Physiological and metabolic adaptations of Potamogeton pectinatus L. tubers support rapid elongation of stem tissue in the absence of oxygen

Differences in the metabolic and functional mechanisms used to tolerate flooding in Guazuma ulmifolia (Lam.) from flood-prone Amazonian and dry Cerrado savanna populations

Flood tolerance is crucial to the survival of tree species subject to long periods of flooding, such as those present in the Amazonian várzea. Tolerance can be mediated by adjustments of metabolism, physiology and morphology, reinforcing the need to investigate the physiological and biochemical mechanisms used by tropical tree species to survive this stress. Moreover, such mechanisms may vary between populations that are subjected to differences in the frequency of flooding events. Here, we aimed to identify the mechanisms used by two populations of the tropical tree Guazuma ulmifolia (Lam.) to tolerate flooding: an Amazonian population frequently exposed to flooding and a Cerrado population, adapted to a dry environment. Young plants were subjected to a flooding of the roots and lower stem for 32 days, followed by 17 days of recovery. Amazonian plants exhibited greater increases in shoot length and higher maximum photosynthetic rate (Amax) compared with non-flooded plants from 7 days of flooding onwards, whereas increased Amax occurred later in flooded Cerrado plants and was not accompanied by increased shoot length. Lactate accumulated in roots of Cerrado plants after 24 h flooding, together with transcripts coding for lactate dehydrogenase in roots of both Cerrado and Amazonian plants. After 7 days of flooding, lactate decreased and alcohol dehydrogenase activity increased transiently, together with concentrations of alanine, γ-aminobutyric acid and succinate, indicating activation of metabolic processes associated with low oxygen availability. Other amino acids also increased in flooded Cerrado plants, revealing more extensive metabolic changes than in Amazonian plants. Wetland and dryland populations of G. ulmifolia revealed the great capacity to tolerate flooding stress through a suite of alterations in photosynthetic gas exchange and metabolism. However, the integrated physiological, biochemical and molecular analyses realized here indicated that wetland plants acclimatized more efficiently with increased shoot elongation and more rapid restoration of normal metabolism.

Tolerant mechanisms to O2 deficiency under submergence conditions in plants

Wetland plants can tolerate long-term strict hypoxia and anoxic conditions and the subsequent re-oxidative stress compared to terrestrial plants. During O2 deficiency, both wetland and terrestrial plants use NAD(P)+ and ATP that are produced during ethanol fermentation, sucrose degradation, and major amino acid metabolisms. The oxidation of NADH by non-phosphorylating pathways in the mitochondrial respiratory chain is common in both terrestrial and wetland plants. As the wetland plants enhance and combine these traits especially in their roots, they can survive under long-term hypoxic and anoxic stresses. Wetland plants show two contrasting strategies, low O2 escape and low O2 quiescence strategies (LOES and LOQS, respectively). Differences between two strategies are ascribed to the different signaling networks related to phytohormones. During O2 deficiency, LOES-type plants show several unique traits such as shoot elongation, aerenchyma formation and leaf acclimation, whereas the LOQS-type plants cease their growth and save carbohydrate reserves. Many wetland plants utilize NH4+ as the nitrogen (N) source without NH4+-dependent respiratory increase, leading to efficient respiratory O2 consumption in roots. In contrast, some wetland plants with high O2 supply system efficiently use NO3- from the soil where nitrification occurs. The differences in the N utilization strategies relate to the different systems of anaerobic ATP production, the NO2--driven ATP production and fermentation. The different N utilization strategies are functionally related to the hypoxia or anoxia tolerance in the wetland plants.

