Understanding plant cold hardiness: an opinion

Integrated anatomical structure, physiological, and transcriptomic analyses to identify differential cold tolerance responses of Ziziphus jujuba mill. 'Yueguang' and its autotetraploid 'Hongguang'

Cold stress is a limiting stress factor that limits plant distribution and development; however, polyploid plants have specific characteristics such as higher resistance to abiotic stress, especially cold stress, that allow them to overcome this challenge. The cultivated cultivar Ziziphus jujuba Mill. 'Yueguang' (YG) and its autotetraploid counterpart 'Hongguang' (HG) exhibit differential cold tolerance. However, the underlying molecular mechanism and methods to enhance their cold tolerance remain unknown. Anatomical structure and physiological analysis indicated YG had a higher wood bark ratio, and xylem ratio under cold treatment compared to HG. However, the half-lethal temperature (LT50), cortex ratio, and malondialdehyde (MDA) content were significantly decreased in YG than HG, which indicated YG was cold tolerant than HG. Transcriptome analysis showed that 2084, 1725, 2888, and 2934 differentially expressed genes (DEGs) were identified in HC vs YC, H20 vs Y20, Y20 vs YC, and H20 vs HC treatment, respectively. Meanwhile, KEGG enrichment analysis of DEGs showed that several metabolic pathways, primarily plant hormone signal transduction and the MAPK signaling pathway, were involved in the differential regulation of cold tolerance between YG and HG. Furthermore, exogenous abscisic acid (ABA) and brassinolide (BR) treatments could improve their cold tolerance through increased SOD and POD activities, decreased relative electrical conductivity, and MDA content. All of these findings suggested that plant hormone signal transduction, particularly ABA and BR, might have an important role in the regulation of differential cold tolerance between YG and HG, laying the foundation for further improving cold tolerance in jujube and examining the molecular mechanisms underlying differences in cold tolerance among different ploidy cultivars.

Experimental investigation of freeze injury temperatures in trees and their contributing factors based on electrical impedance spectroscopy

In trees, injuries resulting from subfreezing temperatures can cause damage to the cellular biofilm system, metabolic functions, and fibrous reticulum, and even cell death. Investigating the occurrence of freezing damage and its contributing factors could help understand the mechanisms underlying freezing injury and prevent the subsequent damage in trees. To achieve this, a laboratory experiment was conducted using cut wood samples from Korean pine (Pinus koraiensis Siebold & Zucc) and Simon poplar (Populus simonii Carr.), and the effects of environmental freezing factors, including freezing temperatures, freezing duration, and cooling rate, on the temperature at which freezing injuries occur were examined using the electrical impedance spectroscopy (EIS) method. The semi-lethal temperature (LT50), as an indicator of freezing injury in wood tissue, was theoretically deduced based on the measured extracellular resistance (r e) using EIS. The contributory factors to changes in LT50 were determined and their relationship was established. The results revealed that all freezing factors exhibited significant effects on electrical impedance characteristics (r e, r i, and τ), significantly influencing the LT50 of the wood. Random forest (RF) and support vector machine (SVM) models were used to assess the contribution of the freezing factors and moisture content (MC). Among the factors examined, freezing duration had the greatest impact on LT50, followed by the MC, whereas the contribution of the cooling rate was minimal. The model accuracies were 0.89 and 0.86 for Korean pine and Simon poplar, respectively. The findings of our study illustrate that the occurrence of freezing injury in trees is primarily influenced by the duration of freezing at specific subzero temperatures. Slow cooling combined with prolonged freezing at low subzero temperatures leads to earlier and more severe freezing damage.

Physiochemical Responses and Ecological Adaptations of Peach to Low-Temperature Stress: Assessing the Cold Resistance of Local Peach Varieties from Gansu, China

In recent years, extreme weather events have become increasingly frequent, and low winter temperatures have had a significant impact on peach cultivation. The selection of cold-resistant peach varieties is an effective solution to mitigate freezing damage. To comprehensively and accurately evaluate the cold resistance of peaches and screen for high cold resistance among Gansu local resources, nine different types of peach were selected as test resources to assess physiological, biochemical, and anatomical indices. Subsequently, 28 peach germplasms were evaluated using relevant indices. The semi-lethal temperature (LT50) was calculated by fitting the change curve of the electrolyte leakage index (ELI) with the Logistic equation; this can be used as an important index for identifying and evaluating the cold resistance of peach trees. The LT50 values ranged from -28.22 °C to -17.22 °C among the 28 tested resources; Dingjiaba Liguang Tao exhibited the lowest LT50 value at -28.22 °C, indicating its high level of cold resistance. The LT50 was positively correlated with the ELI and malondialdehyde (MDA) content with correlation coefficients of 0.894 and 0.863, respectively, while it was negatively correlated with the soluble sugar (SS), soluble protein (SP), and free proline (Pro) contents with correlation coefficients of -0.894, -0.721, and -0.863, respectively. The thicknesses of the xylem, cork layer, cork layer ratio (CLR) and thickness/cortex thickness (X/C) showed negative correlations (-0.694, -0.741, -0.822, -0.814, respectively). Finally, the membership function method was used to evaluate cold resistance based on the ELI, MDA, Pro, SP, SS, CLR, and xylem thickness/cortex thickness (X/C) indices. The average membership degree among all tested resources ranged from 0.17 to 0.61. Dingjiaba Liguang Tao emerged prominently in terms of high-cold-resistance (HR) membership value (0.61).

Identification of Genomic Regions Associated with Seedling Frost Tolerance in Sorghum

Sorghum bicolor (L.) Moench is the fifth most valuable cereal crop globally. Although sorghum is tolerant to drought and elevated temperatures, it is susceptible to chilling, frost, and freezing stresses. Sorghum seeds planted in April may encounter frequent frost during late April and early May. Early spring freezing temperatures adversely affect crop development and yield. This study aims to identify genomic regions associated with frost tolerance at the seedlings stage. Breeding freeze-tolerant cultivars require selection for freeze tolerance in nurseries. However, the unpredictability of environmental conditions complicates the identification of freeze-tolerant genotypes. An indoor selection protocol has been developed to investigate the genetic determinism of freeze tolerance at the seedling stages and its correlation with several developmental traits. To accomplish this, we used two populations of recombinant inbred lines (RIL) developed from crosses between cold-tolerant (CT19, ICSV700) and cold-sensitive (TX430, M81E) parents. The derived RIL populations were evaluated for single nucleotide polymorphism (SNP) using genotype-by-sequencing (GBS) under controlled environments for their response to freezing stress. Linkage maps were constructed with 464 and 875 SNPs for the CT19 X TX430 (C1) and ICSV700 X M81E(C2) populations. Using quantitative trait loci (QTL) mapping, we identified six QTLs conferring tolerance to freezing temperatures. One QTL in the C1 population and four QTLs in the C2 population, explain 17.75-98% of the phenotypic variance of traits measured. Proline leaf content was increased in response to exposing the seedlings to low temperatures. Candidate QTLs identified in this study could be further exploited to develop frost-tolerant cultivars as proxies in marker-assisted breeding, genomic selection, and genetic engineering.

Dynamics of Polyamines, Proline, and Ethylene Metabolism under Increasing Cold in Winter Oilseed Rape

Cold stress is among the most important environmental factors reducing the yield of crops. The present study aimed to investigate the impact of increasing cold stress conditions on winter oilseed rape polyamines, proline, and ethylene metabolism in acclimated and non-acclimated winter oilseed rape. This study was carried out under controlled conditions in the laboratory. The winter oilseed rape hybrid 'Visby' was used in the experiment. Acclimated and non-acclimated plants were subjected to a two-day-long increasing cold (from -1 °C to -3 °C) treatment. HPTLC, RT-qPCR, spectral analysis, and gas chromatography methods were used to analyse the levels of polyamines, gene expression, proline, and ethylene, respectively. This study showed a decrease in putrescine, spermidine, and spermine content during cold acclimation and a decrease in putrescine and spermidine levels at sub-zero temperatures. There were intensive changes in ADC2 gene expression, proline, and ethylene levels in non-acclimated plants: a substantial increase after exposure to -1 °C temperature and a sharp decrease after exposure to -3 °C temperature. The changes in these parameters were lower or absent in acclimated plants. The phenomena observed in this study add new insights to the knowledge about the plant stress response and suggest questions to be answered in the future.

