The Role of Endoplasmic Reticulum Stress Response in Pollen Development and Heat Stress Tolerance

Deciphering the Vulnerability of Pollen to Heat Stress for Securing Crop Yields in a Warming Climate

Climate change is leading to more frequent and severe extreme temperature events, negatively impacting agricultural productivity and threatening global food security. Plant reproduction, the process fundamental to crop yield, is highly susceptible to heatwaves, which disrupt pollen development and ultimately affect seed-set and crop yields. Recent research has increasingly focused on understanding how pollen grains from various crops react to heat stress at the molecular and cellular levels. This surge in interest over the last decade has been driven by advances in genomic technologies, such as single-cell RNA sequencing, which holds significant potential for revealing the underlying regulatory reprogramming triggered by heat stress throughout the various stages of pollen development. This review focuses on how heat stress affects gene regulatory networks, including the heat stress response, the unfolded protein response, and autophagy, and discusses the impact of these changes on various stages of pollen development. It highlights the potential of pollen selection as a key strategy for improving heat tolerance in crops by leveraging the genetic variability among pollen grains. Additionally, genome-wide association studies and population screenings have shed light on the genetic underpinnings of traits in major crops that respond to high temperatures during male reproductive stages. Gene-editing tools like CRISPR/Cas systems could facilitate precise genetic modifications to boost pollen heat resilience. The information covered in this review is valuable for selecting traits and employing molecular genetic approaches to develop heat-tolerant genotypes.

Molecular Mechanisms of Gene Expression Regulation in Response to Heat Stress in Hemerocallis fulva

Hemerocallis fulva is one of the three major flowers in the world; its flower type and color are very rich, with high ornamental value and economic value. Heat stress severely limits the cultivation and geographical distribution of H. fulva. Genetic resources and their underlying molecular mechanisms constitute the cornerstone of contemporary breeding technologies. However, research on the response of H. fulva to heat stress remains relatively scant. In this study, we used the heat-resistant 'Dan Yang' variety and heat-sensitive 'Nuo Mi Lu' variety with phenotypic expression as experimental materials to determine the changes in substance and gene expression levels, and used bioinformatics technology to study the molecular mechanisms and gene resource mining of H. fulva in response to heat stress. We identified several thousand differentially expressed genes (DEGs) in different comparison groups. At the same time, 1850 shared DEGs were identified in two H. fulva genotypes responding to heat stress. The dynamic cutting algorithm was used to cluster the genes, and 23 gene co-expression modules were obtained. The MEorangered, MElightpink, and MEmagenta modules were significantly correlated with physiological and biochemical traits. We identified ten key genes closely related to the response of H. fulva to heat stress, including plant-pathogen interactions, plant hormone signal transduction, oxidative transduction phosphorylation, and the plant hormone signal transduction pathway. This study not only analyzes the molecular mechanism of H. fulva response to heat stress, but also provides genetic resources for breeding H. fulva heat tolerance.

Heat Stress Inhibits Pollen Development by Degrading mRNA Capping Enzyme ARCP1 and ARCP2

Pollen development and germination are critical for successful generation of offspring in plants, yet they are highly susceptible to heat stress (HS). However, the molecular mechanism underlying this process has not been fully elucidated. In this study, we highlight the essential roles of two mRNA capping enzymes, named Arabidopsis mRNA capping phosphatase (ARCP) 1 and 2, in regulating male and female gamete development. The transmission efficiencies of gametes carrying arcp1 arcp2 from arcp1+/- arcp2-/- and arcp1-/- arcp2+/- mutants are 30% and zero, respectively. These mutants exhibited a significant increase in misshaped pollen, with germination rates approximately half of those in wild type. ARCP1/2 exhibit RNA triphosphatase and RNA guanylyltransferase activities, which are required for proper pollen development. Through RNA-seq analysis, genes involved in pollen development/germination and HS response were identified as downregulated genes in pollen from arcp1+/- arcp2-/- mutant. Furthermore, ARCP2 protein is degraded under HS condition, and inducing the expression of ARCP2 can increase the pollen germination rate under elevated temperature. We propose that HS triggers the degradation of mRNA capping enzymes, which in turn disrupts the transcriptome that required for pollen development and pollen germination and ultimately leads to male sterility.

Melatonin confers thermotolerance and antioxidant capacity in Chinese cabbage

Due to the damaging effect of high temperatures on plant development, global warming is predicted to increase agricultural risks. Chinese cabbage holds considerable importance as a leafy vegetable that is extensively consumed and cultivated worldwide. Its year-round production also encounters severe challenges in the face of high temperatures. In this study, melatonin (MT), a pivotal multifunctional signaling molecule that coordinates responses to diverse environmental stressors was used to mitigate the harmful effects of high temperatures on Chinese cabbage. Through the utilization of growth indices, cytological morphology, physiological and biochemical responses, and RNA-Seq analysis, alongside an examination of the influence of crucial enzymes in the endogenous MT synthesis pathway on the thermotolerance of Chinese cabbage, we revealed that MT pretreatment enhanced photosynthetic activity, maintained signaling pathways associated with endoplasmic reticulum protein processing, and preserved circadian rhythm in Chinese cabbage under high temperatures. Furthermore, pretreatment with MT resulted in increased levels of soluble sugar, vitamin C, proteins, and antioxidant enzyme activity, along with decreased levels of malondialdehyde, nitrate, flavonoids, and bitter glucosinolates, ultimately enhancing the capacity of the organism to mitigate oxidative stress. The knockdown of the tryptophan decarboxylase gene, which encodes a key enzyme responsible for MT biosynthesis, resulted in a significant decline in the ability of transgenic Chinese cabbage to alleviate oxidative damage under high temperatures, further indicating an important role of MT in establishing the thermotolerance. Taken together, these results provide a mechanism for MT to improve the antioxidant capacity of Chinese cabbage under high temperatures and suggest beneficial implications for the management of other plants subjected to global warming.

