Physiological Responses and Expression Profile of NADPH Oxidase in Rice (Oryza Sativa) Seedlings under Different Levels of Submergence

Psammophytes Alyssum desertorum Stapf and Secale sylvestre Host Are Sensitive to Soil Flooding

While morphological and functional traits enable hydrophytes to survive under waterlogging and partial or complete submergence, the data on responses of psammophytes-sand plants-to flooding are very limited. We analyzed the effect of 5- and 10-day soil flooding on the photosynthetic apparatus and the synthesis of alcohol dehydrogenase (ADH), heat shock proteins 70 (HSP70), and ethylene in seedlings of psammophytes Alyssum desertorum and Secale sylvestre using electron microscopy, chlorophyll a fluorescence induction, and biochemical methods. It was found that seedlings growing under soil flooding differed from those growing in stationary conditions with such traits as chloroplast ultrastructure, pigment content, chlorophyll fluorescence induction, and the dynamics of ADH, HSP, and ethylene synthesis. Although flooding caused no apparent damage to the photosynthetic apparatus in all the variants, a significant decrease in total photosynthesis efficiency was observed in both studied plants, as indicated by decreased values of φR0 and PIABS,total. More noticeable upregulation of ADH in S. sylvestre, as well as increasing HSP70 level and more intensive ethylene emission in A. desertorum, indicate species-specific differences in these traits in response to short-term soil flooding. Meanwhile, the absence of systemic anaerobic metabolic adaptation to prolonged hypoxia causes plant death.

Phosphorylation of Thr-225 and Ser-262 on ERD7 Promotes Age-Dependent and Stress-Induced Leaf Senescence through the Regulation of Hydrogen Peroxide Accumulation in Arabidopsis thaliana

As the final stage of leaf development, leaf senescence is affected by a variety of internal and external signals including age and environmental stresses. Although significant progress has been made in elucidating the mechanisms of age-dependent leaf senescence, it is not clear how stress conditions induce a similar process. Here, we report the roles of a stress-responsive and senescence-induced gene, ERD7 (EARLY RESPONSIVE TO DEHYDRATION 7), in regulating both age-dependent and stress-induced leaf senescence in Arabidopsis. The results showed that the leaves of erd7 mutant exhibited a significant delay in both age-dependent and stress-induced senescence, while transgenic plants overexpressing the gene exhibited an obvious accelerated leaf senescence. Furthermore, based on the results of LC-MS/MS and PRM quantitative analyses, we selected two phosphorylation sites, Thr-225 and Ser-262, which have a higher abundance during senescence, and demonstrated that they play a key role in the function of ERD7 in regulating senescence. Transgenic plants overexpressing the phospho-mimetic mutant of the activation segment residues ERD7T225D and ERD7T262D exhibited a significantly early senescence, while the inactivation segment ERD7T225A and ERD7T262A displayed a delayed senescence. Moreover, we found that ERD7 regulates ROS accumulation by enhancing the expression of AtrbohD and AtrbohF, which is dependent on the critical residues, i.e., Thr-225 and Ser-262. Our findings suggest that ERD7 is a positive regulator of senescence, which might function as a crosstalk hub between age-dependent and stress-induced leaf senescence.

Identification of NADPH Oxidase Genes Crucial for Rice Multiple Disease Resistance and Yield Traits

Reactive oxygen species (ROS) act as a group of signaling molecules in rice functioning in regulation of development and stress responses. Respiratory burst oxidase homologues (Rbohs) are key enzymes in generation of ROS. However, the role of the nine Rboh family members was not fully understood in rice multiple disease resistance and yield traits. In this study, we constructed mutants of each Rboh genes and detected their requirement in rice multiple disease resistance and yield traits. Our results revealed that mutations of five Rboh genes (RbohA, RbohB, RbohE, RbohH, and RbohI) lead to compromised rice blast disease resistance in a disease nursery and lab conditions; mutations of five Rbohs (RbohA, RbohB, RbohC, RbohE, and RbohH) result in suppressed rice sheath blight resistance in a disease nursery and lab conditions; mutations of six Rbohs (RbohA, RbohB, RbohC, RbohE, RbohH and RbohI) lead to decreased rice leaf blight resistance in a paddy yard and ROS production induced by PAMPs and pathogen. Moreover, all Rboh genes participate in the regulation of rice yield traits, for all rboh mutants display one or more compromised yield traits, such as panicle number, grain number per panicle, seed setting rate, and grain weight, resulting in reduced yield per plant except rbohb and rbohf. Our results identified the Rboh family members involved in the regulation of rice resistance against multiple pathogens that caused the most serious diseases worldwide and provide theoretical supporting for breeding application of these Rbohs to coordinate rice disease resistance and yield traits.