Estimating daily minimum grass temperature to quantify frost damage to winter wheat during stem elongation in the central area of Huang-Huai plain in China

Frost damage to winter wheat during stem elongation frequently occurred in the Huang-Huai plain of China, leading to considerable yield losses. Minimum Stevenson screen temperature (ST_min) and minimum grass temperature (GT_min) have long been used to quantify frost damage. Although GT_min has higher accuracy than ST_min, it is limited in application due to the lack of data. Therefore, this study aimed to select appropriate environmental variables to estimate GT_min, as well as to quantify the frost damage. Shangqiu, a frost-prone winter wheat area in the central Huang-Hui plain, was selected as the study area. From the descriptive statistics of ST, air relative humidity (RH), wind speed (WS), cloud fraction (CF), and volumetric soil water content (VWC) during temperature decreasing and increasing, seven variables significantly correlated with GT_min were selected, including ST_min, maximum reduction of ST (RST), maximum increase of ST (IST), minimum RH during temperature increasing (RH_min), WS at ST_min occurrence (WS), minimum VWC during temperature decreasing (VWC_min), and nightly CF. Multiple linear regression (MLR), support vector regression (SVR), random forest (RF), and K-nearest neighbor (KNN) were adopted for estimating GT_min based on the various combinations of the variables. Results showed the more variables, the higher the accuracy for the MLR and SVR. However, this pattern was not always true for the KNN and RF. The KNN based on ST_min, RST, IST, RH_min, and WS achieved the highest accuracy, with R² of 0.9992, RMSE of 0.14 ℃, and MAE of 0.076 ℃. The overall classification accuracy for frost damage identified by the estimated GT_min reached 97.1% during stem elongation of winter wheat from 2017 to 2021. The integrated frost stress (IFS) index calculated by the estimated and measured GT_min maintained high linear fitting accuracy. The KNN with fewer variables demonstrated good applicability at the regional scale.

Protease inhibitor ASP enhances freezing tolerance by inhibiting protein degradation in kumquat

Cold acclimation is a complex biological process leading to the development of freezing tolerance in plants. In this study, we demonstrated that cold-induced expression of protease inhibitor FmASP in a Citrus-relative species kumquat [Fortunella margarita (Lour.) Swingle] contributes to its freezing tolerance by minimizing protein degradation. Firstly, we found that only cold-acclimated kumquat plants, despite extensive leaf cellular damage during freezing, were able to resume their normal growth upon stress relief. To dissect the impact of cold acclimation on this anti-freezing performance, we conducted protein abundance assays and quantitative proteomic analysis of kumquat leaves subjected to cold acclimation (4°C), freezing treatment (-10°C) and post-freezing recovery (25°C). FmASP (Against Serine Protease) and several non-specific proteases were identified as differentially expressed proteins induced by cold acclimation and associated with stable protein abundance throughout the course of low-temperature treatment. FmASP was further characterized as a robust inhibitor of multiple proteases. In addition, heterogeneous expression of FmASP in Arabidopsis confirmed its positive role in freezing tolerance. Finally, we proposed a working model of FmASP and illustrated how this extracellular-localized protease inhibitor protects proteins from degradation, thereby maintaining essential cellular function for post-freezing recovery. These findings revealed the important role of protease inhibition in freezing response and provide insights on how this role may help develop new strategies to enhance plant freezing tolerance.

Foliar application of putrescine, salicylic acid, and ascorbic acid mitigates frost stress damage in Vitis vinifera cv. ̒Giziluzum̕

BACKGROUND

Cold stress is an effective factor in reducing production and injuring fruit trees. Various materials, such as salicylic acid, ascorbic acid, and putrescine, are used to alleviate the damage of abiotic stress.

RESULTS

The effect of different treatments of putrescine, salicylic acid, and ascorbic acid on alleviating the damage of frost stress (- 3 °C) to grapes 'Giziluzum' was investigated. Frost stress increased the amount of H₂O₂, MDA, proline, and MSI. On the other hand, it decreased the concentration of chlorophyll and carotenoids in the leaves. Putrescine, salicylic acid and ascorbic acid significantly increased the activities of catalase, guaiacol peroxidase, ascorbate peroxidase, and superoxide dismutase under frost stress. Following frost stress, the grapes treated with putrescine, salicylic acid, and ascorbic acid showed higher levels of DHA, AsA, and AsA/DHA than the untreated grapes. Our results showed that the treatment with ascorbic acid outperformed the other treatments in adjusting frost stress damages.

CONCLUSION

The use of compounds, such as ascorbic ac id, salicylic acid, and putrescine, modulates the effects of frost stress, thereby increasing the antioxidant defense system of cells, reducing its damage, and stabilizing stable cell conditions, so it can be used to reduce frost damage to different grape cultivars.

Does phenotyping of Hypericum secondary metabolism reveal a tolerance to biotic/abiotic stressors?

In this review we summarize the current knowledge about the changes in Hypericum secondary metabolism induced by biotic/abiotic stressors. It is known that the extreme environmental conditions activate signaling pathways leading to triggering of enzymatic and non-enzymatic defense systems, which stimulate production of secondary metabolites with antioxidant and protective effects. Due to several groups of bioactive compounds including naphthodianthrones, acylphloroglucinols, flavonoids, and phenylpropanes, the world-wide Hypericum perforatum represents a high-value medicinal crop of Hypericum genus, which belongs to the most diverse genera within flowering plants. The summary of the up-to-date knowledge reveals a relationship between the level of defense-related phenolic compounds and interspecific differences in the stress tolerance. The chlorogenic acid, and flavonoids, namely the amentoflavone, quercetin or kaempferol glycosides have been reported as the most defense-related metabolites associated with plant tolerance against stressful environment including temperature, light, and drought, in association with the biotic stimuli resulting from plant-microbe interactions. As an example, the species-specific cold-induced phenolics profiles of 10 Hypericum representatives of different provenances cultured in vitro are illustrated in the case-study. Principal component analysis revealed a relationship between the level of defense-related phenolic compounds and interspecific differences in the stress tolerance indicating a link between the provenance of Hypericum species and inherent mechanisms of cold tolerance. The underlying metabolome alterations along with the changes in the activities of ROS-scavenging enzymes, and non-enzymatic physiological markers are discussed. Given these data it can be anticipated that some Hypericum species native to divergent habitats, with interesting high-value secondary metabolite composition and predicted high tolerance to biotic/abiotic stresses would attract the attention as valuable sources of bioactive compounds for many medicinal purposes.

Protective Strategies of Haberlea rhodopensis for Acquisition of Freezing Tolerance: Interaction between Dehydration and Low Temperature

Resurrection plants are able to deal with complete dehydration of their leaves and then recover normal metabolic activity after rehydration. Only a few resurrection species are exposed to freezing temperatures in their natural environments, making them interesting models to study the key metabolic adjustments of freezing tolerances. Here, we investigate the effect of cold and freezing temperatures on physiological and biochemical changes in the leaves of Haberlea rhodopensis under natural and controlled environmental conditions. Our data shows that leaf water content affects its thermodynamical properties during vitrification under low temperatures. The changes in membrane lipid composition, accumulation of sugars, and synthesis of stress-induced proteins were significantly activated during the adaptation of H. rhodopensis to both cold and freezing temperatures. In particular, the freezing tolerance of H. rhodopensis relies on a sucrose/hexoses ratio in favor of hexoses during cold acclimation, while there is a shift in favor of sucrose upon exposure to freezing temperatures, especially evident when leaf desiccation is relevant. This pattern was paralleled by an elevated ratio of unsaturated/saturated fatty acids and significant quantitative and compositional changes in stress-induced proteins, namely dehydrins and early light-induced proteins (ELIPs). Taken together, our data indicate that common responses of H. rhodopensis plants to low temperature and desiccation involve the accumulation of sugars and upregulation of dehydrins/ELIP protein expression. Further studies on the molecular mechanisms underlying freezing tolerance (genes and genetic regulatory mechanisms) may help breeders to improve the resistance of crop plants.