Metabolic and biochemical responses of Potamogeton anguillanus Koidz. (Potamogetonaceae) to low oxygen conditions

Oxygen availability in water is considered one of the most important factors for growth and productivity in aquatic submerged macrophytes. In the present study, the growth, stress responses, and metabolic changes in Potamogeton anguillanus Koidz. (Potamogetonaceae) were assessed after a 21-day exposure to low (hypoxia; dissolved oxygen, DO < 1 mg/L) or null (anoxia) oxygen concentrations in water. High growth rates and an increased indole acetic acid (IAA) content in P. anguillanus were observed under the hypoxic conditions (4-fold to control) compared to the anoxic conditions. In addition, the activation of glycolysis and fermentation processes was further recorded, given the increase in alcohol dehydrogenase activity and pyruvate concentration on the studied plants that were exposed to low oxygen concentrations. Moreover, the positive correlations of antioxidative enzyme activities, catalase (CAT) and guaiacol peroxidase (POD) with hydrogen peroxide (H₂O₂) confirmed the species ability to scavenge excess H₂O₂ under low oxygen stress. The capillary electrophoresis-mass spectrometry (CE-MS) analysis of the metabolome identified metabolite accumulations (e.g., glutamate, glutamine, aspartate, asparagine, valine, malate, lactate, citrate, isocitrate, proline and γ-amino butyric acid) in response to the anoxia.

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.

Energy-crises in well-aerated and anoxic tissue: does tolerance require the same specific proteins and energy-efficient transport?

Many of the profound changes in metabolism that are caused by O2 deficiency also occur in well-aerated tissues when oxidative phosphorylation is partially or wholly inhibited. For these well-aerated tissues, reduction in energy formation occurs during exposure to inhibitors of oxidative phosphorylation, cold/chilling and wounding, so we prefer the term 'energy crisis' metabolism over 'anaerobic' metabolism. In this review, we note that the overwhelming body of data on energy crises has been obtained by exposure to hypoxia-anoxia, which we will indicate when discussing the particular experiments. We suggest that even transient survival of an energy crisis requires a network of changes common to a large number of conditions, ranging from changes in development to various adverse conditions such as high salinity, drought and nutrient deficiency, all of which reduce growth. During an energy crisis this general network needs to be complemented by energy specific proteins, including the so called 'anaerobic proteins' and the group of ERFVII transcription factors, which induces the synthesis of these proteins. Crucially, the difference between anoxia-intolerant and -tolerant tissues in the event of a severe energy crisis would mainly depend on changes in some 'key' energy crisis proteins: we suggest these proteins would include phytoglobin, the V-H+PPiase and pyruvate decarboxylase. A second characteristic of a high tolerance to an energy crisis is engagement of energy efficient transport. This feature includes a sharp reduction in rates of solute transport and use of energy-efficient modifications of transport systems by primary H+ transport and secondary H+-solute transport systems. Here we also discuss the best choice of species to study an energy crisis. Further, we consider confounding of the acclimative response by responses to injury, be it due to the use of tissues intolerant to an energy crisis, or to faulty techniques.

Finding the pond through the weeds: eDNA reveals underestimated diversity of pondweeds

PREMISE OF THE STUDY

The detection of environmental DNA (eDNA) using high-throughput sequencing has rapidly emerged as a method to detect organisms from environmental samples. However, eDNA studies of aquatic biomes have focused on surveillance of animal species with less emphasis on plants. Pondweeds are important bioindicators of freshwater ecosystems, although their diversity is underestimated due to difficulties in morphological identification and monitoring.

METHODS

A protocol was developed to detect pondweeds in water samples using atpB-rbcL and ITS2 markers. The water samples were collected from the Grand River within the rare Charitable Research Reserve, Ontario (RARE). Short fragments were amplified using primers targeting pondweeds, sequenced on an Ion Torrent Personal Genome Machine, and assigned to the taxonomy using a local DNA reference library and GenBank.

RESULTS

We detected two species earlier documented at the experimental site during ecological surveys (Potamogeton crispus and Stuckenia pectinata) and three species new to the RARE checklist (P. foliosus, S. filiformis, and Zannichellia palustris).

DISCUSSION

Our targeted approach to track the species composition of pondweeds in freshwater ecosystems revealed underestimation of their diversity. This result suggests that eDNA is an effective tool for monitoring plant diversity in aquatic habitats.

