The crucial role of plant mitochondria in orchestrating drought tolerance

How do trees die? A test of the hydraulic failure and carbon starvation hypotheses

Despite decades of research on plant drought tolerance, the physiological mechanisms by which trees succumb to drought are still under debate. We report results from an experiment designed to separate and test the current leading hypotheses of tree mortality. We show that piñon pine (Pinus edulis) trees can die of both hydraulic failure and carbon starvation, and that during drought, the loss of conductivity and carbohydrate reserves can also co-occur. Hydraulic constraints on plant carbohydrate use determined survival time: turgor loss in the phloem limited access to carbohydrate reserves, but hydraulic control of respiration prolonged survival. Our data also demonstrate that hydraulic failure may be associated with loss of adequate tissue carbohydrate content required for osmoregulation, which then promotes failure to maintain hydraulic integrity.

Regulation of the calcium-sensing receptor in both stomatal movement and photosynthetic electron transport is crucial for water use efficiency and drought tolerance in Arabidopsis

Production per amount of water used (water use efficiency, WUE) is closely correlated with drought tolerance. Although stomatal aperture can regulate WUE, the underlying molecular mechanisms are still unclear. Previous reports revealed that stomatal closure was inhibited in the calcium-sensing receptor (CAS) antisense line of Arabidopsis (CASas). Here it is shown that decreased drought tolerance and WUE of CASas was associated with higher stomatal conductance due to improper regulation of stomatal aperture, rather than any change of stomatal density. CASas plants also had a lower CO2 assimilation rate that was attributed to a lower photosynthetic electron transport rate, leading to higher chlorophyll fluorescence. Gene co-expression combined with analyses of chlorophyll content and transcription levels of photosynthesis-related genes indicate that CAS is involved in the formation of the photosynthetic electron transport system. These data suggest that CAS regulates transpiration and optimizes photosynthesis by playing important roles in stomatal movement and formation of photosynthetic electron transport, thereby regulating WUE and drought tolerance.

A physiological and proteomic study of poplar leaves during ozone exposure combined with mild drought

The occurrence of high-ozone concentrations during drought episodes is common considering that they are partially caused by the same meteorological phenomena. It was suggested that mild drought could protect plants against ozone-induced damage by causing the closure of stomata and preventing the entry of ozone into the leaves. The present experiment attempts to create an overview of the changes in cellular processes in response to ozone, mild drought and a combined treatment based on the use of 2D-DiGE to compare the involved proteins, and a number of supporting analyses. Morphological symptoms were worst in the combined treatment, indicating a severe stress, but fewer proteins were differentially abundant in the combined treatment than for ozone alone. Stomatal conductance was slightly lowered in the combined treatment. Shifts in carbon metabolism indicated that the metabolism changed to accommodate for protective measures and changes in the abundance of proteins involved in redox protection indicated the presence of an oxidative stress. This study allowed identifying a set of proteins that changed similarly during ozone and drought stress, indicative of crosstalk in the molecular response of plants exposed to these stresses. The abundance of other key proteins changed only when the plants are exposed to specific conditions. Together this indicates the coexistence of generalized and specialized responses to different conditions.

Morphological, anatomical and physiological leaf traits of Q. ilex, P. latifolia, P. lentiscus, and M. communis and their response to Mediterranean climate stress factors

BACKGROUND

Limitations to plant growth imposed by the Mediterranean climate are mainly due to carbon balance in response to stress factors. In particular, water stress associated to high air temperature and irradiance in summer causes a marked decrease in CO2 assimilation. Air temperature sensitivity of photosynthesis (PN) differs from that of leaf respiration (RD). PN often decreases sharply at temperature above its optimum while RD increases exponentially over short term rises in temperature. Nevertheless, the impact of water deficit on RD is still far from clear with reports in literature including decreases, maintenance or increases in its rates. The ratio RD/PN can be considered a simple approach to leaf carbon balance because it indicates the percentage of photosynthates that is respired.

RESULTS

The results underline different morphological, anatomical and physiological traits of the evergreen species co-occurring in the Mediterranean maquis which are indicative of their adaptive capability to Mediterranean stress factors. The ratio RD/PN varies from 0.15 ± 0.04 in autumn, 0.24 ± 0.05 in spring through 0.29 ± 0.15 in winter to 0.46 ± 0.11 in summer. The lower RD/PN in autumn and spring underlines the highest PN rates during the favorable periods when resources are not limited and leaves take in roughly three to five times more CO2 than they lose by respiration. On the contrary, the highest RD/PN ratio in summer underlines the lowest sensitivity of respiration to drought. Among the considered species, Quercus ilex and Pistacia lentiscus have the largest tolerance to low winter temperatures while Phillyrea latifolia and Myrtus communis to drought, and Phillyrea latifolia the highest recovery capability after the first rainfall following drought.

CONCLUSIONS

The Mediterranean evergreen specie shows a different tolerance to Mediterranean climate stress factors. The predicted global warming might differently affect carbon balance of the considered species, with a possible change in Mediterranean shrublands composition in the long-term. Understanding the carbon balance of plants in water limited environments is crucial in order to make informed land management decisions. Moreover, our results underline the importance of including seasonal variations of photosynthesis and respiration in carbon balance models.

Peroxidation due to cryoprotectant treatment is a vital factor for cell survival in Arabidopsis cryopreservation

Cryopreservation can be a safe and cost-effective tool for the long-term storage of plant germplasm. In Arabidopsis, the ability to recover from cryogenic treatment was lost as growth progressed. Growth could be restored in 48-h seedlings, whereas 72-h seedlings died after cryogenic treatment. Why seedling age and survival are negatively correlated is an interesting issue. A comparative transcriptomics was performed to screen differentially expressed genes between 48- and 72-h seedlings after exposure to cryoprotectant. Among differentially expressed genes, oxidative stress response genes played important roles in cryoprotectant treatment, and peroxidation was a key factor related to cell survival. Seedlings underwent more peroxidation at 72-h than at 48-h. A comprehensive analysis indicated that peroxidation injured membrane systems leading to photophosphorylation and oxidative phosphorylation damage. Furthermore, the apoptosis-like events were found in cryogenic treatment of Arabidopsis seedlings. 48- and 72-h seedlings underwent different degrees of membrane lipid peroxidation during cryoprotectant treatment, and reducing the injury of oxidative stress was an important factor to successful cryopreservation. This study provided a novel insight of genetic regulatory mechanisms in cryopreservation, and established an excellent model to test and evaluate the effect of exogenous antioxidants and conventional cryoprotectants in plant cryopreservation.

Genome-wide transcriptional analysis of two soybean genotypes under dehydration and rehydration conditions

BACKGROUND

Soybean is an important crop that provides valuable proteins and oils for human use. Because soybean growth and development is extremely sensitive to water deficit, quality and crop yields are severely impacted by drought stress. In the face of limited water resources, drought-responsive genes are therefore of interest. Identification and analysis of dehydration- and rehydration-inducible differentially expressed genes (DEGs) would not only aid elucidation of molecular mechanisms of stress response, but also enable improvement of crop stress tolerance via gene transfer. Using Digital Gene Expression Tag profiling (DGE), a new technique based on Illumina sequencing, we analyzed expression profiles between two soybean genotypes to identify drought-responsive genes.