Cold acclimation threshold induction temperatures of switchgrass ecotypes grown under a long and short photoperiod

Plants can cold acclimate to enhance their freezing tolerance by sensing declining temperature and photoperiod cues. However, the factors influencing genotypic variation in the induction of cold acclimation are poorly understood among perennial grasses. We hypothesized that the more northern upland switchgrass (Panicum virgatum L.) ecotype develops a higher degree of freezing tolerance by initiating cold acclimation at higher temperatures as compared with the coastal and southern lowland ecotypes. First, we determined the optimal method for assessing freezing tolerance and the length of exposure to 8/4°C required to induce the maximum level of freezing tolerance in the most northern upland and most southern lowland genotypes. We characterized the maximum freezing tolerance of eight uplands, three coastal and five lowland genotypes grown for 21 days at 8/4°C and a 10 or 16 h photoperiod. Next, we identified the temperature required to induce cold acclimation by exposing the 16 genotypes for 7 days at 20-6°C constant temperatures under a 10 or 16 h photoperiod. Cold acclimation initiated at temperatures 5 and 7°C higher in upland than in coastal and lowland genotypes. Among upland genotypes the shorter photoperiod induced cold acclimation at a 1°C higher temperature. Genotypes originating from a more northern latitude initiate cold acclimation at higher temperatures and develop higher maximum freezing tolerances. An earlier response to declining temperatures may provide the upland ecotype with additional time to prepare for winter and provide an advantage when plants are subjected to the rapid changes in fall temperature associated with injurious frosts.

Repeated colonization of alpine habitats by Arabidopsis arenosa viewed through freezing resistance and ice management strategies

Success or failure of plants to cope with freezing temperatures can critically influence plant distribution and adaptation to new habitats. Especially in alpine environments, frost is a likely major selective force driving adaptation. In Arabidopsis arenosa (L.) Lawalrée, alpine populations have evolved independently in different mountain ranges, enabling studying mechanisms of acclimation and adaptation to alpine environments. We tested for heritable, parallel differentiation in freezing resistance, cold acclimation potential and ice management strategies using eight alpine and eight foothill populations. Plants from three European mountain ranges (Niedere Tauern, Făgăraș and Tatra Mountains) were grown from seeds of tetraploid populations in four common gardens, together with diploid populations from the Tatra Mountains. Freezing resistance was assessed using controlled freezing treatments and measuring effective quantum yield of photosystem II, and ice management strategies by infrared video thermography and cryomicroscopy. The alpine ecotype had a higher cold acclimation potential than the foothill ecotype, whereby this differentiation was more pronounced in tetraploid than diploid populations. However, no ecotypic differentiation was found in one region (Făgăraș), where the ancient lineage had a different evolutionary history. Upon freezing, an ice lens within a lacuna between the palisade and spongy parenchyma tissues was formed by separation of leaf tissues, a mechanism not previously reported for herbaceous species. The dynamic adjustment of freezing resistance to temperature conditions may be particularly important in alpine environments characterized by large temperature fluctuations. Furthermore, the formation of an extracellular ice lens may be a useful strategy to avoid tissue damage during freezing.

Cold stress regulates accumulation of flavonoids and terpenoids in plants by phytohormone, transcription process, functional enzyme, and epigenetics

Plants make different defense mechanisms in response to different environmental stresses. One common way is to produce secondary metabolites. Temperature is the main environmental factor that regulates plant secondary metabolites, especially flavonoids and terpenoids. Stress caused by temperature decreasing to 4-10 °C is conducive to the accumulation of flavonoids and terpenoids. However, the accumulation mechanism under cold stress still lacks a systematic explanation. In this review, we summarize three aspects of cold stress promoting the accumulation of flavonoids and terpenoids in plants, that is, by affecting (1) the content of endogenous plant hormones, especially jasmonic acid and abscisic acid; (2) the expression level and activity of important transcription factors, such as bHLH and MYB families. This aspect also includes post-translational modification of transcription factors caused by cold stress; (3) key enzyme genes expression and activity in the biosynthesis pathway, in addition, the rate-limiting enzyme and glycosyltransferases genes are responsive to cold stress. The systematic understanding of cold stress regulates flavonoids, and terpenoids will contribute to the future research of genetic engineering breeding, metabolism regulation, glycosyltransferases mining, and plant synthetic biology.

Salicylic Acid, a Multifaceted Hormone, Combats Abiotic Stresses in Plants

In recent decades, many new and exciting findings have paved the way to the better understanding of plant responses in various environmental changes. Some major areas are focused on role of phytohormone during abiotic stresses. Salicylic acid (SA) is one such plant hormone that has been implicated in processes not limited to plant growth, development, and responses to environmental stress. This review summarizes the various roles and functions of SA in mitigating abiotic stresses to plants, including heating, chilling, salinity, metal toxicity, drought, ultraviolet radiation, etc. Consistent with its critical roles in plant abiotic tolerance, this review identifies the gaps in the literature with regard to the complex signalling network between SA and reactive oxygen species, ABA, Ca²⁺, and nitric oxide. Furthermore, the molecular mechanisms underlying signalling networks that control development and stress responses in plants and underscore prospects for future research on SA concerning abiotic-stressed plants are also discussed.

Cold and exogenous calcium alter Allium fistulosum cell wall pectin to depress intracellular freezing temperatures

De-methyl esterification of homogalacturonan and subsequent cross-linking with Ca2+ is hypothesized to enhance the freezing survival of cold acclimated plants by reducing the porosity of primary cell walls. To test this theory, we collected leaf epidermal peels from non- (23/18 °C) and cold acclimated (2 weeks at 12/4 °C) Japanese bunching onion (Allium fistulosum L.). Cold acclimation enhanced the temperature at which half the cells survived freezing injury by 8 °C (LT50 =-20 °C), and reduced tissue permeability by 70-fold compared with non-acclimated epidermal cells. These effects were associated with greater activity of pectin methylesterase (PME) and a reduction in the methyl esterification of homogalacturonan. Non-acclimated plants treated with 50 mM CaCl2 accumulated higher concentrations of galacturonic acid, Ca2+ in the cell wall, and a lower number of visible cell wall pores compared with that observed in cold acclimated plants. Using cryo-microscopy, we observed that 50 mM CaCl2 treatment did not lower the LT50 of non-acclimated cells, but reduced the lethal intracellular ice nucleation to temperatures observed in cold acclimated epidermal cells. We postulate that the PME-homogalacturonan-mediated reduction in cell wall porosity is integral to intracellular freezing avoidance strategies in cold acclimated herbaceous cells.

Cold Stress, Freezing Adaptation, Varietal Susceptibility of Olea europaea L.: A Review

Olive (Olea europaea L.) is an evergreen xerophytic tree characterizing vegetative landscape and historical-cultural identity of the Mediterranean Basin. More than 2600 cultivars constitute the rich genetic patrimony of the species cultivated in approximately 60 countries. As a subtropical species, the olive tree is quite sensitive to low temperatures, and air temperature is the most critical environmental factor limiting olive tree growth and production. In this present review, we explored the detrimental effects caused of low temperatures on olive cultivars, and analyzed the most frequently experimental procedures used to evaluate cold stress. Then, current findings freezing stress physiology and gene are summarized in olive tree, with an emphasis on adaptive mechanisms for cold tolerance. This review might clear the way for new research on adaptive mechanisms for cold acclimation and for improvement of olive growing management.

Genetic loci associated with freezing tolerance in a European rapeseed (Brassica napus L.) diversity panel identified by genome-wide association mapping

Winter biotypes of rapeseed (Brassica napus L.) require a vernalization treatment to enter the reproductive phase and generally produce greater yields than spring rapeseed. To find genetic loci associated with freezing tolerance in rapeseed, we first performed genotyping-by-sequencing (GBS) on a diversity panel consisting of 222 rapeseed accessions originating primarily from Europe, which identified 69,554 high-quality single-nucleotide polymorphisms (SNPs). Model-based cluster analysis suggested that there were eight subgroups. The diversity panel was then phenotyped for freezing survival (visual damage and Fv/Fo and Fv/Fm) after 2 months of cold acclimation (5°C) and a freezing treatment (-15°C for 4 h). The genotypic and phenotypic data for each accession in the rapeseed diversity panel was then used to conduct a genome-wide association study (GWAS). GWAS results showed that 14 significant markers were mapped to seven chromosomes for the phenotypes scored. Twenty-four candidate genes located within the mapped loci were identified as previously associated with lipid, photosynthesis, flowering, ubiquitination, and cytochrome P450 in rapeseed or other plant species.