Plant Adaptation to Flooding Stress under Changing Climate Conditions: Ongoing Breakthroughs and Future Challenges

Climate-change-induced variations in temperature and rainfall patterns are a serious threat across the globe. Flooding is the foremost challenge to agricultural productivity, and it is believed to become more intense under a changing climate. Flooding is a serious form of stress that significantly reduces crop yields, and future climatic anomalies are predicted to make the problem even worse in many areas of the world. To cope with the prevailing flooding stress, plants have developed different morphological and anatomical adaptations in their roots, aerenchyma cells, and leaves. Therefore, researchers are paying more attention to identifying developed and adopted molecular-based plant mechanisms with the objective of obtaining flooding-resistant cultivars. In this review, we discuss the various physiological, anatomical, and morphological adaptations (aerenchyma cells, ROL barriers (redial O2 loss), and adventitious roots) and the phytohormonal regulation in plants under flooding stress. This review comprises ongoing innovations and strategies to mitigate flooding stress, and it also provides new insights into how this knowledge can be used to improve productivity in the scenario of a rapidly changing climate and increasing flood intensity.

Physiological and transcriptomic analysis of the mangrove species Kandelia obovata in response to flooding stress

Flooding stress on mangroves is growing continually with rising sea level. In this study, the physiology and transcriptome of the mangrove species Kandelia obovata under flooding stress were analyzed. With increasing inundation time, malondialdehyde (MDA), superoxide dismutase (SOD), glutathione (GSH), soluble sugar (SS), soluble protein (SP), and proline (Pro) content declined, while peroxidase (POD) and ascorbate peroxidase (APX) activity rose significantly. According to the KEGG pathway enrichment analysis, upregulated differentially expressed genes (DEGs) were enriched in the plant hormone signaling pathway. Furthermore, MYB44 and MYB108 genes from the MYB transcription factor family and RAP2.12, DREB2B, and ERF4 genes from the AP2/ERF family were up-regulated under flooding conditions. A strong correlation was established between the expression levels of 12 DEGs under flooding stress and RNA sequencing data and was verified by qRT-PCR. These results provide new insights into the molecular mechanism of K. obovata in response to flooding stress.

Physiological and transcriptional regulation in Taxodium hybrid 'Zhongshanshan' leaves in acclimation to prolonged partial submergence

MAIN CONCLUSION

Taxodium 703 leaves activate fermentation, amino acids metabolism and ROS detoxification, and reduce TCA cycle and ABA biosynthesis in acclimation to prolonged partial submergence stress. Taxodium hybrid 'Zhongshanshan 703' (T. mucronatum × T. distichum; Taxodium 703) is a highly flooding-tolerant woody plant. To investigate the physiological and transcriptional regulatory mechanisms underlying its leaves in acclimation to long-term flooding, we exposed cuttings of Taxodium 703 to either non-flooding (control) or partial submergence for 2 months. The leaf tissues above (AL) and below (BL) flooding-water were separately harvested. Partial submergence decreased concentrations of chlorophyll (a + b) and dehydroascorbate (DHA) and lactate dehydrogenase (LDH) activity in AL, and reduced biomass, concentrations of succinic acid, fumaric acid and malic acid, and transcript levels of genes involved in tricarboxylic acid (TCA) cycle in BL. Under partial submergence, concentrations of starch, malondialdehyde and abscisic acid (ABA) decreased, and also mRNA levels of nine-cis-epoxycarotenoid dioxygenases that are involved in ABA biosynthesis in AL and BL of Taxodium 703. Partial submergence increased O2- content in AL, and improved concentrations of pyruvate and soluble sugars and activities of LDH and peroxidase in BL. In addition, partial submergence increased concentrations of ethanol, lactate, alanine, γ-aminobutyric acid, total amino acids and ascorbic acid (ASA), and ASA/DHA, activities of alcohol dehydrogenases (ADH) and ascorbate peroxidase, as well as transcript levels of ADH1A, ADH1B and genes involved in alanine biosynthesis and starch degradation in AL and BL of Taxodium 703. Overall, these results suggest that Taxodium 703 leaves activate fermentation, amino acids metabolism and reactive oxygen species detoxification, and maintain a steady supply of sugars, and reduce TCA cycle and ABA biosynthesis in acclimation to prolonged partial submergence stress.

Physiological responses and Ethylene-Response AP2/ERF Factor expression in Indica rice seedlings subjected to submergence and osmotic stress

BACKGROUND

The increased frequency of heavy rains in recent years has led to submergence stress in rice paddies, severely affecting rice production. Submergence causes not only hypoxic stress from excess water in the surrounding environment but also osmotic stress in plant cells. We assessed physiological responses and Ethylene-Response AP2/ERF Factor regulation under submergence conditions alone and with ionic or nonionic osmotic stress in submergence-sensitive IR64 and submergence-tolerant IR64-Sub1 Indica rice cultivars.

