Regulation of Photosynthesis during Abiotic Stress-Induced Photoinhibition

The oxygen-evolving complex: a super catalyst for life on earth, in response to abiotic stresses

The oxygen-evolving complex is integrated into photosystem (PSII). An essential part of oxygenic photosynthetic apparatus, embedded in the thylakoid membrane of chloroplasts. The OEC is a super catalyst to split water into molecular oxygen in the presence of light. The OEC consist of four Mn atoms, one Ca atom and five oxygen atoms (CaMn₄O₅) and this cluster is maintained by its surrounding proteins viz., PsbQ, PsbP, PsbO, PsbR. The function of this super catalyst with a high turnover frequency of 500 s^-1 in standard condition. Chlorophyll a fluorescence (OJIP transients) are used to understand structural and functional cohesion of photosynthetic apparatus. A further K-peak in OJIP curve reflects damage at the OEC donor site in response to salinity, drought, and high temperature. The decline in performance indices (PI, SFI) also revealed structural damage of photosynthetic apparatus that leads to disruption of electron transport rate under abiotic conditions. This review discusses the structural and function cohesion of the OEC in plant against variable abiotic conditions.

Modes of Brassinosteroid Activity in Cold Stress Tolerance

Cold stress is a significant environmental factor that negatively affects plant growth and development in particular when it occurs during the growth phase. Plants have evolved means to protect themselves from damage caused by chilling or freezing temperatures and some plant species, in particular those from temperate geographical zones, can increase their basal level of freezing tolerance in a process termed cold acclimation. Cold acclimation improves plant survival, but also represses growth, since it inhibits activity of the growth-promoting hormones gibberellins (GAs). In addition to GAs, the steroid hormones brassinosteroids (BRs) also take part in growth promotion and cold stress signaling; however, in contrast to Gas, BRs can improve cold stress tolerance with fewer trade-offs in terms of growth and yields. Here we summarize our current understanding of the roles of BRs in cold stress responses with a focus on freezing tolerance and cold acclimation pathways.

Exogenous abscisic acid mediates ROS homeostasis and maintains glandular trichome to enhance artemisinin biosynthesis in Artemisia annua under copper toxicity

One of the major abiotic stresses that cause environmental pollution is heavy metal stress. In the present investigation, copper (Cu) toxicity caused morphological and cellular damages to the Artemisia annua L. plants but supplementation of abscisic acid (ABA) ameliorated the damaging effect of Cu. Copper toxicity significantly reduced the shoot and root lengths; fresh and dry weights of shoot. However, exogenous application of ABA to Cu-treated plants significantly attenuated the damaging effects on plants caused by Cu toxicity. Copper stress also reduced the physiological and biochemical parameters, but ABA application ameliorated the negative effects of Cu in the affected plant. Accumulation of Cu in plant tissues significantly increased the membrane damage and oxidative enzyme activities such as catalase (CAT), peroxidase (POX) and superoxide dismutase (SOD). Further, the impact of high concentration of Cu on density, area and ultrastructure of glandular trichomes and artemisinin content was studied. Moreover, the foliar application of ABA improved the area, density of glandular trichomes and secured the plant cells from Cu toxicity. Therefore, this investigation indicated that the exogenous application of ABA protects A. annua plant by increasing antioxidant enzymes activity, which helps in maintaining cell integrity of leaves and results in increased artemisinin production.

Hydrogen sulfide is required for salicylic acid-induced chilling tolerance of cucumber seedlings

Salicylic acid (SA) and hydrogen sulfide (H₂S) have been proved to be multifunctional signal molecules to participate in the response of plants to abiotic stresses. However, it is still unclear whether there is interaction between SA and H₂S in response to chilling intensity of cucumber seedlings. Here, we found SA was sensitive to chilling intensity. Under normal condition, NaHS (H₂S donor) or removing endogenous H₂S with hypotaurine (HT, a specific scavenger of H₂S) and DL-propargylglycine (PAG, a specific inhibitor of H₂S) has no effect on endogenous SA level; however, SA induced endogenous H₂S content and activated the activities and mRNA level of L-/D-cysteine desulfhydrase (L-/D-CD), and inhibiting endogenous SA with paclobutrazol (PAC) or 2-aminoindan-2-phosphonic acid (AIP) blocked this effect, implying H₂S may play a role after SA signal. Further studies showed that both SA and NaHS notably alleviated chilling injury, which was evidenced by lower electrolyte leakage (EL), MDA content, and ROS accumulation, compared with H₂O treatment. Of note, SA and H₂S improved the activities and mRNA level of antioxidant enzymes (SOD, POD, CAT, APX, and GR) as well as the contents of AsA and GSH. Additionally, the chilling-response genes (ICE, CBF1, and COR) were obviously upregulated by exogenous SA and NaHS. However, the positive effect of SA on chilling tolerance was inhibited by HT, whereas PAC or AIP did not affect NaHS-induced chilling tolerance. Taken together, the data reveals that H₂S acts as a downstream signal of SA-induced chilling tolerance of cucumber via modulating antioxidant system and chilling-response genes.

Physiological mechanism of transglutaminase-mediated improvement in salt tolerance of cucumber seedlings

Transglutaminase (TGase) is inextricably associated with plant growth and development. However, the mechanism by which TGase enhances salt tolerance of higher plants under salt stress is poorly understood. In this study, we investigated the effects of NaCl stress and exogenous o-phenanthroline (o-Phen, a metalloprotease inhibitor) on TGase activity, chlorophyll fluorescence parameters, carbohydrates contents, the reactive oxygen species (ROS) scavenging system, and endogenous polyamines (PAs) contents of salt-sensitive 'Jinyou No. 4' and salt-tolerant 'Inbred Line 9930' cucumber. Salt stress significantly inhibited plant growth of the two cultivars, as well as hindered carbohydrates transport, which was more evident in the salt-sensitive cultivar. TGase activity and expression, ROS scavenging capacity, and bound PAs content were up-regulated by salt stress to some extent, which was more distinct in the salt-tolerant cucumber cultivar. However, o-Phen treatment significantly inhibited TGase expression, and further decreased plant growth and the actual photochemical efficiency of photosystem II in the two cultivars. In addition, application of o-Phen significantly decreased endogenous PAs content in leaves of 'Jinyou No. 4' and 'Inbred Line 9930' seedlings by 9.60% and 42.32% under NaCl stress, respectively. These results suggested that high activity of TGase increases the salt stress tolerance of cucumber plants by increasing endogenous PAs content and ROS scavenging capacity, and promoting carbon assimilation and photosynthetic products.

De novo transcriptome analysis of Chlorella sorokiniana: effect of glucose assimilation, and moderate light intensity

Chlorella can produce an unusually wide range of metabolites under various nutrient availability, carbon source, and light availability. Glucose, an essential molecule for the growth of microorganisms, also contributes significantly to the metabolism of various metabolic compounds produced by Chlorella. In addition, manipulation of light intensity also induces the formation of secondary metabolites such as pigments, and carotenoids in Chlorella. This study will focus on the effect of glucose addition, and moderate light on the regulation of carotenoid, lipid, starch, and other key metabolic pathways in Chlorella sorokiniana. To gain knowledge about this, we performed transcriptome profiling on C. sorokiniana strain NIES-2168 in response to moderate light stress supplemented with glucose under mixotrophic conditions. A total of 60,982,352 raw paired-end (PE) reads 100 bp in length was obtained from both normal, and mixotrophic samples of C. sorokiniana. After pre-processing, 93.63% high-quality PE reads were obtained, and 18,310 predicted full-length transcripts were assembled. Differential gene expression showed that a total of 937, and 1124 genes were upregulated, and downregulated in mixotrophic samples, respectively. Transcriptome analysis revealed that the mixotrophic condition caused upregulation of genes involved in carotenoids production (specifically lutein biosynthesis), fatty acid biosynthesis, TAG accumulation, and the majority of the carbon fixation pathways. Conversely, starch biosynthesis, sucrose biosynthesis, and isoprenoid biosynthesis were downregulated. Novel insights into the pathways that link the enhanced production of valuable metabolites (such as carotenoids in C. sorokiniana) grown under mixotrophic conditions is presented.

