Hypoxic Roots Research Articles

Reciprocal modulation of responses to nitrate starvation and hypoxia in roots and leaves of Arabidopsis thaliana

ABSTRACT The flooding of agricultural land leads to hypoxia and nitrate leaching. While understanding the plant’s response to these conditions is essential for crop improvement, the effect of extended nitrate limitation on subsequent hypoxia has not been studied in an organ-specific manner. We cultivated Arabidopsis thaliana without nitrate for 1 week before inducing hypoxia by bubbling the hydroponic solution with nitrogen gas for 16 h. In the roots, the transcripts of two transcription factor genes (HRA1, HRE2) and three genes involved in fermentation (SUS4, PDC1, ADH1) were ~10- to 100-fold upregulated by simultaneous hypoxia and nitrate starvation compared to the control condition (replete nitrate and oxygen). In contrast, this hypoxic upregulation was ~5 to 10 times stronger when nitrate was available. The phytoglobin genes PGB1 and PGB2, involved in nitric oxide (NO) scavenging, were massively downregulated by nitrate starvation (~1000-fold and 105-fold, respectively), but only under ambient oxygen levels; this was reflected in a 2.5-fold increase in NO concentration. In the leaves, HRA1, SUS4, and RAP2.3 were upregulated ~20-fold by hypoxia under nitrate starvation, whereas this upregulation was virtually absent in the presence of nitrate. Our results highlight that the plant’s responses to nitrate starvation and hypoxia can influence each other.

Growth Responses and Adventitious Root Formation of Cucumber Hybrid Lines in a Waterlogged Condition

Cucumber (Cucumis sativus L.) F1 hybrids are grown commercially in open-field or greenhouse conditions. Hybrids are well adapted to these settings due to directed breeding. In protected cultivation systems, a small rhizosphere volume and intensive, continuous fertigation predispose the roots to waterlogging (WL) conditions and potentially to hypoxia. However, high productivity is expected and achieved under these conditions. The aim of this study was to identify traits that play a role in this surprising behavior. Initial observations revealed the presence of a significantly higher number of adventitious roots (ARs) in three greenhouse (7–14) vs. three open-field cultivars (less than two) grown under normal conditions. Further on, two contrasting representative hybrids typically grown in open-field and in greenhouse conditions were subjected to WL stress. Declining oxygen levels in the media and increased alcohol dehydrogenase activity (ADH) in the roots were experienced during the WL treatment in both hybrids, with anaerobic metabolism triggered less intensively (~4-fold less ADH activity) in the greenhouse-type ‘Oitol’. The induction characteristics of cysteine oxidase (CysOx) genes, key components of the hypoxia sensing pathway, were used to confirm the hypoxic stress experienced by the roots. The lower extent of upregulation in CysOx genes expression agreed with the milder level of hypoxic stress in the roots of ‘Oitol’ than in ‘Joker’. The more efficient induction of AR formation with a ~50% increase upon waterlogging stress was found to be a prominent trait in ‘Oitol’, apparently helping root internal aeration and mitigating hypoxia. The shoot growth of neither hybrid was set back by hypoxic root conditions. ‘Joker’ plants maintained the same growth rate as that of the control, while the growth of ‘Oitol’ accelerated when its root system was flooded with nutrient solution. Acclimation processes to hypoxia were proposed to explain the lack of growth retardation in both varieties. This corresponded well with a general abundance of AR development in greenhouse-type (slicing) cucumbers that are typically cultivated in soilless systems.

Preserving root stem cell functionality under low oxygen stress: the role of nitric oxide and phytoglobins.

The preservation of quiescent center stem cell integrity in hypoxic roots by phytoglobins is exercised through their ability to scavenge nitric oxide and attenuate its effects on auxin transport and cell degradation. Under low oxygen stress, the retention or induction of phytoglobin expression maintains cell viability while loss or lack of induction of phytoglobin leads to cell degradation. Plants have evolved unique attributes to ensure survival in the environment in which they must exist. Common among the attributes is the ability to maintain stem cells in a quiescent (or low proliferation) state in unfriendly environments. From the seed embryo to meristematic regions of the plant, quiescent stem cells exist to regenerate the organism when environmental conditions are suitable to allow plant survival. Frequently, plants dispose of mature cells or organs in the process of acclimating to the stresses to ensure survival of meristems, the stem cells of which are capable of regenerating cells and organs that have been sacrificed, a feature not generally available to mammals. Most of the research on plant stress responses has dealt with how mature cells respond because of the difficulty of specifically examining plant meristem responses to stress. This raises the question as to whether quiescent stem cells behave in a similar fashion to mature cells in their response to stress and what factors within these critical cells determine whether they survive or degrade when exposed to environmental stress. This review attempts to examine this question with respect to the quiescent center (QC) stem cells of the root apical meristem. Emphasis is put on how varying levels of nitric oxide, influenced by the expression of phytoglobins, affect QC response to hypoxic stress.

