Post-translational activation of CBF for inducing freezing tolerance

Recent advances in the multifaceted functions of Cys2/His2-type zinc finger proteins in plant growth, development, and stress responses

Recent research has highlighted the importance of Cys2/His2-type zinc finger proteins (C2H2-ZFPs) in plant growth and in responses to various stressors, and the complex structures of C2H2-ZFP networks and the molecular mechanisms underlying their responses to stress have received considerable attention. Here, we review the structural characteristics and classification of C2H2-ZFPs, and consider recent research advances in their functions. We systematically introduce the roles of these proteins across diverse aspects of plant biology, encompassing growth and development, and responses to biotic and abiotic stresses, and in doing so hope to lay the foundations for further functional studies of C2H2-ZFPs in the future.

The transcription factor ERF110 promotes cold tolerance by directly regulating sugar and sterol biosynthesis in citrus

ERFs (ethylene-responsive factors) are known to play a key role in orchestrating cold stress signal transduction. However, the regulatory mechanisms and target genes of most ERFs are far from being well deciphered. In this study, we identified a cold-induced ERF, designated as PtrERF110, from trifoliate orange (Poncirus trifoliata L. Raf., also known as Citrus trifoliata L.), an elite cold-hardy plant. PtrERF110 is a nuclear protein with transcriptional activation activity. Overexpression of PtrERF110 remarkably enhanced cold tolerance in lemon (Citrus limon) and tobacco (Nicotiana tabacum), whereas VIGS (virus-induced gene silencing)-mediated knockdown of PtrERF110 drastically impaired the cold tolerance. RNA sequence analysis revealed that PtrERF110 overexpression resulted in global transcriptional reprogramming of a range of stress-responsive genes. Three of the genes, including PtrERD6L16 (early responsive dehydration 6-like transporters), PtrSPS4 (sucrose phosphate synthase 4), and PtrUGT80B1 (UDP-glucose: sterol glycosyltransferases 80B1), were confirmed as direct targets of PtrERF110. Consistently, PtrERF110-overexpressing plants exhibited higher levels of sugars and sterols compared to their wild type counterparts, whereas the VIGS plants had an opposite trend. Exogenous supply of sucrose restored the cold tolerance of PtrERF110-silencing plants. In addition, knockdown of PtrSPS4, PtrERD6L16, and PtrUGT80B1 substantially impaired the cold tolerance of P. trifoliata. Taken together, our findings indicate that PtrERF110 positively modulates cold tolerance by directly regulating sugar and sterol synthesis through transcriptionally activating PtrERD6L16, PtrSPS4, and PtrUGT80B1. The regulatory modules (ERF110-ERD6L16/SPS4/UGT80B1) unraveled in this study advance our understanding of the molecular mechanisms underlying sugar and sterol accumulation in plants subjected to cold stress.

OsKASI-2 is required for the regulation of unsaturation levels of membrane lipids and chilling tolerance in rice

Chilling stress has seriously limited the global production and geographical distribution of rice. However, the molecular mechanisms associated with plant responses to chilling stress are less known. In this study, we revealed a member of β-ketoacyl-ACP synthase I family (KASI), OsKASI-2 which confers chilling tolerance in rice. OsKASI-2 encodes a chloroplast-localized KASI enzyme mainly expressed in the leaves and anthers of rice and strongly induced by chilling stress. Disruption of OsKASI-2 led to decreased KAS enzymatic activity and the levels of unsaturated fatty acids, which impairs degree of unsaturation of membrane lipids, thus increased sensitivity to chilling stress in rice. However, the overexpression of OsKASI-2 significantly improved the chilling tolerance ability in rice. In addition, OsKASI-2 may regulate ROS metabolism in response to chilling stress. Natural variation of OsKASI-2 might result in difference in chilling tolerance between indica and japonica accessions, and Hap1 of OsKASI-2 confers chilling tolerance in rice. Taken together, we suggest OsKASI-2 is critical for regulating degree of unsaturation of membrane lipids and ROS accumulation for maintenance of membrane structural homeostasis under chilling stress, and provide a potential target gene for improving chilling tolerance of rice.

