Cold is one of the most significant abiotic stresses that restrict crop growth and productivity worldwide. In order to investigate how spring barley (Aths cultivar) adapts to short-term cold stress, the present study attempted to explore proteomic, physiological, and biochemical changes that occur in leaves. Barley seedlings were exposed to low temperature (4°C) for 48 hours, and third leaves were harvested and compared with plants grown in normal conditions (25°C). In coldstressed plants, results indicated a significant increase in hydrogen peroxide content, associated with a highly significant rise of lipid peroxidation (as demonstrated by MDA measurements) and catalase activity; the latter represents one of the oxidative stress resistance strategies adopted to promote cell survival. Cold stress also caused a significant reduction in chlorophyll b (chlb) content with a parallel increase in the chla/chlb ratio, which is probably related to defense mechanisms against cold-induced production of reactive oxygen species. On the contrary, other physiological and biochemical traits [namely, the membrane stability index (MSI), peroxidase activity, electron leakage, carotenoid and chla content] showed no statistically significant differences. The proteomic analysis revealed fifteen statistically significant cold-modulated spots, seven of which were successfully identified by LC-MS/MS. These responsive proteins are related to the Calvin cycle, photosynthetic electron transport, light reactions, and signal transduction. An increase in abundance of proteins involved in the regulation of the chloroplast system probably reflects plant acclimation to cold, thus confirming that cold stress severely affects photosynthesis in spring barley.

Longo V, Valizadeh Kamran R, Michaletti A, Toorchi M, Zolla L, Rinalducci S (2017) Proteomic and Physiological Response of Spring Barley Leaves to Cold Stress. Int J Plant Biol Res 5(1): 1061.

•    Spring barley
•    Cold stress
•    Proteome
•    LC-MS/MS

Barley is akin to other plants from the tribe Triticeae and exists in spring and winter varieties. In order to avoid heat and drought stress at the end of the season, spring cultivars must be planted earlier [1]. Frost in the late spring during seedling development is a major factor that affects spring barley production, as it can result in mortality, poor crop establishment, and subsequent yield reduction [2]. Spring cold stress can damage wheat and barley, causing yield penalties as high as 30-50% [3]. The risk of spring freeze is likely to be overlooked in a preponderance of breeding programs due to the significance of early maturity. As a result, presently spring freeze is considered much less than cold acclimatization and freeze hardiness [4]. Cold stress leads to a broad range of responses in plants, including physiobiochemical responses, which take place together with gene expression fluctuations [5]. These changes make plants more tolerant to cold conditions. Many studies have been conducted over the last years that explore plant response to cold stress in wheat and barley [6,7].

The cell membrane is considered to be the first possible target of cold. Cold disfigures the membrane and disturbs its activity [8]. It causes membrane electrolyte leakage for tissues in the sensitive plants that are incapable of increasing the fluidity of the living membrane by increasing unsaturated fatty acids [9]. Freeze and cold stress create disorder in plant metabolism and intensify the production of various active oxygen molecules. These free radicals damage cell macromolecules through the oxidation of biotic molecules and impose oxidative stress on the plant [10]. One of these active oxygen molecules is hydrogen peroxide (H2 O2 ), which, through the peroxidation of unsaturated fatty acids in membrane lipids, affects their selective permeability, thereby damaging membranes [11]. The destruction of membrane unsaturated fatty acids leads to the production of malondialdehyde (MDA), which is measured as a suitable marker for identifying the rate and intensity of oxidative damage to biotic membranes [12]. To resist active oxygen radicals and diminish their detrimental effects, the cell produces or activates antioxidant enzymes, among which are catalase [13]. Proteins perform a major role in plant stress response because they participate in both structural and metabolic changes directly. They also participate in energy metabolism, photosynthesis, and reactive oxygen species (ROS) scavenging, storage, protection from stress, regulation of the cell cycle, and plant development during freezing conditions and cold stress [14]. Studies on proteomics have substantially contributed to the identification and characterization of numerous new proteins that are involved in the plant cold response. There is a preponderance of works regarding proteomics that have been published on cold response in the wheat leaf and crown [15- 21], whereas only limited information is available regarding the proteomic response of barley.