Anti-icing strategies of plant surfaces: the ice formation on leaves visualized by Cryo-SEM experiments

This paper presents an experimental study on surface icing on leaves in six plant species having different surface micromorphology and wettability properties. Contrary to previous studies on ice crystallization, which have been mainly performed by using infrared video thermography, we applied a Cryo-SEM approach allowing not only characterization of plant surfaces in their native conditions but also visualization of ice crystal formation on the native plant surfaces at the micro- and nanoscales. The Cryo-SEM was also used as an experimental device to freeze water vapor, thaw ice crystals, and freeze fluid water on the plant surface again. The experiments clearly demonstrate that trichome coverage (especially with several distinct layers) and 3D wax projections can be recognized as anti-icing strategies of plants. Trichomes can prevent and delay ice formation by being nucleation points for the formation of ice from vapor and protect the plant surface from overcooling, when fluid water freezes in contact with the leaf surface. The study shows for the first time two important effects that might reduce plant cell freezing rate: the presence of air pockets between wax projections that protect from direct contact between ice crystals and the plant cuticle and elimination of fluid water after thawing and preventing further re-freezing on the surface. The detailed knowledge obtained here is not only important for plant ecology, evolution, and plant protection but also for looking for potential biomimetic strategies that reduce/avoid icing of cultural plants and artificial technical surfaces.

Molecular Insights into Freezing Stress in Peach Based on Multi-Omics and Biotechnology: An Overview

In nature or field conditions, plants are frequently exposed to diverse environmental stressors. Among abiotic stresses, the low temperature of freezing conditions is a critical factor that influences plants, including horticultural crops, decreasing their growth, development, and eventually quality and productivity. Fortunately, plants have developed a mechanism to improve the tolerance to freezing during exposure to a range of low temperatures. In this present review, current findings on freezing stress physiology and genetics in peach (Prunus persica) were refined with an emphasis on adaptive mechanisms for cold acclimation, deacclimation, and reacclimation. In addition, advancements using multi-omics and genetic engineering approaches unravel the molecular physiological mechanisms, including hormonal regulations and their general perceptions of freezing tolerance in peach were comprehensively described. This review might pave the way for future research to the horticulturalists and research scientists to overcome the challenges of freezing temperature and improvement of crop management in these conditions.

Salicylic acid induces tolerance of Vitisriparia×V.labrusca to chilling stress by altered photosynthetic, antioxidant mechanisms and expression of cold stress responsive genes

The yield and quality of wine grapes are severely persecuted by low-temperaturestresses. Salicylic acid (SA) assists plants in coping with abiotic stresses such as drought, heavy metal toxicity, and osmotic stress. The objective of this study was to evaluate the effect of foliar spraying of different concentrations of SA on the mitigation of cold damage in grapes, which is useful for the cultivation of wine grapes.Vitisriparia×V.labruscaseedlings were treated with foliar-sprayedSA at concentrations of 0-2 mM and then subjected to chilling stress at 4°C for 2 or 4 days, while the expression of relevant physiological indicators and cold response genes (CBF1, CBF2, CBF3) were measured. The findings indicated that low temperature stresses markedly reduced chlorophyll content, and increased proline as well as soluble sugar content, enhanced superoxide dismutase (SOD) and peroxidase (POD) activities, decreased catalase (CAT) activity and inducedCBFgene expression in leaves. Physiologically, foliar spraying of different concentrations of SA greatly increased antioxidant enzyme activity (P < .05), soluble sugars, proline, and chlorophyll content of grapes leave under low temperature stress. With regard to gene expression, SA has significantly regulated the cold response genesCBF1, CBF2, andCBF3. Therefore, SA could reduce cold damage in grapevines under low-temperaturestress, and the effect of SA was most pronounced in the 1 and 2 mM concentrates.

Assessing growth, frost tolerance, and acclimation of pine seedlings with contrasted dormancy strategies as influenced by organic nitrogen supply

Freezing stress is a critical environmental factor affecting survival, distribution, and evolution of plants. Although there is evidence that nitrogen (N) affects frost tolerance of juvenile conifers, the magnitude and direction of such effect can diverge among species. The influence of the N source on frost tolerance has been barely studied. Particularly, how organic N sources could affect the cold acclimation dynamics of seedlings is poorly understood. We studied morpho-physiological responses to organic N supply (amino acids) in comparison to inorganic N in seedlings of two Mediterranean pine species: Pinus halepensis and P. sylvestris. Fertilization was applied at low and high N doses (30 and 130 mg N seedling^-1 ) in the first growing season. Then, tolerance of seedlings to freezing stress was evaluated through the cold season. This study confirmed that organic N supply promotes growth of both species as effectively as inorganic N sources. At low N availability, seedlings had acute phosphorus deficiencies when grown with inorganic N, but not with organic N. Likewise, high organic-N availability improved chlorophylls concentration. Both species increased their frost tolerance through time, especially during late autumn. Although organic N supply did not show clear benefits on frost tolerance, it seemed to enhance cold acclimation via increases of compatible solutes, such as soluble sugars and proline, particularly in P. halepensis. Thus, the effects of organic N supply could depend on the extent that such osmolytes contribute to the dormancy strategy of the species. Other species-specific mechanisms to cope with freezing stress are further discussed.

A Novel Banana Mutant "RF 1" (Musa spp. ABB, Pisang Awak Subgroup) for Improved Agronomic Traits and Enhanced Cold Tolerance and Disease Resistance

Banana is a major fruit crop grown in tropical and subtropical regions worldwide. Among cultivars, "FenJiao, FJ" (Musa spp. ABB, Pisang Awak subgroup) is a popular variety of bananas, due to its better sugar-acid blend and relatively small fruit shape. However, because the traditional FJ variety grows relatively high in height, it is vulnerable to lodging and unsuitable for harvesting. In this study, we sought desirable banana mutants by carrying out ethyl methanesulfonate (EMS) mutagenesis with the FJ cultivar. After the FJ shoot tips had been treated with 0.8% (v/v) EMS for 4 h, we obtained a stably inherited mutant, here called "ReFen 1" (RF1), and also observed a semi-dwarfing phenotype. Compared with the wild type (FJ), this RF1 mutant featured consistently improved agronomic traits during 5-year field experiments conducted in three distinct locations in China. Notably, the RF1 plants showed significantly enhanced cold tolerance and Sigatoka disease resistance, mainly due to a substantially increased soluble content of sugar and greater starch accumulation along with reduced cellulose deposition. Therefore, this study not only demonstrated how a powerful genetic strategy can be used in fruit crop breeding but also provided insight into the identification of novel genes for agronomic trait improvement in bananas and beyond.

The Breeding of Winter-Hardy Malting Barley

In breeding winter malting barley, one recurring strategy is to cross a current preferred spring malting barley to a winter barley. This is because spring malting barleys have the greatest amalgamation of trait qualities desirable for malting and brewing. Spring barley breeding programs can also cycle their material through numerous generations each year-some managing even six-which greatly accelerates combining desirable alleles to generate new lines. In a winter barley breeding program, a single generation per year is the limit when the field environment is used and about two generations per year if vernalization and greenhouse facilities are used. However, crossing the current favored spring malting barley to a winter barley may have its downsides, as winter-hardiness too may be an amalgamation of desirable alleles assembled together that confers the capacity for prolonged cold temperature conditions. In this review I touch on some general criteria that give a variety the distinction of being a malting barley and some of the general trends made in the breeding of spring malting barleys. But the main objective of this review is to pull together different aspects of what we know about winter-hardiness from the seemingly most essential aspect, which is survival in the field, to molecular genetics and gene regulation, and then finish with ideas that might help further our insight for predictability purposes.

