Flooding stress signaling through perturbations in oxygen, ethylene, nitric oxide and light

The effect of exogenous melatonin on waterlogging stress in Clematis

Clematis is the queen of the vines, being an ornamental plant with high economic value. Waterlogging stress reduces the ornamental value of the plant and limits its application. Melatonin plays an important role in plant resistance to abiotic stresses. In this study, the physiological responses and gene expression levels of two wild species, namely, Clematis tientaiensis and Clematis lanuginosa, and two horticultural varieties, namely, 'Sen-No-Kaze' and 'Viva Polonia,' under waterlogging stress were analyzed to determine the effect of melatonin on waterlogging tolerance. The results showed that the waterlogging tolerances of C. lanuginosa and 'Sen-No-Kaze' were relatively poor, but were significantly improved by concentrations of 100 μmol·L-1 and 50 μmol·L-1 melatonin. C. tientaiensis and 'Viva Polonia' had relatively strong tolerance to waterlogging, and this was significantly improved by 200 μmol·L-1 melatonin. Under waterlogging stress, the relative conductivity and H2O2 content of Clematis increased significantly; the photosynthetic parameters and chlorophyll contents were significantly decreased; photosynthesis was inhibited; the contents of soluble protein and soluble sugars were decreased. Effective improvement of waterlogging tolerance after exogenous melatonin spraying, the relative conductivity was decreased by 4.05%-27.44%; the H2O2 content was decreased by 3.84%-23.28%; the chlorophyll content was increased by 35.59%-103.36%; the photosynthetic efficiency was increased by 25.42%-45.86%; the antioxidant enzyme activities of APX, POD, SOD, and CAT were increased by 28.03%-158.61%; the contents of proline, soluble protein, and soluble sugars were enhanced, and cell homeostasis was improved. Transcription sequencing was performed on wild Clematis with differences in waterlogging tolerance, and nine transcription factors were selected that were highly correlated with melatonin and that had the potential to improve waterlogging tolerance, among which LBD4, and MYB4 were significantly positively correlated with the antioxidant enzyme system, and bHLH36, DOF36, and WRKY4 were significantly negatively correlated. Photosynthetic capacity was positively correlated with DOF36 and WRKY4 while being significantly negatively correlated with MYB4, MOF1, DOF47, REV1 and ABR1. Melatonin could enhance the flooding tolerance of Clematis by improving photosynthetic efficiency and antioxidant enzyme activity. This study provides an important basis and reference for the application of melatonin in waterlogging-resistant breeding of Clematis.

Uncovering the effect of waterlogging stress on plant microbiome and disease development: current knowledge and future perspectives

Waterlogging is a constant threat to crop productivity and ecological biodiversity. Plants face multiple challenges during waterlogging stress like metabolic reprogramming, hypoxia, nutritional depletion, reduction in gaseous exchange, pH modifications, microbiome alterations and disease promotion all of which threaten plants survival. Due to global warming and climatic change, the occurrence, frequency and severity of flooding has dramatically increased posing a severe threat to food security. Thus, developing innovative crop management technologies is critical for ensuring food security under changing climatic conditions. At present, the top priority among scientists is to find nature-based solutions to tackle abiotic or biotic stressors in sustainable agriculture in order to reduce climate change hazards to the environment. In this regard, utilizing plant beneficial microbiome is one of the viable nature based remedial tool for mitigating abiotic stressors like waterlogging. Beneficial microbiota provides plants multifaceted benefits which improves their growth and stress resilience. Plants recruit unique microbial communities to shield themselves against the deleterious effects of biotic and abiotic stress. In comparison to other stressors, there has been limited studies on how waterlogging stress affects plant microbiome structure and their functional traits. Therefore, it is important to understand and explore how waterlogging alters plant microbiome structure and its implications on plant survival. Here, we discussed the effect of waterlogging stress in plants and its microbiome. We also highlighted how waterlogging stress promotes pathogen occurrence and disease development in plants. Finally, we highlight the knowledge gaps and areas for future research directions on unwiring how waterlogging affects plant microbiome and its functional traits. This will pave the way for identifying resilient microbiota that can be engineered to promote their positive interactions with plants during waterlogging stress.