This study was carried out in order to explore physiological, biochemical, and proteome changes of barley leaves exposed to low temperature (4 °C) for 48 hours by two-dimensional gel electrophoresis (2D-GE) coupled with peptide mass fingerprinting for protein identification using LC-MS/MS.

Plant growth conditions and cold stress treatments

The plant material (barley line Aths) was a sensitive genotype according to freezing measurements reported in [22]. The seeds were obtained from the Seed and Plant Improvement Institute (SPII) in Karaj, Iran. After initial germination on filter paper, the seeds were planted in pots that were 15 cm in diameter and 50 cm in length, and contained fine sand and perlite (10:1). Plants were grown in an experimental greenhouse under 70% humidity, in 16-hour daylight at 25 °C, and with light intensity of 300 µmol m-2 s-1. They were regularly irrigated up to the threeleaf stage. Afterwards, a number of randomly chosen pots were subjected to 4 °C acclimation for 48 hours (cold treatment) in a freezing machine under the same light regime of control plants. Plant leaves were harvested and compared with those grown at a normal temperature to measure physiological and biochemical traits and perform proteome analysis.

Membrane stability index (MSI)

This index was determined by measuring the electrical conductivity of the substances leaked from the leaf samples into double-distilled water at 40 and 100 °C [23]. The MSI was determined in percentage terms according to the following formula: MSI = [1 - (C1 /C2 )] × 100.

H2O2 content

The H2 O2 content was measured according to the method provided by Gong et al. [24]. In 0.1% (v/w) trichloroacetic acid and liquid nitrogen, 0.2 g of the leaf sample was homogenized.

The homogenized solution was centrifuged at 12000 g for 15 minutes. Then 0.5 ml of a 10 mmol potassium phosphate buffer (pH = 7.5) and 1 ml of 1 mol potassium iodide were added to 0.5 ml of the supernatant. The absorbance was read at 390 nm. The H2 O2 content was determined by using the standard curve for pure H2 O2 .

Lipid peroxidation rate

The level of lipid peroxidation was measured with regard to the thiobarbituric acid reactive content. In 10 ml of 0.1% (v/w) trichloroacetic acid, 0.5 g of the leaf sample was homogenized. The homogenized solution was centrifuged at 15000 g for 5 minutes. Then 4 ml of 0.5% (v/w) thiobarbituric acid in 20% (v/w) trichloroacetic acid was added to 2 ml of the supernatant. The mixture was heated at 95 °C for 30 minutes and then was cooled in an ice bar. After being centrifuged at 10000 g for 10 minutes, supernatant absorbance was read at 532 nm. Nonspecific absorbance at 600 nm was deducted from this content. The total content of thiobarbituric acid was measured with the extinction coefficient of 155 mm−1cm−1 [25].

Pigment content

For the measurement of the chlorophyll and carotenoid content, the leaf sample (0.05 g) was incubated in 5 ml of dimethyl sulfoxide (DMSO) at 65 °C for 4 hours. Absorbance was recorded at 645, 665, and 470 nm, and the quantities of chlorophyll a (chla), chlorophyll b (chlb), and chla/chlb were calculated [26].

Rate of catalase activity

The rate of catalase activity was measured according to the method provided by Aebi [27]. Next, 0.5 g of the leaf sample was homogenized in a 0.1 M cold potassium phosphate buffer (pH = 7.5) with 0.5 M of EDTA. The homogenized sample was centrifuged at 15000 g for 15 minutes at 4 °C. Then, 0.05 ml of the supernatant was added to 1.5 ml of a 0.1 mM phosphate buffer (pH = 7) and 1.45 ml of double-distilled water. The reaction began by adding 0.5 ml of 75 mM H2 O2 . The reduction in absorbance was recorded at 240 nm for 1 minute.

