Different metabolic adaptation strategies after overwintering in Eutrema sp. and Arabidopsis accessions under field conditions in Germany

Successful overwintering is a prerequisite for high fitness in temperate perennials and winter annuals and is highly dependent on increased freezing tolerance and timely balancing of deacclimation with growth resumption in spring. To assess fitness costs associated with overwintering and elucidate metabolic mechanisms underlying winter survival and the switch from acclimated freezing tolerance to growth resumption, we performed a comparative field study using 14 Eutrema salsugineum accessions, E. halophilum, E. botschantzevii and 11 Arabidopsis thaliana accessions differing in freezing tolerance. Winter survival and reproductive fitness parameters were recorded and correlated with phenological stage and metabolite status during growth resumption in spring. The results revealed considerable intraspecific variation in winter survival, but survival rates of the extremophyte Eutrema were not inherently better. In both Eutrema and A. thaliana, improved winter survival was associated with reduced reproductive fitness. Metabolic analysis by GC-MS revealed intrinsic differences in the primary metabolism of the two genera during deacclimation. Eutrema contained higher levels of several amino and chlorogenic acids, while Arabidopsis had higher levels of several sugars and sugar conjugates. In both genera, increased levels of several soluble sugars were associated with increased winter survival, whereas myo-inositol has different roles in overwintering of Eutrema and A. thaliana. In addition, differences in amino acid metabolism and polyhydroxy acids levels after winter survival were found. The results provide strong evidence for a trade-off between increased winter survival and reproductive fitness in both Eutrema and Arabidopsis and document inherent differences in their metabolic strategies to survive winter.

Consistent diurnal pattern of leaf respiration in the light among contrasting species and climates

Leaf daytime respiration (leaf respiration in the light, R_L ) is often assumed to constitute a fixed fraction of leaf dark respiration (R_D ) (i.e. a fixed light inhibition of respiration (R_D )) and vary diurnally due to temperature fluctuations. These assumptions were tested by measuring R_L , R_D and the light inhibition of R_D in the field at a constant temperature using the Kok method. Measurements were conducted diurnally on 21 different species: 13 deciduous, four evergreen and four herbaceous from humid continental and humid subtropical climates. R_L and R_D showed significant diurnal variations and the diurnal pattern differed in trajectory and magnitude between climates, but not between plant functional types (PFTs). The light inhibition of R_D varied diurnally and differed between climates and in trajectory between PFTs. The results highlight the entrainment of leaf daytime respiration to the diurnal cycle and that time of day should be accounted for in studies seeking to examine the environmental and biological drivers of leaf daytime respiration.

Nocturnal plant respiration is under strong non-temperature control

Most biological rates depend on the rate of respiration. Temperature variation is typically considered the main driver of daily plant respiration rates, assuming a constant daily respiration rate at a set temperature. Here, we show empirical data from 31 species from temperate and tropical biomes to demonstrate that the rate of plant respiration at a constant temperature decreases monotonically with time through the night, on average by 25% after 8 h of darkness. Temperature controls less than half of the total nocturnal variation in respiration. A new universal formulation is developed to model and understand nocturnal plant respiration, combining the nocturnal decrease in the rate of plant respiration at constant temperature with the decrease in plant respiration according to the temperature sensitivity. Application of the new formulation shows a global reduction of 4.5 -6 % in plant respiration and an increase of 7-10% in net primary production for the present-day.

Winter warming stimulates vegetative growth and alters fruit quality of blackcurrant (Ribes nigrum)

The rate of global warming varies in magnitude between seasons, with warming being more pronounced in winter and spring than in summer and autumn at high latitudes. Winter warming can have profound effects on dormancy release and spring phenology of perennial fruit crops, but potential follow-on impacts on growth, fruit yield or quality have only rarely been investigated. We studied the effects of mild winter warming on spring phenology, current year shoot growth, cropping performance and fruit quality in four field-grown cultivars of blackcurrant with different chilling requirements. Plants were exposed to ambient or slightly elevated (+ 0.5 °C) temperatures from early October to mid-April the following year. Winter warming had few effects on spring phenology and fruit yield, but caused significant changes in berry contents of phenolic compounds and a reduction in soluble sugars. Increased vegetative growth of warmed plants likely accounts for the changes in berry quality. The results demonstrate a persistent effect of winter warming on shoot growth, which indirectly changes fruit quality.