Discovery of candidate genes involved in ethylene biosynthesis and signal transduction pathways related to peach bud cold resistance

Background: Low temperature pose significant challenges to peach cultivation, causing severe damage to peach buds and restricting production and distribution. Ethylene, an important phytohormone, plays a critical role in enhancing plant cold resistance. Structural genes and transcription factors involved in ethylene biosynthesis and signal transduction pathways are associated with cold resistance. However, no research has specifically addressed their roles in peach cold resistance. Methods: In this study, we aimed for cold-resistance gene discovery in cold-sensitive peach cultivar "21Shiji" (21SJ) and cold-resistance cultivar "Shijizhixing" (SJZX) using RNA-seq and gas chromatography. Results: The findings revealed that under cold stress conditions, ethylene biosynthesis in "SJZX" was significantly induced. Subsequently, a structural gene, PpACO1-1, involved in ethylene biosynthesis in peach buds was significantly upregulated and showed a higher correlation with ethylene release rate. To identify potential transcription factors associated with PpACO1-1 expression and ethylene signal transduction, weighted gene co-expression network analysis was conducted using RNA-seq data. Four transcription factors: PpERF2, PpNAC078, PpWRKY65 and PpbHLH112, were identified. Conclusion: These findings provide valuable theoretical insights for investigating the regulatory mechanisms of peach cold resistance and guiding breeding strategies.

Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development

Reactive oxygen species (ROS) are produced as undesirable by-products of metabolism in various cellular compartments, especially in response to unfavorable environmental conditions, throughout the life cycle of plants. Stress-induced ROS production disrupts normal cellular function and leads to oxidative damage. To cope with excessive ROS, plants are equipped with a sophisticated antioxidative defense system consisting of enzymatic and non-enzymatic components that scavenge ROS or inhibit their harmful effects on biomolecules. Nonetheless, when maintained at relatively low levels, ROS act as signaling molecules that regulate plant growth, development, and adaptation to adverse conditions. Here, we provide an overview of current approaches for detecting ROS. We also discuss recent advances in understanding ROS signaling, ROS metabolism, and the roles of ROS in plant growth and responses to various abiotic stresses.

A C2H2-type zinc finger protein ZAT12 of Poncirus trifoliata acts downstream of CBF1 to regulate cold tolerance

The Cys2/His2 (C2H2)-type zinc finger family has been reported to regulate multiple aspects of plant development and abiotic stress response. However, the role of C2H2-type zinc finger proteins in cold tolerance remains largely unclear. Through RNA-sequence analysis, a cold-responsive zinc finger protein, named as PtrZAT12, was identified and isolated from trifoliate orange (Poncirus trifoliata L. Raf.), a cold-hardy plant closely related to citrus. Furthermore, we found that PtrZAT12 was markedly induced by various abiotic stresses, especially cold stress. PtrZAT12 is a nuclear protein, and physiological analysis suggests that overexpression of PtrZAT12 conferred enhanced cold tolerance in transgenic tobacco (Nicotiana tabacum) plants, while knockdown of PtrZAT12 by virus-induced gene silencing (VIGS) increased the cold sensitivity of trifoliate orange and repressed expression of genes involved in stress tolerance. The promoter of PtrZAT12 harbors a DRE/CRT cis-acting element, which was verified to be specifically bound by PtrCBF1 (Poncirus trifoliata C-repeat BINDING FACTOR1). VIGS-mediated silencing of PtrCBF1 reduced the relative expression levels of PtrZAT12 and decreased the cold resistance of trifoliate orange. Based on these results, we propose that PtrZAT12 is a direct target of CBF1 and plays a positive role in modulation of cold stress tolerance. The knowledge gains new insight into a regulatory module composed of CBF1-ZAT12 in response to cold stress and advances our understanding of cold stress response in plants.