RESULTS

Our results indicate that both IR64 and IR64-Sub1 exhibited shorter plant heights and root lengths under submergence with nonionic osmotic stress than normal condition and submergence alone. IR64-Sub1 seedlings exhibited a significantly lower plant height under submergence conditions alone and with ionic or nonionic osmotic stress than IR64 cultivars. IR64-Sub1 seedlings also presented lower malondialdehyde (MDA) concentration and higher survival rates than did IR64 seedlings after submergence with ionic or nonionic osmotic stress treatment. Sub1A-1 affects reactive oxygen species (ROS) accumulation and antioxidant enzyme activity in rice. The results also show that hypoxia-inducible ethylene response factors (ERF)-VII group and alcohol dehydrogenase 1 (ADH1) and lactate dehydrogenase 1 (LDH1) genes exhibited different expression levels under nonionic or ionic osmotic stress during submergence on rice.

CONCLUSIONS

Together, these results demonstrate that complex regulatory mechanisms are involved in responses to the aforementioned forms of stress and offer new insights into the effects of submergence and osmotic stress on rice.

Changes in terpene biosynthesis and submergence tolerance in cotton

BACKGROUND

Flooding is among the most severe abiotic stresses in plant growth and development. The mechanism of submergence tolerance of cotton in response to submergence stress is unknown.

RESULTS

The transcriptome results showed that a total of 6,893 differentially expressed genes (DEGs) were discovered under submergence stress. Gene Ontology (GO) enrichment analysis showed that DEGs were involved in various stress or stimulus responses. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that DEGs related to plant hormone signal transduction, starch and sucrose metabolism, glycolysis and the biosynthesis of secondary metabolites were regulated by submergence stress. Eight DEGs related to ethylene signaling and 3 ethylene synthesis genes were identified in the hormone signal transduction. For respiratory metabolism, alcohol dehydrogenase (ADH, GH_A02G0728) and pyruvate decarboxylase (PDC, GH_D09G1778) were significantly upregulated but 6-phosphofructokinase (PFK, GH_D05G0280), phosphoglycerate kinase (PGK, GH_A01G0945 and GH_D01G0967) and sucrose synthase genes (SUS, GH_A06G0873 and GH_D06G0851) were significantly downregulated in the submergence treatment. Terpene biosynthetic pathway-related genes in the secondary metabolites were regulated in submergence stress.

CONCLUSIONS

Regulation of terpene biosynthesis by respiratory metabolism may play a role in enhancing the tolerance of cotton to submergence under flooding. Our findings showed that the mevalonate pathway, which occurs in the cytoplasm of the terpenoid backbone biosynthesis pathway (ko00900), may be the main response to submergence stress.

Transcriptome Analysis Reveals Genes Associated with Flooding Tolerance in Mulberry Plants

Mulberry (Morus alba), a widely distributed economic plant, can withstand long-term flooding stress. However, the regulatory gene network underlying this tolerance is unknown. In the present study, mulberry plants were subjected to submergence stress. Subsequently, mulberry leaves were collected to perform quantitative reverse-transcription PCR (qRT-PCR) and transcriptome analysis. Genes encoding ascorbate peroxidase and glutathione S-transferase were significantly upregulated after submergence stress, indicating that they could protect the mulberry plant from flood damage by mediating ROS homeostasis. Genes that regulate starch and sucrose metabolism; genes encoding pyruvate kinase, alcohol dehydrogenase, and pyruvate decarboxylase (enzymes involved in glycolysis and ethanol fermentation); and genes encoding malate dehydrogenase and ATPase (enzymes involved in the TCA cycle) were also obviously upregulated. Hence, these genes likely played a key role in mitigating energy shortage during flooding stress. In addition, genes associated with ethylene, cytokinin, abscisic acid, and MAPK signaling; genes involved in phenylpropanoid biosynthesis; and transcription factor genes also showed upregulation under flooding stress in mulberry plants. These results provide further insights into the adaptation mechanisms and genetics of submergence tolerance in mulberry plants and could aid in the molecular breeding of these plants.

Phytoglobin-NO cycle and AOX pathway play a role in anaerobic germination and growth of deepwater rice

An important and interesting feature of rice is that it can germinate under anoxic conditions. Though several biochemical adaptive mechanisms play an important role in the anaerobic germination of rice but the role of phytoglobin-nitric oxide cycle and alternative oxidase pathway is not known, therefore in this study we investigated the role of these pathways in anaerobic germination. Under anoxic conditions, deepwater rice germinated much higher and rapidly than aerobic condition and the anaerobic germination and growth were much higher in the presence of nitrite. The addition of nitrite stimulated NR activity and NO production. Important components of phytoglobin-NO cycle such as methaemoglobin reductase activity, expression of Phytoglobin1, NIA1 were elevated under anaerobic conditions in the presence of nitrite. The operation of phytoglobin-NO cycle also enhanced anaerobic ATP generation, LDH, ADH activities and in parallel ethylene levels were also enhanced. Interestingly nitrite suppressed the ROS production and lipid peroxidation. The reduction of ROS was accompanied by enhanced expression of mitochondrial alternative oxidase protein and its capacity. Application of AOX inhibitor SHAM inhibited the anoxic growth mediated by nitrite. In addition, nitrite improved the submergence tolerance of seedlings. Our study revealed that nitrite driven phytoglobin-NO cycle and AOX are crucial players in anaerobic germination and growth of deepwater rice.

