Cold stress signalling in female reproductive tissues

Physiological and molecular analysis of pitaya (Hylocereus polyrhizus) reveal up-regulation of secondary metabolites, nitric oxide, antioxidant defense system, and expression of responsive genes under low-temperature stress by the pre-treatment of hydrogen peroxide

Low-temperature events are one of the leading environmental cues that considerably reduce plant growth and shift species biodiversity. Hydrogen peroxide (H2O2) is a signaling molecule that has a distinguished role during unfavorable conditions and shows outstanding perspectives in low-temperature stress. Herein, we elucidated the protective role and regulatory mechanism of H2O2 in alleviating the deleterious effects of low-temperature stress in pitaya plants. Micropropagated pitaya plants were cultured in Murashige and Skoog media supplemented with different levels of H2O2 (0, 5, 10, and 20 mM) and then exposed to low-temperature stress (5 °C for 24 h). H2O2 at 10 mM, improved low-temperature stress tolerance by relieving oxidative injuries and ameliorating growth parameters in terms of fresh weight (66.7%), plant length (16.7%), and pigments content viz., chlorophyll a (157.4%), chlorophyll b (209.1%), and carotenoids (225.9%). H2O2 counteracted the low-temperature stress by increasing amino acids (224.7%), soluble proteins (190.5%), and sugars (126.6%). Simultaneously, secondary metabolites like ascorbic acid (ASA), anthocyanins, phenolics, flavonoids, total antioxidant (TOA), and proline were also up-regulated by H2O2 (104.9%, 128.8%, 166.3%, 141.4%, and 436.4%, respectively). These results corresponded to the stimulative role triggered by H2O2 in boosting the activities of catalase (22.4%), ascorbate peroxidase (20.7%), superoxide dismutase (88.4%), polyphenol oxidase (60.7%), soluble peroxidase (23.8%), and phenylalanine ammonia-lyase (57.1%) as well as the expression level of HpCAT, HpAPX, HpSOD, HpPPO, and HpPAL genes, which may help to moderate low-temperature stress. In conclusion, our findings stipulate new insights into the mechanisms by which H2O2 regulates low-temperature stress tolerance in pitaya plants.

Maternal nitric oxide homeostasis impacts female gametophyte development under optimal and stress conditions

In adverse environments, the number of fertilizable female gametophytes (FGs) in plants is reduced, leading to increased survival of the remaining offspring. How the maternal plant perceives internal growth cues and external stress conditions to alter FG development remains largely unknown. We report that homeostasis of the stress signaling molecule nitric oxide (NO) plays a key role in controlling FG development under both optimal and stress conditions. NO homeostasis is precisely regulated by S-nitrosoglutathione reductase (GSNOR). Prior to fertilization, GSNOR protein is exclusively accumulated in sporophytic tissues and indirectly controls FG development in Arabidopsis (Arabidopsis thaliana). In GSNOR null mutants, NO species accumulated in the degenerating sporophytic nucellus, and auxin efflux into the developing FG was restricted, which inhibited FG development, resulting in reduced fertility. Importantly, restoring GSNOR expression in maternal, but not gametophytic tissues, or increasing auxin efflux substrate significantly increased the proportion of normal FGs and fertility. Furthermore, GSNOR overexpression or added auxin efflux substrate increased fertility under drought and salt stress. These data indicate that NO homeostasis is critical to normal auxin transport and maternal control of FG development, which in turn determine seed yield. Understanding this aspect of fertility control could contribute to mediating yield loss under adverse conditions.

Enhancing cold and drought tolerance in cotton: a protective role of SikCOR413PM1

The present study explored the potential role of cold-regulated plasma membrane protein COR413PM1 isolated from Saussurea involucrata (Matsum. & Koidz)(SikCOR413PM1), in enhancing cotton (Gossypium hirsutum) tolerance to cold and drought stresses through transgenic methods. Under cold and drought stresses, the survival rate and the fresh and dry weights of the SikCOR413PM1-overexpressing lines were higher than those of the wild-type plants, and the degree of leaf withering was much lower. Besides, overexpressing SikCOR413PM1 overexpression increased the relative water content, reduced malondialdehyde content and relative conductivity, and elevated proline and soluble sugar levels in cotton seedlings. These findings suggest that SikCOR413PM1 minimizes cell membrane damage and boosts plant stability under challenging conditions. Additionally, overexpression of this gene upregulated antioxidant enzyme-related genes in cotton seedlings, resulting in enhanced antioxidant enzyme activity, lowered peroxide content, and reduced oxidative stress. SikCOR413PM1 overexpression also modulated the expression of stress-related genes (GhDREB1A, GhDREB1B, GhDREB1C, GhERF2, GhNAC3, and GhRD22). In field trials, the transgenic cotton plants overexpressing SikCOR413PM1 displayed high yields and increased environmental tolerance. Our study thus demonstrates the role of SikCOR413PM1 in regulating stress-related genes, osmotic adjustment factors, and peroxide content while preserving cell membrane stability and improving cold and drought tolerance in cotton.

