Email updates

Keep up to date with the latest news and content from BMC Plant Biology and BioMed Central.

Open Access Research article

Comparison of freezing tolerance, compatible solutes and polyamines in geographically diverse collections of Thellungiella sp. and Arabidopsis thaliana accessions

Yang Ping Lee1, Alexei Babakov2, Bert de Boer3, Ellen Zuther1 and Dirk K Hincha1*

Author affiliations

1 Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, Potsdam, D-14476, Germany

2 All-Russia Research Institute of Agricultural Biotechnology RAAS, Timiryazevskaya St. 42, Moscow, 127550, Russia

3 Department of Structural Biology, Vrije Universiteit Amsterdam, De Boelelaan 1085-1087, Amsterdam, 1081 HV, The Netherlands

For all author emails, please log on.

Citation and License

BMC Plant Biology 2012, 12:131  doi:10.1186/1471-2229-12-131

The electronic version of this article is the complete one and can be found online at: http://www.biomedcentral.com/1471-2229/12/131


Received:24 April 2012
Accepted:13 July 2012
Published:3 August 2012

© 2012 Lee et al.; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background

Thellungiella has been proposed as an extremophile alternative to Arabidopsis to investigate environmental stress tolerance. However, Arabidopsis accessions show large natural variation in their freezing tolerance and here the tolerance ranges of collections of accessions in the two species were compared.

Results

Leaf freezing tolerance of 16 Thellungiella accessions was assessed with an electrolyte leakage assay before and after 14 days of cold acclimation at 4°C. Soluble sugars (glucose, fructose, sucrose, raffinose) and free polyamines (putrescine, spermidine, spermine) were quantified by HPLC, proline photometrically. The ranges in nonacclimated freezing tolerance completely overlapped between Arabidopsis and Thellungiella. After cold acclimation, some Thellungiella accessions were more freezing tolerant than any Arabidopsis accessions. Acclimated freezing tolerance was correlated with sucrose levels in both species, but raffinose accumulation was lower in Thellungiella and only correlated with freezing tolerance in Arabidopsis. The reverse was true for leaf proline contents. Polyamine levels were generally similar between the species. Only spermine content was higher in nonacclimated Thellungiella plants, but decreased during acclimation and was negatively correlated with freezing tolerance.

Conclusion

Thellungiella is not an extremophile with regard to freezing tolerance, but some accessions significantly expand the range present in Arabidopsis. The metabolite data indicate different metabolic adaptation strategies between the species.

Keywords:
Arabidopsis thaliana; Cold acclimation; Compatible solutes; Freezing tolerance; Natural variation; Polyamines; Thellungiella salsuginea

Background

Low temperatures and freezing impose major limitations on plant growth and development and limit the productivity of crop plants in large parts of the world. Plants from temperate regions increase in freezing tolerance during exposure to low but nonfreezing temperatures for a period of days to weeks, a process termed cold acclimation. This is accompanied by massive changes in gene expression and metabolite composition [1-3], including increased levels of compatible solutes such as sugars, proline and polyamines that potentially contribute to cellular freezing tolerance.

The majority of molecular studies of plant freezing tolerance and cold acclimation have been performed in Arabidopsis thaliana. In addition to forward and reverse genetics, the analysis of natural variation has become an increasingly useful approach in the analysis of complex adaptive traits in this species (see [4-6] for reviews). Arabidopsis accessions are widely distributed throughout the Northern hemisphere, spanning diverse growth environments. It can therefore be expected that they harbour phenotypic and genetic variation that is advantageous for adaptation to various climatic conditions. Several studies have shown significant natural variation in the responses of Arabidopsis accessions to low temperature [7-13]. However, Arabidopsis is not an extremophile and it could be expected that more freezing tolerant species have evolved different or additional protective mechanisms that cannot be found in this species.

Thellungiella salsuginea is an emerging plant model species that has been suggested to possess the characteristics of an extremophile, i.e. high tolerance of salinity, freezing, nitrogen-deficiency and drought stress [14-19]. The genus Thellungiella is part of the Brassicaceae family and therefore related to Arabidopsis thaliana[20,21]. T. salsuginea resembles Arabidopsis in many features such as short life cycle, self-fertility, transformation by the floral-dip method and a genome size approximately twice that of Arabidopsis[17]. The genome of the closely related species T. parvula has recently been sequenced [22]. Similar to Arabidopsis, also in T. salsuginea different accessions have been identified and the Shandong and Yukon accessions, which originate from China and Canada, respectively, have frequently been used to investigate responses to abiotic stresses [21]. However, no systematic investigation of natural variation in the stress tolerance of Thellungiella has been published to date.