Tolerance to excess moisture in soybean is enhanced by over-expression of the Glycine max Phytoglobin (GmPgb1)

Excess moisture in the form of waterlogging or full submergence can cause severe conditions of hypoxia or anoxia compromising several physiological and biochemical processes. A decline in photosynthetic rate due to accumulation of ROS and damage of leaf tissue are the main consequences of excess moisture. These effects compromise crop yield and quality, especially in sensitive species, such as soybean (Glycine max.). Phytoglobins (Pgbs) are expressed during hypoxia and through their ability to scavenge nitric oxide participate in several stress-related responses. Soybean plants over-expressing or suppressing the Pgb1 gene GmPgb1 were generated and their ability to cope with waterlogging and full submergence conditions was assessed. Plants over-expressing GmPgb1 exhibited a higher retention of photosynthetic rate during waterlogging and survival rate during submergence relative to wild type plants. The same plants also had lower levels of ROS due to a reduction in expression of Respiratory Burst Oxidase Homologs (RBOH), components of the NADPH oxidase enzyme, and enhanced antioxidant system characterized by higher expression of catalases (CAT) and superoxide dismutase (SOD), as well as elevated expression and activity of ascorbate peroxidase (APX). Plants over-expressing GmPgb1 also exhibited an expression pattern of aquaporins typical of excess moisture resilience. This was in contrast to plants downregulating GmPgb1 which were characterized by the lowest photosynthetic rates, higher ROS signal, and reduced expression and activities of many antioxidant enzymes. Results from these studies suggest that GmPgb1 exercises a protective role during conditions of excess moisture with similar mechanisms operating during waterlogging and submergence.

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

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

RICE ACYL-COA-BINDING PROTEIN6 Affects Acyl-CoA Homeostasis and Growth in Rice

BACKGROUNDS

Acyl-coenzyme A (CoA) esters are important intermediates in lipid metabolism with regulatory properties. Acyl-CoA-binding proteins bind and transport acyl-CoAs to fulfill these functions. RICE ACYL-COA-BINDING PROTEIN6 (OsACBP6) is currently the only one peroxisome-localized plant ACBP that has been proposed to be involved in β-oxidation in transgenic Arabidopsis. The role of the peroxisomal ACBP (OsACBP6) in rice (Oryza sativa) was investigated.

RESULTS

Here, we report on the function of OsACBP6 in rice. The osacbp6 mutant showed diminished growth with reduction in root meristem activity and leaf growth. Acyl-CoA profiling and lipidomic analysis revealed an increase in acyl-CoA content and a slight triacylglycerol accumulation caused by the loss of OsACBP6. Comparative transcriptomic analysis discerned the biological processes arising from the loss of OsACBP6. Reduced response to oxidative stress was represented by a decline in gene expression of a group of peroxidases and peroxidase activities. An elevation in hydrogen peroxide was observed in both roots and shoots/leaves of osacbp6. Taken together, loss of OsACBP6 not only resulted in a disruption of the acyl-CoA homeostasis but also peroxidase-dependent reactive oxygen species (ROS) homeostasis. In contrast, osacbp6-complemented transgenic rice displayed similar phenotype to the wild type rice, supporting a role for OsACBP6 in the maintenance of the acyl-CoA pool and ROS homeostasis. Furthermore, quantification of plant hormones supported the findings observed in the transcriptome and an increase in jasmonic acid level occurred in osacbp6.

CONCLUSIONS

In summary, OsACBP6 appears to be required for the efficient utilization of acyl-CoAs. Disruption of OsACBP6 compromises growth and led to provoked defense response, suggesting a correlation of enhanced acyl-CoAs content with defense responses.

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

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

Chlorophyll Metabolism and Gene Expression in Response to Submergence Stress and Subsequent Recovery in Perennial Ryegrass Accessions Differing in Growth Habits

