Identification and Characterization of the Heat-Induced Plastidial Stress Granules Reveal New Insight Into Arabidopsis Stress Response

A New Phase for WNK Kinase Signaling Complexes as Biomolecular Condensates

The purpose of this review is to highlight transformative advances that have been made in the field of biomolecular condensates, with special emphasis on condensate material properties, physiology, and kinases, using the With-No-Lysine (WNK) kinases as a prototypical example. To convey how WNK kinases illustrate important concepts for biomolecular condensates, we start with a brief history, focus on defining features of biomolecular condensates, and delve into some examples of how condensates are implicated in cellular physiology (and pathophysiology). We then highlight how WNK kinases, through the action of "WNK droplets" that ubiquitously regulate intracellular volume and kidney-specific "WNK bodies" that are implicated in distal tubule salt reabsorption and potassium homeostasis, exemplify many of the defining features of condensates. Finally, this review addresses the controversies within this emerging field and questions to address.

Establishments of G3BP1-GFP stress granule monitoring system for real-time stress assessment in human neuroblastoma cells

Acute stress caused by short-term exposure to deleterious chemicals can induce the aggregation of RNA-binding proteins (RBPs) in the cytosol and the formation of stress granules (SGs). The cytoplasmic RBP, Ras GTPase-activating protein-binding protein 1 (G3BP1) is a critical organizer of SG, and its aggregation is considered a hallmark of cellular stress. However, assembly of SG is a highly dynamic process that involves RBPs; hence, existing methods based on fixation processes or overexpression of RBPs exhibit limited efficacy in detecting the assembly of SG under stress conditions. In this study, we established a G3BP1- Green fluorescent protein (GFP) reporter protein in a human neuroblastoma cell line to overcome these limitations. GFP was introduced into the G3BP1 genomic sequence via homologous recombination to generate a G3BP1-GFP fusion protein and further analyze the aggregation processes. We validated the assembly of SG under stress conditions using the G3BP1-GFP reporter system. Additionally, this system supported the evaluation of bisphenol A-induced SG response in the established human neuroblastoma cell line. In conclusion, the established G3BP1-GFP reporter system enables us to monitor the assembly of the SG complex in a human neuroblastoma cell line in real time and can serve as an efficient tool for assessing potential neurotoxicity associated with short-term exposure to chemicals.

Thermal adaptation in plants: understanding the dynamics of translation factors and condensates

Plants, as sessile organisms, face the crucial challenge of adjusting growth and development with ever-changing environmental conditions. Protein synthesis is the fundamental process that enables growth of all organisms. Since elevated temperature presents a substantial threat to protein stability and function, immediate adjustments of protein synthesis rates are necessary to circumvent accumulation of proteotoxic stress and to ensure survival. This review provides an overview of the mechanisms that control translation under high-temperature stress by the modification of components of the translation machinery in plants, and compares them to yeast and metazoa. Recent research also suggests an important role for cytoplasmic biomolecular condensates, named stress granules, in these processes. Current understanding of the role of stress granules in translational regulation and of the molecular processes associated with translation that might occur within stress granules is also discussed.

Proteomic composition of eukaryotic and bacterial RNA decay condensates suggests convergent evolution

Bacterial cells have a unique challenge to organize their cytoplasm without the use of membrane-bound organelles. Biomolecular condensates (henceforth BMCs) are a class of nonmembrane-bound organelles, which, through the physical process of phase separation, can form liquid-like droplets with proteins/nucleic acids. BMCs have been broadly characterized in eukaryotic cells, and BMCs have been recently identified in bacteria, with the first and best studied example being bacterial ribonucleoprotein bodies (BR-bodies). BR-bodies contain the RNA decay machinery and show functional parallels to eukaryotic P-bodies (PBs) and stress granules (SGs). Due to the finding that mRNA decay machinery is compartmentalized in BR-bodies and in eukaryotic PBs/SGs, we will explore the functional similarities in the proteins, which are known to enrich in these structures based on recent proteomic studies. Interestingly, despite the use of different mRNA decay and post-transcriptional regulatory machinery, this analysis has revealed evolutionary convergence in the classes of enriched enzymes in these structures.

