Yeast is widely used for industrial production of various types of products, such as ethanol and enzymes. However, its fermentation efficiency is strongly reduced by harmful environmental stresses. Specifically, harmful environmental stresses damage important cellular components, such as cell wall, cell membrane, proteins, etc. Then, these damages cause cellular metabolic disorders or even death. In the past decades, there has been a portfolio of studies on the environmental stress tolerance of yeasts, which mainly aimed at cell damages caused by different environmental stresses, different ways to improve yeast environmental stress tolerance or a tolerance mechanism for certain environmental stress. However, a comprehensive overview of how yeasts respond to environmental stresses is lacking, and the correlation of tolerance mechanism between different environmental stresses is unclear. In this review, we summarized the general damages induced by most of environmental stresses, the existing major mechanisms of environmental stress tolerance from the perspective of key signalling pathways, and the common ways to improve the resistance to environmental stresses in yeast cells. The tolerance mechanisms of yeast cells to different environmental stresses are diverse, but sometimes they share the same signalling pathway. Cells use sensors on the cell surface to recognize environmental stresses and transmit signals to the nucleus to cause changes in gene expression. By summarizing the main signalling pathways, including MAPK pathway, cAMP/PKA pathway, YAP1/SKN7 pathway, it will provide a powerful reference for future efforts to promote yeast environmental stress tolerance and study yeast tolerance mechanisms.
As an important cell factory, yeast has been widely used for industrial fermentation , but the fermentation efficiency of yeast is severely affected by the fermentation environment. During the fermentation process, yeast is exposed to various environmental stresses including high temperature, high osmolarity, acid, oxidative stress and so on. The damage in yeast cells under environmental stresses is depicted in Fig. 1. Environmental stresses mainly cause damages to cellular macromolecules, then induce protein denaturation, generate abnormal proteins, accumulate toxic proteins . The accumulation of denatured proteins can compose protein aggregation, then induce endoplasmic reticulum (ER) stress to influence normal protein production. Furthermore, the integrity of cell wall can be damaged by environmental stresses. The environmental stresses also might directly or indirectly cause cell membrane disorder and degrade vacuole membrane . In addition, environmental stresses can impose DNA lesions and even induce lethal DNA damages [4, 5].
Environmental stresses cause damages to yeast cells in different ways. Specifically, osmolarity stress leads to yeast cells shrinking and forming improper cell volume then causes a retard of cellular processes in yeast cells [6,7,8]. Acidity stress unexpectedly improves cellular anions level, degrades intracellular vacuoles, inhibits cell growth, and influences the formation of product [9, 10]. Under oxidative stress, excessive ROS production induces the peroxidation of several essential components, such as protein, lipid, and nucleic acids . High ambient temperature, especially during summer or in tropical countries, causes protein to improperly aggregates and reduces cellular enzymes activity, and it will be costly to maintain normal fermentation .
The mechanisms that yeast cells respond to environmental stresses have been deeply explored and summarized in reviews. Specifically, responses of yeast cells to multiple environmental stresses were analysed from the perspective of transcription factors and functional compounds [13,14,15]. With regard to high temperature, how the cells respond to heat stress is a matter of great concern to researchers . However, heat stress is also the main source of oxidative stress . Oxidative stress mainly induces the accumulation of harmful reactive oxygen species (ROS), so the source of ROS and cell response have received much attention . The ability of yeast cells to adapt to a high osmolarity environment is of great significance for alcoholic beverages and biofuels industries. The high osmolarity glycerol (HOG) pathway plays an important role in hyperosmotic stress tolerance [7, 19]. Yeast plays an important role in food industry may have to overcome acid stress, the remodelling of cell wall and cell membrane structure, regulated by transcription factors especially Haa1 . The production of a fuel alcohol or brewing industry rely on yeast, so ethanol tolerance has received much attention. A comprehensive review of ethanol stress tolerance mechanism is given that the cell membrane fluidity and protein folding, membrane lipid composition play important roles in resistance to ethanol [21,22,23]. How S. cerevisiae responds to toxic aldehyde compounds has been well summarized . Even an in-depth review of the methods using global regulators and cell surface properties to enhance stress tolerance is given by Kuroda et al. .