Submergence-induced leaf senescence may alter chlorophyll metabolism. The objective of this study was to characterize chlorophyll biosynthesis and degradation in contrasting perennial ryegrass (Lolium perenne) in response to submergence stress and recovery. The light-green and fast-growing accession PI238938 and the darker-green and slow-growing cultivar BrightStar SLT were exposed to 0, 6 h, 1-, 3-, and 7-d of submergence stress and 1- and 5- d of de-submergence, respectively. Plant growth of PI238938 were more severely inhibited by submergence stress and recovery. Both accessions showed increased leaf malondialdehyde under stress and recovery, but reduced chlorophyll (Chl) concentrations were observed at 3- and 7-d of stress and at recovery. The reduction in Chl was more severe in BrightStar SLT at 7 d of stress. The concentration of 5-aminolevulenic acid was unaffected by stress but increased at 1d of recovery. Activities of 5-aminolevulinic acid dehydratase (ALAD) involved in Chl biosynthesis remained unchanged under stress and recovery, while the activities of Chl degrading enzymes chlorophyllase (CHL) and pheophytinase (PPH) increased at 3 d or 7 d of stress, and returned to the control level after recovery in both accessions. The downregulation of Chl-biosynthetic genes CHLI, POR, and CHLP and the upregulation of Chl-degrading genes CLH, PPH, and SGR were observed in both accessions under most of the stress periods. BrightStar SLT exhibited much lower expressions of the Chl-biosynthetic genes PBGD, CHS, and CHID under stress, while PI238938 had remarkably higher expressions of genes involved in Chl breakdown including CLH, PPH, PAO, RCCR, and SGR, and the expressions of these genes remained at a higher level at 1 d of recovery. The results indicated that submergence-induced leaf senescence and declines in Chl were associated with downregulation of more Chl-biosynthetic genes in slow-growing genotype and upregulation of more Chl-degrading genes in fast-growing genotype of perennial ryegrass.

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

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

Mechanisms of Waterlogging Tolerance in Plants: Research Progress and Prospects

Waterlogging is one of the main abiotic stresses suffered by plants. Inhibition of aerobic respiration during waterlogging limits energy metabolism and restricts growth and a wide range of developmental processes, from seed germination to vegetative growth and further reproductive growth. Plants respond to waterlogging stress by regulating their morphological structure, energy metabolism, endogenous hormone biosynthesis, and signaling processes. In this updated review, we systematically summarize the changes in morphological structure, photosynthesis, respiration, reactive oxygen species damage, plant hormone synthesis, and signaling cascades after plants were subjected to waterlogging stress. Finally, we propose future challenges and research directions in this field.

Transcriptome, physiological and biochemical analysis of Triarrhena sacchariflora in response to flooding stress

Background

In recent decades, the frequency of flooding is increasing with the change of global climate. Flooding has become one of the major abiotic stresses that seriously affect growth and development of plants. Triarrhena sacchariflora Nakai has been considered a promising energy crop for utilization in ethanol production. Flooding stress is among the most severe abiotic stressors in the production of Nakai. However, the physiological and molecular biological mechanisms of Nakai response to flooding is still unclear. In the present study, in order to understand the molecular mechanisms of Nakai in response to flooding stress, the transcriptome, physiological and biochemical were investigated.

Results

The results demonstrated that significant physiological changes were observed in photosynthetic system, antioxidative enzyme activity, chlorophyll, carotenoid, proline, lipid peroxidation and soluble sugar content under normal and flooding treatments. Such as, the chlorophyll, carotenoid contents and photosynthetic system were significantly decreased. Whereas, the antioxidative enzyme activity, proline, lipid peroxidation and soluble sugar has increased first and then decreased under treatments compared with the normal plants. Additionally, a total of 8832, 6608 and 3649 unigenes were validated to be differentially expressed under different treatments, respectively. Besides, gene ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of the different expression levels of genes also presented processes, which involved in photosynthesis, sucrose catabolism, glycolysis, stress response and defense, phytohormone biosynthesis and signal transduction.

Conclusions

The results provide a comprehensive view of the complex molecular events involved in the response to flooding stress of Nakai leaves, which also will promote the research in the development of flood-resistant crops and provide new tools for Nakai breeders.

GABA-Alleviated Oxidative Injury Induced by Salinity, Osmotic Stress and their Combination by Regulating Cellular and Molecular Signals in Rice

This study was conducted in order to determine the effect of priming with γ-aminobutyric acid (GABA) at 0.5 mM on rice (Oryza sativa L.) seed germination under osmotic stress (OS) induced by polyethylene glycol (30 g/L PEG 6000); and salinity stress (S, 150 mM NaCl) and their combination (OS+S). Priming with GABA significantly alleviated the detrimental effects of OS, S and OS+S on seed germination and seedling growth. The photosynthetic system and water relation parameters were improved by GABA under stress. Priming treatment significantly increased the GABA content, sugars, protein, starch and glutathione reductase. GABA priming significantly reduced Na⁺ concentrations, proline, free radical and malonaldehyde and also significantly increased K⁺ concentration under the stress condition. Additionally, the activities of antioxidant enzymes, phenolic metabolism-related enzymes, detoxification-related enzymes and their transcription levels were improved by GABA priming under stress. In the GABA primed-plants, salinity stress alone resulted in an obvious increase in the expression level of Calcineurin B-like Protein-interacting protein Kinases (CIPKs) genes such as OsCIPK01, OsCIPK03, OsCIPK08 and OsCIPK15, and osmotic stress alone resulted in obvious increase in the expression of OsCIPK02, OsCIPK07 and OsCIPK09; and OS+S resulted in a significant up-regulation of OsCIPK12 and OsCIPK17. The results showed that salinity, osmotic stresses and their combination induced changes in cell ultra-morphology and cell cycle progression resulting in prolonged cell cycle development duration and inhibitory effects on rice seedlings growth. Hence, our findings suggested that the high tolerance to OS+S is closely associated with the capability of GABA priming to control the reactive oxygen species (ROS) level by inducing antioxidant enzymes, secondary metabolism and their transcription level. This knowledge provides new evidence for better understanding molecular mechanisms of GABA-regulating salinity and osmotic-combined stress tolerance during rice seed germination and development.