Climate Change-The Rise of Climate-Resilient Crops

Climate change disrupts food production in many regions of the world. The accompanying extreme weather events, such as droughts, floods, heat waves, and cold snaps, pose threats to crops. The concentration of carbon dioxide also increases in the atmosphere. The United Nations is implementing the climate-smart agriculture initiative to ensure food security. An element of this project involves the breeding of climate-resilient crops or plant cultivars with enhanced resistance to unfavorable environmental conditions. Modern agriculture, which is currently homogeneous, needs to diversify the species and cultivars of cultivated plants. Plant breeding programs should extensively incorporate new molecular technologies, supported by the development of field phenotyping techniques. Breeders should closely cooperate with scientists from various fields of science.

Genome-wide identification of DREB1 transcription factors in perennial ryegrass and functional profiling of LpDREB1H2 in response to cold stress

Perennial ryegrass (Lolium perenne L.) is an outstanding turfgrass and forage cultivated in temperate regions worldwide. However, poor tolerance to extreme cold, heat, or drought limits wide extension and cultivation. DEHYDRATION-RESPONSIVE ELEMENT BINDING FACTOR1s (DREB1s) play a vital role in enhancing plant tolerance to abiotic stress, specifically for low-temperature stress. In this study, a total of 24 LpDREB1 family members were identified from the released genome of perennial ryegrass. Phylogenetic analysis showed that the LpDREB1 genes are divided into 7 groups that have close relationships with rice homologues. Conserved motif analysis revealed that members within the same group have similar conserved motif compositions. All LpDREB1s lack introns, and the promoter sequences of LpDREB1 genes contain multiple cis-acting elements associated with stress response, phytohormone signal transduction and plant growth and development. The majority of LpDREB1 genes were upregulated by drought, submergence, heat and cold stress treatments, including LpDREB1H2. Further investigation showed that LpDREB1H2 is localized in the nucleus. Overexpression of LpDREB1H2 in Arabidopsis induced the expression of cold-responsive (COR) genes, increased the levels of osmotic adjusting substances, and enhanced antioxidant enzyme activities, thus improving the cold tolerance of Arabidopsis. This study lays a foundation for further understanding the function of LpDREB1 genes in perennial ryegrass and provides insights for plant stress tolerance breeding.

CsBPC2 is essential for cucumber survival under cold stress

Cold stress affects the growth and development of cucumbers. Whether the BPC2 transcription factor participates in cold tolerance and its regulatory mechanism in plants have not been reported. Here, we used wild-type (WT) cucumber seedlings and two mutant Csbpc2 lines as materials. The underlying mechanisms were studied by determining the phenotype, physiological and biochemical indicators, and transcriptome after cold stress. The results showed that CsBPC2 knockout reduced cucumber cold tolerance by increasing the chilling injury index, relative electrical conductivity and malondialdehyde (MDA) content and decreasing antioxidant enzyme activity. We then conducted RNA sequencing (RNA-seq) to explore transcript-level changes in Csbpc2 mutants. A large number of differentially expressed genes (1032) were identified and found to be unique in Csbpc2 mutants. However, only 489 down-regulated genes related to the synthesis and transport of amino acids and vitamins were found to be enriched through GO analysis. Moreover, both RNA-seq and qPT-PCR techniques revealed that CsBPC2 knockout also decreased the expression of some key cold-responsive genes, such as CsICE1, CsCOR413IM2, CsBZR1 and CsBZR2. These results strongly suggested that CsBPC2 knockout not only affected cold function genes but also decreased the levels of some key metabolites under cold stress. In conclusion, this study reveals for the first time that CsBPC2 is essential for cold tolerance in cucumber and provides a reference for research on the biological function of BPC2 in other plants.