Cold stress and freezing tolerance negatively affect the fitness of Arabidopsis thaliana accessions under field and controlled conditions

Higher acclimated freezing tolerance improved winter survival, but reduced reproductive fitness of Arabidopsis thaliana accessions under field and controlled conditions. Low temperature is one of the most important abiotic factors influencing plant fitness and geographical distribution. In addition, cold stress is known to influence crop yield and is therefore of great economic importance. Increased freezing tolerance can be acquired by the process of cold acclimation, but this may be associated with a fitness cost. To assess the influence of cold stress on the fitness of plants, long-term field trials over 5 years were performed with six natural accessions of Arabidopsis thaliana ranging from very tolerant to very sensitive to freezing. Fitness parameters, as seed yield and 1000 seed mass, were measured and correlation analyses with temperature and freezing tolerance data performed. The results were compared with fitness parameters from controlled chamber experiments over 3 years with application of cold priming and triggering conditions. Winter survival and seed yield per plant were positively correlated with temperature in field experiments. In addition, winter survival and 1000 seed mass were correlated with the cold-acclimated freezing tolerance of the selected Arabidopsis accessions. The results provide strong evidence for a trade-off between higher freezing tolerance and reproductive fitness in A. thaliana, which might have ecological impacts in the context of global warming.

Measuring Freezing Tolerance of Leaves and Rosettes: Electrolyte Leakage and Chlorophyll Fluorescence Assays

Quantitative assessment of freezing tolerance is essential to unravel plant adaptations to cold temperatures. Not only the survival of whole plants, but also impairment of detached leaves or small rosettes after a freeze-thaw cycle can be used to accurately quantify plant freezing tolerance in terms of LT₅₀ values. Here we describe two methods to determine the freezing tolerance of detached leaves or rosettes using a full or selected set of freezing temperatures and an additional method using chlorophyll fluorescence as a different physiological parameter. Firstly, we illustrate how to assess the integrity of (predominantly) the plasma membrane during freezing using an electrolyte leakage assay. Secondly, we provide a chlorophyll fluorescence imaging protocol to determine the freezing tolerance of the photosynthetic apparatus.

Transcriptional and Post-Transcriptional Regulation and Transcriptional Memory of Chromatin Regulators in Response to Low Temperature

Chromatin regulation ensures stable repression of stress-inducible genes under non-stress conditions and transcriptional activation and memory of stress-related genes after stress exposure. However, there is only limited knowledge on how chromatin genes are regulated at the transcriptional and post-transcriptional level upon stress exposure and relief from stress. We reveal that the repressive modification histone H3 lysine 27 trimethylation (H3K27me3) targets genes which are quickly activated upon cold exposure, however, H3K27me3 is not necessarily lost during a longer time in the cold. In addition, we have set-up a quantitative reverse transcription polymerase chain reaction-based platform for high-throughput transcriptional profiling of a large set of chromatin genes. We find that the expression of many of these genes is regulated by cold. In addition, we reveal an induction of several DNA and histone demethylase genes and certain histone variants after plants have been shifted back to ambient temperature (deacclimation), suggesting a role in the memory of cold acclimation. We also re-analyze large scale transcriptomic datasets for transcriptional regulation and alternative splicing (AS) of chromatin genes, uncovering an unexpected level of regulation of these genes, particularly at the splicing level. This includes several vernalization regulating genes whose AS may result in cold-regulated protein diversity. Overall, we provide a profiling platform for the analysis of chromatin regulatory genes and integrative analyses of their regulation, suggesting a dynamic regulation of key chromatin genes in response to low temperature stress.

Deacclimation after cold acclimation-a crucial, but widely neglected part of plant winter survival

Temperate and boreal plants show natural low temperature acclimation during autumn. This cold acclimation process results in increased freezing tolerance. Global climate change is leading to increasing spring and autumn temperatures that can trigger deacclimation and loss of freezing tolerance, making plants susceptible to both late-autumn and late-spring freezing events. In particular, spring frosts can have devastating effects on whole ecosystems and can significantly reduce the yield of crop plants. Although the timing and speed of deacclimation are clearly of crucial importance for plant winter survival, the molecular basis of this process is still largely unknown. The regulation of deacclimation is, however, not only related to freezing tolerance, but also to the termination of dormancy, and the initiation of growth and development. In this paper, we provide an overview of what is known about deacclimation in both woody and herbaceous plants. We use publicly available transcriptome data to identify a core set of deacclimation-related genes in Arabidopsis thaliana that highlight physiological determinants of deacclimation, and suggest important directions for future research in this area.