Rate of peroxidase activity

The activity of this enzyme was assessed according to the method described by Macadam et al. [28]. Next, 0.5 g of the leaf sample was homogenized in a 0.1 M cold potassium phosphate buffer (pH = 7.5) with 0.5 M of EDTA. The homogenized sample was centrifuged at 15000 g at 4 °C for 15 minutes. Then, 20 μl of the supernatant was added to 0.81 ml of a 50 M potassium phosphate buffer (pH = 6.6). Next, 90 ml of 1% guaiacol as an electron donor was added to the resultant mixture. The reaction mixture was added to the cuvette. Moreover, before measuring the rate of the reaction, 90 ml of 3% H2 O2 was added to the reaction mixture as an electron acceptor. Also, absorbance was measured at 470 nm for 60 seconds at 25 °C.

Data analysis for physiological and biochemical traits

A t-test was used to test the statistical difference between cold stress and control pots after clearing homogeneity of within-group variances. Data analysis was performed using SPSS software.

Extraction of proteins

Total protein extracts were isolated from approximately 0.5 g of a frozen root per biological replicate (n = 3) to obtain a fine powder, which was then suspended in cold acetone containing 10% TCA and 0.07% 2-Mercapthoetanol. The resultant powder was dissolved in the lysis buffer containing 7 M Urea, 2 M thiourea, 2% CHAPS, 60 mM DDT, and 1% ampholyte (pH = 3-10). Protein concentration was determined by the Bradford method [29].

2DE

To remove lipids, 600 μg of protein was precipitated from a desired volume of each sample with a cold mix of tri-n-butyl phosphate/acetone/methanol (1:12:1). After incubation at 4 °C for 90 min, the precipitate was pelleted by centrifugation at 13500 rpm for 20 min at 4 °C. The pellet was air-dried and then solubilized in a buffer containing 7 M urea, 2 M thiourea, 4% (w/v) CHAPS, and 40 mM Tris-HCL. The sample was subsequently reduced (5 mM tributylphosphine, 1 h) and alkylated (7.7 mM IAA, 1 h). To prevent over-alkylation, iodoacetamide (IAA) excess was neutralized by adding 10 mM DTE. The sample was included in the rehydration solution (7M Urea, 2 M Thiourea, 4% CHAPS and 0.5% w/v pH 3-10 carrier ampholyte) (Bio-Rad, CA,USA) and was taken up into the 17 cm IPG strips pH 3-10 (Bio-Rad, CA, USA) passively during rehydration over night. IEF was run on a Protean IEF Cell (Bio-Rad, CA, USA) at a 20 °C constant temperature and the total product time×voltage applied was 80,000 V-h. After IEF, the IPG gel strips were incubated at room temperature for 30 min in 6 M Urea, 30% w/v glycerol, 2% w/v SDS, 50 mM Tris-HCL, and pH 8.8. The strips were sealed at the top of a 1.5 mm vertical second dimensional gel home-made with 0.5% agarose in 25 mM Tris, 192 mM glycine, 0.1% SDS, and pH 8.3. SDS-PAGE was carried out on homogeneous running gels 12% T, 2.6% C. Run conditions were 40mA/gel until the bromophenol blue reached the bottom of the gel. Protein spots were stained by sensitive Coomassie brilliant blue G-250 stain.

Image analysis

Three biological replicates and two technical replicates were carried out, making a total of 12 gels per growth conditions. 2DE maps were digitalized using a ChemiDoc™ XRS+ Systems with the Image Lab™ Software (Bio-Rad Hercules, CA). The scanned images were processed and statistically evaluated with the Progenesis SameSpots software (Nonlinear Dynamics, UK). All spots were pre-filtered and manually checked before applying the statistical criteria (ANOVA p < 0.05 and fold ≥ 1.5).

In-gel digestion

Gel bands were carefully excised from the gel and subjected to in-gel trypsin digestion according to Shevchenko et al. [30], with minor modifications. The gel pieces were swollen in a digestion buffer containing 50 mM NH4 HCO3 and 12.5 ng/ml trypsin (modified porcine trypsin, sequencing grade, Promega, Madison, and WI) in an ice bath. After 30 min, the supernatant was removed and discarded; then, 20 ml of 50 mM NH4 HCO3 was added to the gel pieces, and digestion was allowed to proceed overnight at 37 °C. The supernatant containing tryptic peptides was dried by vacuum centrifugation.

