The NADPH-oxidase AtrbohB plays a role in Arabidopsis seed after-ripening

Regulation of seed germination: ROS, epigenetic, and hormonal aspects

BACKGROUND

The whole life of a plant is regulated by complex environmental or hormonal signaling networks that control genomic stability, environmental signal transduction, and gene expression affecting plant development and viability. Seed germination, responsible for the transformation from seed to seedling, is a key initiation step in plant growth and is controlled by unique physiological and biochemical processes. It is continuously modulated by various factors including epigenetic modifications, hormone transport, ROS signaling, and interaction among them. ROS showed versatile crucial functions in seed germination including various physiological oxidations to nucleic acid, protein, lipid, or chromatin in the cytoplasm, cell wall, and nucleus.

AIM

of review: This review intends to provide novel insights into underlying mechanisms of seed germination especially associated with the ROS, and considers how these versatile regulatory mechanisms can be developed as useful tools for crop improvement.

KEY SCIENTIFIC CONCEPTS OF REVIEW

We have summarized the generation and elimination of ROS during seed germination, with a specific focus on uncovering and understanding the mechanisms of seed germination at the level of phytohormones, ROS, and epigenetic switches, as well as the close connections between them. The findings exhibit that ROS plays multiple roles in regulating the ethylene, ABA, and GA homeostasis as well as the Ca2+ signaling, NO signaling, and MAPK cascade in seed germination via either the signal trigger or the oxidative modifier agent. Further, ROS shows the potential in the nuclear genome remodeling and some epigenetic modifiers function, although the detailed mechanisms are unclear in seed germination. We propose that ROS functions as a hub in the complex network regulating seed germination.

Regulatory Mechanisms of Bud Dormancy: Environmental, Hormonal, and Genetic Perspectives

Dormancy is a vital adaptive strategy in temperate and boreal plants, particularly fruit trees, enabling them to withstand harsh winter conditions and ensure survival and synchronized growth resumption in spring. This review comprehensively examines dormancy, focusing on its physiological, environmental, and molecular mechanisms. Dormancy is characterized by two distinct phases: endodormancy, which is regulated by internal plant signals and requires cold temperatures for release, and ecodormancy, which is influenced by external environmental factors. These stages are intricately linked to seasonal temperature fluctuations and the plant's ability to synchronize growth cycles, ensuring survival through harsh winters and optimal growth in warmer seasons. The review delves into the role of chilling requirements, temperature thresholds, and hormonal regulation in the dormancy process, highlighting how these factors influence critical growth events such as budbreak, flowering, and fruiting. Plant hormones, including abscisic acid, gibberellins, and cytokinins, regulate dormancy by modulating gene expression and growth activity. Additionally, we explore the historical development of dormancy research, from early observations of chilling requirements to the formulation of the chilling hours model. Considering ongoing climate change, the review examines how rising winter temperatures may disrupt dormancy cycles, potentially affecting the timing of flowering, fruiting, and overall crop productivity. This shift necessitates new strategies for managing dormancy, particularly in regions experiencing inconsistent or insufficient chilling. The review concludes by discussing practical approaches to enhance dormancy release and mitigate the impact of environmental stress on deciduous fruit tree growth, offering insights into improving agricultural practices amidst a changing climate.

Redox Control of Seed Germination is Mediated by the Crosstalk of Nitric Oxide and Reactive Oxygen Species

Significance: Seed germination and seedling establishment are characterized by changes in the intracellular redox state modulated by accelerated production of nitric oxide (NO) and reactive oxygen species (ROS). Redox regulation and enhanced accumulation of NO and ROS, approaching excessively high levels during seed imbibition, are critically important for breaking endodormancy and inducing germination. Recent Advances: Upon depletion of oxygen under the seed coat, NO is produced anaerobically in the reductive pathway associated mainly with mitochondria, and it participates in the energy metabolism of the seed until radicle protrusion. NO turnover involves nitrate reduction to nitrite in the cytosol, nitrite reduction to NO in mitochondria, and NO oxygenation in the cytosol in the reaction involving the hypoxically induced class 1 phytoglobin. In postgerminative degradation of seed tissues, NO and ROS are involved in redox signaling via post-translational modification of proteins and mediation of phytohormonal responses. Critical Issues: The crosstalk between the cellular redox potential, NO, ROS, and phytohormones integrates major physiological processes related to seed germination. Intensive accumulation of NO and ROS during imbibition is critically important for breaking seed dormancy. Upon oxygen depletion, NO and other nitrous oxides (NOx) are produced anaerobically and support energy metabolism prior to radicle protrusion. Future Directions: The turnover of NOx and ROS is determined by the intracellular redox balance, and it self-controls redox and energy levels upon germination. The particular details, regulation of this process, and its physiological significance remain to be established. Antioxid. Redox Signal. 42, 442-461.

ROS, an Important Plant Growth Regulator in Root Growth and Development: Functional Genes and Mechanism

Roots are fundamental to the growth, development, and survival of plants. Beyond anchoring the plant, roots absorb water and nutrients, supporting the plant's ability to grow and function normally. Root systems, originating from the apical meristem, exhibit significant diversity depending on the plant species. ROS are byproducts of aerobic metabolism, present in both above- and below-ground plant tissues. While ROS were once considered merely harmful byproducts of oxygen metabolism, they are now recognized as critical signaling molecules that regulate plant growth and development. Under stress conditions, plants produce elevated levels of ROS, which can inhibit growth. However, moderate ROS levels act as signals that integrate various regulatory pathways, contributing to normal plant development. However, there is still a lack of comprehensive and systematic research on how ROS precisely regulate root growth and development. This review provides an overview of ROS production pathways and their regulatory mechanisms in plants, with a particular focus on their influence on root development.

Stress induced factor 2 is a dual regulator for defense and seed germination in Arabidopsis

Receptor-like kinases (RLKs) constitute a diverse superfamily of proteins pivotal for various plant physiological processes, including responses to pathogens, hormone perception, growth, and development. Their ability to recognize conserved epitopes for general elicitors and specific pathogens marked significant advancements in plant pathology research. Emerging evidence suggests that RLKs and associated components also act as modulators in hormone signaling and cellular trafficking, showcasing their multifunctional roles in growth and development. Notably, STRESS INDUCED FACTOR 2 (SIF2) stands out as a representative with distinct expression patterns in different Arabidopsis organs. Our prior work highlighted the specific induction of SIF2 expression in guard cells, emphasizing its positive contribution to stomatal immunity. Expanding on these findings, our present study delves into the diverse functions of SIF2 expression in root tissues. Utilizing comprehensive physiology, molecular biology, protein biochemistry, and genetic analyses, we reveal that SIF2 modulates abscisic acid (ABA) signaling in Arabidopsis roots. SIF2 is epistatic with key regulators in the ABA signaling pathway, thereby governing the expression of genes crucial for dormancy release and, consequently, Arabidopsis seed germination. This study sheds light on the intricate roles of SIF2 as a multi-functional RLK, underscoring its organ-specific contributions to plant immunity, hormonal regulation, and seed germination.

Overexpression of OsRbohH Enhances Heat and Drought Tolerance through ROS Homeostasis and ABA Mediated Pathways in Rice (Oryza sativa L.)

Respiratory burst oxidase homologs (Rbohs) are the primary producers of reactive oxygen species (ROS), which have been demonstrated to play critical roles in plant responses to abiotic stress. Here, we explored the function of OsRbohH in heat and drought stress tolerance by generating overexpression lines (OsRbohH-OE). OsRbohH was highly induced by various abiotic stress and hormone treatments. Compared to wild-type (WT) controls, OsRbohH-OE plants exhibited enhanced tolerance to heat and drought, as determined by survival rate analyses and total chlorophyll content. Histochemical staining revealed that OsRbohH-OE accumulated less ROS. This is consistent with the observed increase in catalase (CAT) and peroxidase (POD) activities, as well as a reduced electrolyte leakage rate and malondialdehyde (MDA) content. Moreover, OsRbohH-OE exhibited enhanced sensitivity to exogenous abscisic acid (ABA), accompanied by altered expression levels of ABA synthesis and catabolic genes. Further analysis indicated that transgenic lines had lower transcripts of ABA signaling-related genes (OsDREB2A, OsLEA3, OsbZIP66, and OsbZIP72) under heat but higher levels under drought than WT. In conclusion, these results suggest that OsRbohH is a positive regulator of heat and drought tolerance in rice, which is probably performed through OsRbohH-mediated ROS homeostasis and ABA signaling.