Composition and function of stress granules and P-bodies in plants

Stress Granules (SGs) and Processing-bodies (P-bodies) are biomolecular condensates formed in the cell with the highly conserved purpose of maintaining balance between storage, translation, and degradation of mRNA. This balance is particularly important when cells are exposed to different environmental conditions and adjustments have to be made in order for plants to respond to and tolerate stressful conditions. While P-bodies are constitutively present in the cell, SG formation is a stress-induced event. Typically thought of as protein-RNA aggregates, SGs and P-bodies are formed by a process called liquid-liquid phase separation (LLPS), and both their function and composition are very dynamic. Both foci are known to contain proteins involved in translation, protein folding, and ATPase activity, alluding to their roles in regulating mRNA and protein expression levels. From an RNA perspective, SGs and P-bodies primarily consist of mRNAs, though long non-coding RNAs (lncRNAs) have also been observed, and more focus is now being placed on the specific RNAs associated with these aggregates. Recently, metabolites such as nucleotides and amino acids have been reported in purified plant SGs with implications for the energetic dynamics of these condensates. Thus, even though the field of plant SGs and P-bodies is relatively nascent, significant progress has been made in understanding their composition and biological role in stress responses. In this review, we discuss the most recent discoveries centered around SG and P-body function and composition in plants.

Liquid-liquid phase separation in plants: Advances and perspectives from model species to crops

Membraneless biomolecular condensates play important roles in both normal biological activities and responses to environmental stimuli in living organisms. Liquid‒liquid phase separation (LLPS) is an organizational mechanism that has emerged in recent years to explain the formation of biomolecular condensates. In the past decade, advances in LLPS research have contributed to breakthroughs in disease fields. By contrast, although LLPS research in plants has progressed over the past 5 years, it has been concentrated on the model plant Arabidopsis, which has limited relevance to agricultural production. In this review, we provide an overview of recently reported advances in LLPS in plants, with a particular focus on photomorphogenesis, flowering, and abiotic and biotic stress responses. We propose that many potential LLPS proteins also exist in crops and may affect crop growth, development, and stress resistance. This possibility presents a great challenge as well as an opportunity for rigorous scientific research on the biological functions and applications of LLPS in crops.

Multifaceted functions of histone deacetylases in stress response

Histone deacetylases (HDACs) are important chromatin regulators essential for plant tolerance to adverse environments. In addition to histone deacetylation and epigenetic regulation, HDACs deacetylate non-histone proteins and thereby regulate multiple pathways. Like other post-translational modifications (PTMs), acetylation/deacetylation is a reversible switch regulating different cellular processes in plants. Here, by focusing on results obtained in arabidopsis (Arabidopsis thaliana) and rice plants, we analyze the different aspects of HDAC functions and the underlying regulatory mechanisms in modulating plant responses to stress. We hypothesize that, in addition to epigenetic regulation of gene expression, HDACs can also control plant tolerance to stress by regulating transcription, translation, and metabolic activities and possibly assembly-disassembly of stress granules (SGs) through lysine deacetylation of non-histone proteins.

Stress-related biomolecular condensates in plants

Biomolecular condensates are membraneless organelle-like structures that can concentrate molecules and often form through liquid-liquid phase separation. Biomolecular condensate assembly is tightly regulated by developmental and environmental cues. Although research on biomolecular condensates has intensified in the past 10 years, our current understanding of the molecular mechanisms and components underlying their formation remains in its infancy, especially in plants. However, recent studies have shown that the formation of biomolecular condensates may be central to plant acclimation to stress conditions. Here, we describe the mechanism, regulation, and properties of stress-related condensates in plants, focusing on stress granules and processing bodies, 2 of the most well-characterized biomolecular condensates. In this regard, we showcase the proteomes of stress granules and processing bodies in an attempt to suggest methods for elucidating the composition and function of biomolecular condensates. Finally, we discuss how biomolecular condensates modulate stress responses and how they might be used as targets for biotechnological efforts to improve stress tolerance.

