Transcriptome Analysis and Identification of a Transcriptional Regulatory Network in the Response to H2O2

H2O2-mediated signaling in plant stress caused by herbicides: its role in metabolism and degradation pathways

Systemic acquired acclimation and resistance are vital physiological mechanisms, essential for plants to survive challenging conditions, including herbicide stress. Harmonizing this adaptation involves a series of complex communication pathways. Hydrogen peroxide (H2O2) metabolism might play pivotal roles in orchestrating weeds' acclimation and defense responses. In the context of herbicide resistance, the interaction between H2O2 and key stress signaling pathways is crucial in understanding weed physiology and developing effective management strategies. This dynamic interplay might significantly influence how weeds develop resistance to the various challenges posed by herbicides. Moreover, the production and eradication of H2O2 can be highly compartmentalized, depending on the type of herbicide exposure. Till date there have been no studies aiming to explore/discuss these possibilities. Therefore, in this mini-review, our objective is to delve into the potentialities and recent advancements regarding H2O2-mediated signaling of transcriptomic changes during herbicide stress.

CRF transcription factors in the trade-off between abiotic stress response and plant developmental processes

Climate change-induced environmental stress significantly affects crop yield and quality. In response to environmental stressors, plants use defence mechanisms and growth suppression, creating a resource trade-off between the stress response and development. Although stress-responsive genes have been widely engineered to enhance crop stress tolerance, there is still limited understanding of the interplay between stress signalling and plant growth, a research topic that can provide promising targets for crop genetic improvement. This review focuses on Cytokinin Response Factors (CRFs) transcription factor's role in the balance between abiotic stress adaptation and sustained growth. CRFs, known for their involvement in cytokinin signalling and abiotic stress responses, emerge as potential targets for delaying senescence and mitigating yield penalties under abiotic stress conditions. Understanding the molecular mechanisms regulated by CRFs paves the way for decoupling stress responses from growth inhibition, thus allowing the development of crops that can adapt to abiotic stress without compromising development. This review highlights the importance of unravelling CRF-mediated pathways to address the growing need for resilient crops in the face of evolving climatic conditions.

Global transcriptome profiling reveals root- and leaf-specific responses of barley (Hordeum vulgare L.) to H2O2

In cereal crops, such as barley (Hordeum vulgare L.), the ability to appropriately respond to environmental cues is an important factor for yield stability and thus for agricultural production. Reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), are key components of signal transduction cascades involved in plant adaptation to changing environmental conditions. H2O2-mediated stress responses include the modulation of expression of stress-responsive genes required to cope with different abiotic and biotic stresses. Despite its importance, knowledge of the effects of H2O2 on the barley transcriptome is still scarce. In this study, we identified global transcriptomic changes induced after application of 10 mM H2O2 to five-day-old barley plants. In total, 1883 and 1001 differentially expressed genes (DEGs) were identified in roots and leaves, respectively. Most of these DEGs were organ-specific, with only 209 DEGs commonly regulated and 37 counter-regulated between both plant parts. A GO term analysis further confirmed that different processes were affected in roots and leaves. It revealed that DEGs in leaves mostly comprised genes associated with hormone signaling, response to H2O2 and abiotic stresses. This includes many transcriptions factors and small heat shock proteins. DEGs in roots mostly comprised genes linked to crucial aspects of H2O2 catabolism and oxidant detoxification, glutathione metabolism, as well as cell wall modulation. These categories include many peroxidases and glutathione transferases. As with leaves, the H2O2 response category in roots contains small heat shock proteins, however, mostly different members of this family were affected and they were all regulated in the opposite direction in the two plant parts. Validation of the expression of the selected commonly regulated DEGs by qRT-PCR was consistent with the RNA-seq data. The data obtained in this study provide an insight into the molecular mechanisms of oxidative stress responses in barley, which might also play a role upon other stresses that induce oxidative bursts.

Drought-Induced Xylem Sulfate Activates the ABA-Mediated Regulation of Sulfate Assimilation and Glutathione Redox in Brassica napus Leaves