LC-MS/MS analysis

Peptide extracts were analyzed by using a split-free nanoflow liquid chromatography system (EASY-nLC II, Proxeon, Odense, Denmark) coupled with a 3D-ion trap (model AmaZon ETD, Bruker Daltonik, Germany) equipped with an online ESI nanosprayer (the spray capillary was a fused silica capillary, 0.090 mm OD, 0.020 mm ID) in the positive-ion mode. For all experiments, a sample volume of 15 μl was loaded by the autosampler onto a homemade 2-cm fused silica precolumn (100 μm I.D.; 375 μm O.D.; Reprosil C18-AQ, 5 μm, Dr. Maisch GmbH, Ammerbuch-Entringen, Germany). Sequential elution of peptides was accomplished by using a flow rate of 300 nl/min and a linear gradient from Solution A (2% acetonitrile; 0.1% formic acid) to 50% of Solution B (98% acetonitrile; 0.1% formic acid) in 40 min over the precolumn on-line with a homemade 15-cm resolving column (75 μm ID; 375 μm OD; Reprosil C18-AQ, 3 μm, Dr. Maisch GmbH, Ammerbuch-Entringen, Germany). The acquisition parameters for the mass spectrometer were as follows: dry gas temperature, 220 °C; dry gas, 4.0 l/min; nebulizer gas, 10 psi; electrospray voltage, 4000 v; high-voltage end-plate offset, -200 v; capillary exit, 140 v; trap drive, 63.2; funnel 1 in 100 v out of 35 v and funnel 2 in 12 v out of 10 v; ICC target, 200,000; and maximum accumulation time, 50 ms. The sample was measured with the Enhanced Resolution Mode at 8100 m/z per second (which allows monoisotopic resolution up to four charge stages), scan range from m/z 300 to 1500, 5 spectra averaged, and rolling average of 1. The “Smart Decomposition” was set to “auto.” Automatic MS/MS fragmentation was performed on the three most intense precursor ions of each spectrum. Precursor ions were excluded from MS/MS fragmentation after appearance in 2 spectra and released after 1 minute. Acquired MS/MS spectra were processed in Data Analysis 4.0 and submitted to the Mascot search program (in-house version 2.2, Matrix Science, London, UK). The following parameters were adopted for database searches: NCBInr database; taxonomy = Viridiplantae; peptide and fragment mass tolerance = ± 0.3 Da; enzyme specificity trypsin with 1 missed cleavages considered; fixed modifications: carbamidomethyl (C); variable modifications: oxidation (M).

Cold effects on physiological and biochemical traits

After verifying the normality of the data, the t-test was performed, and the results showed that the difference between the control condition (25 °C) and the cold stress condition (4 °C) in five physiological and biochemical traits, namely H2 O2 , catalase activity, MDA, chlorophyll-b content, and chl-a/chl-b ratio, was significant (Figure 1, panels A-C, H, I); however, the peroxidase activity, MSI, carotenoids, and chl-a content had no statistically significant differences ((Figure 1), panels D-G).

