NADPH oxidase inhibitor diphenyleneiodonium and reduced glutathione mitigate ethephon-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression in sweet potato (Ipomoea batatas)

Ethylene-dependent regulation of oxidative stress in the leaves of fusaric acid-treated tomato plants

The mycotoxin fusaric acid (FA) induces rapid oxidative burst leading to cell death in plants. At the same time, plant defence reactions are mediated by several phytohormones for instance ethylene (ET). However, previously conducted studies leave research gaps on how ET plays a regulatory role under mycotoxin exposure. Therefore, this study aims to the time-dependent effects of two FA concentrations (0.1 mM and 1 mM) were explored on the regulation of reactive oxygen species (ROS) in leaves of wild-type (WT) and ET receptor mutant Never ripe (Nr) tomatoes. FA induced superoxide and H₂O₂ accumulation in both genotypes in a mycotoxin dose- and exposure time-dependent pattern. 1 mM FA activated NADPH oxidase (+34% compared to the control) and RBOH1 transcript levels in WT leaves. However, superoxide production was significantly higher in Nr with 62% which could contribute to higher lipid peroxidation in this genotype. In parallel, the antioxidative defence mechanisms were also activated. Both peroxidase and superoxide dismutase activities were lower in Nr but ascorbate peroxidase showed one-fold higher activity under 1 mM FA stress than in WT leaves. Interestingly, catalase (CAT) activity decreased upon FA in a time- and concentration-dependent manner and the encoding CAT genes were also downregulated, especially in Nr leaves at 20%. Ascorbate level was decreased and glutathione remained lower in Nr than WT plants under FA exposure. Conclusively, Nr genotype showed more sensitivity to FA-induced ROS suggesting that ET serves defence reactions of plants by activating several enzymatic and non-enzymatic antioxidants to detoxify excess ROS accumulation.

Glutathione Is Required for the Early Alert Response and Subsequent Acclimation in Cadmium-Exposed Arabidopsis thaliana Plants

Pollution by cadmium (Cd) is a worldwide problem, posing risks to human health and impacting crop yield and quality. Cadmium-induced phytotoxicity arises from an imbalance between antioxidants and pro-oxidants in favour of the latter. The Cd-induced depletion of the major antioxidant glutathione (GSH) strongly contributes to this imbalance. Rather than being merely an adverse effect of Cd exposure, the rapid depletion of root GSH levels was proposed to serve as an alert response. This alarm phase is crucial for an optimal stress response, which defines acclimation later on. To obtain a better understanding on the importance of GSH in the course of these responses and how these are defined by the rapid GSH depletion, analyses were performed in the GSH-deficient cadmium-sensitive 2-1 (cad2-1) mutant. Cadmium-induced root and leaf responses related to oxidative challenge, hydrogen peroxide (H2O2), GSH, ethylene, and 1-aminocyclopropane-1-carboxylic acid (ACC) were compared between wild-type (WT) and mutant Arabidopsis thaliana plants. Although the cad2-1 mutant has significantly lower GSH levels, root GSH depletion still occurred, suggesting that the chelating capacity of GSH is prioritised over its antioxidative function. We demonstrated that responses related to GSH metabolism and ACC production were accelerated in mutant roots and that stress persisted due to suboptimal acclimation. In general, the redox imbalance in cad2-1 mutant plants and the lack of proper transient ethylene signalling contributed to this suboptimal acclimation, resulting in a more pronounced Cd effect.

Increased Drought Resistance 1 Mutation Increases Drought Tolerance of Upland Rice by Altering Physiological and Morphological Traits and Limiting ROS Levels