Arabidopsis root apical meristem survival during waterlogging is determined by phytoglobin through nitric oxide and auxin.

Over-expression of phytoglobin mitigates the degradation of the root apical meristem (RAM) caused by waterlogging through changes in nitric oxide and auxin distribution at the root tip. Plant performance to waterlogging is ameliorated by the over-expression of the Arabidopsis Phytoglobin 1 (Pgb1) which also contributes to the maintenance of a functional RAM. Hypoxia induces accumulation of ROS and damage in roots of wild type plants; these events were preceded by the exhaustion of the RAM resulting from the loss of functionality of the WOX5-expressing quiescent cells (QCs). These phenotypic deviations were exacerbated by suppression of Pgb1 and attenuated when the same gene was up-regulated. Genetic and pharmacological studies demonstrated that degradation of the RAM in hypoxic roots is attributed to a reduction in the auxin maximum at the root tip, necessary for the specification of the QC. This reduction was primarily caused by alterations in PIN-mediated auxin flow but not auxin synthesis. The expression and localization patterns of several PINs, including PIN1, 2, 3 and 4, facilitating the basipetal translocation of auxin and its distribution at the root tip, were altered in hypoxic WT and Pgb1-suppressing roots but mostly unchanged in those over-expressing Pgb1. Disruption of PIN1 and PIN2 signal in hypoxic roots suppressing Pgb1 initiated in the transition zone at 12h and was specifically associated to the absence of Pgb1 protein in the same region. Exogenous auxin restored a functional RAM, while inhibition of the directional auxin flow exacerbated the degradation of the RAM. The regulation of root behavior by Pgb1 was mediated by nitric oxide (NO) in a model consistent with the recognized function of Pgbs as NO scavengers. Collectively, this study contributes to our understanding of the role of Pgbs in preserving root meristem function and QC niche during conditions of stress, and suggests that the root transition zone is most vulnerable to hypoxia.

Fermentation-mediated growth, signaling, and defense in plants.

While traditionally considered important mainly in hypoxic roots during flooding, upregulation of fermentation pathways in plants has recently been described as an evolutionarily conserved drought survival strategy, with acetate signaling mediating reprograming of transcription and cellular carbon and energy metabolism from roots to leaves. The amount of acetate produced directly correlates with survival through potential mechanisms including defense gene activation, biosynthesis of primary and secondary metabolites, and aerobic respiration. Here, we review root ethanolic fermentation responses to hypoxia during saturated soil conditions and summarize studies highlighting acetate fermentation under aerobic conditions coupled with respiration during growth and drought responses. Recent work is discussed demonstrating long-distance transport of acetate via the transpiration stream as a respiratory substrate. While maintenance and growth respiration are often modeled separately in terrestrial models, here we propose the concept of 'Defense Respiration' fueled by acetate fermentation in which upregulation of acetate fermentation contributes acetate substrate for alternative energy production via aerobic respiration, biosynthesis of primary and secondary metabolites, and the acetylation of proteins involved in defense gene regulation. Finally, we highlight new frontiers in leaf-atmosphere emission measurements as a potential way to study acetate fermentation responses of individual leaves, branches, ecosystems, and regions.

Metabolic responses of sugar beet to the combined effect of root hypoxia and NaCl-salinity

The combined occurrence of salt stress and hypoxia leads to increased growth reduction and severe toxic effects compared to salt stress alone. In the present work, we analyzed the metabolic response of sugar beet (Beta vulgaris L.) to salt stress combined with hypoxia in roots as well as in young and mature leaves. B. vulgaris plants were grown in a hydroponic culture under low and high salt concentrations combined with normoxic and hypoxic conditions. A non-targeted metabolic approach was used to identify the biochemical pathways underlying the metabolic and physiological adaptation mechanisms.Young and mature leaves showed a similar metabolic response to salt stress alone and combined stresses, accumulating sugar compounds. Osmoprotectants such as proline and pinitol were accumulated under combined stress. Roots exposed to hypoxic conditions showed increased TCA (tricarboxylic acid cycle) intermediates levels such as succinate, fumarate and malate. During hypoxia, the concentration of free amino acids as well as intermediates of the GABA (gamma-aminobutyric acid) shunt increased in roots as well as in leaves. The combination of salt stress and hypoxia results in a severe stress response in roots and leaves. A partial flux of the TCA cycle linked with the GABA shunt might be activated during hypoxia to regain reduction equivalents.

