An NADPH-Oxidase/Polyamine Oxidase Feedback Loop Controls Oxidative Burst Under Salinity

HB52-PUT2 Module-Mediated Polyamine Shoot-to-Root Movement Regulates Salt Stress Tolerance in Tomato

Soil salinity severely restricts crop quality and yields. Plants have developed various strategies to alleviate salinity stress's negative effects, including polyamine redistribution by polyamine uptake transporters (PUTs). However, the mechanisms by which PUTs alter polyamine translocation processes during salt stress have not been fully elucidated. Here, we show that disruption of PUT2, which is involved in polyamine shoot-to-root transport, results in salt sensitivity phenotypes in tomato. Moreover, yeast one-hybrid screened for an HD-Zip transcription factor HB52 that interacts with PUT2, and loss of function of HB52 also led to increased sensitivity to salt stress, whereas HB52-overexpression lines exhibited improved salt tolerance. Furthermore, molecular analyses demonstrated that HB52 directly activated the expression of PUT2 and facilitated Na+ efflux by promoting polyamine shoot-to-root mobility. This study uncovers a synergistic transcriptional regulatory network associated with a homeobox protein regulator that promotes polyamine long-distance transport under salt stress.

Stimulatory Effect of Delonix regia Flower Extract in Protecting Syzygium cumini Seedlings from Salinity

Syzygium cumini (L.) Skeels (jamun) is an ornamental tree species that is sensitive to salinity. Salinity stress is a major challenge, particularly in regions with saline irrigation water. In the present study, the ameliorative potential of foliar application of an aqueous extract of Delonix regia (Poinciana) flowers (PFE) to saline water-irrigated jamun seedlings was investigated over a period of two years. PFE was effective in mitigating the harmful effects of salinity on plant growth, physiology, and biochemistry. Salinity-induced reductions in plant height, leaf area, and biomass which were significantly alleviated by PFE foliar application. The extract also enhanced antioxidant activity, as indicated by increased ferric reducing antioxidant potential (FRAP) and phenolic content, while also reducing hydrogen peroxide (H2O2) levels and membrane damage as indicated by the accumulation of malondialdehyde (MDA). Additionally, the foliar application of PFE promoted the accumulation of free proline, an essential osmo-protectant, further enhancing the plant's resilience to salinity stress. These findings highlight the potential of PFE as an eco-friendly bio-stimulant to improve salinity tolerance in jamun and pave the way for sustainable salinity management strategies in other crops as well.

Vernicia fordii leaf extract inhibited anthracnose growth by downregulating reactive oxygen species (ROS) levels in vitro and in vivo

Background

Colletotrichum fructicola is a predominant anthracnose species in Camellia oleifera, causing various adverse effects. Traditional intercropping Vernicia fordii with C. oleifera may enhance anthracnose resistance, but the mechanism remains elusive.

Methods

We utilized UPLC-MS/MS and acid-base detection to identify the major antimicrobial alkaloid components in the V. fordii leaf extract. Subsequently, by adding different concentrations of V. fordii leaf extract for cultivating C. fructicola, with untreated C. fructicola as a control, we investigated the impact of the V. fordii leaf extract, cell wall integrity, cell membrane permeability, MDA, and ROS content changes. Additionally, analysis of key pathogenic genes of C. fructicola confirmed that the V. fordii leaf extract inhibits the growth of the fungus through gene regulation.

Results

This study discovered the alkaloid composition of V. fordii leaf extract by UPLC-MS/MS and acid-base detection, such as trigonelline, stachydrine, betaine, and O-Phosphocholine. V. fordii leaf extract successfully penetrated C. fructicola mycelia, damaged cellular integrity, and increased ROS and MDA levels by 1.75 and 2.05 times respectively, thereby inhibiting C. fructicola proliferation. By analyzing the key pathogenic genes of C. fructicola, it was demonstrated that the antifungal function of V. fordii leaf extract depends mainly on the regulation of RAB7 and HAC1 gene expression. Therefore, this study elucidates the mechanism of V. fordii -C. oleifera intercropping in strengthening anthracnose resistance in C. oleifera, contributing to efficient C. oleifera cultivation.

Transcriptomics reveal the involvement of reactive oxygen species production and sequestration during stigma development and pollination in Fraxinus mandshurica

Stigma development and successful pollination are essential for the continuous existence of flowering plants. However, the specific mechanisms regulating these important processes are not well understood. In this study, we investigated the development of the stigma in Fraxinus mandshurica, dividing it into three stages: S1, S2, and S3. Transcriptome data were used to analyze the gene expression patterns across these developmental stages, and 6,402 genes were observed to exhibit variable expression levels. Our analysis revealed a significant enrichment of pathways related to reactive oxygen species (ROS) and flavonoids, as indicated by the Kyoto Encyclopedia of Genes and Genomes enrichment analysis of the differentially expressed genes. Further examination by cluster analysis and quantitative polymerase chain reaction revealed that 58 genes were associated with ROS synthesis and seven genes were linked to flavonoid synthesis during the S2 and S3 stages. ROS accumulated during stigma development, which decreased rapidly upon pollen germination and pollen tube elongation, as confirmed by H2DCFDA staining. Moreover, ROS levels in mature stigmas were reduced by treatment with ROS scavengers, such as copper (II) chloride, sodium salicylate, and diphenyleneiodonium, an inhibitor of NADPH oxidases, which enhanced pollen adhesion and germination. These findings suggest that the balance between ROS production and sequestration plays a critical role in regulating stigma development and pollen germination in Fraxinus mandshurica.

RtNAC055 promotes drought tolerance via a stomatal closure pathway linked to methyl jasmonate/hydrogen peroxide signaling in Reaumuria trigyna

The stomata regulate CO2 uptake and efficient water usage, thereby promoting drought stress tolerance. NAC proteins (NAM, ATAF1/2, and CUC2) participate in plant reactions following drought stress, but the molecular mechanisms underlying NAC-mediated regulation of stomatal movement are unclear. In this study, a novel NAC gene from Reaumuria trigyna, RtNAC055, was found to enhance drought tolerance via a stomatal closure pathway. It was regulated by RtMYC2 and integrated with jasmonic acid signaling and was predominantly expressed in stomata and root. The suppression of RtNAC055 could improve jasmonic acid and H2O2 production and increase the drought tolerance of transgenic R. trigyna callus. Ectopic expression of RtNAC055 in the Arabidopsis atnac055 mutant rescued its drought-sensitive phenotype by decreasing stomatal aperture. Under drought stress, overexpression of RtNAC055 in poplar promoted ROS (H2O2) accumulation in stomata, which accelerated stomatal closure and maintained a high photosynthetic rate. Drought upregulated the expression of PtRbohD/F, PtP5CS2, and PtDREB1.1, as well as antioxidant enzyme activities in heterologous expression poplars. RtNAC055 promoted H2O2 production in guard cells by directly binding to the promoter of RtRbohE, thus regulating stomatal closure. The stress-related genes RtDREB1.1/P5CS1 were directly regulated by RtNAC055. These results indicate that RtNAC055 regulates stomatal closure by maintaining the balance between the antioxidant system and H2O2 level, reducing the transpiration rate and water loss, and improving photosynthetic efficiency and drought resistance.