After being subjected to cold stress at 4 °C, the H2 O2 content increased significantly (p < 0.001) ((Figure 1), panel A). This increase may be due to the following reasons: (1) upon the water shortage through plant stress, oxygenation reaction of RuBisCO in the chloroplast increases, thus causing a rise in the glycolate content. Glycolate in peroxisomes is converted into glyoxylate by the glycolate oxidase enzyme, which is a reaction leading to the generation of H2 O2 [31]; (2) under stresses, oxygen in peroxisomes acquires the hydrogens that result from the dehydrogenation of the fatty acids produced from the betaoxidation process and ultimately changes to H2 O2 [32]; (3) the formation of H2 O2 by means of amine oxidase activity in the apoplast is also induced by stress [33]. The increase in the H2 O2 level through cold stress has been previously reported, although in different plants [7]. In parallel with the H2 O2 accumulation we also found a cold-induced increase in the catalase activity (Figure 1, panel B), confirming previous reports that show the plant need of augmenting this antioxidant defense [33]. High levels of H2 O2 directly mediate lipid peroxidation [11], and this is in agreement with our evidence of a statistically significant increase in the MDA content after cold stress ((Figure 1), panel C), as also reported on rice and on rape seeds by other authors [34,35]. As lowtemperature (LT) stress is known to affect plant photosynthesis and reduce the need for the utilization of light [36], we measured chl-a and chl-b content in control and 48h-LT-treated plants. The results showed a significant decrease (p < 0.001) of the chlb content after cold stress ((Figure 1), panel H). During cold stress, the photosynthetic electron transport chain is disturbed, and the electron is transported to the oxygen molecule [37]. At this moment, the high amount of chlorophyll elevates only the level of ROS. One of the methods for reducing the production of ROS is to diminish the leaf chlorophyll content, particularly chlb, which takes place in plants upon the increase in chlorophyllase activity [38]. This enzyme detaches the linear chain of chlorophyll from the cyclic part. This chain is the phytol, which acts as a precursor of alpha-tocopherol. Moreover, during the breakdown of chlorophyll, chlb converts to chla [39]. A greater reduction in chlb than in chla and, subsequently, an increase in the chla/chlb ratio indicate the conversion of chlb to chla for maintaining the content of this flavonoid at a high level during exposure to cold stress. Considering the effect of cold stress on rice [37], oat [40], and rapeseed [35] plants, it has been stated that cold stress reduces the chlorophyll content.

2-DE Analysis

Two-dimensional electrophoresis (2-DE) analysis (Figure 2) led to the detection of 72 reproducible protein spots, 15 of which showed statistically significant differences in abundance between control and cold stress condition (Table 1). Out of the 15 LT-modulated protein spots, 10 showed over-expression and 5 showed a decrease in abundance through cold stress at 4 °C for 48 hours. On the whole, 7 protein spots were successfully identified by using LC-MS/MS (Table 1). The identified proteins were assigned to four functional categories (Figure 3).

Proteins involved in the light reactions of photosynthesis

The protein spots 2115 and 3770 were placed in this group. The spot 2115 is a chloroplast stem-loop binding protein of 41b (CSP41b), which demonstrated up-regulation. Chloroplast gene expression is indispensable for the accumulation of photosynthetic complexes. The processes of transcription and translation significantly rely on chloroplast proteins, which are regulated, for the most part, by nuclear encoding factors. Certain groups of regulators are comprised of proteins that either cleave RNA or are bound to RNA (generally in correspondence to a stem-loop structure present in the 3’-untranslated region) in order to mediate RNA splicing, editing stability, and translation. Among chloroplast proteins that are bound to RNA and have definite biochemical properties, one could refer to CSP41, which has a and b homologs [41]. In the spinach chloroplast as well, CSP41a has been demonstrated both as ribo nuclease and as a RNAbinding protein for its activity in mRNA30s [42]. CSP41b was also purified as a homolog of CSP41a from Nicotiana tabacum [43] and Arabidopsis seedlings, where it was shown that RNA target for this protein is the PetD RNA [44]. The chloroplast petD gene encodes the subunit of the cytochrome b6 complex, which mediates the electron transport between PSII and PSI. Moreover, the expression of this protein is affected by growth regulators, environmental factors, senescence, and water stress [44].

In addition to the above mentioned duties, there are other functions that have been attributed to this protein, such as regulation of the chloroplast system both in the light and the dark, integrity of chloroplast, accumulation of ribosomal units, and regulation of hetero polysaccharides [45,46]. Therefore, the increase in the abundance of this protein in stress conditions can help to resist the effects of cold stress, which brings about the degradation of chloroplast and reduction in transcription. In addition, it can probably increase the electron transport between PSII and PSI indirectly through an increase in the stability of the PetD protein, which might be the attempt by chloroplast to reach a recovery state in the electron transport from PSII and the production of energy.