High Overexpression of SiAAP9 Leads to Growth Inhibition and Protein Ectopic Localization in Transgenic Arabidopsis

Amino acid permeases (AAPs) transporters are crucial for the long-distance transport of amino acids in plants, from source to sink. While Arabidopsis and rice have been extensively studied, research on foxtail millet is limited. This study identified two transcripts of SiAAP9, both of which were induced by NO3- and showed similar expression patterns. The overexpression of SiAAP9L and SiAAP9S in Arabidopsis inhibited plant growth and seed size, although SiAAP9 was found to transport more amino acids into seeds. Furthermore, SiAAP9-OX transgenic Arabidopsis showed increased tolerance to high concentrations of glutamate (Glu) and histidine (His). The high overexpression level of SiAAP9 suggested its protein was not only located on the plasma membrane but potentially on other organelles, as well. Interestingly, sequence deletion reduced SiAAP9's sensitivity to Brefeldin A (BFA), and SiAAP9 had ectopic localization on the endoplasmic reticulum (ER). Protoplast amino acid uptake experiments indicated that SiAAP9 enhanced Glu transport into foxtail millet cells. Overall, the two transcripts of SiAAP9 have similar functions, but SiAAP9L shows a higher colocalization with BFA compartments compared to SiAAP9S. Our research identifies a potential candidate gene for enhancing the nutritional quality of foxtail millet through breeding.

Physiological and proteomic characterization revealed the response mechanisms underlying aluminium tolerance in lentil (Lens culinaris Medikus)

Aluminium (Al) toxicity causes major plant distress, affecting root growth, nutrient uptake and, ultimately, agricultural productivity. Lentil, which is a cheap source of vegetarian protein, is recognized to be sensitive to Al toxicity. Therefore, it is important to dissect the physiological and molecular mechanisms of Al tolerance in lentil. To understand the physiological system and proteome composition underlying Al tolerance, two genotypes [L-4602 (Al-tolerant) and BM-4 (Al-sensitive)] were studied at the seedling stage. L-4602 maintained a significantly higher root tolerance index and malate secretion with reduced Al accumulation than BM-4. Also, label-free proteomic analysis using ultra-performance liquid chromatography-tandem mass spectrometer exhibited significant regulation of Al-responsive proteins associated with antioxidants, signal transduction, calcium homeostasis, and regulation of glycolysis in L-4602 as compared to BM-4. Functional annotation suggested that transporter proteins (transmembrane protein, adenosine triphosphate-binding cassette transport-related protein and multi drug resistance protein), antioxidants associated proteins (nicotinamide adenine dinucleotide dependent oxidoreductase, oxidoreductase molybdopterin binding protein & peroxidases), kinases (calmodulin-domain kinase & protein kinase), and carbohydrate metabolism associated proteins (dihydrolipoamide acetyltransferase) were found to be abundant in tolerant genotype providing protection against Al toxicity. Overall, the root proteome uncovered in this study at seedling stage, along with the physiological parameters measured, allow a greater understanding of Al tolerance mechanism in lentil, thereby assisting in future crop improvement programmes.

Investigating the effects of tauroursodeoxycholic acid (TUDCA) in mitigating endoplasmic reticulum stress and cellular responses in Pak choi

The accumulation of misfolded proteins in the endoplasmic reticulum (ER) within plant cells due to unfavourable conditions leads to ER stress. This activates interconnected pathways involving reactive oxygen species (ROS) and unfolded protein response (UPR), which play vital roles in regulating ER stress. The aim of this study is to investigate the underlying mechanisms of tunicamycin (TM) induced ER stress and explore the potential therapeutic applications of tauroursodeoxycholic acid (TUDCA) in mitigating cellular responses to ER stress in Pak choi (Brassica campestris subsp. chinensis). The study revealed that ER stress in Pak choi leads to detrimental effects on plant morphology, ROS levels, cellular membrane integrity, and the antioxidant defence system. However, treatment with TUDCA in TM-induced ER stressed Pak choi improved morphological indices, pigment contents, ROS accumulation, cellular membrane integrity, and antioxidant defence system restoration. Additionally, TUDCA also modulates the transcription levels of ER stress sensors genes, ER chaperone genes, and ER-associated degradation (ERAD) genes during ER stress in Pak choi. Furthermore, TUDCA has demonstrated its ability to alleviate ER stress, stabilize the UPR, reduce oxidative stress, prevent apoptosis, and positively influence plant growth and development. These results collectively comprehend TUDCA as a promising agent for mitigating ER stress-induced damage in Pak choi plants and provide valuable insights for further research and potential applications in crop protection and stress management.

Identifying long non-coding RNAs involved in heat stress response during wheat pollen development

Introduction

Wheat is a staple food crop for over one-third of the global population. However, the stability of wheat productivity is threatened by heat waves associated with climate change. Heat stress at the reproductive stage can result in pollen sterility and failure of grain development.

Methods

This study used transcriptome data analysis to explore the specific expression of long non-coding RNAs (lncRNAs) in response to heat stress during pollen development in four wheat cultivars.