Stem elongation and gibberellin response to submergence depth in clonal plant Alternanthera philoxeroides

Clonal plants are widely distributed in the riparian zone and play a very important role in the maintenance of wetland ecosystem function. Flooding is an environmental stress for plants in the riparian zone, and the response of plants varies according to the depth and duration of flooding. However, there is a lack of research on the growth response of clonal plants during flooding, and the endogenous hormone response mechanism of clonal plants is still unclear. In the present study, Alternanthera philoxeroides, a clonal plant in the riparian zone, was used to investigate the time-dependent stem elongation, the elongation of different part of the immature internodes, and the relationship between growth elongation and the phytohormone gibberellin (GA) under a series of submergence depths (0 m, 2 m, 5 m, and 9 m). The results showed that stem elongation occurred under all treatments, however, compared to 0 m (control), plants grew more under 2 m and 5 m submergence depth, while grew less under 9 m water depth. Additionally, basal part elongation of the immature internode was the predominant factor contributing to the stem growth of A. philoxeroides under different submergence depths. The phytohormone contents in basal part of the mature and immature internodes showed that GA induced the differential elongation of internode. Plant submerged at depth of 2 m had the highest GA accumulation, but plant submerged at depth of 9 m had the lowest GA concentration. These data suggested that GA biosynthesis are essential for stem elongation in A. philoxeroides, and the basal part of the immature internode was the main position of the GA biosynthesis. This study provided new information about the rapid growth and invasion of the clonal plant A. philoxeroides around the world, further clarified the effects of submergence depth and duration on the elongation of the stem, and deepened our understanding of the growth response of terrestrial plants in deeply flooded environments.

How plant roots respond to waterlogging

Plant submergence is a major abiotic stress that impairs plant performance. Under water, reduced gas diffusion exposes submerged plant cells to an environment that is enriched in gaseous ethylene and is limited in oxygen (O2) availability (hypoxia). The capacity for plant roots to avoid and/or sustain critical hypoxia damage is essential for plants to survive waterlogging. Plants use spatiotemporal ethylene and O2 dynamics as instrumental flooding signals to modulate potential adaptive root growth and hypoxia stress acclimation responses. However, how non-adapted plant species modulate root growth behaviour during actual waterlogged conditions to overcome flooding stress has hardly been investigated. Here we discuss how changes in the root growth rate, lateral root formation, density, and growth angle of non-flood adapted plant species (mainly Arabidopsis) could contribute to avoiding and enduring critical hypoxic conditions. In addition, we discuss current molecular understanding of how ethylene and hypoxia signalling control these adaptive root growth responses. We propose that future research would benefit from less artificial experimental designs to better understand how plant roots respond to and survive waterlogging. This acquired knowledge would be instrumental to guide targeted breeding of flood-tolerant crops with more resilient root systems.

ZmWRKY70 activates the expression of hypoxic responsive genes in maize and enhances tolerance to submergence in Arabidopsis

Hypoxic stress due to submergence is a serious threat to the growth and development of maize. WRKY transcription factors are significant regulators of plant responses to various abiotic and biotic stresses. Nevertheless, their function and regulatory mechanisms in the resistance of maize to submergence stress remain unclear. Here we report the cloning of a maize WRKY transcription factor gene, ZmWRKY70, transcripts of which accumulate under submergence stress in maize seedlings. Subcellular localization analysis and yeast transcriptional activation assay indicated that ZmWRKY70 was localized in the nucleus and had transcriptional activation activity. Heterologous overexpression of ZmWRKY70 in Arabidopsis increased the tolerance of seeds and seedlings to submergence stress by upregulating the transcripts of several key genes involved in anaerobic respiration, such as group VII ethylene-responsive factor (ERFVII) (AtRAP2.2), alcohol dehydrogenase (AtADH1), pyruvate decarboxylase (AtPDC1/2), and sucrose synthase (AtSUS4), under submergence conditions. Moreover, the overexpression of ZmWRKY70 in maize mesophyll protoplasts enhanced the expression of ZmERFVII members (ZmERF148, ZmERF179, and ZmERF193), ZmADH1, ZmPDC2/3, and ZmSUS1. Yeast one-hybrid and dual-luciferase activity assays further confirmed that ZmWRKY70 enhanced the expression of ZmERF148 by binding to the W box motif located in the promoter region of ZmERF148. Together, these results indicate that ZmWRKY70 plays a significant role in tolerance of submergence stress. This work provides a theoretical basis, and suggests excellent genes, for biotechnological breeding to improve the tolerance of maize to submergence through the regulation of ZmWRKY genes.