RESULTS

Two soybean genotypes - drought-tolerant Jindou21 and drought-sensitive Zhongdou33 - were subjected to dehydration and rehydration conditions. For analysis of DEGs under dehydration conditions, 20 cDNA libraries were generated from roots and leaves at two different time points under well-watered and dehydration conditions. We also generated eight libraries for analysis under rehydration conditions. Sequencing of the 28 libraries produced 25,000-33,000 unambiguous tags, which were mapped to reference sequences for annotation of expressed genes. Many genes exhibited significant expression differences among the libraries. DEGs in the drought-tolerant genotype were identified by comparison of DEGs among treatments and genotypes. In Jindou21, 518 and 614 genes were differentially expressed under dehydration in leaves and roots, respectively, with 24 identified both in leaves and roots. The main functional categories enriched in these DEGs were metabolic process, response to stresses, plant hormone signal transduction, protein processing, and plant-pathogen interaction pathway; the associated genes primarily encoded transcription factors, protein kinases, and other regulatory proteins. The seven most significantly expressed (|log2 ratio| ≥ 8) genes - Glyma15g03920, Glyma05g02470, Glyma15g15010, Glyma05g09070, Glyma06g35630, Glyma08g12590, and Glyma11g16000 - are more likely to determine drought stress tolerance. The expression patterns of eight randomly-selected genes were confirmed by quantitative RT-PCR; the results of QRT-PCR analysis agreed with transcriptional profile data for 96 out of 128 (75%) data points.

CONCLUSIONS

Many soybean genes were differentially expressed between drought-tolerant and drought-sensitive genotypes. Based on GO functional annotation and pathway enrichment analysis, some of these genes encoded transcription factors, protein kinases, and other regulatory proteins. The seven most significant DEGs are candidates for improving soybean drought tolerance. These findings will be helpful for analysis and elucidation of molecular mechanisms of drought tolerance; they also provide a basis for cultivating new varieties of drought-tolerant soybean.

Effect of environmental variables and stand structure on ecosystem respiration components in a Mediterranean beech forest

The temporal variability of ecosystem respiration (RECO) has been reported to have important effects on the temporal variability of net ecosystem exchange, the net amount of carbon exchanged between an ecosystem and the atmosphere. However, our understanding of ecosystem respiration is rather limited compared with photosynthesis or gross primary productivity, particularly in Mediterranean montane ecosystems. In order to investigate how environmental variables and forest structure (tree classes) affect different respiration components and RECO in a Mediterranean beech forest, we measured soil, stem and leaf CO2 efflux rates with dynamic chambers and RECO by the eddy-covariance technique over 1 year (2007-2008). Ecosystem respiration showed marked seasonal variation, with the highest rates in spring and autumn and the lowest in summer. We found that the soil respiration (SR) was mainly controlled by soil water content below a threshold value of 0.2 m(3) m(-3), above which the soil temperature explained temporal variation in SR. Stem CO2 effluxes were influenced by air temperature and difference between tree classes with higher rates measured in dominant trees than in co-dominant ones. Leaf respiration (LR) varied significantly between the two canopy layers considered. Non-structural carbohydrates were a very good predictor of LR variability. We used these measurements to scale up respiration components to ecosystem respiration for the whole canopy and obtained cumulative amounts of carbon losses over the year. Based on the up-scaled chamber measurements, the relative contributions of soil, stem and leaves to the total annual CO2 efflux were: 56, 8 and 36%, respectively. These results confirm that SR is the main contributor of ecosystem respiration and provided an insight on the driving factors of respiration in Mediterranean montane beech forests.

Elucidating rice cell metabolism under flooding and drought stresses using flux-based modeling and analysis

Rice (Oryza sativa) is one of the major food crops in world agriculture, especially in Asia. However, the possibility of subsequent occurrence of flood and drought is a major constraint to its production. Thus, the unique behavior of rice toward flooding and drought stresses has required special attention to understand its metabolic adaptations. However, despite several decades of research investigations, the cellular metabolism of rice remains largely unclear. In this study, in order to elucidate the physiological characteristics in response to such abiotic stresses, we reconstructed what is to our knowledge the first metabolic/regulatory network model of rice, representing two tissue types: germinating seeds and photorespiring leaves. The phenotypic behavior and metabolic states simulated by the model are highly consistent with our suspension culture experiments as well as previous reports. The in silico simulation results of seed-derived rice cells indicated (1) the characteristic metabolic utilization of glycolysis and ethanolic fermentation based on oxygen availability and (2) the efficient sucrose breakdown through sucrose synthase instead of invertase. Similarly, flux analysis on photorespiring leaf cells elucidated the crucial role of plastid-cytosol and mitochondrion-cytosol malate transporters in recycling the ammonia liberated during photorespiration and in exporting the excess redox cofactors, respectively. The model simulations also unraveled the essential role of mitochondrial respiration during drought stress. In the future, the combination of experimental and in silico analyses can serve as a promising approach to understand the complex metabolism of rice and potentially help in identifying engineering targets for improving its productivity as well as enabling stress tolerance.

High-resolution temperature responses of leaf respiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function

We tested whether snow gum (Eucalyptus pauciflora) trees growing in thermally contrasting environments exhibit generalizable temperature (T) response functions of leaf respiration (R) and fluorescence (Fo). Measurements were made on pot-grown saplings and field-grown trees (growing between 1380 and 2110 m a.s.l.). Using a continuous, high-resolution protocol, we quantified T response curves of R and Fo--these data were used to identify an algorithm for modelling R-T curves at subcritical T's and establish variations in heat tolerance. For the latter, we quantified Tmax [T where R is maximal] and Tcrit [T where Fo rises rapidly]. Tmax ranged from 51 to 57 °C, varying with season (e.g. winter  summer). Tcrit ranged from 41 to 49 °C in summer and from 58 to 63 °C in winter. Thus, surprisingly, leaf energy metabolism was more heat-tolerant in trees experiencing ice-encasement in winter than warmer conditions in summer. A polynomial model fitted to log-transformed R data provided the best description of the T-sensitivity of R (between 10 and 45 °C); using these model fits, we found that the negative slope of the Q10 -T relationship was greater in winter than in summer. Collectively, our results (1) highlight high-T limits of energy metabolism in E. pauciflora and (2) provide a framework for improving representation of T-responses of leaf R in predictive models.

In the mitochondrial CMSII mutant of Nicotiana sylvestris photosynthetic activity remains higher than in the WT under persisting mild water stress

Photosynthetic responses to persisting mild water stress were compared between the wild type (WT) and the respiratory complex I mutant CMSII of Nicotiana sylvestris. In both genotypes, plants kept at 80% leaf-RWC (WT80 and CMSII80) had lower photosynthetic activity and stomatal/mesophyll conductances compared to well-watered controls. While the stomatal conductance and the chloroplastic CO2 molar ratio were similar in WT80 and CMSII80 leaves, net photosynthesis was higher in CMSII80. Carboxylation efficiency was lowest in WT80 leaves both, on the basis of the same internal and chloroplastic CO2 molar ratio. Photosynthetic and fluorescence parameters indicate that WT80 leaves were only affected in the presence of oxygen. Photorespiration, as estimated by electron flux to oxygen, increased slightly in CMSII80 and WT80 leaves in accordance with increased glycerate contents but maximum photorespiration at low chloroplastic CO2 was markedly lowest in WT80 leaves. This suggests that carbon assimilation of WT80 leaves is impaired by limited photorespiratory activity. The results are discussed with respect to a possible pre-acclimation of complex I deficient leaves in CMSII to drive photosynthesis and photorespiration at low CO2 partial pressure.

Alternative oxidase: a mitochondrial respiratory pathway to maintain metabolic and signaling homeostasis during abiotic and biotic stress in plants

Alternative oxidase (AOX) is a non-energy conserving terminal oxidase in the plant mitochondrial electron transport chain. While respiratory carbon oxidation pathways, electron transport, and ATP turnover are tightly coupled processes, AOX provides a means to relax this coupling, thus providing a degree of metabolic homeostasis to carbon and energy metabolism. Beside their role in primary metabolism, plant mitochondria also act as "signaling organelles", able to influence processes such as nuclear gene expression. AOX activity can control the level of potential mitochondrial signaling molecules such as superoxide, nitric oxide and important redox couples. In this way, AOX also provides a degree of signaling homeostasis to the organelle. Evidence suggests that AOX function in metabolic and signaling homeostasis is particularly important during stress. These include abiotic stresses such as low temperature, drought, and nutrient deficiency, as well as biotic stresses such as bacterial infection. This review provides an introduction to the genetic and biochemical control of AOX respiration, as well as providing generalized examples of how AOX activity can provide metabolic and signaling homeostasis. This review also examines abiotic and biotic stresses in which AOX respiration has been critically evaluated, and considers the overall role of AOX in growth and stress tolerance.

