Physiology, gene expression, and metabolome of two wheat cultivars with contrasting submergence tolerance

Application of multiomics analysis to plant flooding response

Flooding, as a natural disaster, plays a pivotal role in constraining the growth and development of plants. Flooding stress, including submergence and waterlogging, not only induces oxygen, light, and nutrient deprivation, but also alters soil properties through prolonged inundation, further impeding plant growth and development. However, hypoxia (or anoxia) is the most serious and direct damage to plants caused by flooding. Moreover, flooding disrupts the structural integrity of plant cell walls and compromises endoplasmic reticulum functionality, while hindering nutrient absorption and shifting metabolic processes from normal aerobic respiration to anaerobic respiration. It can be asserted that flooding exerts comprehensive effects on plants encompassing phenotypic changes, transcriptional alterations, protein dynamics, and metabolic shifts. To adapt to flooding environments, plants employ corresponding adaptive mechanisms at the phenotypic level while modulating transcriptomic profiles, proteomic characteristics, and metabolite levels. Hence, this study provides a comprehensive analysis of transcriptomic, proteomic, and metabolomics investigations conducted on flooding stress on model plants and major crops, elucidating their response mechanisms from diverse omics perspectives.

A group VIIIa ethylene-responsive factor, CmERF4, negatively regulates waterlogging tolerance in chrysanthemum

Ethylene-responsive factors (ERF) play an important role in plant responses to waterlogging stress. However, the function and mechanism of action of ERFVIII in response to waterlogging stress remain poorly understood. In this study, we found that expression of the ERF VIIIa gene CmERF4 in chrysanthemum was induced by waterlogging stress. CmERF4 localized to the nucleus when expressed in tobacco leaves. Yeast two-hybrid and luciferase assays showed that CmERF4 is a transcriptional inhibitor. CmERF4 overexpression in chrysanthemum reduced plant waterlogging tolerance, whereas overexpression of the chimeric activator CmERF4-VP64 reversed its transcriptional activity, promoting higher waterlogging tolerance than that observed in wild-type plants, indicating that CmERF4 negatively regulates waterlogging tolerance. Transcriptome profiling showed that energy metabolism and reactive oxygen species (ROS) pathway-associated genes were differentially expressed between CmERF4-VP64 and wild-type plants. RT-qPCR analysis of selected energy metabolism and reactive oxygen species-related genes showed that the gene expression patterns were consistent with the expression levels obtained from RNA-seq analysis. Overall, we identified new functions of CmERF4 in negatively regulating chrysanthemum waterlogging tolerance by modulating energy metabolism and ROS pathway genes.

The mechanisms behind the contrasting responses to waterlogging in black-grass (Alopecurus myosuroides) and wheat (Triticum aestivum)

Black-grass (Alopecurus myosuroides ) is one of the most problematic agricultural weeds of Western Europe, causing significant yield losses in winter wheat (Triticum aestivum ) and other crops through competition for space and resources. Previous studies link black-grass patches to water-retaining soils, yet its specific adaptations to these conditions remain unclear. We designed pot-based waterlogging experiments to compare 13 biotypes of black-grass and six cultivars of wheat. These showed that wheat roots induced aerenchyma when waterlogged whereas aerenchyma-like structures were constitutively present in black-grass. Aerial biomass of waterlogged wheat was smaller, whereas waterlogged black-grass was similar or larger. Variability in waterlogging responses within and between these species was correlated with transcriptomic and metabolomic changes in leaves of control or waterlogged plants. In wheat, transcripts associated with regulation and utilisation of phosphate compounds were upregulated and sugars and amino acids concentrations were increased. Black-grass biotypes showed limited molecular responses to waterlogging. Some black-grass amino acids were decreased and one transcript commonly upregulated was previously identified in screens for genes underpinning metabolism-based resistance to herbicides. Our findings provide insights into the different waterlogging tolerances of these species and may help to explain the previously observed patchiness of this weed's distribution in wheat fields.