Aquatic and terrestrial morphotypes of the aquatic invasive plant, Ludwigia grandiflora, show distinct morphological and metabolomic responses

In the context of expansion of invasive species, survival of invasive plants is conditioned by their ability to adapt. In France, the water primrose Ludwigia grandiflora, an aquatic invasive species, invades yet wet meadows, leading to a depreciation of their fodder value. Understanding its potential adaption is necessary to its management, strong differences between both morphotypes were expected. So morphological and metabolic responses to terrestrial environment were analyzed for aquatic and terrestrial morphotypes. All morphological and biomass variables were greater in the terrestrial morphotype than the aquatic morphotype, independent of conditions. In terrestrial condition, both morphotypes showed a high production of sugars in root tissues, especially in the terrestrial morphotype and both morphotypes produced a low level of amino acids in shoot tissues. All results demonstrate that the terrestrial condition seems a stressful situation for both morphotypes, which activates glycolysis and fermentation pathways to improve their survival under hypoxic stress. But, only the terrestrial morphotype has been able to adjust its metabolism and maintain efficient growth. In the future, a differential transcriptomic analysis will be carried out to confirm this result.

Waterlogging and submergence stress: affects and acclimation

Submergence, whether partial or complete, imparts some serious consequences on plants grown in flood prone ecosystems. Some plants can endure these conditions by embracing various survival strategies, including morphological adaptations and physiological adjustments. This review summarizes recent progress made in understanding of the stress and the acclimation responses of plants under waterlogged or submerged conditions. Waterlogging and submergence are often associated with hypoxia development, which may trigger various morphological traits and cellular acclimation responses. Ethylene, abscisic acid, gibberellic acid and other hormones play a crucial role in the survival process which is controlled genetically. Effects at the cellular level, including ATP management, starch metabolism, elemental toxicity, role of transporters and redox status have been explained. Transcriptional and hormonal interplay during this stress may provide some key aspects in understanding waterlogging and submergence tolerance. The level and degree of tolerance may vary depending on species or climatic variations which need to be studied for a proper understanding of waterlogging stress at the global level. The exploration of regulatory pathways and interplay in model organisms such as Arabidopsis and rice would provide valuable resources for improvement of economically and agriculturally important plants in waterlogging affected areas.

Efficient use of energy in anoxia-tolerant plants with focus on germinating rice seedlings

Anoxia tolerance in plants is distinguished by direction of the sparse supply of energy to processes crucial to cell maintenance and sometimes to growth, as in rice seedlings. In anoxic rice coleoptiles energy is used to synthesise proteins, take up K(+) , synthesise cell walls and lipids, and in cell maintenance. Maintenance of electrochemical H(+) gradients across the tonoplast and plasma membrane is crucial for solute compartmentation and thus survival. These gradients sustain some H(+) -solute cotransport and regulate cytoplasmic pH. Pyrophosphate (PPi ), the alternative energy donor to ATP, allows direction of energy to the vacuolar H(+) -PPi ase, sustaining H(+) gradients across the tonoplast. When energy production is critically low, operation of a biochemical pHstat allows H(+) -solute cotransport across plasma membranes to continue for at least for 18 h. In active (e.g. growing) cells, PPi produced during substantial polymer synthesis allows conversion of PPi to ATP by PPi -phosphofructokinase (PFK). In quiescent cells with little polymer synthesis and associated PPi formation, the PPi required by the vacuolar H(+) -PPi ase and UDPG pyrophosphorylase involved in sucrose mobilisation via sucrose synthase might be produced by conversion of ATP to PPi through reversible glycolytic enzymes, presumably pyruvate orthophosphate dikinase. These hypotheses need testing with species characterised by contrasting anoxia tolerance.