To discover new mutants conferring enhanced tolerance to drought stress, we screened a mutagenized upland rice (Oryza sativa) population (cv. IAPAR9) and identified a mutant, named idr1-1 (increased drought resistance 1-1), with obviously increased drought tolerance under upland field conditions. The idr1-1 mutant possessed a significantly enhanced ability to tolerate high-drought stresses. Map-based cloning revealed that the gene LOC_Os05g26890, residing in the mapping region of IDR1 locus, carried a single-base deletion in the idr1-1 mutant. IDR1 encodes the Gα subunit of the heterotrimeric G protein (also known as RGA1), and this protein was localized in nucleus and to plasma membrane or cell periphery. Further investigations indicated that the significantly increased drought tolerance in idr1-1 mutants stemmed from a range of physiological and morphological changes, including greater leaf potentials, increased proline contents, heightened leaf thickness and upregulation of antioxidant-synthesizing and drought-induced genes, under drought-stressed conditions. Especially, reactive oxygen species (ROS) production might be remarkably impaired, while ROS-scavenging ability appeared to be markedly enhanced due to significantly elevated expression of ROS-scavenging enzyme genes in idr1-1 mutants under drought-stressed conditions. In addition, idr1-1 mutants showed reduced expression of OsBRD1. Altogether, these results suggest that mutation of IDR1 leads to alterations in multiple layers of regulations, which ultimately leads to changes in the physiological and morphological traits and limiting of ROS levels, and thereby confers obviously increased drought tolerance to the idr1-1 mutant.

The effect of NADPH oxidase inhibitor diphenyleneiodonium (DPI) and glutathione (GSH) on Isatis cappadocica, under Arsenic (As) toxicity

The present work was conducted to assess the effects of arsenic (As, 1000 µM), diphenyleneiodonium (DPI, 10 µM) and reduced glutathione (GSH, 500 µM) on Isatis cappadocica. As treatment decreased plant growth and fresh and dry weight of shoot and root and also enhanced the accumulation of As. As stress also enhanced the oxidative stress biomarkers, hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) content. However, the application of GSH decreased the content of H₂O₂ and MDA by 43% and 55%, respectively, as compared to As treatment. The antioxidants like superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR) and glutathione S-transferase (GST) also enhanced with As stress. NADPH oxidase inhibitor, the DPI, enhances the effect of As toxicity by increasing the accumulation of As, H₂O₂, MDA. DPI also enhances the activity of antioxidant enzymes except GR and GST, However, the application GSH increased the plant growth and biomass yield, decreases accumulation of As, H₂O₂ and MDA content in As as well as As + DPI treated plants. The thiols content [total thiol (TT), non-protein thiol (NPT) protein thiols (PT), and glutathione (GSH)] were decreased in the As + DPI treatment but supplementation of GSH enhanced them. Novelty statement: The study reveals the beneficial role of GSH in mitigating the deleterious effects of Arsenic toxicity through its active involvement in the antioxidant metabolism, thiol synthesis and osmolyte accumulation. Apart from As, We provided the plants NADPH oxidase inhibitor, the diphenyleneiodonium (DPI), which boosts the As toxicity. At present, there is dearth of information pertaining to the effects of DPI on plants growth and their responses under heavy metal stress.GSH application reversed the effect of diphenyleneiodonium (DPI) under As stress preventing the oxidative damage to biomolecules through the modulation of different antioxidant enzymes. The application of GSH for As stressed soil could be a sustainable approach for crop production.

Identifying the Pressure Points of Acute Cadmium Stress Prior to Acclimation in Arabidopsis thaliana

The toxic metal cadmium (Cd) is a major soil pollutant. Knowledge on the acute Cd-induced stress response is required to better understand the triggers and sequence of events that precede plant acclimation. Therefore, we aimed to identify the pressure points of Cd stress using a short-term exposure set-up ranging from 0 h to 24 h. Acute responses related to glutathione (GSH), hydrogen peroxide (H₂O₂), 1-aminocyclopropane-1-carboxylic acid (ACC), ethylene and the oxidative challenge were studied at metabolite and/or transcript level in roots and leaves of Arabidopsis thaliana either exposed or not to 5 µM Cd. Cadmium rapidly induced root GSH depletion, which might serve as an alert response and modulator of H₂O₂ signalling. Concomitantly, a stimulation of root ACC levels was observed. Leaf responses were delayed and did not involve GSH depletion. After 24 h, a defined oxidative challenge became apparent, which was most pronounced in the leaves and concerted with a strong induction of leaf ACC synthesis. We suggest that root GSH depletion is required for a proper alert response rather than being a merely adverse effect. Furthermore, we propose that roots serve as command centre via a.o. root-derived ACC/ethylene to engage the leaves in a proper stress response.