Here we present such a study, investigating the freezing tolerance and cold acclimation responses of 14 T. salsuginea accessions and of the two closely related species T. halophila and T. botschantzevii. We compare these data to the results of a recent study on 54 Arabidopsis accessions [13]. Our results suggest that the freezing tolerance after cold acclimation of the Thellungiella accessions extends to lower temperatures than the freezing tolerance of the most tolerant Arabidopsis accessions. In addition, the data provide the first evidence for a different metabolic acclimation strategy in Thellungiella compared to Arabidopsis.

Methods

Plant material

Seeds of the Thellungiella salsuginea ((Pallas) O.E. Schulz) accessions Colorado, Cracker Creek, Dillibrough, Hebei, Henan, Jiangsu, Shandong, Xinjiang and Yukon were kindly provided by Prof. Ray A. Bressan (Purdue University, West Lafayette, IN). Seeds of further T. salsuginea accessions (Altai 1, Altai 2, Buriatia, Tuva and Yakutsk), T. halophila ((C.A. Meyer) O.E. Schulz) (Bayanaul) and T. botschantzevii (D.A. German) (Saratov) were collected in Russia and Kazakhstan. The geographical origins of all accessions are listed in Table 1. The A. thaliana accessions used for polyamine determination are those used in our previous studies [7,13].

Table 1. Thellungiellaaccessions with information on their geographic origins

Seeds of the Thellungiella accessions were sown in soil and exposed to 4°C in a growth cabinet at 16 h day length with 90 μE m-2 s-1 for one week to promote germination. Seedlings were transferred to a greenhouse at 16 h day length with light supplementation to reach at least 200 μE m-2 s-1 at a temperature of 20°C during the day and 18°C during the night for 8 weeks (nonacclimated plants). For cold acclimation, plants were transferred to a 4°C growth cabinet under the conditions described above for an additional 14 days. Arabidopsis plants were grown and acclimated under identical conditions [7,11], but were only grown under nonacclimating conditions for 6 weeks to reach the same developmental state.

Freezing tolerance assays

Freezing damage was determined as electrolyte leakage after freezing of detached leaves to different temperatures as described in detail in previous publications [7,11]. Briefly, series consisting of three rosette leaves taken from three individual plants were placed in glass tubes containing 300 μl of distilled water. The tubes were transferred to a programmable cooling bath set to −1°C, control samples were left on ice during the entire experiment. After 30 min of temperature equilibration at −1°C, ice crystals were added to the tubes to initiate freezing. After another 30 min, the samples were cooled at a rate of 4°C/h. Over a temperature range of −1°C to −30°C samples were taken from the bath and thawed slowly on ice over night. Leaves were then immersed in distilled water and placed on a shaker for 16 h at 4°C. Electrolyte leakage was determined as the ratio of conductivity measured in the water before and after boiling the samples. The temperature of 50% electrolyte leakage (LT50) was calculated as the LOG EC50 value of sigmoidal curves fitted to the leakage values using the software GraphPad Prism3.

Sugar analysis

Two leaves from plants that were also used in the freezing tolerance assays were frozen in liquid nitrogen immediately after sampling and homogenized using a ball mill “Retsch MM 200” (Retsch, Haan, Germany). Soluble sugars were extracted and quantified by high performance anion exchange chromatography (HPAEC) using a CarboPac PA-100 column on an ICS3000 chromatography system (Dionex, Sunnyvale, CA) as described previously [24].

Proline measurements

Proline content was measured from the ethanolic extracts that were also used for sugar determination following a method modified from a previously described procedure [25,26]. The extracts were diluted 10-fold with distilled water and 100 μl were combined with 100 μl of glacial acetic acid and 100 μl of 2.5% (w/v) acid ninhydrine reagent [26]. The mixture was incubated at 95°C for 1 h and then for 10 min on ice. The reaction mixture was extracted with 500 μl of toluene and the ninhydrine absorbance was measured in the toluene phase at 520 nm in a spectrophotometer.

Polyamine measurements

Leaf samples (100–200 mg) were homogenized with a ball mill, extracted in 1 ml of 0.2 N perchloric acid for 1 h at 4°C to extract free polyamines and centrifuged at 16000 x g at 4°C for 30 min. Since we detected only very low levels of bound polyamines in our samples (data not shown), these were not further investigated. To 100 μl aliquots of the supernatants, 110 μl of 1.5 M sodium carbonate and 200 μl of dansyl chloride (7.5 mg/ml in acetone; Sigma, Munich, Germany) were added. In addition, 10 μl of 0.5 mM diaminohexane were added as an internal standard. After 1 h incubation at 60°C in the dark, 50 μl of a 100 mg/ml proline solution was added to bind free dansyl chloride [27]. After 30 min incubation at 60°C in the dark, dansylated polyamines were extracted with 250 μl toluene, dried in a vacuum centrifuge and dissolved in 100 μl methanol. Analyses were performed with a reverse phase LC-18 column (Supelco, Munich, Germany) on a HPLC system (Dionex) consisting of a gradient pump (model P 580), an automated sample injector (ASI-100) and a fluorescence detector (RF 2000). Twenty μl samples were injected, polyamines were eluted with a linear gradient of from 70% to 100% (v/v) methanol in water at a flow rate of 1 ml/min and detected at an excitation wavelength of 365 nm and an emission wavelength of 510 nm. Data were analyzed using the Dionex Chromeleon software and quantification was performed with calibration curves obtained from the pure substances.