Cold deacclimation mechanisms and reacclimation potential in flower buds of blackcurrant (Ribes nigrum)

As a consequence of global climate change, cold acclimation and deacclimation cycles are becoming increasingly frequent during winter in temperate regions. However, little is known about plant deacclimation and in particular reacclimation mechanisms, although deacclimation resistance and the ability to reacclimate may have wide-ranging consequences regarding plant productivity in a changing climate. Here, we report time-dependent responses of freezing tolerance, respiration rates, metabolite contents (high-resolution magic angle spinning NMR) and fatty acid levels (gas chromatography) in flower buds of two ecodormant Ribes nigrum cultivars exposed to three different deacclimation temperatures followed by a reacclimation treatment at 4°C. The data reveal that despite differences in the progression of deacclimation, the capacity of blackcurrant flower buds to reharden in late winter is virtually non-existing, implying that increasingly irregular temperature patterns is critical for blackcurrant fruit yield. The early phase of deacclimation is associated with a transient increase in respiration and decreasing contents of amino acids, tricarboxylic acid (TCA) cycle intermediates and sugars, indicating an increased need for carbon sources and respiratory energy production for the activation of growth. Decreasing sugar levels may additionally cause loss of freezing tolerance. Deacclimation also involves desaturation of membrane lipids, which likely also contributes to decreased freezing tolerance but may also reflect biosynthesis of signaling molecules stimulating growth and floral organ differentiation. These data provide new insights into the under-researched deacclimation mechanisms and the ability of blackcurrant to reacclimate following different advancements of deacclimation and contribute to our understanding of plant responses to increasingly irregular temperature patterns.

Molecular signatures associated with increased freezing tolerance due to low temperature memory in Arabidopsis

Alternating temperatures require fast and coordinated adaptation responses of plants. Cold acclimation has been extensively investigated and results in increased freezing tolerance in Arabidopsis thaliana. Here, we show that the two Arabidopsis accessions, Col-0 and N14, which differ in their freezing tolerance, showed memory of cold acclimation, that is, cold priming. Freezing tolerance was higher in plants exposed to cold priming at 4°C, a lag phase at 20°C, and a second triggering cold stress (4°C) than in plants that were only cold primed. To our knowledge, this is the first report on cold memory improving plant freezing tolerance. The triggering response was distinguishable from the priming response at the levels of gene expression (RNA-Seq), lipid (ultraperformance liquid chromatography-mass spectrometry), and metabolite composition (gas chromatography-mass spectrometry). Transcriptomic responses pointed to induced lipid, secondary metabolism, and stress in Col-0 and growth-related functions in N14. Specific accumulation of lipids included arabidopsides with possible functions as signalling molecules or precursors of jasmonic acid. Whereas cold-induced metabolites such as raffinose and its precursors were maintained in N14 during the lag phase, they were strongly accumulated in Col-0 after the cold trigger. This indicates genetic differences in the transcriptomic and metabolic patterns during cold memory.

Metabolic Fingerprinting of Dormant and Active Flower Primordia of Ribes nigrum Using High-Resolution Magic Angle Spinning NMR

Global warming may modify the timing of dormancy release and spring growth of buds of temperate fruit crops. Environmental regulation of the activity-dormancy cycle in perennial plants remains poorly understood at the metabolic level. Especially, the fine-scale metabolic dynamics in the meristematic zone within buds has received little attention. In this work we performed metabolic profiling of intact floral primordia of Ribes nigrum isolated from buds differing in dormancy status using high-resolution magic angle spinning (HR-MAS) NMR. The technique proved useful in monitoring different groups of metabolites, e.g., carbohydrates and amino acids, in floral primordia and allowed metabolic separation of primordia from endo- and ecodormant buds. In addition, due to its nondestructive character, HR-MAS NMR may provide novel insights into cellular compartmentation of individual biomolecules that cannot be obtained using liquid-state NMR. Out results show that HR-MAS NMR may be an important method for metabolomics of intact plant structures.

Rapid transcriptional and metabolic regulation of the deacclimation process in cold acclimated Arabidopsis thaliana

Background

During low temperature exposure, temperate plant species increase their freezing tolerance in a process termed cold acclimation. This is accompanied by dampened oscillations of circadian clock genes and disrupted oscillations of output genes and metabolites. During deacclimation in response to warm temperatures, cold acclimated plants lose freezing tolerance and resume growth and development. While considerable effort has been directed toward understanding the molecular and metabolic basis of cold acclimation, much less information is available about the regulation of deacclimation.