Gene editing of the E3 ligase PIRE1 fine-tunes ROS production for enhanced bacterial disease resistance in tomato

Reactive oxygen species (ROS) accumulation is required for effective plant defense. Accumulation of the Arabidopsis NADPH oxidase RBOHD is regulated by phosphorylation of a conserved C-terminal residue (T912) leading to ubiquitination by the RING E3 ligase PIRE. Arabidopsis PIRE knockouts exhibit enhanced ROS production and resistance to the foliar pathogen Pseudomonas syringae. Here, we identified 170 PIRE homologs, which emerged in Tracheophytes and expanded in Angiosperms. We investigated the role of Solanum lycopersicum (tomato) PIRE homologs in regulating ROS production, RBOH stability, and disease resistance. Mutational analyses of residues corresponding to T912 in the tomato RBOHD ortholog, SlRBOHB, affected protein accumulation and ROS production in a PIRE-dependent manner. Using CRISPR-cas9, we generated mutants in two S. lycopersicum PIRE homologs (SlPIRE). SlPIRE1 edited lines (Slpire1) in the tomato cultivar M82 displayed enhanced ROS production upon treatment with flg22, an immunogenic epitope of flagellin. Furthermore, Slpire1 exhibited decreased disease symptoms and bacterial accumulation when inoculated with foliar bacterial pathogens Pseudomonas syringae and Xanthomonas campestris. However, Slpire1 exhibited similar levels of colonization as wild type upon inoculation with diverse soilborne pathogens. These results indicate that phosphorylation and ubiquitination crosstalk regulate RBOHs in multiple plant species, and PIRE is a promising target for foliar disease control. This study also highlights the pathogen-specific role of PIRE, indicating its potential for targeted manipulation to enhance foliar disease resistance without affecting root-associated interactions, positioning PIRE as a promising target for improving overall plant health.

Transcriptomic analysis of the defense response in "Cabernet Sauvignon" grape leaf induced by Apolygus lucorum feeding

To investigate the molecular mechanism of the defense response of "Cabernet Sauvignon" grapes to feeding by Apolygus lucorum, high-throughput sequencing technology was used to analyze the transcriptome of grape leaves under three different treatments: feeding by A. lucorum, puncture injury, and an untreated control. The research findings indicated that the differentially expressed genes were primarily enriched in three aspects: cellular composition, molecular function, and biological process. These genes were found to be involved in 42 metabolic pathways, particularly in plant hormone signaling metabolism, plant-pathogen interaction, MAPK signaling pathway, and other metabolic pathways associated with plant-induced insect resistance. Feeding by A. lucorum stimulated and upregulated a significant number of genes related to jasmonic acid and calcium ion pathways, suggesting their crucial role in the defense molecular mechanism of "Cabernet Sauvignon" grapes. The consistency between the gene expression and transcriptome sequencing results further supports these findings. This study provides a reference for the further exploration of the defense response in "Cabernet Sauvignon" grapes by elucidating the expression of relevant genes during feeding by A. lucorum.

H2O2 promotes trimming-induced tillering by regulating energy supply and redox status in bermudagrass

Tillering/branching pattern plays a significant role in determining the structure and diversity of grass, and trimming has been found to induce tillering in turfgrass. Recently, it has been reported that hydrogen peroxide (H2O2) regulates axillary bud development. However, the role of H2O2 in trimming-induced tillering in bermudagrass, a kind of turfgrass, remains unclear. Our study unveils the significant impact of trimming on promoting the sprouting and growth of tiller buds in stolon nodes, along with an increase in the number of tillers in the main stem. This effect is accompanied by spatial-temporal changes in cytokinin and sucrose content, as well as relevant gene expression in axillary buds. In addition, the partial trimming of new-born tillers results in an increase in sucrose and starch reserves in their leaves, which can be attributed to the enhanced photosynthesis capacity. Importantly, trimming promotes a rapid H2O2 burst in the leaves of new-born tillers and axillary stolon buds. Furthermore, exogenous application of H2O2 significantly increases the number of tillers after trimming by affecting the expression of cytokinin-related genes, bolstering photosynthesis potential, energy reserves and antioxidant enzyme activity. Taken together, these results indicate that both endogenous production and exogenous addition of H2O2 enhance the inductive effects of trimming on the tillering process in bermudagrass, thus helping boost energy supply and maintain the redox state in newly formed tillers.

Melatonin as a key regulator in seed germination under abiotic stress

Seed germination (SG) is the first stage in a plant's life and has an immense importance in sustaining crop production. Abiotic stresses reduce SG by increasing the deterioration of seed quality, and reducing germination potential, and seed vigor. Thus, to achieve a sustainable level of crop yield, it is important to improve SG under abiotic stress conditions. Melatonin (MEL) is an important biomolecule that interplays in developmental processes and regulates many adaptive responses in plants, especially under abiotic stresses. Thus, this review specifically summarizes and discusses the mechanistic basis of MEL-mediated SG under abiotic stresses. MEL regulates SG by regulating some stress-specific responses and some common responses. For instance, MEL induced stress specific responses include the regulation of ionic homeostasis, and hydrolysis of storage proteins under salinity stress, regulation of C-repeat binding factors signaling under cold stress, starch metabolism under high temperature and heavy metal stress, and activation of aquaporins and accumulation of osmolytes under drought stress. On other hand, MEL mediated regulation of gibberellins biosynthesis and abscisic acid catabolism, redox homeostasis, and Ca2+ signaling are amongst the common responses. Nonetheless factors such as endogenous MEL contents, plant species, and growth conditions also influence above-mentioned responses. In conclusion, MEL regulates SG under abiotic stress conditions by interacting with different physiological mechanisms.

The Plasma Membrane Purinoreceptor P2K1/DORN1 Is Essential in Stomatal Closure Evoked by Extracellular Diadenosine Tetraphosphate (Ap4A) in Arabidopsis thaliana

Dinucleoside polyphosphates (NpnNs) are considered novel signalling molecules involved in the induction of plant defence mechanisms. However, NpnN signal recognition and transduction are still enigmatic. Therefore, the aim of our research was the identification of the NpnN receptor and signal transduction pathways evoked by these nucleotides. Earlier, we proved that purine and pyrimidine NpnNs differentially affect the phenylpropanoid pathway in Vitis vinifera suspension-cultured cells. Here, we report, for the first time, that both diadenosine tetraphosphate (Ap4A) and dicytidine tetraphosphate (Cp4C)-induced stomatal closure in Arabidopsis thaliana. Moreover, we showed that plasma membrane purinoreceptor P2K1/DORN1 (does not respond to nucleotide 1) is essential for Ap4A-induced stomata movements but not for Cp4C. Wild-type Col-0 and the dorn1-3 A. thaliana knockout mutant were used. Examination of the leaf epidermis dorn1-3 mutant provided evidence that P2K1/DORN1 is a part of the signal transduction pathway in stomatal closure evoked by extracellular Ap4A but not by Cp4C. Reactive oxygen species (ROS) are involved in signal transduction caused by Ap4A and Cp4C, leading to stomatal closure. Ap4A induced and Cp4C suppressed the transcriptional response in wild-type plants. Moreover, in dorn1-3 leaves, the effect of Ap4A on gene expression was impaired. The interaction between P2K1/DORN1 and Ap4A leads to changes in the transcription of signalling hubs in signal transduction pathways.

Enhanced Ca2+ binding to EF-hands through phosphorylation of conserved serine residues activates MpRBOHB and chitin-triggered ROS production

NADPH oxidases/RBOHs catalyze apoplastic ROS production and act as key signaling nodes, integrating multiple signal transduction pathways regulating plant development and stress responses. Although RBOHs have been suggested to be activated by Ca2+ binding and phosphorylation by various protein kinases, a mechanism linking Ca2+ binding and phosphorylation in the activity regulation remained elusive. Chitin-triggered ROS production required cytosolic Ca2+ elevation and Ca2+ binding to MpRBOHB in a liverwort Marchantia polymorpha. Heterologous expression analysis of truncated variants revealed that a segment of the N-terminal cytosolic region highly conserved among land plant RBOHs encompassing the two EF-hand motifs is essential for the activation of MpRBOHB. Within the conserved regulatory domain, we have identified two Ser residues whose phosphorylation is critical for the activation in planta. Isothermal titration calorimetry analyses revealed that phosphorylation of the two Ser residues increased the Ca2+ binding affinity of MpRBOHB, while Ca2+ binding is indispensable for the activation, even if the two Ser residues are phosphorylated. Our findings shed light on a mechanism through which phosphorylation potentiates the Ca2+ -dependent activation of MpRBOHB, emphasizing the pivotal role of Ca2+ binding in mediating the Ca2+ and phosphorylation-driven activation of MpRBOHB, which is likely to represent a fundamental mechanism conserved among land plant RBOHs.

