Understanding the cellular mechanism of recovery from freeze-thaw injury in spinach: possible role of aquaporins, heat shock proteins, dehydrin and antioxidant system

Exploring freeze-injury mechanism through ion-specific analysis of leachate from reversibly versus irreversibly injured spinach (Spinacia oleracea L.) leaves

The present study analyzed four cations (K+, Ca2+, Mg2+, Fe2+) in leachate from freeze-injured spinach (Spinacia oleracea L. 'Reflect') leaves exposed for four freezing-durations (FDs) (0.5, 3.0, 5.5, 10.5 h) at -4.8 °C. Comparison of electrolyte leakage from right-after-thaw with that after 6-d recovery revealed that injury at 0.5 or 3 h FDs was recoverable but irreversible at 5.5 or 10.5 h FDs. Data suggests leakage of K+, the most abundant cation in leachate, can serve as a proxy for total electrolyte-leakage in determining plant freezing-tolerance and an ionic marker discerning moderate vs. severe injury. Quantitative correspondence between Ca2+- and K+-leakage supports earlier proposition that leaked K+ induces loss of membrane-Ca2+, which, in turn, promotes further K+-leakage due to weakened membrane. Reduced/undetectable Fe2+ in leachate at longer FDs suggests activation of Fenton reaction converting soluble Fe2+ into insoluble Fe3+. Enhanced Mg2+-leakage at greater freeze-injury suggests structural/functional impairment of chlorophyll/chloroplast complex.

Pre-stress salicylic-acid treatment as an intervention strategy for freeze-protection in spinach: Foliar versus sub-irrigation application and duration of efficacy

Exogenous application of salicylic acid (SA) to plant tissues has been shown to confer tolerance against various abiotic stresses. Recently, SA application through sub-irrigation was shown to improve plant freezing tolerance (FT). For SA treatment to be employable as an effective intervention strategy for frost protection under field conditions, it is important to study its effect on FT when applied as a foliar spray to whole plants. It is also important to determine for how long the FT-improvement by SA lasts. Present study was conducted to compare SA-induced FT of spinach (Spinacia oleracea L. 'Reflect') seedlings following SA-application by foliar spray vs. sub-irrigation. Durability of FT-promotive effect of SA was evaluated using three freeze-tests over a 4-d period, i.e., at 10-d, 12-d, and 14-d after the SA application. Freezing stress was applied using a temperature-controlled freeze-thaw protocol, and FT was assessed by visual observations (leaf flaccidness vs. turgidity) as well as ion-leakage assay. Data indicated that both foliar spray and sub-irrigation methods improved FT of the seedlings against a relatively moderate (-5.5 °C) as well as severe stress (-6.5 °C). Moreover, improved FT against moderate stress was sustained over a 4-d period, whereas such benefit waned somewhat against the severe stress. SA-treated leaves' growth performance was similar to the non-treated control based on dry weight, fresh weight, leaf area, and dry weight/leaf area parameters. Our results suggest that SA application as a foliar spray can potentially be used to protect field-grown transplants against episodic frosts.

Exogenous Glycine Betaine Application Improves Freezing Tolerance of Cabbage (Brassica oleracea L.) Leaves

Exogenous glycine betaine (GB) application has been reported to improve plant tolerance to various abiotic stresses, but its effect on freezing tolerance has not been well studied. We investigated the effect of exogenous GB on freezing tolerance of cabbage (Brassica oleracea L.) leaves. Seedlings fed with 30 mM GB via sub-irrigation showed effectively assimilated GB as evident by higher GB concentration. Exogenous GB did not retard leaf-growth (fresh weight, dry weight, and leaf area) rather slightly promoted it. Temperature controlled freeze-thaw tests proved GB-fed plants were more freeze-tolerant as indicated by lower electrolyte leakage (i.e., indication of less membrane damage) and alleviating oxidative stress (less accumulation of O₂^•- and H₂O₂, as well as of malondialdehyde (MDA)) following a relatively moderate or severe freeze-thaw stress, i.e., -2.5 and -3.5 °C. Improved freezing tolerance induced by exogenous GB application may be associated with accumulation of compatible solute (proline) and antioxidant (glutathione). GB-fed leaves also had higher activity of antioxidant enzymes, catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD). These changes, together, may improve freezing tolerance through membrane protection from freeze-desiccation and alleviation of freeze-induced oxidative stress.

