Proteomic analysis of black poplar (Populus nigra L.) seed storability

Abstract

Key message

Adequate storability of black poplar ( Populus nigra L.) seeds at temperatures below 0 °C was associated with preservation of proteins of energy and carbohydrate metabolism, protein turnover, and proteins maintaining long-term stability of dehydrated tissue.

Context

Understanding seed storability is a key factor for effective seed preservation and conservation. Black poplar is an endangered tree species and its seed loses rapidly viability during storage.

Aims

The aim of this study was to determine, and functionally characterise, the proteins associated with storability of black poplar seeds.

Methods

Dried seeds (7.1% MC) were stored at 3 °C, − 3 °C, − 20 °C, and − 196 °C, for a period of 12 and 24 months. Proteins were extracted and separated according to their isoelectric point (pI) and mass using 2-dimensional electrophoresis. Proteins that varied in abundance for temperature and time of storage were identified by mass spectrometry.

Results

A germination test showed that seeds remained viable at − 3, − 20, and − 196 °C. Storage at 3 °C caused loss of seed viability. This loss in seed vigour was related to the largest changes in protein abundance. As storage temperature decreased, a smaller number of proteins displayed changed abundance.

Conclusion

Good storability of black poplar seeds under freezing conditions may be associated with the following: an inhibition of synthesis of energy and carbohydrate metabolism and protein turnover proteins, LEA proteins that maintain long-term stability of dehydrated tissue, GDSL esterases/lipases that inhibit hydrolysing activity, and by chaperonins that protect protein functionality. Storage conditions influence the preservation of protein function that governs seed viability.

1 Introduction

Preservation of genetic diversity is one of the major issues in seed science. Seed longevity, the period over which seeds remain viable, is an important trait for plant adaptation to changing environments and conservation of biodiversity (Sano et al. 2016). Seed longevity largely relies on the viability of embryo (Sano et al. 2016). Black poplar (Populus nigra L.) produces short-lived seeds, classified as intermediate (Suszka et al. 2014; Michalak et al. 2015). This tree forms a vital part of floodplain forest ecosystems (Wyckoff and Zasada 2005). It is very important to preserve genetic material due to the devastation of these habitats, with black poplar in particular being one of the most threatened tree species in Europe (Suszka et al. 2014).

Proteomic approaches enable the evaluation of protein markers which can be useful in determination of optimal storage conditions (Baginsky 2009). Protein markers can give information about physiological changes occurring during seed conservation, and about the condition of stored seeds. Changes in the proteome can show otherwise invisible signs of seed deterioration, what can be useful in predication of loss of seed vigour. Such a loss of vigour precedes the loss of the ability to germinate (van de Venter 2001). The germination tests do not reflect adequately the degree of seed deterioration (van de Venter 2001). Proteomic research has mainly concentrated on the negative influence of external and internal factors causing loss of seed viability (see for review Wang et al. 2015), such as drying (Rajjou and Debeaujon 2008), high temperature (Zhang and Li 2015; Chen et al. 2016), and high humidity and temperature (ageing) (Rajjou et al. 2008; Kalemba and Pukacka 2014; Nguyen et al. 2015; Zhang et al. 2015; Li et al. 2017; Yin et al. 2017). Proteomic data has shown that reduction of seed longevity is often associated with oxidation of cellular macromolecules such as nucleic acids, proteins, and lipids (Sano et al. 2016). Gao et al. (2016) indicated that redox regulation proteins, mainly glutathione-related proteins, and defence proteins, including DNA-damage-repair proteins, and a late embryogenesis abundant (LEA) protein might have positive influence on seed storability. Min et al. (2017) also highlighted the positive role of antioxidant enzymes dehydroascorbate reductase (DHAR), ascorbate peroxidase (APX), and superoxide dismutase (SOD), as well as peroxiredoxin, for seed longevity (Yin et al. 2015). Nguyen et al. (2015) confirmed the role of antioxidant systems, and additionally illustrated the role of translation machinery and energy pathways in seed longevity. Yin et al. (2017) showed that loss of seed viability is also associated with a decrease in heat shock proteins. For elm (Ulmus pumila L.), it was found that increased production of ROS during seed ageing induced alteration of specific mitochondrial proteins that may be involved in the process of mitochondrial deterioration, which led to loss of seed viability (Li et al. 2017). These data concerned generally seeds exposed to ageing protocols which were based on high-humidity and high-temperature storage conditions. The present study illustrates the influence of low temperatures, including cryopreservation, on seed viability and protein composition.