Results

We report metabolic (gas chromatography-mass spectrometry) and transcriptional (microarrays, quantitative RT-PCR) responses underlying deacclimation during the first 24 h after a shift of Arabidopsis thaliana (Columbia-0) plants cold acclimated at 4 °C back to warm temperature (20 °C). The data reveal a faster response of the transcriptome than of the metabolome and provide evidence for tightly regulated temporal responses at both levels. Metabolically, deacclimation is associated with decreasing contents of sugars, amino acids, glycolytic and TCA cycle intermediates, indicating an increased need for carbon sources and respiratory energy production for the activation of growth. The early phase of deacclimation also involves extensive down-regulation of protein synthesis and changes in the metabolism of lipids and cell wall components. Hormonal regulation appears particularly important during deacclimation, with extensive changes in the expression of genes related to auxin, gibberellin, brassinosteroid, jasmonate and ethylene metabolism. Members of several transcription factor families that control fundamental aspects of morphogenesis and development are significantly regulated during deacclimation, emphasizing that loss of freezing tolerance and growth resumption are transcriptionally highly interrelated processes. Expression patterns of some clock oscillator components resembled those under warm conditions, indicating at least partial re-activation of the circadian clock during deacclimation.

Conclusions

This study provides the first combined metabolomic and transcriptomic analysis of the regulation of deacclimation in cold acclimated plants. The data indicate cascades of rapidly regulated genes and metabolites that underlie the developmental switch resulting in reduced freezing tolerance and the resumption of growth. They constitute a large-scale dataset of genes, metabolites and pathways that are crucial during the initial phase of deacclimation. The data will be an important reference for further analyses of this and other important but under-researched stress deacclimation processes.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4126-3) contains supplementary material, which is available to authorized users.

Winter warming delays dormancy release, advances budburst, alters carbohydrate metabolism and reduces yield in a temperate shrub

Global climate models predict an increase in the mean surface air temperature, with a disproportionate increase during winter. Since temperature is a major driver of phenological events in temperate woody perennials, warming is likely to induce changes in a range of these events. We investigated the impact of slightly elevated temperatures (+0.76 °C in the air, +1.35 °C in the soil) during the non-growing season (October-April) on freezing tolerance, carbohydrate metabolism, dormancy release, spring phenology and reproductive output in two blackcurrant (Ribes nigrum) cultivars to understand how winter warming modifies phenological traits in a woody perennial known to have a large chilling requirement and to be sensitive to spring frost. Warming delayed dormancy release more in the cultivar 'Narve Viking' than in the cultivar 'Titania', but advanced budburst and flowering predominantly in 'Titania'. Since 'Narve Viking' has a higher chilling requirement than 'Titania', this indicates that, in high-chilling-requiring genotypes, dormancy responses may temper the effect of warming on spring phenology. Winter warming significantly reduced fruit yield the following summer in both cultivars, corroborating the hypothesis that a decline in winter chill may decrease reproductive effort in blackcurrant. Elevated winter temperatures tended to decrease stem freezing tolerance during cold acclimation and deacclimation, but it did not increase the risk of freeze-induced damage mid-winter. Plants at elevated temperature showed decreased levels of sucrose in stems of both cultivars and flower buds of 'Narve Viking', which, in buds, was associated with increased concentrations of glucose and fructose. Hence, winter warming influences carbohydrate metabolism, but it remains to be elucidated whether decreased sucrose levels account for any changes in freezing tolerance. Our results demonstrate that even a slight increase in winter temperature may alter phenological traits in blackcurrant, but to various extents depending on genotype-specific differences in chilling requirement.

Global changes in gene expression, assayed by microarray hybridization and quantitative RT-PCR, during acclimation of three Arabidopsis thaliana accessions to sub-zero temperatures after cold acclimation

During cold acclimation plants increase in freezing tolerance in response to low non-freezing temperatures. This is accompanied by many physiological, biochemical and molecular changes that have been extensively investigated. In addition, plants of many species, including Arabidopsis thaliana, become more freezing tolerant during exposure to mild, non-damaging sub-zero temperatures after cold acclimation. There is hardly any information available about the molecular basis of this adaptation. Here, we have used microarrays and a qRT-PCR primer platform covering 1,880 genes encoding transcription factors (TFs) to monitor changes in gene expression in the Arabidopsis accessions Columbia-0, Rschew and Tenela during the first 3 days of sub-zero acclimation at -3 °C. The results indicate that gene expression during sub-zero acclimation follows a tighly controlled time-course. Especially AP2/EREBP and WRKY TFs may be important regulators of sub-zero acclimation, although the CBF signal transduction pathway seems to be less important during sub-zero than during cold acclimation. Globally, we estimate that approximately 5% of all Arabidopsis genes are regulated during sub-zero acclimation. Particularly photosynthesis-related genes are down-regulated and genes belonging to the functional classes of cell wall biosynthesis, hormone metabolism and RNA regulation of transcription are up-regulated. Collectively, these data provide the first global analysis of gene expression during sub-zero acclimation and allow the identification of candidate genes for forward and reverse genetic studies into the molecular mechanisms of sub-zero acclimation.