NADPH Oxidases: The Vital Performers and Center Hubs during Plant Growth and Signaling

NADPH oxidases (NOXs), mostly known as respiratory burst oxidase homologs (RBOHs), are the key producers of reactive oxygen species (ROS) in plants. A lot of literature has addressed ROS signaling in plant development regulation and stress responses as well as on the enzyme's structure, evolution, function, regulation and associated mechanisms, manifesting the role of NOXs/RBOHs as the vital performers and center hubs during plant growth and signaling. This review focuses on recent advances of NOXs/RBOHs on cell growth, hormone interaction, calcium signaling, abiotic stress responses, and immunity. Several primary particles, including Ca2+, CDPKs, BIK1, ROPs/RACs, CERK, FER, ANX, SnRK and SIK1-mediated regulatory mechanisms, are fully summarized to illustrate the signaling behavior of NOXs/RBOHs and their sophisticated and dexterous crosstalks. Diverse expression and activation regulation models endow NOXs/RBOHs powerful and versatile functions in plants to maintain innate immune homeostasis and development integrity. NOXs/RBOHs and their related regulatory items are the ideal targets for crop improvement in both yield and quality during agricultural practices.

Generation of Superoxide by OeRbohH, a NADPH Oxidase Activity During Olive (Olea europaea L.) Pollen Development and Germination

Reactive oxygen species (ROS) are produced in the olive reproductive organs as the result of intense metabolism. ROS production and pattern of distribution depend on the developmental stage, supposedly playing a broad panel of functions, which include defense and signaling between pollen and pistil. Among ROS-producing mechanisms, plasma membrane NADPH-oxidase activity is being highlighted in plant tissues, and two enzymes of this type have been characterized in Arabidopsis thaliana pollen (RbohH and RbohJ), playing important roles in pollen physiology. Besides, pollen from different species has shown distinct ROS production mechanism and patterns of distribution. In the olive reproductive tissues, a significant production of superoxide has been described. However, the enzymes responsible for such generation are unknown. Here, we have identified an Rboh-type gene (OeRbohH), mainly expressed in olive pollen. OeRbohH possesses a high degree of identity with RbohH and RbohJ from Arabidopsis, sharing most structural features and motifs. Immunohistochemistry experiments allowed us to localize OeRbohH throughout pollen ontogeny as well as during pollen tube elongation. Furthermore, the balanced activity of tip-localized OeRbohH during pollen tube growth has been shown to be important for normal pollen physiology. This was evidenced by the fact that overexpression caused abnormal phenotypes, whereas incubation with specific NADPH oxidase inhibitor or gene knockdown lead to impaired ROS production and subsequent inhibition of pollen germination and pollen tube growth.

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.

Identification and characterization of MAGO and Y14 genes in Hevea brasiliensis

Mago nashi (MAGO) and Y14 proteins are highly conserved among eukaryotes. In this study, we identified two MAGO (designated as HbMAGO1 andHbMAGO2) and two Y14 (designated as HbY14aand HbY14b) genes in the rubber tree (Hevea brasiliensis) genome annotation. Multiple amino acid sequence alignments predicted that HbMAGO and HbY14 proteins are structurally similar to homologous proteins from other species. Tissue-specific expression profiles showed that HbMAGO and HbY14 genes were expressed in at least one of the tissues (bark, flower, latex, leaf and root) examined. HbMAGOs and HbY14s were predominately located in the nucleus and were found to interact in yeast two-hybrid analysis (YTH) and bimolecular fluorescence complementation (BiFC) assays. HbMAGOs and HbY14s showed the highest transcription in latex and were regulated by ethylene and jasmonate. Interaction between HbMAGO2 and gp91^phox (a large subunit of nicotinamide adenine dinucleotide phosphate) was identified using YTH and BiFC assays. These findings suggested that HbMAGO may be involved in the aggregation of rubber particles in H. brasiliensis.