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

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

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

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

24-Epibrassinolide Facilitates Adventitious Root Formation by Coordinating Cell-Wall Polyamine Oxidase- and Plasma Membrane Respiratory Burst Oxidase Homologue-Derived Reactive Oxygen Species in Capsicum annuum L

Adventitious root (AR) formation is a critical process in cutting propagation of horticultural plants. Brassinosteroids (BRs) have been shown to regulate AR formation in several plant species; however, little is known about their exact effects on pepper AR formation, and the downstream signaling of BRs also remains elusive. In this study, we showed that treatment of 24-Epibrassinolide (EBL, an active BR) at the concentrations of 20-100 nM promoted AR formation in pepper (Capsicum annuum). Furthermore, we investigated the roles of apoplastic reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂) and superoxide radical (O₂^•-), in EBL-promoted AR formation, by using physiological, histochemical, bioinformatic, and biochemical approaches. EBL promoted AR formation by modulating cell-wall-located polyamine oxidase (PAO)-dependent H₂O₂ production and respiratory burst oxidase homologue (RBOH)-dependent O₂^•- production, respectively. Screening of CaPAO and CaRBOH gene families combined with gene expression analysis suggested that EBL-promoted AR formation correlated with the upregulation of CaPAO1, CaRBOH2, CaRBOH5, and CaRBOH6 in the AR zone. Transient expression analysis confirmed that CaPAO1 was able to produce H₂O₂, and CaRBOH2, CaRBOH5, and CaRBOH6 were capable of producing O₂^•-. The silencing of CaPAO1, CaRBOH2, CaRBOH5, and CaRBOH6 in pepper decreased the ROS accumulation and abolished the EBL-induced AR formation. Overall, these results uncover one of the regulatory pathways for BR-regulated AR formation, and extend our knowledge of the functions of BRs and of the BRs-ROS crosstalk in plant development.

Genome-Wide Identification and Characterization of the Polyamine Uptake Transporter (Put) Gene Family in Tomatoes and the Role of Put2 in Response to Salt Stress

The polyamine uptake transporter (Put), an important polyamines-related protein, is involved in plant cell growth, developmental processes, and abiotic stimuli, but no research on the Put family has been carried out in the tomato. Herein, eight tomato Put were identified and scattered across four chromosomes, which were classified into three primary groups by phylogenetic analysis. Protein domains and gene structural organization also showed a significant degree of similarity, and the Put genes were significantly induced by various hormones and polyamines. Tissue-specific expression analysis indicated that Put genes were expressed in all tissues of the tomato. The majority of Put genes were induced by different abiotic stresses. Furthermore, Put2 transcription was found to be responsive to salt stress, and overexpression of Put2 in yeast conferred salinity tolerance and polyamine uptake. Moreover, overexpression of Put2 in tomatoes promoted salinity tolerance accompanied by a decrease in the Na⁺/K⁺ ratio, restricting the generation of reactive oxygen and increasing polyamine metabolism and catabolism, antioxidant enzyme activity (SOD, CAT, APX, and POD), and nonenzymatic antioxidant activity (GSH/GSSG and ASA/DHA ratios, GABA, and flavonoid content); loss of function of put2 produced opposite effects. These findings highlight that Put2 plays a pivotal role in mediating polyamine synthesis and catabolism, and the antioxidant capacity in tomatoes, providing a valuable gene for salinity tolerance in plants.

Distinct role of AtCuAOβ- and RBOHD-driven H2O2 production in wound-induced local and systemic leaf-to-leaf and root-to-leaf stomatal closure

Polyamines (PAs) are ubiquitous low-molecular-weight aliphatic compounds present in all living organisms and essential for cell growth and differentiation. The developmentally regulated and stress-induced copper amine oxidases (CuAOs) oxidize PAs to aminoaldehydes producing hydrogen peroxide (H₂O₂) and ammonia. The Arabidopsis thaliana CuAOβ (AtCuAOβ) was previously reported to be involved in stomatal closure and early root protoxylem differentiation induced by the wound-signal MeJA via apoplastic H₂O₂ production, suggesting a role of this enzyme in water balance, by modulating xylem-dependent water supply and stomata-dependent water loss under stress conditions. Furthermore, AtCuAOβ has been shown to mediate early differentiation of root protoxylem induced by leaf wounding, which suggests a whole-plant systemic coordination of water supply and loss through stress-induced stomatal responses and root protoxylem phenotypic plasticity. Among apoplastic ROS generators, the D isoform of the respiratory burst oxidase homolog (RBOH) has been shown to be involved in stress-mediated modulation of stomatal closure as well. In the present study, the specific role of AtCuAOβ and RBOHD in local and systemic perception of leaf and root wounding that triggers stomatal closure was investigated at both injury and distal sites exploiting Atcuaoβ and rbohd insertional mutants. Data evidenced that AtCuAOβ-driven H₂O₂ production mediates both local and systemic leaf-to-leaf and root-to-leaf responses in relation to stomatal movement, Atcuaoβ mutants being completely unresponsive to leaf or root wounding. Instead, RBOHD-driven ROS production contributes only to systemic leaf-to-leaf and root-to-leaf stomatal closure, with rbohd mutants showing partial unresponsiveness in distal, but not local, responses. Overall, data herein reported allow us to hypothesize that RBOHD may act downstream of and cooperate with AtCuAOβ in inducing the oxidative burst that leads to systemic wound-triggered stomatal closure.

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.

Review of the Mechanisms by Which Transcription Factors and Exogenous Substances Regulate ROS Metabolism under Abiotic Stress

Reactive oxygen species (ROS) are signaling molecules that regulate many biological processes in plants. However, excess ROS induced by biotic and abiotic stresses can destroy biological macromolecules and cause oxidative damage to plants. As the global environment continues to deteriorate, plants inevitably experience abiotic stress. Therefore, in-depth exploration of ROS metabolism and an improved understanding of its regulatory mechanisms are of great importance for regulating cultivated plant growth and developing cultivars that are resilient to abiotic stresses. This review presents current research on the generation and scavenging of ROS in plants and summarizes recent progress in elucidating transcription factor-mediated regulation of ROS metabolism. Most importantly, the effects of applying exogenous substances on ROS metabolism and the potential regulatory mechanisms at play under abiotic stress are summarized. Given the important role of ROS in plants and other organisms, our findings provide insights for optimizing cultivation patterns and for improving plant stress tolerance and growth regulation.

CsPAO2 Improves Salt Tolerance of Cucumber through the Interaction with CsPSA3 by Affecting Photosynthesis and Polyamine Conversion

Polyamine oxidases (PAOs) are key enzymes in polyamine metabolism and are related to the tolerance of plants to abiotic stresses. In this study, overexpression of cucumber (Cucumis sativus L.) PAO2 (CsPAO2) in Arabidopsis resulted in increased activity of the antioxidant enzyme and accelerated conversion from Put to Spd and Spm, while malondialdehyde content (MDA) and electrolyte leakage (EL) was decreased when compared with wild type, leading to enhanced plant growth under salt stress. Photosystem Ⅰ assembly 3 in cucumber (CsPSA3) was revealed as an interacting protein of CsPAO2 by screening yeast two-hybrid library combined with in vitro and in vivo methods. Then, CsPAO2 and CsPSA3 were silenced in cucumber via virus-mediated gene silencing (VIGS) with pV190 as the empty vector. Under salt stress, net photosynthetic rate (Pn) and transpiration rate (Tr) of CsPAO2-silencing plants were lower than pV190-silencing plants, and EL in root was higher than pV190-silencing plants, indicating that CsPAO2-silencing plants suffered more serious salt stress damage. However, photosynthetic parameters of CsPSA3-silencing plants were all higher than those of CsPAO2 and pV190-silencing plants, thereby enhancing the photosynthesis process. Moreover, CsPSA3 silencing reduced the EL in both leaves and roots when compared with CsPAO2-silencing plants, but the EL only in leaves was significantly lower than the other two gene-silencing plants, and conversion from Put to Spd and Spm in leaf was also promoted, suggesting that CsPSA3 interacts with CsPAO2 in leaves to participate in the regulation of salt tolerance through photosynthesis and polyamine conversion.