Statistics

Correlation tests were performed using Pearson's product–moment correlation analysis in the R statistics package [28].

Results

Establishment of a collection of natural Thellungiella spec. accessions

We investigated the cold acclimation and freezing tolerance of 16 different Thellungiella accessions (Table 1). Of these, 14 belong to the species T. salsuginea and one each to T. halophila (Bayanaul) and T. botschantzevii (Saratov). Four of the accessions originate from the continental USA or Canada and five from China and substantial work has been performed previously on the accessions Yukon and Shandong (see [21] for a review). In addition, seven accessions were collected for this study from different sites in Russia and Kazakhastan to enrich our collection for accessions from very cold climates (Table 1). Thus the geographical origins of these accessions span the Northern hemisphere (between 33°N and 61°N) from 130°E to 135°W.

Natural variation in the freezing tolerance of Thellungiella accessions

The freezing tolerance of the Thellungiella accessions was determined before (nonacclimated; NA) and after (acclimated; ACC) two weeks of cold acclimation at 4°C (Figure 1). The results show strong natural variation in the freezing tolerance of Thellungiella. Higher variation was found in acclimated (LT50 from −9.12°C (Jiangsu) to −15.21°C (Tuva)), than in nonacclimated plants (LT50 from −5.70°C (Xinjiang) to −7.40°C (Bayanaul)). In addition, Tuva showed the highest acclimation capacity (8.22°C difference in LT50 between NA and ACC plants) and Jiangsu the lowest (3.28°C).

thumbnailFigure 1. Freezing tolerance of leaves from 16Thellungiellaaccessions before (NA) and after (ACC) 14 days of cold acclimation at 4°C. Freezing tolerance was measured with an electrolyte leakage assay and is expressed as the LT50, i.e. the temperature that resulted in 50% ion leakage from the leaves. All accessions and information on their geographical origins are listed in Table 1. The bars in the top panel represent the means ± SE from five replicate measurements where each replicate comprised leaves from three plants. The accessions are ordered from the lowest LT50 after cold acclimation on the left to the highest on the right. The bottom panel shows the range of LT50 values before and after cold acclimation for 54 Arabidopsis accessions [13] and the 16 Thellungiella accessions investigated in the present study.

Thellungiella is generally considered to be much more freezing tolerant than Arabidopsis [29]. The fact that we have recently determined the freezing tolerance of 54 Arabidopsis accessions under exactly the same conditions as used here for Thellungiella[13] provided a unique opportunity to test this assumption. Figure 1 clearly shows that the range of LT50 values was not different between Arabidopsis and Thellungiella in the nonacclimated state, but that some Thellungiella accessions (Tuva, Saratov, Altai 1 and 2, Bayanaul) reached lower LT50 values after cold acclimation.

No significant correlations at p < 0.05 were found between the latitude of the geographical origin of the accessions and their LT50 either before or after cold acclimation. However, LT50 ACC was significantly correlated with the average minimum habitat temperature recorded during the coldest month of the growth season, while no such correlation was found before cold acclimation (Figure 2).

thumbnailFigure 2. Correlation between the average minimum habitat temperature recorded during the coldest month of the growth season (Table1) and the LT50of the leaves from either nonacclimated (NA) or cold acclimated plants (ACC). The lines were fitted to the data by linear regression analysis and the correlation coefficients and p-values are shown in the figure.

Accumulation of sugars and proline in response to cold

The accumulation of compatible solutes such as sugars and proline is commonly observed during cold acclimation [2,3]. We therefore measured the amounts of glucose (Glc), fructose (Fru), sucrose (Suc), raffinose (Raf) and proline (Pro). Figures 3 and 4 show that the contents of sugars and Pro increased strongly in leaf samples of most Thellungiella accessions during cold acclimation. As observed previously for Arabidopsis[13], there were also some Thellungiella accessions that failed to accumulate a particular solute. For instance, Yakutsk showed an extremely low level of Fru after acclimation, while Dillibrough did not accumulate any Pro in the cold.

thumbnailFigure 3. Contents of soluble sugars in the leaves of all investigatedThellungiellaaccessions. Leaves were harvested either before (NA) or after (ACC) cold acclimation. Note the different scales of the ordinates in the different panels. The accessions are ordered from the lowest LT50 after cold acclimation on the left to the highest on the right. The bars represent means ± SE from measurements of seven to 10 samples from two independent experiments.

thumbnailFigure 4. Proline contents in the leaves of all investigatedThellungiellaaccessions. Leaves were harvested either before (NA) or after (ACC) cold acclimation. The accessions are ordered from the lowest LT50 after cold acclimation on the left to the highest on the right. The bars represent means ± SE from measurements of nine or 10 samples from two independent experiments.

