. These results provide a good direction for further elucidation of the detailed interplay between polyamine metabolism and H2O2-mediated signaling, which would help to improve abiotic stress tolerance in plants.

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

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

Cold adaptation strategies in plants—An emerging role of epigenetics and antifreeze proteins to engineer cold resilient plants

Cold stress adversely affects plant growth, development, and yield. Also, the spatial and geographical distribution of plant species is influenced by low temperatures. Cold stress includes chilling and/or freezing temperatures, which trigger entirely different plant responses. Freezing tolerance is acquired via the cold acclimation process, which involves prior exposure to non-lethal low temperatures followed by profound alterations in cell membrane rigidity, transcriptome, compatible solutes, pigments and cold-responsive proteins such as antifreeze proteins. Moreover, epigenetic mechanisms such as DNA methylation, histone modifications, chromatin dynamics and small non-coding RNAs play a crucial role in cold stress adaptation. Here, we provide a recent update on cold-induced signaling and regulatory mechanisms. Emphasis is given to the role of epigenetic mechanisms and antifreeze proteins in imparting cold stress tolerance in plants. Lastly, we discuss genetic manipulation strategies to improve cold tolerance and develop cold-resistant plants.

Concentration-dependent effects of effusol and juncusol from Juncus compressus on seedling development of Arabidopsis thaliana

Juncus species are valuable sources of phenanthrene compounds that have been used in traditional Chinese medicine for thousands of years. Effusol and juncusol are the most investigated compounds reported to have antimicrobial and anticancer effects; however, to date, their effects on higher plants have not been investigated. In this study, we examined the effects of effusol and juncusol on the growth and other biochemical parameters of the dicot model plant Arabidopsis thaliana in a concentration-dependent manner with a focus on polyamine metabolism. Phenanthrene induced toxic effects on plant growth and development, while effusol and juncusol induced higher biomass and maintained antioxidant defence mechanisms associated with reduced polyamine degradation. Taken together, our results suggest that these compounds could be good candidates for new biopesticide or biostimulant plant growth regulators in the future.

Improvement of Drought Tolerance by Exogenous Spermidine in Germinating Wheat (Triticum aestivum L.) Plants Is Accompanied with Changes in Metabolite Composition

Drought is one of the most important environmental factors reducing the yield and production of crops, including wheat. Polyamines are closely associated with plant stress tolerance. The present study investigated the mechanisms through seed germination with spermidine protecting wheat varieties from drought stress. In the first experiment, the effects of spermidine on the germination of wheat varieties, namely Rakhshan, Mihan, Sirvan and Pishgam, were investigated in three drought levels, namely 0, −2, and −4 MPa induced by polyethylene glycol 6000. Analysis of variance indicated that spermidine, drought stress and interaction between varieties and drought stress were significant for all traits, and with severity of stress, all traits significantly decreased. In the second experiment, detailed gene expression and non-targeted metabolomics analyses were carried out using the Rakhshan and Mihan varieties after germination, with or without spermidine treatment and/or drought stress. According to the biomass parameters, the Mihan variety showed relatively better growth compared to the other variety, but the Rakhshan one showed more pronounced responses at gene expression level to exogenous spermidine than the Mihan variety. Overall, these results showed that spermidine increased the drought tolerance of wheat at the germination stage, due to specific role of polyamine metabolism in the development of effective responses under drought stress.

Comprehensive Analysis of Differentially Expressed Genes and Epigenetic Modification-Related Expression Variation Induced by Saline Stress at Seedling Stage in Fiber and Oil Flax, Linum usitatissimum L

The ability of different germplasm to adapt to a saline-alkali environment is critical to learning about the tolerance mechanism of saline-alkali stress in plants. Flax is an important oil and fiber crop in many countries. However, its molecular tolerance mechanism under saline stress is still not clear. In this study, we studied morphological, physiological characteristics, and gene expression variation in the root and leaf in oil and fiber flax types under saline stress, respectively. Abundant differentially expressed genes (DEGs) induced by saline stress, tissue/organ specificity, and different genotypes involved in plant hormones synthesis and metabolism and transcription factors and epigenetic modifications were detected. The present report provides useful information about the mechanism of flax response to saline stress and could lead to the future elucidation of the specific functions of these genes and help to breed suitable flax varieties for saline/alkaline soil conditions.

The humidity level matters during the desiccation of Norway spruce somatic embryos

In Norway spruce, as in many other conifers, the germination capacity of somatic embryos is strongly influenced by the desiccation phase inserted after maturation. The intensity of drying during desiccation eminently affected the formation of emblings (i.e., seedlings developed from somatic embryos). Compared to non-desiccated embryos, the germination capacity of embryos desiccated at 100% relative humidity was about three times higher, but the reduction of relative humidity to 95 and 90% had a negative effect on the subsequent embryo development. The water loss observed in these embryos did not lead to an increase in lipid peroxidation, as shown by malondialdehyde levels. Another metabolic pathway in plants that mediates a response to abiotic stresses is directed toward the biosynthesis of polyamines (PAs). The activities of PA biosynthetic enzymes increased steadily in embryos during desiccation at 100% relative humidity, whereas they decreased at lower humidity. The total content of free PAs in the embryos gradually decreased throughout desiccation. The increase in free putrescine (Put) and perchloric acid-insoluble Put conjugates was observed in embryos desiccated at lower humidity. These changes were accompanied to some extent by the transcription of the genes for the PA biosynthesis enzymes. Desiccation at 100% relative humidity increased the activity of the cell wall-modifying enzymes β-1,3-glucanases and chitinases; the activities of these enzymes were also significantly suppressed at reduced humidity. The same pattern was observed in the transcription of some β-1,3-glucanase and chitinase genes. Desiccation treatments triggered metabolic processes that responded to water availability, suggesting an active response of the embryo to the reduction in humidity. A positive effect was demonstrated only for desiccation at high relative humidity. Some of the physiological characteristics described can be used as markers of inappropriate relative humidity during somatic embryo desiccation.