The response of yeasts to environmental stresses includes a complex system of signalling pathways. These signalling pathways allow the recognising of the external environmental changes and the signalling from the cell surface to the nucleus. Then, the environmental stress signals are transformed into alterations in gene expression. The functional factors synthesized under the control of these signalling pathways allow damaged cells to recover and avoid further damage. Summarizing the signalling pathways of yeast cells in response to environmental stresses is conducive to a full understanding of the cellular environmental stress response mechanism, and facilitates the subsequent rational improvement of the environmental stress tolerance of yeast cells.
At present, the mechanism of yeast environmental stress tolerance is still a long-standing puzzle. It is known that the response of yeast cells to environmental stresses is controlled by transcription regulators (TFs) . Specifically, transcription factors Msn2/4 play dominating roles in variable environmental stresses, such as osmotic stress, thermal stress, and high ethanol concentration . Transcription factors Hsf1 and Yap1 specifically mediate heat shock response and oxidative stress response, respectively [26, 27]. Many researchers have used omics analysis to study the tolerance mechanism of yeasts that are resistant to environmental stresses [28, 29]. By comparing the expression patterns of environmental stress-tolerant strains and environmental stress-sensitive strains, functional signalling pathways were identified. These results indicate that the tolerance mechanisms of different environmental stresses are diverse, but sometimes they share the same signalling pathway, which makes the mechanisms of environmental stress tolerance more complex. Therefore, this review aims to summarize key signalling pathways and attempts to figure out the main pathways that are resistant to various environmental stresses and the interactions between these pathways, which will provide a fundamental summary for future studies on yeast environmental stress tolerance.
Intracellular accumulation of denatured proteins and reactive oxygen species (ROS) that can be induced by most environmental stresses
Yeast cells can be provoked by environmental stresses during fermentation, which can cause the formation of denatured proteins. More seriously, misfolded proteins are prone to aggregate, leading to cell damage and affecting fermentation efficiency. Since all secreted and membrane proteins are synthesized and begin the process of maturation in the ER, the accumulation of misfolded proteins in the ER may induce ER stress. Then ER stress activates the unfolded protein response (UPR), endoplasmic reticulum-associated degradation (ERAD) pathway and stimulates the assembly of the pre-autophagosome structure, and triggers autophagy [30,31,32]. Maintaining protein homeostasis involves the degradation of harmful denatured proteins and the refolding of misfolded and damaged proteins  (Fig. 2). Induction of rapid function of multiple factors, including heat shock proteins (HSPs), ubiquitin–proteasome system (UPS), and autophagy act to maintain intracellular homeostasis in threatened yeast cells (Fig. 2). Different sizes of HSPs play different roles in yeast against environmental stress which have been well summarized in other reviews [34, 35]. Rapid clearance of denatured proteins can be achieved by those efficient functional factors. Because of the similar function, increased ubiquitin-dependent degradation can substitute the vital requirement for HSP induction .
In general, environmental stresses may cause the accumulation of excess ROS in cells (Fig. 1). Forrester et al.  well summarized the ROS production in compartments, such as the cytoplasm, mitochondria, peroxisome, and endoplasmic reticulum, in mammalian cells. The ROS production sites summarized in that review are also applicable to yeast. Deregulated ROS generation results in DNA damage, even cause cells the induction of apoptosis in yeast cells. These two main consequences, denatured proteins, and ROS, caused by environmental stress tightly interact. ER stress is prolonged or acute, cell death may occur, accompanied by the generation of mitochondrial ROS . In turn, ROS induces the production of misfolded proteins leading to ER stress .
Heat shock response (HSR) and oxidative stress response (OSR) are two main cellular responses against environmental stresses in yeasts. The HSR and OSR in S. cerevisiae have been well summarized in . HSR mainly regulates the biosynthesis of HSPs to eliminate stress-induced denatured proteins. OSR mainly mediates the biosynthesis of antioxidants to scavenge cellular ROS and eliminate oxidative stress. Especially, in our previous work, we found that high temperature induced OSR and HSR simultaneously in a heat-sensitive Pichia pastoris strain .