We further explored the functional significance of these compatible solutes in leaf freezing tolerance by correlation analysis. The contents of sugars and Pro were not significantly correlated with LT50 under nonacclimated condition except for Glc (r = −0.619, p = 0.011). After cold acclimation, only the contents of Suc (Figure 5) and Pro (Figure 6) were significantly positively correlated with freezing tolerance (i.e. negative correlation with LT50), while the content of Fru was negatively correlated. In other words, the contents of Suc and Pro were higher in the more freezing tolerant accessions, while the contents of Fru was higher in the more sensitive accessions.

thumbnailFigure 5. Correlations among the contents of different soluble sugars inArabidopsisandThellungiellaand their freezing tolerance after cold acclimation. The lines were fitted to the data by linear regression analysis and the correlation coefficients and p-values are shown in the panels. The data for Thellungiella (solid symbols) are the same as those shown in Fig. 1 for LT50 and in Fig. 3 for sugar contents. The data for Arabidopsis (open symbols) are taken from [13].

thumbnailFigure 6. Correlations between the proline content ofArabidopsisandThellungiellaleaves and their freezing tolerance after cold acclimation. The lines were fitted to the data by linear regression analysis and the correlation coefficients and p-values are shown in the panels. The data for Thellungiella (solid symbols) are the same as those shown in Fig. 1 for LT50 and in Fig. 4 for proline content. The data for Arabidopsis (open symbols) are taken from [13].

Since we had previously also determined the sugar and Pro contents of the leaves of 54 Arabidopsis accessions [13], we could now directly compare the role of compatible solutes in the acclimated freezing tolerance of these species (Figures 5 and 6). While Glc, Fru and Suc contents were significantly positively correlated with freezing tolerance in Arabidopsis, this was only true for Suc in Thellungiella. However, the overall pool sizes of these sugars were similar, although some Arabidopsis accessions accumulated two- to three-fold higher amounts of Glc. The most striking differences were found for Raf and Pro. The amounts of Raf in the leaves of the most freezing tolerant acclimated Arabidopsis accessions were several-fold higher than those of any Thellungiella accessions. For example, the most freezing tolerant Arabidopsis accession (N14) contained about 10.5 μmol Raf g-1 FW, while all Thellungiella accessions accumulated less than 3 μmol g-1 FW. On the other hand, Pro levels were much higher in Thellungiella than in Arabidopsis leaves and there was no significant correlation between Pro contents and LT50 ACC in Arabidopsis (Figure 6). Some Thellungiella accessions already contained more Pro in their leaves in the nonacclimated state (up to 18.5 μmol g FW-1) than any Arabidopsis accession after cold acclimation (up to 14.9 μmol g FW-1).

Polyamine contents in Thellungiella and Arabidopsis accessions

There is evidence from several studies that polyamines may play important roles in the development of plant freezing tolerance (see [30] for a recent review). We have therefore measured the amounts of free putrescine (Put), spermidine (Spd) and spermine (Spm) in leaf samples from all Thellungiella accessions both before and after cold acclimation (Figure 7). Since no published data on the polyamine contents of different Arabidopsis accessions under these conditions were available, we also determined the respective polyamines in nine Arabidopsis accessions that span a wide range of freezing tolerance [7,13]. In general, the levels of Put and Spd were similar in Thellungiella and Arabidopsis and they either increased during cold acclimation or remained unaltered in some accessions (e.g. Dillibrough and Hebei; Te-0 and Can-0). However, the levels of Spm were much higher in nonacclimated Thellungiella leaves, but were drastically reduced during cold acclimation. In Arabidopsis, Spm levels were generally lower and only decreased in a few accessions during acclimation. In both species free Spd was the predominant polyamine under both conditions.

thumbnailFigure 7. Contents of soluble polyamines in the leaves of all investigatedThellungiellaand nineArabidopsisaccessions. Leaves were harvested either before (NA) or after (ACC) cold acclimation. Note the different scales of the ordinates in the different panels. The accessions are ordered from the lowest LT50 after cold acclimation on the left to the highest on the right separately for Thellungiella and Arabidopsis. The bars represent means ± SE from measurements of eight to 10 samples from two independent experiments for Thellungiella and three samples from one experiment for Arabidopsis.