The spot 3770 is the photosystem I reaction IV center subunit protein, which decreased under cold stress conditions. This protein is abbreviated as PsaE; different functions have been reported for PsaE, including assistance in binding ferredoxin to PSI and interaction with Ferredoxin-NADP+ oxidoreductase (FNR) [47]. Numerous pieces of evidence demonstrate that PSI can be the target of photoinhibition, particularly under cold and freezing conditions, where the electron transport chain is unbalanced [48]. Moreover, the decrease in the activity or sensitivity of PSI to cold conditions has been attributed to the decrease in the capacity of antioxidant enzymes to detoxify ROS [49]. In other words, the increase in ROS leads to a decrease in the activity of PSI. As a result, the PsaE protein abundance reduction in the sensitive cultivar Aths could be due to an increase in the production of H2 O2 and hydroxyl radicals in the Mahler cycle. This decrease in expression may lead to a decrease in the binding of ferredoxin to PSI and may create impairment in the electron transport chain, thereby reducing the production of NADPH. There are several reports on the reduction in the activity of PSI and its components in various type of stresses, such as drought, salinity, and metal [50,52].

Electron transport

The up-regulated spot 2173 was identified as the beta subunit of the chloroplast ATP synthase. The beta subunit plays a major role in the energy metabolism by converting ADP to ATP when there is the intramembrane proton gradient. By proteomic analysis, Wu et al. [54] demonstrated an over-expression of the ATPase beta subunit in birch leaves under cold stress. An increase in abundance of the same protein has also been observed in Arabidopsis as a part of defensive processes against cold stress [55]. Accordingly, the 48-h-cold-induced up-regulation of this enzyme found in our study is probably due to the fact that the cells require a large quantity of energy to increase the tolerance of plants to cold stress. A similar increase has been reported in rice under cold stress for 24 hours [56]. However, there are also several reports that demonstrate a decrease in the expression of this protein under cold stress [17,19,57,58]. The reason for this is said to be the effect of cold on the degradation of chloroplast or the strategy of the plant for acclimatizing to photosynthesis. Another protein involved in this pathway is the spot 3248, which contained the mitochondrial thioredoxin protein and displayed up-modulation under LT stress. Thioredoxins (Trxs) are ubiquitous and evolutionarily conserved enzymes involved in regulating the redox activation/inactivation of several metabolic and antioxidant proteins [59]. Barranco-Medina et al. [60] declared the existence of a heterocomplex between plant mitochondrial Trx-o and peroxiredoxin-IIF, which ensures a high and efficient detoxification of H2 O2 . Furthermore, Trx has a negative impact on the activity of ascorbate peroxidase [61]. Considering physiological findings reported in the current research (i.e., accumulation of H2 O2 ), the rise in Trx content after cold treatment of Aths leaves could be ascribed to a possible defense mechanism for detoxifying free radicals in cold stress and developing resistance in this cultivar. The increase in chloroplast Trx under cold and freezing conditions has previously been reported in wheat leaves as well [62]. Upon exploring cold stress effects on a resistant and semi-resistant wheat cultivar, Sarhadi and coworkers [16] also demonstrated that the M-type Trx content was higher in semi-hardy plants.

The signal transduction pathway 

The spot 2447 was identified as the 14-3-3 protein, which displayed a decrease in abundance under cold stress. 14-3-3 proteins constitute a family of conserved regulatory proteins that are uniquely present in eukaryotes and involved in protein-protein interactions mediating signal transduction pathways [63]. In Arabidopsis, acclimatization to cold is done by a complex network of signal transduction pathways, including those mediated by the expression of C-Repeated/DRE Binding Factors (CBFs) or by the Abscisic acid (ABA) and ethylene hormones [64]. It has been reported that the expression of 14-3-3 proteins in Arabidopsis increases the production of ethylene by regulating the stability of the 1-aminocyclopropane-1-carboxylic acid synthase (ACS), which catalyzes the rate-limiting step of ethylene biosynthesis [65]. Specifically, if on one hand a decrease in expression of the 14-3-3 protein has been correlated to the accumulation of ABA and thus to a decrease in cold tolerance, on the other hand its down-regulation negatively influences ethylene production, thereby causing sensitivity to cold stress.