Drought intensity modifies the assimilatory pathway of glutathione (GSH) synthesis. Abscisic acid (ABA) is a representative signaling hormone involved in regulating plant stress responses. This study aimed to investigate an interactive regulation of sulfate and/or ABA in GSH metabolism and redox. The drought-responsive alterations in sulfate assimilation and GSH-based redox reactions were assessed relative to ABA responses on the time-course of drought intensity. Drought-responsive H₂O₂ concentrations were divided into two distinct phases-an initial 4 days of no change (Ψ_w ≥ -0.49 MPa) and a phase of higher accumulation during the late phase of the drought (days 10-14; Ψ_w ≤ -1.34 MPa). During the early phase of the drought, GSH/GSSG redox state turned to the slightly reduced state with a transient increase in GSH, resulting from a strong activation of H₂O₂ scavenging enzymes, ascorbate peroxidase (APOX) and glutathione reductase (GR). The late phase of the drought was characterized by a decrease in GSH due to cysteine accumulation, shifting GSH- and NADPH-based redox states to higher oxidization, increasing sulfate and ABA in xylem, and causing ABA accumulation in leaves. Regression analysis revealed that sulfate in xylem sap was positively correlated with H₂O₂ concentrations and ABA was closely related to decreases in the GSH pool and the oxidation of GSH catalyzed by glutathione peroxidase (GPOX). These results indicate that drought-induced oxidation proceeds through the suppression of GSH synthesis and further GSH oxidation in a sulfate-activated ABA-dependent manner.

Dimethylthiourea Alleviates Drought Stress by Suppressing Hydrogen Peroxide-Dependent Abscisic Acid-Mediated Oxidative Responses in an Antagonistic Interaction with Salicylic Acid in Brassica napus Leaves

In plants, prolonged drought induces oxidative stress, leading to a loss of reducing potential in redox components. Abscisic acid (ABA) is a representative hormonal signal regulating stress responses. This study aimed to investigate the physiological significance of dimethylthiourea (DMTU, an H₂O₂ scavenger) in the hormonal regulation of the antioxidant system and redox control in rapeseed (Brassica napus L.) leaves under drought stress. Drought treatment for 10 days provoked oxidative stress, as evidenced by the increase in O₂^•- and H₂O₂ concentrations, and lipid peroxidation levels, and a decrease in leaf water potential. Drought-induced oxidative responses were significantly alleviated by DMTU treatment. The accumulation of O₂^•- and H₂O₂ in drought-treated plants coincided with the enhanced expression of the NADPH oxidase and Cu/Zn-SOD genes, leading to an up-regulation in oxidative signal-inducible 1 (OXI1) and mitogen-activated protein kinase 6 (MAPK6), with a concomitant increase in ABA levels and the up-regulation of ABA-related genes. DMTU treatment under drought largely suppressed the drought-responsive up-regulation of these genes by depressing ABA responses through an antagonistic interaction with salicylic acid (SA). DMTU treatment also alleviated the drought-induced loss of reducing potential in GSH- and NADPH-based redox by the enhanced expression of glutathione reductase 1 (GR1) and up-regulation of oxidoreductase genes (TRXh5 and GRXC9). These results indicate that DMTU effectively alleviates drought-induced oxidative responses by suppressing ABA-mediated oxidative burst signaling in an antagonistic regulation of SA.

Arabidopsis thaliana Accessions from the Chernobyl Exclusion Zone Show Decreased Sensitivity to Additional Acute Irradiation

Chronic ionising radiation exposure is a main consequence of radioactive pollution of the environment. The development of functional genomics approaches coupled with morphological and physiological studies allows new insights into plant adaptation to life under chronic irradiation. Using morphological, reproductive, physiological, and transcriptomic experiments, we evaluated the way in which Arabidopsis thaliana natural accessions from the Chernobyl exclusion zone recover from chronic low-dose and acute high-dose γ-irradiation of seeds. Plants from radioactively contaminated areas were characterized by lower germination efficiency, suppressed growth, decreased chlorophyll fluorescence, and phytohormonal changes. The transcriptomes of plants chronically exposed to low-dose radiation indicated the repression of mobile genetic elements and deregulation of genes related to abiotic stress tolerance. Furthermore, these chronically irradiated natural accessions showed higher tolerance to acute 150 Gy γ-irradiation of seeds, according to transcriptome and phytohormonal profiles. Overall, the lower sensitivity of the accessions from radioactively contaminated areas to acute high-dose irradiation may come at the cost of their growth performance under normal conditions.

Comparative proteomics analysis of Arabidopsis thaliana response to light-emitting diode of narrow wavelength 450 nm, 595 nm, and 650 nm