No significant correlations were found among the Put, Spd or Spm contents and LT50 NA (not shown) or Put or Spd contents and LT50 ACC in Thellungiella (Figure 8). However, there was a significant correlation between Spm content and LT50 ACC in Thellungiella, indicating that higher leaf freezing tolerance was correlated with a lower pool size of free Spm. In Arabidopsis, no correlations among LT50 and polyamine pool sizes were observed under either condition.

thumbnailFigure 8. Correlations among the contents of different soluble polyamines in theThellungiellaandArabidopsisaccessions and their freezing tolerance after cold acclimation. The lines were fitted to the data by linear regression analysis and the correlation coefficients and p-values are shown in the panels. Solid symbols denote data from Thellungiella, open symbols data from Arabidopsis.

Discussion

Thellungiella has been proposed as an alternative model species to Arabidopsis to investigate plant abiotic stress tolerance mechanisms. Thellungiella shares many features with Arabidopsis that make it an attractive candidate for both physiological and molecular studies [14,21,29]. The main argument in favor of Thellungiella, however, is that it is considered an “extremophile” that is much more tolerant to various stresses than Arabidopsis. On the other hand, it has been shown that there is considerable natural variation between different accessions of Arabidopsis that results in different levels of tolerance under various environmental growth and stress conditions (see e.g. [6] for a recent review). This natural variation has been investigated most extensively for cold acclimation and freezing tolerance [7,8,10,12,13]. Since natural accessions are also available for Thellungiella this opens the unique possibility to directly compare the range of stress tolerance and possible differences in adaptive mechanisms between these species.

In the present study, we have for the first time compared the range of natural variation in the freezing tolerance of Arabidopsis and Thellungiella. We conclude from the wide overlap in the freezing tolerance that at least with regard to this trait Thellungiella should not be considered an extremophile. Its range of freezing tolerance, however, extends to lower temperatures than that of Arabidopsis with about one-third of the available Thellungiella accessions more freezing tolerant than any Arabidopsis accession. The acclimated freezing tolerance of Thellungiella was positively correlated with the average minimum habitat temperature recorded during the coldest month of the growth season, consistent with previous results for Arabidopsis[7,12].

Only the freezing tolerance of the Yukon accession of Thellungiella has previously been reported in the literature [16]. LT50 values of −13°C for nonacclimated and −18.5°C for cold acclimated plants were recorded when whole-plant survival was evaluated. These temperatures are substantially lower than the −6.4°C (NA) and −11.7°C (ACC) obtained from our electrolyte leakage measurements. However, corresponding electrolyte leakage data in [16] suggest a similar temperature range to our results although no LT50 values were given. In addition, since no direct comparison with Arabidopsis was presented, any comparison between the species remained speculative in this paper.

From the comparison presented here we suggest that although Thellungiella may not be an extremophile with regard to freezing tolerance, its range of freezing tolerance after cold acclimation clearly extends beyond Arabidopsis. We therefore consider Thellungiella a useful additional model species to identify superior or alternative freezing tolerance mechanisms.

During cold acclimation in Arabidopsis, the composition of the metabolome is strongly changed (see [1] for a review). The pool sizes of several metabolites are increased and there are significant differences in the cold-responsive metabolomes of different Arabidopsis accessions [7,31,32]. Significantly, the leaf contents of the four sugars Glc, Fru, Suc and Raf were linearly correlated with leaf freezing tolerance [8,11,13] and these sugars were also found among a small group of metabolites that could be used to predict the freezing tolerance of several Arabidopsis genotypes with high accuracy [32]. In addition, although the Pro contents of the leaves also increased during cold acclimation, there was no correlation with freezing tolerance among the 54 accessions investigated previously [13] and Pro was also not among the predictive metabolites [32].

The present data suggest that the role of these five compatible solutes may be significantly different between Arabidopsis and Thellungiella. Among the sugars, a positive correlation with acclimated freezing tolerance was only observed for Suc, while there was actually a negative correlation for Fru. In addition, the Thellungiella accessions did not accumulate Raf to the same extent as Arabidopsis. Instead, Thellungiella accumulated much higher amounts of Pro during cold acclimation and we found a significant correlation with acclimated freezing tolerance. The accumulation of compatible solutes, particularly Suc and Pro, was not only found in Thellungiella plants during cold acclimation. Especially Pro contents also increased much more than in Arabidopsis when plants were challenged with high NaCl concentrations [15,33,34] suggesting a different metabolic adaptation strategy between the species under abiotic stress conditions. Obviously, this hypothesis has to be tested in the future by metabolomic approaches using appropriate collections of accessions from both species.

