Regulation of Plant Responses to Heat and Drought Stress

A special issue of Plants (ISSN 2223-7747). This special issue belongs to the section "Plant Response to Abiotic Stress and Climate Change".

Deadline for manuscript submissions: closed (31 July 2023) | Viewed by 30133

Special Issue Editors

Agronomy College, Qingdao Agricultural University, Qingdao 266109, China
Interests: plant science; abiotic stress; heat stress; maize; RNA alternative splicing
Special Issues, Collections and Topics in MDPI journals

E-Mail Website
Guest Editor
1. USDA-ARS Plant Science Research Unit, Raleigh, NC 27607, USA
2. Crop and Soil Sciences, North Carolina State University, Raleigh, NC 27695, USA
Interests: environmental stress; ozone; plant physiology and biochemistry; heat stress; soybeans; wheat
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear colleagues,

High temperatures and drought stresses disturb cellular homeostasis and impede growth and development in plants. Extensive agricultural losses are attributed to heat stress or drought stress, often in combination or with other stresses. Plants have evolved a variety of responses to heat and drought stress to minimize damage and to protect themselves from further stress. Plant scientists and breeders are challenged to understand how plants, especially crop plants, can better tolerate heat and drought stress. The responses of plants to heat and drought stress have been extensively studied, but the means and mechanisms to confer tolerance are less well understood. This Special Issue of Plants will highlight the regulatory mechanisms of plant response to heat and drought stress, including but not limited to transcriptional and post-transcriptional regulation, metabolism adjustment, and physiological processes that improve the heat and drought stress tolerance of plants.

Dr. Zhaoxia Li
Prof. Dr. Kent Burkey
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Plants is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • heat stress
  • drought stress
  • Arabidopsis
  • crop plants
  • tolerance
  • yield
  • transcriptional regulation
  • post-transcriptional regulation
  • metabolism adjustment

Benefits of Publishing in a Special Issue

  • Ease of navigation: Grouping papers by topic helps scholars navigate broad scope journals more efficiently.
  • Greater discoverability: Special Issues support the reach and impact of scientific research. Articles in Special Issues are more discoverable and cited more frequently.
  • Expansion of research network: Special Issues facilitate connections among authors, fostering scientific collaborations.
  • External promotion: Articles in Special Issues are often promoted through the journal's social media, increasing their visibility.
  • e-Book format: Special Issues with more than 10 articles can be published as dedicated e-books, ensuring wide and rapid dissemination.

Further information on MDPI's Special Issue polices can be found here.

Published Papers (11 papers)

Order results
Result details
Select all
Export citation of selected articles as:

Research

Jump to: Review, Other

17 pages, 3796 KiB  
Article
Proline Enhances Resistance and Recovery of Oilseed Rape after a Simulated Prolonged Drought
by Sigita Jurkonienė, Rima Mockevičiūtė, Virgilija Gavelienė, Vaidevutis Šveikauskas, Mariam Zareyan, Elžbieta Jankovska-Bortkevič, Jurga Jankauskienė, Tautvydas Žalnierius and Liudmyla Kozeko
Plants 2023, 12(14), 2718; https://doi.org/10.3390/plants12142718 - 21 Jul 2023
Cited by 10 | Viewed by 1997
Abstract
This study was carried out to evaluate the effect of exogenous proline on the growth, biochemical responses, and plant recovery of drought-stressed oilseed rape plants after renewed irrigation. The experiment was conducted under controlled laboratory conditions. After 21 days of cultivation, 3–4 leaf [...] Read more.
This study was carried out to evaluate the effect of exogenous proline on the growth, biochemical responses, and plant recovery of drought-stressed oilseed rape plants after renewed irrigation. The experiment was conducted under controlled laboratory conditions. After 21 days of cultivation, 3–4 leaf stage seedlings were sprayed with proline (1 mM), then subjected to prolonged drought stress for 8 days to achieve a severe water deficit, next, irrigation was resumed and recovery was assessed after 4 days. The results show that exogenous application of proline reduced the drought-induced growth inhibition of seedlings while maintaining relative water content (RWC) and growth parameters closer to those of irrigated plants. Proline had a positive effect on chlorophyll accumulation and membrane permeability while decreasing ethylene, H2O2, and MDA levels. Moreover, after 4 days of recovery, the H2O2 content of the proline-treated plants was significantly lower (2-fold) and the MDA content was close to that of continuously irrigated plants. Thus, all these biochemical reactions influenced plant survival: after drought + proline treatment, the number of surviving plants was two times higher than that of drought-treated plants. The findings show that exogenous proline has antioxidant, osmotic, and growth-promoting properties that improve the drought tolerance of winter oilseed rape plants and is, therefore, beneficial for drought adaptation in oilseed rape. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