Photosynthetic responses of Arabidopsis to SO2 were related to photosynthetic pigments, photosynthesis gene expression and redox regulation

Sulfur dioxide (SO₂) is one of the most common and harmful air pollutants. High concentrations of SO₂ can induce a series of defensive responses in Arabidopsis plants. However, the role of photosynthesis in the plant response to SO₂ stress is not clear. Here, we report the photosynthetic responses of Arabidopsis plants to SO₂ stress. Exposure to 30 mg/m³ SO₂ decreased stomatal conductance (Gs) and transpiration rate (Tr) but increased photosynthetic pigments and net photosynthetic rate (Pn). The contents of carbohydrates and sucrose were not altered. The transcript levels of most genes related to photosystem II (PSII), cytochrome b6/f (Cytb6f), photosystem I (PSI) and carbon fixation were upregulated, revealing one important regulatory circuit for the maintenance of chloroplast homeostasis under SO₂ stress. Exposure to SO₂ triggered reactive oxygen species (ROS) generation, accompanied by increases in superoxide dismutase (SOD) activity and the contents of cysteine (Cys), glutathione (GSH) and non-protein thiol (NPT), which maintained cellular redox homeostasis. Together, our results indicated that chloroplast photosynthesis was involved in the plant response to SO₂ stress. The photosynthetic responses were related to photosynthetic pigments, photosynthesis gene expression and redox regulation.

Terpenes and Terpenoids in Plants: Interactions with Environment and Insects

The interactions of plants with environment and insects are bi-directional and dynamic. Consequently, a myriad of mechanisms has evolved to engage organisms in different types of interactions. These interactions can be mediated by allelochemicals known as volatile organic compounds (VOCs) which include volatile terpenes (VTs). The emission of VTs provides a way for plants to communicate with the environment, including neighboring plants, beneficiaries (e.g., pollinators, seed dispersers), predators, parasitoids, and herbivores, by sending enticing or deterring signals. Understanding terpenoid distribution, biogenesis, and function provides an opportunity for the design and implementation of effective and efficient environmental calamity and pest management strategies. This review provides an overview of plant-environment and plant-insect interactions in the context of terpenes and terpenoids as important chemical mediators of these abiotic and biotic interactions.

Proteomic profiles of young and mature cocoa leaves subjected to mechanical stress caused by wind

Cocoa is a perennial and arboreal species intolerant to strong and frequent winds and, for this reason, is usually grown with windbreaks of trees. The mechanical alterations caused by the wind in the field have a great impact on the growth, development and productivity of cocoa. The present work had a main objective to understand the molecular mechanisms of responses to mechanical stress, caused by the action of constant wind flow in young plants of cocoa through alterations of the proteomic profile in young (YL) and mature leaves (ML). Plants were exposed to constant wind (CW) at a speed of 4.5 m s^-1 for 12 h. There was a reduction in the accumulation of proteins in YL and a significant increase in ML submitted to CW in relation to the control. Differentially accumulated proteins, identified in YL and ML, belong to a broad functional group, related to energy production and carbon metabolism. Besides that, there was a higher efficiency in the protein relative abundance associated to energy production and the assimilation of carbon in the ML exposed to CW, in relation to the control. It was observed the appearance of new isoforms and, or post-transitional changes, which represent an acclimatization and tolerance response of these leaves to the stressor factor. In contrast, in YL, the energy production and the synthesis of gene products essential for their growth and development were affected by the mechanical stress caused by the wind, making them more intolerant.

Identification and Analysis of microRNAs in Chlorella sorokiniana Using High-Throughput Sequencing

Chlorella is a popular microalga with robust physiological and biochemical characteristics, which can be cultured under various conditions. The exploration of the small RNA content of Chlorella could improve strategies for the enhancement of metabolite production from this microalga. In this study, stress was introduced to the Chlorella sorokiniana culture to produce high-value metabolites such as carotenoids and phenolic content. The small RNA transcriptome of C. sorokiniana was sequenced, focusing on microRNA (miRNA) content. From the analysis, 98 miRNAs were identified in cultures subjected to normal and stress conditions. The functional analysis result showed that the miRNA targets found were most often involved in the biosynthesis of secondary metabolites, followed by protein metabolism, cell cycle, and porphyrin and chlorophyll metabolism. Furthermore, the biosynthesis of secondary metabolites such as carotenoids, terpenoids, and lipids was found mostly in stress conditions. These results may help to improve our understanding of regulatory mechanisms of miRNA in the biological and metabolic process of Chlorella species. It is important and timely to determine the true potential of this microalga species and to support the potential for genetic engineering of microalgae as they receive increasing focus for their development as an alternative source of biofuel, food, and health supplements.

Differential responses of photosynthetic parameters and its influence on carbohydrate metabolism in some contrasting rice (Oryza sativa L.) genotypes under arsenate stress

Influence of arsenic (As) in As tolerant and sensitive rice genotypes based chloroplastic pigments, leaf gas exchange attributes and their influence on carbohydrate metabolism were investigated in the present study. As retards growth of crop plants and increase several health ailments by contaminating food chain. Photosynthetic inhibition is known to be the prime target of As toxicity due to over-production of ROS. Hydroponically grown rice seedlings of twelve cultivars were exposed to 25, 50, and 75 μM arsenate (AsV) that exerted negative impact on plastidial pigments content and resulted into inhibition of Hill activity. Internal CO₂ concentration lowered gradually due to interference of As with stomatal conductance and transpiration rate that subsequently led to drop in net photosynthesis. Twelve contrasting rice genotypes responded differentially to As(V) stress. Present study evaluated As tolerant and sensitive rice cultivars with respect to As(V) imposed alterations in pigments content, photosynthetic attributes along with sugar metabolism. Starch contents, the principle carbohydrate storage declined differentially among As(V) stressed test cultivars, being more pronounced in cvs. Swarnadhan, Tulaipanji, Pusa basmati, Badshabhog, Tulsibhog and IR-20 compared to cvs. Bhutmuri, Kumargore, Binni, Vijaya, TN-1 and IR-64. Therefore, the six former cultivars tried to adapt defensive mechanisms by accumulating higher levels of reducing and non-reducing sugars to carry out basal metabolism to withstand As(V) induced alterations in photosynthesis. This study could help to screen As tolerant and sensitive rice genotypes based on their photosynthetic efficiency in As polluted agricultural fields to reduce As contamination assisted ecotoxicological risk.

Barley strigolactone signalling mutant hvd14.d reveals the role of strigolactones in abscisic acid-dependent response to drought

Strigolactones (SLs) are a group of plant hormones involved in many aspects of plant development and stress adaptation. Here, we investigated the drought response of a barley (Hordeum vulgare L.) mutant carrying a missense mutation in the gene encoding the SL-specific receptor HvD14. Our results clearly showed that hvd14.d mutant is hyper-sensitive to drought stress. This was illustrated by a lower leaf relative water content (RWC), impaired photosynthesis, disorganization of chloroplast structure, altered stomatal density and slower closure of stomata in response to drought in the mutant compared to the wild type parent cultivar Sebastian. Although the content of abscisic acid (ABA) and its derivatives remained unchanged in the mutant, significant differences in expression of genes related to ABA biosynthesis were observed. Moreover, hvd14.d was insensitive to ABA during seed germination. Analysis of Arabidopsis thaliana mutant atd14-1 also demonstrated that mutation in the SL receptor resulted in increased sensitivity to drought. Our results indicate that the drought-sensitive phenotype of barley SL mutant might be caused by a disturbed ABA metabolism and/or signalling pathways. These results together uncovered a link between SL signalling and ABA-dependent drought stress response in barley.

High-throughput phenotyping platform for analyzing drought tolerance in rice

A new imaging platform was constructed to analyze drought-tolerant traits of rice. Rice was used to quantify drought phenotypes through image-based parameters and analyzing tools. Climate change has increased the frequency and severity of drought, which limits crop production worldwide. Developing new cultivars with increased drought tolerance and short breeding cycles is critical. However, achieving this goal requires phenotyping a large number of breeding populations in a short time and in an accurate manner. Novel cutting-edge technologies such as those based on remote sensors are being applied to solve this problem. In this study, new technologies were applied to obtain and analyze imaging data and establish efficient screening platforms for drought tolerance in rice using the drought-tolerant mutant osphyb. Red-Green-Blue images were used to predict plant area, color, and compactness. Near-infrared imaging was used to determine the water content of rice, infrared was used to assess plant temperature, and fluorescence was used to examine photosynthesis efficiency. DroughtSpotter technology was used to determine water use efficiency, plant water loss rate, and transpiration rate. The results indicate that these methods can detect the difference between tolerant and susceptible plants, suggesting their value as high-throughput phenotyping methods for short breeding cycles as well as for functional genetic studies of tolerance to drought stress.