Tree seed quality, development, and germination have been considered using proteomic approaches (Staszak and Pawłowski 2014; Pawłowski and Staszak 2016; Staszak et al. 2019). In the present study, proteome changes were investigated in black poplar seeds under different storage conditions (temperature and time), to reveal differentially abundant proteins, in order to identify those putatively responsible for successful storability and stress resistance. We hypothesised that the preservation of black poplar seeds is accounted for by the smallest changes in protein abundance variability, and seed vitality is associated with proteins maintaining primary metabolism, protein and dehydrated tissue stability.

2 Material and methods

2.1 Plant material and experimental design

Mature seeds enclosed in catkins were collected from several trees of black poplar (Populus nigra L.) growing in floodplains located near Czeszewo, Poland (N 52° 8′ and E 17° 30′) in 2010 (27 May–16 June). After collection, catkins were placed in an environmental chamber at a constant temperature of 15 °C for 72 h to promote full opening. Seeds were then separated from cotton.

2.2 Desiccation of seeds

Freshly extracted seeds had a water content (WC) in the range of 0.14–0.17 g H₂O g⁻¹ dry mass (g g⁻¹), which represents a moisture content (MC) of 12.5–14.4% on a fresh weight basis. Prior to storage, seeds were dried in a seed dryer at 20 °C for about 1 h to 0.071 g g⁻¹ (7.1% MC). Seed WC (3 replications of 50 seeds each) was determined by drying seeds at 103 ± 2 °C for 17 h (ISTA 2013).

2.3 Seed storage conditions

Details of the storage protocols were described in Suszka et al. (2014). Briefly, desiccated seeds were stored at different temperatures: 3 °C, − 3 °C, − 20 °C in cold rooms. Seeds were packed in tightly closed polyethylene vials during storage. Seeds were also cryostored at − 196 °C in liquid nitrogen. After storage in liquid N, seeds in the cryovials were thawed at 40 °C in a water bath for 5 min. Seeds were stored conventionally for a period of 12 and 24 months, and in liquid N for 24 months. The controls were desiccated non-stored seeds. The germination test was performed on four samples of 100 seeds each as described by Suszka et al. (2014). Analysis of variance (ANOVA) and a Tukey–Kramer HSD was used to assess the influence of storage temperature and time on the level of seed germination, at p < 0.05 (JMP software, SAS Institute, Cary, NC, USA). Details of the results of statistical analysis, because of the use of the same batch of seeds in present and that study, were described in Suszka et al. (2014).

2.4 Protein extraction

All analyses were conducted using four biological replicates. Proteins were extracted and precipitated overnight at – 20 °C in a 10% (w/v) solution of TCA in acetone containing 20 mM dithiothreitol (DTT) (Pawłowski 2009). After centrifugation (16,000g for 5 min at 4 °C), the resulting pellets were washed three times with 1 ml of acetone supplemented with 20 mM DTT and were then re-centrifuged. After vacuum drying, pellets were suspended in lysis buffer (7 M urea, 2 M thiourea, 2% [w/v] CHAPS, 1.5% [w/v] DTT, 0.5% [v/v] IPG buffer pH 4–7, supplemented with a Protease Inhibitor Cocktail (Roche, Mannheim, Germany) according to the manufacturer’s suggestions. Protein concentrations were determined using the Bradford assay (Ramagli and Rodriguez 1985).