Integrated analysis of transcriptome and microRNAs associated with exogenous calcium-mediated enhancement of hypoxic tolerance in cucumber seedlings (Cucumis sativus L.)

Plants often suffer from hypoxic stress due to flooding caused by extreme weather. Hypoxia usually leads to restricted oxygen supply and alters metabolic patterns from aerobic to anaerobic. Cucumber roots are fragile and highly sensitive to damage from hypoxic stress. The purpose of this study was to investigate the regulatory mechanism of exogenous calcium alleviating hypoxic stress in cucumber through transcriptome and small RNAs analysis. Three treatments were performed in this paper, including untreated-control (CK), hypoxic stress (H), and hypoxic stress + exogenous calcium treatment (H + Ca²⁺). A large number of differentially expressed genes (DEGs) were identified, 1,463 DEGs between CK vs H, 3,399 DEGs between H vs H + Ca²⁺, and 5,072 DEGs between CK vs H + Ca²⁺, respectively. KEGG analysis of DEGs showed that exogenous calcium could activate hormone signaling pathways (ethylene, ABA, IAA and cytokinin), transcription factors (MYB, MYB-related, bHLH, bZIP, and WRKY), calcium signaling and glycolysis pathway to mitigating hypoxic stress in cucumber seedlings. Additionally, miRNA and their target genes were detected and predicted between treatments. The target genes of these miRNAs revealed that auxin, cellulose synthase, and mitochondrial ribosomal related genes (Csa2G315390, Csa6G141390, Csa4G053280, and Csa6G310480) probably play in the improvement of the hypoxic tolerance of cucumber seedlings through exogenous calcium application. In short, our data adds new information to the mechanism of exogenous calcium mitigation of hypoxic stress injury in cucumber seedlings at transcriptional and post-transcriptional levels.

The vacuolar H+/Ca transporter CAX1 participates in submergence and anoxia stress responses

A plant's oxygen supply can vary from normal (normoxia) to total depletion (anoxia). Tolerance to anoxia is relevant to wetland species, rice (Oryza sativa) cultivation, and submergence tolerance of crops. Decoding and transmitting calcium (Ca) signals may be an important component to anoxia tolerance; however, the contribution of intracellular Ca transporters to this process is poorly understood. Four functional cation/proton exchangers (CAX1-4) in Arabidopsis (Arabidopsis thaliana) help regulate Ca homeostasis around the vacuole. Our results demonstrate that cax1 mutants are more tolerant to both anoxic conditions and submergence. Using phenotypic measurements, RNA-sequencing, and proteomic approaches, we identified cax1-mediated anoxia changes that phenocopy changes present in anoxia-tolerant crops: altered metabolic processes, diminished reactive oxygen species production post anoxia, and altered hormone signaling. Comparing wild-type and cax1 expressing genetically encoded Ca indicators demonstrated altered cytosolic Ca signals in cax1 during reoxygenation. Anoxia-induced Ca signals around the plant vacuole are involved in the control of numerous signaling events related to adaptation to low oxygen stress. This work suggests that cax1 anoxia response pathway could be engineered to circumvent the adverse effects of flooding that impair production agriculture.

Full-Length Transcriptome and RNA-Seq Analyses Reveal the Mechanisms Underlying Waterlogging Tolerance in Kiwifruit (Actinidia valvata)

Actinidia valvata possesses waterlogging tolerance; however, the mechanisms underlying this trait are poorly characterized. Here, we performed a transcriptome analysis by combining single-molecule real-time (SMRT) sequencing and Illumina RNA sequencing and investigated the physiological responses of the roots of KR5 (A. valvata, a tolerant genotype) after 0, 12, 24 and 72 h of waterlogging stress. KR5 roots responded to waterlogging stress mainly via carbohydrate and free amino acids metabolism and reactive oxygen species (ROS) scavenging pathways. Trehalose-6-phosphate synthase (TPS) activity, alcohol dehydrogenase (ADH) activity and the total free amino acid content increased significantly under waterlogging stress. The nicotinamide adenine dinucleotide-dependent glutamate synthase/alanine aminotransferase (NADH-GOGAT/AlaAT) cycle was correlated with alanine accumulation. Levels of genes encoding peroxidase (POD) and catalase (CAT) decreased and enzyme activity increased under waterlogging stress. Members of the LATERAL ORGAN BOUNDARIES (LOB), AP2/ERF-ERF, Trihelix and C3H transcription factor families were identified as potential regulators of the transcriptional response. Several hub genes were identified as key factors in the response to waterlogging stress by a weighted gene co-expression network analysis (WGCNA). Our results provide insights into the factors contributing to waterlogging tolerance in kiwifruit, providing a basis for further studies of interspecific differences in an important plant trait and for molecular breeding.