SMRT and Illumina RNA sequencing reveal novel insights into the heat stress response and crosstalk with leaf senescence in tall fescue

BACKGROUND

As a cool-season grass species, tall fescue (Festuca arundinacea) is challenged by increasing temperatures. Heat acclimation or activation of leaf senescence, are two main strategies when tall fescue is exposed to heat stress (HS). However, lacking a genome sequence, the complexity of hexaploidy nature, and the short read of second-generation sequencing hinder a comprehensive understanding of the mechanism. This study aims to characterize the molecular mechanism of heat adaptation and heat-induced senescence at transcriptional and post-transcriptional levels.

RESULTS

Transcriptome of heat-treated (1 h and 72 h) and senescent leaves of tall fescue were generated by combining single-molecular real-time and Illumina sequencing. In total, 4076; 6917, and 11,918 differentially expressed genes (DEGs) were induced by short- and long-term heat stress (HS), and senescence, respectively. Venn and bioinformatics analyses of DEGs showed that short-term HS strongly activated heat shock proteins (Hsps) and heat shock factors (Hsfs), as well as specifically activated FK506-binding proteins (FKBPs), calcium signaling genes, glutathione S-transferase genes, photosynthesis-related genes, and phytohormone signaling genes. By contrast, long-term HS shared most of DEGs with senescence, including the up-regulated chlorophyll catabolic genes, phytohormone synthesis/degradation genes, stress-related genes, and NACs, and the down-regulated photosynthesis-related genes, FKBPs, and catalases. Subsequently, transient overexpression in tobacco showed that FaHsfA2a (up-regulated specifically by short-term HS) reduced cell membrane damages caused by HS, but FaNAC029 and FaNAM-B1 (up-regulated by long-term HS and senescence) increased the damages. Besides, alternative splicing was widely observed in HS and senescence responsive genes, including Hsps, Hsfs, and phytohormone signaling/synthesis genes.

CONCLUSIONS

The short-term HS can stimulate gene responses and improve thermotolerance, but long-term HS is a damage and may accelerate leaf senescence. These results contribute to our understanding of the molecular mechanism underlying heat adaptation and heat-induced senescence.

Diclofenac shifts the role of root glutamine synthetase and glutamate dehydrogenase for maintaining nitrogen assimilation and proline production at the expense of shoot carbon reserves in Solanum lycopersicum L

The continuous increase of the human population worldwide has led to an increase of pharmaceuticals' consumption, such as diclofenac (DCF), a widely used non-steroidal anti-inflammatory drug (NSAID), that is not removed by wastewater treatment processes. Although there is some research regarding the effects of DCF on animals and aquatic invertebrates, information concerning its influence on plants' metabolism is still scarce. Through an integrated approach, using combined biochemical and molecular biology techniques, this work aimed to evaluate the phytotoxicity of DCF in Solanum lycopersicum L., focusing on the primary plant processes: nitrogen (N) assimilation and photosynthesis. The exposure of tomato plants to increasing concentrations of DCF (0, 0.5, and 5 mg L^-1) revealed that glutamine synthetase (GS) was differentially affected, in an organ-dependent manner, by this contaminant at the gene expression, protein, and activity levels, with an increased activity of 0.2-fold in shoots of plants treated with the lowest concentration of DCF although a general decrease was registered for the SlGS gene family expression, revealing that post-translational regulation was in order, since GS2 polypeptide content did not change. Glutamate dehydrogenase (GDH) activity was generally enhanced, accompanied by increases of 0.4- to 1.9-fold in proline levels, revealing GDH as an important compensatory route for both N assimilation and proline production under stressful conditions. No alterations in most photosynthetic endpoints were noticed after DCF treatments, but small decreases of 0.1- to 0.8-fold in the accumulation of RuBisCO-encoding transcripts were observed, along with a reduction in starch content. Some alterations in the soluble polypeptide profile were also detected in response to DCF, evidencing the participation of some stress-related proteins in the plant's response to DCF.

Photosynthetic characterization of flavodoxin-expressing tobacco plants reveals a high light acclimation-like phenotype

Flavodoxins are electron carrier flavoproteins present in bacteria and photosynthetic microorganisms which duplicate the functional properties of iron-sulphur containing ferredoxins and replace them under adverse environmental situations that lead to ferredoxin decline. When expressed in plant chloroplasts, flavodoxin complemented ferredoxin deficiency and improved tolerance to multiple sources of biotic, abiotic and xenobiotic stress. Analysis of flavodoxin-expressing plants grown under normal conditions, in which the two carriers are present, revealed phenotypic effects unrelated to ferredoxin replacement. Flavodoxin thus provided a tool to alter the chloroplast redox poise in a customized way and to investigate its consequences on plant physiology and development. We describe herein the effects exerted by the flavoprotein on the function of the photosynthetic machinery. Pigment analysis revealed significant increases in chlorophyll a, carotenoids and chlorophyll a/b ratio in flavodoxin-expressing tobacco lines. Results suggest smaller antenna size in these plants, supported by lower relative contents of light-harvesting complex proteins. Chlorophyll a fluorescence and P700 spectroscopy measurements indicated that transgenic plants displayed higher quantum yields for both photosystems, a more oxidized plastoquinone pool under steady-state conditions and faster plastoquinone dark oxidation after a pulse of saturating light. Many of these effects resemble the phenotypes exhibited by leaves adapted to high irradiation, a most common environmental hardship faced by plants growing in the field. The results suggest that flavodoxin-expressing plants would be better prepared to cope with this adverse situation, and concur with earlier observations reporting that hundreds of stress-responsive genes were induced in the absence of stress in these lines.

Differential expression between drought-tolerant and drought-sensitive sugarcane under mild and moderate water stress as revealed by a comparative analysis of leaf transcriptome

Sugarcane contributes 80% of global sugar production and to bioethanol generation for the bioenergy industry. Its productivity is threatened by drought that can cause up to 60% yield loss. This study used RNA-Seq to gain a better understanding of the underlying mechanism by which drought-tolerant sugarcane copes with water stress. We compared gene expression in KPS01-12 (drought-tolerant genotype) and UT12 (drought-sensitive genotype) that have significantly different yield loss rates under drought conditions. We treated KPS01-12 and UT12 with mild and moderate water stress and found differentially expressed genes in various biological processes. KPS01-12 had higher expression of genes that were involved in water retention, antioxidant secondary metabolite biosynthesis, and oxidative and osmotic stress response than UT12. In contrast, the sensitive genotype had more down-regulated genes that were involved in photosynthesis, carbon fixation and Calvin cycle than the tolerant genotype. Our obtained expression profiles suggest that the tolerant sugarcane has a more effective genetic response than the sensitive genotype at the initiation of drought stress. The knowledge gained from this study may be applied in breeding programs to improve sugarcane production in drought conditions.

Technical benefit on apple fruit of controlled atmosphere influenced by 1-MCP at molecular levels

The apple is a highly perishable fruit after harvesting and, therefore, several storage technologies have been studied to provide the consumer market with a quality product with a longer shelf life. However, little is known about the apple genome that is submitted to the storage, and even less with the application of ripening inhibitors. Due to these factors, this study sought to elucidate the transcriptional profile of apple cultivate Gala stored in a controlled atmosphere (AC) treated and not treated with 1-methyl cyclopropene (1-MCP). Through the genetic mapping of the apple, applying the microarray technique, it was possible to verify the action of treatments on transcripts related to photosynthesis, carbohydrate metabolism, response to hormonal stimuli, nucleic acid metabolism, reduction of oxidation, regulation of transcription and metabolism of cell wall and lipids. The results showed that the transcriptional profile in the entire genome of the fruit showed significant differences in the relative expression of the gene, this in response to CA in the presence and absence of 1-MCP. It should be noted that the transcription genes involved in the anabolic pathway were only maintained after six months in fruits treated with 1-MCP. The data in this work suggests that the apple in the absence of 1-MCP begins to prepare its metabolism to mature, even during the storage period in AC. Meanwhile, in the presence of the inhibitor, the transcriptional profile of the fruit is similar to that at the time of harvest. It was also found that a set of genes that code for ethylene receptors, auxin homeostasis, MADS Box, and NAC transcription factors may be involved in the regulation of post-harvest ripening after storage and in the absence of 1-MCP.

Exogenous 2-(3,4-Dichlorophenoxy) triethylamine alleviates salinity stress in maize by enhancing photosynthetic capacity, improving water status and maintaining K+/Na+ homeostasis

BACKGROUND

Soil salinity restricts plant growth and productivity. 2-(3,4-dichlorophenoxy) triethylamine (DCPTA) can alleviate salinity stress in plants. However, the mechanism of DCPTA-mediated salinity tolerance has not been fully clarified. We aimed to investigate its role in enhancing photosynthetic capacity, improving water status, maintaining K+/Na+ homeostasis and alleviating salinity stress in maize (Zea mays L.).