Involvement of small molecules and metabolites in regulation of biomolecular condensate properties

Biomolecular condensate (BMCs) formation facilitates the grouping of molecules, including proteins, nucleic acids, and small molecules, creating specific microenvironments with particular functions. They are often assembled through liquid-liquid phase separation (LLPS), a phenomenon that arises when specific proteins, nucleic acids, and small molecules demix from the aqueous environment into another phase with different physiochemical properties. BMCs assemble and disassemble in response to external and internal stimuli such as temperature, molecule concentration, ionic strength, pH, and cellular redox state. Likewise, the nature of the regulatory stimuli may affect the lifespan, morphology, and content of BMCs. In humans, compelling evidence points to the critical role of BMCs in diseases. By contrast, the link between BMC formation, stress resistance, and cell survival has not been revealed in plants. Recent studies have pointed out the nascent roles of small molecules in the assembly and dynamics of BMCs; however, this is still an emerging field of study. This review briefly highlights the most significant efforts to identify the molecular mechanisms between small molecules and BMC formation and regulation in plants and other organisms. We then discuss (i) how small molecules exert control over the BMC assembly and dynamics in plants and (ii) how small molecules can influence the formation and material properties of plant BMCs. Finally, we propose novel alternatives that might help to understand the relationship between chemicals and condensation dynamics and their possible application to plant biotechnology.

Identification of genetic and biochemical mechanisms associated with heat shock and heat stress adaptation in grain amaranths

Heat stress is poised to become a major factor negatively affecting plant performance worldwide. In terms of world food security, increased ambient temperatures are poised to reduce yields in cereals and other economically important crops. Grain amaranths are known to be productive under poor and/or unfavorable growing conditions that significantly affect cereals and other crops. Several physiological and biochemical attributes have been recognized to contribute to this favorable property, including a high water-use efficiency and the activation of a carbon starvation response. This study reports the behavior of the three grain amaranth species to two different stress conditions: short-term exposure to heat shock (HS) conditions using young plants kept in a conditioned growth chamber or long-term cultivation under severe heat stress in greenhouse conditions. The latter involved exposing grain amaranth plants to daylight temperatures that hovered around 50°C, or above, for at least 4 h during the day and to higher than normal nocturnal temperatures for a complete growth cycle in the summer of 2022 in central Mexico. All grain amaranth species showed a high tolerance to HS, demonstrated by a high percentage of recovery after their return to optimal growing conditions. The tolerance observed coincided with increased expression levels of unknown function genes previously shown to be induced by other (a)biotic stress conditions. Included among them were genes coding for RNA-binding and RNA-editing proteins, respectively. HS tolerance was also in accordance with favorable changes in several biochemical parameters usually induced in plants in response to abiotic stresses. Conversely, exposure to a prolonged severe heat stress seriously affected the vegetative and reproductive development of all three grain amaranth species, which yielded little or no seed. The latter data suggested that the usually stress-tolerant grain amaranths are unable to overcome severe heat stress-related damage leading to reproductive failure.

The Karrikin Receptor Karrikin Insensitive2 Positively Regulates Heat Stress Tolerance in Arabidopsis thaliana

In this study, we investigated the potential role of the karrikin receptor KARRIKIN INSENSITIVE2 (KAI2) in the response of Arabidopsis seedlings to high-temperature stress. We performed phenotypic, physiological and transcriptome analyses of Arabidopsis kai2 mutants and wild-type (WT) plants under control (kai2_C and WT_C, respectively) and 6- and 24-h heat stress conditions (kai2_H6, kai2_H24, WT_H6 and WT_H24, respectively) to understand the basis for KAI2-regulated heat stress tolerance. We discovered that the kai2 mutants exhibited hypersensitivity to high-temperature stress relative to WT plants, which might be associated with a more highly increased leaf surface temperature and cell membrane damage in kai2 mutant plants. Next, we performed comparative transcriptome analysis of kai2_C, kai2_H6, kai2_H24, WT_C, WT_H6 and WT_H24 to identify transcriptome differences between WT and kai2 mutants in response to heat stress. K-mean clustering of normalized gene expression separated the investigated genotypes into three clusters based on heat-treated and non-treated control conditions. Within each cluster, the kai2 mutants were separated from WT plants, implying that kai2 mutants exhibited distinct transcriptome profiles relative to WT plants. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses showed a repression in 'misfolded protein binding', 'heat shock protein binding', 'unfolded protein binding' and 'protein processing in endoplasmic reticulum' pathways, which was consistent with the downregulation of several genes encoding heat shock proteins and heat shock transcription factors in the kai2 mutant versus WT plants under control and heat stress conditions. Our findings suggest that chemical or genetic manipulation of KAI2 signaling may provide a novel way to improve heat tolerance in plants.