Evaluation of Reference Genes for Normalizing RT-qPCR and Analysis of the Expression Patterns of WRKY1 Transcription Factor and Rhynchophylline Biosynthesis-Related Genes in Uncaria rhynchophylla

Uncaria rhynchophylla (Miq.) Miq. ex Havil, a traditional medicinal herb, is enriched with several pharmacologically active terpenoid indole alkaloids (TIAs). At present, no method has been reported that can comprehensively select and evaluate the appropriate reference genes for gene expression analysis, especially the transcription factors and key enzyme genes involved in the biosynthesis pathway of TIAs in U. rhynchophylla. Reverse transcription quantitative PCR (RT-qPCR) is currently the most common method for detecting gene expression levels due to its high sensitivity, specificity, reproducibility, and ease of use. However, this methodology is dependent on selecting an optimal reference gene to accurately normalize the RT-qPCR results. Ten candidate reference genes, which are homologues of genes used in other plant species and are common reference genes, were used to evaluate the expression stability under three stress-related experimental treatments (methyl jasmonate, ethylene, and low temperature) using multiple stability analysis methodologies. The results showed that, among the candidate reference genes, S-adenosylmethionine decarboxylase (SAM) exhibited a higher expression stability under the experimental conditions tested. Using SAM as a reference gene, the expression profiles of 14 genes for key TIA enzymes and a WRKY1 transcription factor were examined under three experimental stress treatments that affect the accumulation of TIAs in U. rhynchophylla. The expression pattern of WRKY1 was similar to that of tryptophan decarboxylase (TDC) under ETH treatment. This research is the first to report the stability of reference genes in U. rhynchophylla and provides an important foundation for future gene expression analyses in U. rhynchophylla. The RT-qPCR results indicate that the expression of WRKY1 is similar to that of TDC under ETH treatment. It may coordinate the expression of TDC, providing a possible method to enhance alkaloid production in the future through synthetic biology.

Advances in Rice Seed Shattering

Seed shattering is an important trait that wild rice uses to adapt to the natural environment and maintain population reproduction, and weedy rice also uses it to compete with the rice crop. The loss of shattering is a key event in rice domestication. The degree of shattering is not only one of the main reasons for rice yield reduction but also affects its adaptability to modern mechanical harvesting methods. Therefore, it is important to cultivate rice varieties with a moderate shattering degree. In this paper, the research progress on rice seed shattering in recent years is reviewed, including the physiological basis, morphological and anatomical characteristics of rice seed shattering, inheritance and QTL/gene mapping of rice seed shattering, the molecular mechanism regulating rice seed shattering, the application of seed-shattering genes, and the relationship between seed-shattering genes and domestication.

Cold stress triggers freezing tolerance in wheat (Triticum aestivum L.) via hormone regulation and transcription of related genes

Low temperatures limit the geographic distribution and yield of plants. Hormones play an important role in coordinating the growth and development of plants and their tolerance to low temperatures. However, the mechanisms by which hormones affect plant resistance to extreme cold stress in the natural environment are still unclear. In this study, two winter wheat varieties with different cold resistances, Dn1 and J22, were used to conduct targeted plant hormone metabolome analysis on the tillering nodes of winter wheat at 5 °C, -10 °C and -25 °C using an LC-ESI-MS/MS system. We screened 39 hormones from 88 plant hormone metabolites and constructed a partial regulatory network of auxin, jasmonic acid and cytokinin. GO analysis and enrichment of KEGG pathways in different metabolites showed that the 'plant hormone signal transduction' pathway was the most common. Our study showed that extreme low temperature increased the most levels of auxin, cytokinin and salicylic acid, and decreased levels of jasmonic acid and abscisic acid, and that levels of auxin, jasmonic acid and cytokinin in Dn1 were higher than those in J22. These changes in hormone levels were associated with changes in gene expression in synthesis, catabolism, transport and signal transduction pathways. These results differ from the previous hormone regulation mechanisms, which were mostly obtained at 4 °C. Our results provide a basis for further understanding the molecular mechanisms by which plant endogenous hormones regulate plant freezing stress tolerance.