RESULTS

In present study, maize seedlings were grown in nutrient solutions with a combination of NaCl (0, 150 mM) and DCPTA (0, 20, 100, and 400 μM). And photosynthesis, water status, ion homeostasis and the expression of genes involved in ion uptake and transport were evaluated in the maize seedlings. The results demonstrated that DCPTA alleviated the growth inhibition of maize seedlings exposed to salinity stress by increasing the net photosynthetic rate (Pn) and the quantum efficiency of photosystem II (PSII) photochemistry. DCPTA improved the root hydraulic conductivity, which help maintained the water status. A relatively high K+ concentration but a relatively low Na+ concentration and the Na+/K+ ratio were observed in the presence of DCPTA under salinity stress. Additionally, DCPTA altered the expression of four genes (ZmSOS1, ZmHKT1, ZmNHX1 and ZmSKOR) that encode membrane transport proteins responsible for K+/Na+ homeostasis.

CONCLUSIONS

DCPTA improved the salinity tolerance of maize may be associated with enhanced photosynthetic capacity, maintenance of water status and altered expression of genes involved in ion uptake and transport.

Growth and physiological responses of three warm-season legumes to water stress

Novel drought-tolerant grain legumes like mothbean (Vigna acontifolia), tepary bean (Phaseolus acutifolius), and guar (Cyamopsis tetragonoloba) may also serve as summer forages, and add resilience to agricultural systems in the Southern Great Plains (SGP). However, limited information on the comparative response of these species to different water regimes prevents identification of the most reliable option. This study was conducted to compare mothbean, tepary bean and guar for their vegetative growth and physiological responses to four different water regimes: 100% (control), and 75%, 50% and 25% of control, applied from 27 to 77 days after planting (DAP). Tepary bean showed the lowest stomatal conductance (g_s) and photosynthetic rate (A), but also maintained the highest instantaneous water use efficiency (WUE_i) among species at 0.06 and 0.042 m³ m^-3 soil moisture levels. Despite maintaining higher A, rates of vegetative growth by guar and mothbean were lower than tepary bean due to their limited leaf sink activity. At final harvest (77 DAP), biomass yield of tepary bean was 38-60% and 41-56% greater than guar and mothbean, respectively, across water deficits. Tepary bean was the most drought-tolerant legume under greenhouse conditions, and hence future research should focus on evaluating this species in extensive production settings.

Na2CO3-responsive Photosynthetic and ROS Scavenging Mechanisms in Chloroplasts of Alkaligrass Revealed by Phosphoproteomics

Alkali-salinity exerts severe osmotic, ionic, and high-pH stresses to plants. To understand the alkali-salinity responsive mechanisms underlying photosynthetic modulation and reactive oxygen species (ROS) homeostasis, physiological and diverse quantitative proteomics analyses of alkaligrass (Puccinellia tenuiflora) under Na₂CO₃ stress were conducted. In addition, Western blot, real-time PCR, and transgenic techniques were applied to validate the proteomic results and test the functions of the Na₂CO₃-responsive proteins. A total of 104 and 102 Na₂CO₃-responsive proteins were identified in leaves and chloroplasts, respectively. In addition, 84 Na₂CO₃-responsive phosphoproteins were identified, including 56 new phosphorylation sites in 56 phosphoproteins from chloroplasts, which are crucial for the regulation of photosynthesis, ion transport, signal transduction, and energy homeostasis. A full-length PtFBA encoding an alkaligrass chloroplastic fructose-bisphosphate aldolase (FBA) was overexpressed in wild-type cells of cyanobacterium Synechocystis sp. Strain PCC 6803, leading to enhanced Na₂CO₃ tolerance. All these results indicate that thermal dissipation, state transition, cyclic electron transport, photorespiration, repair of photosystem (PS) II, PSI activity, and ROS homeostasis were altered in response to Na₂CO₃ stress, which help to improve our understanding of the Na₂CO₃-responsive mechanisms in halophytes.

Methyl Jasmonate Affects Photosynthesis Efficiency, Expression of HvTIP Genes and Nitrogen Homeostasis in Barley

Jasmonates modulate many growth and developmental processes and act as stress hormones that play an important role in plant tolerance to biotic and abiotic stresses. Therefore, there is a need to identify the genes that are regulated through the jasmonate signalling pathway. Aquaporins, and among them the Tonoplast Intrinsic Proteins (TIPs), form the channels in cell membranes that are responsible for the precise regulation of the movement of water and other substrates between cell compartments. We identified the cis-regulatory motifs for the methyl jasmonate (MeJA)-induced genes in the promoter regions of all the HvTIP genes, which are active in barley seedlings, and thus we hypothesised that the HvTIP expression could be a response to jasmonate signalling. In the presented study, we determined the effect of methyl jasmonate on the growth parameters and photosynthesis efficiency of barley seedlings that had been exposed to different doses of MeJA (15–1000 µM × 120 h) in a hydroponic solution. All of the applied MeJA concentrations caused a significant reduction of barley seedling growth, which was most evident in the length of the first leaf sheath and dry leaf weight. The observed decrease of the PSII parameters after the exposure to high doses of MeJA (500 µM or higher) was associated with the downregulation of HvPsbR gene encoding one of the extrinsic proteins of the Oxygen Evolving Complex. The reduced expression of HvPsbR might lead to the impairment of the OEC action, manifested by the occurrence of the K-band in an analysis of fluorescence kinetics after MeJA treatment as well as reduced photosynthesis efficiency. Furthermore, methyl jasmonate treatment caused a decrease in the nitrogen content in barley leaves, which was associated with an increased expression the four tonoplast aquaporin genes (HvTIP1;2, HvTIP2;2, HvTIP4;1 and HvTIP4;2) predicted to transport the nitrogen compounds from the vacuole to the cytosol. The upregulation of the nitrogen-transporting HvTIPs might suggest their involvement in the vacuolar unloading of ammonia and urea, which both could be remobilised when the nitrogen content in the leaves decreases. Our research provides tips on physiological role of the individual TIP subfamily members of aquaporins under methyl jasmonate action.

Parthenium hysterophorus steps up Ca-regulatory pathway in defence against highlight intensities

Parthenium hysterophorus exhibits tolerance to a great extent against abiotic stresses including high light intensities. In this study, P. hysterophorus was subjected to three different light intensities viz. control (CL, 250 µmol photons m^-2 s^-1), moderately high (ML, 500 µmol photons m^-2 s^-1) and high (HL, 1000 µmol photons m^-2 s^-1) for assessment of biochemical and physiological responses at 3 and 5 days after treatment (DAT). Proteomic responses were also observed at 5 DAT. Level of oxidative stress marker, abundance of H₂O₂ and O₂^- was highest in leaves exposed to HL followed by ML treatment. Biomass accumulation, photosynthetic parameters, chloroplast and mitochondrial integrity were also affected by both ML and HL treatments. Differential protein expression data showed modulation of thirty-eight proteins in ML and HL intensities. P. hysterophorus exhibited good ability to survive in ML then HL treatment as demonstrated by enhancement of the antioxidant system and photosynthesis. Furthermore, P. hysterophorus mobilized some key proteins related to calcium signaling, which in turn coordinate physiological homeostasis under stress. Proline and total soluble sugar content were high under stress; however, results of simulated experiment of our study indicate such accumulation of osmolytes may inhibit photon-availability to chloroplast. These results clarify our understanding of the mechanisms underlying the light stress tolerance of P. hysterophorus.

Biochar-induced immobilization and transformation of silver-nanoparticles affect growth, intracellular-radicles generation and nutrients assimilation by reducing oxidative stress in maize

Silver nanoparticles (AgNPs) are used in a wide range of consumer products inevitably releases in massive quantities in the natural environment, posing a potential thread to ecosystem-safety and plant health. Here, the impact of AgNPs (100-1000 mg L^-1) without and with biochar (@2 % w/v) amendment on maize plants was assessed in hydroponics exposure medium. AgNPs exposure to plants induced dose-dependent phytotoxicity by suppressing plant growth, disturbing photosynthesis and gas exchange traits and alteration in macro- and micronutrients assimilation. At the same time, AgNPs with addition of biochar alleviated the phyto-toxic effects of AgNPs through approximately 4-8 times reduction in uptake and tissue accumulation of Ag. Moreover, activities of antioxidant enzymes in AgNPs + biochar treated plants indicated the lower oxidative stress. Electron paramagnetic resonance (EPR) spectroscopy confirmed that superoxide (O₂^-) radical was the dominant reactive oxygen species. Fourier-transform infrared spectroscopic (FTIR) and X-ray photoelectron spectroscopic (XPS) results revealed that biochar surface carboxyl and sulfur functional groups were involved in complexation process with NPs, which inhibited the oxidative dissolution and release of Ag⁺ ions besides of biochar space shield effect. Thus, the interaction of biochar with AgNPs immobilizes these NPs and can effectively reduce their bioavailability in the environmental matrix.