Results and discussion

We identified 11,054 lncRNA-producing loci, of which 5,482 lncRNAs showed differential expression in response to heat stress. Heat-responsive lncRNAs could target protein-coding genes in cis and trans and in lncRNA-miRNA-mRNA regulatory networks. Gene ontology analysis predicted that target protein-coding genes of lncRNAs regulate various biological processes such as hormonal responses, protein modification and folding, response to stress, and biosynthetic and metabolic processes. We also noted some paired lncRNA/protein-coding gene modules and some lncRNA-miRNA-mRNA regulatory modules shared in two or more wheat cultivars. These modules were related to regulating plant responses to heat stress, such as heat-shock proteins and transcription factors, and protein domains, such as MADS-box, Myc-type, and Alpha crystallin/Hsp20 domain.

Conclusion

Our results provide the basic knowledge and molecular resources for future functional studies investigating wheat reproductive development under heat stress.

Dynamic organelle changes and autophagic processes in lily pollen germination

Pollen germination is a crucial process in the life cycle of flowering plants, signifying the transition of quiescent pollen grains into active growth. This study delves into the dynamic changes within organelles and the pivotal role of autophagy during lily pollen germination. Initially, mature pollen grains harbor undifferentiated organelles, including amyloplasts, mitochondria, and the Golgi apparatus. However, germination unveils remarkable transformations, such as the redifferentiation of amyloplasts accompanied by starch granule accumulation. We investigate the self-sustained nature of amylogenesis during germination, shedding light on its association with osmotic pressure. Employing BODIPY 493/503 staining, we tracked lipid body distribution throughout pollen germination, both with or without autophagy inhibitors (3-MA, NEM). Typically, lipid bodies undergo polarized movement from pollen grains into elongating pollen tubes, a process crucial for directional growth. Inhibiting autophagy disrupted this essential lipid body redistribution, underscoring the interaction between autophagy and lipid body dynamics. Notably, the presence of tubular endoplasmic reticulum (ER)-like structures associated with developing amyloplasts and lipid bodies implies their participation in autophagy. Starch granules, lipid bodies, and membrane remnants observed within vacuoles further reinforce the involvement of autophagic processes. Among the autophagy inhibitors, particularly BFA, significantly impede germination and growth, thereby affecting Golgi morphology. Immunogold labeling substantiates the pivotal role of the ER in forming autophagosome-like compartments and protein localization. Our proposed speculative model of pollen germination encompasses proplastid differentiation and autophagosome formation. This study advances our understanding of organelle dynamics and autophagy during pollen germination, providing valuable insights into the realm of plant reproductive physiology.

Membrane-Bound Transcription Factor ZmNAC074 Positively Regulates Abiotic Stress Tolerance in Transgenic Arabidopsis

Maize (Zea mays L.) is one of the most widely cultivated crops for humans, making a vital contribution to human nutrition and health. However, in recent years, due to the influence of external adverse environments, the yield and quality of maize have been seriously affected. NAC (NAM, ATAF1/2 and CUC2) transcription factors (TFs) are important plant-unique TFs, which are crucial for regulating the abiotic stress response of plants. Therefore, it is of great biological significance to explore the underlying regulatory function of plant NAC TFs under various abiotic stresses. In this study, wild-type and ZmNAC074-overexpressed transgenic Arabidopsis were used as experimental materials to dissect the stress-resistant function of ZmNAC074 in transgenic Arabidopsis at phenotypic, physiological and molecular levels. The analyses of seed germination rate, survival rate, phenotype, the content of chlorophyll, carotenoids, malondialdehyde (MDA), proline and other physiological indexes induced by distinct abiotic stress conditions showed that overexpression of ZmNAC074 could confer the enhanced resistance of salt, drought, and endoplasmic reticulum (ER) stress in transgenic Arabidopsis, indicating that ZmNAC074 plays an important regulatory role in plant response to abiotic stress, which provides an important theoretical foundation for further uncovering the molecular regulation mechanism of ZmNAC074 under abiotic stresses.

The transcriptome of soybean reproductive tissues subjected to water deficit, heat stress, and a combination of water deficit and heat stress

Global warming and climate change are driving an alarming increase in the frequency and intensity of extreme climate events, such as droughts, heat waves, and their combination, inflicting heavy losses to agricultural production. Recent studies revealed that the transcriptomic responses of different crops to water deficit (WD) or heat stress (HS) are very different from that to a combination of WD + HS. In addition, it was found that the effects of WD, HS, and WD + HS are significantly more devastating when these stresses occur during the reproductive growth phase of crops, compared to vegetative growth. As the molecular responses of different reproductive and vegetative tissues of plants to WD, HS, or WD + HS could be different from each other and these differences could impact many current and future attempts to enhance the resilience of crops to climate change through breeding and/or engineering, we conducted a transcriptomic analysis of different soybean (Glycine max) tissues to WD, HS, and WD + HS. Here we present a reference transcriptomic dataset that includes the response of soybean leaf, pod, anther, stigma, ovary, and sepal to WD, HS, and WD + HS conditions. Mining this dataset for the expression pattern of different stress response transcripts revealed that each tissue had a unique transcriptomic response to each of the different stress conditions. This finding is important as it suggests that enhancing the overall resilience of crops to climate change could require a coordinated approach that simultaneously alters the expression of different groups of transcripts in different tissues in a stress-specific manner.