Abiotic stress-induced anthocyanins in plants: Their role in tolerance to abiotic stresses

Abiotic stresses, such as heat, drought, salinity, low temperature, and heavy metals, inhibit plant growth and reduce crop productivity. Abiotic stresses are becoming increasingly extreme worldwide due to the ongoing deterioration of the global climate and the increase in agrochemical utilization and industrialization. Plants grown in fields are affected by one or more abiotic stresses. The consequent stress response of plants induces reactive oxygen species (ROS), which are then used as signaling molecules to activate stress-tolerance mechanism. However, under extreme stress conditions, ROS are overproduced and cause oxidative damage to plants. In such conditions, plants produce anthocyanins after ROS signaling via the transcription of anthocyanin biosynthesis genes. These anthocyanins are then utilized in antioxidant activities by scavenging excess ROS for their sustainability. In this review, we discuss the physiological, biochemical, and molecular mechanisms underlying abiotic stress-induced anthocyanins in plants and their role in abiotic stress tolerance. In addition, we highlight the current progress in the development of anthocyanin-enriched transgenic plants and their ability to increase abiotic stress tolerance. Overall, this review provides valuable information that increases our understanding of the mechanisms by which anthocyanins respond to abiotic stress and protect plants against it. This review also provides practical guidance for plant biologists who are engineering stress-tolerant crops using anthocyanin biosynthesis or regulatory genes.

Standardization of electrolyte leakage data and a novel liquid nitrogen control improve measurements of cold hardiness in woody tissue

BACKGROUND

A variety of basic and applied research programs in plant biology require the accurate and reliable determination of plant tissue cold hardiness. Over the past 50 years, the electrolyte leakage method has emerged as a popular and practical method for quantifying the amount of damage inflicted on plant tissue by exposure to freezing temperatures. Numerous approaches for carrying out this method and analyzing the resultant data have emerged. These include multiple systems for standardizing and modeling raw electrolyte leakage data and multiple protocols for boiling or autoclaving samples in order to maximize leakage as a positive control. We compare four different routines for standardization of leakage data and assess a novel control method-immersion in liquid nitrogen in lieu of traditional autoclaving-and apply them to woody twigs collected from 12 maple (Acer) species in early spring. We compare leakage data from these samples using each of four previously published forms of data analysis and autoclaving vs. liquid nitrogen controls and validate each of these approaches against visual estimates of freezing damage and differential thermal analysis.

RESULTS

Through presentation of our own data and re-analysis of previously published findings, we show that standardization of raw data against estimates of both minimum and maximum attainable freezing damage allows for reliable estimation of cold hardiness at the species level and across studies in diverse systems. Furthermore, use of our novel liquid nitrogen control produces data commensurate across studies and enhances the consistency and realism of the electrolyte leakage method, especially for very cold hardy samples.

CONCLUSION

Future leakage studies that relativize data against minimum and maximum leakage and that employ our updated liquid nitrogen control will contribute generalizable, repeatable, and realistic data to the existing body of cold hardiness research in woody plants. Data from studies conducted using a liquid nitrogen (and not an autoclaving) control can still be compared to previously published data, especially when raw data are standardized using the best-performing approach among those we assessed. Electrolyte leakage of woody twigs emerges as a useful technique for quickly assessing the probability of tissue death in response to freezing in dormant plants. Differential thermal analysis may provide different and complementary information on cold hardiness.

Altitudinal differentiation in the leaf wax-mediated flowering bud protection against frost in a perennial Arabidopsis

An altitudinal gradient of leaf water repellency is often observed between and within species. In a previous study of Arabidopsis halleri, cauline leaves (stem leaves that wrap flowering buds) showed higher water repellency in exposed semi-alpine plants than in understory low-elevation plants. Here, we examined altitudinal variations in the cuticular wax content of the leaf surface and experimentally evaluated the role of high water repellency of cauline leaves. Leaf cuticular wax was analysed using comprehensive two-dimensional gas chromatography (GC)-mass spectrometry and a GC-flame ionisation detector. Young flowering buds wrapped by cauline leaves were exposed to freezing temperatures with or without water, and frost damage to the flowering buds was compared between plants from semi-alpine and low-elevation habitats. Higher amounts of C29, C31, and C33 alkanes were observed in the cauline leaves of semi-alpine plants than in those of low-elevation plants. In the freezing experiment, water application increased damage to the flowering buds of low-elevation plants, and the extent of damage to the flowering buds was lower in semi-alpine plants than in low-elevation plants when water was applied to the plant surface. Genetic variations in the amounts of alkanes on the leaf surface depending on the altitude occurred specifically in cauline leaves. Our results indicate that the water repellency of cauline leaves presumably minimises frost damage to flowering buds at high altitudes.

Potential benefits of promoting snowmelt by artificial snow blacking on the growth of winter wheat and their dependence upon regional climate

An earlier onset of regrowth after snow disappearance can enable wheat cultivars to avoid the hotter grain-filling period, without the need for early sowing in snowy regions. A blackened snow surface easily accelerates snow melting by absorbing solar radiation. In this study, we compare the yield components associated with snowmelt acceleration over 4 years and in 2 locations (Sapporo, SP, and Memuro, MM) in Japan, which exhibit contrasting autumn and spring climates. Early snow melting by snow-blackening accelerated wheat growth in MM by a maximum of 4 days for heading and 3 days for anthesis. Moreover, accelerating wheat phenological growth improved the grain yield in MM in 2016. This is because wheat plants were less likely to experience the localised cool and rainy weather that typically occurs during anthesis in mid-June. Early anthesis would decrease the likelihood that wheat plants experiencing lower sunlight intensity during the grain-filling period owing to exposure to rainy weather. However, warmer autumn conditions in SP likely hindered the development of high-level cold resistance in overwintering wheat. Accelerating snowmelt is one possible tool for mitigating the fluctuations in regional wheat production; however, the effectiveness of snow-blackening depends on the regional climate.

Formation of adaptive responses of grapes to the action of abiotic stressors of the winter period

The purpose of this work is to study the formation of adaptive reactions of grapes to the action of stressors of the winter period, by physiological and biochemical parameters, to identify varieties that have increased adaptive abilities for use in breeding. Objects of research are grape varieties of different origins: Kristall (control), Dostoynyi, Krasnostop AZOS, Vostorg, Aligote, Zarif. It was revealed that the increased resistance to winter stress conditions in the varieties Kristall, Dostoyny is achieved by a decrease in the water content of the buds by 9.09-10.40 %. In the varieties Vostorg, Dostoyny, Krasnostop AZOS adaptive resistance is achieved by increasing the starch content in the tissues of the shoots by 2.81-5.50 times in the pre-winter period. In varieties Krasnostop AZOS, Vostorg an important contribution to the formation of adaptive processes was made by water-soluble sugars, the content of which increased 2.82 and 2.89 times as a result of starch hydrolysis. An increase in the activity of peroxidase (2.49-2.75 times) indicated the instability of varieties Zarif, Aligote. Varieties Vostorg, Dostoynyi, Krasnostop AZOS have increased adaptive abilities in comparison with other studied varieties and are recommended for use in the breeding process.

Adaptive resistance of apple trees in the unstable climate of southern Russia

The purpose of these studies is to assess the adaptive resistance of the apple tree to winter stresses in terms of physiological and biochemical parameters, to identify the most resistant varieties for cultivation in the Krasnodar region. Objects of research are apple varieties of various origins: Orfey, Idared, Ligol, Prikubanskoe. It was found that the cryoprotective function of water-soluble sugars was maximally manifested in the varieties Orfey and Ligol, their content increased by 2.56 and 2.7 times, respectively. Soluble proteins made the greatest contribution to the formation of a protective response in Orfey and Prikubanskoye varieties in January, their content increased 1.8 and 1.9 times, respectively, compared to December. The protective function of anthocyanins was maximally manifested in December in the Orfey variety, in the bark of which their content increased by 5.6 times. Physiological and biochemical adaptation of apple varieties to low negative temperatures of the winter period is achieved by increasing the content of water-soluble sugars, proteins in anthocyanins in the bark and buds. According to the obtained data, the apple varieties Orfey and Prikubanskoye showed themselves to be more adaptive in comparison with other studied varieties and are recommended for cultivation in the Krasnodar region.