Transcriptomic analysis reveals potential pathways associated with salt resistance in pecan (Carya illinoensis K. Koch)

Soil salinity is a serious abiotic stress worldwide. Pecan plants (Carya illinoensis K. Koch) have been suggested for cultivation in soils with high levels of salinity owing to their huge demand. To understand the potential molecular mechanisms in pecan in response to salt stress, RNA-sequencing technology was used to compare the transcriptomes of pecan plants treated with 0, 0.3 %, or 0.6 % NaCl solutions. The results indicated that 170,086 unigenes were obtained from pecan leaf cDNA samples. Based on the assembled de novo transcriptome, 53, 535, and 7358 differentially expressed genes (DEGs) were detected between untreated and salt-treated leaves at 8, 24, and 48 h, respectively. Because of the large number of DEGs across different contrasts, a Gene Set Enrichment Analysis was selected to identify gene pathways associated with salt treatment. A total of 1858 DEGs were enriched in 66 gene sets, including 22 up-regulated and 47 down-regulated gene sets in the salt treatment groups, compared with those in the control groups. The up-regulated gene sets were mainly involved in the response to salicylic acid; the regulation of the jasmonic acid-mediated signalling pathway during the short-term treatment (8 h); and the cellular response to hypoxia, cellular respiration, and RNA modification during the long-term treatment (24-48 h). The down-regulated gene sets were predominately associated with photosynthesis, water transport, and the metabolic biosynthetic process under salt stress. Genes related to the Really Interesting New Gene superfamily protein and F-box domain protein in the ubiquitin-dependent degradation pathway were significantly up-regulated or down-regulated in different periods of the regulating process. Overall, these results not only enrich genomic resources but also provide insights into the molecular mechanism in pecan under salt stress.

Freezing Tolerance and Expression of β-amylase Gene in Two Actinidia arguta Cultivars with Seasonal Changes

Low temperature causes injuries to plants during winter, thereby it affects kiwi fruit quality and yield. However, the changes in metabolites and gene expression during cold acclimation (CA) and deacclimation (DA) in kiwi fruit remain largely unknown. In this study, freezing tolerance, carbohydrate metabolism, and β-amylase gene expression in two Actinidia arguta cv. "CJ-1" and "RB-3" were detected from CA to DA stages. In all acclimation stages, the "CJ-1" was hardier than "RB-3" and possessed lower semi-lethal temperature (LT50). Furthermore, "CJ-1" had a more rapid acclimation speed than "RB-3". Changes of starch, β-amylase, and soluble sugars were associated with freezing tolerance in both cultivars. Starch contents continued to follow a declining trend, while soluble sugars contents continuously accumulated in both cultivars during CA stages (from October to January). To investigate the possible molecular mechanism underlying cold response in A. arguta, in total, 16 AcBAMs genes for β-amylase were identified in the kiwi fruit genome. We carried out localization of chromosome, gene structure, the conserved motif, and the analysis of events in the duplication of genes from AcBAMs. Finally, a strong candidate gene named AaBAM3 from AcBAMs was cloned in Actinidia arguta (A. arguta), The real-time qPCR showed that AaBAM3 gene expression in seasonal changes was consistent with changes of soluble sugars. These results reveal that AaBAM3 may enhance the freezing tolerance of A. arguta through increasing soluble sugar content.