We would like to stress at this point that it is highly unlikely that the differences in compatible solute content are the only reason for the observed differences in freezing tolerance. Although the constitutively freezing tolerant esk1 mutant in Arabidopsis shows a high accumulation of Pro under nonacclimated conditions [35], it also shows hundreds of changes in gene expression, making it impossible to attribute the higher freezing tolerance to a single factor [36]. Similarly, although freezing tolerance in Arabidopsis is strongly correlated with Raf content, a knock-out mutant of the raffinose synthase gene in Col-0 resulted in the absence of Raf in the cold acclimated leaves without an impairment of freezing tolerance [37]. All these findings emphasize the well-known fact that plant freezing tolerance is a multigenic, quantitative trait. In addition, the present data indicate that even in closely related species, different metabolites may be important.

One additional class of metabolites that has frequently been implicated in plant freezing tolerance are polyamines [30]. They are thought to be involved in many aspects of plant growth, development and stress tolerance (see [38-40] for reviews). Their exact functions in these processes have not been completely elucidated, but it was demonstrated that Put is an essential component of the cold acclimation process in Arabidopsis[41]. This is at least in part mediated through a role in the regulation of ABA biosynthesis.

The measurement of free polyamine levels in several accessions of both Arabidopsis and Thellungiella revealed that not all accessions showed an increase in the content of Put or Spd during cold acclimation. Also, the levels of free Put and Spd were not correlated with leaf freezing tolerance. In fact, the most freezing tolerant Arabidopsis accession in this study (Te-0) showed no increase in the pool size of either polyamine. In addition, the overall amounts of Put and Spd were very similar in all studied plants. Only the contents of free Spm showed higher levels in Thellungiella under nonacclimating conditions than in Arabidopsis. This was, however, strongly decreased during cold acclimation, leading to similar pool sizes between the species in the acclimated state. In Thellungiella we found a negative correlation between Spm contents and LT50 ACC, indicating that low levels of Spm may be a requirement for efficient cold acclimation. A similar reduction of Spm levels was previously already observed in the Arabidopsis accession Col-0 [41] and in wheat [42] in response to cold exposure. However, the functional relevance of this reduction of free Spm levels is currently unknown. The natural variation in Spm content revealed in this study may offer an interesting possibility to elucidate the molecular basis and functional significance of this phenomenon.

Conclusion

While Thellungiella is generally assumed to be an extremophile with regard to its abiotic stress tolerance, the presented data indicate that this is not true with regard to its freezing tolerance. Some accessions, however, significantly expand the range present in Arabidopsis, stressing the utility of Thellungiella as an additional model species. The metabolite data indicate different metabolic adaptation strategies between these rather closely related species that need to be followed up with appropriate profiling technologies.

Competing interests

The authors declare that they have no competing interests.

Authors´ contributions

YPL carried out the freezing tolerance experiments and the proline measurements, YPL and EZ performed the sugar and polyamine determinations. AB and BdB collected and provided Thellungiella seeds. YPL, EZ and DKH designed the study and analyzed the data. YPL and DKH drafted the manuscript. All authors read and approved the manuscript.

Acknowledgements

We would like to thank Prof. Ray Bressan (Purdue University, West Lafayette, IN) for making his Thellungiella salsuginea seed collection available to us, Dr. Dmitry German (University Heidelberg, Germany) for collecting the seeds from T. halophila (Bayanaul) and Ulrike Seider and Astrid Basner for excellent technical assistance. YPL thanks the Swiss National Science Foundation and the Max Planck Society for post-doctoral fellowships. AB was supported by a grant from the Russian Foundation of Basic Research (05-04-89005-NWO-a) and BdB by a grant from The Netherlands Organization for Scientific Research (047.017.004).

References

  1. Guy CL, Kaplan F, Kopka J, Selbig J, Hincha DK: Metabolomics of temperature stress.

    Physiol Plant 2008, 132:220-235. PubMed Abstract | Publisher Full Text OpenURL

  2. Smallwood M, Bowles DJ: Plants in a cold climate.

    Phil Trans R Soc Lond B 2002, 357:831-847. Publisher Full Text OpenURL

  3. Xin Z, Browse J: Cold comfort farm: the acclimation of plants to freezing temperatures.

    Plant Cell Environ 2000, 23:893-902. Publisher Full Text OpenURL

  4. de Meaux J, Koornneef M: The cause and consequences of natural variation: the genomic era takes off!

    Curr Opin Plant Biol 2008, 11:99-102. PubMed Abstract | Publisher Full Text OpenURL

  5. Koornneef M, Alonso-Blanco C, Vreugdenhil D: Naturally occurring genetic variation in Arabidopsis thaliana.

    Annu Rev Plant Biol 2004, 55:141-172. PubMed Abstract | Publisher Full Text OpenURL

  6. Weigel D: Natural variation in Arabidopsis: from molecular genetics to ecological genomics.

    Plant Physiol 2012, 158:2-22. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  7. Hannah MA, Wiese D, Freund S, Fiehn O, Heyer AG, Hincha DK: Natural genetic variation of freezing tolerance in Arabidopsis.