Hence, one of the factors responsible for the sensitivity of the cultivar Aths to cold stress is likely to be a decrease in abundance of this protein. A down-regulation of the 14-3-3 protein has been previously declared in wheat lines under cold stress [15,18]. However, Han et al. [62], demonstrated an up-modulation of this protein in common wheat under spring cold conditions, supporting that it plays a major role in signal transduction pathways for the regulation of the gene expression under cold stress.

The Calvin cycle

The spot 180 was identified as the RuBisCO large subunit (LSU). RuBisCO represents 12-35% of total leaf proteins and 50% of soluble proteins, particularly in C3 plants [66]. Compared with the control, this spot demonstrates a 2-fold increase in abundance under cold conditions. Cold-induced fragmentation of the RuBisCO LSU was shown by Yan et al. [67], whereas the up-regulation of the intact protein has been reported on the growth stage of wheat at the end of autumn [57] but also in a resistant wheat cultivar under freeze stress at -8 °C [19] and after cold stress at 4 °C for 7 days in Alfalfa leaves [68]. The large subunit of RuBisCO has a documented role in various processes such as cell death, protein folding, and response to cold and acquired resistance [57]. Additionally, this increase could be due to the low ratio of carbon dioxide (CO2 ) to oxygen that results from a reduction in stomatal conductance during stress, where this enzyme begins its oxygenase activity for photorespiration [69]. The spot 2362 contained the RuBisCO activase isoform I, which displayed overexpression. As chaperonin molecules, this enzyme could provide assistance in accumulating or isolating protein fragments. Upon hydrolyzing ATP, RuBisCO activase promotes the dissociation of sugar-phosphate inhibitors (e.g., Ribulose-1,5-bisphosphate) from the active site of RuBisCO, so that CO2 keeps the enzyme RuBisCO active though carbamylation [70,71]. In the present study, both RuBisCO and RuBisCO activase showed up-regulation. Therefore, it could be stated that the upregulation of RuBisCO is unlikely to be due to an increase in photorespiration, but rather it may be due to the increase in the fixation of CO2 for resisting cold stress or removing any remaining CO2 , which prevents energy loss by the oxygenase reaction of photorespiration. The overexpression of RuBisCO activase in the strawberry [72] and wheat [17] under cold stress has been reported, and it has been attributed to the role of this enzyme in stabilizing the active form of RuBisCO. On the other hand, PSI activity decreased under cold stress in the sensitive cultivar Aths and photosynthetic light reactions were inhibited; therefore, it is possible that RuBisCO entered the photo respiratory cycle and displayed over-expression. As a consequence, respiratory products close the active site of RuBisCO [73], and the RuBisCO activase over-expression keeps this site free [71], so that the remaining CO2 is fixed and the plant displays resistance against cold stress.

Table 1: List of differentially abundant proteins.

In conclusion, our data demonstrate that cold stress (48 h of exposure at 4 °C) induces physiological and proteome-level responses in the spring barley Aths cultivar. To counteract cold effects, plants performed the following: (i) strengthened antioxidant defenses by acting on free radical detoxification through an increase of catalase activity and a reduction of chlb content; (ii) up-modulated chloroplast proteins involved in the light/dark system and in its integrity; (iii) increased transcription through the accumulation of ribosomal units; and (iv) responded to the necessity of more energy by the up-regulation of ATPase. The identification of cold tolerance mechanisms that are adapted to each given plant is necessary to improve breeding programs. To accomplish this goal, intensive in vitro and in vivo analyses of plant responses to cold conditions are needed.

The authors acknowledge the support of the Interuniversity Consortium for Biotechnologies (CIB, Italy) in this work.

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