Coupling VIGS with Short- and Long-Term Stress Exposure to Understand the Fiskeby III Iron Deficiency Stress Response

Yield loss due to abiotic stress is an increasing problem in agriculture. Soybean is a major crop for the upper Midwestern United States and calcareous soils exacerbate iron deficiency for growers, resulting in substantial yield losses. Fiskeby III is a soybean variety uniquely resistant to a variety of abiotic stresses, including iron deficiency. Previous studies identified a MATE transporter (Glyma.05G001700) associated with iron stress tolerance in Fiskeby III. To understand the function of this gene in the Fiskeby III response to iron deficiency, we coupled its silencing using virus-induced gene silencing with RNAseq analyses at two timepoints. Analyses of these data confirm a role for the MATE transporter in Fiskeby III iron stress responses. Further, they reveal that Fiskeby III induces transcriptional reprogramming within 24 h of iron deficiency stress, confirming that like other soybean varieties, Fiskeby III is able to quickly respond to stress. However, Fiskeby III utilizes novel genes and pathways in its iron deficiency response. Identifying and characterizing these genes and pathways in Fiskeby III provides novel targets for improving abiotic stress tolerance in elite soybean lines.

Landscape of biomolecular condensates in heat stress responses

High temperature is one of the abiotic stresses that plants face and acts as a major constraint on crop production and food security. Plants have evolved several mechanisms to overcome challenging environments and respond to internal and external stimuli. One significant mechanism is the formation of biomolecular condensates driven by liquid-liquid phase separation. Biomolecular condensates have received much attention in the past decade, especially with regard to how plants perceive temperature fluctuations and their involvement in stress response and tolerance. In this review, we compile and discuss examples of plant biomolecular condensates regarding their composition, localization, and functions triggered by exposure to heat. Bioinformatic tools can be exploited to predict heat-induced biomolecular condensates. As the field of biomolecular condensates has emerged in the study of plants, many intriguing questions have arisen that have yet to be solved. Increased knowledge of biomolecular condensates will help in securing crop production and overcoming limitations caused by heat stress.

A proteostasis network safeguards the chloroplast proteome

Several protein homeostasis (proteostasis) pathways safeguard the integrity of thousands of proteins that localize in plant chloroplasts, the indispensable organelles that perform photosynthesis, produce metabolites, and sense environmental stimuli. In this review, we discuss the latest efforts directed to define the molecular process by which proteins are imported and sorted into the chloroplast. Moreover, we describe the recently elucidated protein folding and degradation pathways that modulate the levels and activities of chloroplast proteins. We also discuss the links between the accumulation of misfolded proteins and the activation of signalling pathways that cope with folding stress within the organelle. Finally, we propose new research directions that would help to elucidate novel molecular mechanisms to maintain chloroplast proteostasis.

How do plants feel the heat and survive?

Climate change is increasingly affecting the quality of life of organisms on Earth. More frequent, extreme, and lengthy heat waves are contributing to the sixth mass extinction of complex life forms in the Earth's history. From an anthropocentric point of view, global warming is a major threat to human health because it also compromises crop yields and food security. Thus, achieving agricultural productivity under climate change calls for closer examination of the molecular mechanisms of heat-stress resistance in model and crop plants. This requires a better understanding of the mechanisms by which plant cells can sense rising temperatures and establish effective molecular defenses, such as molecular chaperones and thermoprotective metabolites, as reviewed here, to survive extreme diurnal variations in temperature and seasonal heat waves.

A New Approach to the Study of Plastidial Stress Granules: The Integrated Use of Arabidopsis thaliana and Chlamydomonas reinhardtii as Model Organisms

The field of stress granules (SGs) has recently emerged in the study of the plant stress response, yet these structures, their dynamics and importance remain poorly characterized. There is currently a gap in our understanding of the physiological function of SGs during stress. Since there are only a few studies addressing SGs in planta, which are primarily focused on cytoplasmic SGs. The recent observation of SG-like foci in the chloroplast (cpSGs) of Arabidopsis thaliana opened even more questions regarding the role of these subcellular features. In this opinion article, we review the current knowledge of cpSGs and propose a workflow for the joint use of the long-established model organisms Chlamydomonas reinhardtii and A. thaliana to accelerate the evaluation of individual plant cpSGs components and their impact on stress responses. Finally, we present a short outlook and what we believe are the significant gaps that need to be addressed in the following years.

