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Temperate and boreal plants show natural low temperature acclimation during autumn. This cold acclimation process results in increased freezing tolerance. Global climate change is leading to increasing spring and autumn temperatures that can trigger deacclimation and loss of freezing tolerance, making plants susceptible to both late-autumn and late-spring freezing events. In particular, spring frosts can have devastating effects on whole ecosystems and can significantly reduce the yield of crop plants. Although the timing and speed of deacclimation are clearly of crucial importance for plant winter survival, the molecular basis of this process is still largely unknown. The regulation of deacclimation is, however, not only related to freezing tolerance, but also to the termination of dormancy, and the initiation of growth and development. In this paper, we provide an overview of what is known about deacclimation in both woody and herbaceous plants. We use publicly available transcriptome data to identify a core set of deacclimation-related genes in Arabidopsis thaliana that highlight physiological determinants of deacclimation, and suggest important directions for future research in this area.

Key messageWinter reddening of young Douglas-fir (Pseudotsuga menziesii Mirb. Franco), triggered by large thermal fluctuations in late winter, is a critical problem for European forestry. A literature review identified certain climatic conditions that are characteristic of ‘reddening’ years, including warm daily temperatures, high daily temperature amplitude, low relative humidity, moderate wind speeds, as well as the occurrence of freeze-thaw cycles with cold night temperatures.By describing the triggering environmental and stand factors, we propose three hypotheses for the physiological processes leading to winter reddening, namely (i) hydraulic failure due to winter drought stress, (ii) photo-oxidative stress in shade-acclimated trees, and (iii) early cold deacclimation during warm periods.i) Low soil temperature, by reducing root water uptake, combined with anticyclonic conditions, by increasing water losses, can induce hydraulic failure in the xylem. Hydraulic failure may be further accelerated by night frosts.ii) Winter reddening can occur when low temperature and high irradiance coincide, disrupting photostasis. Overwhelming of winter photo-protection may lead to photodamage and subsequent reddening.iii) Warm periods, by inducing cold deacclimation, make trees susceptible to frost damage.Finally, the three processes may interact under atypical anticyclonic conditions in late winter (e.g. cold or dry soils, warm days, high irradiance and/or freezing nights). Indeed, trees under water stress would develop a higher sensitivity to freezing night and photooxidative stress. We therefore proposed mitigation actions to avoid exposing trees to stressful conditions based on e.g. stand characteristics, understorey vegetation and planting.

Cold acclimation is crucial for the overwintering process of plants. Cold deacclimation is also important for plant survival in winter, which results in loss of freezing tolerance and initiation of growth. MicroRNAs (miRNAs) play crucial roles in regulating various physiological activities including cold response in plants. However, there is no study on miRNAs and their target genes in response to cold deacclimation in a cold-tolerant crop – barley (Hordeum vulgare L.). Here, we performed high-throughput sequencing of miRNAs of leaves during the cold deacclimation process using two barley cultivars with contrasting cold tolerance (Nure, tolerant and Tremois, sensitive). We found a total of 36 known and 267 novel miRNAs, including 12 known and 112 novel ones that are differentially expressed during cold deacclimation. The number of detected differentially expressed miRNAs was larger in Nure than that in Tremois, and the expression profile of miRNAs was dramatically different between Nure and Tremois. Moreover, we identified 13 known and 97 novel miRNAs, which have putative target genes during cold deacclimation. The putative targets of the novel miRNAs included genes encoding C-repeat binding factor (CBF) transcription factors, phytohormones, antioxidant, osmopretectant and flower development. Our results suggest that barley miRNAs respond quickly to cold deacclimation, and the larger number of miRNAs differentially expressed in the cold tolerant cv. Nure indicating that miRNAs might play an important role in the process of deacclimation. It sets a solid foundation for future studies and breeding programs on low temperature tolerance in barley.