Biochemistry, proteomics, and phosphoproteomics of plant mitochondria from non-photosynthetic cells

Mitochondria fulfill some basic roles in all plant cells. They supply the cell with energy in the form of ATP and reducing equivalents [NAD(P)H] and they provide the cell with intermediates for a range of biosynthetic pathways. In addition to this, mitochondria contribute to a number of specialized functions depending on the tissue and cell type, as well as environmental conditions. We will here review the biochemistry and proteomics of mitochondria from non-green cells and organs, which differ from those of photosynthetic organs in a number of respects. We will briefly cover purification of mitochondria and general biochemical properties such as oxidative phosphorylation. We will then mention a few adaptive properties in response to water stress, seed maturation and germination, and the ability to function under hypoxic conditions. The discussion will mainly focus on Arabidopsis cell cultures, etiolated germinating rice seedlings and potato tubers as model plants. It will cover the general proteome as well as the posttranslational modification protein phosphorylation. To date 64 phosphorylated mitochondrial proteins with a total of 103 phosphorylation sites have been identified.

Increased susceptibility to drought-induced mortality in Sequoia sempervirens (Cupressaceae) trees under Cenozoic atmospheric carbon dioxide starvation

PREMISE OF THE STUDY

Climate-induced forest retreat has profound ecological and biogeochemical impacts, but the physiological mechanisms underlying past tree mortality are poorly understood, limiting prediction of vegetation shifts with climate variation. Climate, drought, fire, and grazing represent agents of tree mortality during the late Cenozoic, but the interaction between drought and declining atmospheric carbon dioxide ([CO2]a) from high to near-starvation levels ∼34 million years (Ma) ago has been overlooked. Here, this interaction frames our investigation of sapling mortality through the interdependence of hydraulic function, carbon limitation, and defense metabolism. •

METHODS

We recreated a changing Cenozoic [CO2]a regime by growing Sequoia sempervirens trees within climate-controlled growth chambers at 1500, 500, or 200 ppm [CO2]a, capturing the decline toward minimum concentrations from 34 Ma. After 7 months, we imposed drought conditions and measured key physiological components linking carbon utilization, hydraulics, and defense metabolism as hypothesized interdependent mechanisms of tree mortality. •

KEY RESULTS

Catastrophic failure of hydraulic conductivity, carbohydrate starvation, and tree death occurred at 200 ppm, but not 500 or 1500 ppm [CO2]a. Furthermore, declining [CO2]a reduced investment in carbon-rich foliar defense compounds that would diminish resistance to biotic attack, likely exacerbating mortality. •

CONCLUSIONS

Low-[CO2]a-driven tree mortality under drought is consistent with Pleistocene pollen records charting repeated Californian Sequoia forest contraction during glacial periods (180-200 ppm [CO2]a) and may even have contributed to forest retreat as grasslands expanded on multiple continents under low [CO2]a over the past 10 Ma. In this way, geologic intervals of low [CO2]a coupled with drought could impose a demographic bottleneck in tree recruitment, driving vegetation shifts through forest mortality.

Responses of the Mediterranean seagrass Posidonia oceanica to hypersaline stress duration and recovery

We studied the hypersaline stress responses of the Mediterranean seagrass Posidonia oceanica to determine if the species was tolerant to salinity increases that occur in coastal waters by the desalination industry. Water relations, amino acids, carbohydrates, ions, photosynthesis, respiration, chlorophyll a fluorescence, leaf growth and morphology, and plant mortality were analysed after exposing the mesocosm P. oceanica to a salinity level of 43 for one and three months followed by a month for recovery. One-month saline-stressed plants exhibited sub-lethal effects, including a leaf cell turgor pressure reduction, loss of ionic equilibrium and decreased leaf growth. There were also changes in photoprotective mechanisms, increased concentrations of organic osmolytes in leaves and reduced leaf ageing. All these dysfunctions recovered after removing the stress. After the longer exposure of three months, stress symptoms were much more acute and plants showed an excessive ionic exclusion capacity, increased leaf cell turgor, reduced plant carbon balance, increased leaf aging and leaf decay and increased plant mortality, which indicated that the plant had entered a stage of severe physiological stress. In addition, the long-term saline-stressed plants were not able to recover, still showing sustained injury after the one-month recovery period as reflected by unbalanced leaf ionic content, persistently impaired photosynthesis, decline in internal carbon resources and decreased leaf growth that resulted in undersized plants. In conclusion, P. oceanica was not able to acclimate to the saline conditions tested since it could not reach a new physiological equilibrium or recover after a chronic exposure of 3 months.

Modulation of bud survival in Populus nigra sprouts in response to water stress-induced embolism

Understanding drought tolerance mechanisms requires knowledge about the induced weakness that leads to tree death. Bud survival is vital to sustain tree growth across seasons. We hypothesized that the hydraulic connection of the bud to stem xylem structures was critical for its survival. During an artificial drastic water stress, we carried out a census of bud metabolic activity of young Populus nigra L. trees by microcalorimetry. We monitored transcript expression of aquaporins (AQPs; plasma membrane intrinsic proteins (PIPs), X intrinsic proteins (XIPs) and tonoplast membrane intrinsic proteins (TIPs)) and measured local water status within the bud and tissues in the bearer shoot node by nuclear magnetic resonance (NMR) imaging. We found that the bud respiration rate was closely correlated with its water content and decreased concomitantly in buds and their surrounding bearer tissues. At the molecular level, we observed a modulation of AQP pattern expressions (PIP, TIP and XIP subfamilies) linked to water movements in living cells. However, AQP functions remain to be investigated. Both the bud and tree died beyond a threshold water content and respiration rate. Nuclear magnetic resonance images provided relevant local information about the various water reservoirs of the stem, their dynamics and their interconnections. Comparison of pith, xylem and cambium tissues revealed that the hydraulic connection between the bud and saturated parenchyma cells around the pith allowed bud desiccation to be delayed. At the tree death date, NMR images showed that the cambium tissues remained largely hydrated. Overall, the respiration rate (Rco2) and a few AQP isoforms were found to be two suitable, complementary criteria to assess the bud metabolic activity and the ability to survive a severe drought spell. Bud moisture content could be a key factor in determining the capacity of poplar to recover from water stress.

Acclimation of foliar respiration and photosynthesis in response to experimental warming in a temperate steppe in northern China

BACKGROUND

Thermal acclimation of foliar respiration and photosynthesis is critical for projection of changes in carbon exchange of terrestrial ecosystems under global warming.

METHODOLOGY/PRINCIPAL FINDINGS

A field manipulative experiment was conducted to elevate foliar temperature (Tleaf) by 2.07°C in a temperate steppe in northern China. Rd/Tleaf curves (responses of dark respiration to Tleaf), An/Tleaf curves (responses of light-saturated net CO2 assimilation rates to Tleaf), responses of biochemical limitations and diffusion limitations in gross CO2 assimilation rates (Ag) to Tleaf, and foliar nitrogen (N) concentration in Stipa krylovii Roshev. were measured in 2010 (a dry year) and 2011 (a wet year). Significant thermal acclimation of Rd to 6-year experimental warming was found. However, An had a limited ability to acclimate to a warmer climate regime. Thermal acclimation of Rd was associated with not only the direct effects of warming, but also the changes in foliar N concentration induced by warming.

CONCLUSIONS/SIGNIFICANCE

Warming decreased the temperature sensitivity (Q10) of the response of Rd/Ag ratio to Tleaf. Our findings may have important implications for improving ecosystem models in simulating carbon cycles and advancing understanding on the interactions between climate change and ecosystem functions.