13 pages, 2998 KiB  
Article
Exogenous Sorbitol Application Confers Drought Tolerance to Maize Seedlings through Up-Regulating Antioxidant System and Endogenous Sorbitol Biosynthesis
by Jun Li, Meiai Zhao, Ligong Liu, Xinmei Guo, Yuhe Pei, Chunxiao Wang and Xiyun Song
Plants 2023, 12(13), 2456; https://doi.org/10.3390/plants12132456 - 26 Jun 2023
Cited by 3 | Viewed by 1791
Abstract
This study aims to explore the impacts of exogenous sorbitol on maize seedlings under polyethylene glycol (PEG)-simulated drought stress. Six treatments were set: normal condition (CK), PEG (P), 10 mM sorbitol (10S), PEG plus 10 mM sorbitol (10SP), 100 mM sorbitol (100S) and [...] Read more.
This study aims to explore the impacts of exogenous sorbitol on maize seedlings under polyethylene glycol (PEG)-simulated drought stress. Six treatments were set: normal condition (CK), PEG (P), 10 mM sorbitol (10S), PEG plus 10 mM sorbitol (10SP), 100 mM sorbitol (100S) and PEG plus 100 mM sorbitol (100SP). Maize seedlings’ growth under PEG-simulated drought stress was significantly inhibited and exogenous sorbitol largely alleviated this growth inhibition. The seedlings under 10SP treatment grew much better than those under P, 100S and 100SP treatments and no significant difference in growth parameters was observed between the control and 10S treatment. The seedlings treated with 10SP had higher contents of soluble sugar, soluble protein, proline, ascorbic acid (AsA), reduced glutathione (GSH), sorbitol and relative water content, higher activities of antioxidant enzymes and aldose reductase, but lower contents of malondialdehyde (MDA), H2O2 and relative electrical conductivity than those treated with P, 100S and 100SP. qRT-PCR analysis showed that the transcript levels of genes encoding putative aldose reductase (AR) under P treatment were significantly up-regulated in sorbitol-applied treatments. Taken together, the results demonstrated that exogenous sorbitol application conferred drought tolerance to maize seedlings by up-regulating the expression levels of AR-related genes to enhance the accumulation of intracellular osmotic substances such as sorbitol and improve antioxidant systems to tone down the damage caused by drought stress. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