Physiological, Epigenetic, and Transcriptome Analyses Provide Insights into the Responses of Wheat Seedling Leaves to Different Water Depths under Flooding Conditions

Flooding stress, including waterlogging and submergence, is one of the major abiotic stresses that seriously affects the growth and development of plants. In the present study, physiological, epigenetic, and transcriptomic analyses were performed in wheat seedling leaves under waterlogging (WL), half submergence (HS), and full submergence (FS) treatments. The results demonstrate that FS increased the leaves' hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents and reduced their chlorophyll contents (SPAD), photosynthetic efficiency (Fv/Fm), and shoot dry weight more than HS and WL. In addition, FS increased catalase (CAT) and peroxidase (POD) activities more than HS and WL. However, there were no significant differences in the contents of H2O2, MDA, SPAD, and Fv/Fm, and the activities of superoxide dismutase (SOD) and POD between the HS and WL treatments. The changes in DNA methylation were related to stress types, increasing under the WL and HS treatments and decreasing under the FS treatment. Additionally, a total of 9996, 10,619, and 24,949 genes were differentially expressed under the WL, HS, and FS treatments, respectively, among which the 'photosynthesis', 'phenylpropanoid biosynthesis', and 'plant hormone signal transduction' pathways were extensively enriched under the three flooding treatments. The genes involved in these pathways showed flooding-type-specific expression. Moreover, flooding-type-specific responses were observed in the three conditions, including the enrichment of specific TFs and response pathways. These results will contribute to a better understanding of the molecular mechanisms underlying the responses of wheat seedling leaves to flooding stress and provide valuable genetic and epigenetic information for breeding flood-tolerant varieties of wheat.

Plant Life with and without Oxygen: A Metabolomics Approach

Oxygen deficiency is an environmental challenge which affects plant growth, the development and distribution in land and aquatic ecosystems, as well as crop yield losses worldwide. The capacity to exist in the conditions of deficiency or the complete lack of oxygen depends on a number of anatomic, developmental and molecular adaptations. The lack of molecular oxygen leads to an inhibition of aerobic respiration, which causes energy starvation and the acceleration of glycolysis passing into fermentations. We focus on systemic metabolic alterations revealed with the different approaches of metabolomics. Oxygen deprivation stimulates the accumulation of glucose, pyruvate and lactate, indicating the acceleration of the sugar metabolism, glycolysis and lactic fermentation, respectively. Among the Krebs-cycle metabolites, only the succinate level increases. Amino acids related to glycolysis, including the phosphoglycerate family (Ser and Gly), shikimate family (Phe, Tyr and Trp) and pyruvate family (Ala, Leu and Val), are greatly elevated. Members of the Asp family (Asn, Lys, Met, Thr and Ile), as well as the Glu family (Glu, Pro, Arg and GABA), accumulate as well. These metabolites are important members of the metabolic signature of oxygen deficiency in plants, linking glycolysis with an altered Krebs cycle and allowing alternative pathways of NAD(P)H reoxidation to avoid the excessive accumulation of toxic fermentation products (lactate, acetaldehyde, ethanol). Reoxygenation induces the downregulation of the levels of major anaerobically induced metabolites, including lactate, succinate and amino acids, especially members of the pyruvate family (Ala, Leu and Val), Tyr and Glu family (GABA and Glu) and Asp family (Asn, Met, Thr and Ile). The metabolic profiles during native and environmental hypoxia are rather similar, consisting in the accumulation of fermentation products, succinate, fumarate and amino acids, particularly Ala, Gly and GABA. The most intriguing fact is that metabolic alterations during oxidative stress are very much similar, with plant response to oxygen deprivation but not to reoxygenation.

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.

Integrated Transcriptomic and Metabolomics Analysis of the Root Responses of Orchardgrass to Submergence Stress

Submergence stress can severely affect plant growth. Orchardgrass (Dactylis glomerata L.) is an important forage grass, and the molecular mechanisms of orchardgrass to submergence stress are not well understood. The roots of the flood-tolerant cultivar "Dian Bei" were harvested at 0 h, 8 h and 24 h of submergence stress. The combined transcriptomic and metabolomic analyses showed that β-alanine metabolism, flavonoid biosynthesis, and biosynthesis of amino acid pathways were significantly enriched at 8 h and 24 h of submergence stress and were more pronounced at 24 h. Most of the flavonoid biosynthesis-related genes were down-regulated for the synthesis of metabolites such as naringenin, apigenin, naringin, neohesperidin, naringenin chalcone, and liquiritigenin in response to submergence stress. Metabolites such as phenylalanine, tyrosine, and tryptophan were up-regulated under stress. The predominant response of flavonoid and amino acids biosynthesis to submergence stress suggests an important role of these pathways in the submergence tolerance of orchardgrass.