Drought response strategies define the relative contributions of hydraulic dysfunction and carbohydrate depletion during tree mortality

Plant survival during drought requires adequate hydration in living tissues and carbohydrate reserves for maintenance and recovery. We hypothesized that tree growth and hydraulic strategy determines the intensity and duration of the 'physiological drought', thereby affecting the relative contributions of loss of hydraulic function and carbohydrate depletion during mortality. We compared patterns in growth rate, water relations, gas exchange and carbohydrate dynamics in three tree species subjected to prolonged drought. Two Eucalyptus species (E. globulus, E. smithii) exhibited high growth rates and water-use resulting in rapid declines in water status and hydraulic conductance. In contrast, conservative growth and water relations in Pinus radiata resulted in longer periods of negative carbon balance and significant depletion of stored carbohydrates in all organs. The ongoing demand for carbohydrates from sustained respiration highlighted the role that duration of drought plays in facilitating carbohydrate consumption. Two drought strategies were revealed, differentiated by plant regulation of water status: plants maximized gas exchange, but were exposed to low water potentials and rapid hydraulic dysfunction; and tight regulation of gas exchange at the cost of carbohydrate depletion. These findings provide evidence for a relationship between hydraulic regulation of water status and carbohydrate depletion during terminal drought.

The existence of phospholipase A(2) activity in plant mitochondria and its activation by hyperosmotic stress in durum wheat (Triticum durum Desf.)

The activity of mitochondrial phospholipase A(2) (PLA(2)) was shown for the first time in plants. It was observed in etiolated seedlings from durum wheat, barley, tomato, spelt and green seedlings of maize, but not in potato and topinambur tubers and lentil etiolated seedlings. This result was achieved by a novel spectrophotometric assay based on the coupled PLA(2)/lipoxygenase reactions using 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine as substrate; the mitochondrial localisation was assessed by checking recovery of marker enzymes. Durum wheat mitochondrial PLA(2) (DWM-PLA(2)) showed maximal activity at pH 9.0 and 1mM Ca(2+), hyperbolic kinetics (K(m)=90±6μM, V(max)=29±1nmolmin(-1)mg(-1) of protein) and inhibition by methyl arachidonyl fluorophosphonate, 5-(4-benzyloxyphenyl)-4S-(7-phenylheptanoylamino)pentanoic acid and palmityl trifluoromethyl ketone. Reactive oxygen species had no effect on DWM-PLA(2), that instead was activated by about 50% and 95%, respectively, under salt (0.21M NaCl) and osmotic (0.42M mannitol) stress imposed during germination. Contrarily, a secondary Ca(2+)-independent activity, having optimum at pH 7.0, was stress-insensitive. We propose that the activation of DWM-PLA(2) is responsible for the strong increase of free fatty acids recently measured in mitochondria under the same stress conditions [Laus, et al., J. Exp. Bot. 62 (2011) 141-154] that, in turn, activate potassium channel and uncoupling protein, able to counteract hyperosmotic stress.

The β-cyanoalanine pathway is involved in the response to water deficit in Arabidopsis thaliana

The β-cyanoalanine pathway is primarily responsible for detoxification of excess cyanide produced by plants. Recent evidence suggests that cyanide detoxification via this pathway may be involved in the response and tolerance to water deficit in plants. The aim of this study was to explore this role in Arabidopsis thaliana in greater detail. The first objective was to establish responsiveness of the pathway to the magnitude and duration of water deficit. The second objective was to examine how interruption of single genes (AtCysA1, AtCysC1 and AtNIT4) encoding enzymes of the pathway influenced the ability to metabolize cyanide and withstand water deficit. Arabidopsis plants were exposed to conditions which emulated acute and chronic water deficit, followed by measurement of tissue cyanide concentration, activity of enzymes, and physiological parameters. The results for wild-type Arabidopsis demonstrated a transient increase in cyanide concentration and β-cyanoalanine synthase activity, followed by a decrease in both. The increase in enzyme activity was localized to the tissue in direct proximity to the stress. The knockdown AtCysA1 mutant did not differ from wild-type while AtCysC1 mutants were slightly more sensitive to water deficit. The AtNIT4 mutant was the most sensitive showing decreased growth along with altered chlorophyll content under water deficit as compared to wild-type. Collectively, the results indicated that the pathway is responsive to water deficit although the severity of stress did not alter the nature of the response, implying that the capacity to remove cyanide generated during water deficit may contribute to tolerance to this stress in Arabidopsis.

Late embryogenesis abundant proteins protect human hepatoma cells during acute desiccation

Expression of late embryogenesis abundant (LEA) proteins is highly correlated with desiccation tolerance in anhydrobiotic animals, selected land plants, and bacteria. Genes encoding two LEA proteins, one localized to the cytoplasm/nucleus (AfrLEA2) and one targeted to mitochondria (AfrLEA3m), were stably transfected into human HepG2 cells. A trehalose transporter was used for intracellular loading of this disaccharide. Cells were rapidly and uniformly desiccated to low water content (<0.12 g H(2)O/g dry weight) with a recently developed spin-drying technique. Immediately on rehydration, control cells without LEA proteins or trehalose exhibited 0% membrane integrity, compared with 98% in cells loaded with trehalose and expressing AfrLEA2 or AfrLEA3m; surprisingly, AfrLEA3m without trehalose conferred 94% protection. Cell proliferation across 7 d showed an 18-fold increase for cells dried with AfrLEA3m and trehalose, compared with 27-fold for nondried controls. LEA proteins dramatically enhance desiccation tolerance in mammalian cells and offer the opportunity for engineering biostability in the dried state.

Metabolic profiling reveals metabolic shifts in Arabidopsis plants grown under different light conditions

Plants have tremendous capacity to adjust their morphology, physiology and metabolism in response to changes in growing conditions. Thus, analysis solely of plants grown under constant conditions may give partial or misleading indications of their responses to the fluctuating natural conditions in which they evolved. To obtain data on growth condition-dependent differences in metabolite levels, we compared leaf metabolite profiles of Arabidopsis thaliana growing under three constant laboratory light conditions: 30 [low light (LL)], 300 [normal light (NL)] and 600 [high light (HL)]µmol photons m(-2) s(-1). We also shifted plants to the field and followed their metabolite composition for 3 d. Numerous compounds showed light intensity-dependent accumulation, including: many sugars and sugar derivatives (fructose, sucrose, glucose, galactose and raffinose); tricarboxylic acid (TCA) cycle intermediates; and amino acids (ca. 30% of which were more abundant under HL and 60% under LL). However, the patterns differed after shifting NL plants to field conditions. Levels of most identified metabolites (mainly amino acids, sugars and TCA cycle intermediates) rose after 2 h and peaked after 73 h, indicative of a 'biphasic response' and 'circadian' effects. The results provide new insight into metabolomic level mechanisms of plant acclimation, and highlight the role of known protectants under natural conditions.

A novel stress-induced sugarcane gene confers tolerance to drought, salt and oxidative stress in transgenic tobacco plants

BACKGROUND

Drought is a major abiotic stress that affects crop productivity worldwide. Sugarcane can withstand periods of water scarcity during the final stage of culm maturation, during which sucrose accumulation occurs. Meanwhile, prolonged periods of drought can cause severe plant losses.

METHODOLOGY/PRINCIPAL FINDINGS

In a previous study, we evaluated the transcriptome of drought-stressed plants to better understand sugarcane responses to drought. Among the up-regulated genes was Scdr1 (sugarcane drought-responsive 1). The aim of the research reported here was to characterize this gene. Scdr1 encodes a putative protein containing 248 amino acids with a large number of proline (19%) and cysteine (13%) residues. Phylogenetic analysis showed that ScDR1is in a clade with homologs from other monocotyledonous plants, separate from those of dicotyledonous plants. The expression of Scdr1 in different varieties of sugarcane plants has not shown a clear association with drought tolerance.

