Involvement of ascorbate peroxidase and heat shock proteins on citrus tolerance to combined conditions of drought and high temperatures

Systematic analysis and functional verification of citrus ascorbate peroxidases reveal that CsAPX01 and CsAPX02 negatively regulate citrus bacterial canker through the hydrogen peroxide regulation

Ascorbate peroxidases (APXs) are antioxidant enzymes that play vital roles in redox homeostasis in plants. Citrus is susceptible to infection by Xanthomonas citri subsp. citri (Xcc), resulting in citrus bacterial canker (CBC). The present study used bioinformatic and expression analyses to investigate the APX family in Citrus sinensis. Bioinformatic research revealed the chromosomal locations, phylogeny, gene structure, promoter elements, functional domains, conserved motifs, and most likely physicochemical properties of the sequences. Six APXs clustered in three groups were identified, with each protein containing a single peroxidase domain. The promoter regions contained a variety of transcription factor-binding and hormone-response components. Xcc infection induced different CsAPX01 and CsAPX02 expressions in the CBC-susceptible Wanjincheng and CBC-resistant Kumquat varieties. Subcellular localization and transient expression showed that CsAPX01 and CsAPX02 were expressed in the cytoplasm and nucleus and had hydrogen peroxide (H2O2)-scavenging activity. Virus-induced gene silencing (VIGS) of CsAPX01 and CsAPX02 resulted in strong resistance to CBC and H2O2 bursts without effects on the plant phenotype. The current study focused on investigating and characterizing the citrus APX family. It was found that CsAPX01 and CsAPX02 exacerbated CBC by altering the balance of H2O2. These findings emphasize the importance of APXs in enhancing plant resistance to pathogens.

Proline accumulation and antioxidant response are crucial for citrus tolerance to UV-B light-induced stress

Plants face a wide range of biotic and abiotic stress conditions, which are further intensified by climate change. Among these stressors, increased irradiation in terms of intensity and wavelength range can lead to detrimental effects, such as chlorophyll degradation, destruction of the PSII reaction center, generation of ROS, alterations to plant metabolism, and even plant death. Here, we investigated the responses of two citrus genotypes, Citrus macrophylla (CM), and Troyer citrange (TC) to UV-B light-induced stress, by growing plants of both genotypes under control and UV-B stress conditions for 5 days to evaluate their tolerance mechanisms. TC seedlings had higher sensitivity to UV-B light than CM seedlings, as they showed more damage and increased levels of oxidative harm (indicated by the accumulation of MDA). In contrast, CM seedlings exhibited specific adaptive mechanisms, including accumulation of higher levels of proline under stressful conditions, and enhanced antioxidant capacity, as evidenced by increased ascorbate peroxidase activity and upregulation of the CsAPX2 gene. Phytohormone accumulation patterns were similar in both genotypes, with a decrease in ABA content in response to UV-B light. Furthermore, expression of genes involved in light perception and response was specifically affected in the tolerant CM seedlings, which exhibited higher expression of CsHYH/CsHY5 and CsRUP1-2 genes. These findings underscore the importance of the antioxidant system in citrus plants subjected to UV-B light-induced stress and suggest that CsHYH/CsHY5 and CsRUP1-2 could be considered genes associated with tolerance to such challenging conditions.

Complex plant responses to drought and heat stress under climate change

Global climate change is predicted to result in increased yield losses of agricultural crops caused by environmental conditions. In particular, heat and drought stress are major factors that negatively affect plant development and reproduction, and previous studies have revealed how these stresses induce plant responses at physiological and molecular levels. Here, we provide a comprehensive overview of current knowledge concerning how drought, heat, and combinations of these stress conditions affect the status of plants, including crops, by affecting factors such as stomatal conductance, photosynthetic activity, cellular oxidative conditions, metabolomic profiles, and molecular signaling mechanisms. We further discuss stress-responsive regulatory factors such as transcription factors and signaling factors, which play critical roles in adaptation to both drought and heat stress conditions and potentially function as 'hubs' in drought and/or heat stress responses. Additionally, we present recent findings based on forward genetic approaches that reveal natural variations in agricultural crops that play critical roles in agricultural traits under drought and/or heat conditions. Finally, we provide an overview of the application of decades of study results to actual agricultural fields as a strategy to increase drought and/or heat stress tolerance. This review summarizes our current understanding of plant responses to drought, heat, and combinations of these stress conditions.

Evaluation of protein's interaction and the regulatory network of some drought-responsive genes in Canola under drought and re-watering conditions

Drought stress is one of the most important environmental stresses that severely limits the growth and yield of Canola. The re-watering can compensate for the damage caused by drought stress. Investigation of protein's interaction of genes involved in important drought-responsive pathways and their regulatory network by microRNAs (miRNAs) under drought and re-watering conditions are helpful approaches to discovering drought-stress tolerance and recovery mechanisms. In this study, the protein's interaction and functional enrichment analyses of glycolysis, pentose phosphate, glyoxylate cycle, fatty acid biosynthesis, heat shock factor main genes, and the regulatory network of key genes by miRNAs were investigated by in silico analysis. Then, the relative expression of key genes and their related miRNAs were investigated in tolerant and susceptible genotypes of Canola under drought and re-watering conditions by Real-time PCR technique. The bna-miR156b/c/g, bna-miR395d/e/f, bna-miR396a, and all the studied key genes except HSFA1E and PK showed changes in expression levels in one or both genotypes after re-watering. The PPC1 and HSFB2B expression decreased, whereas the MLS and CAC3 expression increased in both genotypes under re-watering treatment after drought stress. It could cause the regulation of oxaloacetate production, the increase of the glyoxylate cycle, lipid biosynthesis, and the reduction of the negative regulation of HSFs under re-watering conditions. It seems that PPC1, G6PD2, MLS, CAC3, and HSFB2B were involved in the recovery mechanisms after drought stress of Canola. They were regulated by drought-responsive miRNAs to respond appropriately to drought stress. Therefore, regulating these genes could be important in plant recovery mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01345-1.