Metabolomic and transcriptomic responses of Adiantum (Adiantum nelumboides) leaves under drought, half-waterlogging, and rewater conditions

Introduction: Adiantum nelumboides (Adiantum) is an endangered fern with a narrow distribution along the Yangtze River in China. Due to its cliff-dwelling habit, it experiences water stress conditions, which further endangers its survival. However, no information is available about its molecular responses to drought and half-waterlogging conditions. Methods: Here, we applied five and ten days of half-waterlogging stress, five days of drought stress, and rewatering after five days of drought stress, and studied the resulting metabolome profiles and transcriptome signatures of Adiantum leaves. Results and Discussion: The metabolome profiling detected 864 metabolites. The drought and half-waterlogging stress induced up-accumulation of primary and secondary metabolites including amino acids and derivatives, nucleotides and derivatives, flavonoids, alkaloids, and phenolic acid accumulation in Adiantum leaves. Whereas, rewatering the drought-stressed seedlings reversed most of these metabolic changes. Transcriptome sequencing confirmed the differential metabolite profiles, where the genes enriched in pathways associated with these metabolites showed similar expression patterns. Overall, the half-waterlogging stress for 10 days induced large-scale metabolic and transcriptomic changes compared to half-waterlogging stress for 05 days, drought stress for 05 days or rewatering for 05 days. Conclusion: This pioneering attempt provides a detailed understanding of molecular responses of Adiantum leaves to drought and half-waterlogging stresses and rewater conditions. This study also provides useful clues for the genetic improvement of Adiantum for drought/half-waterlogging stress tolerance.

Evaluation of the water pollution risk of dam and dike-break floods in the inundated area

The inundated area of dam and dike-break floods includes various types of land and factories that release considerable amounts of pollutants into floods, causing serious water pollution and further endangering human health. Many pollution sources and factors affect the water pollution risk in inundated areas. Accurate assessment of the water pollution risk for dam and dike-break floods enables people to take measures in advance to reduce public health problems. The existing evaluation methods cannot effectively analyze the water pollution risk for dam and dike-break floods because partial or all pollution sources and influencing factors are ignored. The main factors affecting flood water quality were summarized into point source (PS), non-point source (NPS), flood depth, velocity, duration, and temperature. The water pollution risk caused by NPSs and PSs were quantified, as well as the impact of all main factors on water pollution risk. The evaluation model proposed for water pollution risk in inundated areas of dam and dike-break floods considers all pollution sources and influencing factors. The WPR was proposed to represent the water pollution risk value. The dam-break flood of Luhun Reservoir was simulated to verify the feasibility of the evaluation model. We concluded that (1) WPR varied with space and time in the inundated area and was seriously affected by PS in local areas; (2) the annual average WPR of different land use types from high to low were construction land, cropland, urban, water, rural area, woodland, and grassland. The evaluation model can be used to evaluate the water pollution risk for dam and dike-break floods at macro and micro scales. People can use this method to evaluate the impact, range, and degree of specific pollution sources or pollutants in the inundated area, thus allowing for measures to be taken in advance to reduce associated damages.

Advances in Plant Metabolomics and Its Applications in Stress and Single-Cell Biology

In the past two decades, the post-genomic era envisaged high-throughput technologies, resulting in more species with available genome sequences. In-depth multi-omics approaches have evolved to integrate cellular processes at various levels into a systems biology knowledge base. Metabolomics plays a crucial role in molecular networking to bridge the gaps between genotypes and phenotypes. However, the greater complexity of metabolites with diverse chemical and physical properties has limited the advances in plant metabolomics. For several years, applications of liquid/gas chromatography (LC/GC)-mass spectrometry (MS) and nuclear magnetic resonance (NMR) have been constantly developed. Recently, ion mobility spectrometry (IMS)-MS has shown utility in resolving isomeric and isobaric metabolites. Both MS and NMR combined metabolomics significantly increased the identification and quantification of metabolites in an untargeted and targeted manner. Thus, hyphenated metabolomics tools will narrow the gap between the number of metabolite features and the identified metabolites. Metabolites change in response to environmental conditions, including biotic and abiotic stress factors. The spatial distribution of metabolites across different organs, tissues, cells and cellular compartments is a trending research area in metabolomics. Herein, we review recent technological advancements in metabolomics and their applications in understanding plant stress biology and different levels of spatial organization. In addition, we discuss the opportunities and challenges in multiple stress interactions, multi-omics, and single-cell metabolomics.