CONCLUSIONS/SIGNIFICANCE

The overexpression of Scdr1 in transgenic tobacco plants increased their tolerance to drought, salinity and oxidative stress, as demonstrated by increased photosynthesis, water content, biomass, germination rate, chlorophyll content and reduced accumulation of ROS. Physiological parameters, such as transpiration rate (E), net photosynthesis (A), stomatal conductance (gs) and internal leaf CO(2) concentration, were less affected by abiotic stresses in transgenic Scdr1 plants compared with wild-type plants. Overall, our results indicated that Scdr1 conferred tolerance to multiple abiotic stresses, highlighting the potential of this gene for biotechnological applications.

Isolation of Arabidopsis ahg11, a weak ABA hypersensitive mutant defective in nad4 RNA editing

The phytohormone abscisic acid (ABA) plays pivotal roles in the regulation of developmental and environmental responses in plants. Identification of cytoplasmic ABA receptors enabled the elucidation of the main ABA signalling pathway, connecting ABA perception to either nuclear events or the action of several transporters. However, the physiological functions of ABA in cellular processes largely remain unknown. To obtain greater insight into the ABA response, genetic screening was performed to isolate ABA-related mutants of Arabidopsis and several novel ABA-hypersensitive mutants were isolated. One of those mutants--ahg11--was characterized further. Map-based cloning showed that AHG11 encodes a PPR type protein, which has potential roles in RNA editing. An AHG11-GFP fusion protein indicated that AHG11 mainly localized to the mitochondria. Consistent with this observation, the nad4 transcript, which normally undergoes RNA editing, lacks a single RNA editing event conferring a conversion of an amino acid residue in ahg11 mutants. The geminating ahg11 seeds have higher levels of reactive-oxygen-species-responsive genes. Presumably, partial impairment of mitochondrial function caused by an amino acid conversion in one of the complex I components induces redox imbalance which, in turn, confers an abnormal response to the plant hormone.

Mutations in an Arabidopsis mitochondrial transcription termination factor-related protein enhance thermotolerance in the absence of the major molecular chaperone HSP101

The molecular chaperone heat shock protein101 (HSP101) is required for acquired thermotolerance in plants and other organisms. To identify factors that interact with HSP101 or that are involved in thermotolerance, we screened for extragenic suppressors of a dominant-negative allele of Arabidopsis thaliana HSP101, hot1-4. One suppressor, shot1 (for suppressor of hot1-4 1), encodes a mitochondrial transcription termination factor (mTERF)-related protein, one of 35 Arabidopsis mTERFs about which there is limited functional data. Missense (shot1-1) and T-DNA insertion (shot1-2) mutants suppress the hot1-4 heat-hypersensitive phenotype. Furthermore, shot1-2 suppresses other heat-sensitive mutants, and shot1-2 alone is more heat tolerant than the wild type. SHOT1 resides in mitochondria, indicating it functions independently of cytosolic/nuclear HSP101. Microarray analysis suggests altered mitochondrial function and/or retrograde signaling in shot1-2 increases transcripts of other HSPs and alters expression of redox-related genes. Reduced oxidative damage is the likely cause of shot1 thermotolerance, indicating HSP101 repairs protein oxidative damage and/or reduced oxidative damage allows recovery in the absence of HSP101. Changes in organelle-encoded transcripts in shot1 demonstrate that SHOT1 is involved in organelle gene regulation. The heat tolerance of shot1 emphasizes the importance of mitochondria in stress tolerance, and defining its function may provide insights into control of oxidative damage for engineering stress-resistant plants.

Light inhibition of leaf respiration in field-grown Eucalyptus saligna in whole-tree chambers under elevated atmospheric CO2 and summer drought

We investigated whether the degree of light inhibition of leaf respiration (R) differs among large Eucalyptus saligna grown in whole-tree chambers and exposed to present and future atmospheric [CO(2) ] and summer drought. Associated with month-to-month changes in temperature were concomitant changes in R in the light (R(light) ) and darkness (R(dark) ), with both processes being more temperature dependent in well-watered trees than under drought. Overall rates of R(light) and R(dark) were not significantly affected by [CO(2) ]. By contrast, overall rates of R(dark) (averaged across both [CO(2) ]) were ca. 25% lower under drought than in well-watered trees. During summer, the degree of light inhibition of leaf R was greater in droughted (ca. 80% inhibition) than well-watered trees (ca. 50% inhibition). Notwithstanding these treatment differences, an overall positive relationship was observed between R(light) and R(dark) when data from all months/treatments were combined (R(2)  = 0.8). Variations in R(light) were also positively correlated with rates of Rubisco activity and nitrogen concentration. Light inhibition resulted in a marked decrease in the proportion of light-saturated photosynthesis respired (i.e. reduced R/A(sat) ). Collectively, these results highlight the need to account for light inhibition when assessing impacts of global change drivers on the carbon economy of tree canopies.

Respiratory complex I deficiency induces drought tolerance by impacting leaf stomatal and hydraulic conductances

To investigate the role of plant mitochondria in drought tolerance, the response to water deprivation was compared between Nicotiana sylvestris wild type (WT) plants and the CMSII respiratory complex I mutant, which has low-efficient respiration and photosynthesis, high levels of amino acids and pyridine nucleotides, and increased antioxidant capacity. We show that the delayed decrease in relative water content after water withholding in CMSII, as compared to WT leaves, is due to a lower stomatal conductance. The stomatal index and the abscisic acid (ABA) content were unaffected in well-watered mutant leaves, but the ABA/stomatal conductance relation was altered during drought, indicating that specific factors interact with ABA signalling. Leaf hydraulic conductance was lower in mutant leaves when compared to WT leaves and the role of oxidative aquaporin gating in attaining a maximum stomatal conductance is discussed. In addition, differences in leaf metabolic status between the mutant and the WT might contribute to the low stomatal conductance, as reported for TCA cycle-deficient plants. After withholding watering, TCA cycle derived organic acids declined more in CMSII leaves than in the WT, and ATP content decreased only in the CMSII. Moreover, in contrast to the WT, total free amino acid levels declined whilst soluble protein content increased in CMSII leaves, suggesting an accelerated amino acid remobilisation. We propose that oxidative and metabolic disturbances resulting from remodelled respiration in the absence of Complex I activity could be involved in bringing about the lower stomatal and hydraulic conductances.

Influence of drought stress on the cellular ultrastructure and antioxidant system in leaves of drought-tolerant and drought-sensitive apple rootstocks

We compared two apple rootstocks -Malus prunifolia and Malus hupehensis - that differ in their tolerance to this abiotic stress. The former is considered drought-tolerant, the latter, sensitive. We monitored changes in their leaf ultrastructure and responses by their antioxidant defense systems. Irrigation was withheld for 12 d from two-year-old potted plants. Compared with the control, this treatment led to considerable ultrastructural alterations in organelles. Plants of M. prunifolia maintained their structural cell integrity longer than did M. hupehensis. M. hupehensis was more vulnerable to drought than was M. prunifolia, resulting in larger increases in the levels of H(2)O(2), O(2)(-), and MDA from the former. Except for catalase (CAT) and monodehydroascorbate reductase (MDHAR), the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), and dehydroascorbate reductase (DHAR) analyzed here were enhanced to a greater extent in M. prunifolia than in M. hupehensis in response to drought. This was also true for levels of ascorbic acid (AsA) and glutathione (GSH). Under well-watered conditions, changes in lipid peroxidation and relevant antioxidant parameters were not significantly different between the two species throughout the experimental period. These results demonstrate that, in order to minimize oxidative damage, both the activities of antioxidant enzymes and antioxidant concentrations are increased in the leaves of M. prunifolia and M. hupehensis in response to water stress. Moreover, plants of M. prunifolia exhibit higher antioxidant capacity and a stronger protective mechanism, such that their cell structural integrity is better maintained during exposure to drought.