Cross-species transcriptomic analyses reveals common and opposite responses in Arabidopsis, rice and barley following oxidative stress and hormone treatment

BACKGROUND

For translational genomics, a roadmap is needed to know the molecular similarities or differences between species, such as model species and crop species. This knowledge is invaluable for the selection of target genes and pathways to alter downstream in response to the same stimuli. Here, the transcriptomic responses to six treatments including hormones (abscisic acid - ABA and salicylic acid - SA); treatments that cause oxidative stress (3-amino-1,2,4-triazole - 3AT, methyl viologen - MV); inhibit respiration (antimycin A - AA) or induce genetic damage (ultraviolet radiation -UV) were analysed and compared between Arabidopsis (Arabidopsis thaliana), barley (Hordeum vulgare) and rice (Oryza sativa).

RESULTS

Common and opposite responses were identified between species, with the number of differentially expressed genes (DEGs) varying greatly between treatments and species. At least 70% of DEGs overlapped with at least one other treatment within a species, indicating overlapping response networks. Remarkably, 15 to 34% of orthologous DEGs showed opposite responses between species, indicating diversity in responses, despite orthology. Orthologous DEGs with common responses to multiple treatments across the three species were correlated with experimental data showing the functional importance of these genes in biotic/abiotic stress responses. The mitochondrial dysfunction response was revealed to be highly conserved in all three species in terms of responsive genes and regulation via the mitochondrial dysfunction element.

CONCLUSIONS

The orthologous DEGs that showed a common response between species indicate conserved transcriptomic responses of these pathways between species. However, many genes, including prominent salt-stress responsive genes, were oppositely responsive in multiple-stresses, highlighting fundamental differences in the responses and regulation of these genes between species. This work provides a resource for translation of knowledge or functions between species.

To Fight or to Grow: The Balancing Role of Ethylene in Plant Abiotic Stress Responses

Plants often live in adverse environmental conditions and are exposed to various stresses, such as heat, cold, heavy metals, salt, radiation, poor lighting, nutrient deficiency, drought, or flooding. To adapt to unfavorable environments, plants have evolved specialized molecular mechanisms that serve to balance the trade-off between abiotic stress responses and growth. These mechanisms enable plants to continue to develop and reproduce even under adverse conditions. Ethylene, as a key growth regulator, is leveraged by plants to mitigate the negative effects of some of these stresses on plant development and growth. By cooperating with other hormones, such as jasmonic acid (JA), abscisic acid (ABA), brassinosteroids (BR), auxin, gibberellic acid (GA), salicylic acid (SA), and cytokinin (CK), ethylene triggers defense and survival mechanisms thereby coordinating plant growth and development in response to abiotic stresses. This review describes the crosstalk between ethylene and other plant hormones in tipping the balance between plant growth and abiotic stress responses.

Physiological and transcriptional changes provide insights into the effect of root waterlogging on the aboveground part of Pterocarya stenoptera

Pterocarya stenoptera is a tree species that occurs along rivers and has high tolerance to waterlogging. Identification of waterlogging response genes in the aboveground part of P. stenoptera will increase understanding of tolerance mechanisms under root waterlogging conditions. In this study, we employed four physiological indicators and comparative transcriptome sequencing to investigate the waterlogging tolerance mechanism in P. stenoptera. The physiological results showed that the aboveground part of P. stenoptera was not obviously affected by waterlogging. P. stenoptera enhanced waterlogging tolerance by increasing the synthesis of alpha-Linolenic acids and flavonoids and activating the jasmonic acid, ethylene, and auxin signaling pathways. Our results confirmed our hypothesis that P. stenoptera, a species that is widely distributed along rivers, has evolved a range of mechanisms in response to waterlogging. Our research will provide new insights for understanding the tolerance mechanism of species to waterlogging.