Heat-shock responses: systemic and essential ways of malaria parasite survival

Fever is a part of the human innate immune response that contributes to limiting microbial growth and development in many infectious diseases. For the parasite Plasmodium falciparum, survival of febrile temperatures is crucial for its successful propagation in human populations as well as a fundamental aspect of malaria pathogenesis. This review discusses recent insights into the biological complexity of the malaria parasite's heat-shock response, which involves many cellular compartments and essential metabolic processes to alleviate oxidative stress and accumulation of damaged and unfolded proteins. We highlight the overlap between heat-shock and artemisinin resistance responses, while also explaining how the malaria parasite adapts its fever response to fight artemisinin treatment. Additionally, we discuss how this systemic and essential fight for survival can also contribute to parasite transmission to mosquitoes.

Alpha-lipoic acid attenuates heat stress-induced apoptosis via upregulating the heat shock response in porcine parthenotes

Heat stress (HS) is a long-standing hurdle that animals face in the living environment. Alpha-lipoic acid (ALA) is a strong antioxidant synthesized by plants and animals. The present study evaluated the mechanism of ALA action in HS-induced early porcine parthenotes development. Parthenogenetically activated porcine oocytes were divided into three groups: control, high temperature (HT) (42 °C for 10 h), and HT + ALA (with 10 µM ALA). The results show that HT treatment significantly reduced the blastocyst formation rate compared to the control. The addition of ALA partially restored the development and improved the quality of blastocysts. Moreover, supplementation with ALA not only induced lower levels of reactive oxygen species and higher glutathione levels but also markedly reduced the expression of glucose regulatory protein 78. The protein levels of heat shock factor 1 and heat shock protein 40 were higher in the HT + ALA group, which suggests activation of the heat shock response. The addition of ALA reduced the expression of caspase 3 and increased the expression of B-cell lymphoma-extra-large protein. Collectively, this study revealed that ALA supplementation ameliorated HS-induced apoptosis by suppressing oxidative and endoplasmic reticulum stresses via activating the heat shock response, which improved the quality of HS-exposed porcine parthenotes.

Oat AsDA1-2D enhances heat stress tolerance and negatively regulates seed-storage globulin

The importance of oats has increased because of their high nutritional value and health benefits in the human diet. High-temperature stress during the reproductive growth period has a detrimental effect on grain morphology by changing the structure and concentration of several seed-storage proteins. DA1, a conserved ubiquitin-proteasome pathway component, plays an important role in regulating grain size by controlling cell proliferation in maternal integuments during the grain-filling stage. However, there have been no reports or studies on oat DA1 genes. In this study, we identified three DA1-like genes (AsDA1-2D, AsDA1-5A, and AsDA1-1D) using genome-wide analysis. Among these, AsDA1-2D was found to be responsible for high-temperature stress tolerance via a yeast thermotolerance assay. The physical interaction of AsDA1-2D with oat-storage-globulin (AsGL-4D) and a protease inhibitor (AsPI-4D) was observed using yeast two-hybrid screening. A subcellular localization assay revealed that AsDA1-2D and its interacting proteins are localized in the cytosol and plasma membrane. An in vitro pull-down assay showed that AsDA1-2D forms a complex with both AsPI-4D and AsGL-4D. An in vitro cell-free degradation assay showed that AsGL-4D was degraded by AsDA1-2D under high-temperature conditions and that AsPI-4D inhibited the function of AsDA1-2D. These results suggest that AsDA1-2D acts as a cysteine protease and negatively regulates oat-grain-storage-globulin under heat stress.

Melatonin and Abiotic Stress Tolerance in Crop Plants

Increasing food demand by the growing human population and declining crop productivity due to climate change affect global food security. To meet the challenges, developing improved crops that can tolerate abiotic stresses is a priority. Melatonin in plants, also known as phytomelatonin, is an active component of the various cellular mechanisms that alleviates oxidative damage in plants, hence supporting the plant to survive abiotic stress conditions. Exogenous melatonin strengthens this defence mechanism by enhancing the detoxification of reactive by-products, promoting physiological activities, and upregulating stress-responsive genes to alleviate damage during abiotic stress. In addition to its well-known antioxidant activity, melatonin protects against abiotic stress by regulating plant hormones, activating ER stress-responsive genes, and increasing protein homoeostasis, heat shock transcription factors and heat shock proteins. Under abiotic stress, melatonin enhances the unfolded protein response, endoplasmic reticulum-associated protein degradation, and autophagy, which ultimately protect cells from programmed cell death and promotes cell repair resulting in increased plant survival.

Proteome and Ubiquitylome Analyses of Maize Endoplasmic Reticulum under Heat Stress

High temperatures severely affect plant growth and pose a threat to global crop production. Heat causes the accumulation of misfolded proteins in the endoplasmic reticulum(ER), as well as triggering the heat-shock response (HSR) in the cytosol and the unfolded protein response (UPR) in the ER. Excessive misfolded proteins undergo further degradation through ER-associated degradation (ERAD). Although much research on the plant heat stress response has been conducted, the regulation of ER-localized proteins has not been well-studied thus far. We isolated the microsome fraction from heat-treated and untreated maize seedlings and performed proteome and ubiquitylome analyses. Of the 8306 total proteins detected in the proteomics analysis, 1675 proteins were significantly up-regulated and 708 proteins were significantly down-regulated. Global ubiquitination analysis revealed 1780 proteins with at least one ubiquitination site. Motif analysis revealed that alanine and glycine are the preferred amino acids upstream and downstream of ubiquitinated lysine sites. ERAD components were found to be hyper-ubiquitinated after heat treatment, implying the feedback regulation of ERAD activity through protein degradation.