Incident light is a central modulator of plant growth and development. However, there are still open questions surrounding wavelength-specific plant proteomic responses. Here we applied tandem mass tag based quantitative proteomics technology to acquire an in-depth view of proteome changes in Arabidopsis thaliana response to narrow wavelength blue (B; 450 nm), amber (A; 595 nm), or red (R; 650 nm) light treatments. A total of 16,707 proteins were identified with 9120 proteins quantified across all three light treatments in three biological replicates. This enabled examination of changes in the abundance for proteins with low abundance and important regulatory roles including transcription factors and hormone signaling. Importantly, 18% (1631 proteins) of the A. thaliana proteome is differentially abundant in response to narrow wavelength lights, and changes in proteome correlate well with different morphologies exhibited by plants. To showcase the usefulness of this resource, data were placed in the context of more than thirty published datasets, providing orthogonal validation and further insights into light-specific biological pathways, including Systemic Acquired Resistance and Shade Avoidance Syndrome. This high-resolution resource for A. thaliana provides baseline data and a tool for defining molecular mechanisms that control fundamental aspects of plant response to changing light conditions, with implications in plant development and adaptation. SIGNIFICANCE: Understanding of molecular mechanisms involved in wavelength-specific response of plant is question of widespread interest both to basic researchers and to those interested in applying such knowledge to the engineering of novel proteins, as well as targeted lighting systems. Here we sought to generate a high-resolution labeling proteomic profile of plant leaves, based on exposure to specific narrow-wavelength lights. Although changes in plant physiology in response to light spectral composition is well documented, there is limited knowledge on the roles of specific light wavelengths and their impact. Most previous studies have utilized relatively broad wavebands in their experiments. These multi-wavelengths lights function in a complex signaling network, which provide major challenges in inference of wavelength-specific molecular processes that underly the plant response. Besides, most studies have compared the effect of blue and red wavelengths comparing with FL, as control. As FL light consists the mixed spectra composition of both red and blue as well as numerous other wavelengths, comparing undeniably results in inconsistent and overlapping responses that will hamper effects to elucidate the plant response to specific wavelengths [1, 2]. Monitoring plant proteome response to specific wavelengths and further compare the changes to one another, rather than comparing plants proteome to FL, is thus necessary to gain the clear insights to specific underlying biological pathways and their effect consequences in plant response. Here, we employed narrow wavelength LED lights in our design to eliminate the potential overlap in molecular responses by ensuring non-overlapping wavelengths in the light treatments. We further applied TMT-labeling technology to gain a high-resolution view on the associates of proteome changes. Our proteomics data provides an in-depth coverage suitable for system-wide analyses, providing deep insights on plant physiological processes particularly because of the tremendous increase in the amount of identified proteins which outreach the other biological data.

Photoperiod Stress in Arabidopsis thaliana Induces a Transcriptional Response Resembling That of Pathogen Infection

Plants are exposed to regular diurnal rhythms of light and dark. Changes in the photoperiod by the prolongation of the light period cause photoperiod stress in short day-adapted Arabidopsis thaliana. Here, we report on the transcriptional response to photoperiod stress of wild-type A. thaliana and photoperiod stress-sensitive cytokinin signaling and clock mutants and identify a core set of photoperiod stress-responsive genes. Photoperiod stress caused altered expression of numerous reactive oxygen species (ROS)-related genes. Photoperiod stress-sensitive mutants displayed similar, but stronger transcriptomic changes than wild-type plants. The alterations showed a strong overlap with those occurring in response to ozone stress, pathogen attack and flagellin peptide (flg22)-induced PAMP triggered immunity (PTI), which have in common the induction of an apoplastic oxidative burst. Interestingly, photoperiod stress triggers transcriptional changes in jasmonic acid (JA) and salicylic acid (SA) biosynthesis and signaling and results in increased JA, SA and camalexin levels. These responses are typically observed after pathogen infections. Consequently, photoperiod stress increased the resistance of Arabidopsis plants to a subsequent infection by Pseudomonas syringae pv. tomato DC3000. In summary, we show that photoperiod stress causes transcriptional reprogramming resembling plant pathogen defense responses and induces systemic acquired resistance (SAR) in the absence of a pathogen.

AetSRG1 contributes to the inhibition of wheat Cd accumulation by stabilizing phenylalanine ammonia lyase

Cadmium (Cd) is a toxic heavy metal that poses a serious threat to crop safety, productivity, and human health. Aegilops tauschii is the D genome donor of common wheat and shows abundant genetic variation. However, the tolerance of Ae. tauschii toward Cd at the molecular level is poorly understood. In this study, key factors involved in the Cd stress response of Ae. tauschii were investigated by RNA sequencing. Differentially expressed genes (DEGs) under Cd stress were identified in Ae. tauschii roots and shoots. A Fe(II)/2-oxoglutarate dependent dioxygenase (designated as AetSRG1), with an unknown function in Cd stress, was of particular interest. The open reading frame of AetSRG1 was cloned and overexpressed in wheat, which resulted in reduced Cd accumulation along with a lower Cd²⁺ flux, decreased electrolyte leakage, and higher reactive oxygen species production. The protein of AetSRG1 interacted with phenylalanine ammonia lyase (PAL). Finally, we found that AetSRG1 stabilizes PAL and promotes the synthesis of endogenous salicylic acid. This study provides novel insights into the molecular mechanisms underlying the response of Ae. tauschii toward Cd stress. The key genes identified in this work serve as potential targets for developing low cadmium wheat.