The Triticeae CBF Gene Cluster-To Frost Resistance and Beyond

The pivotal role of CBF/DREB1 transcriptional factors in Triticeae crops involved in the abiotic stress response has been highlighted. The CBFs represent an important hub in the ICE-CBF-COR pathway, which is one of the most relevant mechanisms capable of activating the adaptive response to cold and drought in wheat, barley, and rye. Understanding the intricate mechanisms and regulation of the cluster of CBF genes harbored by the homoeologous chromosome group 5 entails significant potential for the genetic improvement of small grain cereals. Triticeae crops seem to share common mechanisms characterized, however, by some peculiar aspects of the response to stress, highlighting a combined landscape of single-nucleotide variants and copy number variation involving CBF members of subgroup IV. Moreover, while chromosome 5 ploidy appears to confer species-specific levels of resistance, an important involvement of the ICE factor might explain the greater tolerance of rye. By unraveling the genetic basis of abiotic stress tolerance, researchers can develop resilient varieties better equipped to withstand extreme environmental conditions. Hence, advancing our knowledge of CBFs and their interactions represents a promising avenue for improving crop resilience and food security.

Low temperature-induced regulatory network rewiring via WRKY regulators during banana peel browning

Banana (Musa spp.) fruits, as typical tropical fruits, are cold sensitive, and lower temperatures can disrupt cellular compartmentalization and lead to severe browning. How tropical fruits respond to low temperature compared to the cold response mechanisms of model plants remains unknown. Here, we systematically characterized the changes in chromatin accessibility, histone modifications, distal cis-regulatory elements, transcription factor binding, and gene expression levels in banana peels in response to low temperature. Dynamic patterns of cold-induced transcripts were generally accompanied by concordant chromatin accessibility and histone modification changes. These upregulated genes were enriched for WRKY binding sites in their promoters and/or active enhancers. Compared to banana peel at room temperature, large amounts of banana WRKYs were specifically induced by cold and mediated enhancer-promoter interactions regulating critical browning pathways, including phospholipid degradation, oxidation, and cold tolerance. This hypothesis was supported by DNA affinity purification sequencing, luciferase reporter assays, and transient expression assay. Together, our findings highlight widespread transcriptional reprogramming via WRKYs during banana peel browning at low temperature and provide an extensive resource for studying gene regulation in tropical plants in response to cold stress, as well as potential targets for improving cold tolerance and shelf life of tropical fruits.

Integrated Full-Length Transcriptome and Metabolome Profiling Reveals Flavonoid Regulation in Response to Freezing Stress in Potato

Cold stress impairs plant growth and development, resulting in crop failure. Cultivated potato (Solanum tuberosum L.) is sensitive to freezing, while its wild relative, S. commersonii, has a strong freezing tolerance. To decipher the anti-freezing mechanism of CM, we carried out a transcriptomic and metabolomic analysis of an anti-freezing variety of CM (a type of S. commersonii) and a freeze-sensitive variety of DM (a type of Solanum tuberosum L.). A total of 49,232 high-quality transcripts from 12,811 gene loci, including 46,772 coding sequences and 2018 non-coding RNAs, were identified. KEEG enrichment analysis of differentially expressed genes (DEGs) between the two varieties showed that the flavonoid biosynthesis pathway was strongly induced by freezing stress, which was proven by flavonoid metabolome analysis. Consistent with the accumulation of more flavonoids, nearly all the pathway genes were significantly upregulated in CM than those in DM. The transcript levels of two chalcone synthase (CHS-1) isoforms and four isoforms of flavonoid 3'-hydroxylase (F3'H-1) were confirmed by qRT-PCR. Co-expression analysis identified one Myb-related and three UGTs (UDP-glycosyltransferase) that were significantly upregulated in CM during freezing stress. Our findings support that the flavonoid pathway was significantly enhanced by freezing stress and the greater accumulation ofglycosylatedflavonoids in resistant types than that of sensitive types, maybe accounting for the increased freezing tolerance of freeze-resistant potato varieties.