Cytosolic Ascorbate Peroxidases Plays a Critical Role in Photosynthesis by Modulating Reactive Oxygen Species Level in Stomatal Guard Cell

Photosynthetic rate is one of the key factors limiting yield of cotton. Reactive oxygen species (ROS) generated by abiotic stress imposes numerous detrimental effects and causes tremendous loss of yield. It is worth to study whether ROS scavenging enzymes could affect yield through regulating photosynthetic rate in cotton. In this study, we created transgenic cotton with changes of endogenous ROS by overexpressing or suppressing the expression of cytosolic ascorbate peroxidases (APXs), which are hydrogen peroxide (H₂O₂) scavenging enzymes in plants. The suppression of cytosolic APXs by RNAi brings about a great influence on plant growth and development. Plant height and leaf size declined, and yield-related traits including single boll weight, seed weight, seed size, and lint weight dropped significantly, in IAO lines (cytosolic APX-suppressed lines). The stunted plant growth was due to the decrease of plant photosynthetic rate. The evidences showed that increased ROS level in guard cells inhibited stomatal opening and suppressed the absorption of CO₂ and H₂O in IAO line. The decrease of water content and the increase of water loss rate in leaf exacerbated the decline of photosynthetic rate in cytosolic APX-suppressed lines. Based on these results, it implies that cytosolic APXs as a whole play an important role in maintaining REDOX balance to regulate photosynthetic rate and yield in cotton.

Impact of Long-Term Copper Exposure on Growth, Photosynthesis, Antioxidant Defence System and Artemisinin Biosynthesis in Soil-Grown Artemisia annua Genotypes

The effects of copper (Cu) exposure on growth and physiological characteristics of three genotypes (CN-12, Cim-Sanjeevani and Cim-Arogya) of Artemisia annua L. were elucidated. The plants were grown under naturally illuminated greenhouse conditions and were harvested after physiological maturity (120 days after sowing). Results suggest that 10 mg kg^- 1 Cu significantly enhanced the growth and physiological parameters like enzyme activities, photosynthesis. At higher concentrations, Cu inhibited the growth, biomass, photosynthetic parameters; while increased lipid peroxidation in all the genotypes. The activities of antioxidant enzymes viz. catalase, peroxidase and superoxide dismutase were upregulated by the Cu stress. The highest applied concentration of Cu (60 mg kg^- 1) proved most toxic for plants. Moreover, artemisinin content was increased upto 10 mg kg^- 1 of Cu treatment, compared with control, however, the artemisinin accumulation decreased at higher doses of Cu in all the genotypes. On the basis of studied parameters, Cim-Arogya was found to be most tolerant among all for Cu toxicity.

Photosynthetic activity of indica rice sister lines with contrasting cold tolerance

Incidence of cold during early stages is an obstacle for the growing progress of rice plants. Cold stress has strong negative effects on photosynthetic activity. Previously, our group evaluated plant survival of 90 indica rice genotypes after cold treatment. Two sister lines were characterized as cold-tolerant and cold-sensitive. Transcriptomic analyses of the same genotypes had indicated differential expression of genes related to photosynthesis. Previous work with japonica rice had suggested that cold sensitivity was more related to photosystem II (PSII) than to photosystem I (PSI). Using our previously identified contrasting genotypes, we investigated the role of specific steps of the photosynthetic process in cold tolerance/sensitivity of indica rice plants during and after (recovery period) cold exposure. During both cold treatment and recovery period, the photochemical activity (including PSII and PSI) presented higher levels in the low temperature-tolerant genotype, when compared with the sensitive one. The higher photochemical efficiency during the cold treatment appears to be related to a lower fraction of reduced QA - in PSII. We also observed lower transpiration rates and higher water use efficiency in the cold-tolerant genotype, due to stomatal closure. After the recovery period, the higher efficiency in the cold-tolerant genotype seems to be related to a lower fraction of reduced QA - and a larger pool of final electron acceptors at the PSI. This work uncovered changes in photosynthetic performance including both photosystems and improved water use efficiency which may be important components of cold tolerance mechanisms in indica rice.

Nuclear-encoded synthesis of the D1 subunit of photosystem II increases photosynthetic efficiency and crop yield

In photosynthetic organisms, the photosystem II (PSII) complex is the primary target of thermal damage. Plants have evolved a repair process to prevent the accumulation of damaged PSII. The repair of PSII largely involves de novo synthesis of proteins, particularly the D1 subunit protein encoded by the chloroplast gene psbA. Here we report that the allotropic expression of the psbA complementary DNA driven by a heat-responsive promoter in the nuclear genome sufficiently protects PSII from severe loss of D1 protein and dramatically enhances survival rates of the transgenic plants of Arabidopsis, tobacco and rice under heat stress. Unexpectedly, we found that the nuclear origin supplementation of the D1 protein significantly stimulates transgenic plant growth by enhancing net CO2 assimilation rates with increases in biomass and grain yield. These findings represent a breakthrough in bioengineering plants to achieve efficient photosynthesis and increase crop productivity under normal and heat-stress conditions.

Higher order photoprotection mutants reveal the importance of ΔpH-dependent photosynthesis-control in preventing light induced damage to both photosystem II and photosystem I

Although light is essential for photosynthesis, when in excess, it may damage the photosynthetic apparatus, leading to a phenomenon known as photoinhibition. Photoinhibition was thought as a light-induced damage to photosystem II; however, it is now clear that even photosystem I may become very vulnerable to light. One main characteristic of light induced damage to photosystem II (PSII) is the increased turnover of the reaction center protein, D1: when rate of degradation exceeds the rate of synthesis, loss of PSII activity is observed. With respect to photosystem I (PSI), an excess of electrons, instead of an excess of light, may be very dangerous. Plants possess a number of mechanisms able to prevent, or limit, such damages by safe thermal dissipation of light energy (non-photochemical quenching, NPQ), slowing-down of electron transfer through the intersystem transport chain (photosynthesis-control, PSC) in co-operation with the Proton Gradient Regulation (PGR) proteins, PGR5 and PGRL1, collectively called as short-term photoprotection mechanisms, and the redistribution of light between photosystems, called state transitions (responsible of fluorescence quenching at PSII, qT), is superimposed to these short term photoprotective mechanisms. In this manuscript we have generated a number of higher order mutants by crossing genotypes carrying defects in each of the short-term photoprotection mechanisms, with the final aim to obtain a direct comparison of their role and efficiency in photoprotection. We found that mutants carrying a defect in the ΔpH-dependent photosynthesis-control are characterized by photoinhibition of both photosystems, irrespectively of whether PSBS-dependent NPQ or state transitions defects were present or not in the same individual, demonstrating the primary role of PSC in photoprotection. Moreover, mutants with a limited capability to develop a strong PSBS-dependent NPQ, were characterized by a high turnover of the D1 protein and high values of Y(NO), which might reflect energy quenching processes occurring within the PSII reaction center.

Melatonin foliar sprays elicit salinity stress tolerance and enhance fruit yield and quality in strawberry (Fragaria × ananassa Duch.)

The increasing salinity in soils and irrigation water is a major concern for growers of strawberry, a salt-sensitive horticultural crop. The hormone melatonin (N-acetyl-5-methoxytryptamine) is involved in many biological processes and affects plant responses to environmental stresses. The effects of weekly 100 and 200 μM melatonin sprays on leaf composition parameters (photosynthetic pigment and macronutrient concentrations, oxidative stress markers, antioxidant defense systems and abscisic acid concentrations), fruit yield and quality parameters (soluble solids, total acidity, ascorbic acid, total antioxidants and phenolics and sugars), and leaf and fruit melatonin have been studied in strawberry grown under non-saline, moderate and intense salinity conditions (0, 40 and 80 mM NaCl, respectively). Salinity led to decreases in yield, fruit quality parameters and leaf photosynthetic pigments and macronutrient concentrations, as well as to increases in oxidative stress, with melatonin foliar application alleviating all these changes. On the other hand, salinity led to increases in the leaf levels of antioxidant enzymes, abscisic acid and melatonin, with foliar applications of melatonin boosting those increases. In the absence of salinity stress, melatonin led to smaller changes in all parameters in the same direction to that observed in the presence of salinity. Furthermore, melatonin resulted in increases in strawberry fruit yield and quality, especially in plants grown under salinity. Results indicate that the effects of melatonin application are associated with a boost in leaf antioxidant enzymes and abscisic acid, and support that the application of melatonin is a promising tool for mitigating salt stress in strawberry.