Ethylene enhances root water transport and aquaporin expression in trembling aspen (Populus tremuloides) exposed to root hypoxia

BackgroundRoot hypoxia has detrimental effects on physiological processes and growth in most plants. The effects of hypoxia can be partly alleviated by ethylene. However, the tolerance mechanisms contributing to the ethylene-mediated hypoxia tolerance in plants remain poorly understood.ResultsIn this study, we examined the effects of root hypoxia and exogenous ethylene treatments on leaf gas exchange, root hydraulic conductance, and the expression levels of several aquaporins of the plasma membrane intrinsic protein group (PIP) in trembling aspen (Populus tremuloides) seedlings. Ethylene enhanced net photosynthetic rates, transpiration rates, and root hydraulic conductance in hypoxic plants. Of the two subgroups of PIPs (PIP1 and PIP2), the protein abundance of PIP2s and the transcript abundance of PIP2;4 and PIP2;5 were higher in ethylene-treated trembling aspen roots compared with non-treated roots under hypoxia. The increases in the expression levels of these aquaporins could potentially facilitate root water transport. The enhanced root water transport by ethylene was likely responsible for the increase in leaf gas exchange of the hypoxic plants.ConclusionsExogenous ethylene enhanced root water transport and the expression levels of PIP2;4 and PIP2;5 in hypoxic roots of trembling aspen. The results suggest that ethylene facilitates the aquaporin-mediated water transport in plants exposed to root hypoxia.

Genotypic variations in nitrate respiration along with potassium nitrate treatment - accountable for water logging tolerance in maize

The present investigation was undertaken to study the effects of water logging and potassium nitrate treatment on the status of nitrate metabolism in maize genotypes, I 167 and LM 5, showing differential tolerance towards water logging stress. Water logging reduced length and biomass of LM 5 (stress susceptible) seedlings to a higher extent than those of I 167 (stress tolerant). Foliar spray of 1% KNO3 increased seedling growth in both the genotypes. The water logged I 167 seedlings maintained their nitrate levels more efficiently than those of LM 5 seedlings. The parallel increase in NO content in the hypoxic roots showed the effective utilization of NO in nitrate respiration pathway of I 167 seedlings. The increased NO content in the shoots of water logged LM 5 seedlings irrespective of their reduced NR activity showed that the susceptible genotype might have preferentially utilized nitrite ions for NO generation rather than ammonium assimilation during early stage of water logging stress. KNO3 spray increased alanine amino transferase (AlaAT) and glutamine oxoglutarate amino transferase (GOGAT) activities along with an increase in amino acid and protein content of I 167 shoots that might be responsible for better assimilation of amino acids in the tolerant genotype. It may thus be concluded that there were genotypic differences in nitrate metabolism under water logging stress and potassium nitrate significantly improved ammonium assimilation of maize genotypes.

Foliar treatment of potassium nitrate modulates the fermentative and sucrose metabolizing pathways in contrasting maize genotypes under water logging stress.