ROS interplay between plant growth and stress biology: Challenges and future perspectives

In plants, reactive oxygen species (ROS) have emerged as a multifunctional signaling molecules that modulate diverse stress and growth responses. Earlier studies on ROS in plants primarily focused on its toxicity and ROS-scavenging processes, but recent findings are offering new insights on its role in signal perception and transduction. Further, the interaction of cell wall receptors, calcium channels, HATPase, protein kinases, and hormones with NADPH oxidases (respiratory burst oxidase homologues (RBOHs), provides concrete evidence that ROS regulates major signaling cascades in different cellular compartments related to stress and growth responses. However, at the molecular level there are many knowledge gaps regarding how these players influence ROS signaling and how ROS regulate them during growth and stress events. Furthermore, little is known about how plant sensors or receptors detect ROS under various environmental stresses and induce subsequent signaling cascades. In light of this, we provided an update on the role of ROS signaling in plant growth and stress biology. First, we focused on ROS signaling, its production and regulation by cell wall receptor like kinases. Next, we discussed the interplay between ROS, calcium and hormones, which forms a major signaling trio regulatory network of signal perception and transduction. We also provided an overview on ROS and nitric oxide (NO) crosstalk. Furthermore, we emphasized the function of ROS signaling in biotic, abiotic and mechanical stresses, as well as in plant growth and development. Finally, we conclude by highlighting challenges and future perspectives of ROS signaling in plants that warrants future investigation.

Metabolic and Molecular Rearrangements of Sauvignon Blanc (Vitis vinifera L.) Berries in Response to Foliar Applications of Specific Dry Yeast

Dry yeast extracts (DYE) are applied to vineyards to improve aromatic and secondary metabolic compound content and wine quality; however, systematic information on the underpinning molecular mechanisms is lacking. This work aimed to unravel, through a systematic approach, the metabolic and molecular responses of Sauvignon Blanc berries to DYE treatments. To accomplish this, DYE spraying was performed in a commercial vineyard for two consecutive years. Berries were sampled at several time points after the treatment, and grapes were analyzed for sugars, acidity, free and bound aroma precursors, amino acids, and targeted and untargeted RNA-Seq transcriptional profiles. The results obtained indicated that the DYE treatment did not interfere with the technological ripening parameters of sugars and acidity. Some aroma precursors, including cys-3MH and GSH-3MH, responsible for the typical aromatic nuances of Sauvignon Blanc, were stimulated by the treatment during both vintages. The levels of amino acids and the global RNA-seq transcriptional profiles indicated that DYE spraying upregulated ROS homeostatic and thermotolerance genes, as well as ethylene and jasmonic acid biosynthetic genes, and activated abiotic and biotic stress responses. Overall, the data suggested that the DYE reduced berry oxidative stress through the regulation of specific subsets of metabolic and hormonal pathways.

OsRbohI Regulates Rice Growth and Development via Jasmonic Acid Signalling

Reactive oxygen species (ROS) are highly reactive molecules, generated by nicotinamide adenine dinucleotide phosphate oxidases encoded by respiratory burst oxidase homologs. The functions of the OsRbohs gene family in rice are diverse and poorly understood. OsRbohI was recently identified as a newly evolved gene in the rice OsRbohs gene family. However, the function of OsRbohI in regulating rice growth is not yet reported. In this study, our results indicate that knockout (KO) OsRbohI mutants showed significantly shorter shoot and primary roots, along with lower ROS content than the control lines, whereas the overexpression (OE) lines displayed contrasting results. Further experiments showed that the abnormal length of the shoot and root is mainly caused by altered cell size. These results indicate that OsRbohI regulates rice shoot and root growth through the ROS signal. More importantly, RNA-seq analysis and jasmonic acid (JA) treatment demonstrated that OsRbohI regulates rice growth via the JA synthesis and signaling pathways. Compared with the control, the results showed that the KO mutants were more sensitive to JA, whereas the OE lines were less sensitive to JA. Collectively, our results reveal a novel pathway in which OsRbohI regulates rice growth and development by affecting their ROS homeostasis through JA synthesis and signaling pathway.

Regulation of reactive oxygen species and phytohormones in osmotic stress tolerance during seed germination in indica rice

Climate change due to global warming is now affecting agricultural production worldwide. In rice, one of the most important crops, water limitation due to irregular rainfall in rainfed lowlands during crop growth limits yield. Dry direct-sowing has been proposed as a water-efficient approach to cope with water stress during rice growth, but poor seedling establishment due to drought during germination and emergence is a problem. Here, we germinated indica rice cultivars Rc348 (drought tolerant) and Rc10 (drought sensitive) under osmotic stress induced by PEG to elucidate mechanisms of germination under drought. Rc348 had higher germination rate and germination index under severe osmotic stress of -1.5 MPa, above those of Rc10. Rc348 showed up-regulated GA biosynthesis, down-regulated ABA catabolism, and up-regulated α-amylase gene expression in imbibed seeds under PEG treatment compared to that of Rc10. During germination, reactive oxygen species (ROS) play important roles in antagonism between gibberellic acid (GA) and abscisic acid (ABA). Embryo of Rc348 treated with PEG had significantly greater expression of NADPH oxidase genes and higher endogenous ROS levels, together with significantly increased endogenous GA₁, GA₄ and ABA contents compared to that of Rc10. In aleurone layers treated with exogenous GA, expression of α-amylase genes was higher in Rc348 than in Rc10, and expression of NADPH oxidase genes was enhanced with significantly higher ROS content in Rc348, suggesting higher sensitivity of GA to ROS production and starch degradation in aleurone cells of Rc348. These results suggest that the osmotic stress tolerance of Rc348 is due to enhancement of ROS production, GA biosynthesis, and GA sensitivity, resulting in a higher germination rate under osmotic stress.

Seed pretreatment with melatonin confers cadmium tolerance to chickpea seedlings through cellular redox homeostasis and antioxidant gene expression improvement

Several phytoremediation strategies have been undertaken to alleviate cadmium (Cd)-mediated injury to crop yield resulting from agricultural land pollution. In the present study, the potentially beneficial effect of melatonin (Me) was appraised. Therefore, chickpea (Cicer arietinum L.) seeds were imbibed for 12 H in distilled water or Me (10 µM) solution. Then, the seeds germinated in the presence or the absence of 200 µM CdCl₂ for 6 days. Seedlings obtained from Me-pretreated seeds exhibited enhanced growth traits, reflected by fresh biomass and length increase. This beneficial effect was associated with a decreased Cd accumulation in seedling tissues (by 46 and 89% in roots and shoots, respectively). Besides, Me efficiently protected the cell membrane integrity of Cd-subjected seedlings. This protective effect was manifested by the decreased lipoxygenase activity and the subsequently reduced accumulation of 4-hydroxy-2-nonenal. Melatonin counteracted the Cd-mediated stimulation of the pro-oxidant NADPH-oxidase (90 and 45% decrease compared to non-pretreated Cd-stressed roots and shoots, respectively) and NADH-oxidase activities (almost 40% decrease compared to non-pretreated roots and shoots), preventing, thus, hydrogen peroxide overaccumulation (50 and 35% lesser than non-pretreated roots and shoots, respectively). Furthermore, Me enhanced the cellular content of pyridine nicotinamide reduced forms [NAD(P)H] and their redox state. This effect was associated with the Me-mediated stimulation of the glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase activities, concomitantly with the inhibition of NAD(P)H-consuming activities. These effects were accompanied by the up-regulation of G6PDH gene expression (45% increase in roots) and the down-regulation of the respiratory burst oxidase homolog protein F (RBOHF) gene expression (53% decrease in roots and shoots). Likewise, Me induced an increased activity and gene transcription of the Asada-Halliwell cycle, namely ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase, concomitantly with a reduction of the glutathione peroxidase activity. This modulating effect led to the restoration of the redox homeostasis of the ascorbate and the glutathione pools. Overall, current results attest that seed pretreatment with Me is effective in Cd stress relief and can be a beneficial crop-protective approach.