16 pages, 1806 KiB  
Article
Effects of Exogenous Sodium Nitroprusside Spraying on Physiological Characteristics of Soybean Leaves at the Flowering Stage under Drought Stress
by Zhipeng Qu, Yumei Tian, Xinyu Zhou, Xiaomei Li, Qi Zhou, Xiyue Wang and Shoukun Dong
Plants 2023, 12(8), 1598; https://doi.org/10.3390/plants12081598 - 10 Apr 2023
Cited by 6 | Viewed by 1733
Abstract
Nitric oxide (NO) plays a significant role in plant drought resistance. However, the effects of the exogenous application of NO to crops under drought stress vary within and among species. In this study, we explored the influence of exogenous sodium nitroprusside (SNP) on [...] Read more.
Nitric oxide (NO) plays a significant role in plant drought resistance. However, the effects of the exogenous application of NO to crops under drought stress vary within and among species. In this study, we explored the influence of exogenous sodium nitroprusside (SNP) on the drought resistance of soybean leaves in the full flowering stage using two varieties: drought-tolerant HN44 and non-drought-tolerant HN65. Spraying SNP on soybean leaves at the full flowering period under drought stress improved the NO content in soybean leaves. The activities of nitrite reductase (NiR) and nitrate reductase (NR) in leaves were affected by NO inhibition. The activity of antioxidant enzymes in leaves increased with the extension of SNP application time. Contents of osmomodulatory substances, including proline (Pro), soluble sugar (SS), and soluble protein (SP) increased gradually with the extension of SNP application time. The malondialdehyde (MDA) content decreased as the NO content increased, thus reducing membrane system damage. Overall, spraying SNP reduced damage and improved the ability of soybean to cope with drought. This study explored the physiological changes of SNP soybean under drought stress and provided theoretical basis for improving drought-resistant cultivation of soybean. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

23 pages, 3647 KiB  
Article
Proteomic Investigation of Molecular Mechanisms in Response to PEG-Induced Drought Stress in Soybean Roots
by Ying Zhou, Huiying Li, Haoran Chen, Xiaoqin Yang, Tingting Yu, Yushuang Wang, Yujue Wang, Keting Jiang, Yan Wang, Zhanyu Chen and Xiyan Cui
Plants 2022, 11(9), 1173; https://doi.org/10.3390/plants11091173 - 26 Apr 2022
Cited by 10 | Viewed by 2846
Abstract
Roots are generally the critical drought sensors, but little is known about their molecular response to drought stress. We used the drought-tolerant soybean variety ‘Jiyu 47’ to investigate the differentially expressed proteins (DEPs) in soybean roots during the seedling stage based on the [...] Read more.
Roots are generally the critical drought sensors, but little is known about their molecular response to drought stress. We used the drought-tolerant soybean variety ‘Jiyu 47’ to investigate the differentially expressed proteins (DEPs) in soybean roots during the seedling stage based on the tandem mass tag (TMT) proteomics analysis. Various expression patterns were observed in a total of six physiological parameters. A total of 468 DEPs (144 up-regulated and 324 down-regulated) among a total of 8687 proteins were identified in response to drought stress in 24 h. The expression of DEPs was further validated based on quantitative real-time PCR of a total of five genes (i.e., GmGSH, GmGST1, GmGST2 k GmCAT, and Gm6PGD) involved in the glutathione biosynthesis. Results of enrichment analyses revealed a coordinated expression pattern of proteins involved in various cellular metabolisms responding to drought stress in soybean roots. Our results showed that drought stress caused significant alterations in the expression of proteins involved in several metabolic pathways in soybean roots, including carbohydrate metabolism, metabolism of the osmotic regulation substances, and antioxidant defense system (i.e., the glutathione metabolism). Increased production of reduced glutathione (GSH) enhanced the prevention of the damage caused by reactive oxygen species and the tolerance of the abiotic stress. The glutathione metabolism played a key role in modifying the antioxidant defense system in response to drought stress in soybean roots. Our proteomic study suggested that the soybean plants responded to drought stress by coordinating their protein expression during the vegetative stage, providing novel insights into the molecular mechanisms regulating the response to abiotic stress in plants. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Graphical abstract