2.5 Protein electrophoresis, 2-DE IEF/SDS-PAGE

Proteins (600 μg for colloidal Coomassie Blue) were first separated according to their isoelectric point on rehydrated Immobiline dry strips (24 cm, with a linear pH gradient of 4–7) in rehydration buffer (6 M urea, 2 M thiourea, 2% [w/v] CHAPS, 20 mM [w/v] DTT, 0.5% [v/v] IPG buffer pH 4–7) using an Ettan IPGphor 3 IEF System (GE Healthcare). The electrophoresis program used for isoelectrofocusing was according to the manufacturer’s directions for 24-cm strips. The strips containing the separated proteins were immersed in equilibration solution I (6 M urea, 1.5 M Tris-HCl, pH 8.8, 30% [v/v] glycerol, 2% [w/v] SDS, 1% [w/v] DTT) and then in equilibration solution II (equilibration solution I supplemented with 2.5% [w/v] iodoacetamide, without DTT) for 10 min and subjected to second dimension (SDS-PAGE) separation.

Pre-cast Ettan DALT 12.5% (w/v) polyacrylamide gels (GE Healthcare) and the Ettan Dalt Six electrophoresis chamber (capable of processing six gels) were used for SDS-PAGE. Conditions were as follows: 1 h at 80 V and 5 h at 500 V. A mixture of molecular weight markers (GE Healthcare) was loaded next to the Immobiline strip. Four replicate gels were run for every treatment (n = 4). After electrophoresis, the gels were stained with colloidal Coomassie blue, which, in addition to visualisation and quantification, also allowed MS analyses (Neuhoff et al. 1988).

2.6 Gel analysis

The gels were scanned and analysed using 2D Image Master 7 Platinum software (GE Healthcare). After spot detection, gels from the four independent biological samples were aligned, and normalized spot volumes of the identified spots were determined. Spots with variations in abundance (statistics between gel sets/classes, i.e. central tendency, dispersion, and overlapping measures was used) were subjected to ANOVA and a Tukey–Kramer HSD test (JMP software, SAS Institute, Cary, NC, USA) to select spots that significantly varied (p < 0.05) in abundance for two factors: temperature and time of storage. Fold change was calculated for temperature influence as log₂ (Meunier et al. 2005). The significantly variable proteins were subsequently identified by MS.

2.7 Mass spectrometry

The gel spots were subjected to a standard “in-gel digestion” procedure in which proteins were reduced with 10 mM (w/v) DTT (for 30 min at 56 °C), alkylated with 55 mM iodoacetamide (45 min in the dark at room temperature) and digested overnight with trypsin (Sequencing Grade Modified Trypsin, Promega V5111) in 25 mM ammonium bicarbonate (25 ng μl⁻¹ of trypsin). The resulting peptides were eluted from the gel matrix with 0.1% (v/v) TFA in 2% (v/v) ACN.

Peptide mixtures were analysed by liquid chromatography coupled to the mass spectrometer in the Laboratory of Mass Spectrometry, Institute of Biochemistry and Biophysics, Polish Academy of Sciences (Warsaw, Poland). Samples were concentrated and desalted on a RP-C18 pre-column (nanoACQUITY Symmetry® C18, Waters), and further peptide separation was achieved on a nano-Ultra Performance Liquid Chromatography (UPLC) RP-C18 column (Waters, BEH130 C18 column, 75 μm i.d., 250 mm long) of a nanoACQUITY UPLC system, using a linear acetonitrile gradient (0–60% [v/v] ACN for 120 min.) in the presence of 0.05% (v/v) formic acid with a flow rate of 150 nl/min. The column outlet was directly coupled to the Electrospray Ionization (ESI) ion source of the Orbitrap Velos type mass spectrometer (Thermo Electron Corp., San Jose, CA, USA), working in the regime of data-dependent MS to MS/MS switch. An electrospray voltage of 1.5 kV was used. A blank run preceded each analysis ensuring lack of cross contamination from previous samples.

2.8 Protein identification and classification

The mascot search algorithm (http://www.matrixscience.com) was used for protein identification against the NCBInr (http://www.ncbi.nig.gov) databases. Protein identification was performed using the Mascot search probability-based molecular weight search—MOWSE score. Ions score was –10 x log(P), where P was the probability that the observed match was a random event. Peptides with a Mascot score exceeding the threshold value corresponding to < 5% false positive rate, calculated by the Mascot procedure, were considered to be positively identified.

Identified proteins were grouped due to biological process, molecular function, and subcellular localization according to the gene ontology (GO) annotation using UniProt database and QuickGO search (https://www.ebi.ac.uk/QuickGO/). A Venn diagram of protein quantification according to seed storage condition was prepared using Bioinformatics and Evolutionary Genomics tool (http://bioinformatics.psb.ugent.be/webtools/Venn/).