Evolutionary Analysis of Respiratory Burst Oxidase Homolog (RBOH) Genes in Plants and Characterization of ZmRBOHs

The respiratory burst oxidase homolog (RBOH), as the key producer of reactive oxygen species (ROS), plays an essential role in plant development. In this study, a bioinformatic analysis was performed on 22 plant species, and 181 RBOH homologues were identified. A typical RBOH family was identified only in terrestrial plants, and the number of RBOHs increased from non-angiosperms to angiosperms. Whole genome duplication (WGD)/segmental duplication played a key role in RBOH gene family expansion. Amino acid numbers of 181 RBOHs ranged from 98 to 1461, and the encoded proteins had molecular weights from 11.1 to 163.6 kDa, respectively. All plant RBOHs contained a conserved NADPH_Ox domain, while some of them lacked the FAD_binding_8 domain. Plant RBOHs were classified into five main subgroups by phylogenetic analysis. Most RBOH members in the same subgroup showed conservation in both motif distribution and gene structure composition. Fifteen ZmRBOHs were identified in maize genome and were positioned in eight maize chromosomes. A total of three pairs of orthologous genes were found in maize, including ZmRBOH6/ZmRBOH8, ZmRBOH4/ZmRBOH10 and ZmRBOH15/ZmRBOH2. A Ka/Ks calculation confirmed that purifying selection was the main driving force in their evolution. ZmRBOHs had typical conserved domains and similar protein structures. cis-element analyses together with the expression profiles of the ZmRBOH genes in various tissues and stages of development suggested that ZmRBOH was involved in distinct biological processes and stress responses. Based on the RNA-Seq data and qRT-PCR analysis, the transcriptional response of ZmRBOH genes was examined under various abiotic stresses, and most of ZmRBOH genes were up-regulated by cold stress. These findings provide valuable information for further revealing the biological roles of ZmRBOH genes in plant development and abiotic stress responses.

Multicopper oxidases GbAO and GbSKS are involved in the Verticillium dahliae resistance in Gossypium barbadense

Ascorbate oxidase (AO) and skewed5 (SKU5)-similar (SKS) proteins belong to the multicopper oxidase (MCO) family and play important roles in plants in response to environmental stress via modulation of oxidoreduction homeostasis. Currently, reports on the response of Gossypium barbadense MCO to Verticillium wilt (VW) caused by Verticillium dahliae are still limited. Herein, RNA sequencing of two G. barbadense cultivars of VW-resistant XH21 and VW-susceptible XH7 under V. dahliae treatment, combined with physiological and genetic analysis, was performed to analyze the function and mechanism of multicopper oxidases GbAO and GbSKS involved in V. dahliae resistance. The identified differentially expressed genes are mainly involved in the regulation of oxidoreduction reaction, and extracellular components and signaling. Interestingly, ascorbate oxidase family members were discovered as the most significantly upregulated genes after V. dahliae treatment, including GbAO3A/D, GbSKS3A/D, and GbSKS16A/D. H₂O₂ and Asc contents, especially reductive Asc in both XH21 and XH7, were shown to be increased. Silenced expression of respective GbAO3A/D, GbSKS3A/D, and GbSKS16A/D in virus-induced gene silencing (VIGS) cotton plants significantly decreased the resistance to V. dahliae, coupled with the reduced contents of pectin and lignin. Our results indicate that AO might be involved in cotton VW resistance via the regulation of cell wall components.

Reactive oxygen species function as signaling molecules in controlling plant development and hormonal responses

Reactive oxygen species (ROS) serve as second messengers in plant signaling pathways to remodel plant growth and development. New insights into how enzymatic ROS-producing machinery is regulated by hormones or localized during development have provided a framework for understanding the mechanisms that control ROS accumulation patterns. Signaling-mediated increases in ROS can then modulate the activity of proteins through reversible oxidative modification of specific cysteine residues. Plants also control the synthesis of antioxidants, including plant-specialized metabolites, to further define when, where, and how much ROS accumulate. The availability of sophisticated imaging capabilities, combined with a growing tool kit of ROS detection technologies, particularly genetically encoded biosensors, sets the stage for improved understanding of ROS as signaling molecules.

Cold stratification-induced dormancy removal in apple (Malus domestica Borkh.) seeds is accompanied by an increased glutathione pool in embryonic axes

A reduced form of glutathione (GSH) is an essential metabolite that participates in the control of reactive oxygen species (ROS) levels in cells. GSH plays a pivotal role in seed biology as a modulator of seed viability and germination. The GSH:GSSG ratio and half-cell reduction potential (E_GSSG/2GSH) serve as indicators of the oxidative status in seeds. Apple (Malus domestica Borkh.) seeds are deeply dormant, and this state is removed by long-term cold stratification. The aim of our work was to examine the modification of GSH and GSSG content, GSH:GSSG ratio and E_GSSG/2GSH in the embryonic axes isolated from apple seeds subjected to cold stratification for 7, 14, 21 and 40 d. Our data indicated that cold stratification increased the generation of free radicals in the embryonic axes, which correlated with an alteration in the expression of genes encoding Rboh, particularly RbohC. GSH and GSSG levels increased during prolonged cold stratification of apple seeds. This was accompanied by the modification of glutathione reductase and glutathione peroxidase-like activities, which did not match their transcript levels. The steady-state GSH:GSSG ratio and E_GSSG/2GSH in the axes of embryos subjected to cold stratification indicated no impact of the dormancy removal treatment on apple seed viability. We suggest that the glutathione system is an important component of the redox network and is involved in the management of the seed transition from dormant to nondormant states.

Endosperm weakening: The gateway to a seed's new life

Seed germination is a crucial stage in a plant's life cycle, during which the embryo, surrounded by several tissues, undergoes a transition from the quiescent to a highly active state. Endosperm weakening, a key step in this transition, plays an important role in radicle protrusion. Endosperm weakening is initiated upon water uptake, followed by multiple key molecular events occurring within and outside endosperm cells. Although available transcriptomes have provided information about pivotal genes involved in this process, a complete understanding of the signaling pathways are yet to be elucidated. Much remains to be learnt about the diverse intercellular signals, such as reactive oxygen species-mediated redox signals, phytohormone crosstalk, environmental cue-dependent oxidative phosphorylation, peroxisomal-mediated pectin degradation, and storage protein mobilization during endosperm cell wall loosening. This review discusses the evidences from recent researches into the mechanism of endosperm weakening. Further, given that the endosperm has great potential for manipulation by crop breeding and biotechnology, we offer several novel insights, which will be helpful in this research field and in its application to the improvement of crop production.

Linking Reactive Oxygen Species (ROS) to Abiotic and Biotic Feedbacks in Plant Microbiomes: The Dose Makes the Poison

In nature, plants develop in complex, adaptive environments. Plants must therefore respond efficiently to environmental stressors to maintain homeostasis and enhance their fitness. Although many coordinated processes remain integral for achieving homeostasis and driving plant development, reactive oxygen species (ROS) function as critical, fast-acting orchestrators that link abiotic and biotic responses to plant homeostasis and development. In addition to the suite of enzymatic and non-enzymatic ROS processing pathways that plants possess, they also rely on their microbiota to buffer and maintain the oxidative window needed to balance anabolic and catabolic processes. Strong evidence has been communicated recently that links ROS regulation to the aggregated function(s) of commensal microbiota and plant-growth-promoting microbes. To date, many reports have put forth insightful syntheses that either detail ROS regulation across plant development (independent of plant microbiota) or examine abiotic–biotic feedbacks in plant microbiomes (independent of clear emphases on ROS regulation). Here we provide a novel synthesis that incorporates recent findings regarding ROS and plant development in the context of both microbiota regulation and plant-associated microbes. Specifically, we discuss various roles of ROS across plant development to strengthen the links between plant microbiome functioning and ROS regulation for both basic and applied research aims.