The Role of Phytohormones in Plant Response to Flooding

Climatic variations influence the morphological, physiological, biological, and biochemical states of plants. Plant responses to abiotic stress include biochemical adjustments, regulation of proteins, molecular mechanisms, and alteration of post-translational modifications, as well as signal transduction. Among the various abiotic stresses, flooding stress adversely affects the growth of plants, including various economically important crops. Biochemical and biological techniques, including proteomic techniques, provide a thorough understanding of the molecular mechanisms during flooding conditions. In particular, plants can cope with flooding conditions by embracing an orchestrated set of morphological adaptations and physiological adjustments that are regulated by an elaborate hormonal signaling network. With the help of these findings, the main objective is to identify plant responses to flooding and utilize that information for the development of flood-tolerant plants. This review provides an insight into the role of phytohormones in plant response mechanisms to flooding stress, as well as different mitigation strategies that can be successfully administered to improve plant growth during stress exposure. Ultimately, this review will expedite marker-assisted genetic enhancement studies in crops for developing high-yield lines or varieties with flood tolerance.

Omics-Facilitated Crop Improvement for Climate Resilience and Superior Nutritive Value

Novel crop improvement approaches, including those that facilitate for the exploitation of crop wild relatives and underutilized species harboring the much-needed natural allelic variation are indispensable if we are to develop climate-smart crops with enhanced abiotic and biotic stress tolerance, higher nutritive value, and superior traits of agronomic importance. Top among these approaches are the "omics" technologies, including genomics, transcriptomics, proteomics, metabolomics, phenomics, and their integration, whose deployment has been vital in revealing several key genes, proteins and metabolic pathways underlying numerous traits of agronomic importance, and aiding marker-assisted breeding in major crop species. Here, citing several relevant examples, we appraise our understanding on the recent developments in omics technologies and how they are driving our quest to breed climate resilient crops. Large-scale genome resequencing, pan-genomes and genome-wide association studies are aiding the identification and analysis of species-level genome variations, whilst RNA-sequencing driven transcriptomics has provided unprecedented opportunities for conducting crop abiotic and biotic stress response studies. Meanwhile, single cell transcriptomics is slowly becoming an indispensable tool for decoding cell-specific stress responses, although several technical and experimental design challenges still need to be resolved. Additionally, the refinement of the conventional techniques and advent of modern, high-resolution proteomics technologies necessitated a gradual shift from the general descriptive studies of plant protein abundances to large scale analysis of protein-metabolite interactions. Especially, metabolomics is currently receiving special attention, owing to the role metabolites play as metabolic intermediates and close links to the phenotypic expression. Further, high throughput phenomics applications are driving the targeting of new research domains such as root system architecture analysis, and exploration of plant root-associated microbes for improved crop health and climate resilience. Overall, coupling these multi-omics technologies to modern plant breeding and genetic engineering methods ensures an all-encompassing approach to developing nutritionally-rich and climate-smart crops whose productivity can sustainably and sufficiently meet the current and future food, nutrition and energy demands.

Metabolomics of Chlorophylls and Carotenoids: Analytical Methods and Metabolome-Based Studies

Chlorophylls and carotenoids are two families of antioxidants present in daily ingested foods, whose recognition as added-value ingredients runs in parallel with the increasing number of demonstrated functional properties. Both groups include a complex and vast number of compounds, and extraction and analysis methods evolved recently to a modern protocol. New methodologies are more potent, precise, and accurate, but their application requires a better understanding of the technical and biological context. Therefore, the present review compiles the basic knowledge and recent advances of the metabolomics of chlorophylls and carotenoids, including the interrelation with the primary metabolism. The study includes material preparation and extraction protocols, the instrumental techniques for the acquisition of spectroscopic and spectrometric properties, the workflows and software tools for data pre-processing and analysis, and the application of mass spectrometry to pigment metabolomics. In addition, the review encompasses a critical description of studies where metabolomics analyses of chlorophylls and carotenoids were developed as an approach to analyzing the effects of biotic and abiotic stressors on living organisms.