Ethylene-mediated signaling confers thermotolerance and regulates transcript levels of heat shock factors in rice seedlings under heat stress

BACKGROUND

Agriculture is highly dependent on climate. Increases in temperature caused by global warming pose challenges for crop production. Heat stress induces oxidative damage to cell membranes and then causes cell death. Plants have developed various responses to elevated temperatures, including hormone signaling pathways and heat shock factors that elevate their thermotolerance. In response to heat stress, the gaseous hormone ethylene is produced through regulation of the expression of signaling-related genes to modulate resource allocation dynamics. For comprehensive understanding of the role of ethylene, this study used an ethylene precursor to analyze the ethylene signaling pathway involved in adjustment of the homeostasis of the antioxidant system and to evaluate heat shock factor expression in rice seedlings under heat stress.

RESULTS

Levels of cell membrane oxidation and ion leakage were reduced in rice seedlings under heat treatment combined with ethylene precursor treatment, conferring enhanced thermotolerance. Reduction of the fresh weight and chlorophyll a/b ratio in rice seedlings was lower in rice seedlings under heat stress with ethylene precursor treatment than in those under heat stress only. Moreover, reduction of antioxidant response caused by heat stress was ameliorated by treatment with ethylene precursors such as catalase and total peroxidase. Quantitative reverse transcriptase-polymerase chain reaction showed higher expression levels of heat shock factors such as HSFA1a and HSFA2a, c, d, e, and f and ethylene-signaling-related genes such as ethylene insensitive 2, ethylene insensitive-like 1, and ethylene insensitive-like 2 in rice seedlings under heat stress with ethylene precursor treatment than in rice seedlings under heat stress only.

CONCLUSION

Ethylene-mediated signaling was involved in the reduction of oxidative damage, maintenance of chlorophyll content, and enhancement of thermotolerance in rice seedlings under heat stress. Furthermore, this study revealed heat shock factors and ethylene-signaling-related genes involved in complex network regulation that confers thermotolerance to rice seedlings.

Growth responses of eight wetland species to water level fluctuation with different ranges and frequencies

Fluctuation range and frequency are two important components of water level fluctuation, but their effects on wetland plants have not been evaluated separately. We subjected eight wetland species to a control treatment with static water level and fluctuation treatments with different ranges or frequencies to examine their effects on plant growth. Acorus calamus, Butomus umbellatus and Iris wilsonii showed high survival rates in all treatments with various fluctuation ranges and frequencies. Their survival rates were higher at the medium fluctuation frequency than at the low and high frequencies, suggesting beneficial effects of the medium frequency. In the experiment comparing the fluctuation ranges, A. calamus and I. wilsonii could maintain the capacity for asexual propagation and accumulate higher biomass compared with the control plants, while biomass of the other six species dramatically decreased. In the experiment comparing fluctuation frequency, species with relatively high survival rates (≥ 50%) maintained or increased the capacity of asexual propagation, and A. calamus and I. wilsonii allocated relatively more biomass to roots, which may enhance plant growth and survival. In contrast, these species did not show increased biomass allocation to shoots in response to both fluctuation range and frequency, presumably because shoots are prone to mechanical damage caused by streaming floodwater. Taken together, biomass accumulation in roots rather than in shoots and the ability to asexually propagate are important for the survival of these species during water fluctuation.

Ethylene signaling involves in seeds germination upon submergence and antioxidant response elicited confers submergence tolerance to rice seedlings

BACKGROUND

Flooding has negative impact on agriculture. The plant hormone ethylene is involved in plant growth and stress responses, which are important role in tolerance and adaptation regulatory mechanisms during submergence stress. Ethylene signaling crosstalk with gibberellin signaling enhances tolerance in lowland rice (Flood Resistant 13A) through a quiescence strategy or in deepwater rice through an escape strategy when rice is submerged. Information regarding ethylene-mediated priming in submergence stress tolerance in rice is scant. Here, we used 1-aminocyclopropane-1-carboxylic acid, an ethylene precursor, to evaluate the response in submerged rice seedlings.