Nitric Oxide as a Remedy against Oxidative Damages in Apple Seeds Undergoing Accelerated Ageing

Seed ageing is associated with a high concentration of reactive oxygen species (ROS). Apple (Malus domestica Borkh.) seeds belong to the orthodox type. Due to a deep dormancy, they may be stored in dry condition at 5 °C for a long time, without viability loss. In the laboratory, artificial ageing of apple seeds is performed by imbibition in wet sand at warm temperature (33 °C). The aim of the work was to study nitric oxide (NO) as a seed vigour preservation agent. Embryos isolated from apple seeds subjected to accelerated ageing for 7, 14, 21 or 40 days were fumigated with NO. Embryo quality was estimated by TTC and MDA tests. ROS level was confirmed by NBT staining. We analysed the alteration in transcript levels of CAT, SOD and POX. NO fumigation of embryos of seeds aged for 21 days stimulated germination and increased ROS level which correlated to the elevated expression of RBOH. The increased total antioxidant capacity after NO fumigation was accompanied by the increased transcript levels of genes encoding enzymatic antioxidants, that could protect against ROS overaccumulation. Moreover, post-aged NO application diminished the nitro-oxidative modification of RNA, proving NO action as a remedy in oxidative remodelling after seeds ageing.

Production of reactive oxygen species by PuRBOHF is critical for stone cell development in pear fruit

The production of reactive oxygen species (ROS) by NADPH oxidase, which is also referred to as respiratory burst oxidase homolog (RBOH), affects several processes in plants. However, the role of RBOHs in cell wall lignification is not well understood. In this study, we show that PuRBOHF, an RBOH isoform, plays an important role in secondary wall formation in pear stone cells. ROS were closely associated with lignin deposition and stone cell formation according to microscopy data. In addition, according to the results of an in situ hybridization analysis, the stage-specific expression of PuRBOHF was higher in stone cells than in cells of other flesh tissues. Inhibitors of RBOH activity suppressed ROS accumulation and stone cell lignification in pear fruit. Moreover, transient overexpression of PuRBOHF caused significant changes in the amount of ROS and lignin that accumulated in pear fruit and flesh calli. We further showed that PuMYB169 regulates PuRBOHF expression, while PuRBOHF-derived ROS induces the transcription of PuPOD2 and PuLAC2. The findings of this study indicate that PuRBOHF-mediated ROS production, which is regulated by a lignin-related transcriptional network, is essential for monolignol polymerization and stone cell formation in pear fruit.

Changes in Reactive Oxygen Species, Antioxidants and Carbohydrate Metabolism in Relation to Dormancy Transition and Bud Break in Apple (Malus × domestica Borkh) Cultivars

Understanding the biochemical mechanisms underlying bud dormancy and bloom time regulation in deciduous woody perennials is critical for devising effective strategies to protect these species from spring frost damage. This study investigated the accumulation profiles of carbohydrates, ROS and antioxidants during dormancy in ‘Cripps Pink’ and ‘Honeycrisp’, two apple cultivars representing the early and late bloom cultivars, respectively. Our data showed that starch levels generally declined during dormancy, whereas soluble sugars increased. However, the present study did not record significant alternations in the carbohydrate accumulation profiles between the two cultivars that could account for the differences in their bloom dates. On the other hand, H₂O₂ accumulation patterns revealed an apparent correlation with the dormancy stage and bloom dates in both cultivars; peaking early in the early-blooming cultivar, sustaining high levels for a longer time in the late-blooming cultivars, and fading by the time of bud burst in both cultivars. Also, the redox balance during dormancy appeared to be maintained mainly by catalase and, to a lesser extent, by glutathione (GSH). Overall, the present study concludes that differences in ROS and the bud redox balance could, at least partially, explain the differences in dormancy duration and bloom date among apple cultivars.

A New Role for Plastid Thioredoxins in Seed Physiology in Relation to Hormone Regulation

In Arabidopsis seeds, ROS have been shown to be enabling actors of cellular signaling pathways promoting germination, but their accumulation under stress conditions or during aging leads to a decrease in the ability to germinate. Previous biochemical work revealed that a specific class of plastid thioredoxins (Trxs), the y-type Trxs, can fulfill antioxidant functions. Among the ten plastidial Trx isoforms identified in Arabidopsis, Trx y1 mRNA is the most abundant in dry seeds. We hypothesized that Trx y1 and Trx y2 would play an important role in seed physiology as antioxidants. Using reverse genetics, we found important changes in the corresponding Arabidopsis mutant seeds. They display remarkable traits such as increased longevity and higher and faster germination in conditions of reduced water availability or oxidative stress. These phenotypes suggest that Trxs y do not play an antioxidant role in seeds, as further evidenced by no changes in global ROS contents and protein redox status found in the corresponding mutant seeds. Instead, we provide evidence that marker genes of ABA and GAs pathways are perturbed in mutant seeds, together with their sensitivity to specific hormone inhibitors. Altogether, our results suggest that Trxs y function in Arabidopsis seeds is not linked to their previously identified antioxidant roles and reveal a new role for plastid Trxs linked to hormone regulation.

Differential Regulation of NAPDH Oxidases in Salt-Tolerant Eutrema salsugineum and Salt-Sensitive Arabidopsis thaliana

Reactive oxygen species (ROS) signalling is crucial in modulating stress responses in plants, and NADPH oxidases (NOXs) are an important component of signal transduction under salt stress. The goal of this research was to investigate whether the regulation of NOX-dependent signalling during mild and severe salinity differs between the halophyte Eutrema salsugineum and the glycophyte Arabidopsis thaliana. Gene expression analyses showed that salt-induced expression patterns of two NOX genes, RBOHD and RBOHF, varied between the halophyte and the glycophyte. Five days of salinity stimulated the expression of both genes in E. salsugineum leaves, while their expression in A. thaliana decreased. This was not accompanied by changes in the total NOX activity in E. salsugineum, while the activity in A. thaliana was reduced. The expression of the RBOHD and RBOHF genes in E. salsugineum leaves was induced by abscisic acid (ABA) and ethephon spraying. The in silico analyses of promoter sequences of RBOHD and RBOHF revealed multiple cis-acting elements related to hormone responses, and their distribution varied between E. salsugineum and A. thaliana. Our results indicate that, in the halophyte E. salsugineum, the maintenance of the basal activity of NOXs in leaves plays a role during acclimation responses to salt stress. The different expression patterns of the RBOHD and RBOHF genes under salinity in E. salsugineum and A. thaliana point to a modified regulation of these genes in the halophyte, possibly through ABA- and/or ethylene-dependent pathways.

Superoxide is promoted by sucrose and affects amplitude of circadian rhythms in the evening

Plants must coordinate photosynthetic metabolism with the daily environment and adapt rhythmic physiology and development to match carbon availability. Circadian clocks drive biological rhythms which adjust to environmental cues. Products of photosynthetic metabolism, including sugars and reactive oxygen species (ROS), are closely associated with the plant circadian clock, and sugars have been shown to provide metabolic feedback to the circadian oscillator. Here, we report a comprehensive sugar-regulated transcriptome of Arabidopsis and identify genes associated with redox and ROS processes as a prominent feature of the transcriptional response. We show that sucrose increases levels of superoxide (O₂ ^-), which is required for transcriptional and growth responses to sugar. We identify circadian rhythms of O₂ ^--regulated transcripts which are phased around dusk and find that O₂ ^- is required for sucrose to promote expression of TIMING OF CAB1 (TOC1) in the evening. Our data reveal a role for O₂ ^- as a metabolic signal affecting transcriptional control of the circadian oscillator in Arabidopsis.

Protein Carbonylation: Emerging Roles in Plant Redox Biology and Future Prospects

Plants are sessile in nature and they perceive and react to environmental stresses such as abiotic and biotic factors. These induce a change in the cellular homeostasis of reactive oxygen species (ROS). ROS are known to react with cellular components, including DNA, lipids, and proteins, and to interfere with hormone signaling via several post-translational modifications (PTMs). Protein carbonylation (PC) is a non-enzymatic and irreversible PTM induced by ROS. The non-enzymatic feature of the carbonylation reaction has slowed the efforts to identify functions regulated by PC in plants. Yet, in prokaryotic and animal cells, studies have shown the relevance of protein carbonylation as a signal transduction mechanism in physiological processes including hydrogen peroxide sensing, cell proliferation and survival, ferroptosis, and antioxidant response. In this review, we provide a detailed update on the most recent findings pertaining to the role of PC and its implications in various physiological processes in plants. By leveraging the progress made in bacteria and animals, we highlight the main challenges in studying the impacts of carbonylation on protein functions in vivo and the knowledge gap in plants. Inspired by the success stories in animal sciences, we then suggest a few approaches that could be undertaken to overcome these challenges in plant research. Overall, this review describes the state of protein carbonylation research in plants and proposes new research avenues on the link between protein carbonylation and plant redox biology.