Fox Hunting in Wild Apples: Searching for Novel Genes in Malus Sieversii

Malus sieversii is considered the progenitor of modern apple (Malus pumila) cultivars and to represent a valuable source of genetic diversity. Despite the importance of M. sieversii as a source of disease resistance, stress tolerance, and novel fruit traits, little is known about gene function and diversity in M. sieversii. Notably, a publicly annotated genome sequence for this species is not available. In the current study, the FOX (Full-length cDNA OvereXpressing) gene hunting system was used to construct a library of transgenic lines of Arabidopsis in which each transgenic line overexpresses a full-length gene obtained from a cDNA library of the PI619283 accession of M. sieversii. The cDNA library was constructed from mRNA obtained from bark tissues collected in late fall–early winter, a time at which many abiotic stress-adaptative genes are expressed. Over 4000 apple FOX Arabidopsis lines have been established from the pool of transgenic seeds and cDNA inserts corresponding to various Gene Ontology (GO) categories have been identified. A total of 160 inserts appear to be novel, with no or limited homology to M. pumila, Arabidopsis, or poplar. Over 1300 lines have also been screened for freezing resistance. The constructed library of transgenic lines provides a valuable genetic resource for exploring gene function and diversity in Malus sieversii. Notably, no such library of t-DNA lines currently exists for any Malus species.

Fusion of Mitochondria to 3-D Networks, Autophagy and Increased Organelle Contacts are Important Subcellular Hallmarks during Cold Stress in Plants

Low temperature stress has a severe impact on the distribution, physiology, and survival of plants in their natural habitats. While numerous studies have focused on the physiological and molecular adjustments to low temperatures, this study provides evidence that cold induced physiological responses coincide with distinct ultrastructural alterations. Three plants from different evolutionary levels and habitats were investigated: The freshwater alga Micrasterias denticulata, the aquatic plant Lemna sp., and the nival plant Ranunculus glacialis. Ultrastructural alterations during low temperature stress were determined by the employment of 2-D transmission electron microscopy and 3-D reconstructions from focused ion beam-scanning electron microscopic series. With decreasing temperatures, increasing numbers of organelle contacts and particularly the fusion of mitochondria to 3-dimensional networks were observed. We assume that the increase or at least maintenance of respiration during low temperature stress is likely to be based on these mitochondrial interconnections. Moreover, it is shown that autophagy and degeneration processes accompany freezing stress in Lemna and R. glacialis. This might be an essential mechanism to recycle damaged cytoplasmic constituents to maintain the cellular metabolism during freezing stress.

What drives grassland-forest boundaries? Assessing fire and frost effects on tree seedling survival and architecture

Fire and frost represent two major hurdles for the persistence of trees in open grassy biomes and have both been proposed as drivers of grassland-forest boundaries in Africa.We assess the response of young tree seedlings, which represent a vulnerable stage in tree recruitment, to traumatic fire and frost disturbances.In a greenhouse experiment, we investigated how seedling traits predicted survival and resprouting ability in response to fire versus frost; we characterized survival strategies of seedlings in response to the two disturbances, and we documented how the architecture of surviving seedlings is affected by fire versus frost injury.Survival rates were similar under both treatments. However, different species displayed different levels of sensitivity to fire and frost. Seedling survival was higher for older seedlings and seedlings with more basal leaves. Survivors of a fire event lost more biomass than the survivors of a frost event. However, the architecture of recovered fire- and frost-treated seedlings was mostly similar. Seedlings that recovered from fire and frost treatments were often shorter than those that had not been exposed to any disturbance, with multiple thin branches, which may increase vulnerability to the next frost or fire event. Synthesis. Fire caused more severe aboveground damage compared with a single frost event, suggesting that fire is an important driver of tree distribution in these open grassland systems. However, the impact of repeated frost events may be equally severe and needs to be investigated. Also, woody species composition may be influenced by phenomena that affect the timing and frequency of seedling exposure to damage, as mortality was found to be dependent on seedling age. Therefore, changes in fire regime and climate are likely to result in changes in the composition and the structure of the woody components of these systems.

Evaluation of G × E × M Interactions to Increase Harvest Index and Yield of Early Sown Wheat

Harvest index (HI) is the ratio of grain to total shoot dry matter and is as a measure of reproductive efficiency. HI is determined by interactions between genotypes (G), environment (E), and crop management (M). Historic genetic yield gains due to breeding in wheat have largely been achieved by increasing HI. Environmental factors are important for HI and include seasonal pattern of water supply and extreme temperatures during crop reproductive development. Wheat production in Australia has been dominated by fast-developing spring cultivars that when sown in late-autumn will flower at an optimal time in early spring. Water limited potential yield can be increased by sowing slower developing wheats with a vernalization requirement (winter wheat) earlier than currently practiced such that their development is matched to environment and they flower at the optimal time. This means a longer vegetative phase which increases rooting depth, proportion of water-use transpired, and transpiration efficiency by allowing more growth during winter when vapour pressure deficit is low. All these factors can increase biomass accumulation, grain number and thus grain yield potential. However higher yields are not always realized due to a lower HI of early sown slow developing wheats compared to fast developing wheats sown later. Here, we evaluate genotype × management practices to improve HI and yield in early sown slow developing wheat crops using 6 field experiments conducted across south eastern Australia from 2014 to 2018 in yield environments ranging from ~1 to ~4.7 t/ha. Practices included low plant densities (30-50 plants/m²), mechanical defoliation, and deferred application of nitrogen fertilizer. Lower plant densities had similar yield and HI to higher plant densities. Defoliation tended to increase HI but reduce yield except when there was severe stem frost damage. Deferring nitrogen had a variable effect depending on starting soil N and in crop rainfall. All management strategies evaluated gave variable HI and yield responses with small effect sizes, and we conclude that none of them can reliably increase HI in early sown wheat. We propose that genetic improvement is the most promising avenue for increasing HI and yield in early sown wheat, and postulate that this could be achieved more rapidly through early generation screening for HI in slow developing genotypes than by crop management.

Impact of Processing Factors on Quality of Frozen Vegetables and Fruits

In this paper I review the production of frozen vegetables and fruits from a chain perspective. I argue that the final quality of the frozen product still can be improved via (a) optimization of the complete existing production chain towards quality, and/or (b) introduction of some promising novel processing technology. For this optimization, knowledge is required how all processing steps impact the final quality. Hence, first I review physicochemical and biochemical processes underlying the final quality, such as water holding capacity, ice crystal growth and mechanical damage. Subsequently, I review how each individual processing step impacts the final quality via these fundamental physicochemical and biochemical processes. In this review of processing steps, I also review the potential of novel processing technologies. The results of our literature review are summarized via a causal network, linking processing steps, fundamental physicochemical and biochemical processes, and their correlation with final product quality. I conclude that there is room for optimization of the current production chains via matching processing times with time scales of the fundamental physicochemical and biochemical processes. Regarding novel processing technology, it is concluded in general that they are difficult to implement in the context of existing production chains. I do see the potential for novel processing technology combined with process intensification, incorporating the blanching pretreatment—but which involves quite a change of the production chain.

Site selection for subtropical thicket restoration: mapping cold-air pooling in the South African sub-escarpment lowlands

Restoration of subtropical thicket in South Africa using the plant Portulacaria afra (an ecosystem engineer) has been hampered, in part, by selecting sites that are frost prone—this species is intolerant of frost. Identifying parts of the landscape that are exposed to frost is often challenging. Our aim is to calibrate an existing cold-air pooling (CAP) model to predict where frost is likely to occur in the valleys along the sub-escarpment lowlands (of South Africa) where thicket is dominant. We calibrated this model using two valleys that have been monitored during frost events. To test the calibrated CAP model, model predictions of frost-occurrence for six additional valleys were assessed using a qualitative visual comparison of existing treelines in six valleys—we observe a strong visual match between the predicted frost and frost-free zones with the subtropical thicket (frost-intolerant) and Nama-Karoo shrubland (frost-tolerant) treelines. In addition, we tested the model output using previously established transplant experiments; ∼300 plots planted with P. afra (known as the Thicket-Wide Plots) were established across the landscape—without consideration of frost—to assess the potential factors influencing the survival and growth of P. afra. Here we use a filtered subset of these plots (n = 70), and find that net primary production of P. afra was significantly lower in plots that the model predicted to be within the frost zone. We suggest using this calibrated CAP model as part of the site selection process when restoring subtropical thicket in sites that lie within valleys—avoiding frost zones will greatly increase the likelihood of restoration success.