Significant advances have been made in our understanding of the regulation of cold hardiness. The existence of numerous biophysical and biochemical adaptive mechanisms in perennial woody plants and the complexity their regulation has made the development of methods for managing and improving cold hardiness in perennial woody plants has been very difficult. This may be partially attributed to viewing cold hardiness as a single dimensional response, rather than as a complex phenomenon, involving different mechanisms (avoidance and tolerance), different stages (mid-winter vs. late winter), and having an intimate overlap with the genetic regulation of dormancy. In particular separating the molecular regulation of cold hardiness from growth processes has been challenging. ICE and C-repeat binding factor (CBF), transcription factors (Inducer of CBF expression and CRT-binding factor) have been shown to be an important aspect in the regulation of cold-induced gene expression. Evidence has emerged, however, that they are also intimately involved in the regulation of growth, flowering, dormancy, and stomatal development. This evidence includes the presence of CBF binding motifs in genes regulating these processes, or through cross-talk between the pathways that regulate them. Recent changes in climate that have resulted in erratic episodes of unseasonal warming followed by more seasonal patterns of low temperatures has also highlighted the need to better understand the genetic and molecular regulation of deacclimation, a topic of research that is only more recently being addressed. Environmentally-induced epigenetic regulation of stress responses and seasonal processes such as cold acclimation, deacclimation, and dormancy have been documented but are still poorly understood. Advances in the ability to efficiently generate large DNA and RNA datasets and genetic transformation technologies have greatly increased our ability to explore the regulation of gene expression and explore genetic diversity. Greater knowledge of the interplay between epigenetic and genetic regulation of cold hardiness, along with the application of advanced genetic analyses, such as genome-wide-association-studies (GWAS), are needed to develop strategies for addressing the complex processes associated with cold hardiness in woody plants. A cautionary note is also indicated regarding the time-scale needed to examine and interpret plant response to freezing temperatures if progress is to be made in developing effective approaches for manipulating and improving cold hardiness.

Plants such as winter crops are able to acclimate to low temperatures through complex physiological and biochemical modifications that enhance their frost tolerance. Cold acclimation involves changes in, e.g., photosynthetic efficiency, carbohydrate metabolism, the accumulation of osmoprotectants, the remodelling of membrane lipid composition, and the activation of the antioxidant system. Now, due to ongoing global climate change, temperature fluctuations have become more frequent, particularly during the autumn–winter period. Episodes of warm breaks (mainly above 9 °C) during winter disrupt the cold acclimation process and induce deacclimation, leading to a decrease in frost tolerance and a partial reversal of cold-induced metabolic adjustments. However, deacclimation is not just the reversal of acclimation, as evidenced by distinct responses in metabolites and hormones. Moreover, plants are able to regain lost freezing tolerance through reacclimation upon re-exposure to low temperatures. The article aimed to summarize the current knowledge on the basics underlying cold acclimation, deacclimation, and reacclimation. An explanation of these processes is crucial for protecting winter crop plants under the increasing frequency of variable temperatures during their growth.

Climate change has become a topic of increasing significance in viticulture, severely challenged by this issue. Average global temperatures are increasing, but frost events, with a large variability depending on geographical locations, have been predicted to be a potential risk for grapevine cultivation. Grape cold hardiness encompasses both midwinter and spring frost hardiness, whereas the avoidance of spring frost damage due to late budbreak is crucial in cold resilience. Cold hardiness kinetics and budbreak phenology are closely related and affected by bud’s dormancy state. On the other hand, budbreak progress is also affected by temperatures during both winter and spring. Genetic control of bud phenology in grapevine is still largely undiscovered, but several studies have recently aimed at identifying the molecular drivers of cold hardiness loss and the mechanisms that control deacclimation and budbreak. A review of these related traits and their variability in different genotypes is proposed, possibly contributing to develop the sustainability of grapevine production as climate-related challenges rise.

Cold acclimation (CA) is a crucial survival mechanism for deciduous fruit trees during winter. In grapevine ( Vitis vinifera L) buds, cold hardiness development and CA occur during endodormancy, triggered by low temperatures (LT) and abscisic acid (ABA). Conversely, the loss of cold hardiness known as deacclimation (DA), takes place during ecodormancy due to rising temperatures. The ICE-CBF-COR signalling pathway is pivotal in regulating CA in plants. This study investigates into the intricate relationship between ICE-CBF-COR genes, cold hardiness and the influence of LT and ABA at two different dormancy stage of grapevine buds: endodormancy and ecodormancy. Our findings reveal that the response of grapevine bud cold hardiness and the expression of the ICE-CBF-COR genes to LT and ABA vary based on dormancy state of the buds. During endodormancy, the joint application of LT and ABA synergistically augments cold hardiness and triggers the expression of ICE-CBF-COR genes. In contrast, during ecodormancy, LT alone suppresses the expression of these genes, while maintaining cold hardiness at levels like to ABA/LT-treated buds in endodormancy. These results suggest: (1) that the downregulation of ICE-CBF-COR genes by LT is not associated with the DA process and does not contribute to the loss of cold hardiness. Instead, it prevents premature DA, ensuring budbreak in spring. (2) Increased temperatures stimulate DA and the upregulation of VvCBF genes. (3) Temperature significantly regulates ICE-CBF-COR genes, influencing both CA and DA processes, ultimately impacting grapevine budbreak in spring. These findings underscore the complexity of the relationship between temperature, dormancy stages, gene expression and the intricate mechanisms underlying both CA and DA in grapevine buds.