The effect of potassium nitrate on the status of fermentative and sucrose metabolizing pathways was studied in two maize (Zea mays L.) genotypes, viz., LM 5 (relatively susceptible to flooding) and I 167 (relatively tolerant to flooding) under water logging stress. The higher increase in pyruvate decarboxylase, alcohol dehydrogenase and aldehyde dehydrogenase activities in the hypoxic roots of I 167 seedlings over LM 5 showed the former's efficient tolerance mechanism towards anaerobic conditions. Foliar application of KNO3 reduced these enzymatic activities in the roots of both the genotypes. The shoots of I 167 seedlings also showed a parallel increase in alcohol dehydrogenase and pyruvate decarboxylase activities under water logging stress. These enzymatic activities, however, remained unaffected in shoots of water logged LM 5 seedlings. There was a higher decrease in acid and alkaline invertase activities in the hypoxic roots of I 167 seedlings. KNO3 treatment led to higher acid invertase activity in roots of I 167 seedlings than those of LM 5. Sucrose synthase (synthesis) and sucrose phosphate synthase activities decreased, but sucrose synthase (breakdown) activity increased in the roots of both the genotypes, during water logging. KNO3 increased sucrose synthesizing activities with a parallel increase in the sucrose content of the roots. Sucrose synthesis was comparatively unaffected in I 167 shoots under water logging stress while LM 5 shoots showed higher reduction in its sucrose synthase (synthesis) and sucrose phosphate synthase activities. It may thus be concluded that KNO3 induced a network of reactions for improving water logging tolerance. The nitrate ions acted as an alternate electron acceptor and thus reduced the activities of fermentative enzymes. It promoted the funneling of sugars into the glycolytic pathway by inducing the activities of acid and alkaline invertases in the roots and shoots of maize genotypes. It also directed the hexoses towards biosynthetic pathway by increasing the activities of sucrose synthesizing enzymes.

Radial gradient of the mitochondrial ultrastructure in Pisum sativum L. roots

Tissue hypoxia in roots of terrestrial plants in the environment saturated with oxygen is conditioned by the high tissue density and oxygen utilization during the cellular respiration, and it is followed by a gradual decrease in the oxygen concentration from the organ surface. We used this natural model on the example of the main roots of 5-day-old seedlings of Pisum sativum L. growing under well-aerated conditions to study the ultrastructure of mitochondria in tissue with reducing oxygen content. In the direction from the root surface to the endodermis, it was found a gradual increase in the size of mitochondria due to swelling and partial fusion of the organelles. The formation of one dominant invagination led to the appearance of cup-shaped organelles in inner cortex cell layers. Such successive changes in the structure of organelles were called the radial gradient of the mitochondrial ultrastructure under a gradual decrease in the cell oxygen content. It is suggested that the described transformations in the mitochondrial ultrastructure could be an unspecific response to conditions that limit their energy and / or metabolic functions.

Stem cell fate in hypoxic root apical meristems is influenced by phytoglobin expression.

Root survival to flooding-induced hypoxic stress is dependent upon maintaining the functionality of the root apical meristem quiescent center (QC), a process that is governed by the basipetal flow of auxin leading to the formation of an auxin maximum, which is needed for the establishment of a highly oxidized environment specifying the QC niche. Perturbations in auxin flow and distribution along the root profile occurring during hypoxia can shift the redox state of the QC towards a more reduced environment, leading to the activation of the QC, degradation of the meristem, and root abortion. The maize phytoglobin gene ZmPgb1.1 is involved in minimizing these damaging effects during hypoxia in processes that result in sustaining the PIN-mediated auxin maximum and an oxidized environment in the QC. The oxidized environment is accomplished by maintaining the activity of redox enzymes oxidizing ascorbate and glutathione. These events, compromised in QCs suppressing ZmPgb1.1, ensure the functionality of the QC and root meristems under conditions of low oxygen, resulting in stable root performance.

Products of anaerobic metabolism in waterlogged roots of soybean are exported in the xylem

Waterlogging leads to hypoxia of the root system. Metabolic changes occur that enable the plant to tolerate the hypoxic stress. We investigated the export of organic acids, products of anaerobic metabolism, via xylem of waterlogged soybean (Glycine max) plants. Organic acids were quantified by GC–MS and their formation via aspartate metabolism investigated using [4-13C]aspartate. Elevated levels of malate were found together with variable amounts of other organic acids, notably lactate and succinate. Addition of [4-13C]aspartate to the medium led to isotopic enrichment of several organic acids in the xylem sap. Quantitatively, malate carried the highest amount of label among the organic acids. Labelling of succinate indicates its formation by reversal of the TCA-cycle from oxaloacetate. Since aspartate was a prominent amino acid of the phloem sap, it is suggested that this is an important source of malate exported in the xylem. The export of these organic acids will play the role of removing electrons from the hypoxic roots, representing an additional mechanism in the metabolic response to root hypoxia. Malate, normally considered an intermediate in succinate formation, is definitively a product of anaerobic metabolism.