H2O2-Responsive Hormonal Status Involves Oxidative Burst Signaling and Proline Metabolism in Rapeseed Leaves

Drought alters the level of endogenous reactive oxygen species (ROS) and hormonal status, which are both involved in the regulation of stress responses. To investigate the interplay between ROS and hormones in proline metabolism, rapeseed (Brassica napus L.) plants were exposed to drought or exogenous H₂O₂ (Exo-H₂O₂) treatment for 10 days. During the first 5 days, the enhanced H₂O₂ concentrations in drought treatment were associated with the activation of superoxide dismutase (SOD) and NADPH oxidase, with enhanced ABA and SA levels, while that in Exo-H₂O₂ treatment was mainly associated with SA-responsive POX. During the latter 5 days, ABA-dependent ROS accumulation was predominant with an upregulated oxidative signal-inducible gene (OXI1) and MAPK6, leading to the activation of ABA synthesis and the signaling genes (NCED3 and MYC2). During the first 5 days, the enhanced levels of P5C and proline were concomitant with SA-dependent NDR1-mediated signaling in both drought and Exo-H₂O₂ treatments. In the latter 5 days of drought treatment, a distinct enhancement in P5CR and ProDH expression led to higher proline accumulation compared to Exo-H₂O₂ treatment. These results indicate that SA-mediated P5C synthesis is highly activated under lower endogenous H₂O₂ levels, and ABA-mediated OXI1-dependent proline accumulation mainly occurs with an increasing ROS level, leading to ProDH activation as a hypersensitive response to ROS and proline overproduction under severe stress.

Hydrogen peroxide-induced stress acclimation in plants

Among all reactive oxygen species (ROS), hydrogen peroxide (H2O2) takes a central role in regulating plant development and responses to the environment. The diverse role of H2O2 is achieved through its compartmentalized synthesis, temporal control exerted by the antioxidant machinery, and ability to oxidize specific residues of target proteins. Here, we examine the role of H2O2 in stress acclimation beyond the well-studied transcriptional reprogramming, modulation of plant hormonal networks and long-distance signalling waves by highlighting its global impact on the transcriptional regulation and translational machinery.

Improving oxidative stress resilience in plants

Originally conceived as harmful metabolic byproducts, reactive oxygen species (ROS) are now recognized as an integral part of numerous cellular programs. Thanks to their diverse physicochemical properties, compartmentalized production, and tight control exerted by the antioxidant machinery they activate signaling pathways that govern plant growth, development, and defense. Excessive ROS levels are often driven by adverse changes in environmental conditions, ultimately causing oxidative stress. The associated negative impact on cellular constituents have been a major focus of decade-long research efforts to improve the oxidative stress resilience by boosting the antioxidant machinery in model and crop species. We highlight the role of enzymatic and non-enzymatic antioxidants as integral factors of multiple signaling cascades beyond their mere function to prevent oxidative damage under adverse abiotic stress conditions.

Permissive action of H2O2 mediated ClUGT75 expression for auxin glycosylation and Al3+- tolerance in watermelon

Although Al³⁺-toxicity is one of the limiting factors for crop production in acidic soils, little is known about the Al³⁺-tolerance mechanism in watermelon, a fairly acid-tolerant crop. This work aimed to identify the interaction between the H₂O₂ scavenging pathway and auxin glycosylation relevant to watermelon Al³⁺-tolerance. By analyzing expressions of hormone-related ClUGTs and antioxidant enzyme genes in Al³⁺-tolerant (ZJ) and Al³⁺-sensitive (NBT) cultivars, we identified ClUGT75s (B1, B2, and D1) and ClSOD1-2-ClCAT as crucial components associated with Al³⁺-tolerance. Al³⁺-stress significantly increased H₂O₂ content by 92.7% in NBT and 42.3% in ZJ, accompanied by less Al³⁺-, auxin (IAA and IBA), and MDA contents in ZJ than NBT. These findings coincided with significant ClSOD1-2 expression and stable dismutation activity in NBT than ZJ. Hence, higher H₂O₂ content in the root apex of NBT than ZJ correlated with a significant increase in auxin content and ClSOD1-2 up-regulation. Moreover, Al³⁺-activated ClUGT75D1 and ClUGT75B2 in ZJ coincided with no considerable change in IBA content, suggesting that glycosylation-mediated changes in IBA content might be relevant to Al³⁺-tolerance in watermelon. Furthermore, exogenous H₂O₂ and IBA indicated ClUGT75D1 modulating IBA is likely dependent on H₂O₂ background. We hypothesize that a higher H₂O₂ level in NBT represses ClUGT75, resulting in increased auxin than those in ZJ roots. Thus, excess in both H₂O₂ and auxin aggravated the inhibition of root elongation under Al³⁺-stress. Our findings provide insights on the permissive action of H₂O₂ in the mediation of auxin glycosylation by ClUGT75 in root apex for Al³⁺-tolerance in watermelon.