Role of Plasma Membrane NADPH Oxidase in Response to Salt Stress in Cucumber Seedlings

Plasma membrane NADPH oxidases (RBOHs, EC 1.6.3.1) are known as the main ROS generators involved in plant adaptation to stress conditions. In the present work, regulation of NADPH oxidase was analyzed in cucumber (Cucumis sativus L. var. Krak) seedlings exposed to salinity. RBOH activity and gene expression, as well as H₂O₂ content, were determined in the roots of plants treated with 50 or 100 mM NaCl for 1 h, and 50 mM NaCl for 1 or 6 days. It was found that enzyme activity increased in parallel with an enhancement in the H₂O₂ level in roots exposed to 100 mM NaCl for 1 h, and to 50 mM NaCl for 1 day. The expression of some CsRboh genes was induced by salt. Moreover, an increase in the activity of G6PDH, providing the substrate for the NADPH oxidase, was observed. In seedlings subjected to salinity for a longer time, antioxidant enzymes-including superoxide dismutase, catalase, and ascorbate peroxidase-were activated, participating in maintaining a steady-state H₂O₂ content in the root cells. In conclusion, NADPH oxidase and endogenous H₂O₂ up-regulation seem to be early events in cucumber response to salinity.

Helicobacter pylori promotes gastric cancer progression through the tumor microenvironment

Gastric cancer (GC) is a leading type of cancer. Although immunotherapy has yielded important recent progress in the treatment of GC, the prognosis remains poor due to drug resistance and frequent recurrence and metastasis. There are multiple known risk factors for GC, and infection with Helicobacter pylori is one of the most significant. The mechanisms underlying the associations of H. pylori and GC remain unclear, but it is well known that infection can alter the tumor microenvironment (TME). The TME and the tumor itself constitute a complete ecosystem, and the TME plays critical roles in tumor progression, metastasis, and drug resistance. H. pylori infection can act synergistically with the TME to cause DNA damage and abnormal expression of multiple genes and activation of signaling pathways. It also modulates the host immune system in ways that enhance the proliferation and metastasis of tumor cells, promote epithelial-mesenchymal transition, inhibit apoptosis, and provide energy support for tumor growth. This review elaborates myriad ways that H. pylori infections promote the occurrence and progression of GC by influencing the TME, providing new directions for immunotherapy treatments for this important disease. KEY POINTS: • H. pylori infections cause DNA damage and affect the repair of the TME to DNA damage. • H. pylori infections regulate oncogenes or activate the oncogenic signaling pathways. • H. pylori infections modulate the immune system within the TME.

The Defensive Role of Endogenous H2S in Brassica rapa against Mercury-Selenium Combined Stress

Plants are always exposed to the environment, polluted by multiple trace elements. Hydrogen sulfide (H₂S), an endogenous gaseous transmitter in plant cells, can help plant combat single elements with excess concentration. Until now, little has been known about the regulatory role of H₂S in response to combined stress of multiple elements. Here we found that combined exposure of mercury (Hg) and selenium (Se) triggered endogenous H₂S signal in the roots of Brasscia rapa. However, neither Hg nor Se alone worked on it. In roots upon Hg + Se exposure, the defensive role of endogenous H₂S was associated to the decrease in reactive oxygen species (ROS) level, followed by alleviating cell death and recovering root growth. Such findings extend our knowledge of plant H₂S in response to multiple stress conditions.

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

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

Molecular Dissection of the Gene OsGA2ox8 Conferring Osmotic Stress Tolerance in Rice

Gibberellin 2-oxidase (GA2ox) plays an important role in the GA catabolic pathway and the molecular function of the OsGA2ox genes in plant abiotic stress tolerance remains largely unknown. In this study, we functionally characterized the rice gibberellin 2-oxidase 8 (OsGA2ox8) gene. The OsGA2ox8 protein was localized in the nucleus, cell membrane, and cytoplasm, and was induced in response to various abiotic stresses and phytohormones. The overexpression of OsGA2ox8 significantly enhanced the osmotic stress tolerance of transgenic rice plants by increasing the number of osmotic regulators and antioxidants. OsGA2ox8 was differentially expressed in the shoots and roots to cope with osmotic stress. The plants overexpressing OsGA2ox8 showed reduced lengths of shoots and roots at the seedling stage, but no difference in plant height at the heading stage was observed, which may be due to the interaction of OsGA2ox8 and OsGA20ox1, implying a complex feedback regulation between GA biosynthesis and metabolism in rice. Importantly, OsGA2ox8 was able to indirectly regulate several genes associated with the anthocyanin and flavonoid biosynthetic pathway and the jasmonic acid (JA) and abscisic acid (ABA) biosynthetic pathway, and overexpression of OsGA2ox8 activated JA signal transduction by inhibiting the expression of jasmonate ZIM domain-containing proteins. These results provide a basis for a future understanding of the networks and respective phenotypic effects associated with OsGA2ox8.

Reactive oxygen species and organellar signaling

The evolution of photosynthesis and its associated metabolic pathways has been crucial to the successful establishment of plants, but has also challenged plant cells in the form of production of reactive oxygen species (ROS). Intriguingly, multiple forms of ROS are generated in virtually every plant cell compartment through diverse pathways. As a result, a sophisticated network of ROS detoxification and signaling that is simultaneously tailored to individual organelles and safeguards the entire cell is necessary. Here we take an organelle-centric view on the principal sources and sinks of ROS across the plant cell and provide insights into the ROS-induced organelle to nucleus retrograde signaling pathways needed for operational readjustments during environmental stresses.

Impact of Ascorbate-Glutathione Cycle Components on the Effectiveness of Embryogenesis Induction in Isolated Microspore Cultures of Barley and Triticale

Enhanced antioxidant defence plays an essential role in plant survival under stress conditions. However, excessive antioxidant activity sometimes suppresses the signal necessary for the initiation of the desired biological reactions. One such example is microspore embryogenesis (ME)-a process of embryo-like structure formation triggered by stress in immature male gametophytes. The study focused on the role of reactive oxygen species and antioxidant defence in triticale (×Triticosecale Wittm.) and barley (Hordeum vulgare L.) microspore reprogramming. ME was induced through various stress treatments of tillers and its effectiveness was analysed in terms of ascorbate and glutathione contents, total activity of low molecular weight antioxidants and activities of glutathione-ascorbate cycle enzymes. The most effective treatment for both species was a combination of low temperature and exogenous application of 0.3 M mannitol, with or without 0.3 mM reduced glutathione. The applied treatments induced genotype-specific defence responses. In triticale, both ascorbate and glutathione were associated with ME induction, though the role of glutathione did not seem to be related to its function as a reducing agent. In barley, effective ME was accompanied by an accumulation of ascorbate and high activity of enzymes regulating its redox status, without direct relation to glutathione content.

Ectopic Expression of GhSAMDC3 Enhanced Salt Tolerance Due to Accumulated Spd Content and Activation of Salt Tolerance-Related Genes in Arabidopsis thaliana

Polyamines (PAs), especially spermidine and spermine (which are involved in various types of abiotic stress tolerance), have been reported in many plant species. In this study, we identified 14 putative S-adenosylmethionine decarboxylase genes (GhSAMDC1-14) in upland cotton. Based on phylogenetic and expression analyses conducted under different abiotic stresses, we selected and transferred GhSAMDC3 into Arabidopsis thaliana. Compared to the wild type, transgenic plants displayed rapid growth and increases in average leaf area and leaf number of 52% and 36%, respectively. In transgenic plants, the germination vigor and rate were markedly enhanced under NaCl treatment, and the plant survival rate increased by 50% under 300 mM NaCl treatment. The spermidine content was significantly increased, possibly due to the synthesis of a series of PAs and oxidant and antioxidant genes, resulting in improved salinity tolerance in Arabidopsis. Various salinity resistance-related genes were upregulated in transgenic plants. Together, these results indicate that ectopic expression of GhSAMDC3 raised salinity tolerance by the accumulation of spermidine and activation of salinity tolerance-related genes in A. thaliana.

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.

Morphological, Physiological and Molecular Markers for Salt-Stressed Plants

Plant growth and development is adversely affected by different kind of stresses. One of the major abiotic stresses, salinity, causes complex changes in plants by influencing the interactions of genes. The modulated genetic regulation perturbs metabolic balance, which may alter plant's physiology and eventually causing yield losses. To improve agricultural output, researchers have concentrated on identification, characterization and selection of salt tolerant varieties and genotypes, although, most of these varieties are less adopted for commercial production. Nowadays, phenotyping plants through Machine learning (deep learning) approaches that analyze the images of plant leaves to predict biotic and abiotic damage on plant leaves have increased. Here, we review salinity stress related markers on molecular, physiological and morphological levels for crops such as maize, rice, ryegrass, tomato, salicornia, wheat and model plant, Arabidopsis. The combined analysis of data from stress markers on different levels together with image data are important for understanding the impact of salt stress on plants.