Heritable Effect of Plant Water Availability Conditions on Restoration of Male Fertility in the "9E" CMS-Inducing Cytoplasm of Sorghum

Heritable changes of phenotype arising in plant ontogenesis by the influence of environmental factors belong to the most intriguing genetic phenomena. An unusual inheritance pattern was detected during examination of male fertility restoration in the CMS-inducing "9E" type cytoplasm of sorghum: Rf-genes were functional in self-pollinated progeny of F(1) hybrids yet were either not expressed or poorly expressed in backcrosses of these hybrids to CMS-lines with the same cytoplasm type. In experiments on parallel growing of the same F(1) hybrid combinations in the "dry plot" and in the "irrigated plot," it was found that high level of plant water availability during panicle and pollen developmental stages significantly increased male fertility of F(1) and test-cross hybrid populations, in which fertility-restoring genes were in heterozygote state, whereas in F(2) populations the influences of water availability conditions cause less pronounce effects. Similarly, male-sterile F(1) plants, being transferred from the "dry plot" to greenhouse, produced male-fertile panicles. In addition, male-sterile plants from F(2) families, which segregated-out as recessives, being transferred to greenhouse also produced male-fertile panicles. In the progenies of these revertants that were grown in field conditions and in the "dry plot," stable inheritance of male fertility for three cycles of self-pollination was observed, and a number of stable fertile lines in the "9E" cytoplasm were obtained. However, in test-crosses of these fertile lines to CMS-lines with the "9E" cytoplasm restoration of male fertility was not observed, except the progeny of one revertant that behaved as fertility-restorer line. These data suggest that the functional state of fertility-restoring genes for the "9E" sorghum cytoplasm is epigenetically regulated trait established by the influence of environmental factors and is transmitted to sexual generations.

Opposite changes in leaf dark respiration and soluble sugars with drought in two Mediterranean oaks

The decline in net photosynthetic CO2 uptake (An) caused by drought could reduce the availability of soluble sugars and thus limit leaf dark respiration (Rd). We investigated the response of leaf gas exchange and nonstructural carbohydrates to drought by stopping watering to 2-year-old plants of Quercus ilex L. and Quercus pubescens Willd. grown in large pots. An declined with increasing water deficit more rapidly than Rd, and Rd declined slightly more steeply in Q. ilex than in Q. pubescens. Soluble sugars increased in drought-treated plants relative to control well watered plants, and the opposite pattern was found for starch. After rewatering, Rd returned to pre-drought rates within 2 days and An within 1 week. Soluble sugars tended to recover pre-drought values after rewatering but continued to be significantly higher in drought-treated than control plants of Q. pubescens, for which the increase in the concentration of soluble sugars had been higher. These results suggest that the relative production of soluble sugars is upregulated when An is limited, and that soluble sugars do not control respiratory rates in response to and recovery from water deficit. Rather, we suggest that the decline in Rd contributes to drought tolerance by reducing the consumption of soluble sugars, which play an important role as osmoprotectants during water deficit stress.

Potassium channel-oxidative phosphorylation relationship in durum wheat mitochondria from control and hyperosmotic-stressed seedlings

Durum wheat mitochondria (DWM) possess an ATP-inhibited K(+) channel, the plant mitoK(ATP) (PmitoK(ATP) ), which is activated under environmental stress to control mitochondrial ROS production. To do this, PmitoK(ATP) collapses membrane potential (ΔΨ), thus suggesting mitochondrial uncoupling. We tested this point by studying oxidative phosphorylation (OXPHOS) in DWM purified from control seedlings and from seedlings subjected both to severe mannitol and NaCl stress. In severely-stressed DWM, the ATP synthesis via OXPHOS, continuously monitored by a spectrophotometric assay, was about 90% inhibited when the PmitoK(ATP) was activated by KCl. Contrarily, in control DWM, although PmitoK(ATP) collapsed ΔΨ, ATP synthesis, as well as coupling [respiratory control (RC) ratio and ratio between phosphorylated ADP and reduced oxygen (ADP/O)] checked by oxygen uptake experiments, were unaffected. We suggest that PmitoK(ATP) may play an important defensive role at the onset of the environmental/oxidative stress by preserving energy in a crucial moment for cell and mitochondrial bioenergetics. Consistently, under moderate mannitol stress, miming an early stress condition, the channel may efficiently control reactive oxygen species (ROS) generation (about 35-fold from fully open to closed state) without impairing ATP synthesis. Anyway, if the stress significantly proceeds, the PmitoK(ATP) becomes fully activated by decrease of ATP concentration (25-40%) and increase of activators [free fatty acids (FFAs) and superoxide anion], thus impairing ATP synthesis.

The role of mitochondrial respiration in salinity tolerance

NaCl is the most abundant salt in salinity-affected land. The ability of plants to sift the water table, limit NaCl uptake, compartmentalise Na⁺/Cl⁻ ions and prevent negative ionic and osmotic effects on cell function, are the foundations of salinity tolerance mechanisms. In this review, we show that although the quantitative response of respiratory rate to changes in salt concentration is complex, the properties of respiratory processes are crucial for tolerance during ion exclusion and tissue tolerance. We consider whole-plant gas exchange and carbon balance analysis alongside the salt responses of mitochondrial properties and genetic studies manipulating respiratory processes. We showcase the importance of efficient ATP generation, dampened reactive oxygen species and mitochondrial osmolytes for salinity tolerance in plants.

Slowly developing drought stress increases photosynthetic acclimation of Catharanthus roseus

Our understanding of plant responses to drought has improved over the decades. However, the importance of the rate of drought imposition on the response is still poorly understood. To test the importance of the rate at which drought stress develops, whole-plant photosynthesis (P(net) ), respiration (R(dark) ), daily carbon gain (DCG), daily evapotranspiration (DET) and water use efficiency (WUE) of vinca (Catharanthus roseus), subjected to different drought imposition rates, were investigated. We controlled the rate at which the substrate dried out with an automated irrigation system that allowed pot weight to decrease gradually throughout the drying period. Fast, intermediate and slow drying treatments reached their final pot weight [500 g, substrate water content (θ) ≈ 0.10 m³ m(-3) ] after 3.1, 6.6 and 10 days, respectively. Although all drying treatments decreased P(net) and R(dark) , slow drying reduced P(net) and R(dark) less than fast drying. At a θ < 0.10 m³ m(-3) , DCG and DET in the slow drying treatment were reduced by ≈50%, whereas DCG and DET in the fast drying treatment were reduced by 85 and 70% at a θ of 0.16 m(3) m(-3) . Plants exposed to slow drought imposition maintained a high WUE, even at θ < 0.10 m³ m(-3) . Overall, physiological responses to low θ were less severe in plants subjected to slow drying as compared with fast drying, even though the final θ was lower for plants exposed to slow drying. This suggests that the rate at which drought stress develops has important implications for the level of acclimation that occurs.

Essential role of tissue-specific proline synthesis and catabolism in growth and redox balance at low water potential

To better define the still unclear role of proline (Pro) metabolism in drought resistance, we analyzed Arabidopsis (Arabidopsis thaliana) Δ(1)-pyrroline-5-carboxylate synthetase1 (p5cs1) mutants deficient in stress-induced Pro synthesis as well as proline dehydrogenase (pdh1) mutants blocked in Pro catabolism and found that both Pro synthesis and catabolism were required for optimal growth at low water potential (ψ(w)). The abscisic acid (ABA)-deficient mutant aba2-1 had similar reduction in root elongation as p5cs1 and p5cs1/aba2-1 double mutants. However, the reduced growth of aba2-1 but not p5cs1/aba2-1 could be complemented by exogenous ABA, indicating that Pro metabolism was required for ABA-mediated growth protection at low ψ(w). PDH1 maintained high expression in the root apex and shoot meristem at low ψ(w) rather than being repressed, as in the bulk of the shoot tissue. This, plus a reduced oxygen consumption and buildup of Pro in the root apex of pdh1-2, indicated that active Pro catabolism was needed to sustain growth at low ψ(w). Conversely, P5CS1 expression was most highly induced in shoot tissue. Both p5cs1-4 and pdh1-2 had a more reduced NADP/NADPH ratio than the wild type at low ψ(w). These results indicate a new model of Pro metabolism at low ψ(w) whereby Pro synthesis in the photosynthetic tissue regenerates NADP while Pro catabolism in meristematic and expanding cells is needed to sustain growth. Tissue-specific differences in Pro metabolism and function in maintaining a favorable NADP/NADPH ratio are relevant to understanding metabolic adaptations to drought and efforts to enhance drought resistance.