Arbuscular mycorrhizal fungi enhanced drought resistance in apple by regulating genes in the MAPK pathway

Arbuscular mycorrhizal fungi (AMF) can form a symbiotic relationships with most terrestrial plants and play an important role in plant growth and adaptation to various stresses. To study the role of AMF in regulating drought resistance in apple, the effects of drought stress on Malus hupehensis inoculated with AMF were investigated. Inoculation of AMF enhanced apple plants growth. Mycorrhizal plants had higher total chlorophyll concentrations but lower relative electrolyte leakage under drought stress. Mycorrhizal plants increased net photosynthetic rate, stomatal conductance, and transpiration rate under drought stress, however, they showed lower inhibition in the quantum yield of PSII photochemistry. Mycorrhizal plants also had higher superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzyme activities under drought conditions. Thus, mycorrhizal plants had lower accumulated MDA, H₂O₂, and O₂^- than non-mycorrhizal seedlings. Total sugar and proline concentrations also significantly increased, helping maintain the osmotic balance. Furthermore, mitogen-activated protein kinase (MAPK) cascades, which participate in the regulation of responses of plants and microorganisms to biotic and abiotic stress, were up-regulated in apple plants and AMF during drought. We saw that there were at least two motifs that were identical in MAPK proteins and many elements that responded to hormones and stress from these MAPK genes. In summary, our results showed that mycorrhizal colonization enhanced apple drought tolerance by improving gas exchange capacity, increasing chlorophyll fluorescence parameters, creating a greater osmotic adjustment capacity, increasing scavenging of reactive oxygen species (ROS), and using MAPK signals for interactions between AMF and their apple plant hosts.

Specific functions for Mediator complex subunits from different modules in the transcriptional response of Arabidopsis thaliana to abiotic stress

Adverse environmental conditions are detrimental to plant growth and development. Acclimation to abiotic stress conditions involves activation of signaling pathways which often results in changes in gene expression via networks of transcription factors (TFs). Mediator is a highly conserved co-regulator complex and an essential component of the transcriptional machinery in eukaryotes. Some Mediator subunits have been implicated in stress-responsive signaling pathways; however, much remains unknown regarding the role of plant Mediator in abiotic stress responses. Here, we use RNA-seq to analyze the transcriptional response of Arabidopsis thaliana to heat, cold and salt stress conditions. We identify a set of common abiotic stress regulons and describe the sequential and combinatorial nature of TFs involved in their transcriptional regulation. Furthermore, we identify stress-specific roles for the Mediator subunits MED9, MED16, MED18 and CDK8, and putative TFs connecting them to different stress signaling pathways. Our data also indicate different modes of action for subunits or modules of Mediator at the same gene loci, including a co-repressor function for MED16 prior to stress. These results illuminate a poorly understood but important player in the transcriptional response of plants to abiotic stress and identify target genes and mechanisms as a prelude to further biochemical characterization.

Photosynthetic Response Mechanism of Soil Salinity-Induced Cross-Tolerance to Subsequent Drought Stress in Tomato Plants

Soil salinization and water shortage cause ion imbalance and hyperosmoticity in plant cells, adversely impairing photosynthesis efficiency. How soil salinity-induced photosynthetic acclimation influences the cross-tolerance to drought conditions represents a promising research topic. This study was to reveal the photosynthetic mechanism of soil salinity-induced resistance to the subsequent drought stress in tomato leaves through comprehensive photosynthesis-related spectroscopy analysis. We conducted soil salinity pretreatment and subsequent drought stress experiments, including irrigation with 100 mL water, 100 mL 100 mM NaCl solution (NaCl100), 50 mL water, and 50 mL 100 mM NaCl solution (NaCl50) for five days, followed by five-day drought stress. The results showed that soil salinity treatment by NaCl decreased the rate of photosynthetic gas exchange but enhanced CO₂ assimilation, along with photosystem II [PS(II)] and photosystem I [PS(I)] activity and photochemical efficiency in tomato plants compared with water pretreatment after subsequent drought stress. NaCl100 and NaCl50 had the capacity to maintain non-photochemical quenching (NPQ) of chlorophyll fluorescence and the cyclic electron (CEF) flow around PSI in tomato leaves after being subjected to subsequent drought stress, thus avoiding the decrease of photosynthetic efficiency under drought conditions. NaCl100 and NaCl50 pretreatment induced a higher proton motive force (pmf) and also alleviated the damage to the thylakoid membrane and adenosine triphosphate (ATP) synthase of tomato leaves caused by subsequent drought stress. Overall, soil salinity treatment could enhance drought resistance in tomato plants by inducing NPQ, maintaining CEF and pmf that tradeoff between photoprotection and photochemistry reactions. This study also provides a photosynthetic perspective for salt and drought cross-tolerance.

Stress Responses of Shade-Treated Tea Leaves to High Light Exposure after Removal of Shading

High-quality green tea is produced from buds and young leaves grown by the covering-culture method, which employs shading treatment for tea plants (Camellia sinensis L.). Shading treatment improves the quality of tea, but shaded tea plants undergo sudden exposures to high light (HL) at the end of the treatment by shade removal. In this study, the stress response of shaded tea plants to HL illumination was examined in field condition. Chl a/b ratio was lower in shaded plants than nonshaded control, but it increased due to exposure to HL after 14 days. Rapid decline in Fv/Fm values and increases in carbonylated protein level were induced by HL illumination in the shaded leaves on the first day, and they recovered thereafter between a period of one and two weeks. These results revealed that shaded tea plants temporarily suffered from oxidative damages caused by HL exposure, but they could also recover from these damages in 2 weeks. The activities of antioxidant enzymes, total ascorbate level, and ascorbate/dehydroascorbate ratio were decreased and increased in response to low light and HL conditions, respectively, suggesting that the upregulation of antioxidant defense systems plays a role in the protection of the shaded tea plants from HL stress.

Ethylene insensitive mutation improves Arabidopsis plant tolerance to NO2 exposure

Ethylene signaling was addressed, for the first time, in plant responses to nitrogen dioxide (NO₂) by comparatively analyzing the performance of Arabidopsis ethylene insensitive 2 (ein2-1) with wild-type (WT) plants. Following NO₂ fumigation, severe leaf wilting and chlorosis occurred in WT plants, but much less symptoms were observed in ein2-1. The activities of superoxide dismutase (SOD), peroxidase (PRX) and catalase (CAT) were 39%, 92%, and 11% higher, respectively, in ein2-1 than in WT following NO₂ exposure. Although glutathione contents and the ratio of its reduced form (GSH) to oxidized form (GSSG) were decreased by NO₂, an obviously alleviated degree was detected in ein2-1 relative to WT. Correspondingly, the contents of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), and electrolyte leakage were 25%, 24%, and 29% lower, respectively, in ein2-1 than in WT. The difference of oxidative stress between two tested genotypes was also revealed by the leaf staining regarding the production and distribution of H₂O₂, superoxide anion (O₂˙-), and cell death. The genes involved in antioxidation or oxidation-reduction processes mostly presented a stronger expression in ein2-1 than in WT under NO₂ stress. The photosynthesis-related parameters including chlorophyll and soluble sugar contents, net photosynthetic rate (Pn), and ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity and gene expression, and chlorophyll fluorescence parameters were affected, generally, to a lesser degree in ein2-1 than in WT following NO₂ fumigation. The enzymatic activities and gene expressions of invertases mostly displayed a higher level in ein2-1 relative to WT following NO₂ fumigation. For example, the activities of cytoplasmic, cell wall and vacuolar invertases were 76%, 26%, and 26% higher, respectively, in ein2-1 than in WT. Together, these data suggest that ethylene signal insensitivity efficiently improves plant tolerance to NO₂ exposure, and the possible mechanisms might be correlated with leaf antioxidative defense, photosynthesis-related processes, and sucrose metabolisms.