Comparative Transcriptomics and Metabolomics Reveal an Intricate Priming Mechanism Involved in PGPR-Mediated Salt Tolerance in Tomato

Plant-associated beneficial strains inhabiting plants grown under harsh ecosystems can help them cope with abiotic stress factors by positively influencing plant physiology, development, and environmental adaptation. Previously, we isolated a potential plant growth promoting strain (AXSa06) identified as Pseudomonas oryzihabitans, possessing 1-aminocyclopropane-1-carboxylate deaminase activity, producing indole-3-acetic acid and siderophores, as well as solubilizing inorganic phosphorus. In this study, we aimed to further evaluate the effects of AXSa06 seed inoculation on the growth of tomato seedlings under excess salt (200 mM NaCl) by deciphering their transcriptomic and metabolomic profiles. Differences in transcript levels and metabolites following AXSa06 inoculation seem likely to have contributed to the observed difference in salt adaptation of inoculated plants. In particular, inoculations exerted a positive effect on plant growth and photosynthetic parameters, imposing plants to a primed state, at which they were able to respond more robustly to salt stress probably by efficiently activating antioxidant metabolism, by dampening stress signals, by detoxifying Na⁺, as well as by effectively assimilating carbon and nitrogen. The primed state of AXSa06-inoculated plants is supported by the increased leaf lipid peroxidation, ascorbate content, as well as the enhanced activities of antioxidant enzymes, prior to stress treatment. The identified signatory molecules of AXSa06-mediated salt tolerance included the amino acids aspartate, threonine, serine, and glutamate, as well as key genes related to ethylene or abscisic acid homeostasis and perception, and ion antiporters. Our findings represent a promising sustainable solution to improve agricultural production under the forthcoming climate change conditions.

Methyl salicylate is the most effective natural salicylic acid ester to close stomata while raising reactive oxygen species and nitric oxide in Arabidopsis guard cells

Modulation by salicylic acid (SA) and its six esters of stomatal closure was evaluated in Arabidopsis thaliana. The seven compounds tested are salicylic acid (SA), acetylsalicylate (ASA), methyl salicylate (MeSA), propyl salicylate (PrSA), amyl salicylate, benzyl salicylate, and salicin. Among these, MeSA was the most effective to induce stomatal closure, followed by salicin and SA, while ASA was the least effective. Since SA, ASA, and MeSA could modulate plant function, the effects of these three compounds on the levels of reactive oxygen species (ROS) or nitric oxide (NO) in guard cells were studied. MeSA and SA raised the content of ROS or NO in as with ABA. The extent of ROS/NO production in response to ASA was the lowest. Reversal by cPTIO or catalase of stomatal closure by MeSA indicated the essentiality of NO and ROS for stomatal closure. Further studies revealed peroxidase as the ROS source during stomatal closure by MeSA, unlike the dominant role of NADPH oxidase in ROS production induced by ABA. The rise in NO production by ABA or MeSA was dependent on nitrate reductase and NO synthase-like enzyme. Given its most effective nature, MeSA can be an excellent tool to examine the signaling components in guard cells and other plant tissues. The ability of MeSA to induce stomatal closure is physiologically relevant because of its volatile nature, stability, and systemic action.

Antisense-mediated S-adenosyl-L-methionine decarboxylase silencing affects heat stress responses of tobacco plants

Understanding the molecular mode(s) of plant tolerance to heat stress (HS) is crucial since HS is a potential threat to sustainable agriculture and global crop production. Polyamines (PAs) seem to exert multifaceted effects in plant growth and development and responses to abiotic and biotic stresses, presumably via their homeostasis, chemical interactions and contribution to hydrogen peroxide (H2O2) cellular 'signatures'. Downregulation of the apoplastic POLYAMINE OXIDASE (PAO) gene improved thermotolerance in tobacco (Nicotiana tabacum L.) transgenics. However, in the present work we show that transgenic tobacco plants with antisense-mediated S-ADENOSYL-L-METHIONINE DECARBOXYLASE silencing (AS-NtSAMDC) exhibited enhanced sensitivity and delayed responses to HS which was accompanied by profound injury upon HS removal (recovery), as assessed by phenological, physiological and biochemical characteristics. In particular, the AS-NtSAMDC transgenics exhibited significantly reduced rate of photosynthesis, as well as enzymatic and non-enzymatic antioxidants. These transgenics suffered irreversible damage, which significantly reduced their growth potential upon return to normal conditions. These data reinforce the contribution of increased PA homeostasis to tolerance, and can move forward our understanding on the PA-mediated mechanism(s) conferring tolerance to HS that might be targeted via traditional or biotechnological breeding for developing HS tolerant plants.

Putrescine metabolism modulates the biphasic effects of brassinosteroids on canola and Arabidopsis salt tolerance

Brassinosteroids (BRs) are known to improve salt tolerance of plants, but not in all situations. Here, we show that a certain concentration of 24-epibrassinolide (EBL), an active BR, can promote the tolerance of canola under high-salt stress, but the same concentration is disadvantageous under low-salt stress. We define this phenomenon as hormonal stress-level-dependent biphasic (SLDB) effects. The SLDB effects of EBL on salt tolerance in canola are closely related to H₂ O₂ accumulation, which is regulated by polyamine metabolism, especially putrescine (Put) oxidation. The inhibition of EBL on canola under low-salt stress can be ameliorated by repressing Put biosynthesis or diamine oxidase activity to reduce H₂ O₂ production. Genetic and phenotypic results of bri1-9, bak1, bes1-D, and bzr1-1D mutants and overexpression lines of BRI1 and BAK1 in Arabidopsis indicate that a proper enhancement of BR signaling benefits plants in countering salt stress, whereas excessive enhancement is just as harmful as a deficiency. These results highlight the involvement of crosstalk between BR signaling and Put metabolism in H₂ O₂ accumulation, which underlies the dual role of BR in plant salt tolerance.

Functional switching of NPR1 between chloroplast and nucleus for adaptive response to salt stress

Salt stress causes rapid accumulation of nonexpressor of pathogenesis-related genes 1 (NPR1) protein, known as the redox-sensitive transcription coactivator, which in turn elicits many adaptive responses. The NPR1 protein transiently accumulates in chloroplast stroma under salt stress, which attenuates stress-triggered down-regulation of photosynthetic capability. We observed that oligomeric NPR1 in chloroplasts and cytoplasm had chaperone activity, whereas monomeric NPR1 in the nucleus did not. Additionally, NPR1 overexpression resulted in reinforcement of morning-phased and evening-phased circadian clock. NPR1 overexpression also enhanced antioxidant activity and reduced stress-induced reactive oxygen species (ROS) generation at early stage, followed with transcription levels for ROS detoxification. These results suggest a functional switch from a molecular chaperone to a transcriptional coactivator, which is dependent on subcellular localization. Our findings imply that dual localization of NPR1 is related to proteostasis and redox homeostasis in chloroplasts for emergency restoration as well as transcriptional coactivator in the nucleus for adaptation to stress.