The Molecular Regulatory Pathways and Metabolic Adaptation in the Seed Germination and Early Seedling Growth of Rice in Response to Low O2 Stress

As sessile organisms, flooding/submergence is one of the major abiotic stresses for higher plants, with deleterious effects on their growth and survival. Therefore, flooding/submergence is a large challenge for agriculture in lowland areas worldwide. Long-term flooding/submergence can cause severe hypoxia stress to crop plants and can result in substantial yield loss. Rice has evolved distinct adaptive strategies in response to low oxygen (O₂) stress caused by flooding/submergence circumstances. Recently, direct seeding practice has been increasing in popularity due to its advantages of reducing cultivation cost and labor. However, establishment and growth of the seedlings from seed germination under the submergence condition are large obstacles for rice in direct seeding practice. The physiological and molecular regulatory mechanisms underlying tolerant and sensitive phenotypes in rice have been extensively investigated. Here, this review focuses on the progress of recent advances in the studies of the molecular mechanisms and metabolic adaptions underlying anaerobic germination (AG) and coleoptile elongation. Further, we highlight the prospect of introducing quantitative trait loci (QTL) for AG into rice mega varieties to ensure the compatibility of flooding/submergence tolerance traits and yield stability, thereby advancing the direct seeding practice and facilitating future breeding improvement.

Potential Pathway of Nitrous Oxide Formation in Plants

Plants can produce and emit nitrous oxide (N₂O), a potent greenhouse gas, into the atmosphere, and several field-based studies have concluded that this gas is emitted at substantial amounts. However, the exact mechanisms of N₂O production in plant cells are unknown. Several studies have hypothesised that plants might act as a medium to transport N₂O produced by soil-inhabiting microorganisms. Contrarily, aseptically grown plants and axenic algal cells supplied with nitrate (NO₃) are reported to emit N₂O, indicating that it is produced inside plant cells by some unknown physiological phenomena. In this study, the possible sites, mechanisms, and enzymes involved in N₂O production in plant cells are discussed. Based on the experimental evidence from various studies, we determined that N₂O can be produced from nitric oxide (NO) in the mitochondria of plants. NO, a signaling molecule, is produced through oxidative and reductive pathways in eukaryotic cells. During hypoxia and anoxia, NO₃ in the cytosol is metabolised to produce nitrite (NO₂), which is reduced to form NO via the reductive pathway in the mitochondria. Under low oxygen condition, NO formed in the mitochondria is further reduced to N₂O by the reduced form of cytochrome c oxidase (CcO). This pathway is active only when cells experience hypoxia or anoxia, and it may be involved in N₂O formation in plants and soil-dwelling animals, as reported previously by several studies. NO can be toxic at a high concentration. Therefore, the reduction of NO to N₂O in the mitochondria might protect the integrity of the mitochondria, and thus, protect the cell from the toxicity of NO accumulation under hypoxia and anoxia. As NO₃ is a major source of nitrogen for plants and all plants may experience hypoxic and anoxic conditions owing to soil environmental factors, a significant global biogenic source of N₂O may be its formation in plants via the proposed pathway.

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.

Molecular cloning and expression analysis of three ThERFs involved in the response to waterlogging stress of Taxodium 'Zhongshanshan406', and subcellular localization of the gene products

As a subfamily of the APETALA 2/ethylene response element binding protein (AP2/EREBP) transcription factor superfamily, the ethylene response factor (ERF) is widely involved in the regulation of growth and response to various abiotic stresses in plants, and has been shown to be the main transcription factor regulating transcription of the genes related to hypoxia and waterlogging stress. In this study, three ThERF genes, with significant differences in expression profile in response to flooding stress, were identified from the transcriptomics data acquired from Taxodium hybrid ‘Zhongshanshan 406’ (T. mucronatum Tenore × T. distichum (L.) Rich) under waterlogging stress: ThERF15, ThERF39 and ThRAP2.3 (GenBank ID: KY463467, KY463468 and KY463470, respectively).The full-length cDNA of each of the three ERFs was obtained using the RACE (rapid amplification cDNA ends) method, and all three were intron-free. Multiple protein sequence alignments indicated that ThERF15, ThERF39 and ThRAP2.3 proteins all had only one AP2-ERF domain and belonged to the ERF subfamily. A transient gene expression assay demonstrated that ThERF15, ThERF39 and ThRAP2.3 were all localized to the nucleus. Real-time quantitative PCR (qPCR) revealed that the expression of ThERF15, ThERF39 and ThRAP2.3 exhibited significant differences, compared with the control, in response to two levels of flooding treatment (half-flooding or total-submergence) of ‘Zhongshanshan 406’. Quantification of ethylene concentration revealed that ethylene was more relevant to the level of expression than the period of flooding treatment. Based on the experimental results above, ThERF15, ThERF39 and ThRAP2.3 were identified as being related to the regulation of downstream flooding- responsive gene expression in ‘Zhongshanshan 406’. ThRAP2.3 is most likely to be a key downstream-response ERF gene to respond to the output of the ethylene signal generated by flooding stress.