Effect of UV-B priming on the abiotic stress tolerance of stress-sensitive rice seedlings: Priming imprints and cross-tolerance

Ultraviolet (UV)-B priming can boost the abiotic stress tolerance of plants by activating stress-responsive pathways. The main objective of the present study was to investigate the persistence of priming imprints and cross-tolerance inducing effects of UV-B priming in abiotic stress-sensitive rice (Oryza sativa L. 'Aiswarya') when subjected to various abiotic stressors (NaCl, PEG, and UV-B). The UV-B priming of rice seeds and seedlings effectively enhanced photosynthetic efficiency, antioxidant machinery activity, and antioxidative enzyme production, especially when seedlings were exposed to NaCl, followed by UV-B and PEG. The ability of UV-B priming to induce cross-tolerance against NaCl stress was substantiated by the greater antioxidant activity of the primed and NaCl-stressed seedlings. The greater performance and stress tolerance of the seedlings from UV-B-primed seeds were attributed to the carryover of priming imprints from seeds into the seedlings. Indeed, UV-B priming activated the antioxidant systems of the seedlings, even under non-stress conditions, and resulted in greater responses upon subsequent stress exposure, which suggested that preparedness for encountering imminent stress was attained by UV-B priming in a stress-sensitive rice.

Transcriptomic analysis of the phytotoxic effects of 1-allyl-3-methylimidazolium chloride on the growth and plant hormone metabolic pathways of maize (Zea mays L.) seedlings

In this study, we investigated the phytotoxicity of an imidazolium-based ionic liquid, 1-allyl-3-methylimidazolium chloride ([Amim]Cl), against maize seedlings. It was found that in response to an increase in [Amim]Cl treatment concentrations, there were significant decreases in growth parameters (fresh weights and lengths) and the photosynthetic pigment contents of maize plants, whereas in contrast, the malondialdehyde content increased. In order to determine the molecular basis of [Amim]Cl-induced plant growth inhibition, an RNA-Seq analysis to examine the gene expression profiles of selected central biological pathways was performed. And a total of 4024 genes that were differentially expressed between control and 400 mg/L [Amim]Cl-treated plants were accordingly identified. Pathway enrichment analysis for the differentially expressed genes revealed that 12 of 15 genes in the porphyrin and chlorophyll metabolic pathways were down-regulated in response to [Amim]Cl treatment. Moreover, all six genes encoding key chlorophyll synthetic enzymes were down-regulated by [Amim]Cl. With regards to plant hormone metabolic pathways, the genes encoding key enzymes involved in ethybilene and abscisic acid (ABA) biosynthesis were up-regulated in response to [Amim]Cl treatment. Genes responsible for gibberellin (GA) inactivation were also stimulated by [Amim]Cl. These observations indicate that [Amim]Cl may promote the biosynthesis of senescence-related hormones (ethylene and ABA) as well as inactivation of growth-promoting hormones (GAs). It might be concluded that the observed [Amim]Cl-induced inhibition of maize seedling growth could be associated with changes in the gene expression profiles of these metabolic pathways.

Concomitant phytonutrient and transcriptome analysis of mature fruit and leaf tissues of tomato (Solanum lycopersicum L. cv. Oregon Spring) grown using organic and conventional fertilizer

Enhanced levels of antioxidants, phenolic compounds, carotenoids and vitamin C have been reported for several crops grown under organic fertilizer, albeit with yield penalties. As organic agricultural practices continue to grow and find favor it is critical to gain an understanding of the molecular underpinnings of the factors that limit the yields in organically farmed crops. Concomitant phytochemical and transcriptomic analysis was performed on mature fruit and leaf tissues derived from Solanum lycopersicum L. ‘Oregon Spring’ grown under organic and conventional fertilizer conditions to evaluate the following hypotheses. 1. Organic soil fertilizer management results in greater allocation of photosynthetically derived resources to the synthesis of secondary metabolites than to plant growth, and 2. Genes involved in changes in the accumulation of phytonutrients under organic fertilizer regime will exhibit differential expression, and that the growth under different fertilizer treatments will elicit a differential response from the tomato genome. Both these hypotheses were supported, suggesting an adjustment of the metabolic and genomic activity of the plant in response to different fertilizers. Organic fertilizer treatment showed an activation of photoinhibitory processes through differential activation of nitrogen transport and assimilation genes resulting in higher accumulation of phytonutrients. This information can be used to identify alleles for breeding crops that allow for efficient utilization of organic inputs.

Regulation of carbon metabolism in two maize sister lines contrasted for chilling tolerance

Maize can grow in cool temperate climates but is often exposed to spring chilling temperatures that can affect early seedling growth. Here, we used two sister double-haploid lines displaying a contrasted tolerance to chilling to identify major determinants of long-term chilling tolerance. The chilling-sensitive (CS) and the chilling-tolerant (CT) lines were grown at 14 °C day/10 °C night for 60 d. CS plants displayed a strong reduction in growth and aerial biomass compared with CT plants. Photosynthetic efficiency was affected with an increase in energy dissipation in both lines. Chilling tolerance in CT plants was associated with higher chlorophyll content, glucose-6-phosphate dehydrogenase activity, and higher sucrose to starch ratio. Few changes in cell wall composition were observed in both genotypes. There was no obvious correlation between nucleotide sugar content and cell wall polysaccharide composition. Our findings suggest that the central starch-sucrose metabolism is one major determinant of the response to low temperature, and its modulation accounts for the ability of CT plants to cope with low temperature. This modulation seemed to be linked to a strong alteration in the biosynthesis of nucleotide sugars that, at a high level, could reflect the remobilization of carbon in response to chilling.

Modulatory Role of Reactive Oxygen Species in Root Development in Model Plant of Arabidopsis thaliana

Reactive oxygen species (ROS), a type of oxygen monoelectronic reduction product, have a higher chemical activity than O₂. Although ROS pose potential risks to all organisms via inducing oxidative stress, indispensable role of ROS in individual development cannot be ignored. Among them, the role of ROS in the model plant Arabidopsis thaliana is deeply studied. Mounting evidence suggests that ROS are essential for root and root hair development. In the present review, we provide an updated perspective on the latest research progress pertaining to the role of ROS in the precise regulation of root stem cell maintenance and differentiation, redox regulation of the cell cycle, and root hair initiation during root growth. Among the different types of ROS, O₂ ^•- and H₂O₂ have been extensively investigated, and they exhibit different gradient distributions in the roots. The concentration of O₂ ^•- decreases along a gradient from the meristem to the transition zone and the concentration of H₂O₂ decreases along a gradient from the differentiation zone to the elongation zone. These gradients are regulated by peroxidases, which are modulated by the UPBEAT1 (UPB1) transcription factor. In addition, multiple transcriptional factors, such as APP1, ABO8, PHB3, and RITF1, which are involved in the brassinolide signaling pathway, converge as a ROS signal to regulate root stem cell maintenance. Furthermore, superoxide anions (O₂ ^•-) are generated from the oxidation in mitochondria, ROS produced during plasmid metabolism, H₂O₂ produced in apoplasts, and catalysis of respiratory burst oxidase homolog (RBOH) in the cell membrane. Furthermore, ROS can act as a signal to regulate redox status, which regulates the expression of the cell-cycle components CYC2;3, CYCB1;1, and retinoblastoma-related protein, thereby controlling the cell-cycle progression. In the root maturation zone, the epidermal cells located in the H cell position emerge to form hair cells, and plant hormones, such as auxin and ethylene regulate root hair formation via ROS. Furthermore, ROS accumulation can influence hormone signal transduction and vice versa. Data about the association between nutrient stress and ROS signals in root hair development are scarce. However, the fact that ROBHC/RHD2 or RHD6 is specifically expressed in root hair cells and induced by nutrients, may explain the relationship. Future studies should focus on the regulatory mechanisms underlying root hair development via the interactions of ROS with hormone signals and nutrient components.

Plant growth regulators improve growth, photosynthesis, mineral nutrient and antioxidant system under cadmium stress in menthol mint (Mentha arvensis L.)

Menthol mint (Mentha arvensis L.) cultivation is significantly affected by the heavy metals like cadmium (Cd) which also imposes severe health hazards. Two menthol mint cultivars namely Kosi and Kushal were evaluated under Cd stress conditions. Impact of plant growth regulators (PGRs) like salicylic acid (SA), gibberellic acid (GA₃) and triacontanol (Tria) on Cd stress tolerance was assessed. Reduced growth, photosynthetic parameters, mineral nutrient concentration, and increased oxidative stress biomarkers like electrolyte leakage, malondialdehyde, and hydrogen peroxide contents were observed under Cd stress. Differential upregulation of proline content and antioxidant activities under Cd stress was observed in both the cultivars. Interestingly, low electrolyte leakage, lipid peroxidation, hydrogen peroxide and Cd concentration in leaves were observed in Kushal compared to Kosi. Among all the PGRs tested, SA proved to be the best in improving Cd-stress tolerance in both the cultivars but Kushal responded better than Kosi.