Enzymes contributing to the hydrogen peroxide signal dynamics that regulate wall labyrinth formation in transfer cells

Transfer cells are characterized by an amplified plasma membrane area supported on a wall labyrinth composed of a uniform wall layer (UWL) from which wall ingrowth (WI) papillae arise. Adaxial epidermal cells of developing Vicia faba cotyledons, when placed in culture, undergo a rapid (hours) trans-differentiation to a functional epidermal transfer cell (ETC) phenotype. The trans-differentiation event is controlled by a signalling cascade comprising auxin, ethylene, apoplasmic reactive oxygen species (apoROS), and cytosolic Ca2+. Apoplasmic hydrogen peroxide (apoH2O2) was confirmed as the apoROS regulating UWL and WI papillae formation. Informed by an ETC-specific transcriptome, a pharmacological approach identified a temporally changing cohort of H2O2 biosynthetic enzymes. The cohort contained a respiratory burst oxidase homologue, polyamine oxidase, copper amine oxidase, and a suite of class III peroxidases. Collectively these generated two consecutive bursts in apoH2O2 production. Spatial organization of biosynthetic/catabolic enzymes was deduced from responses to pharmacologically blocking their activities on the cellular and subcellular distribution of apoH2O2. The findings were consistent with catalase activity constraining the apoH2O2 signal to the outer periclinal wall of the ETCs. Strategic positioning of class III peroxidases in this outer domain shaped subcellular apoH2O2 signatures that differed during assembly of the UWL and WI papillae.

Polyamines treatment during pollen germination and pollen tube elongation in tobacco modulate reactive oxygen species and nitric oxide homeostasis

Several signaling pathways have been shown to be involved in the regulation of pollen germination and pollen tube elongation. Among others, exogenously applied polyamines were found to strongly affect pollen maturation, pollen tube emergence and elongation. In this study, our aim was to investigate the regulatory relation among exogenous polyamines, and endogenous reactive oxygen species and nitric oxide under pollen germination and the apical growth of pollen tube in tobacco plants. We have found that the various polyamines differentially affected the metabolism of nitric oxide and reactive oxygen species during the processes of pollen germination in the grain and the lengthening pollen tube. It is hypothesized that their differential effects might be related to their distinct influence on the endogenous nitric oxide and reactive oxygen species levels.

The NADPH oxidase in Volvariella volvacea and its differential expression in response to mycelial ageing and mechanical injury

NADPH oxidases are enzymes that have been reported to generate reactive oxygen species (ROS) in animals, plants and many multicellular fungi in response to environmental stresses. Six genes of the NADPH oxidase complex components, including vvnoxa, vvnoxb, vvnoxr, vvbema, vvrac1 and vvcdc24, were identified based on the complete genomic sequence of the edible fungus Volvariella volvacea. The number of vvnoxa, vvrac1, vvbema and vvcdc24 transcripts fluctuated with ageing, and the gene expression patterns of vvnoxa, vvrac1 and vvbema were significantly positively correlated. However, the expression of vvnoxb and vvnoxr showed no significant difference during ageing. In hyphae subjected to mechanical injury stress, both O₂^- and H₂O₂ concentrations were increased. The expression of vvnoxa, vvrac1, vvbema and vvcdc24 was substantially upregulated, but vvnoxb and vvnoxr showed no response to mechanical injury stress at the transcriptional level. Additionally, the transcription of vvnoxa, vvrac1, vvbema and vvcdc24 could be repressed when the intracellular ROS were eliminated by diphenyleneiodonium (DPI) chloride and reduced glutathione (GSH) treatments. These results indicated a positive feedback loop involving NADPH oxidase and intracellular ROS, which might be the reason for the oxidative burst during injury stress.

Melatonin facilitates lateral root development by coordinating PAO-derived hydrogen peroxide and Rboh-derived superoxide radical

Melatonin, a phytochemical, can regulate lateral root (LR) formation, but the downstream signaling of melatonin remains elusive. Here we investigated the roles of hydrogen peroxide (H2O2) and superoxide radical (O2•‾) in melatonin-promoted LR formation in tomato (Solanum lycopersicum) roots by using physiological, histochemical, bioinformatic, and biochemical approaches. The increase in endogenous melatonin level stimulated reactive oxygen species (ROS)-dependent development of lateral root primordia (LRP) and LR. Melatonin promoted LRP/LR formation and modulated the expression of cell cycle genes (SlCDKA1, SlCYCD3;1, and SlKRP2) by stimulating polyamine oxidase (PAO)-dependent H2O2 production and respiratory burst oxidase homologue (Rboh)-dependent O2•‾ production, respectively. Screening of SlPAOs and SlRbohs gene family combined with gene expression analysis suggested that melatonin-promoted LR formation was correlated to the upregulation of SlPAO1, SlRboh3, and SlRboh4 in LR-emerging zone. Transient expression analysis confirmed that SlPAO1 was able to produce H2O2 while SlRboh3 and SlRboh4 were capable of producing O2•‾. Melatonin-ROS signaling cassette was also found in the regulation of LR formation in rice root and lateral hyphal branching in fungi. These results suggested that SlPAO1-H2O2 and SlRboh3/4-O2•‾ acted as downstream of melatonin to regulate the expression of cell cycle genes, resulting in LRP initiation and LR development. Such findings uncover one of the regulatory pathways for melatonin-regulated LR formation, which extends our knowledge for melatonin-regulated plant intrinsic physiology.

Increasing Polyamine Contents Enhances the Stress Tolerance via Reinforcement of Antioxidative Properties

The diamine putrescine and the polyamines (PAs), spermidine (Spd) and spermine (Spm), are ubiquitously occurring polycations associated with several important cellular functions, especially antisenescence. Numerous studies have reported increased levels of PA in plant cells under conditions of abiotic and biotic stress such as drought, high salt concentrations, and pathogen attack. However, the physiological mechanism of elevated PA levels in response to abiotic and biotic stresses remains undetermined. Transgenic plants having overexpression of SAMDC complementary DNA and increased levels of putrescine (1.4-fold), Spd (2.3-fold), and Spm (1.8-fold) under unstressed conditions were compared to wild-type (WT) plants in the current study. The most abundant PA in transgenic plants was Spd. Under salt stress conditions, enhancement of endogenous PAs due to overexpression of the SAMDC gene and exogenous treatment with Spd considerably reduces the reactive oxygen species (ROS) accumulation in intra- and extracellular compartments. Conversely, as compared to the WT, PA oxidase transcription rapidly increases in the S16-S-4 transgenic strain subsequent to salt stress. Furthermore, transcription levels of ROS detoxifying enzymes are elevated in transgenic plants as compared to the WT. Our findings with OxyBlot analysis indicate that upregulated amounts of endogenous PAs in transgenic tobacco plants show antioxidative effects for protein homeostasis against stress-induced protein oxidation. These results imply that the increased PAs induce transcription of PA oxidases, which oxidize PAs, which in turn trigger signal antioxidative responses resulting to lower the ROS load. Furthermore, total proteins from leaves with exogenously supplemented Spd and Spm upregulate the chaperone activity. These effects of PAs for antioxidative properties and antiaggregation of proteins contribute towards maintaining the physiological cellular functions against abiotic stresses. It is suggested that these functions of PAs are beneficial for protein homeostasis during abiotic stresses. Taken together, these results indicate that PA molecules function as antisenescence regulators through inducing ROS detoxification, antioxidative properties, and molecular chaperone activity under stress conditions, thereby providing broad-spectrum tolerance against a variety of stresses.

Genetically Modified Heat Shock Protein90s and Polyamine Oxidases in Arabidopsis Reveal Their Interaction under Heat Stress Affecting Polyamine Acetylation, Oxidation and Homeostasis of Reactive Oxygen Species

One Sentence Summary

Heat shock proteins90 (HSP90s) induce acetylation of polyamines (PAs) and interact with polyamine oxidases (PAOs) affecting oxidation of PAs and hydrogen peroxide (H₂O₂) homeostasis in Arabidopsis thaliana.