Transcriptome profiling reveals the crucial biological pathways involved in cold response in Moso bamboo (Phyllostachys edulis)

Most bamboo species including Moso bamboo (Phyllostachys edulis) are tropical or subtropical plants that greatly contribute to human well-being. Low temperature is one of the main environmental factors restricting bamboo growth and geographic distribution. Our knowledge of the molecular changes during bamboo adaption to cold stress remains limited. Here, we provided a general overview of the cold-responsive transcriptional profiles in Moso bamboo by systematically analyzing its transcriptomic response under cold stress. Our results showed that low temperature induced strong morphological and biochemical alternations in Moso bamboo. To examine the global gene expression changes in response to cold, 12 libraries (non-treated, cold-treated 0.5, 1 and 24 h at -2 °C) were sequenced using an Illumina sequencing platform. Only a few differentially expressed genes (DEGs) were identified at early stage, while a large number of DEGs were identified at late stage in this study, suggesting that the majority of cold response genes in bamboo are late-responsive genes. A total of 222 transcription factors from 24 different families were differentially expressed during 24-h cold treatment, and the expressions of several well-known C-repeat/dehydration responsive element-binding factor negative regulators were significantly upregulated in response to cold, indicating the existence of special cold response networks. Our data also revealed that the expression of genes related to cell wall and the biosynthesis of fatty acids were altered in response to cold stress, indicating their potential roles in the acquisition of bamboo cold tolerance. In summary, our studies showed that both plant kingdom-conserved and species-specific cold response pathways exist in Moso bamboo, which lays the foundation for studying the regulatory mechanisms underlying bamboo cold stress response and provides useful gene resources for the construction of cold-tolerant bamboo through genetic engineering in the future.

Exogenous abscisic acid enhances physiological, metabolic, and transcriptional cold acclimation responses in greenhouse-grown grapevines

Previous studies have demonstrated that the freezing tolerance (FT) of grapevine was enhanced by foliar application of exogenous abscisic acid (exo-ABA), a treatment which might be incorporated into cultural practices to mitigate cold damage in vineyards. To investigate the underlying mechanisms of this response, a two-year (2017 and 2018) study was conducted to characterize the effects of exo-ABA on greenhouse-grown 'Cabernet franc' grapevine. In control grapevines, both physiological (deeper dormancy) and biochemical (sugar accumulation in buds) changes occurred, indicating that grapevines initiated cold acclimation in the greenhouse. Compared to control, exo-ABA decreased stomatal conductance 2 h after application. Two weeks post application, exo-ABA treated grapevines showed accelerated transition of grapevine physiology during cold acclimation (increased depth of dormancy, decreased bud water content and enhanced bud FT), relative to control. Exo-ABA induced the accumulation of several sugars in buds including the raffinose family oligosaccharides (RFOs), and the RFO precursor, galactinol. The expression of raffinose and galactinol synthase genes was higher in exo-ABA treated grapevine buds, compared to control. The new findings from this study have advanced our understanding of the role of ABA in grapevine FT, which will be useful to develop future strategies to protect grapevines from cold damage.

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.

Candidate Genes for Freezing and Drought Tolerance Selected on the Basis of Proteome Analysis in Doubled Haploid Lines of Barley

Plant tolerance to environmental stress is determined by a very complicated network composed of many intra- and extracellular factors. The aim of this study was to select candidate genes involved in responses to freezing and drought in barley on the basis of previous proteomic studies and to analyze changes in their expression caused by application of both stress factors. Six candidate genes for freezing tolerance (namely the genes encoding elongation factor 1 alpha (EF1A), ferredoxin-NADP reductase, a 14-3-3a protein, β-fructofuranosidase, CBF2A and CBF4B) and six for drought tolerance (encoding transketolase, periplasmic serine protease, triosephosphate isomerase, a protein with a co-chaperon region (GroEs), pfam14200 and actin) were chosen arbitrarily on the basis of in silico bioinformatic analyses. The expression levels of these genes were measured under control and stress conditions in six DH (doubled haploid) lines with differing freezing and drought tolerance. The results of gene expression analysis confirmed the roles of the candidate genes preselected in this study on the basis of previous proteome analysis in contributing to the differences in freezing and drought tolerance observed in the studied population of DH lines of winter barley.

Studying of the resistance to winter stresses of grapevine varieties of different ecological and geographical origin

In terms of climate change researches about adaptation mechanism of perennial crop plant to summer and winter stresses are actual. Physiological-biochemical assessment of grapevine varieties allows to clarify the mechanisms of protective and adaptive reaction on stresses and identify perspective genotypes for cultivation in stressful terms. Ecological and geographical origin are important in this case. Winterhardiness of following grapevine varieties: ‘Kristal’, ‘Aligote’, ‘Dostoyniy’ ‘Krasnostop AZOS’, ‘Zarif’ and ‘Vostorg’, are studied in conditions of Anapa-Taman region of the Russian Federation. At the beginning of winter reduced water content of vine tissue are noted for ‘Kristal’, ‘Krasnostop AZOS’ and ‘Dostoyniy’ (41,4-45,3 %). Also, for this varieties are found elevated accumulation of sucrose (2 times more) which means high resistance to low temperature. Intensification of antioxidative protective system’s functioning in response on stress are discovered for varieties ‘Kristal’, ‘Krasnostop AZOS’, ‘Dostoyniy’ and ‘Vostorg’. High containing of anthocyanins and chalcones in varieties ‘Kristal’, ‘Dostoyniy’, ‘Krasnostop AZOS’ and ascorbic acid in varieties ‘Krasnostop AZOS’, ‘Dostoyniy’ are revealed. According to generalized assessment of physiological-biochemical research results in conditions of climate change in Anapa-Taman region of Russia more winterhardiness were varieties ‘Krasnostop AZOS’, ‘Dostoyniy’, ‘Kristal’ than varieties ‘Aligote’, ‘Vostorg’, ‘Zarif’.

A new technical approach for preparing frozen biological samples for electron microscopy

BACKGROUND

Many methodological approaches have focused so far on physiological and molecular responses of plant tissues to freezing but only little knowledge is available on the consequences of extracellular ice-formation on cellular ultrastructure that underlies physiological reactions. In this context, the preservation of a defined frozen state during the entire fixation procedure is an essential prerequisite. However, current techniques are not able to fix frozen plant tissues for transmission electron microscopy (TEM) without interrupting the cold chain. Chemical fixation by glutaraldehyde and osmium tetroxide is not possible at sub-zero temperatures. Cryo-fixation methods, such as high pressure freeze fixation (HPF) representing the state-of-the-art technique for best structural preservation, are not equipped for freezing frozen samples. In order to overcome this obstacle, a novel technical approach for maintaining the cold chain of already frozen plant samples prior and during HPF is presented.

RESULTS

Different algae (Micrasterias denticulata, Klebsormidium crenulatum) and higher plant tissues (Lemna sp., Ranunculus glacialis, Pinus mugo) were successfully frozen and prepared for HPF at freezing temperatures (- 2 °C, - 5 °C, - 6 °C) within a newly developed automatic freezing unit (AFU), that we manufactured from a standard laboratory freezer. Preceding tests on photosynthetic electron transport and ability to plasmolyse show that the temperatures applied did not impair electron transport in PSII nor cell vitality. The transfer of the frozen specimen from the AFU into the HPF-device and subsequently cryo-fixation were performed without intermediate thawing. After cryo-substitution and further processing, the resulting TEM-micrographs showed excellent ultrastructure preservation of the different organisms when compared to specimens fixed at ambient temperature.