Systemic Expression of Genes Involved in the Plant Defense Response Induced by Wounding in Senna tora

Wounds in tissues provide a pathway of entry for pathogenic fungi and bacteria in plants. Plants respond to wounding by regulating the expression of genes involved in their defense mechanisms. To analyze this response, we investigated the defense-related genes induced by wounding in the leaves of Senna tora using RNA sequencing. The genes involved in jasmonate and ethylene biosynthesis were strongly induced by wounding, as were a large number of genes encoding transcription factors such as ERFs, WRKYs, MYBs, bHLHs, and NACs. Wounding induced the expression of genes encoding pathogenesis-related (PR) proteins, such as PR-1, chitinase, thaumatin-like protein, cysteine proteinase inhibitor, PR-10, and plant defensin. Furthermore, wounding led to the induction of genes involved in flavonoid biosynthesis and the accumulation of kaempferol and quercetin in S. tora leaves. All these genes were expressed systemically in leaves distant from the wound site. These results demonstrate that mechanical wounding can lead to a systemic defense response in the Caesalpinioideae, a subfamily of the Leguminosae. In addition, a co-expression analysis of genes induced by wounding provides important information about the interactions between genes involved in plant defense responses.

Repressed OsMESL expression triggers reactive oxygen species-mediated broad-spectrum disease resistance in rice

A few reports have indicated that a single gene confers resistance to bacterial blight, sheath blight and rice blast. In this study, we identified a novel disease resistance mutant gene, methyl esterase-like (osmesl) in rice. Mutant rice with T-DNA insertion displayed significant resistance to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo), sheath blight caused by Rhizoctonia solani and rice blast caused by Magnaporthe oryzae. Additionally, CRISPR-Cas9 knockout mutants and RNAi lines displayed resistance to these pathogens. Complementary T-DNA mutants demonstrated a phenotype similar to the wild type (WT), thereby indicating that osmesl confers resistance to pathogens. Protein interaction experiments revealed that OsMESL affects reactive oxygen species (ROS) accumulation by interacting with thioredoxin OsTrxm in rice. Moreover, qRT-PCR results showed significantly reduced mRNA levels of multiple ROS scavenging-related genes in osmesl mutants. Nitroblue tetrazolium staining showed that the pathogens cause ROS accumulation, and quantitative detection revealed significantly increased levels of H₂ O₂ in the leaves of osmesl mutants and RNAi lines after infection. The abundance of JA, a hormone associated with disease resistance, was significantly more in osmesl mutants than in WT plants. Overall, these results suggested that osmesl enhances disease resistance to Xoo, R. solani and M. oryzae by modulating the ROS balance.

Drought Intensity-Responsive Salicylic Acid and Abscisic Acid Crosstalk with the Sugar Signaling and Metabolic Pathway in Brassica napus

The aim of this study was to characterize hormonal crosstalk with the sugar signaling and metabolic pathway based on a time course analysis of drought intensity. Drought intensity-responsive changes in the assimilation of newly fixed carbon (C) into soluble sugar, the content of sugar and starch, and expression of genes involved in carbohydrate metabolism were interpreted as being linked to endogenous abscisic acid (ABA) and salicylic acid (SA) levels and their signaling genes. The ABA and SA levels in the drought-stressed leaves increased together during the early drought period (days 0–6), and additional ABA accumulation occurred with depressed SA during the late period (days 6–14). Although drought treatment decreased the assimilation of newly fixed C into soluble sugar, representing a 59.9%, 33.1%, and 62.9% reduction in ¹³C-glucose, ¹³C-fructose, and ¹³C-sucrose on day 14, respectively, the drought-responsive accumulation of soluble sugars was significant. During the early period, the drought-responsive accumulation of hexose and sucrose was concurrent with the upregulated expression of hexokinase 1 (HXK1), which, in turn, occurred parallel to the upregulation of ABA synthesis gene 9-sis-epoxycarotenoid dioxygenase (NCED3) and SA-related genes (isochorismate synthase 1 (ICS1) and non-expressor of pathogenesis-related gene (NPR1)). During the late period, hexose accumulation, sucrose phloem loading, and starch degradation were dominant, with a highly enhanced expression of the starch degradation-related genes β-amylase 1 (BAM1) and α-amylase 3 (AMY3), which were concomitant with the parallel enhancement of sucrose non-fermenting−1 (Snf1)-related protein kinase 2 (SnRK2).2 and ABA-responsive element binding 2 (AREB2) expression in an ABA-dependent manner. These results indicate that the drought-responsive accumulation of sugars (especially SA-mediated sucrose accumulation) is part of the acclamatory process during the early period. Conversely, ABA-responsive hexose accumulation and sucrose phloem loading represent severe drought symptoms during the late drought period.