Nitric Oxide and Proline Modulate Redox Homeostasis and Photosynthetic Metabolism in Wheat Plants under High Temperature Stress Acclimation

The effects of exogenously-sourced NO (nitric oxide, as 100 µM SNP) and proline (50 mM) in the protection of the photosynthetic performance of wheat (Triticum aestivum L.) plants against heat stress were investigated. The study focused on the mechanisms of proline accumulation, activity, gene expression of antioxidant enzymes, and NO generation. Plants were exposed to a temperature of 40 °C for 6 h per day over 15 days, then allowed to recover at 28 °C. Heat-stressed plants showed increased oxidative stress, with higher levels of H₂O₂ and TBARS (thiobarbituric acid reactive substances) and increased proline accumulation, ACS activity, ethylene evolution, and NO generation, which in turn leads to increased accumulation of antioxidant enzymes and reduced photosynthetic attributes. In the tested wheat cultivar, the exogenous application of SNP and proline under heat stress improved the photosynthesis and reduced oxidative stress by enhancing the enzymatic antioxidant defense system. Potentially, the promoter AOX (alternative oxidase) played a role in maintaining redox homeostasis by lowering H₂O₂ and TBARS levels. The genes for GR antioxidant and photosystem II core protein encoding psbA and psbB were highly up-regulated in nitric oxide and proline treated heat-stressed plants, indicating that ethylene positively impacted photosynthesis under high temperature stress. Moreover, nitric oxide supplementation under high temperature stress optimized ethylene levels to regulate the assimilation and metabolism of proline and the antioxidant system, lowering the adverse effects. The study showed that nitric oxide and proline increased high temperature stress tolerance in wheat by increasing the osmolytes accumulation and the antioxidant system, resulting in enhanced photosynthesis.

Cold Stress Response Mechanisms in Anther Development

Unlike animals that can escape threats, plants must endure and adapt to biotic and abiotic stresses in their surroundings. One such condition, cold stress, impairs the normal growth and development of plants, in which most phases of reproductive development are particularly susceptible to external low temperature. Exposed to uncomfortably low temperature at the reproductive stage, meiosis, tapetal programmed cell death (PCD), pollen viability, and fertilization are disrupted, resulting in plant sterility. Of them, cold-induced tapetal dysfunction is the main cause of pollen sterility by blocking nutrition supplements for microspore development and altering their timely PCD. Further evidence has indicated that the homeostatic imbalances of hormones, including abscisic acid (ABA) and gibberellic acid (GA), and sugars have occurred in the cold-treated anthers. Among them, cold stress gives rise to the accumulation of ABA and the decrease of active GA in anthers to affect tapetal development and represses the transport of sugar to microspores. Therefore, plants have evolved lots of mechanisms to alleviate the damage of external cold stress to reproductive development by mainly regulating phytohormone levels and sugar metabolism. Herein, we discuss the physiological and metabolic effects of low temperature on male reproductive development and the underlying mechanisms from the perspective of molecular biology. A deep understanding of cold stress response mechanisms in anther development will provide noteworthy references for cold-tolerant crop breeding and crop production under cold stress.

Rationally Designed Heptamethine Cyanine Photosensitizers that Amplify Tumor-Specific Endoplasmic Reticulum Stress and Boost Antitumor Immunity

Cancer phototherapy activates immunogenic cell death (ICD) and elicits a systemic antitumor immune response, which is an emerging approach for tumor treatment. Most available photosensitizers require a combination of immune adjuvants or checkpoint inhibitors to trigger antitumor immunity because of the immunosuppressive tumor microenvironment and the limited phototherapeutic effect. A class of tumor-targeting heptamethine cyanine photosensitizers modified with an endoplasmic reticulum (ER)-targeting group (benzenesulfonamide) are synthesized. Phototherapy of tumor cells markedly amplifies ER stress and promotes tumor antigen release, as the ER is required for protein synthesis, secretion, and transport. More importantly, different electron-donating or -withdrawing substitutions are introduced into benzenesulfonamide to modulate the nonradiative decay pathways through intramolecular charge transfer, including singlet-triplet intersystem crossing (photodynamic effect) and internal thermal conversion (photothermal effect). Thus, a heptamethine cyanine photosensitizer containing a binitro-substituted benzenesulfonamide (ER-Cy-poNO₂ ) is identified that preferentially accumulates in the ER of tumor cells. It significantly enhances the phototherapeutic effect by inducing excessive ER stress and robust ICD. Consequently, this small molecular photosensitizer triggers a sufficient antitumor immune response and effectively suppresses the growth of both primary and distant metastatic tumors, whereas no apparent toxicity is observed. This heptamethine cyanine photosensitizer has the potential to enhance cancer-targeted immunotherapy.

Long-Term Mild Heat Causes Post-Mitotic Pollen Abortion Through a Local Effect on Flowers

Crop reproductive success is significantly challenged by heatwaves, which are increasing in frequency and severity globally. Heat-induced male sterility is mainly due to aborted pollen development, but it is not clear whether this is through direct or systemic effects. Here, long-term mild heat (LTMH) treatment, mimicking a heatwave, was applied locally to tomato flowers or whole plants and followed up by cytological, transcriptomic, and biochemical analyses. By analyzing pollen viability, LTMH was shown to act directly on the flowers and not via effects on other plant tissue. The meiosis to early microspore stage of pollen development was the most sensitive to LTMH and 3 days of exposure around this period was sufficient to significantly reduce pollen viability at the flower anthesis stage. Extensive cytological analysis showed that abnormalities in pollen development could first be observed after pollen mitosis I, while no deviations in tapetum development were observed. Transcriptomic and biochemical analyses suggested that pollen development suffered from tapetal ER stress and that there was a limited role for oxidative stress. Our results provide the first evidence that heat acts directly on flowers to induce pollen sterility, and that the molecular-physiological responses of developing anthers to the LTMH are different from those to severe heat shock.