Short versus prolonged freezing differentially impacts freeze - thaw injury in spinach leaves: mechanistic insights through metabolite profiling

Plant tissues subjected to short or prolonged freezing to a fixed sub-freezing temperature are expected to undergo similar freeze-desiccation but the former causes substantially less injury than the latter. To gain metabolic insight into this differential response, metabolome changes in spinach (Spinacia oleracea L.) leaves were determined following short-term (0.5 and 3.0 h) vs. prolonged freezing (5.5 and 10.5 h) at -4.5°C resulting in reversible or irreversible injury, respectively. LD₅₀ , the freezing duration causing 50% injury, was estimated to be ∼3.1 h and defined as the threshold beyond which tissues were irreversibly injured. From 39 identified metabolites, 19 were selected and clustered into 3 groups: (1) signaling-related (salicylic acid, aliphatic and aromatic amino acids), (2) injury-related (GABA, lactic acid, maltose, fatty acids, policosanols, TCA intermediates) and (3) recovery-related (ascorbic acid, α-tocopherol). Initial accumulation of salicylic acid during short-term freezing followed by a decline may be involved in triggering tolerance mechanisms in moderately injured tissues, while its resurgence during prolonged freezing may signal programmed cell death. GABA accumulated with increasing freezing duration, possibly to serve as a 'pH-stat' against cytoplasmic acidification resulting from lactic acid accumulation. Mitochondria seem to be more sensitive to prolonged freezing than chloroplasts since TCA intermediates decreased after LD₅₀ while salicylic acid and maltose, produced in chloroplasts, accumulate even at 10.5-h freezing. Fatty acids and policosanols accumulation with increasing freezing duration indicates greater injury to membrane lipids and epicuticular waxes. Ascorbic acid and α-tocopherol accumulated after short-term freezing, supposedly facilitating recovery while their levels decreased in irreversibly injured tissues.

A metabolomics study of ascorbic acid-induced in situ freezing tolerance in spinach (Spinacia oleracea L.)

Freeze-thaw stress is one of the major environmental constraints that limit plant growth and reduce productivity and quality. Plants exhibit a variety of cellular dysfunctions following freeze-thaw stress, including accumulation of reactive oxygen species (ROS). This means that enhancement of antioxidant capacity by exogenous application of antioxidants could potentially be one of the strategies for improving freezing tolerance (FT) of plants. Exogenous application of ascorbic acid (AsA), as an antioxidant, has been shown to improve plant tolerance against abiotic stresses but its effect on FT has not been investigated. We evaluated the effect of AsA-feeding on FT of spinach (Spinacia oleracea L.) at whole plant and excised-leaf level, and conducted metabolite profiling of leaves before and after AsA treatment to explore metabolic explanation for change in FT. AsA application did not impede leaf growth, instead slightly promoted it. Temperature-controlled freeze-thaw tests revealed AsA-fed plants were more freezing tolerant as indicated by: (a) less visual damage/mortality; (b) lower ion leakage; and (c) less oxidative injury, lower abundance of free radicals ( O 2 · - and H2O2). Comparative leaf metabolite profiling revealed clear separation of metabolic phenotypes for control versus AsA-fed leaves. Specifically, AsA-fed leaves had greater abundance of antioxidants (AsA, glutathione, alpha- & gamma-tocopherol) and compatible solutes (proline, galactinol, and myo-inositol). AsA-fed leaves also had higher activity of antioxidant enzymes (superoxide dismutase, ascorbate peroxidase, and catalase). These changes, together, may improve FT via alleviating freeze-induced oxidative stress as well as protecting membranes from freeze desiccation. Additionally, improved FT by AsA-feeding may potentially include enhanced cell wall/lignin augmentation and bolstered secondary metabolism as indicated by diminished level of phenylalanine and increased abundance of branched amino acids, respectively.