Determining Cellular Responses: Phytoglobins May Direct the Traffic

How stem cells retain their undifferentiated state or how differentiated cells are capable of having dissimilar responses to perturbations are major open questions in plant biology. Cell-specific phytoglobin expression may be one mechanism determining cell fate by the modulation of nitric oxide (NO), affecting cellular hormonal responses and processes such as cell differentiation.

A new major-effect QTL for waterlogging tolerance in wild barley (H. spontaneum)

We report the first study on the unique allele from wild barley that can improve waterlogging tolerance in cultivated barley with a substantially higher contribution to aerenchyma formation. Waterlogging is one of the major abiotic stresses that dramatically reduce barley crop yield. Direct selection on waterlogging tolerance in the field is less effective due to its viability to environment. The most effective way of selection is to choose traits that make significant contributions to the overall tolerance and are easy to score. Aerenchyma formation under waterlogging stress is one of the most effective mechanisms to provide adequate oxygen supply and overcome stress-induced hypoxia imposed on plants. In this study, a new allele for aerenchyma formation was identified from a wild barley accession TAM407227 on chromosome 4H. Compared to that identified in cultivated barley, this allele not only produced a greater proportion of aerenchyma but made a greater contribution to the overall waterlogging tolerance. The QTL explained 76.8% of phenotypic variance in aerenchyma formation with a LOD value of 51.4. Markers co-segregating with the trait were identified and can be effectively used in marker assisted selection.

Meta-analysis of major QTL for abiotic stress tolerance in barley and implications for barley breeding

We projected meta-QTL (MQTL) for drought, salinity, and waterlogging tolerance to the physical map of barley through meta-analysis. The positions of these MQTL were refined and candidate genes were identified. Drought, salinity and waterlogging are three major abiotic stresses limiting barley yield worldwide. Breeding for abiotic stress-tolerant crops has drawn increased attention, and a large number of quantitative trait loci (QTL) for drought, salinity, and waterlogging tolerance in barley have been detected. However, very few QTL have been successfully used in marker-assisted selection (MAS) in breeding. In this study, we summarized 632 QTL for drought, salinity and waterlogging tolerance in barley. Among all these QTL, only 195 major QTL were used to conduct meta-analysis to refine QTL positions for MAS. Meta-analysis was used to map the summarized major QTL for drought, salinity, and waterlogging tolerance from different mapping populations on the barley physical map. The positions of identified meta-QTL (MQTL) were used to search for candidate genes for drought, salinity, and waterlogging tolerance in barley. Both MQTL3H.4 and MQTL6H.2 control drought tolerance in barley. Fine-mapped QTL for salinity tolerance, HvNax4 and HvNax3, were validated on MQTL1H.4 and MQTL7H.2, respectively. MQTL2H.1 and MQTL5H.3 were also the target regions for improving salinity tolerance in barley. MQTL4H.4 is the main region controlling waterlogging tolerance in barley with fine-mapped QTL for aerenchyma formation under waterlogging conditions. Detected and refined MQTL and candidate genes are crucial for future successful MAS in barley breeding.

Are avoidance and acclimation responses during hypoxic stress modulated by distinct cell-specific mechanisms?

Plants respond to hypoxic stress through either acclimation to the stress or avoidance of it, as they do to most environmental stresses. The hypothesis that has general consensus among the community is that ethylene response factors (ERFs) are central elements that control both types of responses to hypoxia. Recent studies suggest that this may not be the case for all cells experiencing hypoxic stress. Mature maize root cells undergoing hypoxic stress were found to undergo acclimation and avoidance mechanisms involving ERFs, whereas meristematic root cells and cells still undergoing differentiation acclimated to the response without the involvement of ethylene synthesis or ERFs. Phytoglobins (PGBs) and NO were demonstrated to be components critical to the acclimation response. These findings are discussed relative to the possibility that PGBs may be acting as molecular switches controlling cellular stress responses and hormonal changes and responses in cells.