Signalling pathways associated with environmental stress tolerance of yeast
An adverse fermentation environment may disturb yeast cells, thereby activating signalling pathways to maintain cell homeostasis. Signalling pathways including MAPK pathway , cAMP/PKA pathway , YAP1/SKN7 pathway , were depicted in Fig. 3. The MAPK pathways have three parts including the HOG pathway, the CWI pathway, and the Fus3/Kss1 pathway . Other than the Fus3/Kss1 pathway, the HOG pathway and the CWI pathway are responsible for the response of yeast to environmental stresses . The HOG pathway  rapidly regulates the synthesis of glycerol to maintain cell osmotic pressure balance, while the CWI pathway responds to repair cell wall. The cAMP/PKA pathway, as an upstream regulatory pathway, can negatively regulate the transcript factor Msn2/4 in the environmental stress response (ESR) mechanism . The YAP1/SKN7 pathway is primarily responsible for regulating intracellular oxidative stress responses.
Intriguingly, through intricate hierarchical regulatory networks, yeast can quickly respond to environmental changes (Fig. 3). These pathways dramatically interact with each other. Specifically, the sensors of the CWI pathway sense environmental stresses and repress the cAMP/PKA pathway simultaneously . Nevertheless, the cAMP/PKA pathway negatively regulates the YAP1/SKN7 pathway . Rho1 as the upstream regulator of the CWI pathway can control YAP1/SKN7 pathway and then help yeast cells resist oxidative stress . Furthermore, Ypd1 in the HOG pathway can also regulate the YAP1/SKN7 pathway that Ypd1 transfers its phosphate to aspartyl residues within the receiver domains Skn7, which activates Skn7 . The functional factors involved in signalling pathways that respond to environmental stresses were depicted in Table 1.
Environmental stresses (e.g., high osmolarity), induce plasma membrane stretch and then activate the HOG pathway . The HOG pathway is composed of membrane-associated sensors , cytoplasmic and nuclear effectors  (Table 2). Furthermore, any single osmosensor is enough to improve osmoadaptation . A comprehensive review of the HOG pathway is given in Tong et al. . The Hog1 shuttles between the cytoplasm and nucleus to activate the transcription factors Msn2/4 , then activate the expression of glycerol phosphate dehydrogenase 1 Gpd1, leading to glycerol accumulation . Then the accumulation of glycerol rapidly balances the intracellular and extracellular osmotic pressure. Conversely, knocking out the Hog1 gene causes yeast cells to be sensitive to NaCl, sorbitol, and high temperatures . Cells often suffer from multiple environmental stresses at the same time. For example, ethanol may cause high osmotic stress in yeast cells and enhances the production of ROS in mitochondria simultaneously . In addition, the activation of the HOG pathway may induce other protection mechanisms. For instance, Hog1 plays an important role in acid stress by activating the antioxidant defence system . Msn2/4 mediating environmental stress responses are crucial TFs downstream of the HOG pathway. The HOG pathway regulates the synthesis of glycerol, which plays an essential role in maintaining an osmotic equilibrium with the external environment.
By adjusting the percentage of membrane compounds, yeasts effectively improve environmental stress tolerance. Sterols are important membrane compounds of yeast cells. Under the hyperosmolarity condition, yeast cells considerably lower their sterol by involving the HOG pathway . The Hog1 activates the transcription repressor Mot3 and Rox1 to shut down transcription of sterol biosynthesis genes ERG2 and ERG11 under osmotic stress. In addition, by adjusting the proportion of the sterol compositions, the thermotolerance of yeasts may be enhanced .
In view of the role of the HOG pathway under osmotic stress, its main regulator Ypd1 is localized and expressed in the plasma membrane and nucleus to effectively improve the signal transduction efficiency of this pathway . The activation of the HOG pathway under high osmotic stress is conducive to cell growth, while the activation of the HOG pathway under non-stress conditions may cause growth defects on yeast cells . Hence, a robust network structure of the Sln1-Ypd1-Ssk1 three-component phospho-relay with a large excess pool of Ypd1 avoids unexpected activation of the HOG pathway in S. cerevisiae .
Morphological and structural properties of the cell wall are important factors affecting the yeast environmental stress tolerance. The expression of cell wall biogenesis and remodelling genes is mediated by the CWI pathway . When the cell is challenged by environmental stresses, the CWI pathway is activated . Commonly, environmental stresses cause cell wall damage including high temperature, ethanol stress , and toxic chemical compounds. The environmental stress signal is systematically conveyed to Rlm1 transcription factor, then the cell wall remodelling starts. Cell wall remodelling includes changes in the proportion of cell wall compositions and changes in cross-linking in these compositions .
When the yeast cells are subjected to certain environmental stresses, their cell wall components also change. Upon ethanol stress, the expression of cell wall-remodelling proteins including β-1,3-glucan synthase, chitin trans-glycosylase, and O-glycosylated cell wall protein is induced by the CWI pathway . Schiavone et al.  found that heat shock can cause an increase in chitin content, a decrease in β-1,3-glucan content, and an increase in β-1,6-glucan content. Conversely, other researchers have found that the putative glycosylphosphatidylinositol-linked aspartyl protease gene mutant of P. pastoris shows an increased osmotic tolerance, while the chitin content in the cell wall is reduced, the β-1,3-glucan content is increased, and the inner cell wall is thicker . Interestingly, the phenomenon that the inner cell wall thickens happened in a lager yeast which exhibited a multiple stress tolerance . Fascinatingly, in our research on P. pastoris, it was also found that the environmental stress tolerance was increased, and the inner cell wall became thicker . Interestingly, the CWI pathway is not the only way to maintain cell wall integrity. The HOG pathway and the invasive growth pathway are also required to cope with the cell wall damage depending on the nature of the environmental stresses [51, 76]. It is noteworthy that Cdc19 encoding pyruvate kinase plays a crucial role in the viability of yeast at high temperatures . A change in pyruvate kinase activity may influence glycolytic flux which triggers an increase in ATP metabolism. Afterward, the process of cell wall remodelling can be accelerated. In addition, the yeast O-Man glycoproteome plays a major role in maintaining yeast cell wall integrity by activating the CWI pathway [78, 79].
The Rho1/Pkc1 pathway is the upstream regulating pathway of MAPK pathway , blocks polarized cell growth, and repairs the environmental stress-induced wounds by activating the CWI pathways . Rho1 targets effectors including Pkc1, Skn7, and the glucan synthase which directly impact the biogenesis of cell wall . Pkc1 including phospholipid-dependent protein kinases and serine/threonine-specific protein kinases, responds to extracellular signals . Loss of Pkc1 function may cause cell lysis at elevated temperatures due to a deficiency in cell wall construction that both the inner and outer layers of cell wall are thinner .
To modulate protein homeostasis, ubiquitin–proteasome system (UPS) is one of the critical pathways for protein degradation in eukaryotic cells . Similarly, the phosphorylation–de-phosphorylation of Ubc1 encoding ubiquitin-conjugating enzymes was necessary for thermotolerance in the absence of either the HOG or CWI pathways , which means the HOG pathway and the CWI pathway have overlap functions with the UPS. In contradiction of anticipations, the UPS is not capable of compensating for a deficiency in autophagy . Interestingly, both ubiquitin and HSPs are responsible for maintaining protein homeostasis. However, ubiquitin and HSPs have an obscure connection. As ubiquitin expression increases, the demand for HSP decreases, resulting in restoration in HSP synthesis .
Overexpression of Pkc1 activated CWI pathway that expression levels of Slt2 and Rlm1 upregulated [85, 86]. Overexpression of a hyperactive allele of MKK1 can also induce the activation of CWI pathway by increasing the expression of Slt2 and Rlm1 [85, 87]. Rlm1 mediates positive autoregulatory transcriptional feedback by binding at the promoter of Slt2 and Rlm1 . Therefore, overexpression of Rlm1 may mediate the activation of CWI pathway.
In yeast, the global regulators cAMP and PKA play central roles in metabolism regulation, environmental stress resistance, and cell cycle progression . It targets transcription factors Msn2/4 , which govern vital stress response signalling pathways. Ras2p is a small GTP binding protein and homologous to the mammalian Ras protooncogenes . Cdc25 encodes a membrane-bound guanine nucleotide exchange factor, which activates Ras1 and Ras2, is a major regulator of cAMP signalling . The cAMP negatively regulates the Msn2/4 . When the level of intracellular cAMP is low, the Msn2/4 can be activated, and then yeast cells are able to fight against heat stress . Adenylate cyclase Cyr1 synthesizes cAMP, then regulates the cAMP/PKA pathway [88, 92]. It is worth mentioning that cell sensor Wsc1 activates the CWI pathway and parallel inhibits the PKA signalling .
The PKA signalling pathway is of vital significance in response to environmental stresses. In yeast cells, thioredoxins partner with peroxiredoxin in H2O2 signalling . Oxidized thioredoxins constrain the PKA pathway, in part through inhibiting the nuclear holding of the PKA catalytic subunits, allowing Msn2 to converge in the nucleus . Negative feedback regulation periodically renovates PKA activity, causing Msn2 to exit the nucleus . Upon calcium stress, lacking the function of PKA resulted in an elevated expression of transcriptional regulator Prz1 which causes a CaCl2-sensitive phenotype . Inhibiting cAMP/PKA helps yeast resist environmental stresses. The major cytosolic peroxiredoxin, Tsa1 is required for both improving tolerance to H2O2 and prolonging lifespan under caloric restriction by inhibiting the cAMP/PKA pathway .
The YAP1/SKN7 pathway mainly involves the regulation of antioxidant genes to oxidative stress . Moreover, Yap1 and Skn7 as significant ROS response signalling factors also have been proved to participate in DNA double-strand break repair . The YAP1/SKN7 pathway could be provoked by environmental stresses causing ER stress, but in the absence of oxygen, the damages will be repaired . When Yap1 gene was knockout, cells failed to survive under oxidative stress . Overexpression of Yap1 enhances the resistance of yeast cells to diverse toxic compounds . Moreover, overexpression of Yap1 enhanced resistance to lignocellulose-derived fermentation inhibitors in S. cerevisiae . Most YAP1-regulated genes are classified in the numerous functional categories of redox metabolism, amino acid metabolism, stress response, DNA repair, and co-regulate with chaperones [99, 100].
As an important regulatory factor, Skn7 regulates multi-functional responses in fungi and other organisms, and assists cells to resist oxidative stress and maintain cell wall integrity in yeast . Evidence proved that the mutants lacking Skn7 were sensitive to H2O2 . Besides, Skn7 is also governed by Rho1, sensor Sln1, and response factor Ypd1 , which then plays an important role in cell wall biogenesis when yeast cells undergo hypo-osmolarity stress .
Environmental stresses always induce the generation of cellular ROS and then cause oxidative stress on cells. The antioxidant defence system plays a very important role in cell response to environmental stresses. Antioxidant GSH is the most abundant thiol and serves as a protectant in eukaryotic cells in redox unbalanced conditions . Yap gene family members mediate numerous genes in a wide range of functional categories . Interestingly, the concentration of GSH is found to be higher in Yap1 over-expression yeast , revealing that GSH is one of the downstream factors of Yap gene family members. In addition, peroxiredoxin is one of the activators of the oxidant-sensing transcription factor Yap1 and other stress-responsive transcription factors, such as Msn2 and Skn7 [105, 106]. The maintenance of the thioredoxin system is of great significance that too many oxidized thioredoxins will cause UPR and then diminish cell growth rate .
SODs are responsible for reducing the cellular ROS level. Specifically, when superoxide anions (O2·−) are generated as coproducts of the mitochondrial electron transport chain, they are changed into hydrogen peroxide (H2O2) by SODs . Overexpression of SOD1 in P. pastoris reduced the ROS level caused by methanol exposure . H2O2 is then transformed into water and oxygen by catalase, glutathione peroxidase, and thioredoxin peroxidase, to prevent the formation of oxidant hydroxyl radicals (∙OH) . Previously, the Δsod1 mutant lacking cytosolic Cu/Zn-SOD has been shown to have a higher O2·− level under various environmental stresses conditions . Additionally, the repression of SOD1 and SOD2 causes yeast cells sensitive to ethanol and heat stress . Moreover, SOD2 activates oxidative response regulator Yap1 , so when SOD1 and SOD2 are knocked out, the ability to survive under oxidative impulse is severely suppressed . In addition, the inactivation of the Yap1 cancelled the activation of SOD and catalase .
Evident correlation among functional factors
Many cellular functional compounds are helpful to the environmental stress tolerance of yeast. In addition to the end-products of the signalling pathways, such as glycerol, cell wall components, and YAP1 pathway-regulated antioxidants (Table 1). The integrity of cell membranes has also attracted much attention. For example, Yang et al.  found that S. cerevisiae strain with increasing content of unsaturated fatty acid (UFA) and cell membrane fluidity can tolerate high concentrations (up to 25 vol%) of ethanol. Researchers have found that the increasing content of some amino acids helps yeast to resist environmental stresses. For instance, Wang et al.  proved that overexpressing key gene PRO1 encoding proline improved the tolerance of S. cerevisiae to furfural, acetic acid, and phenol stress. Trehalose is a vital functional compound being proved to help yeasts to resist various environmental stresses, such as desiccation , freezing , heat stress , etc. The pentose phosphate pathway  is the main source of NADPH, an important cofactor for cellular ROS elimination.
These compounds can be biosynthesized in yeast cells. Generally, the increase in concentration of one functional compound might correspondingly reduce the synthesis of the other compound with a similar function, unless the sole improvement of one compound is not enough to meet the demand for stress resistance. Interestingly, we found that the synthesis of these compounds is interrelated (Fig. 4). Understanding the synthesis of stress-resistant compounds in yeast cells can help improve the stress tolerance of yeast through a combination of genetic engineering and metabolic engineering.
How to improve the environmental stress tolerance of yeasts?
Resistance to environmental stresses varies from strain to strain, also relates to the certain type of environmental stresses and the duration of exposure to environmental stresses conditions. To improve yeast fermentation ability, several methods have been previously employed to improve the environmental stress tolerance of yeasts (Table 3). Since environmental stress tolerance mechanism is still obscure, the majority of the methods depend on random mutagenesis, such as adaptive laboratory evolution , UV mutagenesis  (Table 3). A new genome shuffling technique with CRISPR system was used to realize environmental stress tolerance by genome evolution under environmental stress conditions . Increasing the ploidy of yeast is also an option for researchers to improve yeast tolerance .
Regarding rational methods, the principle of improving cell tolerance mainly relies on the existing understanding of the mechanism of yeast cell tolerance. Therefore, rational mutation of key genes and increasing the content of functional factors are usually used as effective means to improve the environmental stress tolerance of yeasts (Table 3). Based on the understanding of the yeast stress response mechanism, the researchers might use many rational methods to improve the stress tolerance of yeast. Regrettably, strains exhibiting multi-tolerance simultaneously have not been developed yet. In addition to the way cells are modified based on the cellular environmental stress response mechanism, the direct addition of functional compounds through exogenous sources can also improve the strain’s environmental stress tolerance to a certain extent.
During the fermentation process, yeast cells inevitably encounter many environmental stresses that seriously affect the fermentation efficiency. Improving the environmental stress tolerance of yeasts is beneficial to the smooth completion of the fermentation. In ethanol fermentation, the yeast cells are exposed to high concentrations of ethanol, which are toxic and hinder the further fermentation process of yeast cells . Irrational strain breeding methods are very common that a superior S. cerevisiae strain F23 was acquired using strategies of ethanol domestication, UV mutagenesis, and protoplast fusion . Thanks to the ethanol tolerant yeast obtained irrationally, researchers can study its tolerance mechanism through omics methods, etc. Understanding the environmental stress tolerance mechanism of yeast is one of the foundations for rationally constructing tolerant yeast. By validating the function of OLE1 in the variability of membrane fluidity in strain F23, OLE1-overexpressed transformant successfully resists ethanol stress . In addition, fermentation inhibitors involved in lignocellulose hydrolysates are unavoidable problems to produce biofuels and biochemicals by yeast cells. Kim et al. constructed robust S. cerevisiae with improved furan derivatives and acetic acid tolerance through modulation of spermidine contents by metabolic engineering method .
Conclusion and perspectives
Concerning sustainable development, environmental stress-tolerant yeasts are extremely vital for industrial fermentation. Yeast cells may encounter multiple environmental stresses at the same time. Mechanisms required for one environmental stress tolerance may also be responsible for other environmental stress tolerance. At present, a lot of articles have well summarized the damages caused by environmental stresses. However, the understanding of the tactics that yeast cells respond to environmental stresses is still incomplete. We summarized from the perspectives of signalling pathways and their cross-talk mechanism to figure out the cell response in yeasts under environmental stresses. Additionally, we summarized the functional compounds and their inter-correlations in response to environmental stresses. This review provided a general awareness and understanding of how yeast cells respond to environmental stresses. Besides, it has initially sorted out the associations between the various environmental stress tolerance mechanisms and the associations between functional factors in yeast. It provides a reference for improving the environmental stress tolerance of yeast from the perspective of overall regulation.
Based on the summary of the mechanisms of environmental stress tolerance in yeast, we would like to give some advice to other researchers. When studying several environmental stresses simultaneously, you can start with the cell wall integrity pathway and changes in cell membrane compositions. Also, HSPs and UPS can be the focus of research since almost all kinds of environmental stresses can lead to misfolding and aggregation of intracellular proteins. When studying osmotic tolerance, attention can be paid to the synthesis of the HOG pathway and the accumulation of intracellular glycerol. Under the conditions of oxidative stress, attention should be paid to the YAP1/SKN7 pathway and the antioxidant defence system. In addition to focusing on intracellular trehalose content, studies on the mechanism of thermotolerance in yeast can refer to the anti-oxidative stress mechanism in most cases. The cellular level of amino acid can be changed to acquire an environmental stress-tolerant yeast.
The environmental stress tolerance mechanism of yeast is mainly based on two aspects: cells resist damages caused by environmental stresses and cell self-repairing after being damaged. This understanding provides ideas for rational transformation to improve yeast stress tolerance. Through the continuous understanding of the cellular environmental stress tolerance mechanism, more and more key genes and key pathways have been unearthed. With the development of gene editing technology, more and more researchers use CRISPR system for genome editing in yeast. Lian et al.  described a combinatorial metabolic engineering strategy that combines transcriptional activation, transcriptional interference, and gene deletion in S. cerevisiae. In the future, through CRISPR system, it may be possible to simultaneously knock out unfavourable genes, activate favourable genes, and inhibit genes suitable for low-dose expression in yeast to achieve the environmental stress tolerance of yeast.
Despite several signalling pathways that promote environmental stress tolerance in yeasts have been demonstrated, the inter-relationship of these pathways is not clear enough, and there may be some key links that have not been found. In the future, researches on the environmental stress tolerance of yeasts need to be carried out in terms of finding more functional pathways and modifying existing metabolic pathways, then trying to resolve the completed environmental stress tolerance mechanism.
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This work is supported by the National Key Research and Development Program of China [grant number 2021YFC2100203]; the National Natural Science Foundation of China [grant number 32072162]; the Postgraduate Research & Practice Innovation Program of Jiangsu Province [grant number KYCX18_1791], and the National First-Class Discipline Program of Light Industry Technology and Engineering [grant number LITE2018-09].
Conflict of interest
The authors declare that they have no conflict of interest.
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Lin, NX., Xu, Y. & Yu, XW. Overview of yeast environmental stress response pathways and the development of tolerant yeasts. Syst Microbiol and Biomanuf (2021). https://doi.org/10.1007/s43393-021-00058-4
- Environmental stress
- Signalling pathway
- Protein aggregation