Utilizing the physiological adaptation mechanisms of coastal plants for vegetative restoration of barrier islands

Since 1932, there has been increased coastal erosion accounting for over one million acres of land loss along the Louisiana coastline resulting in a major economic loss in the United States economy. The objective of this research was to determine the physiological and morphological adaptations of barrier island vegetation to better plan future planting and restoration efforts. Plant growth, biomass, and nutrient load changes were evaluated for the purpose of barrier island restoration. Plants flooded for eight weeks accumulated greater biomass than the unflooded groups showing adaptability to hypoxia and flooding stress. Tissue analysis of coastal edge plants resulted in no nutrient content differences, demonstrating their ability to exclude excessive quantities of Fe and Mn uptake in hypoxic conditions. Arenchyma cells were abundant in black mangrove (Avicinia germinans) and S. alterniflora flooded roots transporting oxygen to the hypoxic root zone. Beach plants were found to help increase sand elevation, creating increased contour heights. Additionally, rooting strength research indicated greater soil stabilization after eight weeks of seashore paspalum establishment on simulated beach plots. Coastal edge plants were found to be suitable for the uptake of high nutrient loads from water. Carefully selecting barrier island restoration plant populations based on their adaptive mechanisms could help improve the success of barrier island restoration efforts.

Root Bending Is Antagonistically Affected by Hypoxia and ERF-Mediated Transcription via Auxin Signaling.

When plants encounter soil water logging or flooding, roots are the first organs to be confronted with reduced gas diffusion resulting in limited oxygen supply. Since roots do not generate photosynthetic oxygen, they are rapidly faced with oxygen shortage rendering roots particularly prone to damage. While metabolic adaptations to low oxygen conditions, which ensure basic energy supply, have been well characterized, adaptation of root growth and development have received less attention. In this study, we show that hypoxic conditions cause the primary root to grow sidewise in a low oxygen environment, possibly to escape soil patches with reduced oxygen availability. This growth behavior is reversible in that gravitropic growth resumes when seedlings are returned to normoxic conditions. Hypoxic root bending is inhibited by the group VII ethylene response factor (ERFVII) RAP2.12, as rap2.12-1 seedlings show exaggerated primary root bending. Furthermore, overexpression of the ERFVII member HRE2 inhibits root bending, suggesting that primary root growth direction at hypoxic conditions is antagonistically regulated by hypoxia and hypoxia-activated ERFVIIs. Root bending is preceded by the establishment of an auxin gradient across the root tip as quantified with DII-VENUS and is synergistically enhanced by hypoxia and the auxin transport inhibitor naphthylphthalamic acid. The protein abundance of the auxin efflux carrier PIN2 is reduced at hypoxic conditions, a response that is suppressed by RAP2.12 overexpression, suggesting antagonistic control of auxin flux by hypoxia and ERFVII. Taken together, we show that hypoxia triggers an escape response of the primary root that is controlled by ERFVII activity and mediated by auxin signaling in the root tip.

Mapping quantitative trait loci for root development under hypoxia conditions in soybean (Glycine max L. Merr.).

Greatest potential, QTLs for hypoxia and waterlogging tolerance in soybean roots were detected using a new phenotypic evaluation method. Waterlogging is a major environmental stress limiting soybean yield in wet parts of the world. Root development is an important indicator of hypoxia tolerance in soybean. However, little is known about the genetic control of root development under hypoxia. This study was conducted to identify quantitative trait loci (QTLs) responsible for root development under hypoxia. Recombinant inbred lines (RILs) developed from a cross between a hypoxia-sensitive cultivar, Tachinagaha, and a tolerant landrace, Iyodaizu, were used. Seedlings were subjected to hypoxia, and root development was evaluated with the value change in root traits between after and before treatments. We found 230 polymorphic markers spanning 2519.2cM distributed on all 20 chromosomes (Chrs.). Using these, we found 11 QTLs for root length (RL), root length development (RLD), root surface area (RSA), root surface area development (RSAD), root diameter (RD), and change in average root diameter (CARD) on Chrs. 11, 12, 13 and 14, and 7 QTLs for hypoxia tolerance of these root traits. These included QTLs for RLD and RSAD between markers Satt052 and Satt302 on Chr. 12, which are important markers of hypoxia tolerance in soybean; those QTLs were stable between 2years. To validate the QTLs, we developed a near-isogenic line with the QTL region derived from Iyodaizu. The line performed well under both hypoxia and waterlogging, suggesting that the region contains one or more genes with large effects on root development. These findings may be useful for fine mapping and positional cloning of gene responsible for root development under hypoxia.

Phytoglobins Improve Hypoxic Root Growth by Alleviating Apical Meristem Cell Death.