An Allelopathic Role for Garlic Root Exudates in the Regulation of Carbohydrate Metabolism in Cucumber in a Hydroponic Co-Culture System

Garlic is considered to have a strong positive effect on the growth and yield of receptors under soil cultivation conditions. However, how this positive promotion is produced by changing the growth environment of the receptors or directly acting on the receptors is still not very clear. The direct influence of co-culturing with different quantities of garlic plants (the control 5, 10, 15, 20) on the growth and biochemical processes of cucumber plants was studied using a hydroponic co-culture system. Different numbers of garlic bulbs inhibited the growth of cucumber plants and increased the production and induction of reactive oxygen species, which accompanied the enhancement of lipid peroxidation and oxidative damage to cucumber. This allelopathic exposure further reduced the chlorophyll contents and photosynthesis rate, and consequently impaired the photosynthetic performance of photosystem II (PSII). Garlic root exudates increased the leaves' carbohydrates accumulation, such as soluble sugar contents and sucrose levels by regulating the activities of metabolismic enzymes; however, no such accumulation was observed in the roots. Our results suggested that garlic root exudates can mediate negative plant-plant interactions and its phytotoxic influence on cucumber plants may have occurred through the application of oxidative stress, which consequently imbalanced the source-to-sink photo-assimilate flow.

An Overview of Hazardous Impacts of Soil Salinity in Crops, Tolerance Mechanisms, and Amelioration through Selenium Supplementation

Soil salinization is one of the major environmental stressors hampering the growth and yield of crops all over the world. A wide spectrum of physiological and biochemical alterations of plants are induced by salinity, which causes lowered water potential in the soil solution, ionic disequilibrium, specific ion effects, and a higher accumulation of reactive oxygen species (ROS). For many years, numerous investigations have been made into salinity stresses and attempts to minimize the losses of plant productivity, including the effects of phytohormones, osmoprotectants, antioxidants, polyamines, and trace elements. One of the protectants, selenium (Se), has been found to be effective in improving growth and inducing tolerance against excessive soil salinity. However, the in-depth mechanisms of Se-induced salinity tolerance are still unclear. This review refines the knowledge involved in Se-mediated improvements of plant growth when subjected to salinity and suggests future perspectives as well as several research limitations in this field.

Genetic and Physiological Dissection of Photosynthesis in Barley Exposed to Drought Stress

Balanced photosynthesis under drought is essential for better survival and for agricultural benefits in terms of biomass and yield. Given the current attempts to improve the photosynthetic efficiency for greater crop yield, the explanation of the genetic basis of that process, together with the phenotypic analysis, is significant in terms of both basic studies and potential agricultural application. Therefore, the main objective of this study was to uncover the molecular basis of the photosynthesis process under drought stress in barley. To address that goal, we conducted transcriptomic examination together with detailed photosynthesis analysis using the JIP-test. Using this approach, we indicated that photosynthesis is a process that is very early affected in barley seedlings treated with severe drought stress. Rather than focusing on individual genes, our strategy was pointed to the identification of groups of genes with similar expression patterns. As such, we identified and annotated almost 150 barley genes as crucial core-components of photosystems, electron transport components, and Calvin cycle enzymes. Moreover, we designated 17 possible regulatory interactions between photosynthesis-related genes and transcription factors in barley. Summarizing, our results provide a list of candidate genes for future genetic research and improvement of barley drought tolerance by targeting photosynthesis.

Melatonin Positively Influences the Photosynthetic Machinery and Antioxidant System of Avena sativa during Salinity Stress

Recent studies have demonstrated melatonin protects various crops against abiotic stresses. However, the effects of melatonin on the photosynthetic apparatus of stressed plants is poorly characterized. We investigated the effects of melatonin pretreatment on photosynthesis and tolerance to salinity stress in Avena sativa (oat) plants. Oat plants were exposed to four treatments (three replicate pots per treatment): well-watered (WW; control); watered with 300 mM salt solution for 10 days (NaCl); pretreated with 100 µM melatonin solution for 7 days then watered normally for 10 days (Mel+W); or pretreated with 100 µM melatonin for 7 days then 300 mM salt for 10 days (Mel+NaCl). Considerable differences in growth parameters, chlorophyll content, stomatal conductance, proline accumulation, lipid peroxidation, electrolyte leakage, and growth parameters were observed between groups. Genes encoding three major antioxidant enzymes were upregulated in the Mel+NaCl group compared to the other groups. Chlorophyll-a fluorescence kinetic analyses revealed that almost all photosynthetic parameters were improved in Mel+NaCl plants compared to the other treatments. Analysis of genes encoding the major extrinsic proteins of photosystem II (PSII) revealed that PsbA, PsbB, PsbC, and PsbD (but not PsbO) were highly upregulated in Mel+NaCl plants compared to the other groups, indicating melatonin positively influenced photosynthesis under control conditions and salt stress. In addition, melatonin upregulated stress-responsive NAC transcription factor genes in plants exposed to salt stress. These findings suggest melatonin pretreatment improves photosynthesis and enhances salt tolerance in oat plants.

Overexpression of Banana ATG8f Modulates Drought Stress Resistance in Arabidopsis

Autophagy is essential for plant growth, development, and stress resistance. However, the involvement of banana autophagy-related genes in drought stress response and the underlying mechanism remain elusive. In this study, we found that the transcripts of 10 banana ATG8s responded to drought stress in different ways, and MaATG8f with the highest transcript in response to drought stress among them was chosen for functional analysis. Overexpression of MaATG8f improved drought stress resistance in Arabidopsis, with lower malonaldehyde level and higher level of assimilation rate. On the one hand, overexpression of MaATG8f activated the activities of superoxide dismutase, catalase, and peroxidase under drought stress conditions, so as to regulate reactive oxygen species accumulation. On the other hand, MaATG8f-overexpressing lines exhibited higher endogenous abscisic acid (ABA) level and more sensitivity to abscisic acid. Notably, the autophagosomes as visualized by CaMV35S::GFP–MaATG8f was activated after ABA treatment. Taken together, overexpression of MaATG8f positively regulated plant drought stress resistance through modulating reactive oxygen species metabolism, abscisic acid biosynthesis, and autophagic activity.

Transcriptome profiling analysis of the seagrass, Zostera muelleri under copper stress

Copper (Cu) in an essential trace metal but it can also contaminate coastal waters at high concentrations mainly from agricultural run-off and mining activities which are detrimental to marine organisms including seagrasses. The molecular mechanisms driving Cu toxicity in seagrasses are not clearly understood yet. Here, we investigated the molecular responses of the Australian seagrass, Z. muelleri at the whole transcriptomic level after 7 days of exposure to 250 μg Cu L^-1 and 500 μg Cu L^-1. The leaf-specific whole transcriptome results showed a concentration-dependent disturbance in chloroplast function, regulatory stress responses and defense mechanisms. This study provided new insights into the responses of seagrasses to trace metal stress and reports possible candidate genes which can be considered as biomarkers to improve conservation and management of seagrass meadows.

Protein Carbonylation As a Biomarker of Heavy Metal, Cd and Pb, Damage in Paspalum fasciculatum Willd. ex Flüggé

Heavy metal tolerant plants have phytoremediation potential for the recovery of contaminated soils, and the characterization of their metabolic adaptation processes is an important starting point to elucidate their tolerance mechanisms at molecular, biochemical and physiological levels. In this research, the effects of Cd and Pb on growth and protein carbonylation in tissues of Paspalum fasciculatum exposed to 30 and 50 mg·Kg⁻¹ Cd and Pb respectively were determined. P. fasciculatum seedlings exposed to metals grew more than controls until 60 days of cultivation and limited their oxidative effects to a reduced protein group. Carbonyl indexes in leaf and root proteins reached a significant increase concerning their controls in plants exposed 30 days to Cd and 60 days to Pb. From the combined approach of Western Blot with Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE) and protein analysis by Matrix Asisted Laser Desorption/Ionisation-Time Of Flight (MALDI-TOF/TOF) mass spectrometry, chloroplastic proteins were identified into the main oxidative stress-inducible proteins to Cd and Pb, such as subunits α, γ of ATP synthetase, Chlorophyll CP26 binding protein, fructose-bisphosphate aldolase and long-chain ribulose bisphosphate carboxylase (RuBisCO LSU). Cd generated damage in the photosynthetic machinery of the leaves of P. fasciculatum into the first 30 days of treatment; five of the oxidized proteins are involved in photosynthesis processes. Moreover, there was a proteolytic fragmentation of the RuBisCO LSU. Results showed that intrinsic tolerance of P. fasciculatum to these metals reached 60 days in our conditions, along with the bioaccumulating appreciable quantities of metals in their roots.