Ethylene and Metal Stress: Small Molecule, Big Impact

The phytohormone ethylene is known to mediate a diverse array of signaling processes during abiotic stress in plants. Whereas many reports have demonstrated enhanced ethylene production in metal-exposed plants, the underlying molecular mechanisms are only recently investigated. Increasing evidence supports a role for ethylene in the regulation of plant metal stress responses. Moreover, crosstalk appears to exist between ethylene and the cellular redox balance, nutrients and other phytohormones. This review highlights our current understanding of the key role ethylene plays during responses to metal exposure. Moreover, particular attention is paid to the integration of ethylene within the broad network of plant responses to metal stress.

Cadmium-Induced Hydrogen Accumulation Is Involved in Cadmium Tolerance in Brassica campestris by Reestablishment of Reduced Glutathione Homeostasis

Hydrogen gas (H2) was recently proposed as a therapeutic antioxidant and signaling molecule in clinical trials. However, the underlying physiological roles of H2 in plants remain unclear. In the present study, hydrogen-rich water (HRW) was used to characterize the physiological roles of H2 in enhancing the tolerance of Brassica campestris against cadmium (Cd). The results showed that both 50 μM CdCl2 and 50%-saturated HRW induced an increase of endogenous H2 in Brassica campestris seedlings, and HRW alleviated Cd toxicity related to growth inhibition and oxidative damage. Seedlings supplied with HRW exhibited increased root length and reduced lipid peroxidation, similar to plants receiving GSH post-treatment. Additionally, seedlings post-treated with HRW accumulated higher levels of reduced glutathione (GSH) and ascorbic acid (AsA) and showed increased GST and GPX activities in roots. Molecular evidence illustrated that the expression of genes such as GS, GR1 and GR2, which were down-regulated following the addition of Cd, GSH or BSO, could be reversed to varying degrees by the addition of HRW. Based on these results, it could be proposed that H2 might be an important regulator for enhancing the tolerance of Brassica campestris seedlings against Cd, mainly by governing reduced glutathione homeostasis.

Ethylene signalling is mediating the early cadmium-induced oxidative challenge in Arabidopsis thaliana

Cadmium (Cd) induces the generation of reactive oxygen species (ROS) and stimulates ethylene biosynthesis. The phytohormone ethylene is a regulator of many developmental and physiological plant processes as well as stress responses. Previous research indicated various links between ethylene signalling and oxidative stress. Our results support a correlation between the Cd-induced oxidative challenge and ethylene signalling in Arabidopsis thaliana leaves. The effects of 24 or 72 h exposure to 5 μM Cd on plant growth and several oxidative stress-related parameters were compared between wild-type (WT) and ethylene insensitive mutants (etr1-1, ein2-1, ein3-1). Cadmium-induced responses observed in WT plants were mainly affected in etr1-1 and ein2-1 mutants, of which the growth was less inhibited by Cd exposure as compared to WT and ein3-1 mutants. Both etr1-1 and ein2-1 showed a delayed response in the glutathione (GSH) metabolism, including GSH levels and transcript levels of GSH synthesising and recycling enzymes. Furthermore, the expression of different oxidative stress marker genes was significantly lower in Cd-exposed ein2-1 mutants, evidencing that ethylene signalling is involved in early responses to Cd stress. A model for the cross-talk between ethylene signalling and oxidative stress is proposed.

Sweet potato SPAP1 is a typical aspartic protease and participates in ethephon-mediated leaf senescence

Plant aspartic proteases are generally divided into three categories: typical, nucellin-like, and atypical aspartic proteases based on their gene and protein structures. In this report, a full-length cDNA SPAP1 was cloned from sweet potato leaves, which contained 1515 nucleotides (504 amino acids) and exhibited high amino acid sequence identity (ca. 51-72%) with plant typical aspartic proteases, including tomato LeAspP, potato StAsp, and wheat WAP2. SPAP1 also contained conserved DTG and DSG amino acid residues within its catalytic domain and plant specific insert (PSI) at the C-terminus. The cDNA corresponding to the mature protein (starting from the 66th to 311th amino acid residues) without PSI domain was constructed with pET30a expression vector for fusion protein and antibody production. RT-PCR and protein blot hybridization showed that SPAP1 expression level was the highest in L3 mature leaves, then gradually declined until L5 completely yellow leaves. Ethephon, an ethylene-releasing compound, also enhanced SPAP1 expression at the time much earlier than the onset of leaf senescence. Exogenous application of SPAP1 fusion protein promoted ethephon-induced leaf senescence, which could be abolished by pre-treatment of SPAP1 fusion protein with (a) 95 °C for 5 min, (b) aspartic protease inhibitor pepstatin A, and (c) anti-SPAP1 antibody, respectively. Exogenous SPAP1 fusion protein, whereas, did not significantly affect leaf senescence under dark. These data conclude that sweet potato SPAP1 is a functional typical aspartic protease and participates in ethephon-mediated leaf senescence. The SPAP1-promoted leaf senescence and its activity are likely not associated with the PSI domain. Interaction of ethephon-inducible components for effective SPAP1 promotion on leaf senescence is also suggested.