Hypoxic root growth in maize (Zea mays) is influenced by the expression of phytoglobins (ZmPgbs). Relative to the wild type, suppression of ZmPgb1.1 or ZmPgb1.2 inhibits the growth of roots exposed to 4% oxygen, causing structural abnormalities in the root apical meristems. These effects were accompanied by increasing levels of reactive oxygen species (ROS), possibly through the transcriptional induction of four Respiratory Burst Oxidase Homologs TUNEL-positive nuclei in meristematic cells indicated the involvement of programmed cell death (PCD) in the process. These cells also accumulated nitric oxide and stained heavily for ethylene biosynthetic transcripts. A sharp increase in the expression level of several 1-aminocyclopropane synthase (ZmAcs2, ZmAcs6, and ZmAcs7), 1-aminocyclopropane oxidase (Aco15, Aco20, Aco31, and Aco35), and ethylene-responsive (ZmErf2 and ZmEbf1) genes was observed in hypoxic ZmPgb-suppressing roots, which overproduced ethylene. Inhibiting ROS synthesis with diphenyleneiodonium or ethylene perception with 1-methylcyclopropene suppressed PCD, increased BAX inhibitor-1, an effective attenuator of the death programs in eukaryotes, and restored root growth. Hypoxic roots overexpressing ZmPgbs had the lowest level of ethylene and showed a reduction in ROS staining and TUNEL-positive nuclei in the meristematic cells. These roots retained functional meristems and exhibited the highest growth performance when subjected to hypoxic conditions. Collectively, these results suggest a novel function of Pgbs in protecting root apical meristems from hypoxia-induced PCD through mechanisms initiated by nitric oxide and mediated by ethylene via ROS.

Flooding tolerance of forage legumes.

We review waterlogging and submergence tolerances of forage (pasture) legumes. Growth reductions from waterlogging in perennial species ranged from >50% for Medicago sativa and Trifolium pratense to <25% for Lotus corniculatus, L. tenuis, and T. fragiferum. For annual species, waterlogging reduced Medicago truncatula by ~50%, whereas Melilotus siculus and T. michelianum were not reduced. Tolerant species have higher root porosity (gas-filled volume in tissues) owing to aerenchyma formation. Plant dry mass (waterlogged relative to control) had a positive (hyperbolic) relationship to root porosity across eight species. Metabolism in hypoxic roots was influenced by internal aeration. Sugars accumulate in M. sativa due to growth inhibition from limited respiration and low energy in roots of low porosity (i.e. 4.5%). In contrast, L. corniculatus, with higher root porosity (i.e. 17.2%) and O2 supply allowing respiration, maintained growth better and sugars did not accumulate. Tolerant legumes form nodules, and internal O2 diffusion along roots can sustain metabolism, including N2 fixation, in submerged nodules. Shoot physiology depends on species tolerance. In M. sativa, photosynthesis soon declines and in the longer term (>10 d) leaves suffer chlorophyll degradation, damage, and N, P, and K deficiencies. In tolerant L. corniculatus and L. tenuis, photosynthesis is maintained longer, shoot N is less affected, and shoot P can even increase during waterlogging. Species also differ in tolerance of partial and complete shoot submergence. Gaps in knowledge include anoxia tolerance of roots, N2 fixation during field waterlogging, and identification of traits conferring the ability to recover after water subsides.

Waterlogging-induced changes in fermentative metabolism in roots and nodules of soybean genotypes

Waterlogging blocks the oxygen supply to the root system which inhibits respiration, and greatly reduces the energy status of cells that affect important metabolic processes. This study evaluated fermentative metabolism and carbohydrate contents in the root system of two soybean (Glycine max L. Merril) genotypes under hypoxic and post-hypoxic conditions. Nodulated plants (genotypes Fundacep 53 RR and BRS Macota) were grown in vermiculite and transferred to a hydroponic system at the reproductive stage. The root system was submitted to hypoxia by flowing N2 (nitrogen) gas in a solution for 24 and 72 h. For recovery, plants returned to normoxia condition by transfer to vermiculite for 24 and 72 h. Fermentative enzyme activity, levels of anaerobic metabolites and carbohydrate content were all quantified in roots and nodules. The activity of alcohol dehydrogenase, pyruvate decarboxylase and lactate dehydrogenase enzymes, as well as the content of ethanol and lactate, increased with hypoxia in roots and nodules, and subsequently returned to pre-hypoxic levels in the recovery phase in both genotypes. Pyruvate content increased in nodules and decreased in roots. Sugar and sucrose levels increased in roots and decreased in nodules under hypoxia in both genotypes. Fundacep RR 53 was more responsive to the metabolic effects caused by hypoxia and post-hypoxia than BRS Macota, and it is likely that these characteristics contribute positively to improving adaptation to oxygen deficiency.