Impact of extreme floods on plants considering various influencing factors downstream of Luhun Reservoir, China

Extreme floods caused by dike or dam breaks have led to substantial damage to various types of vegetation, including forests, orchards, grass, and crops. Many factors affect the impacts of extreme floods on plants, e.g., flood parameters, plant characteristics and natural factors. However, these factors have never been systematically analyzed or considered when evaluating the impacts of extreme floods on plants. Firstly, we summarized the main influencing factors and simplified them into six categories: temperature, geomorphic change, plant age, flood velocity, ratio of the flood depth to the plant height, and ratio of the flood duration to the plant waterlogging tolerance time. Secondly, we proposed the two indices of unit risk biomass (URB) and total risk biomass (TRB) to represent the impacts of floods on plants regionally and over the entire inundated area, respectively. In addition, the calculation methods of URB and TRB considering plant biomass and the comprehensive influence coefficient (I) were put forward. To calculate I, we considered the six influencing factors with different weights according to their importance and varying conditions. The flood parameters and geomorphic changes caused by a simulated dam-break flood of Luhun Reservoir in China were then calculated. Furthermore, we divided a year into six time periods according to the species and growth characteristics of the plants in the inundated area. Then we evaluated the impacts of the dam-break flood on the plants during each period. The results showed that: (a) the URB varied with space in the inundated area; (b) because of the large inundation area of crops, the TRB was far greater than that of forests and orchards and affected the TRB of the whole inundated area; and (c) both the URB and TRB changed with time with the changes in crop species, crop parameters and temperature.

Involvement of the miR156/SPL module in flooding response in Medicago sativa

The highly conserved plant microRNA, miR156, affects plant development, metabolite composition, and stress response. Our previous research revealed the role of miR156 in abiotic stress response in Medicago sativa exerted by downregulating SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE transcription factors. Here we investigated the involvement and possible mechanism of action of the miR156/SPL module in flooding tolerance in alfalfa. For that, we used miR156 overexpressing, SPL13RNAi, flood-tolerant (AAC-Trueman) and -sensitive (AC-Caribou) alfalfa cultivars exposed to flooding. We also used Arabidopsis ABA insensitive (abi1-2, abi5-8) mutants and transgenic lines with either overexpressed (KIN10-OX1, KIN10-OX2) or silenced (KIN10RNAi-1, KIN10RNAi-2) catalytic subunit of SnRK1 to investigate a possible role of ABA and SnRK1 in regulating miR156 expression under flooding. Physiological analysis, hormone profiling and global transcriptome changes revealed a role for miR156/SPL module in flooding tolerance. We also identified nine novel alfalfa SPLs (SPL1, SPL1a, SPL2a, SPL7, SPL7a, SPL8, SPL13a, SPL14, SPL16) responsive to flooding. Our results also showed a possible ABA-dependent SnRK1 upregulation to enhance miR156 expression, resulting in downregulation of SPL4, SPL7a, SPL8, SPL9, SPL13, and SPL13a. We conclude that these effects induce flooding adaptive responses in alfalfa and modulate stress physiology by affecting the transcriptome, ABA metabolites and secondary metabolism.

Review: Metabolomics as a prediction tool for plants performance under environmental stress

Metabolomics as a diagnosis tool for plant performance has shown good features for breeding and crop improvement. Additionally, due to limitations in land area and the increasing climate changes, breeding projects focusing on abiotic stress tolerance are becoming essential. Nowadays no universal method is available to identify predictive metabolic markers. As a result, research aims must dictate the best method or combination of methods. To this end, we will introduce the key aspects to consider regarding growth scenarios and sampling strategies and discuss major analytical and data treatment approaches that are available to find metabolic markers of plant performance.

Transcriptomic and anatomic profiling reveal the germination process of different wheat varieties in response to waterlogging stress

BACKGROUND

Waterlogging is one of the most serious abiotic stresses affecting wheat-growing regions in China. Considerable differences in waterlogging tolerance have been found among different wheat varieties, and the mechanisms governing the waterlogging tolerance of wheat seeds during germination have not been elucidated.