3 Results

Effect of storage time and temperature on black poplar seed germination is illustrated in Fig. 1. Germination ability was highest after seed storage at − 20 °C for 1 year as well as for 2 years. A slight decrease in germination, which was not statistically significant, was indicated in seeds stored for 1 and 2 years at − 3 °C. A significant decrease in germination was observed after 1 year, and even more so after 2 years of seed storage at 3 °C.

Fig. 1

Effect of storage time and temperature on Populus nigra L. seed germination as a function of time (0, 12, 24 months) and temperature (3, − 3, − 20, − 196 °C). Data with different letters are significantly different, p < 0.05 (Suszka et al. 2014, with modification)

An average of 420 Coomassie blue stained spots was detected on each gel using Image Master 7 Platinum software. A total of 37 spots exhibiting significant changes in volume (i.e. abundance), representing approximately 9% of the total number of spots on a reference gel (Fig. 2), were identified by MS.

Fig. 2

Reference gel presenting positions of statistically variable spots from Populus nigra L. seeds stored at different temperature conditions of 3, − 3, − 20, and − 196 °C for 12 and 24 months. The numbers of identified proteins correspond to those listed in Table 1. Spots which increased in abundance under the influence of temperature in comparison with control are marked in green; spots which decreased in abundance are marked in red

Among the 37 identified proteins (Table 1; Fig. 2), 19 increased and 18 decreased in abundance under the influence of storage temperature, compared with the control (ANOVA, the Tukey–Kramer HSD test, and fold change). Twenty five proteins were variable for 3 °C (14 increased and 11 decreased in abundance), 16 proteins for − 3 °C (half increased and half decreased in abundance), 13 proteins for − 20 °C (4 increased and 9 decreased in abundance), and 3 proteins for − 196 °C (2 increased and 1 decreased in abundance). One protein was variable in abundance for time of storage (it decreased after 1 year) and 9 for time and temperature (3 increased and 6 decreased in abundance). Among 25 proteins responsive to 3 °C, 15 were only responsive to this temperature (Fig. 3). Six proteins were only responsive to − 3 °C, 5 only to − 20 °C, and none only to LN. Only 2 proteins were responsive to all temperatures.

Table 1 Identification of differentially abundant proteins of Populus nigra seeds stored in different temperature (3, − 3, − 20, and − 196 °C (LN)) and time (12 and 24 months) conditions

Fig. 3

Venn diagram of the 37 identified proteins grouped based on Populus nigra L. seed storage temperature of 3, − 3, − 20, and − 196 °C

As listed in Table 1, the spots represented 37 non-redundant proteins. The percentage of sequence coverage ranged from 7 to 61%, and the number of identified peptides varied from 4 to 76. Among the 37 spots, 23 corresponded to P. trichocarpa, 9 to other Populus family species, 4 to other woody plants, and 1 to annual plants. Homologous proteins were found for all of the spots.

To further understand the biological regulations and functions of identified proteins in storability of black poplar seeds, we carried out gene ontology (GO) and subcellular localization analyses (https://www.ebi.ac.uk/QuickGO/). The results of biological process analysis showed that a large number of identified proteins were associated with cellular process (28 proteins), metabolic process (27 proteins), and response to stimulus (14 proteins) (Table 2). The results of molecular function analysis showed that the majority of proteins was related to catalytic activity (24 proteins) and binding (20 proteins). The determination of subcellular localization showed that a large number of identified proteins were present in the cytosol (13 proteins), chloroplast (10 proteins), and mitochondria (9 proteins).