De novo sequencing, assembly, and analysis of the Taxodium 'Zhongshansa' roots and shoots transcriptome in response to short-term waterlogging.

BackgroundTaxodium is renowned for its strong tolerance to waterlogging stress, thus it has great ecological and economic potential. However, the scant genomic resources in genus Taxodium have greatly hindered further exploration of its underlying flood-tolerance mechanism. Taxodium ‘Zhongshansa’ is an interspecies hybrid of T. distichum and T. mucronatum, and has been widely planted in southeastern China. To understand the genetic basis of its flood tolerance, we analyzed the transcriptomes of Taxodium ‘Zhongshansa’ roots and shoots in response to short-term waterlogging.ResultsRNA-seq was used to analyze genome-wide transcriptome changes of Taxodium ‘Zhongshansa 406’ clone root and shoot treated with 1 h of soil-waterlogging stress. After de novo assembly, 108,692 unigenes were achieved, and 70,260 (64.64%) of them were annotated. There were 2090 differentially expressed genes (DEGs) found in roots and 394 in shoots, with 174 shared by both of them, indicating that the aerial parts were also affected. Under waterlogging stress, the primary reaction of hypoxic-treated root was to activate the antioxidative defense system to prevent cells experiencing reactive oxygen species (ROS) poisoning. As respiration was inhibited and ATP decreased, another quick coping mechanism was repressing the energy-consuming biosynthetic processes through the whole plant. The glycolysis and fermentation pathway was activated to maintain ATP production in the hypoxic root. Constantly, the demand for carbohydrates increased, and carbohydrate metabolism were accumulated in the root as well as the shoot, possibly indicating that systemic communications between waterlogged and non-waterlogged tissues facilated survival. Amino acid metabolism was also greatly influenced, with down-regulation of genes involvedin serine degradation and up-regulation of aspartic acid degradation. Additionally, a non-symbiotic hemoglobin class 1 gene was up-regulated, which may also help the ATP production. Moreover, the gene expression pattern of 5 unigenes involving in the glycolysis pathway revealed by qRT-PCR confirmed the RNA-Seq data.ConclusionsWe conclude that ROS detoxification and energy maintenance were the primary coping mechanisms of ‘Zhongshansa’ in surviving oxygen deficiency, which may be responsible for its remarkable waterlogging tolerance. Our study not only provided the first large-scale assessment of genomic resources of Taxodium but also guidelines for probing the molecular mechanism underlying ‘Zhongshansa’ waterlogging tolerance.

Photoinhibition-like damage to the photosynthetic apparatus in plant leaves induced by submergence treatment in the dark.

Submergence is a common type of environmental stress for plants. It hampers survival and decreases crop yield, mainly by inhibiting plant photosynthesis. The inhibition of photosynthesis and photochemical efficiency by submergence is primarily due to leaf senescence and excess excitation energy, caused by signals from hypoxic roots and inhibition of gas exchange, respectively. However, the influence of mere leaf-submergence on the photosynthetic apparatus is currently unknown. Therefore, we studied the photosynthetic apparatus in detached leaves from four plant species under dark-submergence treatment (DST), without influence from roots and light. Results showed that the donor and acceptor sides, the reaction center of photosystem II (PSII) and photosystem I (PSI) in leaves were significantly damaged after 36 h of DST. This is a photoinhibition-like phenomenon similar to the photoinhibition induced by high light, as further indicated by the degradation of PsaA and D1, the core proteins of PSI and PSII. In contrast to previous research, the chlorophyll content remained unchanged and the H2O2 concentration did not increase in the leaves, implying that the damage to the photosynthetic apparatus was not caused by senescence or over-accumulation of reactive oxygen species (ROS). DST-induced damage to the photosynthetic apparatus was aggravated by increasing treatment temperature. This type of damage also occurred in the anaerobic environment (N2) without water, and could be eliminated or restored by supplying air to the water during or after DST. Our results demonstrate that DST-induced damage was caused by the hypoxic environment. The mechanism by which DST induces the photoinhibition-like damage is discussed below.