Abstract

The chaperones, heat shock proteins (HSPs), stabilize proteins to minimize proteotoxic stress, especially during heat stress (HS) and polyamine (PA) oxidases (PAOs) participate in the modulation of the cellular homeostasis of PAs and reactive oxygen species (ROS). An interesting interaction of HSP90s and PAOs was revealed in Arabidopsis thaliana by using the pLFY:HSP90RNAi line against the four AtHSP90 genes encoding cytosolic proteins, the T-DNA Athsp90-1 and Athsp90-4 insertional mutants, the Atpao3 mutant and pharmacological inhibitors of HSP90s and PAOs. Silencing of all cytosolic HSP90 genes resulted in several-fold higher levels of soluble spermidine (S-Spd), acetylated Spd (N⁸-acetyl-Spd) and acetylated spermine (N¹-acetyl-Spm) in the transgenic Arabidopsis thaliana leaves. Heat shock induced increase of soluble-PAs (S-PAs) and soluble hydrolyzed-PAs (SH-PAs), especially of SH-Spm, and more importantly of acetylated Spd and Spm. The silencing of HSP90 genes or pharmacological inhibition of the HSP90 proteins by the specific inhibitor radicicol, under HS stimulatory conditions, resulted in a further increase of PA titers, N⁸-acetyl-Spd and N¹-acetyl-Spm, and also stimulated the expression of PAO genes. The increased PA titers and PAO enzymatic activity resulted in a profound increase of PAO-derived hydrogen peroxide (H₂O₂) levels, which was terminated by the addition of the PAO-specific inhibitor guazatine. Interestingly, the loss-of-function Atpao3 mutant exhibited increased mRNA levels of selected AtHSP90 genes. Taken together, the results herein reveal a novel function of HSP90 and suggest that HSP90s and PAOs cross-talk to orchestrate PA acetylation, oxidation, and PA/H₂O₂ homeostasis.

Zinc-induced root architectural changes of rhizotron-grown B. napus correlate with a differential nitro-oxidative response

Roots have a noteworthy plasticity: due to different stress conditions their architecture can change to favour seedling vigour and yield stability. The development of the root system is regulated by a complex and diverse signalling network, which besides hormonal factors, includes reactive oxygen (ROS) - and nitrogen species (RNS). The delicate balance of the endogenous signal system can be affected by various environmental stimuli, such as the excess of essential heavy metals, like zinc (Zn). Zn at low concentration, is able to induce the morphological and physiological adaptation of the root system, but in excess it exerts toxic effects on plants. In this study the effect of a low, growth-inducing, and a high, growth inhibiting Zn concentrations on the early development of Brassica napus (L.) root architecture and the underlying nitro-oxidative mechanisms were studied in a soil-filled rhizotron system. The growth-inhibiting Zn treatment resulted in elevated protein tyrosine nitration due to the imbalance in ROS and RNS homeostasis, however its pattern was not changed compared to the control. This nitro-oxidative stress was accompanied by serious changes in the cell wall composition and decrease in the cell proliferation and viability, due to the high Zn uptake and disturbed microelement homeostasis in the root tips. During the positive root growth response, a tyrosine nitration-pattern reorganisation was observed; there were no substantial changes in ROS and RNS balance and the viability and proliferation of the root tips' meristematic zone decreased to a lesser extent, as a result of a lower Zn uptake. The obtained results suggest that Zn in different amounts triggers different root growth responses accompanied by distinct changes in the pattern and strength of tyrosine nitration, proposing that nitrosative processes have an important role in the stress-induced root growth responses.

The Interplay among Polyamines and Nitrogen in Plant Stress Responses

The interplay between polyamines (PAs) and nitrogen (N) is emerging as a key factor in plant response to abiotic and biotic stresses. The PA/N interplay in plants connects N metabolism, carbon (C) fixation, and secondary metabolism pathways. Glutamate, a pivotal N-containing molecule, is responsible for the biosynthesis of proline (Pro), arginine (Arg) and ornithine (Orn) and constitutes a main common pathway for PAs and C/N assimilation/incorporation implicated in various stresses. PAs and their derivatives are important signaling molecules, as they act largely by protecting and preserving the function/structure of cells in response to stresses. Use of different research approaches, such as generation of transgenic plants with modified intracellular N and PA homeostasis, has helped to elucidate a plethora of PA roles, underpinning their function as a major player in plant stress responses. In this context, a range of transgenic plants over-or under-expressing N/PA metabolic genes has been developed in an effort to decipher their implication in stress signaling. The current review describes how N and PAs regulate plant growth and facilitate crop acclimatization to adverse environments in an attempt to further elucidate the N-PAs interplay against abiotic and biotic stresses, as well as the mechanisms controlling N-PA genes/enzymes and metabolites.

The Copper Amine Oxidase AtCuAOδ Participates in Abscisic Acid-Induced Stomatal Closure in Arabidopsis

Plant copper amine oxidases (CuAOs) are involved in wound healing, defense against pathogens, methyl-jasmonate-induced protoxylem differentiation, and abscisic acid (ABA)-induced stomatal closure. In the present study, we investigated the role of the Arabidopsis thaliana CuAOδ (AtCuAOδ; At4g12290) in the ABA-mediated stomatal closure by genetic and pharmacological approaches. Obtained data show that AtCuAOδ is up-regulated by ABA and that two Atcuaoδ T-DNA insertional mutants are less responsive to this hormone, showing reduced ABA-mediated stomatal closure and H₂O₂ accumulation in guard cells as compared to the wild-type (WT) plants. Furthermore, CuAO inhibitors, as well as the hydrogen peroxide (H₂O₂) scavenger N,N¹-dimethylthiourea, reversed most of the ABA-induced stomatal closure in WT plants. Consistently, AtCuAOδ over-expressing transgenic plants display a constitutively increased stomatal closure and increased H₂O₂ production compared to WT plants. Our data suggest that AtCuAOδ is involved in the H₂O₂ production related to ABA-induced stomatal closure.

Characterization, expression, and functional analysis of polyamine oxidases and their role in selenium-induced hydrogen peroxide production in Brassica rapa

BACKGROUND

Selenium (Se)-induced phytotoxicity has been linked to oxidative injury triggered by the accumulation of reactive oxygen species (ROS) due to the disturbance of anti-oxidative systems. However, the way Se stress induces hydrogen peroxide (H2 O2 ) production in plants is a long-standing question. Here we identified the role of polyamine oxidase (PAO) in H2 O2 production in the root of Brassica rapa upon Se stress.

RESULTS

Studying Se-induced growth inhibition, H2 O2 accumulation, and oxidative injury in the root of Brassica rapa, we found that excessive Se exposure resulted in a remarkable increase in PAO activity. Inhibition of PAO activity led to decreased H2 O2 content and alleviated oxidative injury in the Se-treated root. These results indicated that Se stress induced PAO-dependent H2 O2 production. A total of six BrPAO family members were discovered in the genome of B. rapa by in silico analysis. Se stress pronouncedly upregulated the expression of most BrPAOs and further transient expression analysis proved that it could lead to H2 O2 production.

CONCLUSION

These results suggest that Se stress upregulates the expression of a set of BrPAOs which further enhances PAO activity, contributing to H2 O2 generation in roots. © 2019 Society of Chemical Industry.

Physiological and transcriptomic analyses reveal the molecular networks of responses induced by exogenous trehalose in plant

It is well known that exogenous trehalose can improve resistances of plants to some abiotic and biotic stresses. Nonetheless, information respecting the molecular responses of tobacco leaves to Tre treatment is limited. Here we show that exogenous Tre can rapidly reduce stomatal aperture, up-regulate NADPH oxidase genes and increase O2•-andH2O2 on tobacco leaves at 2 h after treatment. We further demonstrated that imidazole and DPI, inhibitors of NADPH oxidase, can promote recovery of stomatal aperture of tobacco leaves upon trehalose treatment. Exogenous trehalose increased tobacco leaf resistance to tobacco mosaic disease significantly in a concentration-dependent way. To elucidate the molecular mechanisms in response to exogenous trehalose, the transcriptomic responses of tobacco leaves with 10 (low concentration) or 50 (high concentration) mM of trehalose treatment at 2 or 24h were investigated through RNA-seq approach. In total, 1288 differentially expressed genes (DEGs) were found with different conditions of trehalose treatments relative to control. Among them, 1075 (83.5%) were triggered by low concentration of trehalose (10mM), indicating that low concentration of Tre is a better elicitor. Functional annotations with KEGG pathway analysis revealed that the DEGs are involved in metabolic pathway, biosynthesis of secondary metabolites, plant hormone signal transduction, plant-pathogen interaction, protein processing in ER, flavonoid synthesis and circadian rhythm and so on. The protein-protein interaction networks generated from the core DEGs regulated by all conditions strikingly revealed that eight proteins, including ClpB1, HSP70, DnaJB1-like protein, universal stress protein (USP) A-like protein, two FTSH6 proteins, GolS1-like protein and chloroplastics HSP, play a core role in responses to exogenous trehalose in tobacco leaves. Our data suggest that trehalose triggers a signal transduction pathway which involves calcium and ROS-mediated signalings. These core components could lead to partial resistance or tolerance to abiotic and biotic stresses. Moreover, 19 DEGs were chosen for analysis of quantitative real-time polymerase chain reaction (qRT-PCR). The qRT-PCR for the 19 candidate genes coincided with the DEGs identified via the RNA-seq analysis, sustaining the reliability of our RNA-seq data.