Bryophytes have long been understood to be desiccation tolerant (capable of reviving from an air-dry state, DT). While mechanistic studies focusing on the expression of this complex trait are numerous over the last 50 years, a firm conceptual framework for the ecology of desiccation tolerance in bryophytes has lagged behind. As a result of this, mechanistic studies have gone ahead without an ecological foundation, and this path can be scientifically risky. The motivating elements for this article include (1) the low visibility and lack of implementation of a conceptual framework for studying the ecology of DT in bryophytes, (2) the lack of a protocol for the step-by-step handling of field-collected or laboratory cultured specimens for study of desiccation tolerance with particular attention to the deacclimation of plants, (3) a recognition that most studies of desiccation tolerance tend to conflate two of the most important factors of desiccation, rate of desiccation and water content, and (4) recognizing that the ecological factors of rehydration, aside from recovery, are nearly always ignored in experiments on desiccation tolerance in bryophytes. Therefore this review postulates a conceptual framework for the ecology of desiccation tolerance, presenting the principal factors of desiccation (rate of drying, water content at equilibration, duration dry, and rate of rehydration) and how they may interact, presenting the principal physiological phases of rehydration (recovery, hardening, dehardening), and highlighting the importance of specimen handling in preparation for an experiment on DT. Useful metrics for assessment are derived that include the minimum rate of drying tolerated at a given water content, the minimum or maximum water content tolerated, the maximum duration dry tolerated, and the minimum dehardening time exhibited.

Chromatin regulation ensures stable repression of stress-inducible genes under non-stress conditions and transcriptional activation and memory of stress-related genes after stress exposure. However, there is only limited knowledge on how chromatin genes are regulated at the transcriptional and post-transcriptional level upon stress exposure and relief from stress. We reveal that the repressive modification histone H3 lysine 27 trimethylation (H3K27me3) targets genes which are quickly activated upon cold exposure, however, H3K27me3 is not necessarily lost during a longer time in the cold. In addition, we have set-up a quantitative reverse transcription polymerase chain reaction-based platform for high-throughput transcriptional profiling of a large set of chromatin genes. We find that the expression of many of these genes is regulated by cold. In addition, we reveal an induction of several DNA and histone demethylase genes and certain histone variants after plants have been shifted back to ambient temperature (deacclimation), suggesting a role in the memory of cold acclimation. We also re-analyze large scale transcriptomic datasets for transcriptional regulation and alternative splicing (AS) of chromatin genes, uncovering an unexpected level of regulation of these genes, particularly at the splicing level. This includes several vernalization regulating genes whose AS may result in cold-regulated protein diversity. Overall, we provide a profiling platform for the analysis of chromatin regulatory genes and integrative analyses of their regulation, suggesting a dynamic regulation of key chromatin genes in response to low temperature stress.

Global climate change is causing increasing fluctuations in winter temperatures, including episodes of warm conditions above 9 °C. Such events disrupt cold acclimation in plants and can induce deacclimation, reducing frost tolerance and altering, among other things, hormonal regulation. This study investigated hormonal and molecular changes associated with cold acclimation and deacclimation in oilseed rape (Brassica napus L.) cultivars Kuga and Thure. Plants were grown under different conditions: non-acclimated (17 °C for three weeks), cold-acclimated (4 °C for three weeks), and deacclimated (16/9 °C day/night for one week). Detailed hormone analysis included auxins, gibberellins, cytokinins, stress-related hormones, and the expression of hormone-related genes (BnABF2, BnAOS, BnARF1, BnARR6, BnICS1, BnRGA, and BnWRKY57). Hormone concentrations in leaves changed dynamically in response to deacclimation with increased amounts of growth-promoting hormones and decreased amounts of stress hormones. Additionally, alterations in gene expression during deacclimation, such as in BnABF2 and BnICS1, may function as protective mechanisms to help maintain or regain frost tolerance during reacclimation when temperatures decline again after the warm period. These findings improve the understanding of hormonal and molecular responses involved in the deacclimation of oilseed rape.