Overexpression of TaMYC2 confers freeze tolerance by ICE-CBF-COR module in Arabidopsis thaliana

Dongnongdongmai No.1 (Dn1) is one of the few winter wheat varieties that can successfully overwinter at temperatures as low as -25°C or even lower. To date, few researches were carried to identify the freeze tolerance genes in Dn1 and applied them to improve plant resistance to extreme low temperatures. The basic helix-loop-helix (bHLH) transcription factor MYC2 is a master regulator in JA signaling, which has been reported to involve in responses to mild cold stress (2°C and 7°C). We hypothesized that MYC2 might be part of the regulatory network responsible for the tolerance of Dn1 to extreme freezing temperatures. In this study, we showed that wheat MYC2 (TaMYC2) was induced under both extreme low temperature (-10°C and-25°C) and JA treatments. The ICE-CBF-COR transcriptional cascade, an evolutionary conserved cold resistance pathway downstream of MYC2, was also activated in extreme low temperatures. We further showed that overexpression of any of the MYC2 genes from Dn1 TaMYC2A, B, D in Arabidopsis led to enhanced freeze tolerance. The TaMYC2 overexpression lines had less electrolyte leakage and lower malondialdehyde (MDA) content, and an increase in proline content, an increases antioxidant defences, and the enhanced expression of ICE-CBF-COR module under the freezing temperature. We further verified that TaMYC2 might function through physical interaction with TaICE41 and TaJAZ7, and that TaJAZ7 physically interacts with TaICE41. These results elucidate the molecular mechanism by which TaMYC2 regulates cold tolerance and lay the foundation for future studies to improve cold tolerance in plants.

F-box protein EBF1 and transcription factor ABI5-like regulate banana fruit chilling-induced ripening disorder

Cold stress adversely affects plant production, both qualitatively and quantitatively. Banana (Musa acuminata) is sensitive to cold stress and suffers chilling injury (CI) when stored under 11°C, causing abnormal fruit softening. However, the mechanism underlying the abnormal fruit softening due to CI remains obscure. This study uncovered the coordinated transcriptional mechanism of ethylene F-box (EBF1) protein and abscisic acid-insensitive 5 (ABI5)-like protein in regulating chilling-induced softening disorders of Fenjiao banana. Cold stress severely inhibited the transcript and protein levels of EBF1, ABI5-like, and fruit softening-related genes. The ABI5-like protein bound to the promoters of key starch and cell wall degradation-related genes such as β-amylase 8 (BAM8), pectate lyase 8 (PL8), and β-D-xylosidase23-like (XYL23-like) and activated their activities. EBF1 physically interacted with ABI5-like and enhanced the transcriptional activity of the key starch and cell wall degradation-related genes but did not ubiquitinate or degrade ABI5-like protein. This promoted fruit ripening and ameliorated fruit CI in a manner similar to the effect of exogenous abscisic acid treatment. The ectopic and transient overexpression of EBF1 and ABI5-like genes in tomato (Solanum lycopersicum) and Fenjiao banana accelerated fruit ripening and softening by promoting ethylene production, starch and cell wall degradation, and decreasing fruit firmness. EBF1 interacted with EIL4 but did not ubiquitinate or degrade EIL4, which is inconsistent with the typical role of EBF1/2 in Arabidopsis (Arabidopsis thaliana). These results collectively highlight that the interaction of EBF1 and ABI5-like controls starch and cell wall metabolism in banana, which is strongly inhibited by chilling stress, leading to fruit softening and ripening disorder.

Brown Adipose Tissue Rescues Bone Loss Induced by Cold Exposure

Cold temperature activates the sympathetic nervous system (SNS) to induce bone loss by altering bone remodeling. Brown adipose tissue (BAT) is influenced by the SNS in cold environments. Many studies have confirmed a positive relationship between BAT volume and bone mass, but the influence and mechanism of BAT on bone in vivo and in vitro is still unknown. Two-month-old C57/BL6j male mice were exposed to cold temperature (4°C) to induce BAT generation. BAT volume, bone remodeling and microstructure were assessed after 1 day, 14 days and 28 days of cold exposure. CTX-1, P1NP and IL-6 levels were detected in the serum by ELISA. To determine the effect of BAT on osteoclasts and osteoblasts in vitro, brown adipocyte conditional medium (BAT CM) was collected and added to the differentiation medium of bone marrow-derived macrophages (BMMs) and bone marrow mesenchymal stem cells (BMSCs). Micro-CT results showed that the bone volume fraction (BV/TV, %) significantly decreased after 14 days of exposure to cold temperature but recovered after 28 days. Double labeling and TRAP staining in vivo showed that bone remodeling was altered during cold exposure. BAT volume enlarged after 14 days of cold stimulation, and IL-6 increased. BAT CM promoted BMSC mineralization by increasing osteocalcin (Ocn), RUNX family transcription factor 2 (Runx2) and alkaline phosphatase (Alp) expression, while bone absorption was inhibited by BAT CM. In conclusion, restoration of bone volume after cold exposure may be attributed to enlarged BAT. BAT has a beneficial effect on bone mass by facilitating osteogenesis and suppressing osteoclastogenesis.