Endoplasmic Reticulum Stress and Reactive Oxygen Species in Plants

The endoplasmic reticulum (ER) is a key compartment responsible for protein processing and folding, and it also participates in many signal transduction and metabolic processes. Reactive oxygen species (ROS) are important signaling messengers involved in the redox equilibrium and stress response. A number of abiotic and biotic stresses can trigger the accumulation of unfolded or misfolded proteins and lead to ER stress. In recent years, a number of studies have reported that redox metabolism and ROS are closely related to ER stress. ER stress can benefit ROS generation and even cause oxidative burden in plants, finally leading to oxidative stress depending on the degree of ER stress. Moreover, ER stress activates nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-mediated ROS signaling, increases antioxidant defense mechanisms, and alters the glutathione (GSH) redox state. Meanwhile, the accumulation of ROS plays a special role in inducing the ER stress response. Given these factors, plants have evolved a series of complex regulatory mechanisms to interact with ROS in response to ER stress. In this review, we summarize the perceptions and responses of plant ER stress and oxidative protein folding in the ER. In addition, we analyze the production and signaling of ROS under ER stress in detail in order to provide a theoretical basis for reducing ER stress to improve the crop survival rate in agricultural applications.

Reproductive-Stage Heat Stress in Cereals: Impact, Plant Responses and Strategies for Tolerance Improvement

Reproductive-stage heat stress (RSHS) poses a major constraint to cereal crop production by damaging main plant reproductive structures and hampering reproductive processes, including pollen and stigma viability, pollination, fertilization, grain setting and grain filling. Despite this well-recognized fact, research on crop heat stress (HS) is relatively recent compared to other abiotic stresses, such as drought and salinity, and in particular, RSHS studies in cereals are considerably few in comparison with seedling-stage and vegetative-stage-centered studies. Meanwhile, climate change-exacerbated HS, independently or synergistically with drought, will have huge implications on crop performance and future global food security. Fortunately, due to their sedentary nature, crop plants have evolved complex and diverse transient and long-term mechanisms to perceive, transduce, respond and adapt to HS at the molecular, cell, physiological and whole plant levels. Therefore, uncovering the molecular and physiological mechanisms governing plant response and tolerance to RSHS facilitates the designing of effective strategies to improve HS tolerance in cereal crops. In this review, we update our understanding of several aspects of RSHS in cereals, particularly impacts on physiological processes and yield; HS signal perception and transduction; and transcriptional regulation by heat shock factors and heat stress-responsive genes. We also discuss the epigenetic, post-translational modification and HS memory mechanisms modulating plant HS tolerance. Moreover, we offer a critical set of strategies (encompassing genomics and plant breeding, transgenesis, omics and agronomy) that could accelerate the development of RSHS-resilient cereal crop cultivars. We underline that a judicious combination of all of these strategies offers the best foot forward in RSHS tolerance improvement in cereals. Further, we highlight critical shortcomings to RSHS tolerance investigations in cereals and propositions for their circumvention, as well as some knowledge gaps, which should guide future research priorities. Overall, our review furthers our understanding of HS tolerance in plants and supports the rational designing of RSHS-tolerant cereal crop cultivars for the warming climate.

Proteolytic Activation of Plant Membrane-Bound Transcription Factors

Due to the presence of a transmembrane domain, the subcellular mobility plan of membrane-bound or membrane-tethered transcription factors (MB-TFs) differs from that of their cytosolic counterparts. The MB-TFs are mostly locked in (sub)cellular membranes, until they are released by a proteolytic cleavage event or when the transmembrane domain (TMD) is omitted from the transcript due to alternative splicing. Here, we review the current knowledge on the proteolytic activation mechanisms of MB-TFs in plants, with a particular focus on regulated intramembrane proteolysis (RIP), and discuss the analogy with the proteolytic cleavage of MB-TFs in animal systems. We present a comprehensive inventory of all known and predicted MB-TFs in the model plant Arabidopsis thaliana and examine their experimentally determined or anticipated subcellular localizations and membrane topologies. We predict proteolytically activated MB-TFs by the mapping of protease recognition sequences and structural features that facilitate RIP in and around the TMD, based on data from metazoan intramembrane proteases. Finally, the MB-TF functions in plant responses to environmental stresses and in plant development are considered and novel functions for still uncharacterized MB-TFs are forecasted by means of a regulatory network-based approach.

Rapid Transcriptional Reprogramming Associated With Heat Stress-Induced Unfolded Protein Response in Developing Brassica napus Anthers

Climate change associated increases in the frequency and intensity of extreme temperature events negatively impact agricultural productivity and global food security. During the reproductive phase of a plant's life cycle, such high temperatures hinder pollen development, preventing fertilization, and seed formation. At the molecular level, heat stress-induced accumulation of misfolded proteins activates a signaling pathway called unfolded protein response (UPR) in the endoplasmic reticulum (ER) and the cytoplasm to enhance the protein folding capacity of the cell. Here, we report transcriptional responses of Brassica napus anthers exposed to high temperature for 5, 15, and 30 min to decipher the rapid transcriptional reprogramming associated with the unfolded protein response. Functional classification of the upregulated transcripts highlighted rapid activation of the ER-UPR signaling pathway mediated by ER membrane-anchored transcription factor within 5 min of heat stress exposure. KEGG pathway enrichment analysis also identified "Protein processing in ER" as the most significantly enriched pathway, indicating that the unfolded protein response (UPR) is an immediate heat stress-responsive pathway during B. napus anther development. Five minutes of heat stress also led to robust induction of the cytosolic HSF-HSP heat response network. Our results present a perspective of the rapid and massive transcriptional reprogramming during heat stress in pollen development and highlight the need for investigating the nature and function of very early stress-responsive networks in plant cells. Research focusing on very early molecular responses of plant cells to external stresses has the potential to reveal new stress-responsive gene networks that can be explored further for developing climate change resilient crops.