RESULTS

The germination rate and mean germination times of rice seeds was higher in seedlings under submergence only when ethylene signaling was inhibited by supplemented with silver nitrate (AgNO₃). Reduced leaf chlorophyll contents and induced senescence-associated genes in rice seedlings under submergence were relieved by pretreatment with an ethylene precursor. The ethylene-mediated priming by pretreatment with an ethylene precursor enhanced the survival rate and hydrogen peroxide (H₂O₂) and superoxide (O₂^-) anion accumulation and affected antioxidant response in rice seedlings.

CONCLUSIONS

Pretreatment with an ethylene precursor leads to reactive oxygen species generation, which in turn triggered the antioxidant response system, thus improving the tolerance of rice seedlings to complete submergence stress. Thus, H₂O₂ signaling may contribute to ethylene-mediated priming to submergence stress tolerance in rice seedlings.

Ethylene signaling modulates contents of catechin and ability of antioxidant in Camellia sinensis

BACKGROUND

Tea is one of the most popular beverages in the world. There are many secondary metabolites can be found in tea such as anthocyanins, proanthocyanidins, flavonols and catechins. These secondary metabolites in plants are proved to act protective components for human health effect. Plant hormone ethylene is considered to have an important role in regulation of plant development and signal transduction. This study evaluated the effect of ethylene signaling regulation in phenolic compounds in tea plants. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) enhanced contents of total catechin in treated oolong tea seedlings.

RESULTS

The degree of epigallocatechin and epicatechin galloylation was increased after ACC treatment in oolong tea seedlings by high performance liquid chromatography determination. The contents of anthocyanins, flavonoids, and total polyphenol were higher after ACC treatment in comparison with control. Antioxidant enzyme such as catalase, superoxide dismutase, and total peroxidase decreased their antioxidant activities after ACC treatment, yet the activity of ascorbate peroxidase is increased. The ability of oxygen radical absorption and 2,2-diphenyl-1-picrylhydrazyl was used to evaluate the antioxidant activity, which was enhanced by ACC treatment.

CONCLUSIONS

Taken together the results of this study demonstrate that the ethylene signaling is involved in modulation of secondary metabolites accumulation and antioxidant ability that to enhance the benefit of human health in tea products.

Effects of Exogenous Application of Protocatechuic Acid and Vanillic Acid to Chlorophylls, Phenolics and Antioxidant Enzymes of Rice (Oryza sativa L.) in Submergence

In this study, effects from application of protocatechuic acid (PA) and vanillic acid (VA) and their mixture on the submergence tolerance of rice were examined. The treatment of 0.01 mM PA and VA did not show significant increase of rice growth as compared to the controls. However, at higher concentrations (0.1-1.0 mM), rice shoot was elevated in submergence by 20.8-22.4%. The survival percentage of rice seedlings at any dose of PA, VA and their mixture was significantly higher than the controls. In general, the mixture of PA and VA was more active to promote shoot elongation and survival in submergence than sole treatment of either PA or VA. The amount of chlorophyll b by PA was significantly increased, while no change in chlorophyll a content was observed. VA remarkably reduced malondialdehyde quantity at three days of submergence, while no significant difference among treatment was observed in PA, the mixture, and respective controls. The two phenolic acids promoted contents of phenolics and flavonoids in rice leaves and roots, however the quantities of endogenous PA and VA in rice were not markedly differed after PA and VA treated on roots of rice seedlings. The ascorbate peroxidase and superoxide dismutase activities were enhanced, while the expression of genes encoding antioxidant enzymes was favored. VA increased the expression level of ascorbate peroxidase genes in higher levels than PA and their mixture, while no significant difference was observed in the other genes including superoxide dismutase, catalase, glutathione reductase, and peroxidase. Findings of this study showed that PA and VA increased the submergence tolerance of rice by promoting the photosynthetic and anti-oxidative processes in rice seedlings. The treatment of PA and VA mixture on seedling roots was potent to promote the submergence tolerance in rice.

Deep RNAseq indicates protective mechanisms of cold-tolerant indica rice plants during early vegetative stage

Cold-tolerance in rice may be related to increased cellulose deposition in the cell wall, membrane fatty acids unsaturation and differential expression of several newly identified genes. Low temperature exposure during early vegetative stages limits rice plant's growth and development. Most genes previously related to cold tolerance in rice are from the japonica subspecies. To help clarify the mechanisms that regulate cold tolerance in young indica rice plants, comparative transcriptome analysis of 6 h cold-treated (10 °C) leaves from two genotypes, cold-tolerant (CT) and cold-sensitive (CS), was performed. Differentially expressed genes were identified: 831 and 357 sequences more expressed in the tolerant and in the sensitive genotype, respectively. The genes with higher expression in the CT genotype were used in systems biology analyses to identify protein-protein interaction (PPI) networks and nodes (proteins) that are hubs and bottlenecks in the PPI. From the genes more expressed in the tolerant plants, 60% were reported as affected by cold in previous transcriptome experiments and 27% are located within QTLs related to cold tolerance during the vegetative stage. Novel cold-responsive genes were identified. Quantitative RT-PCR confirmed the high-quality of RNAseq libraries. Several genes related to cell wall assembly or reinforcement are cold-induced or constitutively highly expressed in the tolerant genotype. Cold-tolerant plants have increased cellulose deposition under cold. Genes related to lipid metabolism are more expressed in the tolerant genotype, which has higher membrane fatty acids unsaturation, with increasing levels of linoleic acid under cold. The CT genotype seems to have higher photosynthetic efficiency and antioxidant capacity, as well as more effective ethylene, Ca²⁺ and hormone signaling than the CS. These genes could be useful in future biotechnological approaches aiming to increase cold tolerance in rice.