Regulatory Mechanisms of bHLH Transcription Factors in Plant Adaptive Responses to Various Abiotic Stresses

Basic helix-loop-helix proteins (bHLHs) comprise one of the largest families of transcription factors in plants. They have been shown to be involved in responses to various abiotic stresses, such as drought, salinity, chilling, heavy metal toxicity, iron deficiency, and osmotic damages. By specifically binding to cis-elements in the promoter region of stress related genes, bHLHs can regulate their transcriptional expression, thereby regulating the plant's adaptive responses. This review focuses on the structural characteristics of bHLHs, the regulatory mechanism of how bHLHs are involved transcriptional activation, and the mechanism of how bHLHs regulate the transcription of target genes under various stresses. Finally, as increasing research demonstrates that flavonoids are usually induced under fluctuating environments, the latest research progress and future research prospects are described on the mechanisms of how flavonoid biosynthesis is regulated by bHLHs in the regulation of the plant's responses to abiotic stresses.

Modes of Brassinosteroid Activity in Cold Stress Tolerance

Cold stress is a significant environmental factor that negatively affects plant growth and development in particular when it occurs during the growth phase. Plants have evolved means to protect themselves from damage caused by chilling or freezing temperatures and some plant species, in particular those from temperate geographical zones, can increase their basal level of freezing tolerance in a process termed cold acclimation. Cold acclimation improves plant survival, but also represses growth, since it inhibits activity of the growth-promoting hormones gibberellins (GAs). In addition to GAs, the steroid hormones brassinosteroids (BRs) also take part in growth promotion and cold stress signaling; however, in contrast to Gas, BRs can improve cold stress tolerance with fewer trade-offs in terms of growth and yields. Here we summarize our current understanding of the roles of BRs in cold stress responses with a focus on freezing tolerance and cold acclimation pathways.

Abscisic Acid Negatively Modulates Heat Tolerance in Rolled Leaf Rice by Increasing Leaf Temperature and Regulating Energy Homeostasis

BACKGROUND

Abscisic acid (ABA) acts as a signaling hormone in plants against abiotic stress, but its function in energy homeostasis under heat stress is unclear.

RESULTS

Two rice genotypes, Nipponbare (wild-type, WT) with flat leaves and its mutant high temperature susceptibility (hts) plant with semi-rolled leaves, were subjected to heat stress. We found significantly higher tissue temperature, respiration rate, and ABA and H2O2 contents in leaves as well as a lower transpiration rate and stomatal conductance in hts than WT plants. Additionally, increased expression of HSP71.1 and HSP24.1 as well as greater increases in carbohydrate content, ATP, NAD (H), and dry matter weight, were detected in WT than hts plants under heat stress. More importantly, exogenous ABA significantly decreased heat tolerance of hts plants, but clearly enhanced heat resistance of WT plants. The increases in carbohydrates, ATP, NAD (H), and heat shock proteins in WT plants were enhanced by ABA under heat stress, whereas these increases were reduced in hts plants.

CONCLUSION

It was concluded that ABA is a negative regulator of heat tolerance in hts plants with semi-rolled leaves by modulating energy homeostasis.

Stress priming, memory, and signalling in plants

Plants need to cope with changing environmental conditions, be it variable light or temperature, different availability of water or nutrients, or attack by pathogens or insects. Some of these changing conditions can become stressful and require strong countermeasures to ensure plant survival. Plants have evolved numerous distinct sensing and signalling mechanisms to perceive and respond appropriately to a variety of stresses. Because of the unpredictable nature of numerous stresses, resource-saving stress response mechanisms are inducible and become activated only upon a stress experience. Furthermore, plants have evolved mechanisms by which they can remember past stress events and prime their responses in order to react more rapidly or more strongly to recurrent stress. Research over the last decade has revealed mechanisms of this information storage and retrieval, which include epigenetic regulation, transcriptional priming, primed conformation of proteins, or specific hormonal or metabolic signatures. There is also increasing understanding of the ecological constraints and relevance of stress priming and memory. This special issue presents research articles and reviews addressing various aspects of this exciting and growing field of research. Here, we introduce the topic by referring to the articles published in this issue, and we outline open questions and future directions of research.