CONCLUSIONS

The method presented allows preserving the ultrastructure of plant cells in the frozen state during cryo-fixation. The resulting high quality TEM-images represent an important step towards a better understanding of the consequences of extracellular ice formation on cellular ultrastructure. It has the potential to provide new insights into changes of organelle structure, identification of intracellular injuries during ice formation and may help to understand freezing and thawing processes in plant tissues. It may be combined with analytical TEM such as electron energy loss spectroscopy (EELS), X-ray analyses (EDX) and various other electron microscopic techniques.

MUR1-mediated cell-wall fucosylation is required for freezing tolerance in Arabidopsis thaliana

Forward genetic screens play a key role in the identification of genes contributing to plant stress tolerance. Using a screen for freezing sensitivity, we have identified a novel freezing tolerance gene, SENSITIVE-TO-FREEZING8, in Arabidopsis thaliana. We identified SFR8 using recombination-based mapping and whole-genome sequencing. As SFR8 was predicted to have an effect on cell wall composition, we used GC-MS and polyacrylamide gel electrophoresis to measure cell-wall fucose and boron (B)-dependent dimerization of the cell-wall pectic domain rhamnogalacturonan II (RGII) in planta. After treatments to promote borate-bridging of RGII, we assessed freeze-induced damage in wild-type and sfr8 plants by measuring electrolyte leakage from freeze-thawed leaf discs. We mapped the sfr8 mutation to MUR1, a gene encoding the fucose biosynthetic enzyme GDP-d-mannose-4,6-dehydratase. sfr8 cell walls exhibited low cell-wall fucose levels and reduced RGII bridging. Freezing sensitivity of sfr8 mutants was ameliorated by B supplementation, which can restore RGII dimerization. B transport mutants with reduced RGII dimerization were also freezing-sensitive. Our research identifies a role for the structure and composition of the plant primary cell wall in determining basal plant freezing tolerance and highlights the specific importance of fucosylation, most likely through its effect on the ability of RGII pectin to dimerize.

Accumulation of Anthocyanins: An Adaptation Strategy of Mikania micrantha to Low Temperature in Winter

The accumulation of anthocyanins in leaves and stems of Mikania micrantha improves its adaptability to low-temperature environments during winter in areas where this species is invasive. The accumulation of anthocyanins in M. micrantha causes the plants to exhibit red coloration when encountering low-temperature environments during winter. Many studies have reported that the accumulation of anthocyanins near the plant surface filters light and improves photoprotection. However, the results of this study showed that the main role of anthocyanins accumulation in M. micrantha during winter was to increase both antioxidant capability and tolerance to low temperature. The results showed that the anthocyanin contents were significantly higher in red leaves and stems than in green leaves and stems, with more than 60-fold greater content in red leaves than in green leaves. In addition, the total antioxidant capability was significantly greater in red leaves and stems than in green leaves and stems. After 4°C treatment for 12 h, a large amount of reactive oxygen species accumulated in green leaves and stems, and the maximum photochemical efficiency decreased significantly. Compared with that of the green leaves, the net photosynthetic rate of red leaves was significantly higher. The biomass statistics revealed that the dry matter accumulation of M. micrantha plants with relatively large amounts of anthocyanins was significantly greater than that of plants with relatively low anthocyanin levels during the same period. Our results suggest that the accumulation of anthocyanins during winter is an adaptation strategy of M. micrantha to low winter temperatures.

Genomic Prediction for Winter Survival of Lowland Switchgrass in the Northern USA

The lowland ecotype of switchgrass has generated considerable interest because of its higher biomass yield and late flowering characteristics compared to the upland ecotype. However, lowland ecotypes planted in northern latitudes exhibit very low winter survival. Implementation of genomic selection could potentially enhance switchgrass breeding for winter survival by reducing generation time while eliminating the dependence on weather. The objectives of this study were to assess the potential of genomic selection for winter survival in lowland switchgrass by combining multiple populations in the training set and applying the selected model in two independent testing datasets for validation. Marker data were generated using exome capture sequencing. Validation was conducted using (1) indirect indicators of winter adaptation based on geographic and climatic variables of accessions from different source locations and (2) winter survival estimates of the phenotype. The prediction accuracies were significantly higher when the training dataset comprising all populations was used in fivefold cross validation but its application was not useful in the independent validation dataset. Nevertheless, modeling for population heterogeneity improved the prediction accuracy to some extent but the genetic relationship between the training and validation populations was found to be more influential. The predicted winter survival of lowland switchgrass indicated latitudinal and longitudinal variability, with the northeast USA the region for most cold tolerant lowland populations. Our results suggested that GS could provide valuable opportunities for improving winter survival and accelerate the lowland switchgrass breeding programs toward the development of cold tolerant cultivars suitable for northern latitudes.

A Molecular View of Plant Local Adaptation: Incorporating Stress-Response Networks

Ecological specialization in plants occurs primarily through local adaptation to different environments. Local adaptation is widely thought to result in costly fitness trade-offs that result in maladaptation to alternative environments. However, recent studies suggest that such trade-offs are not universal. Further, there is currently a limited understanding of the molecular mechanisms responsible for fitness trade-offs associated with adaptation. Here, we review the literature on stress responses in plants to identify potential mechanisms underlying local adaptation and ecological specialization. We focus on drought, high and low temperature, flooding, herbivore, and pathogen stresses. We then synthesize our findings with recent advances in the local adaptation and plant molecular biology literature. In the process, we identify mechanisms that could cause fitness trade-offs and outline scenarios where trade-offs are not a necessary consequence of adaptation. Future studies should aim to explicitly integrate molecular mechanisms into studies of local adaptation.

RNA Sequencing Characterizes Transcriptomes Differences in Cold Response Between Northern and Southern Alternanthera philoxeroides and Highlight Adaptations Associated With Northward Expansion

Alternanthera philoxeroides recently expanded its range northwards in China. It is unknown if the range expansion has a genetic and/or epigenetic basis, or merely an environmental basis due to a warming climate. To test these possibilities, we used an RNAseq approach with a common greenhouse design to examine gene expression in individuals from the northern edge and central portion of alligator weed range from China to determine if there were differences in their responses to cold temperatures. We hypothesized that if the recent range expansion was primarily environmental, we would observe few differences or only differences unrelated to low-temperature adaptations. We assembled over 75,000 genes of which over 65,000 had long open reading frames with similarity to sequences from arabidopsis. Differences in expression between northern and southern populations that were both exposed to low temperatures showed similar expression among genes in the C-REPEAT/DRE BINDING FACTOR (CBF) regulon. However, gene set and sub-network enrichment analysis indicated differences in the response of photosynthetic processes and oxidative stress responses were different between the two populations and we relate these differences to cold adaptation. The transcriptome differences in response to cold between the individuals from the two populations is consistent with adaptations potentiating or resulting from selection after expansion into colder environments and may indicate that genetic changes have accompanied the recent northward expansion of A. philoxeroides in China. However, we cannot rule out the possibility of epigenetic changes may have a role in this expansion.

Interannual variation in UV-B and temperature effects on bud phenology and growth in Populus tremula

Warming affects phenological processes such as spring bud break and autumnal bud set, and also growth rates of trees. Recently, it has been shown that these physiological processes also may be influenced by the ultraviolet-B (UV-B) part of the solar spectrum, and there are reasons to expect that the two environmental factors induce interactive effects when acting in concert. In this study, our aim was to elucidate how experimental enhancements in temperature and UV-B, alone and in combination, affect growth and seasonal phenology of Eurasian aspen (Populus tremula) over several growing seasons (three years). Moreover, we tested how environmentally induced changes in phenology affect the growth achieved over each season, that is, the importance of a prolonged growing season for growth yield. The plants grew in an outdoor experiment with modulated enhancements of temperature and UV-B during the growing season. Both UV-B and temperature enhancement affected bud set dates, while bud break dates were only affected by temperature enhancement. Temperature delayed bud set in all years, but gradually less over years, while UV-B yielded earlier bud set the first year but showed a delayed response the following years. Bud break was always earlier under temperature enhancement. The experimentally induced extension of the growing season in both ends had a positive effect of growth throughout the three-year period. However, the reduced responsiveness of bud set to both enhancement treatments suggest that the plants gradually acclimated to the modified climate, a finding that should also be investigated for other tree species.