Table 2 Gene ontology (GO) functional annotation of the identified proteins based on biological process, molecular function, and subcellular localization

4 Discussion

Since seed storage is often accompanied by a progressive loss of germination vigour, storage conditions must be optimized for the preservation of genetic resources (Rajjou et al. 2008). Seed storage temperature and time of storage affect black poplar seed storability (Suszka et al. 2014). Desiccated seeds (7.1% MC) can be stored successfully at − 3, − 20 °C, and − 196 °C for at least 2 years. These results are in agreement with the observation that low seed moisture content, low temperature, or cryopreservation result in an increase in storage life span (Walters 2004; Walters et al. 2004). A significant decrease in black poplar seed germination was observed already after 1 year of storage at 3 °C. Thus, a positive temperature implicated loss of seed viability. Proteomic analysis revealed that this loss in seed vigour is accompanied by protein changes. Storage at temperature of 3 °C influenced accumulation of most (25) proteins. With lower storage temperature, the abundance of fewer proteins changed, and cryopreservation in liquid nitrogen influenced only three proteins. Gao et al. (2016) showed that seeds from rice cultivars sensitive to storage also displayed more differentially abundant proteins, in agreement with the present results.

Here, we discuss the role of the proteins that can contribute to the storability of the seeds in relation to system biology approaches (Staszak and Pawłowski 2012).

4.1 Proteins involved in metabolism processes

Metabolism-related proteins, particularly those involved in the carbohydrate metabolic pathways, accumulate in mature dry seeds (Fu et al. 2011; He and Yang 2013; Han and Yang 2015). An increase in seed storage temperature results in rapid resumption of several metabolic activities such as respiration, enzymatic activity, RNA, and protein synthesis (Yang et al. 2007; He et al. 2011; Kolodziejczyk et al. 2016). For germination, energy is primarily supplemented by glycolysis and fermentation followed by the tricarboxylic acid (TCA) cycle, gluconeogenesis and glyoxylate cycle, and pentose phosphate pathway (Jin et al. 2014; He and Yang 2013). The abundance of enolase (spot 140), an enzyme of the glycolysis/gluconeogenesis pathway, and pyruvate dehydrogenase (spot 45), an enzyme linking glycolysis to the TCA cycle, was highest after storage at 3 °C, and lowered with decreasing of storage temperature. A negative (− 20 °C) temperature–associated decrease in protein abundance was also observed for glyceraldehyde-3-phosphate dehydrogenase (spots 80 and 303), an enzyme of the glycolysis/gluconeogenesis pathway. A protein related to photosynthesis (carbon fixation in pentose phosphate cycle), ribulose-1,5-bisphosphate carboxylase/oxygenase (spot 368), decreased in abundance during storage at − 20 °C. This suggests that negative storage temperatures prevent launching synthesis of energy, and in this way preserve metabolic activity until the end of storage to allow appropriate germination. Fructose-bisphosphate aldolase (spots 61 and 62), an enzyme involved in the pentose phosphate cycle as well as in glycolysis/gluconeogenesis, decreased in abundance during storage. In conclusion, energy- and carbohydrate metabolism–related enzyme synthesis is putatively blocked in temperatures below zero, which suggests that these are appropriate conditions for black poplar seed storability.

Enoyl-ACP reductase (83) is an enzyme of fatty acid biosynthesis and catalyzes the last step of fatty acid elongation, in which enoyl-ACP is reduced to fully saturated acyl-ACP (Massengo-Tiassé and Cronan 2009). Its abundance increased at 3 °C. It can be suggested that temperature above zero triggers some metabolic activity but this, probably due to storage reserves consumption, causes a loss of seed viability.

Level of GDSL esterase/lipase (spot 351) was the lowest after storage at a temperature of − 3 °C. Enzymes of the SGNH plant lipase superfamily hydrolyse ester bonds, e.g. of phospholipids, or fatty acyl esters (Teutschbein et al. 2010). A number of SGNH family lipases have been characterised, which participate in plant development and morphogenesis, plant tolerance to environmental stresses, and the metabolism of cutin and wax (Volokita et al. 2011). They can play some role during Brassicaceae seed germination, catalyzing hydrolysis of sinapine (Clauß et al. 2008). It was also hypothesised that these lipases are related to the supply of a carbon source to the embryo through the breakdown of oil/lipid in the endosperm during seed development (Kondou et al. 2008). The role of SGNH plant esterase/lipase in storability of black poplar seeds is unknown; however, it may be associated with low temperature inhibition of hydrolysing activity.