Physiological and molecular responses of seedlings of an upland rice ('Tung Lu 3') to total submergence compared to those of a submergence-tolerant lowland rice ('FR13A')

BACKGROUND

Understanding the responses of rice to environmental stresses such as unscheduled submergence is of pressing important owing to increasing severity of weather thought to arise from global climate change. When rice is completely submerged, different types adopt either a quiescence survival strategy (i.e., minimal shoot elongation) or an escape strategy (i.e., enhanced shoot elongation). Each strategy can prolong survival depending on the circumstances. While submergence responses have been studied in rice typical of lowland and flood-prone areas, few studies have explored the physiological and molecular properties of upland rice under submergence. Here, we use seedlings of the upland rice 'Tung Lu 3' ('TL3') to analyze physiological and molecular responses to submergence. We compare them with those of 'FR13A', a lowland rice that tolerates submergence by adopting the quiescence strategy.

RESULTS

Plant height and distance between leaf sheaths, increased rapidly in 'TL3' under submergence. Although this indicated a strong escape strategy the seedlings remained totally underwater for the duration of the experiments. In contrast, 'FR13A' elongated much less. Consequently, after 4 days complete submergence followed by drainage, 'TL3' lodged much more severely than 'FR13A'. After 10 d complete submergence, 55% of 'TL3' seedlings survived compared to 100% in 'FR13A'. Chlorophyll a, b and total chlorophyll concentrations of the 2nd oldest leaves of 'TL3' were also significantly above those of 'FR13A' (but were lower than 'FR13A' in the 3rd oldest leaves) and less hydrogen peroxide accumulated in 'TL3'. Peroxidase activity in submerged 'TL3' was also greater than in 'FR13A' 1 day after submergence. Quantitative RT-PCR showed increased expression of sucrose synthase 1 and alcohol dehydrogenases 1 after 2 days complete submergence with significantly higher levels in 'TL3' compared to 'FR13A'. Expression was also higher in 'TL3' under non-submerged conditions.

CONCLUSIONS

The upland rice line 'TL3' gave a stronger elongation response than 'FR13A' to complete submergence. This escape strategy is widely considered to prejudice survival when the plant remains totally submerged. However, contrary to expectations, 'TL3' survival rates were substantial although below those for 'FR13A' while physiological, biochemical and molecular parameters linked to adaptation differed in detail but appeared to be broadly comparable. These findings highlight that submergence tolerance is determine not only by the adoption of quiescence or escape strategies but maybe by metabolic and physiological properties unrelated to the underwater elongation rate.

ERF73/HRE1 is involved in H2O2 production via hypoxia-inducible Rboh gene expression in hypoxia signaling

Hypoxia deprives cells of energy and induces severe physical damage in embryophytes. Under hypoxia, the equilibrium between ethylene and H₂O₂ affects the response of the transcription factor AtERF73/HRE1. To evaluate the role of AtERF73/HRE1 during hypoxia signaling, we used three independent AtERF73/HRE1 knockout lines to detect H₂O₂ accumulation. The results revealed that under hypoxia, H₂O₂ accumulation in the AtERF73/HRE1 knockout lines decreased, indicating that AtERF73/HRE1 uses a negative feedback regulation mechanism to influence the production of H₂O₂ induced through hypoxia signal transduction. Quantitative RT-PCR analyses showed that oxygen deficiency had different effects on the expression of the hypoxia-induced genes Rboh B, D, G, and I in the AtERF73/HRE1 knockout lines. In particular, Rboh B and D expression were increased, whereas Rboh G expression was decreased. The expression of Rboh I was increased at 1 h but decreased at 3 h during hypoxia treatment in the AtERF73/HRE1 knockout lines. Similarly, the transcript levels of antioxidant and hypoxia-induced/ethylene response genes in the AtERF73/HRE1 knockout lines were affected by hypoxic stress, indicating that AtERF73/HRE1 is essential to hypoxia signal transduction in embryophytes. Additionally, in histochemical analysis, AtERF73/HRE1 promoter-induced GUS expression was detected in various plant parts throughout the plant growth process (e.g., leaves, inflorescences, siliques), particularly in the edges of mature leaves and guard cells. Taken together, our results confirm that AtERF73/HRE1 plays a role in H₂O₂ production by affecting the hypoxia-induced expression of Rboh genes in hypoxia signal transduction.