2',3'-cAMP treatment mimics the stress molecular response in Arabidopsis thaliana

The role of the RNA degradation product 2',3'-cyclic adenosine monophosphate (2',3'-cAMP) is poorly understood. Recent studies have identified 2',3'-cAMP in plant material and determined its role in stress signaling. The level of 2',3'-cAMP increases upon wounding, in the dark, and under heat, and 2',3'-cAMP binding to an RNA-binding protein, Rbp47b, promotes stress granule (SG) assembly. To gain further mechanistic insights into the function of 2',3'-cAMP, we used a multi-omics approach by combining transcriptomics, metabolomics, and proteomics to dissect the response of Arabidopsis (Arabidopsis thaliana) to 2',3'-cAMP treatment. We demonstrated that 2',3'-cAMP is metabolized into adenosine, suggesting that the well-known cyclic nucleotide-adenosine pathway of human cells might also exist in plants. Transcriptomics analysis revealed only minor overlap between 2',3'-cAMP- and adenosine-treated plants, suggesting that these molecules act through independent mechanisms. Treatment with 2',3'-cAMP changed the levels of hundreds of transcripts, proteins, and metabolites, many previously associated with plant stress responses, including protein and RNA degradation products, glucosinolates, chaperones, and SG components. Finally, we demonstrated that 2',3'-cAMP treatment influences the movement of processing bodies, confirming the role of 2',3'-cAMP in the formation and motility of membraneless organelles.

Hydrogen peroxide-induced stress acclimation in plants

Among all reactive oxygen species (ROS), hydrogen peroxide (H2O2) takes a central role in regulating plant development and responses to the environment. The diverse role of H2O2 is achieved through its compartmentalized synthesis, temporal control exerted by the antioxidant machinery, and ability to oxidize specific residues of target proteins. Here, we examine the role of H2O2 in stress acclimation beyond the well-studied transcriptional reprogramming, modulation of plant hormonal networks and long-distance signalling waves by highlighting its global impact on the transcriptional regulation and translational machinery.

Heat stress reveals a specialized variant of the pachytene checkpoint in meiosis of Arabidopsis thaliana

Plant growth and fertility strongly depend on environmental conditions such as temperature. Remarkably, temperature also influences meiotic recombination and thus, the current climate change will affect the genetic make-up of plants. To better understand the effects of temperature on meiosis, we followed male meiocytes in Arabidopsis thaliana by live cell imaging under three temperature regimes: at 21°C; at heat shock conditions of 30°C and 34°C; after an acclimatization phase of 1 week at 30°C. This work led to a cytological framework of meiotic progression at elevated temperature. We determined that an increase from 21°C to 30°C speeds up meiosis with specific phases being more amenable to heat than others. An acclimatization phase often moderated this effect. A sudden increase to 34°C promoted a faster progression of early prophase compared to 21°C. However, the phase in which cross-overs mature was prolonged at 34°C. Since mutants involved in the recombination pathway largely did not show the extension of this phase at 34°C, we conclude that the delay is recombination-dependent. Further analysis also revealed the involvement of the ATAXIA TELANGIECTASIA MUTATED kinase in this prolongation, indicating the existence of a pachytene checkpoint in plants, yet in a specialized form.

Heat Shock Signaling in Land Plants: From Plasma Membrane Sensing to the Transcription of Small Heat Shock Proteins

Heat stress events are major factors limiting crop productivity. During summer days, land plants must anticipate in a timely manner upcoming mild and severe temperature. They respond by accumulating protective heat-shock proteins (HSPs), conferring acquired thermotolerance. All organisms synthetize HSPs; many of which are members of the conserved chaperones families. This review describes recent advances in plant temperature sensing, signaling, and response. We highlight the pathway from heat perception by the plasma membrane through calcium channels, such as cyclic nucleotide-gated channels, to the activation of the heat-shock transcription factors (HSFs). An unclear cellular signal activates HSFs, which act as essential regulators. In particular, the HSFA subfamily can bind heat shock elements in HSP promoters and could mediate the dissociation of bound histones, leading to HSPs transcription. Although plants can modulate their transcriptome, proteome, and metabolome to protect the cellular machinery, HSP chaperones prevent, use, and revert the formation of misfolded proteins, thereby avoiding heat-induced cell death. Remarkably, the HSP20 family is mostly tightly repressed at low temperature, suggesting that a costly mechanism can become detrimental under unnecessary conditions. Here, the role of HSP20s in response to HS and their possible deleterious expression at non-HS temperatures is discussed.