4.2 Proteins involved in stress responses

ROS are continuously generated within living systems, and the inability to manage ROS load leads to elevated oxidative stress and cell damage (Møller et al. 2007). Oxidative stress increases during black poplar seed storage at the non-freezing temperatures, resulting in a loss of viability (Kalemba et al. 2015). Oxidative stress is coupled to the oxidative degradation of lipid membranes, also known as lipid peroxidation. This process generates over 200 types of aldehydes, many of which are highly reactive and toxic. Aldehyde dehydrogenases (ALDHs, spot 107) metabolize endogenous and exogenous aldehydes and thereby mitigate oxidative stress in organisms (Singh et al. 2013). In our experiments, the abundance of ALDH decreased during storage, reaching a minimum after 12 months of storage at 3 °C, and after 24 months at − 3 °C. The role of rice ALDH7 was illustrated for seed maturation by participation in desiccation tolerance and viability maintenance by detoxifying aldehydes generated by lipid peroxidation (Shin et al. 2009). A decrease in ALDH accumulation in stored black poplar seeds suggests the reduction of seed metabolic activity, and lipid peroxidation but that was not associated with seed viability.

Superoxide dismutase (Cu-Zn SOD, spot 367) takes part in conversion of superoxide to hydrogen peroxide, which is removed by glutathione peroxidase or catalase (Kwasigroch et al. 2008). Overexpression of Cu-Zn-SOD genes in tobacco seeds improved seed longevity and germination by attenuating the effects of oxidative stress produced by elongated storage conditions and harsh environmental stresses (Lee et al. 2010). It was suggested that loss of seed viability with ageing is associated with a decrease in SOD activity (Lehner et al. 2008; Bao et al. 2011; Xia et al. 2015; Sahu et al. 2017b). In the present study, Cu-Zn SOD level decreased at 3 °C, which is in agreement with the above results; however, it increased only at − 3 °C after 12 months of storage. Such changes are difficult to explain, but it is worth pointing out that also in some studies SOD was not directly associated with seed longevity (Bailly et al. 2002; Donà et al. 2013; Kong et al. 2015; Araldi et al. 2016). In the recalcitrant Jatropha curcas L. seeds, the activity of SOD was the lowest at the optimal storage temperature of 4 °C, although at unfavourable temperatures below zero it increased (Gao et al. 2015). For Araucaria angustifolia (Bertol.) Kuntze, SOD activity was low in seeds exposed to 5 and − 18 °C, although at unfavourable room temperatures SOD activity initially increased but afterwards fell (Garcia et al. 2015). Initial increasing in SOD activity in the ageing seeds was associated with the accumulation of H₂O₂ (Sahu et al. 2017a), which was not sufficient for the recovery, and probably with seed deterioration progress SOD was degraded, as is also suggested for black poplar seed Cu-Zn SOD.

Glutaredoxin C4 (class I GRX, spot 327) reached its highest abundance after 12 months at 3 °C. GRXs are small ubiquitous redox enzymes involved in the reduction of oxidative modifications using glutathione (Zaffagnini et al. 2011; Ströher and Millar 2012). GRX may act as a stress-related redox regulator by maintaining the glutathionylation of its target proteins (Couturier et al. 2013). Class I GRXs represent the major enzymes responsible for protein deglutathionylation (Couturier et al. 2013). In the present study, the enzyme promoting glutathionylation, glutathione S-transferase (GST, their abundance increased during storage, most significantly at 3 °C (spot 333) and only significantly at 3 °C (spot 343)), has also been identified. Plant GSTs have the ability to confer tolerance against biotic and abiotic stresses by conjugation of reactive electrophile species with glutathione and the modulation of cellular redox status, biosynthesis, binding, and transport of secondary metabolites and hormones (Nianiou-Obeidat et al. 2017). Glutathionylation itself is a post-translational modification which can constitute an intermediate of some reactions, and can modulate protein function, serve as a signalling factor, and protect cysteine residues from irreversible oxidation (Zaffagnini et al. 2011). High abundance of these enzymes was indicated during black poplar seed storage, particularly at temperatures above zero, suggesting that GRX and GST have a role in oxidative stress protection, and in maintaining redox homeostasis. Their activity, however, was not sufficient to protect the seeds, which resulted in a decrease in germination ability. It cannot be excluded that due to protein glutathionylation/deglutathionylation cycling, these enzymes may be able to regulate developmental events associated with seed quality.