Advances in the study of the function and mechanism of the action of flavonoids in plants under environmental stresses

MAIN CONCLUSION

This review summarizes the anti-stress effects of flavonoids in plants and highlights its role in the regulation of polar auxin transport and free radical scavenging mechanism. As secondary metabolites widely present in plants, flavonoids play a vital function in plant growth, but also in resistance to stresses. This review introduces the classification, structure and synthetic pathways of flavonoids. The effects of flavonoids in plant stress resistance were enumerated, and the mechanism of flavonoids in plant stress resistance was discussed in detail. It is clarified that plants under stress accumulate flavonoids by regulating the expression of flavonoid synthase genes. It was also determined that the synthesized flavonoids are transported in plants through three pathways: membrane transport proteins, vesicles, and bound to glutathione S-transferase (GST). At the same time, the paper explores that flavonoids regulate polar auxin transport (PAT) by acting on the auxin export carrier PIN-FORMED (PIN) in the form of ATP-binding cassette subfamily B/P-glycoprotein (ABCB/PGP) transporter, which can help plants to respond in a more dominant form to stress. We have demonstrated that the number and location of hydroxyl groups in the structure of flavonoids can determine their free radical scavenging ability and also elucidated the mechanism by which flavonoids exert free radical removal in cells. We also identified flavonoids as signaling molecules to promote rhizobial nodulation and colonization of arbuscular mycorrhizal fungi (AMF) to enhance plant-microbial symbiosis in defense to stresses. Given all this knowledge, we can foresee that the in-depth study of flavonoids will be an essential way to reveal plant tolerance and enhance plant stress resistance.

EjFAD8 Enhances the Low-Temperature Tolerance of Loquat by Desaturation of Sulfoquinovosyl Diacylglycerol (SQDG)

Loquat (Eriobotrya japonica Lindl.) is an evergreen fruit tree of Chinese origin, and its autumn-winter flowering and fruiting growth habit means that its fruit development is susceptible to low-temperature stress. In a previous study, the triploid loquat (B431 × GZ23) has been identified with high photosynthetic efficiency and strong resistance under low-temperature stress. Analysis of transcriptomic and lipidomic data revealed that the fatty acid desaturase gene EjFAD8 was closely associated with low temperatures. Phenotypic observations and measurements of physiological indicators in Arabidopsis showed that overexpressing-EjFAD8 transgenic plants were significantly more tolerant to low temperatures compared to the wild-type. Heterologous overexpression of EjFAD8 enhanced some lipid metabolism genes in Arabidopsis, and the unsaturation of lipids was increased, especially for SQDG (16:0/18:1; 16:0/18:3), thereby improving the cold tolerance of transgenic lines. The expression of ICE-CBF-COR genes were further analyzed so that the relationship between fatty acid desaturase and the ICE-CBF-COR pathway can be clarified. These results revealed the important role of EjFAD8 under low-temperature stress in triploid loquat, the increase expression of FAD8 in loquat under low temperatures lead to desaturation of fatty acids. On the one hand, overexpression of EjFAD8 in Arabidopsis increased the expression of ICE-CBF-COR genes in response to low temperatures. On the other hand, upregulation of EjFAD8 at low temperatures increased fatty acid desaturation of SQDG to maintain the stability of photosynthesis under low temperatures. This study not only indicates that the EjFAD8 gene plays an important role in loquat under low temperatures, but also provides a theoretical basis for future molecular breeding of loquat for cold resistance.