Transcription Factor NAC075 Delays Leaf Senescence by Deterring Reactive Oxygen Species Accumulation in Arabidopsis

Leaf senescence is a highly complex genetic process that is finely tuned by multiple layers of regulation. Among them, transcriptional regulation plays a critical role in controlling the initiation and progression of leaf senescence. Here, we found that the NAC transcription factor NAC075 functions as a novel negative regulator of leaf senescence. Loss of function of NAC075 promotes leaf senescence in an age-dependent manner, whereas constitutive overexpression of NAC075 delays senescence in Arabidopsis. Transcriptome analysis revealed that transcript levels of antioxidant enzymes such as catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD) are significantly suppressed in nac075 mutants compared with wild-type plants. Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analyses revealed that NAC075 directly binds the promoter of catalase 2 (CAT2). Moreover, genetic analysis showed that overexpression of CAT2 suppresses the overproduction of reactive oxygen species (ROS) and the early senescence phenotypes of nac075 mutants, suggesting that CAT2 acts downstream of NAC075 to delay leaf senescence by repressing ROS accumulation. Collectively, our findings provide a new regulatory module involving NAC075-CAT2-ROS in controlling leaf senescence in Arabidopsis.

Mitochondrial Small Heat Shock Proteins Are Essential for Normal Growth of Arabidopsis thaliana

Mitochondria play important roles in the plant stress responses and the detoxification of the reactive oxygen species generated in the electron transport chain. Expression of genes encoding stress-related proteins such as the mitochondrial small heat shock proteins (M-sHSP) is upregulated in response to different abiotic stresses. In Arabidopsis thaliana, three M-sHSPs paralogous genes were identified, although their function under physiological conditions remains elusive. The aim of this work is to uncover the in vivo function of all three M-sHSPs at the whole plant level. To accomplish this goal, we analyzed the phenotype, proteomic, and metabolic profiles of Arabidopsis knock-down lines of M-sHSPs (single, double, and triple knock-down lines) during normal plant growth. The triple knock-down plants showed the most prominent altered phenotype at vegetative and reproductive stages without any externally applied stress. They displayed chlorotic leaves, growth arrest, and low seed production. Concomitantly, they exhibited increased levels of sugars, proline, and citric, malic, and ascorbic acid, among other metabolites. In contrast, single and double knock-down plants displayed a few changes in their phenotype. A redundant function among the three M-sHSPs is indicated by the impairment in vegetative and reproductive growth associated with the simultaneous loss of all three M-sHSPs genes. The triple knock-down lines showed alteration of proteins mainly involved in photosynthesis and antioxidant defense compared to the control plants. On the other hand, heat stress triggered a distinct cytosolic response pattern and the upregulation of other sHSP members, in the knock-down plants. Overall, depletion of all three M-sHSPs in Arabidopsis severely impacted fundamental metabolic processes, leading to alterations in the correct plant growth and development. These findings expand our knowledge about the contribution of organelle-specific M-sHSPs to healthy plant growth under non-stress conditions.

Elicitors Modulate Young Sandalwood (Santalum album L.) Growth, Heartwood Formation, and Concrete Oil Synthesis

Five chemical elicitors––6-benzyladenine (BA), ethephon (ETH), methyl jasmonate (MeJA), hydrogen peroxide (H₂O₂) and calcium chloride (CaCl₂)––were used to treat 1- and 5-year-old sandal trees (Santalum album L.) to assess their effects on growth, heartwood formation and concrete oil synthesis. The results showed that some newly formed branches in stems that were induced by BA and ETH displayed leaf senescence and developed new smaller and light-green leaves. The relative percentage of concrete oil from the heartwood of water-treated trees (0.65%) was significantly lower than that from trees treated with 4 mM H₂O₂ (2.85%) and 4 mM BA (2.75%) within one year. Four mM BA, H₂O₂ and CaCl₂ induced a significantly higher level of sesquiterpenoids than heartwood treated with 2 mM of these elicitors. Four mM MeJA induced significantly less sesquiterpenoids than heartwood treated with 2 mM MeJA. Morphological, physiological, and chromatographic–spectrometric technologies were integrated to trace the potential function of these exogenously applied chemical elicitors. The results may have important applications and provide a better understanding of the molecular mechanism of heartwood formation and hardening in young sandalwood trees.