Repair of sub-lethal freezing damage in leaves of Arabidopsis thaliana

BACKGROUND

The detrimental effects of global climate change direct more attention to the survival and productivity of plants during periods of highly fluctuating temperatures. In particular in temperate climates in spring, temperatures can vary between above-zero and freezing temperatures, even during a single day. Freeze-thaw cycles cause cell membrane lesions that can lead to tissue damage and plant death. Whereas the processes of cold acclimation and freeze-thaw injury are well documented, not much is known about the recovery of plants after a freezing event. We therefore addressed the following questions: i. how does the severity of freezing damage influence repair; ii. how are respiration and content of selected metabolites influenced during the repair process; and iii. how do transcript levels of selected genes respond during repair?

RESULTS

We have investigated the recovery from freezing to sub-lethal temperatures in leaves of non-acclimated and cold acclimated Arabidopsis thaliana plants over a period of 6 days. Fast membrane repair and recovery of photosynthesis were observed 1 day after recovery (1D-REC) and continued until 6D-REC. A substantial increase in respiration accompanied the repair process. In parallel, concentrations of sugars and proline, acting as compatible solutes during freezing, remained unchanged or declined, implicating these compounds as carbon and nitrogen sources during recovery. Similarly, cold-responsive genes were mainly down regulated during recovery of cold acclimated leaves. In contrast, genes involved in cell wall remodeling and ROS scavenging were induced during recovery. Interestingly, also the expression of genes encoding regulatory proteins, such as 14-3-3 proteins, was increased suggesting their role as regulators of repair processes.

CONCLUSIONS

Recovery from sub-lethal freezing comprised membrane repair, restored photosynthesis and increased respiration rates. The process was accompanied by transcriptional changes including genes encoding regulatory proteins redirecting the previous cold response to repair processes, e.g. to cell wall remodeling, maintenance of the cellular proteome and to ROS scavenging. Understanding of processes involved in repair of freeze-thaw injury increases our knowledge on plant survival in changing climates with highly fluctuating temperatures.

Physiological response of Secale cereale L. seedlings under freezing-thawing and alkaline salt stress

Freezing-thawing and saline-alkaline are the major abiotic stress for the pasture in most high-latitude areas, which are serious threats to the yield of pasture. In this study, the osmotic adjustment substances, membrane lipid peroxidation, and antioxidant enzymes activities of rye (Secale cereale L., cv. Dongmu-70) seedlings under different treatments: CK (no treatment), SC (Na₂CO₃ treatment), FT (freezing-thawing treatment), and FT+SC (combined Na₂CO₃ and freezing-thawing treatments), were investigated. At the freezing stage, the content of MDA and proline, the activity of APX, SOD, and POD increased with the decrease of the temperature in the leaves of rye seedlings in FT and FT+SC treatments and reached the maximum value at - 5 °C. In addition, the content of protein and H₂O₂, CAT activity reached the maximum value at 0 °C; the damage is larger under low temperature stress at 0 °C and - 5 °C in rye seedling. At the thawing stage, the content of MDA and H₂O₂ in seedling leaves decreased in FT and FT + SC treatments. These results demonstrated that proline content and antioxidant enzymes activities could play an important role in protecting cytomembrane and scavenging ROS respectively in rye under alkaline salt stress and freezing-thawing stress. The result also indicated rye seedlings were subjected to a freezing-thawing stress which resulted in a reversible (recoverable) injury.

Mechanism of freeze-thaw injury and recovery: A cool retrospective and warming up to new ideas

Understanding cellular mechanism(s) of freeze-thaw injury (FTI) is key to the efforts for improving plant freeze-tolerance by cultural methods or molecular/genetic approaches. However, not much work has been done in the last 25+ years to advance our understanding of the nature and cellular loci of FTI. Currently, two FTI lesions are predominantly implicated: 1) structural and functional perturbations in plasma membrane; 2) ROS-induced oxidative damage. While both have stood the test of time, many questions remain unresolved and other potentially significant lesions need to be investigated. Additionally, molecular mechanism of post-thaw recovery (PTR), a critical component of frost-survival, has not been well investigated. Mechanistic understanding of repair after reversible injury could expand the options for strategies to improve frost-hardiness. In this review, without claiming to be exhaustive, I have attempted to synthesize major discoveries from last several decades on the mechanisms of FTI and the relatively little research conducted thus far on PTR mechanisms. It is followed by proposing of hypotheses for mechanism(s) for irreversible FTI or PTR involving cytosolic calcium and ROS signaling. Perspective is presented on some unresolved questions and research on new ideas to fill the knowledge gaps and advance the field.