Cargo receptors and adaptors for selective autophagy in plant cells

Plant selective (macro)autophagy is a highly regulated process whereby eukaryotic cells spatiotemporally degrade some of their constituents that have become superfluous or harmful. The identification and characterization of the factors determining this selectivity make it possible to integrate selective (macro)autophagy into plant cell physiology and homeostasis. The specific cargo receptors and/or scaffold proteins involved in this pathway are generally not structurally conserved, as are the biochemical mechanisms underlying recognition and integration of a given cargo into the autophagosome in different cell types. This review discusses the few specific cargo receptors described in plant cells to highlight key features of selective autophagy in the plant kingdom and its integration with plant physiology, so as to identify evolutionary convergence and knowledge gaps to be filled by future research.

bZIP17 regulates heat stress tolerance at reproductive stage in Arabidopsis

High temperature elicits a well-conserved response called the unfolded protein response (UPR) to bring protein homeostasis in the endoplasmic reticulum (ER). Two key UPR regulators bZIP28 and bZIP60 have been shown to be essential for maintaining fertility under heat stress conditions in Arabidopsis, however, the function of transcriptional activator bZIP17, a paralog of bZIP28, in heat stress response at reproductive stage is not reported. Here we found that bzip17 mutant plants were sensitive to heat stress in terms of silique length and fertility comparing to that of wildtype (WT) Arabidopsis plants, and transcriptomic analysis showed that 1380 genes were specifically up-regulated and 493 genes were specifically down-regulated by heat stress in the flowers of WT plants comparing to that in bzip17 mutant plants. These bZIP17-dependent up-regulated genes were enriched in responses to abiotic stresses such as water deprivation and salt stress. Further chromatin immuno-precipitation coupled with high-throughput sequencing (ChIP-Seq) uncovered 1645 genes that were direct targets of bZIP17 in MYC-bZIP17 expressing seedlings subjected to heat stress. Among these 1645 genes, ERSE-II cis-element was enriched in the binding peaks of their promoters, and the up-regulation of 113 genes by heat stress in flowers was dependent on bZIP17. Our results revealed direct targets of bZIP17 in flowers during heat stress responses and demonstrated the important role of bZIP17 in maintaining fertility upon heat stress in plants.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-021-00062-1.

Functions of Redox Signaling in Pollen Development and Stress Response

Cellular redox homeostasis is crucial for normal plant growth and development. Each developmental stage of plants has a specific redox mode and is maintained by various environmental cues, oxidants, and antioxidants. Reactive oxygen species (ROS) and reactive nitrogen species are the chief oxidants in plant cells and participate in cell signal transduction and redox balance. The production and removal of oxidants are in a dynamic balance, which is necessary for plant growth. Especially during reproductive development, pollen development depends on ROS-mediated tapetal programmed cell death to provide nutrients and other essential substances. The deviation of the redox state in any period will lead to microspore abortion and pollen sterility. Meanwhile, pollens are highly sensitive to environmental stress, in particular to cell oxidative burst due to its peculiar structure and function. In this regard, plants have evolved a series of complex mechanisms to deal with redox imbalance and oxidative stress damage. This review summarizes the functions of the main redox components in different stages of pollen development, and highlights various redox protection mechanisms of pollen in response to environmental stimuli. In continuation, we also discuss the potential applications of plant growth regulators and antioxidants for improving pollen vigor and fertility in sustaining better agriculture practices.

Biological Parts for Engineering Abiotic Stress Tolerance in Plants

It is vital to ramp up crop production dramatically by 2050 due to the increasing global population and demand for food. However, with the climate change projections showing that droughts and heatwaves becoming common in much of the globe, there is a severe threat of a sharp decline in crop yields. Thus, developing crop varieties with inbuilt genetic tolerance to environmental stresses is urgently needed. Selective breeding based on genetic diversity is not keeping up with the growing demand for food and feed. However, the emergence of contemporary plant genetic engineering, genome-editing, and synthetic biology offer precise tools for developing crops that can sustain productivity under stress conditions. Here, we summarize the systems biology-level understanding of regulatory pathways involved in perception, signalling, and protective processes activated in response to unfavourable environmental conditions. The potential role of noncoding RNAs in the regulation of abiotic stress responses has also been highlighted. Further, examples of imparting abiotic stress tolerance by genetic engineering are discussed. Additionally, we provide perspectives on the rational design of abiotic stress tolerance through synthetic biology and list various bioparts that can be used to design synthetic gene circuits whose stress-protective functions can be switched on/off in response to environmental cues.