22 pages, 4300 KiB  
Article
Photoprotection Is Achieved by Photorespiration and Modification of the Leaf Incident Light, and Their Extent Is Modulated by the Stomatal Sensitivity to Water Deficit in Grapevines
by Luis Villalobos-González, Nicolás Alarcón, Roberto Bastías, Cristobal Pérez, René Sanz, Álvaro Peña-Neira and Claudio Pastenes
Plants 2022, 11(8), 1050; https://doi.org/10.3390/plants11081050 - 12 Apr 2022
Cited by 8 | Viewed by 2593 | Correction
Abstract
Absorbed energy in excess of that used by photosynthesis induces photoinhibition, which is common in water deficit conditions, resulting in reductions in stomatal conductance. In grapevines, controlled water deficit is a common field practice, but little is known about the impact of a [...] Read more.
Absorbed energy in excess of that used by photosynthesis induces photoinhibition, which is common in water deficit conditions, resulting in reductions in stomatal conductance. In grapevines, controlled water deficit is a common field practice, but little is known about the impact of a given water shortage on the energy transduction processes at the leaf level in relation to contrasting stomatal sensitivities to drought. Here, we assessed the effect of a nearly similar water deficit condition on four grapevine varieties: Cabernet Sauvignon (CS) and Sauvignon Blanc (SB), which are stomatal sensitive, and Chardonnay (CH) and Carménère (CM), which are less stomatal sensitive, grown in 20 L pots outdoors. Plants were maintained to nearly 94% of field capacity (WW) and 83% field capacity (WD). We have assessed plant water status, photosynthesis (AN), photorespiration, AN vs. PAR, ACi curves, photochemical (qP) and non-photochemical (qN) fluorescence quenching vs. PAR, the photoprotective effectiveness of NPQ (qPd) and light interception by leaves. Photorespiration is important under WD, but to a different extent between varieties. This is related to stomatal sensitivity, maintaining a safe proportion of PSII reaction centres in an open state. Additionally, the capacity for carboxylation is affected by WD, but to a greater extent in more sensitive varieties. As for qN, in WD it saturates at 750 μmol PAR m−2s−1, irrespective of the variety, which coincides with PAR, from which qN photoprotective effectiveness declines, and qP is reduced to risky thresholds. Additionally, that same PAR intensity is intercepted by WD leaves from highly stomatal-sensitive varieties, likely due to a modification of the leaf angle in those plants. Pigments associated with qN, as well as chlorophylls, do not seem to be a relevant physiological target for acclimation. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

11 pages, 10016 KiB  
Article
Foliar Anatomy of Three Native Species of Tillandsia L. from the Atacama Desert, Chile
by Eliana Belmonte, Bernardo Arriaza, Mabel Arismendi and German Sepúlveda
Plants 2022, 11(7), 870; https://doi.org/10.3390/plants11070870 - 24 Mar 2022
Cited by 8 | Viewed by 2432
Abstract
In the extreme north of Chile, the genus Tillandsia L. (Bromeliaceae) is represented by three native species, T. marconae Till & Vitek and T. landbeckii Phil., both of terrestrial atmospheric habit, and T. virescens Ruiz & Pav. of saxicolous habit. There is little [...] Read more.
In the extreme north of Chile, the genus Tillandsia L. (Bromeliaceae) is represented by three native species, T. marconae Till & Vitek and T. landbeckii Phil., both of terrestrial atmospheric habit, and T. virescens Ruiz & Pav. of saxicolous habit. There is little information on the foliar structures that allow its establishment in arid environments. Therefore, we studied the leaf anatomy of each of these terrestrial and saxicolous atmospheric species from different altitudinal levels (1000 and 3000 m) in the Arica and Parinacota regions of the Atacama Desert. All populations are monospecific. The study considered scanning electron microscopy, optical microscopy, and the fingernail polish technique. The surface distribution of stomata and trichomes of the species is described. The studied species presented hypostomatic leaves, with anomocytic stomata and peltate trichomes on the abaxial and adaxial sides. Trichomes are formed by a central disc of four equal-sized empty cells, surrounded by a peripheral series of several concentric rings, the innermost of eight, the second of sixteen and the outermost of multiple elongated cells, one cell thick, that form the flexible asymmetric wings. The number of wing cells varies according to the species. Trichomes are evenly arranged in long lanceolate leaf blades with smooth margins. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