Polyamine Catabolism in Plants: A Universal Process With Diverse Functions

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

Osmotic Stress Activates Two Reactive Oxygen Species Pathways with Distinct Effects on Protein Nanodomains and Diffusion

Physiological acclimation of plants to an everchanging environment is governed by complex combinatorial signaling networks that perceive and transduce various abiotic and biotic stimuli. Reactive oxygen species (ROS) serve as one of the second messengers in plant responses to hyperosmotic stress. The molecular bases of ROS production and the primary cellular processes that they target were investigated in the Arabidopsis (Arabidopsis thaliana) root. Combined pharmacological and genetic approaches showed that the RESPIRATORY BURST OXIDASE HOMOLOG (RBOH) pathway and an additional pathway involving apoplastic ascorbate and iron can account for ROS production upon hyperosmotic stimulation. The two pathways determine synergistically the rate of membrane internalization, within minutes after activation. Live superresolution microscopy revealed at single-molecule scale how ROS control specific diffusion and nano-organization of membrane cargo proteins. In particular, ROS generated by RBOHs initiated clustering of the PLASMA MEMBRANE INTRINSIC PROTEIN2;1 aquaporin and its removal from the plasma membrane. This process is contributed to by clathrin-mediated endocytosis, with a positive role of RBOH-dependent ROS, specifically under hyperosmotic stress.

Hydrogen peroxide metabolism and functions in plants

Contents Summary 1197 I. Introduction 1198 II. Measurement and imaging of H₂ O₂ 1198 III. H₂ O₂ and O₂^·- toxicity 1199 IV. Production of H₂ O₂ : enzymes and subcellular locations 1200 V. H₂ O₂ transport 1205 VI. Control of H₂ O₂ concentration: how and where? 1205 VII. Metabolic functions of H₂ O₂ 1207 VIII. H₂ O₂ signalling 1207 IX. Where next? 1209 Acknowledgements 1209 References 1209 SUMMARY: Hydrogen peroxide (H₂ O₂ ) is produced, via superoxide and superoxide dismutase, by electron transport in chloroplasts and mitochondria, plasma membrane NADPH oxidases, peroxisomal oxidases, type III peroxidases and other apoplastic oxidases. Intracellular transport is facilitated by aquaporins and H₂ O₂ is removed by catalase, peroxiredoxin, glutathione peroxidase-like enzymes and ascorbate peroxidase, all of which have cell compartment-specific isoforms. Apoplastic H₂ O₂ influences cell expansion, development and defence by its involvement in type III peroxidase-mediated polymer cross-linking, lignification and, possibly, cell expansion via H₂ O₂ -derived hydroxyl radicals. Excess H₂ O₂ triggers chloroplast and peroxisome autophagy and programmed cell death. The role of H₂ O₂ in signalling, for example during acclimation to stress and pathogen defence, has received much attention, but the signal transduction mechanisms are poorly defined. H₂ O₂ oxidizes specific cysteine residues of target proteins to the sulfenic acid form and, similar to other organisms, this modification could initiate thiol-based redox relays and modify target enzymes, receptor kinases and transcription factors. Quantification of the sources and sinks of H₂ O₂ is being improved by the spatial and temporal resolution of genetically encoded H₂ O₂ sensors, such as HyPer and roGFP2-Orp1. These H₂ O₂ sensors, combined with the detection of specific proteins modified by H₂ O₂ , will allow a deeper understanding of its signalling roles.

The Polyamine Putrescine Contributes to H2O2 and RbohD/F-Dependent Positive Feedback Loop in Arabidopsis PAMP-Triggered Immunity

Polyamines are involved in defense against pathogenic microorganisms in plants. However, the role of the polyamine putrescine (Put) during plant defense has remained elusive. In this work, we studied the implication of polyamines during pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) in the model species Arabidopsis thaliana. Our data indicate that polyamines, particularly Put, accumulate in response to non-pathogenic Pseudomonas syringae pv. tomato DC3000 hrcC and in response to the purified PAMP flagellin22. Exogenously supplied Put to Arabidopsis seedlings induces defense responses compatible with PTI activation, such as callose deposition and transcriptional up-regulation of several PTI marker genes. Consistent with this, we show that Put primes for resistance against pathogenic bacteria. Through chemical and genetic approaches, we find that PTI-related transcriptional responses induced by Put are hydrogen peroxide and NADPH oxidase (RBOHD and RBOHF) dependent, thus suggesting that apoplastic ROS mediates Put signaling. Overall, our data indicate that Put amplifies PTI responses through ROS production, leading to enhanced disease resistance against bacterial pathogens.

Understanding the Biostimulant Action of Vegetal-Derived Protein Hydrolysates by High-Throughput Plant Phenotyping and Metabolomics: A Case Study on Tomato

Designing and developing new biostimulants is a crucial process which requires an accurate testing of the product effects on the morpho-physiological traits of plants and a deep understanding of the mechanism of action of selected products. Product screening approaches using omics technologies have been found to be more efficient and cost effective in finding new biostimulant substances. A screening protocol based on the use of high-throughput phenotyping platform for screening new vegetal-derived protein hydrolysates (PHs) for biostimulant activity followed by a metabolomic analysis to elucidate the mechanism of the most active PHs has been applied on tomato crop. Eight PHs (A-G, I) derived from enzymatic hydrolysis of seed proteins of Leguminosae and Brassicaceae species were foliarly sprayed twice during the trial. A non-ionic surfactant Triton X-100 at 0.1% was also added to the solutions before spraying. A control treatment foliarly sprayed with distilled water containing 0.1% Triton X-100 was also included. Untreated and PH-treated tomato plants were monitored regularly using high-throughput non-invasive imaging technologies. The phenotyping approach we used is based on automated integrative analysis of photosynthetic performance, growth analysis, and color index analysis. The digital biomass of the plants sprayed with PH was generally increased. In particular, the relative growth rate and the growth performance were significantly improved by PHs A and I, respectively, compared to the untreated control plants. Kinetic chlorophyll fluorescence imaging did not allow to differentiate the photosynthetic performance of treated and untreated plants. Finally, MS-based untargeted metabolomics analysis was performed in order to characterize the functional mechanisms of selected PHs. The treatment modulated the multi-layer regulation process that involved the ethylene precursor and polyamines and affected the ROS-mediated signaling pathways. Although further investigation is needed to strengthen our findings, metabolomic data suggest that treated plants experienced a metabolic reprogramming following the application of the tested biostimulants. Nonetheless, our experimental data highlight the potential for combined use of high-throughput phenotyping and metabolomics to facilitate the screening of new substances with biostimulant properties and to provide a morpho-physiological and metabolomic gateway to the mechanisms underlying PHs action on plants.