An Arabidopsis mitochondrial uncoupling protein confers tolerance to drought and salt stress in transgenic tobacco plants

BACKGROUND

Plants are challenged by a large number of environmental stresses that reduce productivity and even cause death. Both chloroplasts and mitochondria produce reactive oxygen species under normal conditions; however, stress causes an imbalance in these species that leads to deviations from normal cellular conditions and a variety of toxic effects. Mitochondria have uncoupling proteins (UCPs) that uncouple electron transport from ATP synthesis. There is evidence that UCPs play a role in alleviating stress caused by reactive oxygen species overproduction. However, direct evidence that UCPs protect plants from abiotic stress is lacking.

METHODOLOGY/PRINCIPAL FINDINGS

Tolerances to salt and water deficit were analyzed in transgenic tobacco plants that overexpress a UCP (AtUCP1) from Arabidopsis thaliana. Seeds of AtUCP1 transgenic lines germinated faster, and adult plants showed better responses to drought and salt stress than wild-type (WT) plants. These phenotypes correlated with increased water retention and higher gas exchange parameters in transgenic plants that overexpress AtUCP1. WT plants exhibited increased respiration under stress, while transgenic plants were only slightly affected. Furthermore, the transgenic plants showed reduced accumulation of hydrogen peroxide in stressed leaves compared with WT plants.

CONCLUSIONS/SIGNIFICANCE

Higher levels of AtUCP1 improved tolerance to multiple abiotic stresses, and this protection was correlated with lower oxidative stress. Our data support previous assumptions that UCPs reduce the imbalance of reactive oxygen species. Our data also suggest that UCPs may play a role in stomatal closure, which agrees with other evidence of a direct relationship between these proteins and photosynthesis. Manipulation of the UCP protein expression in mitochondria is a new avenue for crop improvement and may lead to crops with greater tolerance for challenging environmental conditions.

The interdependence of mechanisms underlying climate-driven vegetation mortality

Climate-driven vegetation mortality is occurring globally and is predicted to increase in the near future. The expected climate feedbacks of regional-scale mortality events have intensified the need to improve the simple mortality algorithms used for future predictions, but uncertainty regarding mortality processes precludes mechanistic modeling. By integrating new evidence from a wide range of fields, we conclude that hydraulic function and carbohydrate and defense metabolism have numerous potential failure points, and that these processes are strongly interdependent, both with each other and with destructive pathogen and insect populations. Crucially, most of these mechanisms and their interdependencies are likely to become amplified under a warmer, drier climate. Here, we outline the observations and experiments needed to test this interdependence and to improve simulations of this emergent global phenomenon.

Impacts of drought on leaf respiration in darkness and light in Eucalyptus saligna exposed to industrial-age atmospheric CO₂ and growth temperature

Our study assessed the impact of a wide range of industrial-age climate scenarios on leaf respiration (R) in Eucalyptus saligna. Well-watered or sustained drought-treated plants were grown in glasshouses differing in atmospheric CO₂ concentration ([CO₂]) (280, 400 and 640 μl l⁻¹) and temperature (26 and 30°C). Rates of R in darkness (R(dark) ) and light (R(light) ), photosynthesis (A) and related leaf traits (mass : area relationships, and nitrogen, phosphorus, starch and sugar concentrations) were measured. Light inhibited R in all cases (R(light) < R(dark) ) (well-watered: 40%; drought-treated: 73%). Growth [CO₂] and temperature had little impact on area-based rates of R(dark) or R(light) , with R(light) exhibiting minimal thermal acclimation. By contrast, sustained drought resulted in reduced R(dark), R(light) and A, with the inhibitory effect of drought on A and R(light) (c. 50-70%) greater than that on R(dark) (c. 15%). Drought effects were fully reversible after watering. Variability in R(light) appeared to be dependent on the underlying rate of R(dark) and associated Rubisco activity. Collectively, our data suggest that there is an asynchronous response of leaf carbon metabolism to drought, and a tighter coupling between R(light) and A than between R(dark) and A, under both past and future climate scenarios. These findings have important implications for ecosystem/global models seeking to predict carbon cycling.

The floristic heterogeneity of the Pantanal and the occurrence of species with different adaptive strategies to water stress

The Pantanal is characterised by a diversity of environments with areas ranging from periodic or permanent heavy flooding to areas with low flood levels, and even environments that never flood. Plant species which inhabit the floodplain are distributed in specific niches, with influence of various phytogeographic domains, including the Seasonal Semi-deciduous Forest, Amazon Rainforest, Cerrado and Chaco, as well rocky remnants, with a wide ecological span in their components. In intensely flooded areas, aquatic macrophytes are widely distributed, with their dynamics closely linked to time, depth and extent of flooding. Although the term "Pantanal" suggests a huge swamp-type wetland, water level variation during a seasonal cycle does not directly reach the root system of many plants. The landscape diversity of the Pantanal wetland is molded by the flood pulse, which interferes with the dynamics of plant communities. Therefore, the retraction and expansion of populations or communities is reflected in important ecological characteristics, considering the variety of morphological, anatomical and ecophysiological features of the species, whose phenotype is the result of a particular genotype. The present study discusses peculiar issues in the adaptation of species distributed in the Pantanal biome and underscores the importance of multidisciplinary approaches to obtain conclusive data on adaptive studies.

Associations between growth, wood anatomy, carbon isotope discrimination and mortality in a Quercus robur forest

Observations of forest mortality are increasing globally, but relatively little is known regarding the underlying mechanisms driving these events. Tree rings carry physiological signatures that may be used as a tool for retrospective analyses. We capitalized on a local soil water drainage event in 1982 that resulted in increased mortality within a stand of oak trees (Quercus robur), to examine the underlying physiological patterns associated with survival and death in response to soil water limitations. Pre-dawn water potentials showed more negative values for trees in the process of dying compared with those that survived. We used tree rings formed over the 123 years prior to mortality to estimate productivity from basal area increment (BAI, mm(2)), multiple xylem hydraulic parameters via anatomical measurements and crown-level gas exchange via carbon isotope discrimination (Δ, ‰). Oaks that died had significantly higher BAI values than trees that survived until the drainage event, after which the BAI of trees that died declined dramatically. Hydraulic diameter and conductivity of vessels in trees that died were higher than in surviving trees until the last 5 years prior to mortality, at which time both groups had similar values. Trees that died had consistently lower Δ values than trees that survived. Therefore, tree mortality in this stand was associated with physiological differences prior to the onset of soil water reduction. We propose that trees that died may have been hydraulically underbuilt for dry conditions, which predisposes them to severe hydraulic constraints and subsequent mortality. Measurements of above-ground/below-ground dry mass partitioning will be critical to future tests of this hypothesis. Based on these results, it is probable that pedunculate oak trees will experience greater future mortality if climate changes cause more severe droughts than the trees have experienced previously.