Mining and expression analysis of candidate genes involved in regulating the chilling requirement fulfillment of Paeonia lactiflora 'Hang Baishao'

BACKGROUND

The artificial enlargement of the planting area and ecological amplitude of ornamentals for horticultural and landscape applications are significant. Herbaceous peony (Paeonia lactiflora Pall.) is a world-famous ornamental with attractive and fragrant flowers and is mainly planted in temperate and cool areas. Comparatively higher winter temperatures in the subtropical and tropical Northern Hemisphere result in a deficit of chilling accumulation for bud dormancy release, which severely hinders "The southward plantation of herbaceous peony". Studies on the dormancy, chilling requirement (CR) and relevant molecular mechanisms of peony are needed to enhance our ability to extend the range of this valuable horticultural species.

RESULTS

Based on natural and artificial chilling experiments, and chilling hour (CH) and chilling unit (CU) evaluation systems, the lowest CR of 'Hang Baishao' was between 504.00 and 672.00 CHs and the optimal CR was 672.00 CHs and 856.08 CUs for achieving strong sprouting, growth and flowering performance. Transcriptome sequencing and gene identification by RNA-Seq were performed on 'Hang Baishao' buds during the dormancy and sprouting periods. Six gene libraries were constructed, and 66 temperature- and photoperiod-associated unigenes were identified as the potential candidate genes that may regulate or possibly determine CR characteristics. The difference in the expression patterns of SUPPRESSPOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) between the winters of 2012-2013 and 2015-2016, and the difference of CR fulfillment periods also between these two winters represented the interesting congruent relationships. This correlation was also observed for WRKY DNA-BINDING PROTEIN 33 (WRKY 33).

CONCLUSIONS

Combined with the results acquired from all of experiments, 'Hang Baishao' was confirmed to be a superb peony resource that have significantly low CR characteristics. The two genes of SOC1 and WRKY33 are likely involved in determining the CR amount and fulfillment period of 'Hang Baishao'. HEAT SHOCK PROTEIN, OSMOTIN and TIMING OF CAB EXPRESSION 1 also deserve attention for the CR research. This study could contribute to the knowledge of the deep factors and mechanisms that regulate CR characteristics, and may be beneficial for breeding new germplasms that have low CRs for landscape or horticulture applications in subtropical regions.

Exogenous salicylic acid improves freezing tolerance of spinach (Spinacia oleracea L.) leaves

Salicylic acid (SA)-treatment has been reported to improve plant tolerance to various abiotic stresses. However, its effect on freezing tolerance has not been well investigated. We investigated the effect of exogenous SA on freezing tolerance of spinach (Spinacia oleracea L.) leaves. We also explored if nitric oxide (NO) and/or hydrogen peroxide (H₂O₂)-mediation was involved in this response, since these are known as primary signaling molecules involved in many physiological processes. A micro-centrifuge tube-based system used to apply SA to petiolate spinach leaves (0.5 mM over 4-d) was effective, as evident by SA content of leaf tissues. SA-treatment did not hamper leaf growth (fresh and dry weight; equatorial and longitudinal length) and was also not significantly different from 25% Hoagland controls vis-à-vis growth. SA application significantly improved freezing tolerance as evidenced by reduced ion-leakage and alleviated oxidative stress (lower accumulation of O₂^·- and H₂O₂) following freeze-thaw stress treatments (-6.5, -7.5, and -8.5 °C). Improved freezing tolerance of SA-treated leaves was paralleled by increased proline and ascorbic acid (AsA) accumulation. A 9-d cold acclimation (CA) treatment also improved leaf freezing tolerance (compared to non-acclimated control) and was accompanied by accumulation of SA and proline. Our results indicate that increased freezing tolerance may be associated with accumulation of compatible solutes (proline) and antioxidants (AsA). Notably, the beneficial effect of SA on freezing tolerance was abolished when either H₂O₂- or NO-scavenger (1 μM N-acetylneuraminic acid, NANA or 100 μM hemoglobin, HB, respectively) was added to SA as pretreatment. Our data suggest that SA-induced freezing tolerance in spinach may be mediated by NO and H₂O₂ signaling.