Effects of 1-MCP treatment on sprouting and preservation of ginger rhizomes during storage at room temperature

The purpose of this study was to explore the effects of 1-MCP on the sprouting and preservation of ginger rhizomes during storage at room temperature. Ginger rhizomes were treated with 1 µL L-1 1-methylcyclopropene (1-MCP) and stored at 23 ± 0.2 °C. Our data showed that application of 1-MCP reduced the rate of sprouting during storage compared with the control rhizome. Respiration rate and the reducing sugar content were also reduced following 1-MCP treatment, while the starch content increased. 1-MCP treatment increased the total phenol content and inhibited polyphenol oxidase (PPO) activity. 1-MCP treatment was also associated with a higher ascorbic acid content but a reduced crude fiber content. The generation of superoxide anion free radicals (O2-), hydrogen peroxide (H2O2) and malondialdehyde (MDA) was lower following 1-MCP treatment, while the activities of catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) were higher compared with the controls. These results suggested that application of 1-MCP could reduce sprouting rates, decrease the accumulation of ROS, and maintain the quality of ginger rhizomes during storage at room temperature. It would be useful to further explore the role and mechanisms of action of ethylene in regulating the sprouting of ginger rhizomes.

Stress-induced reactive oxygen species compartmentalization, perception and signalling

Reactive oxygen species (ROS) are essential for life and are involved in the regulation of almost all biological processes. ROS production is critical for plant development, response to abiotic stresses and immune responses. Here, we focus on recent discoveries in ROS biology emphasizing abiotic and biotic stress responses. Recent advancements have resulted in the identification of one of the first sensors for extracellular ROS and highlighted waves of ROS production during stress signalling in Arabidopsis. Enzymes that produce ROS, including NADPH oxidases, exhibit precise regulation through diverse post-translational modifications. Discoveries highlight the importance of both amino- and carboxy-terminal regulation of NADPH oxidases through protein phosphorylation and cysteine oxidation. Here, we discuss advancements in ROS compartmentalization, systemic ROS waves, ROS sensing and post-translational modification of ROS-producing enzymes and identify areas where foundational gaps remain.

The Barley (Hordeum vulgare ssp. vulgare) Respiratory Burst Oxidase Homolog (HvRBOH) Gene Family and Their Plausible Role on Malting Quality

Controlled generation of reactive oxygen species (ROS) is pivotal for normal plant development and adaptation to changes in the external milieu. One of the major enzymatic sources of ROS in plants are the plasma-membrane localized NADPH oxidases, also called as Respiratory Burst Oxidase Homologs (RBOH). In addition to the six previously reported, seven new members of RBOH gene family were identified in barley using in silico analysis. Conservation of genomic structure and key residues important for catalytic activity and co-factor binding was observed in barley RBOH genes. Phylogenetic analysis of plant RBOHs revealed distinct clades for monocot and dicot RBOH proteins. Hence, we propose to use the rice nomenclature for naming barley RBOH genes. Temporal changes in ROS profiles were observed during barley malting and was accompanied by changes in protein carbonylation, lipid peroxidation, and antioxidant capacity. Among the nine differentially expressed HvRBOHs during various malting stages, HvRBOHA and HvRBOHC showed most significant sustained changes in expression. RNAi knockdown lines with reduced expression of HvRBOHA/C gene exhibited genetic compensation via inducible expression of other gene family members during malting. However, the physiological consequence of reduced expression of HvRBOHA/C manifested as a poor malting quality profile attributable to low alpha-amylase activity and high levels of beta-glucan. We propose that the HvRBOHs play a critical role in modulating the redox milieu during the early stages of malting, which in turn can significantly impact carbohydrate metabolism.

Arabidopsis BRCA1 represses RRTF1-mediated ROS production and ROS-responsive gene expression under dehydration stress

Reactive oxygen species (ROS) act as important secondary messengers in abscisic acid (ABA) signaling and induce stomatal closure under dehydration stress. The breast cancer susceptibility gene 1 (BRCA1), an important tumor suppressor in animals, functions primarily in the maintenance of genome integrity in animals and plants. However, whether and how the plant BRCA1 regulates intracellular ROS homeostasis in guard cells under dehydration stress remains unknown. Here, we found that Arabidopsis atbrca1 loss-of-function mutants showed dehydration stress tolerance. This stress tolerant phenotype of atbrca1 was a result of ABA- and ROS-induced stomatal closure, which was enhanced in atbrca1 mutants compared with the wild-type. AtBRCA1 downregulated the expression of ROS-responsive and marker genes. Notably, these genes were also the targets of the AP2/ERF transcriptional activator RRTF1/ERF109. Under normal conditions, AtBRCA1 physically interacted with RRTF1 and inhibited its binding to the GCC-box-like sequence in target gene promoters. Under dehydration stress, the expression of AtBRCA1 was dramatically reduced and that of RRTF1 was activated, thus inducing the expression of ROS-responsive genes. Overall, our study reveals a novel molecular function of AtBRCA1 in the transcriptional regulation of intracellular ROS homeostasis under dehydration stress.

OsRbohB-mediated ROS production plays a crucial role in drought stress tolerance of rice

We found that a rice NADPH oxidase gene OsRbohB contributes drought tolerance and its functions are involved in the interaction of the OsRbohB-mediated ROS production and ABA signaling. The plasma membrane NADPH oxidases, also known as respiratory burst oxidase homologs, are the key producers of ROS under both normal and stress conditions in plants. However, their functions in rice development and stress tolerance are still under investigation. Here, we found that a rice NADPH oxidase gene OsRbohB, also named OsNOX1, is expressed in all tissues examined throughout the development stages with higher transcripts in leaves. The transcriptional expression of OsRbohB is also strongly stimulated by dehydration, salt and several phytohormonal treatments. Compared with wide-type and the OsRbohB-overexpressing transgenic plants, osrbohB, a Tos17 insertion knockout mutant of OsRbohB, shows lower ROS production, abscisic acid (ABA) content and transcripts of a series of stress-related genes. The osrbohB mutant also exhibits lower seed germination rate, organ size and thousand seed weight, but higher stomatal aperture and sensitivity to drought. Moreover, a number of genes involved in plant development, stress response, transcriptional regulation, and particularly ABA signaling are differentially expressed in osrbohB plants under both normal growth and drought conditions. All these results suggest the roles of OsRbohB in drought tolerance of rice, which probably performed through the interaction of the OsRbohB-mediated ROS production and ABA signaling.

Wheat Transcription Factor TaSNAC11-4B Positively Regulates Leaf Senescence through Promoting ROS Production in Transgenic Arabidopsis

Senescence is the final stage of leaf development which is accompanied by highly coordinated and complicated reprogramming of gene expression. Genetic manipulation of leaf senescence in major crops including wheat has been shown to be able to increase stress tolerance and grain yield. NAC(No apical meristem (NAM), ATAF1/2, and cup-shaped cotyledon (CUC)) transcription factors (TFs) play important roles in regulating gene expression changes during leaf senescence and in response to abiotic stresses. Here, we report the characterization of TaSNAC11-4B (Uniprot: A0A1D5XI64), a wheat NAC family member that acts as a functional homolog of AtNAP, a key regulator of leaf senescence in Arabidopsis. The expression of TaSNAC11-4B was up-regulated with the progression of leaf senescence, in response to abscisic acid (ABA) and drought treatments in wheat. Ectopic expression of TaSNAC11-4B in Arabidopsis promoted ROS accumulation and significantly accelerated age-dependent as well as drought- and ABA-induced leaf senescence. Results from transcriptional activity assays indicated that the TaSNAC11-4B protein displayed transcriptional activation activities that are dependent on its C terminus. Furthermore, qRT-PCR and dual-Luciferase assay results suggested that TaSNAC11-4B could positively regulate the expression of AtrbohD and AtrbohF, which encode catalytic subunits of the ROS-producing NADPH oxidase. Further analysis of TaSNAC11-4B in wheat senescence and the potential application of this gene in manipulating leaf senescence with the purpose of yield increase and stress tolerance is discussed.