19 pages, 1756 KiB  
Article
Optimization of Potassium Supply under Osmotic Stress Mitigates Oxidative Damage in Barley
by Ershad Tavakol, Bálint Jákli, Ismail Cakmak, Klaus Dittert and Mehmet Senbayram
Plants 2022, 11(1), 55; https://doi.org/10.3390/plants11010055 - 25 Dec 2021
Cited by 17 | Viewed by 3002
Abstract
Potassium (K) is the most abundant cation in plants, playing an important role in osmoregulation. Little is known about the effect of genotypic variation in the tolerance to osmotic stress under different K treatments in barley. In this study, we measured the interactive [...] Read more.
Potassium (K) is the most abundant cation in plants, playing an important role in osmoregulation. Little is known about the effect of genotypic variation in the tolerance to osmotic stress under different K treatments in barley. In this study, we measured the interactive effects of osmotic stress and K supply on growth and stress responses of two barley cultivars (Hordeum vulgare L.) and monitored reactive oxygen species (ROS) along with enzymatic antioxidant activity and their respective gene expression level. The selected cultivars (cv. Milford and cv. Sahin-91Sahin-91) were exposed to osmotic stress (−0.7 MPa) induced by polyethylene glycol 6000 (PEG) under low (0.04 mM) and adequate (0.8 mM) K levels in the nutrient solution. Leaf samples were collected and analyzed for levels of K, ROS, kinetic activity of antioxidants enzymes and expression levels of respective genes during the stress period. The results showed that optimal K supply under osmotic stress significantly decreases ROS production and adjusts antioxidant activity, leading to the reduction of oxidative stress in the studied plants. The cultivar Milford had a lower ROS level and a better tolerance to stress compared to the cultivar Sahin-91. We conclude that optimized K supply is of great importance in mitigating ROS-related damage induced by osmotic stress, specifically in drought-sensitive barley cultivars. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

17 pages, 3294 KiB  
Article
Water Regime Monitoring of the Royal Walnut (Juglans regia L.) Using Sap Flow and Dendrometric Measurements
by Viliam Bárek, Martina Kováčová, Vladimír Kišš and Oleg Paulen
Plants 2021, 10(11), 2354; https://doi.org/10.3390/plants10112354 - 30 Oct 2021
Cited by 5 | Viewed by 2615
Abstract
Changes in the distribution of annual rainfall totals, together with the increase in temperature over the last 40 years, are causing more frequent periods of drought, and plants are more often exposed to water stress. The aim of this study was to monitor [...] Read more.
Changes in the distribution of annual rainfall totals, together with the increase in temperature over the last 40 years, are causing more frequent periods of drought, and plants are more often exposed to water stress. The aim of this study was to monitor the effect of different water regimes (irrigated and non-irrigated) of individuals of walnut tree (Juglans regia L.) in a private orchard located in the West of Slovakia. Our research was focused on dendrometric and sap flow measurements in the period from 28 March to 2 June 2019. The results showed differences in the sap flow of walnut trees during the budbreak period: when trees were irrigated, sap flow in the diurnal cycle was around 130 g·h−1 (20.48%), higher than in the non-irrigated treatment. Dendrometric differences between the irrigated and non-irrigated treatments were not significant. The sap flow data in the flowering period of the irrigated variant were slightly higher at 150 g·h−1 (35.62%) than non-irrigated. Dendrometric differences were more significant when the difference between the variants was more than 1.5 mm. Continuation of this research and analysis of the data obtained in the coming years will allow us to evaluate the effects of the environment on fruit trees in the long term. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