Different sensitivity of isoprene emission, respiration and photosynthesis to high growth temperature coupled with drought stress in black poplar (Populus nigra) saplings

The effects of the interaction between high growth temperatures and water stress on gas-exchange properties of Populus nigra saplings were investigated. Water stress was expressed as a function of soil water content (SWC) or fraction of transpirable soil water (FTSW). Isoprene emission and photosynthesis (A) did not acclimate in response to elevated temperature, whereas dark (R(n)) and light (R(d)) respiration underwent thermal acclimation. R(d) was ~30% lower than R(n) irrespective of growth temperature and water stress level. Water stress induced a sharp decline, but not a complete inhibition, of both R(n) and R(d). There was no significant effect of high growth temperature on the responses of A, stomatal conductance (g(s)), isoprene emission, R(n) or R(d) to FTSW. High growth temperature resulted in a significant increase in the SWC endpoint. Photosynthesis was limited mainly by CO(2) acquisition in water-stressed plants. Impaired carbon metabolism became apparent only at the FTSW endpoint. Photosynthesis was restored in about a week following rewatering, indicating transient biochemical limitations. The kinetics of isoprene emission in response to FTSW confirmed that water stress uncouples the emission of isoprene from A, isoprene emission being unaffected by decreasing g(s). The different kinetics of A, respiration and isoprene emission in response to the interaction between high temperature and water stress led to rising R(d)/A ratio and amount of carbon lost as isoprene. Since respiration and isoprene sensitivity are much lower than A sensitivity to water stress, temperature interactions with water stress may dominate poplar acclimatory capability and maintenance of carbon homeostasis under climate change scenarios. Furthermore, predicted temperature increases in arid environments may reduce the amount of soil water that can be extracted before plant gas exchange decreases, exacerbating the effects of water stress even if soil water availability is not directly affected.

A reciprocal 15N-labeling proteomic analysis of expanding Arabidopsis leaves subjected to osmotic stress indicates importance of mitochondria in preserving plastid functions

Plants respond to environmental stress by dynamically reprogramming their growth. Whereas stress onset is accompanied by rapid growth inhibition leading to smaller organs, growth will recover and adapt once the stress conditions become stable and do no threaten plant survival. Here, adaptation of growing Arabidopsis thaliana leaves to mild and prolonged osmotic stress was investigated by means of a complete metabolic labeling strategy with the (15)N-stable isotope as a complement to a previously published transcript and metabolite profiling. Global analysis of protein changes revealed that plastidial ATPase, Calvin cycle, and photorespiration were down-regulated, but mitochondrial ATP synthesis was up-regulated, indicating the importance of mitochondria in preserving plastid functions during water stress. Although transcript and protein data correlated well with the stable and prolonged character of the applied stress, numerous proteins were clearly regulated at the post-transcriptional level that could, at least partly, be related to changes in protein synthesis and degradation. In conclusion, proteomics using the (15)N labeling helped understand the mechanisms underlying growth adaptation to osmotic stress and allowed the identification of candidate genes to improve plant growth under limited water.

Photosynthesis and drought: can we make metabolic connections from available data?

Photosynthesis is one of the key processes to be affected by water deficits, via decreased CO2 diffusion to the chloroplast and metabolic constraints. The relative impact of those limitations varies with the intensity of the stress, the occurrence (or not) of superimposed stresses, and the species we are dealing with. Total plant carbon uptake is further reduced due to the concomitant or even earlier inhibition of growth. Leaf carbohydrate status, altered directly by water deficits or indirectly (via decreased growth), acts as a metabolic signal although its role is not totally clear. Other relevant signals acting under water deficits comprise: abscisic acid (ABA), with an impact on stomatal aperture and the regulation at the transcription level of a large number of genes related to plant stress response; other hormones that act either concurrently (brassinosteroids, jasmonates, and salycilic acid) or antagonistically (auxin, cytokinin, or ethylene) with ABA; and redox control of the energy balance of photosynthetic cells deprived of CO2 by stomatal closure. In an attempt to systematize current knowledge on the complex network of interactions and regulation of photosynthesis in plants subjected to water deficits, a meta-analysis has been performed covering >450 papers published in the last 15 years. This analysis shows the interplay of sugars, reactive oxygen species (ROS), and hormones with photosynthetic responses to drought, involving many metabolic events. However, more significantly it highlights (i) how fragmented and often non-comparable the results are and (ii) how hard it is to relate molecular events to plant physiological status, namely photosynthetic activity, and to stress intensity. Indeed, the same data set usually does not integrate these different levels of analysis. Considering these limitations, it was hard to find a general trend, particularly concerning molecular responses to drought, with the exception of the genes ABI1 and ABI3. These genes, irrespective of the stress type (acute versus chronic) and intensity, show a similar response to water shortage in the two plant systems analysed (Arabidopsis and barley). Both are associated with ABA-mediated metabolic responses to stress and the regulation of stomatal aperture. Under drought, ABI1 transcription is up-regulated while ABI3 is usually down-regulated. Recently ABI3 has been hypothesized to be essential for successful drought recovery.

Genotypic variation in drought stress response and subsequent recovery of wheat (Triticum aestivum L.)

Three wheat (Triticum aestivum L.) genotypes, Sadovo, Katya and Prelom, with different tolerance to drought were comparatively evaluated in terms of leaf respiratory responses to progressing dehydration and consecutive rewatering. Under drought stress, the respiration of all varieties gradually decreased, as the drought-tolerant Katya showed the most pronounced decline at earlier stages of dehydration. When water stress intensified, this genotype gave relatively stable respiration rates compared with the drought-sensitive varieties. Additionally, dehydrated Katya leaves displayed lower stomatal conductance and higher photosynthesis values, which resulted in greater water use efficiency during the dehydration period. Combination of drought stress and short-term changes in leaf temperature also induced genotype-specific response that differed from the response to drought only. Over the whole temperature range, the leaves of Katya exposed to dehydration for 14 days, showed higher respiration rates compared to the drought-sensitive varieties. The sensitive varieties maintained higher respiration rates under control conditions and mild dehydration, and very low rates under severe drought. In Katya, respiration and photosynthesis were fully restored from the stress within the first day of rewatering. The drought-sensitive genotypes displayed a considerably slower recovering capacity. The results are discussed in terms of possible physiological mechanisms underlying plant tolerance to drought.

Activation of the plant mitochondrial potassium channel by free fatty acids and acyl-CoA esters: a possible defence mechanism in the response to hyperosmotic stress

The effect of free fatty acids (FFAs) and acyl-CoA esters on K(+) uptake was studied in mitochondria isolated from durum wheat (Triticum durum Desf.), a species that has adapted well to the semi-arid Mediterranean area and possessing a highly active mitochondrial ATP-sensitive K(+) channel (PmitoK(ATP)), that may confer resistance to environmental stresses. This was made by swelling experiments in KCl solution under experimental conditions in which PmitoK(ATP) activity was monitored. Linoleate and other FFAs (laurate, palmitate, stearate, palmitoleate, oleate, arachidonate, and the non-physiological 1-undecanesulphonate and 5-phenylvalerate), used at a concentration (10 μM) unable to damage membranes of isolated mitochondria, stimulated K(+) uptake by about 2-4-fold. Acyl-CoAs also promoted K(+) transport to a much larger extent with respect to FFAs (about 5-12-fold). In a different experimental system based on safranin O fluorescence measurements, the dissipation of electrical membrane potential induced by K(+) uptake via PmitoK(ATP) was found to increase in the presence of 5-phenylvalerate and palmitoyl-CoA, both unable to elicit the activity of the Plant Uncoupling Protein. This result suggests a direct activation of PmitoK(ATP). Stimulation of K(+) transport by FFAs/acyl-CoAs resulted in a widespread phenomenon in plant mitochondria from different mono/dicotyledonous species (bread wheat, barley, triticale, maize, lentil, pea, and topinambur) and from different organs (root, tuber, leaf, and shoot). Finally, an increase in mitochondrial FFAs up to a content of 50 nmol mg(-1) protein, which was able to activate PmitoK(ATP) strongly, was observed under hyperosmotic stress conditions. Since PmitoK(ATP) may act against environmental/oxidative stress, its activation by FFAs/acyl-CoAs is proposed to represent a physiological defence mechanism.