Reactive oxygen species produced via plasma membrane NADPH oxidase regulate anthocyanin synthesis in apple peel

Solar ultraviolet irradiation regulates anthocyanin synthesis in apple peel by modulating the production of reactive oxygen species via plasma membrane NADPH oxidase instead of other pathways. The synthesis of anthocyanin in apple peels is dependent upon solar irradiation. Using 3-mm commercial glass to attenuate solar UV-A and UV-B light, we confirmed that solar UV irradiation regulated anthocyanin synthesis in apple peels after exposing previously bagged fruit to sunlight. During sunlight exposure, UV attenuation did not affect the expression of MdHY5, MdCOP1, or MdCRY2, but significantly lowered plasma membrane NADPH oxidase activity and superoxide anion concentrations. UV attenuation also reduced the expression levels of MdMYB10, MdPAL, MdCHS, MdF3H, MdDFR, MdANS and MdUFGT1, UDP-glycose:flavonoid 3-O-glycosyltransferase (UFGT) activity, and local concentrations of anthocyanin and quercetin-3-glycoside. In contrast, exogenous application of hydrogen peroxide could enhance anthocyanin and quercetin-3-glycoside synthesis. Xanthophyll cycle pool size on a chlorophyll basis was higher but its de-epoxidation was lower under direct sunlight irradiation than that under UV-attenuating conditions. This suggests that reactive oxygen species (ROS) produced in chloroplast are not major contributors to anthocyanin synthesis regulation. Inhibition of plasma membrane NADPH oxidase activity lowered the production of ROS through this mechanism, significantly inhibited the synthesis of anthocyanin, and increased the total production of ROS in apple peel under direct sunlight irradiation, suggesting that ROS produced via plasma membrane NADPH oxidase regulates anthocyanin synthesis. In summary, solar UV irradiation regulated anthocyanin synthesis in apple peels by modulating the production of ROS via plasma membrane NADPH oxidase.

Switchgrass (Panicum virgatum L) flag leaf transcriptomes reveal molecular signatures of leaf development, senescence, and mineral dynamics

Switchgrass flag leaves can be expected to be a source of carbon to the plant, and its senescence is likely to impact the remobilization of nutrients from the shoots to the rhizomes. However, many genes have not been assigned a function in specific stages of leaf development. Here, we characterized gene expression in flag leaves over their development. By merging changes in leaf chlorophyll and the expression of genes for chlorophyll biosynthesis and degradation, a four-phase molecular roadmap for switchgrass flag leaf ontogeny was developed. Genes associated with early leaf development were up-regulated in phase 1. Phase 2 leaves had increased expression of genes for chlorophyll biosynthesis and those needed for full leaf function. Phase 3 coincided with the most active phase for leaf C and N assimilation. Phase 4 was associated with the onset of senescence, as observed by declining leaf chlorophyll content, a significant up-regulation in transcripts coding for enzymes involved with chlorophyll degradation, and in a large number of senescence-associated genes. Of considerable interest were switchgrass NAC transcription factors with significantly higher expression in senescing flag leaves. Two of these transcription factors were closely related to a wheat NAC gene that impacts mineral remobilization. The third switchgrass NAC factor was orthologous to an Arabidopsis gene with a known role in leaf senescence. Other genes coding for nitrogen and mineral utilization, including ureide, ammonium, nitrate, and molybdenum transporters, shared expression profiles that were significantly co-regulated with the expression profiles of the three NAC transcription factors. These data provide a good starting point to link shoot senescence to the onset of dormancy in field-grown switchgrass.