Plant Stress Granules: Trends and Beyond

Stress granules (SGs) are dynamic membrane-less condensates transiently assembled through liquid-liquid phase separation (LLPS) in response to stress. SGs display a biphasic architecture constituted of core and shell phases. The core is a conserved SG fraction fundamental for its assembly and consists primarily of proteins with intrinsically disordered regions and RNA-binding domains, along with translational-related proteins. The shell fraction contains specific SG components that differ among species, cell type, and developmental stage and might include metabolic enzymes, receptors, transcription factors, untranslated mRNAs, and small molecules. SGs assembly positively correlates with stalled translation associated with stress responses playing a pivotal role during the adaptive cellular response, post-stress recovery, signaling, and metabolic rewire. After stress, SG disassembly releases mRNA and proteins to the cytoplasm to reactivate translation and reassume cell growth and development. However, under severe stress conditions or aberrant cellular behavior, SG dynamics are severely disturbed, affecting cellular homeostasis and leading to cell death in the most critical cases. The majority of research on SGs has focused on yeast and mammals as model organism. Nevertheless, the study of plant SGs has attracted attention in the last few years. Genetics studies and adapted techniques from other non-plant models, such as affinity capture coupled with multi-omics analyses, have enriched our understanding of SG composition in plants. Despite these efforts, the investigation of plant SGs is still an emerging field in plant biology research. In this review, we compile and discuss the accumulated progress of plant SGs regarding their composition, organization, dynamics, regulation, and their relation to other cytoplasmic foci. Lastly, we will explore the possible connections among the most exciting findings of SGs from mammalian, yeast, and plants, which might help provide a complete view of the biology of plant SGs in the future.

Hot topic: Thermosensing in plants

Plants alter their morphology and cellular homeostasis to promote resilience under a variety of heat regimes. Molecular processes that underlie these responses have been intensively studied and found to encompass diverse mechanisms operating across a broad range of cellular components, timescales and temperatures. This review explores recent progress throughout this landscape with a particular focus on thermosensing in the model plant Arabidopsis. Direct temperature sensors include the photosensors phytochrome B and phototropin, the clock component ELF3 and an RNA switch. In addition, there are heat-regulated processes mediated by ion channels, lipids and lipid-modifying enzymes, taking place at the plasma membrane and the chloroplast. In some cases, the mechanism of temperature perception is well understood but in others, this remains an open question. Potential novel thermosensing mechanisms are based on lipid and liquid-liquid phase separation. Finally, future research directions of high temperature perception and signalling pathways are discussed.

Deciphering the Proteotoxic Stress Responses Triggered by the Perturbed Thylakoid Proteostasis in Arabidopsis

Here, we explored heat dependent thylakoid FtsH protease substrates and investigated proteotoxicity induced by thermal damage and processive protease dysfunction on the thylakoid membrane. Through our thylakoid enriched proteome analysis and biochemical experiments, carbonylated stromal proteins were suggested as possible FtsH targets. Furthermore, we observed in the thylakoid fractions in the absence of FtsH stromal reactive oxygen species-detoxifying enzymes, as well as heat shock proteins and chaperones, which are known to be upregulated at the transcriptional level when this protease is absent, which is called the damaged protein response, resembling unfolded protein response in eukaryotic cells. Interestingly, the thylakoid-enriched high-density fractions included stromal translation factors and RNA-binding proteins, along with aminoacyl-tRNA synthetase, reminiscent of the formation of stress granules. Unexpectedly, extraplastid proteins such as mitochondrial chaperones, peroxidase, tricarboxylic acid cycle and respiratory chain enzymes, as well as cytosolic ribosomes, translation factors, heat shock proteins, antioxidants and metabolic enzymes, were also found deposited in the high-density fractions depending on the loss of thylakoid FtsH, with more prominent effects of thermal stress on the cytosolic proteins. This may reflect intracellular adaptation to the proteotoxic influences from the organelle.