Root Adaptation via Common Genetic Factors Conditioning Tolerance to Multiple Stresses for Crops Cultivated on Acidic Tropical Soils

Crop tolerance to multiple abiotic stresses has long been pursued as a Holy Grail in plant breeding efforts that target crop adaptation to tropical soils. On tropical, acidic soils, aluminum (Al) toxicity, low phosphorus (P) availability and drought stress are the major limitations to yield stability. Molecular breeding based on a small suite of pleiotropic genes, particularly those with moderate to major phenotypic effects, could help circumvent the need for complex breeding designs and large population sizes aimed at selecting transgressive progeny accumulating favorable alleles controlling polygenic traits. The underlying question is twofold: do common tolerance mechanisms to Al toxicity, P deficiency and drought exist? And if they do, will they be useful in a plant breeding program that targets stress-prone environments. The selective environments in tropical regions are such that multiple, co-existing regulatory networks may drive the fixation of either distinctly different or a smaller number of pleiotropic abiotic stress tolerance genes. Recent studies suggest that genes contributing to crop adaptation to acidic soils, such as the major Arabidopsis Al tolerance protein, AtALMT1, which encodes an aluminum-activated root malate transporter, may influence both Al tolerance and P acquisition via changes in root system morphology and architecture. However, trans-acting elements such as transcription factors (TFs) may be the best option for pleiotropic control of multiple abiotic stress genes, due to their small and often multiple binding sequences in the genome. One such example is the C2H2-type zinc finger, AtSTOP1, which is a transcriptional regulator of a number of Arabidopsis Al tolerance genes, including AtMATE and AtALMT1, and has been shown to activate AtALMT1, not only in response to Al but also low soil P. The large WRKY family of transcription factors are also known to affect a broad spectrum of phenotypes, some of which are related to acidic soil abiotic stress responses. Hence, we focus here on signaling proteins such as TFs and protein kinases to identify, from the literature, evidence for unifying regulatory networks controlling Al tolerance, P efficiency and, also possibly drought tolerance. Particular emphasis will be given to modification of root system morphology and architecture, which could be an important physiological "hub" leading to crop adaptation to multiple soil-based abiotic stress factors.

WRKY42 transcription factor positively regulates leaf senescence through modulating SA and ROS synthesis in Arabidopsis thaliana

Leaf senescence represents the final stage of leaf growth and development, and its onset and progression are strictly regulated; however, the underlying regulatory mechanisms remain largely unknown. In this study we found that WRKY42 was highly induced during leaf senescence. Loss-of-function wrky42 mutants showed delayed leaf senescence whereas the overexpression of WRKY42 accelerated senescence. Transcriptome analysis revealed 2721 differentially expressed genes between wild-type and WRKY42-overexpressing plants, including genes involved in salicylic acid (SA) and reactive oxygen species (ROS) synthesis as well as several senescence-associated genes (SAGs). Moreover, WRKY42 activated the transcription of isochorismate synthase 1 (ICS1), respiratory burst oxidase homolog F (RbohF) and a few SAG genes. Consistently, the expression of these genes was reduced in wrky42 mutants but was markedly increased in transgenic Arabidopsis overexpressing WRKY42. Both in vitro electrophoretic mobility shift assays (EMSAs) and in vivo chromatin immunoprecipitation and dual luciferase assays demonstrated that WRKY42 directly bound to the promoters of ICS1 and RbohF, as well as a few SAGs, to activate their expression. Genetic analysis further showed that mutations of ICS1 and RbohF suppressed the early senescence phenotype evoked by WRKY42 overexpression. Thus, we have identified WRKY42 as a novel transcription factor positively regulating leaf senescence by directly activating the transcription of ICS1, RbohF and SAGs, without any seed yield penalty.

PeSTZ1 confers salt stress tolerance by scavenging the accumulation of ROS through regulating the expression of PeZAT12 and PeAPX2 in Populus

ZINC FINGER OF ARABIDOPSIS THALIANA12 (ZAT12) plays an important role in stress responses, but the transcriptional regulation of ZAT12 in response to abiotic stress remains unclear. In this study, we confirmed that a SALT TOLERANCE ZINC FINGER1 transcription factor from Populus euphratica (PeSTZ1) could regulate the expression of PeZAT12 by dual-luciferase reporter (DLR) assay and electrophoretic mobility shift assay. The expression of PeSTZ1 was rapidly induced by NaCl and hydrogen peroxide (H2O2) treatments. Overexpressing PeSTZ1 in poplar 84K (Populus alba × Populus glandulosa) plant was endowed with a strong tolerance to salt stress. Under salt stress, transgenic poplar exhibited higher expression levels of PeZAT12 and accumulated a larger amount of antioxidant than the wild-type plants. Meanwhile, ASCORBATE PEROXIDASE2 (PeAPX2) can be activated by PeZAT12 and PeSTZ1, promoting the accumulation of cytosolic ascorbate peroxidase (APX) to scavenge reactive oxygen species (ROS) under salt stress. This new regulatory model (PeSTZ1-PeZAT12-PeAPX2) was found in poplar, providing a new idea and insight for the interpretation of poplar resistance. Transgenic poplar reduced the accumulation of ROS, restrained the degradation of chlorophyll and guaranteed the photosynthesis and electron transport system. On the other hand, transgenic poplar slickly adjusted K+/Na+ homeostasis to alleviate salt toxicity in photosynthetic organs of plants under salt stress and then increased biomass accumulation. In summary, PeSTZ1 confers salt stress tolerance by scavenging the accumulation of ROS through regulating the expression of PeZAT12 and PeAPX2 in poplar.