bHLH57 confers chilling tolerance and grain yield improvement in rice

Chilling stress has become a major limiting factor that reduces crop productivity worldwide. In this study, we identified a new gene bHLH57, whose product enhances chilling tolerance in rice at diverse developmental stages. bHLH57 was mainly expressed in leaves and anthers, and its protein was targeted to the nucleus. Overexpression of bHLH57 enhanced chilling tolerance by increasing trehalose synthesis, whereas its mutants by CRISPR/Cas9-mediated mutagenesis were more sensitive to chilling and had reduced trehalose. Meanwhile, bHLH57 may regulate ROS metabolism and CBFs/DREBs- dependent pathways in response to chilling stress. In addition, the overexpression of bHLH57 resulted in increased grain yield under normal and chilling conditions, however, the disruption of bHLH57 displayed decreased grain size and seed setting rate, thus reduced grain yield. Phylogenetic and nucleotide diversity analyses suggested that bHLH57 is relatively conserved in monocotyledons, and may be selected during indica populations adaptation. Taken together, we have identified a new bHLH regulator involved rice chilling tolerance and grain yield, and provide a potential target gene for improving chilling tolerance and grain yield of rice.

Functional Characterization of Lobularia maritima LmTrxh2 Gene Involved in Cold Tolerance in Tobacco through Alleviation of ROS Damage to the Plasma Membrane

Cold stress is a key environmental factor affecting plant growth and development, crop productivity, and geographic distribution. Thioredoxins (Trxs) are small proteins that are ubiquitously expressed in all organisms and implicated in several cellular processes, including redox reactions. However, their role in the regulation of cold stress in the halophyte plant Lobularia maritima remains unknown. We recently showed that overexpression of LmTrxh2, which is the gene that encodes the h-type Trx protein previously isolated from L. maritima, led to an enhanced tolerance to salt and osmotic stress in transgenic tobacco. This study functionally characterized the LmTrxh2 gene via its overexpression in tobacco and explored its cold tolerance mechanisms. Results of the RT-qPCR and western blot analyses indicated differential temporal and spatial regulation of LmTrxh2 in L. maritima under cold stress at 4 °C. LmTrxh2 overexpression enhanced the cold tolerance of transgenic tobacco, as evidenced by increased germination rate, fresh weight and catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) activities; reduced malondialdehyde levels, membrane leakage, superoxide anion (O₂^-), and hydrogen peroxide (H₂O₂) levels; and higher retention of chlorophyll than in non-transgenic plants (NT). Furthermore, the transcript levels of reactive oxygen species (ROS)-related genes (NtSOD and NtCAT1), stress-responsive late embryogenis abundant protein 5 (NtLEA5), early response to dehydration 10C (NtERD10C), DRE-binding proteins 1A (NtDREB1A), and cold-responsive (COR) genes (NtCOR15A, NtCOR47, and NtKIN1) were upregulated in transgenic lines compared with those in NT plants under cold stress, indicating that LmTrxh2 conferred cold stress tolerance by enhancing the ROS scavenging ability of plants, thus enabling them to maintain membrane integrity. These results suggest that LmTrxh2 promotes cold tolerance in tobacco and provide new insight into the improvement of cold-stress resistance to cold stress in non-halophyte plants and crops.

The genetic basis of cold tolerance in cucumber (Cucumis sativus L.)-the latest developments and perspectives

Cold stress is one of the main causes of yield losses in plant production in temperate climate areas. Cold stress slows down and even stops plant growth and development and causes injuries that may result in the plant's death. Cucumber (Cucumis sativus L.), an economically important vegetable, is sensitive to low temperatures, thus improving cold tolerance in cucumber would benefit cucumber producers, particularly those farming in temperate climates and higher altitude areas. So far, single cucumber accessions showing different degrees of cold tolerance have been identified, and genetic studies have revealed biparentally and maternally inherited genetic factors responsible for chilling tolerance. Paternally transmitted chilling tolerance has also been suggested. Quantitative trait loci (QTL) associated with seed germination ability at low temperature and seedling recovery from chilling have been described. Several transgenic attempts have been made to improve cold tolerance in cucumber. Despite numerous studies, the molecular mechanisms of cold tolerance in cucumber have still not been sufficiently elucidated. In this review, we summarise the results of research focused on understanding the genetic basis of cold tolerance in cucumber and their implications for cucumber breeding.