The Transcriptome of Brassica napus L. Roots under Waterlogging at the Seedling Stage

Although rapeseed (Brassica napus L.) is known to be affected by waterlogging, the genetic basis of waterlogging tolerance by rapeseed is largely unknown. In this study, the transcriptome under 0 h and 12 h of waterlogging was assayed in the roots of ZS9, a tolerant variety, using digital gene expression (DGE). A total of 4432 differentially expressed genes were identified, indicating that the response to waterlogging in rapeseed is complicated. The assignments of the annotated genes based on GO (Gene Ontology) revealed there were more genes induced under waterlogging in “oxidation reduction”, “secondary metabolism”, “transcription regulation”, and “translation regulation”; suggesting these four pathways are enhanced under waterlogging. Analysis of the 200 most highly expressed genes illustrated that 144 under normal conditions were down-regulated by waterlogging, while up to 191 under waterlogging were those induced in response to stress. The expression of genes involved under waterlogging is mediated by multiple levels of transcriptional, post-transcriptional, translational and post-translational regulation, including phosphorylation and protein degradation; in particular, protein degradation might be involved in the negative regulation in response to this stress. Our results provide new insight into the response to waterlogging and will help to identify important candidate genes.

pH regulation in anoxic rice coleoptiles at pH 3.5: biochemical pHstats and net H+ influx in the absence and presence of NOFormula

During anoxia, cytoplasmic pH regulation is crucial. Mechanisms of pH regulation were studied in the coleoptile of rice exposed to anoxia and pH 3.5, resulting in H(+) influx. Germinating rice seedlings survived a combination of anoxia and exposure to pH 3.5 for at least 4 d, although development was retarded and net K(+) efflux was continuous. Further experiments used excised coleoptile tips (7-10 mm) in anoxia at pH 6.5 or 3.5, either without or with 0.2 mM NO(3)(-), which distinguished two processes involved in pH regulation. Net H(+) influx (μmol g(-1) fresh weight h(-1)) for coleoptiles with NO(3)(-) was ∼1.55 over the first 24 h, being about twice that in the absence of NO(3)(-), but then decreased to 0.5-0.9 as net NO(3)(-) uptake declined from ∼1.3 to 0.5, indicating reduced uptake via H(+)-NO(3)(-) symports. NO(3)(-) reduction presumably functioned as a biochemical pHstat. A second biochemical pHstat consisted of malate and succinate, and their concentrations decreased substantially with time after exposure to pH 3.5. In anoxic coleoptiles, K(+) balancing the organic anions was effluxed to the medium as organic anions declined, and this efflux rate was independent of NO(3)(-) supply. Thus, biochemical pHstats and reduced net H(+) influx across the plasma membrane are important features contributing to pH regulation in anoxia-tolerant rice coleoptiles at pH 3.5.

Recovery dynamics of growth, photosynthesis and carbohydrate accumulation after de-submergence: a comparison between two wetland plants showing escape and quiescence strategies

BACKGROUND AND AIMS

The capacity for fast-growth recovery after de-submergence is important for establishment of riparian species in a water-level-fluctuation zone. Recovery patterns of two wetland plants, Alternanthera philoxeroides and Hemarthria altissima, showing 'escape' and 'quiescence' responses, respectively, during submergence were investigated.

METHODS

Leaf and root growth and photosynthesis were monitored continuously during 10 d of recovery following 20 d of complete submergence. Above- and below-ground dry weights, as well as carbohydrate concentrations, were measured several times during the experiment.

KEY RESULTS

Both species remobilized stored carbohydrate during submergence. Although enhanced internode elongation depleted the carbohydrate storage in A. philoxeroides during submergence, this species resumed leaf growth 3 d after de-submergence concomitant with restoration of the maximal photosynthetic capacity. In contrast, some sucrose was conserved in shoots of H. altissima during submergence, which promoted rapid re-growth of leaves 2 d after de-submergence and earlier than the full recovery of photosynthesis. The recovery of root growth was delayed by 1-2 d compared with leaves in both species.

CONCLUSIONS

Submergence tolerance of the escape and quiescence strategies entails not only the corresponding regulation of growth, carbohydrate catabolism and energy metabolism during submergence but also co-ordinated recovery of photosynthesis, growth and carbohydrate partitioning following de-submergence.