WRKY55 transcription factor positively regulates leaf senescence and the defense response by modulating the transcription of genes implicated in the biosynthesis of reactive oxygen species and salicylic acid in Arabidopsis

Reactive oxygen species (ROS) and salicylic acid (SA) are two factors regulating leaf senescence and defense against pathogens. However, how a single gene integrates both ROS and SA pathways remains poorly understood. Here, we show that Arabidopsis WRKY55 transcription factor positively regulates ROS and SA accumulation, and thus leaf senescence and resistance against the bacterial pathogen Pseudomonas syringae WRKY55 is predominantly expressed in senescent leaves and encodes a transcriptional activator localized to nuclei. Both inducible and constitutive overexpression of WRKY55 accelerates leaf senescence, whereas mutants delay it. Transcriptomic sequencing identified 1448 differentially expressed genes, of which 1157 genes are upregulated by WRKY55 expression. Accordingly, the ROS and SA contents in WRKY55-overexpressing plants are higher than those in control plants, whereas the opposite occurs in mutants. Moreover, WRKY55 positively regulates defense against P. syringae Finally, we show that WRKY55 activates the expression of RbohD, ICS1, PBS3 and SAG13 by binding directly to the W-box-containing fragments. Taken together, our work has identified a new WRKY transcription factor that integrates both ROS and SA pathways to regulate leaf senescence and pathogen resistance.

Transcriptomic Analysis of Dark-Induced Senescence in Bermudagrass (Cynodon dactylon)

Leaf senescence induced by prolonged light deficiency is inevitable whenever turfgrass is cultivated in forests, and this negatively influences the survival and aesthetic quality of the turfgrass. However, the mechanism underlying dark-induced senescence in turfgrass remained obscure. In this study, RNA sequencing was performed to analyze how genes were regulated in response to dark-induced leaf senescence in bermudagrass. A total of 159,207 unigenes were obtained with a mean length of 948 bp. The differential expression analysis showed that a total of 59,062 genes, including 52,382 up-regulated genes and 6680 down-regulated genes were found to be differentially expressed between control leaves and senescent leaves induced by darkness. Subsequent bioinformatics analysis showed that these differentially expressed genes (DEGs) were mainly related to plant hormone (ethylene, abscisic acid, jasmonic acid, auxin, cytokinin, gibberellin, and brassinosteroid) signal transduction, N-glycan biosynthesis, and protein processing in the endoplasmic reticulum. In addition, transcription factors, such as WRKY, NAC, HSF, and bHLH families were also responsive to dark-induced leaf senescence in bermudagrass. Finally, qRT-PCR analysis of six randomly selected DEGs validated the accuracy of sequencing results. Taken together, our results provide basic information of how genes respond to darkness, and contribute to the understanding of comprehensive mechanisms of dark-induced leaf senescence in turfgrass.

Assessment of Subcellular ROS and NO Metabolism in Higher Plants: Multifunctional Signaling Molecules

Reactive oxygen species (ROS) and nitric oxide (NO) are produced in all aerobic life forms under both physiological and adverse conditions. Unregulated ROS/NO generation causes nitro-oxidative damage, which has a detrimental impact on the function of essential macromolecules. ROS/NO production is also involved in signaling processes as secondary messengers in plant cells under physiological conditions. ROS/NO generation takes place in different subcellular compartments including chloroplasts, mitochondria, peroxisomes, vacuoles, and a diverse range of plant membranes. This compartmentalization has been identified as an additional cellular strategy for regulating these molecules. This assessment of subcellular ROS/NO metabolisms includes the following processes: ROS/NO generation in different plant cell sites; ROS interactions with other signaling molecules, such as mitogen-activated protein kinases (MAPKs), phosphatase, calcium (Ca²⁺), and activator proteins; redox-sensitive genes regulated by the iron-responsive element/iron regulatory protein (IRE-IRP) system and iron regulatory transporter 1(IRT1); and ROS/NO crosstalk during signal transduction. All these processes highlight the complex relationship between ROS and NO metabolism which needs to be evaluated from a broad perspective.