    Plant Physiol 2006, 142:98-112. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  8. Korn M, Peterek S, Mock H-P, Heyer AG, Hincha DK: Heterosis in the freezing tolerance, and sugar and flavonoid contents of crosses between Arabidopsis thaliana accessions of widely varying freezing tolerance.

    Plant Cell Environ 2008, 31:813-827. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  9. Lee YP, Fleming AJ, Körner C, Meins F: Differential expression of the CBF pathway and cell cycle-related genes in Arabidopsis accessions in response to chronic low-temperature exposure.

    Plant Biol 2009, 11:273-283. PubMed Abstract | Publisher Full Text OpenURL

  10. McKhann HI, Gery C, Berard A, Leveque S, Zuther E, Hincha DK, de Mita S, Brunel D, Teoule E: Natural variation in CBF gene sequence, gene expression and freezing tolerance in the Versailles core collection of Arabidopsis thaliana.

    BMC Plant Biol 2008, 8:105. PubMed Abstract | BioMed Central Full Text | PubMed Central Full Text OpenURL

  11. Rohde P, Hincha DK, Heyer AG: Heterosis in the freezing tolerance of crosses between two Arabidopsis thaliana accessions (Columbia-0 and C24) that show differences in non-acclimated and acclimated freezing tolerance.

    Plant J 2004, 38:790-799. PubMed Abstract | Publisher Full Text OpenURL

  12. Zhen Y, Ungerer MC: Clinal variation in freezing tolerance among natural accessions of Arabidopsis thaliana.

    New Phytol 2008, 177:419-427. PubMed Abstract OpenURL

  13. Zuther E, Schulz E, Childs LH, Hincha DK: Clinal variation in the nonacclimated and cold acclimated freezing tolerance of Arabidopsis thaliana accessions.

    Plant Cell Environ 2012.

    in press

    Publisher Full Text OpenURL

  14. Bressan RA, Zhang C, Zhang H, Hasegawa PM, Bohnert HJ, Zhu JK: Learning from the Arabidopsis experience. The next gene search paradigm.

    Plant Physiol 2001, 127:1354-1360. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  15. Gong Q, Li P, Ma S, Indu RS, Bohnert HJ: Salinity stress adaptation competence in the extremophile Thellungiella halophila in comparison with its relative Arabidopsis thaliana.

    Plant J 2005, 44:826-839. PubMed Abstract | Publisher Full Text OpenURL

  16. Griffith M, Timonin M, Wong ACE, Gray GR, Akhter SR, Saldanha M, Rogers MA, Weretilnyk EA, Moffatt BA: Thellungiella: an Arabidopsis-related model plant adapted to cold temperatures.

    Plant Cell Environ 2007, 30:529-538. PubMed Abstract | Publisher Full Text OpenURL

  17. Inan G, Zhang Q, Li P, Wang Z, Cao Z, Zhang H, Zhang C, Quist TM, Goodwin SM, Zhu J, et al.: Salt cress. A halophyte and cryophyte Arabidopsis relative model system and its applicability to molecular genetic analyses of growth and development of extremophiles.

    Plant Physiol 2004, 135:1718-1737. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  18. Kant S, Bi YM, Weretilnyk E, Barak S, Rothstein SJ: The Arabidopsis halophytic relative Thellungiella halophila tolerates nitrogen-limiting conditions by maintaining growth, nitrogen uptake, and assimilation.

    Plant Physiol 2008, 147:1168-1180. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  19. Wong CE, Li Y, Whitty BR, Díaz-Camino C, Akhter SR, Brandle JE, Golding GB, Weretilnyk EA, Moffatt BA, Griffith M: Expressed sequence tags from the Yukon ecotype of Thellungiella reveal that gene expression in response to cold, drought and salinity shows little overlap.

    Plant Mol Biol 2005, 58:561-574. PubMed Abstract | Publisher Full Text OpenURL

  20. Al-Shehbaz IA, O'Kane SL, Price RA: Generic placement of species excluded from Arabidopsis (Brassicaceae).

    Novon 1999, 9:296-307. Publisher Full Text OpenURL

  21. Amtmann A: Learning from evolution: Thellungiella generates new knowledge on essential and critical components of abiotic stress tolerance in plants.

    Mol Plant 2009, 2:3-12. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  22. Dassanayake M, Oh D-H, Haas JS, Hernandez A, Hong H, Ali S, Yun D-J, Bressan RA, Zhu J-K, Bohnert HJ, et al.: The genome of the extremophile crucifer Thellungiella parvula.