Identification of flooding stress responsible cascades in root and hypocotyl of soybean using proteome analysis

Flooding inducible proteins were analyzed using a proteomic technique to understand the mechanism of soybean response to immersion in water. Soybeans were germinated for 2 days, and then subjected to flooding for 2 days. Proteins were extracted from root and hypocotyl, separated by two-dimensional polyacrylamide gel electrophoresis, stained by Coomassie brilliant blue, and analyzed by protein sequencing and mass spectrometry. Out of 803 proteins, 21 proteins were significantly up-regulated, and seven proteins were down-regulated by flooding stress. Of the total, 11 up-regulated proteins were classified as related to protein destination/storage and three proteins to energy, while four down-regulated proteins were related to protein destination/storage and three proteins to disease/defense. The expression of 22 proteins significantly changed within 1 day after flooding stress. The effects of flooding, nitrogen substitution without flooding, or flooding with aeration were analyzed for 1-4 days. The expression of alcohol dehydrogenase increased remarkably by nitrogen substitution compared to flooding. The expression of many proteins that changed due to flooding showed the same tendencies observed for nitrogen substitution; however, the expression of proteins classified into protein destination/storage did not.

Escape from water or remain quiescent? Lotus tenuis changes its strategy depending on depth of submergence

BACKGROUND AND AIMS

Two main strategies that allow plants to cope with soil waterlogging or deeper submergence are: (1) escaping by means of upward shoot elongation or (2) remaining quiescent underwater. This study investigates these strategies in Lotus tenuis, a forage legume of increasing importance in areas prone to soil waterlogging, shallow submergence or complete submergence.

METHODS

Plants of L. tenuis were subjected for 30 d to well-drained (control), waterlogged (water-saturated soil), partially submerged (6 cm water depth) and completely submerged conditions. Plant responses assessed were tissue porosity, shoot number and length, biomass and utilization of water-soluble carbohydrates (WSCs) and starch in the crown.

KEY RESULTS

Lotus tenuis adjusted its strategy depending on the depth of submergence. Root growth of partially submerged plants ceased and carbon allocation prioritized shoot lengthening (32 cm vs. 24.5 cm under other treatments), without depleting carbohydrate reserves to sustain the faster growth. These plants also developed more shoot and root porosity. In contrast, completely submerged plants became quiescent, with no associated biomass accumulation, new shoot production or shoot elongation. In addition, tissue porosity was not enhanced. The survival of completely submerged plants is attributed to consumption of WSCs and starch reserves from crowns (concentrations 50-75 % less than in other treatments).

CONCLUSIONS

The forage legume L. tenuis has the flexibility either to escape from partial submergence by elongating its shoot more vigorously to avoid becoming totally submerged or to adopt a non-elongating quiescent strategy when completely immersed that is based on utilizing stored reserves. The possession of these alternative survival strategies helps to explain the success of L. tenuis in environments subjected to unpredictable flooding depths.

Flooding tolerance: suites of plant traits in variable environments

Flooding regimes of different depths and durations impose selection pressures for various traits in terrestrial wetland plants. Suites of adaptive traits for different flooding stresses, such as soil waterlogging (short or long duration) and full submergence (short or long duration - shallow or deep), are reviewed. Synergies occur amongst traits for improved internal aeration, and those for anoxia tolerance and recovery, both for roots during soil waterlogging and shoots during submergence. Submergence tolerance of terrestrial species has recently been classified as either the Low Oxygen Quiescence Syndrome (LOQS) or the Low Oxygen Escape Syndrome (LOES), with advantages, respectively, in short duration or long duration (shallow) flood-prone environments. A major feature of species with the LOQS is that shoots do not elongate upon submergence, whereas those with the LOES show rapid shoot extension. In addition, plants faced with long duration deep submergence can demonstrate aspects of both syndromes; shoots do not elongate, but these are not quiescent, as new aquatic-type leaves are formed. Enhanced entries of O₂ and CO₂ from floodwaters into acclimated leaves, minimises O₂ deprivation and improves underwater photosynthesis, respectively. Evolution of 'suites of traits' are evident in wild wetland species and in rice, adapted to particular flooding regimes.

Response of cytoplasmic pH to anoxia in plant tissues with altered activities of fermentation enzymes: application of methyl phosphonate as an NMR pH probe

BACKGROUND AND AIMS

Acidification of the cytoplasm is a commonly observed response to oxygen deprivation in plant tissues that are intolerant of anoxia. The response was monitored in plant tissues with altered levels of lactate dehydrogenase (LDH) and pyruvate decarboxylase (PDC) with the aim of assessing the contribution of the targeted enzymes to cytoplasmic pH (pH(cyt)) regulation.

METHODS

The pH(cyt) was measured by in vivo (31)P nuclear magnetic resonance (NMR) spectroscopy using methyl phosphonate (MeP) as a pH probe. The potential toxicity of MeP was investigated by analysing its effect on the metabolism of radiolabelled glucose.

KEY RESULTS

MeP accumulated to detectable levels in the cytoplasm and vacuole of plant tissues exposed to millimolar concentrations of MeP, and the pH-dependent (31)P NMR signals provided a convenient method for measuring pH(cyt) values in tissues with poorly defined signals from the cytoplasmic inorganic phosphate pool. Pretreatment of potato (Solanum tuberosum) tuber slices with 5 mm MeP for 24 h did not affect the metabolism of [U-(14)C]glucose or the pattern of (14)CO(2) release from specifically labelled [(14)C]-substrates. Time-courses of pH(cyt) measured before, during and after an anoxic episode in potato tuber tissues with reduced activities of LDH, or in tobacco (Nicotiana tabacum) leaves with increased activities of PDC, were indistinguishable from their respective controls.

CONCLUSIONS

MeP can be used as a low toxicity (31)P NMR probe for measuring intracellular pH values in plant tissues with altered levels of fermentation enzymes. The measurements on transgenic tobacco leaves suggest that the changes in pH(cyt) during an anoxic episode are not dominated by fermentation processes; while the pH changes in the potato tuber tissue with reduced LDH activity show that the affected isozymes do not influence the anoxic pH response.

Selective mRNA translation coordinates energetic and metabolic adjustments to cellular oxygen deprivation and reoxygenation in Arabidopsis thaliana

Cellular oxygen deprivation (hypoxia/anoxia) requires an acclimation response that enables survival during an energy crisis. To gain new insights into the processes that facilitate the endurance of transient oxygen deprivation, the dynamics of the mRNA translation state and metabolites were quantitatively monitored in Arabidopsis thaliana seedlings exposed to a short (2 h) or prolonged (9 h) period of oxygen and carbon dioxide deprivation and following 1 h of re-aeration. Hypoxia stress and reoxygenation promoted adjustments in the levels of polyribosomes (polysomes) that were highly coordinated with cellular ATP content. A quantitative comparison of steady-state and polysomal mRNA populations revealed that over half of the cellular mRNAs were restricted from polysome complexes during the stress, with little or no change in abundance. This selective repression of translation was rapidly reversed upon reoxygenation. Comparison of the adjustment in gene transcripts and metabolites demonstrated that profiling of polysomal mRNAs strongly augments the prediction of cellular processes that are altered during cellular oxygen deprivation. The selective translation of a subset of mRNAs promotes the conservation of ATP and facilitates the transition to anaerobic metabolism during low-oxygen stress.

Does anoxia tolerance involve altering the energy currency towards PPi?

Oxygen deficit is an important abiotic stress influencing plants, because this condition results in an 'energy crisis'. Most species only survive short periods of anoxia, but several wetland species tolerate prolonged anoxia. Transcriptomic and proteomic studies, using anoxia-tolerant rice and anoxia-intolerant Arabidopsis, have provided evidence for the selective adoption of pyrophosphate (PPi) over ATP as high-energy donor molecules, which may contribute to anoxia tolerance. The use of PPi in some tolerant plant species is similar to that observed in many anaerobic prokaryotes. Investigations are being performed to better understand the origin and regulation of reversible PPi-dependent glycolytic enzymes such as cytosolic pyruvate phosphate dikinase, as well as PPi-consuming enzymes, which are engaged during the anoxic energy crisis. This will be crucial in unraveling this currency switch and its contribution to anoxia tolerance.

Flooding stress: acclimations and genetic diversity

Flooding is an environmental stress for many natural and man-made ecosystems worldwide. Genetic diversity in the plant response to flooding includes alterations in architecture, metabolism, and elongation growth associated with a low O(2) escape strategy and an antithetical quiescence scheme that allows endurance of prolonged submergence. Flooding is frequently accompanied with a reduction of cellular O(2) content that is particularly severe when photosynthesis is limited or absent. This necessitates the production of ATP and regeneration of NAD(+) through anaerobic respiration. The examination of gene regulation and function in model systems provides insight into low-O(2)-sensing mechanisms and metabolic adjustments associated with controlled use of carbohydrate and ATP. At the developmental level, plants can escape the low-O(2) stress caused by flooding through multifaceted alterations in cellular and organ structure that promote access to and diffusion of O(2). These processes are driven by phytohormones, including ethylene, gibberellin, and abscisic acid. This exploration of natural variation in strategies that improve O(2) and carbohydrate status during flooding provides valuable resources for the improvement of crop endurance of an environmental adversity that is enhanced by global warming.

Anoxia-enhanced expression of genes isolated by suppression subtractive hybridization from pondweed (Potamogeton distinctus A. Benn.) turions

Pondweed (Potamogeton distinctus A. Benn.), a monocot aquatic plant species, has turions, which are overwintering buds forming underground as an asexual reproductive organ. Turions not only survive for more than one month but also elongate under strict anoxia, maintaining high-energy charge by activation of fermentation. We cloned 82 cDNA fragments of genes, that are up-regulated during anoxic growth of pondweed turions, by suppression subtractive hybridization. The transcript levels of 44 genes were confirmed to be higher under anoxia than those in air by both Northern blot analysis and a semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) method. A homology search for their nucleotide sequences revealed that some of them are highly homologous to known sequences of genes from other plants. They included alcohol dehydrogenase, pyruvate decarboxylase (PDC), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), vacuolar H(+)-translocating pyrophosphatase and a plasma membrane intrinsic protein. Time courses of transcript accumulation of some genes under anoxia were different from those in air. The activity of PDC increased under anoxic conditions but the activities of GAPDH and pyrophosphatase remained constant after anoxic treatment. Anoxically up-regulated genes are possibly involved in physiological events to control energy production, pH regulation and cell growth under anoxia. These results suggest that transcriptional regulation of these genes serves as an essential part of survival and growth of pondweed turions under anoxia.

Adaptative response of Vitis root to anoxia

The effect of anoxia on the energy economy of root cells was studied by measuring heat production, ethanol and ATP production, K(+) fluxes and electrical activity in two Vitis species, V. riparia and V. rupestris, that differ in their tolerance to anoxia. Anoxia triggered a marked decrease of metabolic activity (measured by microcalorimetry) and of ATP levels in both species. In V. riparia after the first 2 h of anoxia, the decrease in the rate of heat production was not associated with a further significant decrease in ATP content, whereas in V. rupestris the ATP level continued to decrease until very low values were reached. The concomitant increase in the rate of ethanol production did not compensate for the decreased aerobic ATP supply. In V. rupestris, anoxia typically led to energy deficit and ATP imbalance, together with the subsequent disruption of ion homeostasis and cell death. In V. riparia, the strong decrease in K(+) membrane permeability together with the fast down-regulation of the electrical signals allowed the cells to avoid severe ion imbalances during prolonged anoxic episodes.