Abundance of two temperature-induced lipocalins (TILs) changed significantly after storage at 3 °C (spot 204, increased) and at 3 and − 3 °C and in LN (spot 356, decreased). Lipocalins participate in the transfer of small, hydrophobic molecules. They play an important role in the regulation of developmental processes and are also involved in the reactions of organisms to various stress factors (Grzyb et al. 2006). TILs were recognized to be responsible for cold and heat tolerance by lipid protection (Kawamura and Uemura 2003; Chi et al. 2009). Arabidopsis TIL knockout plants accumulated high level of ROS, while TIL overexpression enhanced tolerance to freezing (Charron et al. 2008). Boca et al. (2014) showed that the Arabidopsis lipocalins AtTIL and AtCHL have overlapping functions essential for lipid protection and seed longevity. The present results show however that TILs were not directly associated with seed longevity.

One of the proteins, the abundance of which is increased during seed storage at 3 °C, was isoflavone reductase (IFR, spot 77). This enzyme is involved in the biosynthesis of various plant defence-related phenylpropanoid-derived compounds, e.g. lignans and isoflavonoids (Gang et al. 1999). A role for the IFRs might be a part of the defence mechanism against oxidative stress because the IFRs contain an NADPH-binding domain related to oxidation/reduction properties (Kim et al. 2010; Cheng et al. 2015). The highest presence of this enzyme in black poplar seeds at temperatures above zero implies its defence role in these adverse storage conditions. Its activity, however, was not sufficient to prevent seed deterioration.

Different late embryogenesis abundant (LEA) proteins were found in the present study. Abundance of the dehydrin DHN 3 (LEA 2 type, spot 323) increased generally during seed storage. LEA D-29 (LEA 3, spot 324) level increased at − 3 °C, besides PM22 (LEA 2, spot 337) at 3 °C. LEA proteins are associated mainly with water deficit tolerance, due to protein protection, membrane stabilisation, and ion sequestration (Battaglia and Covarrubias 2013). They are also involved in seed development, playing a protective role during maturation drying (Kleinwächter et al. 2014). Hundertmark et al. (2011) showed that seed-specific dehydrins play a role against deterioration during storage at low moisture content. RAB18 protein (LEA 2 group) was indicated to be involved in protection of seeds against artificial ageing, suggesting its involvement in seed longevity (Rajjou et al. 2008). Several LEA proteins, including the dehydrin family, were found to be involved in dry storage survival of rice seeds (Galland et al. 2017). The role of LEA proteins in maintaining black poplar seed longevity could be associated with protecting long-term stability of dehydrated tissues.

Abundance of universal stress protein PHOS32 (USP, spot 338) increased after 1 year of storage at 3 °C, besides abundance of USP A-like protein (spot 35) decreased after 1 year of storage at 3 °C. The physiological function of USP in plant has remained unclear. The USP gene expression analysis showed that it is stimulated by a wide variety of stresses, including salt, osmotic stress, wounding, anoxia, and drought (Shinozaki and Yamaguchi-Shinozaki 2007, Isokpehi et al. 2011). It was found that AtUSP functions as a molecular chaperone under oxidative, heat, and cold-shock conditions (Jung et al. 2015; Melencion et al. 2017). Gorshkova et al. (2018) indicated that AtUSP is regulated by phytohormones and is involved in seed germination of Arabidopsis. The role of USPs in maintaining black poplar seed longevity could be associated with chaperon activity, but that should be verified in future experiments.

4.3 Proteins involved in protein turnover

Subtilisin-like protease SBT3.3 (subtilase, spot 10) increased during storage at 3 °C. Subtilases are one of the largest protease families playing a critical role in protein degradation, but also in developmental regulation through protein processing (Schaller et al. 2012). With a broad spectrum of biological functions, subtilases have been gaining increasing attention with regard to their involvement in plant defence (Figueiredo et al. 2017). One of them, senescence-associated subtilase, was involved in reproductive development and determination of silique number in Arabidopsis (Martinez et al. 2015). Subtilases are essential also for seed germination by involvement in cell wall modification (Rautengarten et al. 2008). The present data suggest that in less favourable conditions for seed storage, the role of subtilase can be associated with protein degradation.