    Nat Genet 2011, 43:913-918. PubMed Abstract | Publisher Full Text OpenURL

  23. Fan SJ: Studies on population genetic diversity and molecular evolution of Thellungiella salsuginea. Shandong Normal University, Doctoral dissertation. Jinan; 2007. OpenURL

  24. Zuther E, Kwart M, Willmitzer L, Heyer AG: Expression of a yeast-derived invertase in companion cells results in long-distance transport of a trisaccharide in an apoplastic loader and influences sucrose transport.

    Planta 2004, 218:754-766. OpenURL

  25. Ábrahám E, Hourton-Cabassa C, Erdei L, Szabados L: Methods for determination of proline in plants. In Plant Stress Tolerance: Methods and Protocols. Edited by Sunkar R. Humana Press, New York, NY; 2010:317-331. OpenURL

  26. Bates LS, Waldren RP, Teare ID: Rapid determination of free proline for water-stress studies.

    Plant Soil 1973, 39:205-207. Publisher Full Text OpenURL

  27. Smith MA, Davies PJ: Separation and quantitation of polyamines in plant tissue by high performance liquid chromatography of their dansyl derivatives.

    Plant Physiol 1985, 78:89-91. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  28. R Development Core Team: R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria; 2010. OpenURL

  29. Amtmann A, Bohnert HJ, Bressan RA: Abiotic stress and plant genome evolution. Search for new models.

    Plant Physiol 2005, 138:127-130. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  30. Alcázar R, Cuevas JC, Planas J, Zarza X, Bortolotti C, Carrasco P, Salinas J, Tiburcio AF, Altabella T: Integration of polyamines in the cold acclimation response.

    Plant Sci 2011, 180:31-38. PubMed Abstract | Publisher Full Text OpenURL

  31. Cook D, Fowler S, Fiehn O, Thomashow MF: A prominent role for the CBF cold response pathway in configuring the low-temperature metabolome of Arabidopsis.

    Proc Natl Acad Sci USA 2004, 101:15243-15248. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  32. Korn M, Gärtner T, Erban A, Kopka J, Selbig J, Hincha DK: Predicting Arabidopsis freezing tolerance and heterosis in freezing tolerance from metabolite composition.

    Mol Plant 2010, 3:224-235. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  33. Kant S, Kant P, Raveh E, Barak S: Evidence that differential gene expression between the halophyte, Thellungiella halophila, and Arabidopsis thaliana is responsible for higher levels of the compatible osmolyte proline and tight control of Na + uptake in T. halophila.

    Plant Cell Environ 2006, 29:1220-1234. PubMed Abstract | Publisher Full Text OpenURL

  34. Lugan R, Niogret MF, Leport L, Guégan JP, Larher FR, Savouré A, Kopka J, Bouchereau A: Metabolome and water homeostasis analysis of Thellungiella salsuginea suggests that dehydration tolerance is a key response to osmotic stress in this halophyte.

    Plant J 2010, 64:215-229. PubMed Abstract | Publisher Full Text OpenURL

  35. Xin Z, Browse J: eskimo1 mutants of Arabidopsis are constitutively freezing-tolerant.

    Proc Natl Acad Sci USA 1998, 95:7799-7804. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  36. Xin Z, Mandaokar A, Chen J, Last RL, Browse J: Arabidopsis ESK1 encodes a novel regulator of freezing tolerance.

    Plant J 2007, 49:786-799. PubMed Abstract | Publisher Full Text OpenURL

  37. Zuther E, Büchel K, Hundertmark M, Stitt M, Hincha DK, Heyer AG: The role of raffinose in the cold acclimation response of Arabidopsis thaliana.

    FEBS Lett 2004, 576:169-173. PubMed Abstract | Publisher Full Text OpenURL

  38. Bouchereau A, Aziz A, Larher F, Martin-Tanguy J: Polyamines and environmental challenges: recent development.

    Plant Sci 1999, 140:103-125. Publisher Full Text OpenURL

  39. Galston AW, Sawhney RK: Polyamines in plant physiology.

    Plant Physiol 1990, 94:406-410. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  40. Groppa MD, Benavides MP: Polyamines and abiotic stress: recent advances.

    Amino Acids 2008, 34:35-45. PubMed Abstract | Publisher Full Text OpenURL

  41. Cuevas JC, López-Cobollo R, Alcázar R, Zarza X, Koncz C, Altabella T, Salinas J, Tiburcio AF, Ferrando A: Putrescine is involved in Arabidopsis freezing tolerance and cold acclimation by regulating abscisic acid levels in response to low temperature.

    Plant Physiol 2008, 148:1094-1105. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  42. Nadeau P, Delaney S, Chouinard L: Effects of cold hardening on the regulation of polyamine levels in wheat (Triticum aestivum L.) and Alfalfa (Medicago sativa L.).

    Plant Physiol 1987, 84:73-77. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL