Autophagy is required for tolerance of drought and salt stress in plants

Autophagy is a protein degradation process in which cells recycle cytoplasmic contents when subjected to environmental stress conditions or during certain stages of development. Upon the induction of autophagy, a double membrane autophagosome forms around cytoplasmic components and delivers them to the vacuole or lysosome for degradation. In plants, autophagy has been shown previously to be induced during abiotic stresses including nutrient starvation and oxidative stress. In this paper, we demonstrate the induction of autophagy in high salt and osmotic stress conditions, concomitant with the upregulation of expression of an Arabidopsis thaliana autophagy-related gene AtATG18a. Autophagy-defective RNAi-AtATG18a plants are more sensitive to salt and drought conditions than wild-type plants, demonstrating a role for autophagy in the response to these stresses. NADPH oxidase inhibitors block autophagy induction upon nutrient starvation and salt stress, but not during osmotic stress, indicating that autophagy can be activated by NADPH oxidase-dependent or -independent pathways. Together our results indicate that diverse environmental stresses can induce autophagy and that autophagy is regulated by distinct signaling pathways in different conditions.     

Cytokinin action in response to abiotic and biotic stresses in plants

The phytohormone cytokinin was originally discovered as a regulator of cell division. Later, it was described to be involved in regulating numerous processes in plant growth and development including meristem activity, tissue patterning, and organ size. More recently, diverse functions for cytokinin in the response to abiotic and biotic stresses have been reported. Cytokinin is required for the defence against high light stress and to protect plants from a novel type of abiotic stress caused by an altered photoperiod. Additionally, cytokinin has a role in the response to temperature, drought, osmotic, salt, and nutrient stress. Similarly, the full response to certain plant pathogens and herbivores requires a functional cytokinin signalling pathway. Conversely, different types of stress impact cytokinin homeostasis. The diverse functions of cytokinin in responses to stress and crosstalk with other hormones are described. Its emerging roles as a priming agent and as a regulator of growth‐defence trade‐offs are discussed.     

High osmolarity glycerol (HOG) pathway-induced phosphorylation and activation of 6-phosphofructo-2-kinase are essential for glycerol accumulation and yeast cell proliferation under hyperosmotic stress

In response to changes in the environment, yeast cells coordinate intracellular activities to optimize survival and proliferation. The transductions of diverse extracellular stimuli are exerted through multiple mitogen-activated protein kinase (MAPK) cascades. The high osmolarity glycerol (HOG) MAPK pathway is activated by increased environmental osmolarity and results in a rise of the cellular glycerol concentration to adapt the intracellular osmotic pressure. We studied the importance of the short time regulation of glycolysis under hyperosmotic stress for the survival and proliferation of yeast cells. A stimulation of the HOG-MAPK pathway by increasing the medium osmolarity through addition of salt or glucose to cultivated yeast leads to an activation of 6-phosphofructo-2-kinase (PFK2), which is accompanied by a complex phosphorylation pattern of the enzyme. An increase in medium osmolarity with 5% NaCl activates PFK2 3-fold over the initial value. This change in the activity is the result of a 4-fold phosphorylation of the enzyme mediated by protein kinases from the HOG-MAPK pathway. In the case of hyperosmolar glucose a 5-fold PFK2 activation was achieved by a single phosphorylation with protein kinase A near the carboxyl terminus of the protein on Ser(644) and an additional 5-fold phosphorylation within the same amino-terminal fragment as in the presence of salt. The effect of hyperosmolar glucose is the result of an activation of the Ras-cAMP pathway together with the HOG-MAPK pathway. The activation of PFK2 leads to an activation of the upper part of glycolysis, which is a precondition for glycerol accumulation. Yeast cells containing PFK2 accumulate three times more glycerol than cells lacking PFK2, which are not able to grow under hypertonic stress.     

Mechanosensitive channels protect plastids from hypoosmotic stress during normal plant growth

Cellular response to osmotic stress is critical for survival and involves volume control through the regulated transport of osmolytes. Organelles may respond similarly to abrupt changes in cytoplasmic osmolarity. The plastids of the Arabidopsis thaliana leaf epidermis provide a model system for the study of organellar response to osmotic stress within the context of the cell. An Arabidopsis mutant lacking two plastid-localized homologs of the bacteria mechanosensitive channel MscS (MscS-like [MSL] 2 and 3) exhibits large round epidermal plastids that lack dynamic extensions known as stromules. This phenotype is present under normal growth conditions and does not require exposure to extracellular osmotic stress. Here we show that increasing cytoplasmic osmolarity through a genetic lesion known to produce elevated levels of soluble sugars, exogenously providing osmolytes in the growth media, or withholding water rescues the msl2-1 msl3-1 leaf epidermal plastid phenotype, producing plastids that resemble the wild-type in shape and size. Furthermore, the epidermal plastids in msl2-1 msl3-1 leaves undergo rapid and reversible volume and shape changes in response to extracellular hypertonic or hypotonic challenges. We conclude that plastids are under hypoosmotic stress during normal plant growth and dynamic response to this stress requires MSL2 and MSL3.     

The response regulator BcSkn7 is required for vegetative differentiation and adaptation to oxidative and osmotic stresses in Botrytis cinerea

The high‐osmolarity glycerol pathway plays an important role in the responses of fungi to various environmental stresses. Saccharomyces cerevisiae Skn7 is a response regulator in the high‐osmolarity glycerol pathway, which regulates the oxidative stress response, cell cycle and cell wall biosynthesis. In this study, we characterized an Skn7 orthologue BcSkn7 in Botrytis cinerea. BcSKN7 can partly restore the growth defects of S. cerevisiae SKN7 mutant and vice versa. The BcSKN7 mutant (ΔBcSkn7‐1) revealed increased sensitivity to ionic osmotic and oxidative stresses and to ergosterol biosynthesis inhibitors. In addition, ΔBcSkn7‐1 was also impaired dramatically in conidiation and sclerotial formation. Western blot analysis showed that BcSkn7 positively regulated the phosphorylation of BcSak1 (the orthologue of S. cerevisiae Hog1) under osmotic stress, indicating that BcSkn7 is associated with the high‐osmolarity glycerol pathway in B. cinerea. In contrast with BcSak1, BcSkn7 is not involved in the regulation of B. cinerea virulence. All of the phenotypic defects of ΔBcSkn7‐1 are restored by genetic complementation of the mutant with the wild‐type BcSKN7. The results of this study indicate that BcSkn7 plays an important role in the regulation of vegetative differentiation and in the response to various stresses in B. cinerea.     

Cell-type-specific calcium responses to drought, salt and cold in the Arabidopsis root

Little is known about the signalling processes involved in the response of roots to abiotic stresses. The Arabidopsis root is a model system of root anatomy with a simple architecture and is amenable to genetic manipulation. Although it is known that the root responds to cold, drought and salt stress with increases in cytoplasmic free calcium, there is currently no information about the role(s) of the functionally diverse cell types that comprise the root. Transgenic Arabidopsis with enhancer-trapped GAL4 expression in specific cell types was used to target the calcium reporting protein, aequorin, fused to a modified yellow fluorescent protein (YFP). The luminescence output of targeted aequorin enabled in vivo measurement of changes in cytosolic free calcium concentrations ([Ca2+]cyt) in specific cell types during acute cold, osmotic and salt stresses. In response to an acute cold stress, all cell types tested as well as plants constitutively expressing aequorin displayed rapid [Ca2+]cyt peaks. However, there were significant quantitative differences between different cell types in terms of their response to cold stress, osmotic stress (440 mM mannitol) and salt stress (220 mM NaCl), implying specific roles for certain cell types in the detection and/or response to these stimuli. In response to osmotic and salt stress, the endodermis and pericycle displayed prolonged oscillations in cytosolic calcium that were distinct from the responses of the other cell types tested. Targeted expression of aequorin circumvented the technical difficulties involved in fluorescent dye injection as well as the lack of cell specificity of constitutively expressed aequorin, and revealed a new level of complexity in root calcium signalling.     

Phosphorylation of the zinc finger transcriptional regulator ZAT6 by MPK6 regulates Arabidopsis seed germination under salt and osmotic stress

C₂H₂-type zinc finger proteins (ZFPs) play diverse roles in plant response to abiotic stresses. ZAT6, an Arabidopsis C₂H₂-type ZFP, has been reported to regulate root development and nutrient stress responses. However, its roles in regulation of abiotic stress response are incompletely known. Here, we demonstrate that salt or osmotic stress triggers a strong increase in ZAT6 expression in leaves. Transgenic plants overexpressing ZAT6 showed improved seed germination under salt and osmotic stress. Intriguingly, ZAT6 interacts with a stress-responsive mitogen-activated protein kinase MPK6 in vitro and in planta. ZAT6 is phosphorylated by both recombinant and plant endogenous MPK6. Serine 8 and serine 223 in ZAT6 were identified as the sites phosphorylated by MPK6. In contrast to wild-type form of ZAT6, overexpression of phosphorylation mutant form did not display significantly enhanced salt and osmotic stress tolerance. Altogether, our results suggest that phosphorylation by MPK6 is required for the functional role of ZAT6 in seed germination under salt and osmotic stress.     

Differential regulation of microRNAs in response to osmotic,salt and cold stresses in wheat

MicroRNAs (miRNAs) are tiny non-coding regulatory molecules that modulate plant’s gene expression either by cleaving or repressing their mRNA targets. To unravel the plant actions in response to various environmental factors, identification of stress related miRNAs is essential. For understanding the regulatory behaviour of various abiotic stresses and miRNAs in wheat genotype C-306, we examined expression profile of selected conserved miRNAs viz. miR159, miR164, miR168, miR172, miR393, miR397, miR529 and miR1029 tangled in adapting osmotic, salt and cold stresses. The investigation revealed that two miRNAs (miR168, miR397) were down-regulated and miR172 was up-regulated under all the stress conditions. However, miR164 and miR1029 were up-regulated under cold and osmotic stresses in contrast to salt stress. miR529 responded to cold alone and does not change under osmotic and salt stress. miR393 showed up-regulation under osmotic and salt, and down-regulation under cold stress indicating auxin based differential cold response. Variation in expression level of studied miRNAs in presence of target genes delivers a likely elucidation of miRNAs based abiotic stress regulation. In addition, we reported new stress induced miRNAs Ta-miR855 using computational approach. Results revealed first documentation that miR855 is regulated by salinity stress in wheat. These findings indicate that diverse miRNAs were responsive to osmotic, salt and cold stress and could function in wheat response to abiotic stresses.     

环境胁迫下HOG-MAPK途径对真菌毒素形成的调控机制北大核心CSCD

真菌为了适应在生长侵染食品、饲料等农产品的过程中所面临的各种环境胁迫的考验,包括热胁迫、氧化胁迫、渗透压胁迫、紫外胁迫等,进化出一套高渗透性甘油促分裂原活化蛋白激酶(high osmolarity glycerol mitogen-activated protein kinase,HOG-MAPK)途径。该途径对真菌的生长发育、真菌毒素的产生和致病性都具有重要影响。HOG-MAPK途径共有两个分支,其中SLN1分支相比另一分支(SHO1分支)具有较为敏感的渗透压胁迫感应能力,能在高渗压和高盐浓度下进行渗透压胁迫反应。SHO1分支参与多种信号感应传导,比如氧化胁迫、热胁迫等。本文综述了真菌HOG-MAPK途径中关键基因sln1、sho1、ste11、ssk2、pbs2和hog1在应对渗透压胁迫、氧化胁迫等不同环境胁迫时所发挥的功能,说明HOG-MAPK途径可以响应多种环境信号,并参与调控黄曲霉、赭曲霉等致病真菌的生长和黄曲霉毒素(aflatoxin)、赭曲霉毒素(ochratoxin)等真菌毒素的产生。在不同环境胁迫下,HOG-MAPK途径对真菌毒素调控机制的研究可为食品和饲料等农产品真菌毒素的防控提供理论基础和指导方向。     

Identification of a novel gene family involved in osmotic stress response in Caenorhabditis elegans

Organisms exposed to the damaging effects of high osmolarity accumulate solutes to increase cytoplasmic osmolarity. Yeast accumulates glycerol in response to osmotic stress, activated primarily by MAP kinase Hog1 signaling. A pathway regulated by protein kinase C (PKC1) also responds to changes in osmolarity and cell wall integrity. C. elegans accumulates glycerol when exposed to high osmolarity, but the molecular pathways responsible for this are not well understood. We report the identification of two genes, osm-7 and osm-11, which are related members of a novel gene family. Mutations in either gene lead to high internal levels of glycerol and cause an osmotic resistance phenotype (Osr). These mutants also have an altered defecation rhythm (Dec). Mutations in cuticle collagen genes dpy-2, dpy-7, and dpy-10 cause a similar Osr Dec phenotype. osm-7 is expressed in the hypodermis and may be secreted. We hypothesize that osm-7 and osm-11 interact with the cuticle, and disruption of the cuticle causes activation of signaling pathways that increase glycerol production. The phenotypes of osm-7 are not suppressed by mutations in MAP kinase or PKC pathways, suggesting that C. elegans uses signaling pathways different from yeast to mount a response to osmotic stress.     

An overlap between osmotic and anaerobic stress responses: a potential role for DNA supercoiling in the coordinate regulation of gene expression

The regulation of several genes in response to osmotic and anaerobic stress has been examined. We have demonstrated a clear overlap between these two regulatory signals. Thus, the osmotically induced proU and ompC genes require anaerobic growth for optimum induction while the anaerobically induced tppB gene is also regulated by osmolarity. Furthermore, normal expression of tppB and ompC requires the positive regulatory protein OmpR, yet this requirement can be partially, or even fully, overcome by altering the growth conditions. Finally, the pleiotropic, anaerobic regulatory locus, oxrC, is also shown to affect expression of the osmotically regulated proU gene. The oxrC mutation is shown to affect the level of negative supercoiling of plasmid DNA and its effects on gene expression can be explained as secondary consequences of altered DNA topology. We suggest that there is a class of 'stress-regulated' genes that are regulated by a common mechanism in response to different environmental signals. Furthermore, our data are consistent with the notion that this regulatory overlap is mediated by changes in DNA supercoiling in response to these environmental stresses.     

A Quantitative Study of the Hog1 MAPK Response to Fluctuating Osmotic Stress in Saccharomyces cerevisiae

Background

Yeast cells live in a highly fluctuating environment with respect to temperature, nutrients, and especially osmolarity. The Hog1 mitogen-activated protein kinase (MAPK) pathway is crucial for the adaption of yeast cells to external osmotic changes.

Methodology/Principal Findings

To better understand the osmo-adaption mechanism in the budding yeast Saccharomyces cerevisiae, we have developed a mathematical model and quantitatively investigated the Hog1 response to osmotic stress. The model agrees well with various experimental data for the Hog1 response to different types of osmotic changes. Kinetic analyses of the model indicate that budding yeast cells have evolved to protect themselves economically: while they show almost no response to fast pulse-like changes of osmolarity, they respond periodically and are well-adapted to osmotic changes with a certain frequency. To quantify the signal transduction efficiency of the osmo-adaption network, we introduced a measure of the signal response gain, which is defined as the ratio of output change integral to input (signal) change integral. Model simulations indicate that the Hog1 response gain shows bell-shaped response curves with respect to the duration of a single osmotic pulse and to the frequency of periodic square osmotic pulses, while for up-staircase (ramp) osmotic changes, the gain depends on the slope.

Conclusions/Significance

The model analyses suggest that budding yeast cells have selectively evolved to be optimized to some specific types of osmotic changes. In addition, our work implies that the signaling output can be dynamically controlled by fine-tuning the signal input profiles.     

A history of stress alters drought calcium signalling pathways in Arabidopsis

Environmental stresses commonly encountered by plants lead to rapid transient elevations in cytosolic free calcium concentration ([Ca2+]cyt) (Bush, 1995; Knight et al., 1991). These cellular calcium (Ca2+) signals lead ultimately to the increased expression of stress-responsive genes, including those encoding proteins of protective function (Knight et al., 1996; Knight et al., 1997). The kinetics and magnitude of the Ca2+ signal, or 'calcium signature', differ between different stimuli and are thought to contribute to the specificity of the end response (Dolmetsch et al., 1997; McAinsh and Hetherington, 1998). We measured [Ca2+]cyt changes during treatment with mannitol (to mimic drought stress) in whole intact seedlings of Arabidopsis thaliana. The responses of plants which were previously exposed to osmotic and oxidative stresses were compared to those of control plants. We show here that osmotic stress-induced Ca2+ responses can be markedly altered by previous encounters with either osmotic or oxidative stress. The nature of the alterations in Ca2+ response depends on the identity and severity of the previous stress: oxidative stress pre-treatment reduced the mannitol-induced [Ca2+]cyt response whereas osmotic stress pretreatment increased the [Ca2+]cyt response. Therefore, our data show that different combinations of environmental stress can produce novel Ca2+ signal outputs. These alterations are accompanied by corresponding changes in the patterns of osmotic stress-induced gene expression and, in the case of osmotic stress pre-treatment, the acquisition of stress-tolerance. This suggests that altered Ca2+ responses encode a 'memory' of previous stress encounters and thus may perhaps be involved in acclimation to environmental stresses.     

Compatible solute addition to biological systems treating waste/wastewater to counteract osmotic and other environmental stresses: a review

This study reviews the addition of compatible solutes to biological systems as a strategy to counteract osmolarity and other environmental stresses. At high osmolarity many microorganisms accumulate organic solutes called “compatible solutes” in order to balance osmotic pressure between the cytoplasm and the environment. These organic compounds are called compatible solutes because they can function inside the cell without the need for special adaptation of the intracellular enzymes, and also serve as protein stabilizers in the presence of high ionic strength. Moreover, the compatible solutes strategy is regularly being employed by the cell, not only under osmotic stress at high salinity, but also under other extreme environmental conditions such as low temperature, freezing, heat, starvation, dryness, recalcitrant compounds and solvent stresses. The accumulation of these solutes from the environment has energetically a lower cost than de novo synthesis. Based on this cell mechanism several studies in the field of environmental biotechnology (most of them on biological wastewater treatment) employed this strategy by exogenously adding compatible solutes to the wastewater or medium in order to alleviate environmental stress. This current paper critically reviews and evaluates these studies, and examines the future potential of this approach. In addition to this, a strategy for the successful implementation of compatible solutes in biological systems is proposed.     

Spm1, a stress-activated MAP kinase that regulates morphogenesis in S.pombe.

A gene encoding a novel MAP kinase family member, Spm1, was isolated from the fission yeast Schizosaccharomyces pombe. Overproduction of Spm1 inhibits proliferation. Disruption of the spm1+ gene interferes with cell separation and morphogenesis. Under conditions of nutrient limitation, hypertonic stress or elevated temperature, spm1 delta cells grow as short branched filaments in which the cell walls and septa are thickened, suggesting defects in polarized growth and cell wall remodeling. At high osmolarity, spm1 delta cells fail to form colonies. The Spm1 protein is tyrosine phosphorylated and activated in response to osmotic and heat stress, consistent with a role for Spm1 in adaptation to these conditions. Two other S.pombe MAP kinases are known, Spk1, required for sexual differentiation and sporulation, and Spc1/Sty1/Phh1, which is activated in hypertonic conditions. However, the distinctive features of the spm1 delta mutant phenotype and direct biochemical assays suggest that Spm1 does not lie on other known MAP kinase pathways. Our results demonstrate the existence of a new MAP kinase pathway that regulates cell wall remodeling and cytokinesis in response to environmental stresses.     

Ca2+ signaling in plant responses to abiotic stresses

Adverse variations of abiotic environmental cues that deviate from an optimal range impose stresses to plants. Abiotic stresses severely impede plant physiology and development. Consequently, such stresses dramatically reduce crop yield and negatively impact on ecosystem stability and composition. Physical components of abiotic stresses can be, for example, suboptimal temperature and osmotic perturbations, while representative chemical facets of abiotic stresses can be toxic ions or suboptimal nutrient availability. The sheer complexity of abiotic stresses causes a multitude of diverse components and mechanisms for their sensing and signal transduction. Ca²⁺, as a versatile second messenger, plays multifaceted roles in almost all abiotic stress responses in that, for a certain abiotic stress, Ca²⁺ is not only reciprocally connected with its perception, but also multifunctionally ensures subsequent signal transduction. Here, we will focus on salt/osmotic stress and responses to altered nutrient availability as model cases to detail novel insights into the identity of components that link stress perception to Ca²⁺ signal formation as well as on new insights into mechanisms of Ca²⁺ signal implementation. Finally, we will deduce emerging conceptual consequences of these novel insights and outline arising avenues of future research on the role of Ca²⁺ signaling in abiotic stress responses in plants.     

Effect of osmotic stress on live cell plasma membranes,probed via Laurdan general polarization measurements

Here we seek to gain insight into changes in the plasma membrane of live cells upon the application of osmotic stress using Laurdan, a fluorescent probe that reports on membrane organization, hydration, and dynamics. It is known that the application of osmotic stress to lipid vesicles causes a decrease in Laurdan’s generalized polarization (GP), which has been interpreted as an indication of membrane stretching. In cells, we see the opposite effects, as GP increases when the osmolarity of the solution is decreased. This increase in GP is associated with the presence of caveolae, which are known to disassemble and flatten in response to osmotic stress.