RESULTS

The results showed no significant difference between the germination rate of 'Bainong 207' (BN207) (after 72 h of waterlogging treatment) and that of the control seeds. However, the degree of emulsification and the degradation rate of endosperm cells under waterlogging stress were higher than those obtained with the control treatment, and the number of amyloplasts in the endosperm was significantly reduced by waterlogging. Transcriptomic data were obtained from seed samples (a total of 18 samples) of three wheat varieties, 'Zhoumai 22' (ZM22), BN207 and 'Bainong 607' (BN607), subjected to the waterlogging and control treatments. A comprehensive analysis identified a total of 2775 differentially expressed genes (DEGs). In addition, an analysis of the correlations among the expression difference levels of DEGs and the seed germination rates of the three wheat varieties under waterlogging stress revealed that the relative expression levels of 563 and 398 genes were positively and negatively correlated with the germination rate of the wheat seeds, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that the difference in the waterlogging tolerance among the three wheat varieties was related to the abundance of key genes involved in the glycolysis pathway, the starch and sucrose metabolism pathway, and the lactose metabolism pathway. The alcohol dehydrogenase (ADH) gene in the endosperm of BN607 was induced immediately after short-term waterlogging, and the energy provided by the glycolysis pathway enabled the BN607 seeds to germinate as early as possible; in addition, the expression of the AP2/ERF transcription factor was upregulated to further enhance the waterlogging tolerance of this cultivar.

CONCLUSIONS

Taken together, the results of this study help elucidate the mechanisms through which different wheat varieties respond to waterlogging stress during germination.

Metabolite and Phytohormone Profiling Illustrates Metabolic Reprogramming as an Escape Strategy of Deepwater Rice during Partially Submerged Stress

Rice varieties that can survive under submergence conditions respond to flooding either by enhancing internode elongation or by quiescence of shoot elongation. Despite extensive efforts to identify key metabolites triggered by complete submergence of rice possessing SUBMERGENCE 1 (SUB1) locus, metabolic responses of internode elongation of deepwater rice governed by the SNORKEL 1 and 2 genes remain elusive. This study investigated specific metabolomic responses under partial submergence (PS) to deepwater- (C9285) and non-deepwater rice cultivars (Taichung 65 (T65)). In addition, we examined the response in a near-isogenic line (NIL-12) that has a C9285 genomic fragment on chromosome 12 introgressed into the genetic background of T65. Under short-term submergence (0-24 h), metabolite profiles of C9285, NIL-12, and T65 were compared to extract significantly changed metabolites in deepwater rice under PS conditions. Comprehensive metabolite and phytohormone profiling revealed increases in metabolite levels in the glycolysis pathway in NIL-12 plants. Under long-term submergence (0-288 h), we found decreased amino acid levels. These metabolomic changes were opposite when compared to those in flood-tolerant rice with SUB1 locus. Auxin conjugate levels related to stress response decreased in NIL-12 lines relative to T65. Our analysis helped clarify the complex metabolic reprogramming in deepwater rice as an escape strategy.

Metabolomics: A Way Forward for Crop Improvement

Metabolomics is an emerging branch of "omics" and it involves identification and quantification of metabolites and chemical footprints of cellular regulatory processes in different biological species. The metabolome is the total metabolite pool in an organism, which can be measured to characterize genetic or environmental variations. Metabolomics plays a significant role in exploring environment-gene interactions, mutant characterization, phenotyping, identification of biomarkers, and drug discovery. Metabolomics is a promising approach to decipher various metabolic networks that are linked with biotic and abiotic stress tolerance in plants. In this context, metabolomics-assisted breeding enables efficient screening for yield and stress tolerance of crops at the metabolic level. Advanced metabolomics analytical tools, like non-destructive nuclear magnetic resonance spectroscopy (NMR), liquid chromatography mass-spectroscopy (LC-MS), gas chromatography-mass spectrometry (GC-MS), high performance liquid chromatography (HPLC), and direct flow injection (DFI) mass spectrometry, have sped up metabolic profiling. Presently, integrating metabolomics with post-genomics tools has enabled efficient dissection of genetic and phenotypic association in crop plants. This review provides insight into the state-of-the-art plant metabolomics tools for crop improvement. Here, we describe the workflow of plant metabolomics research focusing on the elucidation of biotic and abiotic stress tolerance mechanisms in plants. Furthermore, the potential of metabolomics-assisted breeding for crop improvement and its future applications in speed breeding are also discussed. Mention has also been made of possible bottlenecks and future prospects of plant metabolomics.