Review

Jump to: Research, Other

16 pages, 2015 KiB  
Review
Regulatory Mechanisms of Heat Stress Response and Thermomorphogenesis in Plants
by Yunzhuan Zhou, Fuxiang Xu, Yanan Shao and Junna He
Plants 2022, 11(24), 3410; https://doi.org/10.3390/plants11243410 - 7 Dec 2022
Cited by 18 | Viewed by 5000
Abstract
As worldwide warming intensifies, the average temperature of the earth continues to increase. Temperature is a key factor for the growth and development of all organisms and governs the distribution and seasonal behavior of plants. High temperatures lead to various biochemical, physiological, and [...] Read more.
As worldwide warming intensifies, the average temperature of the earth continues to increase. Temperature is a key factor for the growth and development of all organisms and governs the distribution and seasonal behavior of plants. High temperatures lead to various biochemical, physiological, and morphological changes in plants and threaten plant productivity. As sessile organisms, plants are subjected to various hostile environmental factors and forced to change their cellular state and morphological architecture to successfully deal with the damage they suffer. Therefore, plants have evolved multiple strategies to cope with an abnormal rise in temperature. There are two main mechanisms by which plants respond to elevated environmental temperatures. One is the heat stress response, which is activated under extremely high temperatures; the other is the thermomorphogenesis response, which is activated under moderately elevated temperatures, below the heat-stress range. In this review, we summarize recent progress in the study of these two important heat-responsive molecular regulatory pathways mediated, respectively, by the Heat Shock Transcription Factor (HSF)–Heat Shock Protein (HSP) pathway and PHYTOCHROME INTER-ACTING FACTOR 4 (PIF4) pathways in plants and elucidate the regulatory mechanisms of the genes involved in these pathways to provide comprehensive data for researchers studying the heat response. We also discuss future perspectives in this field. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

15 pages, 12239 KiB  
Review
Effects of Raised Ambient Temperature on the Local and Systemic Adaptions of Maize
by Zhaoxia Li and Juren Zhang
Plants 2022, 11(6), 755; https://doi.org/10.3390/plants11060755 - 11 Mar 2022
Cited by 3 | Viewed by 3220
Abstract
Maize is a staple food, feed, and industrial crop. One of the major stresses on maize production is heat stress, which is usually accompanied by other stresses, such as drought or salinity. In this review, we compared the effects of high temperatures on [...] Read more.
Maize is a staple food, feed, and industrial crop. One of the major stresses on maize production is heat stress, which is usually accompanied by other stresses, such as drought or salinity. In this review, we compared the effects of high temperatures on maize production in China. Heat stress disturbs cellular homeostasis and impedes growth and development in plants. Plants have evolved a variety of responses to minimize the damage related to high temperatures. This review summarized the responses in different cell organelles at elevated temperatures, including transcriptional regulation control in the nuclei, unfolded protein response and endoplasmic reticulum-associated protein quality control in the endoplasmic reticulum (ER), photosynthesis in the chloroplast, and other cell activities. Cells coordinate their activities to mediate the collective stresses of unfavorable environments. Accordingly, we evaluated heat stress at the local and systemic levels in in maize. We discussed the physiological and morphological changes in sensing tissues in response to heat stress in maize and the existing knowledge on systemically acquired acclimation in plants. Finally, we discussed the challenges and prospects of promoting corn thermotolerance by breeding and genetic manipulation. Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Show Figures

Figure 1

Other

Jump to: Research, Review

1 pages, 212 KiB  
Correction
Correction: Villalobos-González et al. Photoprotection Is Achieved by Photorespiration and Modification of the Leaf Incident Light, and Their Extent Is Modulated by the Stomatal Sensitivity to Water Deficit in Grapevines. Plants 2022, 11, 1050
by Luis Villalobos-González, Nicolás Alarcón, Roberto Bastías, Cristobal Pérez, René Sanz, Álvaro Peña-Neira and Claudio Pastenes
Plants 2022, 11(16), 2096; https://doi.org/10.3390/plants11162096 - 12 Aug 2022
Viewed by 910
Abstract
In the original publication [...] Full article
(This article belongs to the Special Issue Regulation of Plant Responses to Heat and Drought Stress)
Back to TopTop