Winter survival and deacclimation of perennials under warming climate: physiological perspectives

Appropriate timing and rate of cold deacclimation and the ability to reacclimate are important components of winter survival of perennials in temperate and boreal zones. In association with the progressive increase in atmospheric CO₂, temperate and boreal winters are becoming progressively milder, and temperature patterns are becoming irregular with increasing risk of unseasonable warm spells during the colder periods of plants' annual cycle. Because deacclimation is mainly driven by temperature, these changes pose a risk for untimely/premature deacclimation, thereby rendering plant tissue vulnerable to freeze-injury by a subsequent frost. Research also indicates that elevated CO₂ may directly impact deacclimation. Hence, understanding the underlying cellular mechanisms of how deacclimation and reacclimation capacity are affected by changes in environmental conditions is important to ensure winter survival and the sustainability of plant sources under changing climate. Relative to cold acclimation, deacclimation is a little studied process, but the limited evidence points to specific changes occurring in the transcriptome and proteome during deacclimation. Loss of freezing tolerance is additionally associated with substantial changes in cell/tissue-water relations and carbohydrate metabolism; the latter also impacted by temperature-driven, altered respiratory metabolism. This review summarizes recent progress in understanding the physiological mechanisms of deacclimation and how they may be impacted by climate change.

Deacclimation kinetics and carbohydrate changes in stem tissues of Hydrangea in response to an experimental warm spell

Temperate winters are becoming progressively milder due to climate warming, and temperature patterns are becoming increasingly irregular with risk of unseasonable warm spells. Warm spells may cause premature loss of plant cold hardiness and increase the risk of subsequent freezing injury. This study investigated the timing and rate of deacclimation and associated changes in soluble carbohydrates and water status in stems of Hydrangea macrophylla ssp. macrophylla (Thunb.) Ser. 'Alma' and Hydrangea paniculata Sieb. 'Vanille Fraise' in response to a simulated warm spell (22 °C/17 °C day/night). In H. macrophylla, deacclimation kinetics showed a sigmoid course with a short lag-phase followed by a fast deacclimation rate. In H. paniculata, the deacclimation pattern could not be determined precisely, but H. paniculata, the hardier genotype based on mid-winter freezing tolerance, deacclimated to a greater extent than H. macrophylla. These results imply that dehardening resistance is not related to mid-winter hardiness. In both species deacclimation was associated with rehydration and decreasing sugar levels, but species-specific quantitative and qualitative differences in the accumulation patterns of specific sugars were observed. In H. paniculata cold hardiness may be associated with 1-kestose, an oligofructan frequently associated with overwintering in herbaceous plants, but not previously related to freezing tolerance in woody perennials.

Changes in carbohydrates, ABA and bark proteins during seasonal cold acclimation and deacclimation in Hydrangea species differing in cold hardiness

Cold injury is frequently seen in the commercially important shrub Hydrangea macrophylla but not in Hydrangea paniculata. Cold acclimation and deacclimation and associated physiological adaptations were investigated from late September 2006 to early May 2007 in stems of field-grown H. macrophylla ssp. macrophylla (Thunb.) Ser. cv. Blaumeise and H. paniculata Sieb. cv. Kyushu. Acclimation and deacclimation appeared approximately synchronized in the two species, but they differed significantly in levels of mid-winter cold hardiness, rates of acclimation and deacclimation and physiological traits conferring tolerance to freezing conditions. Accumulation patterns of sucrose and raffinose in stems paralleled fluctuations in cold hardiness in both species, but H. macrophylla additionally accumulated glucose and fructose during winter, indicating species-specific differences in carbohydrate metabolism. Protein profiles differed between H. macrophylla and H. paniculata, but distinct seasonal patterns associated with winter acclimation were observed in both species. In H. paniculata concurrent increases in xylem sap abscisic acid (ABA) concentrations ([ABA](xylem)) and freezing tolerance suggests an involvement of ABA in cold acclimation. In contrast, ABA from the root system was seemingly not involved in cold acclimation in H. macrophylla, suggesting that species-specific differences in cold hardiness may be related to differences in [ABA](xylem). In both species a significant increase in stem freezing tolerance appeared long after growth ceased, suggesting that cold acclimation is more regulated by temperature than by photoperiod.