Constitutive expression of a stabilized transcription factor group VII ethylene response factor enhances waterlogging tolerance in wheat without penalizing grain yield

Waterlogging causes oxygen deprivation within plant roots and affects crop growth and yield. In crop wheat (Triticum aestivum), molecular responses to waterlogging are poorly understood. Here, we performed a genome-wide analysis of group VII ethylene response factor (ERFVII) genes in hexaploid wheat and identified 25 genes, which were induced by waterlogging with diverse manner. Among them, TaERFVII.1 exhibited differential expression patterns between waterlogging-tolerant wheat Nonglin46 and susceptible wheat Yangmai16 under waterlogging. Constitutive expression of TaERFVII.1 with an MYC-peptide tag at its N terminus in wheat enhanced tolerance to waterlogging as evidenced by increased grain weight per plant, survival rate, and chlorophyll content of leaves and by increased expression of waterlogging-responsive genes, while silencing of TaERFVII.1 compromised the expression of waterlogging-responsive genes. Notably, constitutive expression of the stabilized TaERFVII.1 did not negatively impact both plant development and grain yield under standard conditions. We further demonstrated that constitutive expression of stabilized TaERFVII.1 elevated the transcriptional level of TaSAB18.1, the ortholog of Arabidopsis HRA1 and rice SAB18, consequently reduced the expression of waterlogging-responsive genes under standard conditions. These results suggest that TaERFVII.1 plays an important role in wheat tolerance to waterlogging, and it could be a candidate for improving crop waterlogging tolerance.

Tolerance to partial and complete submergence in the forage legume Melilotus siculus: an evaluation of 15 accessions for petiole hyponastic response and gas-filled spaces, leaf hydrophobicity and gas films, and root phellem

Background and Aims

Submergence is a severe stress for most plants. Melilotus siculus is a waterlogging- (i.e. root zone hypoxia) tolerant annual forage legume, but data were lacking for the effects of partial and full submergence of the shoots. The aim was to compare the tolerance to partial and full submergence of 15 M. siculus accessions and to assess variation in traits possibly contributing to tolerance. Recovery ability post-submergence was also evaluated.

Methods

A factorial experiment imposed treatments of water level [aerated root zone with shoots in air as controls, stagnant root zone with shoots in air, stagnant root zone with partial (75 %) or full shoot submergence] on 15 accessions, for 7 d on 4-week-old plants in a 20/15 °C day/night phytotron. Measurements included: shoot and root growth, hyponastic petiole responses, petiole gas-filled spaces, leaflet sugars, leaflet surface hydrophobicity, leaflet gas film thickness and phellem area near the base of the main root. Recovery following full submergence was also assessed.

Key Results

Accessions differed in shoot and root growth during partial and full shoot submergence. Traits differing among accessions and associated with tolerance were leaflet gas film thickness upon submergence, gas-filled spaces in petioles and phellem tissue area near the base of the main root. All accessions were able to re-orientate petioles towards the vertical under both partial and full submergence. Petiole extension rates were maintained during partial submergence, but decreased during full submergence. Leaflet sugars accumulated during partial submergence, but were depleted during full submergence. Growth resumption after full submergence differed among accessions and was positively correlated with the number of green leaves retained at desubmergence.

Conclusions

Melilotus siculus is able to tolerate partial and full submergence of at least 7 d. Leaflet surface hydrophobicity and associated gas film retention, petiole gas-filled porosity and root phellem abundance are important traits contributing to tolerance. Post-submergence recovery growth differs among accessions. The ability to retain green leaves is essential to succeed during recovery.

Improving Flooding Tolerance of Crop Plants

A major problem of climate change is the increasing duration and frequency of heavy rainfall events. This leads to soil flooding that negatively affects plant growth, eventually leading to death of plants if the flooding persists for several days. Most crop plants are very sensitive to flooding, and dramatic yield losses occur due to flooding each year. This review summarizes recent progress and approaches to enhance crop resistance to flooding. Most experiments have been done on maize, barley, and soybean. Work on other crops such as wheat and rape has only started. The most promising traits that might enhance crop flooding tolerance are anatomical adaptations such as aerenchyma formation, the formation of a barrier against radial oxygen loss, and the growth of adventitious roots. Metabolic adaptations might be able to improve waterlogging tolerance as well, but more studies are needed in this direction. Reasonable approaches for future studies are quantitative trait locus (QTL) analyses or genome-wide association (GWA) studies in combination with specific tolerance traits that can be easily assessed. The usage of flooding-tolerant relatives or ancestral cultivars of the crop of interest in these experiments might enhance the chances of finding useful tolerance traits to be used in breeding.