Since most plant proteases assume regulatory functions, it is of utmost importance to keep their activity under tight control (Rustgi et al. 2017). To make the regulation highly stringent and specific, plants evolved in terms of protease inhibitors, another layer of control. Kunitz-type protease inhibitors (spot 22, increased at 3 °C) are widespread in the plant realm. They reversibly interact with their target proteases, inhibiting their catalytic activities. Biological significance of protease and protease inhibitors interactions is very high and concerns many aspects of plant development including mobilization of storage proteins, biotic and abiotic stress tolerance, and hormone signalling (Rustgi et al. 2017). Kunitz inhibitors inhibited α-amylase/subtilase involved in mobilization of storage carbohydrates during seed germination (Nielsen et al. 2003; Grosse-Holz and van der Hoorn 2016). During seed storage this mechanism contributes probably in maintaining homeostasis in protein turnover.

Chaperonins fold nascent proteins as well as salvage and recycle stress-denatured proteins (Saibil 2013). Type I chaperonins function as an efficient protein-folding machine, known in plants as chaperonin 60 (Cpn60)–Cpn10 in chloroplasts and heat shock protein 60 (Hsp60)–Hsp10 in mitochondria (see for review Vitlin Gruber et al. 2013). Chaperonins play a part in chloroplast biogenesis (Cpn60), protecting Rubisco activase from thermal denaturation (Cpn60) and ABA signalling (Cpn20) (Vitlin Gruber et al. 2013). The 20 kDa chaperonin protein (spot 353) abundance was the lowest in black poplar seeds stored for 12 months at 3 °C. Abundance of two other chaperonins increased generally, chaperonin CPN60-2 (spot 361) significantly in liquid N, and chaperonin 60 (spot 363) in − 3 °C. Increasing the accumulation of chaperonins in cold temperatures is a consequence of stress response, and it can be suggested that they are putative important factors in seed longevity.

Three other identified proteins were also associated with protein turnover. Eukaryotic translation initiation factor 3 (eIF-3, spot 64) abundance reached the lowest level during black poplar seed storage at − 3 °C. It is a component of the eIF-3 complex, which is involved in protein synthesis of a specialized repertoire of mRNAs and stimulates binding of mRNA and methionyl-tRNA to the 40S ribosome (Hinnebusch 2014). The eIF-3 complex specifically targets and initiates translation of a subset of mRNAs involved in cell proliferation. 60S ribosomal protein L22-1 (spot 373) is directly involved in translation activity as being a part of the ribosomal exit tunnel (Gabashvili et al. 2001). Transport protein SEC13 (spot 358) is known to be involved in transporting of secreted and membrane proteins out of the endoplasmic reticulum (Bhattacharya et al. 2012). The abundance of these proteins was reduced during storage of black poplar seeds for 24 months at − 20 °C and for 12 months at − 3, respectively. This implies that seed storage at temperatures below zero protect translation against undesirable activity, thus inhibiting basal metabolic activity.

5 Conclusion

A germination test of black poplar (Populus nigra L.) seeds showed that they are preserved well at temperatures under zero. Storage at temperature above zero caused loss of seed viability. Each storage condition induced modifications in the abundance of proteins connected to biological processes related to seed storability. The loss in seed vigour was accompanied by the most numerous changes in protein abundance. It is likely that temperatures above zero trigger some metabolic activities, but with time, such temperatures cause loss of seed viability. Storage at 3 °C also caused an increase in abundance of enzymes associated with stress defence. Their presence, however, was not sufficient to prevent seed deterioration. Good storability of seeds in freezing conditions is associated with the following: the decrease in abundance of metabolism, hydrolysing and protein turnover proteins, and the increase in abundance of proteins maintaining long-term stability of dehydrated tissue. Results of present study provide valuable information, revealing the general downregulation and upregulation of significant proteins following the varying temperatures and duration of seed storage. Overall, this data should enhance understanding of the processes associated with seed longevity. The highlighting of potentially important proteins by proteomics provides researchers with starting points for further studies where the next step will be to look at the expression and regulation of the gene encoding the protein of interest, to incorporate it into the seed longevity testing.