Holistic insights from pollen omics: co-opting stress-responsive genes and ER-mediated proteostasis for male fertility

Sexual reproduction in flowering plants takes place without an aqueous environment. Sperm are carried by pollen through air to reach the female gametophyte, though the molecular basis underlying the protective strategy of the male gametophyte is poorly understood. Here we compared the published transcriptomes of Arabidopsis thaliana pollen, and of heat-responsive genes, and uncovered insights into how mature pollen (MP) tolerates desiccation, while developing and germinating pollen are vulnerable to heat stress. Germinating pollen expresses molecular chaperones or "heat shock proteins" in the absence of heat stress. Furthermore, pollen tubes that grew through pistils at basal temperature showed induction of the endoplasmic reticulum (ER) stress response, which is a characteristic of stressed vegetative tissues. Recent studies show MP contains mRNA-protein (mRNP) aggregates that resemble "stress" granules triggered by heat or other stresses to protect cells. Based on these observations, we postulate that mRNP particles are formed in maturing pollen in response to developmentally programmed dehydration. Dry pollen can withstand harsh conditions as it is dispersed in air. We propose that, when pollen lands on a compatible pistil and hydrates, mRNAs stored in particles are released, aided by molecular chaperones, to become translationally active. Pollen responds to osmotic, mechanical, oxidative, and peptide cues that promote ER-mediated proteostasis and membrane trafficking for tube growth and sperm discharge. Unlike vegetative tissues, pollen depends on stress-protection strategies for its normal development and function. Thus, heat stress during reproduction likely triggers changes that interfere with the normal pollen responses, thereby compromising male fertility. This holistic perspective provides a framework to understand the basis of heat-tolerant strains in the reproduction of crops.

Circular RNAs Repertoire and Expression Profile during Brassica rapa Pollen Development

Circular RNAs (circRNAs) are covalently closed RNA molecules generated by the back-splicing of exons from linear precursor mRNAs. Though various linear RNAs have been shown to play important regulatory roles in many biological and developmental processes, little is known about the role of their circular counterparts. In this study, we performed high-throughput RNA sequencing to delineate the expression profile and potential function of circRNAs during the five stages of pollen development in Brassica rapa. A total of 1180 circRNAs were detected in pollen development, of which 367 showed stage-specific expression patterns. Functional enrichment and metabolic pathway analysis showed that the parent genes of circRNAs were mainly involved in pollen-related molecular and biological processes such as mitotic and meiotic cell division, DNA processes, protein synthesis, protein modification, and polysaccharide biosynthesis. Moreover, by predicting the circRNA–miRNA network from our differentially expressed circRNAs, we found 88 circRNAs with potential miRNA binding sites, suggesting their role in post-transcriptional regulation of the genes. Finally, we confirmed the back-splicing sites of nine selected circRNAs using divergent primers and Sanger sequencing. Our study presents the systematic analysis of circular RNAs during pollen development and forms the basis of future studies for unlocking complex gene regulatory networks underpinning reproduction in flowering plants.

Insights into the Mechanisms of Heat Priming and Thermotolerance in Tobacco Pollen

Global warming leads to a progressive rise in environmental temperature. Plants, as sessile organisms, are threatened by these changes; the male gametophyte is extremely sensitive to high temperature and its ability to preserve its physiological status under heat stress is known as acquired thermotolerance. This latter can be achieved by exposing plant to a sub-lethal temperature (priming) or to a progressive increase in temperature. The present research aims to investigate the effects of heat priming on the functioning of tobacco pollen grains. In addition to evaluating basic physiological parameters (e.g., pollen viability, germination and pollen tube length), several aspects related to a correct pollen functioning were considered. Calcium (Ca2+) level, reactive oxygen species (ROS) and related antioxidant systems were investigated, also to the organization of actin filaments and cytoskeletal protein such as tubulin (including tyrosinated and acetylated isoforms) and actin. We also focused on sucrose synthase (Sus), a key metabolic enzyme and on the content of main soluble sugars, including UDP-glucose. Results here obtained showed that a pre-exposure to sub-lethal temperatures can positively enhance pollen performance by altering its metabolism. This can have a considerable impact, especially from the point of view of breeding strategies aimed at improving crop species.

A Ratiometric Calcium Reporter CGf Reveals Calcium Dynamics Both in the Single Cell and Whole Plant Levels Under Heat Stress

Land plants evolved to quickly sense and adapt to temperature changes, such as hot days and cold nights. Given that calcium (Ca²⁺) signaling networks are implicated in most abiotic stress responses, heat-triggered changes in cytosolic Ca²⁺ were investigated in Arabidopsis leaves and pollen. Plants were engineered with a reporter called CGf, a ratiometric, genetically encoded Ca²⁺ reporter with an mCherry reference domain fused to an intensiometric Ca²⁺ reporter GCaMP6f. Relative changes in [Ca²⁺]_cyt were estimated based on CGf's apparent K _D around 220 nM. The ratiometric output provided an opportunity to compare Ca²⁺ dynamics between different tissues, cell types, or subcellular locations. In leaves, CGf detected heat-triggered cytosolic Ca²⁺ signals, comprised of three different signatures showing similarly rapid rates of Ca²⁺ influx followed by differing rates of efflux (50% durations ranging from 5 to 19 min). These heat-triggered Ca²⁺ signals were approximately 1.5-fold greater in magnitude than blue light-triggered signals in the same leaves. In contrast, growing pollen tubes showed two different heat-triggered responses. Exposure to heat caused tip-focused steady growth [Ca²⁺]_cyt oscillations to shift to a pattern characteristic of a growth arrest (22%), or an almost undetectable [Ca²⁺]_cyt (78%). Together, these contrasting examples of heat-triggered Ca²⁺ responses in leaves and pollen highlight the diversity of Ca²⁺ signals in plants, inviting speculations about their differing kinetic features and biological functions.