Polyamines modulate aluminum-induced oxidative stress differently by inducing or reducing H2O2 production in wheat

Polyamines are important bioactive molecules involved in regulating H₂O₂ homeostasis, which is recognized as a major stimulus of oxidative stress under aluminum (Al) exposure. In this study, we investigated the involvement of spermidine oxidation in Al-induced oxidative stress, and its modulation by exogenous putrescine (Put) in two wheat genotypes differing in Al tolerance. Aluminum caused more severe oxidative damage at the root apexes in the Al-sensitive genotype Yangmai-5 than in the tolerant Xi Aimai-1, but these effects were significantly reversed by exogenous Put and polyamine oxidase (PAO) inhibitors. Aluminum caused a more significant increase in cell wall-bound PAO (CW-PAO) activity in Yangmai-5 than in Xi Aimai-1. Inhibiting of CW-PAO reduced H₂O₂ accumulation, restored Spd decline in both genotypes, indicating its potential role in Al-induced H₂O₂ production through catalyzing Spd oxidation. Additionally, Al significantly increased the activity of plasma membrane-NADPH oxidase, another H₂O₂ generator, in wheat roots. Put application significantly inhibited the activity of CW-PAO and plasma membrane-NADPH oxidase, and reduced H₂O₂ accumulation in Al-stressed wheat roots. Antioxidant enzymes were significantly stimulated by Al, but not Put. Overall, Put may protect wheat roots against Al-induced oxidative stress through regulating H₂O₂ production by inhibiting CW-PAO and plasma membrane-NADPH oxidase.

CaWRKY27 Negatively Regulates H2O2-Mediated Thermotolerance in Pepper (Capsicum annuum)

Heat stress, an important and damaging abiotic stress, regulates numerous WRKY transcription factors, but their roles in heat stress responses remain largely unexplored. Here, we show that pepper (Capsicum annuum) CaWRKY27 negatively regulates basal thermotolerance mediated by H2O2 signaling. CaWRKY27 expression increased during heat stress and persisted during recovery. CaWRKY27 overexpression impaired basal thermotolerance in tobacco (Nicotiana tabacum) and Arabidopsis thaliana, CaWRKY27-overexpressing plants had a lower survival rate under heat stress, accompanied by decreased expression of multiple thermotolerance-associated genes. Accordingly, silencing of CaWRKY27 increased basal thermotolerance in pepper plants. Exogenously applied H2O2 induced CaWRKY27 expression, and CaWRKY27 overexpression repressed the scavenging of H2O2 in Arabidopsis, indicating a positive feedback loop between H2O2 accumulation and CaWRKY27 expression. Consistent with this, CaWRKY27 expression was repressed under heat stress in the presence H2O2 scavengers and CaWRKY27 silencing decreased H2O2 accumulation in pepper leaves. These changes may result from changes in levels of reactive oxygen species (ROS)-scavenging enzymes, since the heat stress-challenged CaWRKY27-silenced pepper plants had significantly higher expression of multiple genes encoding ROS-scavenging enzymes, such as CaCAT1, CaAPX1, CaAPX2, CaCSD2, and CaSOD1. Therefore, CaWRKY27 acts as a downstream negative regulator of H2O2-mediated heat stress responses, preventing inappropriate responses during heat stress and recovery.

Calcium transport across plant membranes: mechanisms and functions

Contents Summary 49 I. Introduction 49 II. Physiological and structural characteristics of plant Ca²⁺ -permeable ion channels 50 III. Ca²⁺ extrusion systems 61 IV. Concluding remarks 64 Acknowledgements 64 References 64 SUMMARY: Calcium is an essential structural, metabolic and signalling element. The physiological functions of Ca²⁺ are enabled by its orchestrated transport across cell membranes, mediated by Ca²⁺ -permeable ion channels, Ca²⁺ -ATPases and Ca²⁺ /H⁺ exchangers. Bioinformatics analysis has not determined any Ca²⁺ -selective filters in plant ion channels, but electrophysiological tests do reveal Ca²⁺ conductances in plant membranes. The biophysical characteristics of plant Ca²⁺ conductances have been studied in detail and were recently complemented by molecular genetic approaches. Plant Ca²⁺ conductances are mediated by several families of ion channels, including cyclic nucleotide-gated channels (CNGCs), ionotropic glutamate receptors, two-pore channel 1 (TPC1), annexins and several types of mechanosensitive channels. Key Ca²⁺ -mediated reactions (e.g. sensing of temperature, gravity, touch and hormones, and cell elongation and guard cell closure) have now been associated with the activities of specific subunits from these families. Structural studies have demonstrated a unique selectivity filter in TPC1, which is passable for hydrated divalent cations. The hypothesis of a ROS-Ca²⁺ hub is discussed, linking Ca²⁺ transport to ROS generation. CNGC inactivation by cytosolic Ca²⁺ , leading to the termination of Ca²⁺ signals, is now mechanistically explained. The structure-function relationships of Ca²⁺ -ATPases and Ca²⁺ /H⁺ exchangers, and their regulation and physiological roles are analysed.

Respiratory burst oxidase homologue-dependent H2 O2 and chloroplast H2 O2 are essential for the maintenance of acquired thermotolerance during recovery after acclimation

Thermotolerance is improved by heat stress (HS) acclimation, and the thermotolerance level is "remembered" by plants. However, the underlying signalling mechanisms remain largely unknown. Here, we showed NADPH oxidase-mediated H₂ O₂ (NADPH-H₂ O₂ ), and chloroplast-H₂ O₂ promoted the sustained expression of HS-responsive genes and programmed cell death (PCD) genes, respectively, during recovery after HS acclimation. When spraying the NADPH oxidase inhibitor, diphenylene iodonium, after HS acclimation, the NADPH-H₂ O₂ level significantly decreased, resulting in a decrease in the expression of HS-responsive genes and the loss of maintenance of acquired thermotolerance (MAT). In contrast, compared with HS acclimation, NADPH-H₂ O₂ declined but chloroplast-H₂ O₂ further enhanced during recovery after HS over-acclimation, resulting in the reduced expression of HS-responsive genes and substantial production of PCD. Notably, the further inhibition of NADPH-H₂ O₂ after HS over-acclimation also inhibited chloroplast-H₂ O₂ , alleviating the severe PCD and surpassing the MAT of HS over-acclimation treatment. Due to the change in subcellular H₂ O₂ after HS acclimation, the tomato seedlings maintained a constant H₂ O₂ level during recovery, resulting in stable and lower total H₂ O₂ levels during a tester HS challenge conducted after recovery. We conclude that tomato seedlings increase their MAT by enhancing NADPH-H₂ O₂ content and controlling chloroplast-H₂ O₂ production during recovery, which enhances the expression of HS-responsive genes and balances PCD levels, respectively.

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.

ROS-Activated Ion Channels in Plants: Biophysical Characteristics, Physiological Functions and Molecular Nature

Ion channels activated by reactive oxygen species (ROS) have been found in the plasma membrane of charophyte Nitella flixilis, dicotyledon Arabidopsis thaliana, Pyrus pyrifolia and Pisum sativum, and the monocotyledon Lilium longiflorum. Their activities have been reported in charophyte giant internodes, root trichoblasts and atrichoblasts, pollen tubes, and guard cells. Hydrogen peroxide and hydroxyl radicals are major activating species for these channels. Plant ROS-activated ion channels include inwardly-rectifying, outwardly-rectifying, and voltage-independent groups. The inwardly-rectifying ROS-activated ion channels mediate Ca2+-influx for growth and development in roots and pollen tubes. The outwardly-rectifying group facilitates K⁺ efflux for the regulation of osmotic pressure in guard cells, induction of programmed cell death, and autophagy in roots. The voltage-independent group mediates both Ca2+ influx and K⁺ efflux. Most studies suggest that ROS-activated channels are non-selective cation channels. Single-channel studies revealed activation of 14.5-pS Ca2+ influx and 16-pS K⁺ efflux unitary conductances in response to ROS. The molecular nature of ROS-activated Ca2+ influx channels remains poorly understood, although annexins and cyclic nucleotide-gated channels have been proposed for this role. The ROS-activated K⁺ channels have recently been identified as products of Stellar K⁺ Outward Rectifier (SKOR) and Guard cell Outwardly Rectifying K⁺ channel (GORK) genes.