Analysis of the energy source at the early stage of poplar seed germination: verification of Perl's pathway

Adenosine triphosphate (ATP) is produced at the early stage of seed germination and provides the energy for metabolism. The source of ATP in seeds may be Perl's pathway, but this has not yet been confirmed. In this study, using germinating seeds of poplar as the experimental materials, the transcript levels of genes related to Perl's pathway were determined by real-time PCR. The activities of enzymes in Perl's pathway were also determined. The results were verified by comparison with RNA-Seq and metabolomics data. The results showed that there were high transcript levels of some genes encoding malate dehydrogenase (MDH), phosphoenolpyruvate carboxykinase (PEPCK), pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), and pyruvate kinase (PK) at the early stage of germination (0.75 h). The enzymes MDH, PEPCK, PK, PDC, and ADH showed peaks in activity at around 0.75 h and 6 h during germination. The oxaloacetate concentration was high in poplar seeds at the early stage of germination. This study provides experimental data showing that Perl's pathway participates in supplying energy during the early stages of poplar seed germination, and lays the foundation for further studies on the complex metabolic processes that function during seed germination.

Genome-wide identification and characterization of Respiratory Burst Oxidase Homolog genes in six Rosaceae species and an analysis of their effects on adventitious rooting in apple

Adventitious root formation is essential for plant propagation, development, and response to various stresses. Reactive oxygen species (ROS) are essential for adventitious root formation. However, information on Respiratory Burst Oxidase Homolog (RBOH), a key enzyme that catalyzes the production ROS, remains limited in woody plants. Here, a total of 44 RBOH genes were identified from six Rosaceae species (Malus domestica, Prunus avium, Prunus dulcis 'Texas’, Rubus occidentalis, Fragaria vesca and Rosa chinensis), including ten from M. domestica. Their phylogenetic relationships, conserved motifs and gene structures were analyzed. Exogenous treatment with the RBOH protein inhibitor diphenyleneiodonium (DPI) completely inhibited adventitious root formation, whereas exogenous H₂O₂ treatment enhanced adventitious root formation. In addition, we found that ROS accumulated during adventitious root primordium inducing process. The expression levels of MdRBOH-H, MdRBOH-J, MdRBOH-A, MdRBOH-E1 and MdRBOH-K increased more than two-fold at days 3 or 9 after auxin treatment. In addition, cis-acting element analysis revealed that the MdRBOH-E1 promoter contained an auxin-responsive element and the MdRBOH-K promoter contained a meristem expression element. Based on the combined results from exogenous DPI and H₂O₂ treatment, spatiotemporal expression profiling, and cis-element analysis, MdRBOH-E1 and MdRBOH-K appear to be candidates for the control of adventitious rooting in apple.

Comprehensive Analysis of Respiratory Burst Oxidase Homologs (Rboh) Gene Family and Function of GbRboh5/18 on Verticillium Wilt Resistance in Gossypium barbadense

Respiratory burst oxidase homologs (Rbohs) play a predominant role in reactive oxygen species (ROS) production, which is crucial in plant growth, differentiation, as well as their responses to biotic and abiotic stresses. To date, however, there is little knowledge about the function of cotton Rboh genes. Here, we identified a total of 87 Rbohs from five sequenced Gossypium species (the diploids Gossypium arboreum, Gossypium raimondii, and Gossypium australe, and the allotetraploids Gossypium hirsutum and Gossypium barbadense) via BLAST searching their genomes. Phylogenetic analysis of the putative 87 cotton Rbohs revealed that they were divided into seven clades. All members within the same clade are generally similar to each other in terms of gene structure and conserved domain arrangement. In G. barbadense, the expression levels of GbRbohs in the CladeD were induced in response to a fungal pathogen and to hormones (i.e., jasmonic acid and abscisic acid), based upon which the main functional member in CladeD was discerned to be GbRboh5/18. Further functional and physiological analyses showed that the knock-down of GbRboh5/18 expression attenuates plant resistance to Verticillium dahliae infection. Combined with the molecular and biochemical analyses, we found less ROS accumulation in GbRboh5/18-VIGS plants than in control plants after V. dahliae infection. Overexpression of GbRboh5/18 in G. barbadense resulted in more ROS accumulation than in control plants. These results suggest that GbRboh5/18 enhances the cotton plants' resistance against V. dahliae by elevating the levels of ROS accumulation. By integrating phylogenetic, molecular, and biochemical approaches, this comprehensive study provides a detailed overview of the number, phylogeny, and evolution of the Rboh gene family from five sequenced Gossypium species, as well as elucidating the function of GbRboh5/18 for plant resistance against V. dahliae. This study sheds fresh light on the molecular evolutionary properties and function of Rboh genes in cotton, and provides a reference for improving cotton's responses to the pathogen V. dahliae.

Coordination of five class III peroxidase-encoding genes for early germination events of Arabidopsis thaliana

The Class III peroxidases (CIII Prxs) belong to a plant-specific multigene family. Thanks to their double catalytic cycle they can oxidize compounds or release reactive oxygen species (ROS). They are either involved in different cell wall stiffening processes such as lignification and suberization, in cell wall loosening or defense mechanisms. Germination is an important developmental stage requiring specific peroxidase activity. However, little is known about which isoforms are involved. Five CIII Prx encoding genes: AtPrx04, AtPrx16, AtPrx62, AtPrx69, and AtPrx71 were identified from published microarray data mining. Delayed or induced testa and endosperm rupture were observed for the corresponding CIII Prx mutant lines indicating either a gene-specific inducing or repressing role during germination, respectively. Via in situ hybridization AtPrx16, AtPrx62, AtPrx69 and AtPrx71 transcripts were exclusively localized to the micropylar endosperm facing the radicle, and transcriptomic data analysis enabled positioning the five CIII Prxs in a co-expression network enriched in germination, cell wall, cell wall proteins and xyloglucan hits. Evidence were produced showing that the five CIII Prxs were cell wall-targeted proteins and that the micropylar endosperm displayed a complex cell wall domain topochemistry. Finally, we drew a spatio-temporal model highlighting the fine sequential gene expression and the possible involvement of micropylar endosperm cell wall domains to explain the non-redundant cell wall stiffening and loosening functions of the CIII Prxs in a single cell type. We also highlighted the necessity of a peroxidase homeostasis to accurately control the micropylar endosperm cell wall dynamics during Arabidopsis germination events.

The NADPH-oxidase LsRbohC1 plays a role in lettuce (Lactuca sativa) seed germination

Reactive oxygen species (ROS) are increasingly recognized as non-enzymatic players in the processes of radicle elongation growth and endosperm weakening during seed germination. NADPH oxidases (EC 1.6.3.1), also known as respiratory burst oxidase homologues (Rbohs), are key enzymes for the production of ROS. We previously reported that ROS are involved in the radicle elongation growth and endosperm weakening during lettuce seed germination. However, the function of the gene(s) encoding Rbohs during lettuce seed germination remains to be elucidated. In this study, one lettuce Rboh gene LsRbohC1 was cloned, and over-expression and RNAi-lines of this gene were generated. It was found that LsRbohC1 was abundantly expressed in germinating seeds, especially in the endosperm cap and hypocotyl. Over-expression/knock-down of this gene observably increased/decreased the production of superoxide radicals in the radicle and endosperm cap, and significantly promoted/delayed the germination process. The results suggest that LsRbohC1 plays a role in lettuce seed germination.

The Arabidopsis RboHB Encoded by At1g09090 Is Important for Resistance against Nematodes

Reactive oxygen species are a byproduct of aerobic metabolic processes but are also produced by plants in defense against pathogens. In addition, they can function as signaling molecules that control various aspects of plant life, ranging from developmental processes to responses to abiotic and biotic stimuli. In plants, reactive oxygen species can be produced by respiratory burst oxidase homologues. Arabidopsis contains 10 genes for respiratory burst oxidase homologues that are involved in different aspects of plant life. Plant pathogenic cyst nematodes such as Heterodera schachtii induce a syncytium in the roots of host plants that becomes a feeding site which supplies nutrients throughout the life of the nematode. In line with this function, the transcriptome of the syncytium shows drastic changes. One of the genes that is most strongly downregulated in syncytia codes for respiratory burst oxidase homologue B. This gene is root-specific and we confirm here the downregulation in nematode feeding sites with a promoter::GUS (β-glucuronidase) line. Overexpression of this gene resulted in enhanced resistance against nematodes but also against leaf-infecting pathogens. Thus, respiratory burst oxidase homologue B has a role in resistance. The function of this gene is in contrast to respiratory burst oxidase homologues D and F, which have been found to be needed for full susceptibility of Arabidopsis to H. schachtii. However, our bioinformatic analysis did not find differences between these proteins that could account for the opposed function in the interaction with nematodes.