Abiotic stresses influence the transcript abundance of PIP and TIP aquaporins in Festuca species

Festuca arundinacea and F. pratensis are the models in forage grasses to recognize the molecular basis of drought, salt and frost tolerance, respectively. Transcription profiles of plasma membrane intrinsic proteins (PIPs) and tonoplast intrinsic proteins (TIPs) aquaporin genes were obtained for leaves of Festuca species treated with different abiotic stimuli. F. arundinacea plants were exposed to drought and salt stress, whereas F. pratensis plants were cold-hardened. Changes in genes expression measured with use of real time qRT-PCR method were compared between two genotypes characterized with a significantly different level of each stress tolerance. Under drought the transcript level of PIP1;2 and TIP1;1 aquaporin decreased in both analyzed F. arundinacea genotypes, whereas for PIP2;1 only in a high drought tolerant plant. A salt treatment caused a reduction of PIP1;2 transcript level in a high salt tolerant genotype and an increase of TIP1;1 transcript abundance in both F. arundinacea genotypes, but it did not influence the expression of PIP2;1 aquaporin. During cold-hardening a decrease of PIP1;2, PIP2;1, and TIP1;1 aquaporin transcripts was observed, both in high and low frost tolerant genotypes. The obtained results revealed that the selected genotypes responded in a different way to abiotic stresses application. A reduced level of PIP1;2 transcript in F. arundinacea low drought tolerant genotype corresponded with a faster water loss and a lowering of photosynthesis efficiency and gas exchange during drought conditions. In F. pratensis, cold acclimation was associated with a lower level of aquaporin transcripts in both high and low frost tolerant genotypes. This is the first report on aquaporin transcriptional profiling under abiotic stress condition in forage grasses.

Sodium hydrosulfide induces systemic thermotolerance to strawberry plants through transcriptional regulation of heat shock proteins and aquaporin

BACKGROUND

Temperature extremes represent an important limiting factor to plant growth and productivity. The present study evaluated the effect of hydroponic pretreatment of strawberry (Fragaria x ananassa cv. 'Camarosa') roots with an H2S donor, sodium hydrosulfide (NaHS; 100 μM for 48 h), on the response of plants to acute heat shock treatment (42°C, 8 h).

RESULTS

Heat stress-induced phenotypic damage was ameliorated in NaHS-pretreated plants, which managed to preserve higher maximum photochemical PSII quantum yields than stressed plants. Apparent mitigating effects of H2S pretreatment were registered regarding oxidative and nitrosative secondary stress, since malondialdehyde (MDA), H2O2 and nitric oxide (NO) were quantified in lower amounts than in heat-stressed plants. In addition, NaHS pretreatment preserved ascorbate/glutathione homeostasis, as evidenced by lower ASC and GSH pool redox disturbances and enhanced transcription of ASC (GDH) and GSH biosynthetic enzymes (GS, GCS), 8 h after heat stress imposition. Furthermore, NaHS root pretreatment resulted in induction of gene expression levels of an array of protective molecules, such as enzymatic antioxidants (cAPX, CAT, MnSOD, GR), heat shock proteins (HSP70, HSP80, HSP90) and aquaporins (PIP).

CONCLUSION

Overall, we propose that H2S root pretreatment activates a coordinated network of heat shock defense-related pathways at a transcriptional level and systemically protects strawberry plants from heat shock-induced damage.