Interaction between TaNOX7 and TaCDPK13 Contributes to Plant Fertility and Drought Tolerance by Regulating ROS Production

Reactive oxygen species (ROS) homeostasis is critical for both physiological processes and stress responses of plants. NADPH oxidases (NOXs) are the key producers of ROS in plants. However, their functions in ROS homeostasis and plant growth regulation in wheat (Triticum aestivum) are little investigated. Here, we cloned and characterized a NOX isoform TaNOX7 in wheat. Overexpression of TaNOX7 in rice led to enhanced root length, ROS production, drought tolerance as well as bigger panicles and higher yield but shorter growth period duration. Further results indicate that TaCDPK13, a member of calcium-dependent protein kinases (CDPKs), can directly interact with TaNOX7 and enhance ROS production in plants. These results demonstrate that TaNOX7 plays crucial roles in wheat development, fertility, and drought tolerance via interaction with TaCDPK13, which may act as an upstream regulator of TaNOX7 to regulate ROS production in wheat.

Carbonylation of proteins-an element of plant ageing

Carbonylation-ROS-dependent posttranslational modification of proteins-may be regarded as one of the important events in the process of ageing or senescence in plants. Ageing is the progressive process starting from seed development (plants) and birth (animals). The life-span of living organisms depends on many factors and stresses, which influence reactive oxygen species (ROS) level. The imbalance of their production and scavenging causes pathophysiological conditions that accelerate ageing. ROS modify nucleic acids, lipids, sugars and proteins. The level of carbonylated proteins can serve as an indicator of an oxidative cellular status. Several pathways of protein carbonylation, e.g. the conjugation with reactive carbonyl species, and/or a direct metal-catalysed oxidative attack on amino acids residues are known. Dysfunctional carbonylated proteins are more prone to degradation or form aggregates when the proteolytic machinery is inhibited, as observed in ageing. Protein carbonylation may contribute to formation of organelle-specific signal and to the control of protein quality. Carbonylated proteins are formed during the whole plant life; nevertheless, accelerated ageing stimulates the accumulation of carbonyl derivatives. In the medicine-related literature, concerned ageing and ROS-mediated protein modifications, this topic is extensively analysed, in comparison to the plant science. In plant science, ageing and senescence are considered to describe slightly different processes (physiological events). However, senescence (Latin: senēscere) means "to grow old". This review describes the correlation of protein carbonylation level to ageing or/and senescence in plants. Comparing data from the area of plant and animal research, it is assumed that some basic mechanism of time-dependent alterations in the cellular biochemical processes are common and the protein carbonylation is one of the important causes of ageing.

Danger-Associated Peptide Regulates Root Immune Responses and Root Growth by Affecting ROS Formation in Arabidopsis

Plant elicitor peptides (Peps) are damage/danger-associated molecular patterns (DAMPs) that are perceived by a pair of receptor-like kinases, PEPR1 and PEPR2, to enhance innate immunity and induce the growth inhibition of root in Arabidopsis thaliana. In this study, we show that PEPR1 and PEPR2 function vitally in roots to regulate the root immune responses when treating the roots with bacterial pathogen Pst DC3000. PEPR2, rather than PEPR1, played a predominant role in the perception of Pep1 in the roots and further triggered a strong ROS accumulation—the substance acts as an antimicrobial agent or as a secondary messenger in plant cells. Consistently, seedlings mutating two major ROS-generating enzyme genes, respiratory burst oxidase homologs D and F (RBOHD and RBOHF), abolished the root ROS accumulation and impaired the growth inhibition of the roots induced by Pep1. Furthermore, we revealed that botrytis-induced kinase 1 (BIK1) physically interacted with PEPRs and RBOHD/F, respectively, and served downstream of the Pep1-PEPRs signaling pathway to regulate Pep1-induced ROS production and root growth inhibition. In conclusion, this study demonstrates a previously unrecognized signaling crosstalk between Pep1 and ROS signaling to regulate root immune response and root growth.

Reactive Oxygen Species (ROS) and Nucleic Acid Modifications During Seed Dormancy

The seed is the propagule of higher plants and allows its dissemination and the survival of the species. Seed dormancy prevents premature germination under favourable conditions. Dormant seeds are only able to germinate in a narrow range of conditions. During after-ripening (AR), a mechanism of dormancy release, seeds gradually lose dormancy through a period of dry storage. This review is mainly focused on how chemical modifications of mRNA and genomic DNA, such as oxidation and methylation, affect gene expression during late stages of seed development, especially during dormancy. The oxidation of specific nucleotides produced by reactive oxygen species (ROS) alters the stability of the seed stored mRNAs, being finally degraded or translated into non-functional proteins. DNA methylation is a well-known epigenetic mechanism of controlling gene expression. In Arabidopsis thaliana, while there is a global increase in CHH-context methylation through embryogenesis, global DNA methylation levels remain stable during seed dormancy, decreasing when germination occurs. The biological significance of nucleic acid oxidation and methylation upon seed development is discussed.

Superoxide Radical Metabolism in Sweet Pepper (Capsicum annuum L.) Fruits Is Regulated by Ripening and by a NO-Enriched Environment

Superoxide radical (O2 •-) is involved in numerous physiological and stress processes in higher plants. Fruit ripening encompasses degradative and biosynthetic pathways including reactive oxygen and nitrogen species. With the use of sweet pepper (Capsicum annuum L.) fruits at different ripening stages and under a nitric oxide (NO)-enriched environment, the metabolism of O2 •- was evaluated at biochemical and molecular levels considering the O2 •- generation by a NADPH oxidase system and its dismutation by superoxide dismutase (SOD). At the biochemical level, seven O2 •--generating NADPH-dependent oxidase isozymes [also called respiratory burst oxidase homologs (RBOHs) I-VII], with different electrophoretic mobility and abundance, were detected considering all ripening stages from green to red fruits and NO environment. Globally, this system was gradually increased from green to red stage with a maximum of approximately 2.4-fold increase in red fruit compared with green fruit. Significantly, breaking-point (BP) fruits with and without NO treatment both showed intermediate values between those observed in green and red peppers, although the value in NO-treated fruits was lower than in BP untreated fruits. The O2 •--generating NADPH oxidase isozymes I and VI were the most affected. On the other hand, four SOD isozymes were identified by non-denaturing electrophoresis: one Mn-SOD, one Fe-SOD, and two CuZn-SODs. However, none of these SOD isozymes showed any significant change during the ripening from green to red fruits or under NO treatment. In contrast, at the molecular level, both RNA-sequencing and real-time quantitative PCR analyses revealed different patterns with downregulation of four genes RBOH A, C, D, and E during pepper fruit ripening. On the contrary, it was found out the upregulation of a Mn-SOD gene in the ripening transition from immature green to red ripe stages, whereas a Fe-SOD gene was downregulated. In summary, the data reveal a contradictory behavior between activity and gene expression of the enzymes involved in the metabolism of O2 •- during the ripening of pepper fruit. However, it could be concluded that the prevalence and regulation of the O2 •- generation system (NADPH oxidase-like) seem to be essential for an appropriate control of the pepper fruit ripening, which, additionally, is modulated in the presence of a NO-enriched environment.

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.

Genome-wide identification and expression analysis of NADPH oxidase genes in response to ABA and abiotic stresses, and in fibre formation in Gossypium

Plasma membrane NADPH oxidases, also named respiratory burst oxidase homologues (Rbohs), play pivotal roles in many aspects of growth and development, as well as in responses to hormone signalings and various biotic and abiotic stresses. Although Rbohs family members have been identified in several plants, little is known about Rbohs in Gossypium. In this report, we characterized 13, 13, 26 and 19 Rbohs in G. arboretum, G. raimondii, G. hirsutum and G. barbadense, respectively. These Rbohs were conservative in physical properties, structures of genes and motifs. The expansion and evolution of the Rbohs dominantly depended on segmental duplication, and were under the purifying selection. Transcription analyses showed that GhRbohs were expressed in various tissues, and most GhRbohs were highly expressed in flowers. Moreover, different GhRbohs had very diverse expression patterns in response to ABA, high salinity, osmotic stress and heat stress. Some GhRbohs were preferentially and specifically expressed during ovule growth and fiber formation. These results suggest that GhRbohs may serve highly differential roles in mediating ABA signaling, in acclimation to environmental stimuli, and in fiber growth and development. Our findings are valuable for further elucidating the functions and regulation mechanisms of the Rbohs in adaptation to diverse stresses, and in growth and development in Gossypium.