Redox-mediated responses to high temperature in plants

As sessile organisms, plants are particularly affected by climate change and will face more frequent and extreme temperature variations in the future. Plants have developed a diverse range of mechanisms allowing them to perceive and respond to these environmental constraints, which requires sophisticated signalling mechanisms. Reactive oxygen species (ROS) are generated in plants exposed to various stress conditions including high temperatures and are presumed to be involved in stress response reactions. The diversity of ROS-generating pathways and the ability of ROS to propagate from cell to cell and to diffuse through cellular compartments and even across membranes between subcellular compartments put them at the centre of signalling pathways. In addition, their capacity to modify the cellular redox status and to modulate functions of target proteins, notably through cysteine oxidation, show their involvement in major stress response transduction pathways. ROS scavenging and thiol reductase systems also participate in the transmission of oxidation-dependent stress signals. In this review, we summarize current knowledge on the functions of ROS and oxidoreductase systems in integrating high temperature signals, towards the activation of stress responses and developmental acclimation mechanisms.

Chloroplast-localized GUN1 contributes to the acquisition of basal thermotolerance in Arabidopsis thaliana

Heat stress (HS) severely affects different cellular compartments operating in metabolic processes and represents a critical threat to plant growth and yield. Chloroplasts are crucial for heat stress response (HSR), signaling to the nucleus the environmental challenge and adjusting metabolic and biosynthetic functions accordingly. GENOMES UNCOUPLED 1 (GUN1), a chloroplast-localized protein, has been recognized as one of the main players of chloroplast retrograde signaling. Here, we investigate HSR in Arabidopsis wild-type and gun1 plantlets subjected to 2 hours of HS at 45°C. In wild-type plants, Reactive Oxygen Species (ROS) accumulate promptly after HS, contributing to transiently oxidize the cellular environment and acting as signaling molecules. After 3 hours of physiological recovery at growth temperature (22°C), the induction of enzymatic and non-enzymatic antioxidants prevents oxidative damage. On the other hand, gun1 mutants fail to induce the oxidative burst immediately after HS and accumulate ROS and oxidative damage after 3 hours of recovery at 22°C, thus resulting in enhanced sensitivity to HS. These data suggest that GUN1 is required to oxidize the cellular environment, participating in the acquisition of basal thermotolerance through the redox-dependent plastid-to-nucleus communication.

Reduction of heat stress pressure and activation of photosystem II repairing system are crucial for citrus tolerance to multiple abiotic stress combination

Drought, heat and high irradiance are abiotic stresses that negatively affect plant development and reduce crop productivity. The confluence of these three factors is common in nature, causing extreme situations for plants that compromise their viability. Drought and heat stresses increase the saturation of the photosystem reaction centers, increasing sensitivity to high irradiance. In addition, these stress conditions affect photosystem II (PSII) integrity, alter redox balance of the electron transport chain and decrease the photosynthetic rate. Here, we studied the effect of the stress combinations on the photosynthetic apparatus of two citrus genotypes, Carrizo citrange (Citrus sinensis × Poncirus trifoliata) and Cleopatra mandarin (Citrus reshni). Results obtained showed that physiological responses, such as modulation of stomatal aperture and transpiration rate, aimed to reduce leaf temperature, are key to diminishing heat impact on photosynthetic apparatus and increasing tolerance to double and triple combinations of drought, high irradiance and high temperatures. By using transcriptomic and proteomic analyses, we have demonstrated that under these abiotic stress combinations, Carrizo plants were able to increase expression of genes and proteins related to the photosystem repairing machinery (which better maintained the integrity of PSII) and other components of the photosynthetic apparatus. Our findings reveal crucial physiological and genetic responses in citrus to increase tolerance to the combination of multiple abiotic stresses that could be the basis for breeding programs that ensure a sustainable citrus production.

Wild grapevines as rootstock regulate the oxidative defense system of in vitro grafted scion varieties under drought stress

The narrow genetic base of modern cultivars is becoming a key bottleneck for crop improvement and the use of wild relatives is an appropriate approach to improve the genetic diversity of crops to manage the sustainable production under different abiotic and biotic constraints. In Pakistan, wild germplasm of grapevine viz Dakh, Toran, and Zarishk belong to Vitis vinifera subsp. sylvestris and Fatati belong to Vitis vinifera subsp. sativa is naturally present in humid and sub-humid areas of mountainous and sub-mountainous regions and showed varying level of tolerance against drought stress but have not been evaluated as rootstock. In this study, different tolerant behavior of wild grapevines as rootstock in grafted scion varieties were explored under different levels of PEG-6000 mediated drought stress i.e., -4.00, -6.00, and -8.00 bars. In response to drought stress, wild grapevines evoked several non-enzymatic and enzymatic activities. Among non-enzymatic activities, total chlorophyll contents of commercial varieties were sustained at higher level when grafted on wild grapevines Dakh and Fatati which subsequently reduced the damage of cell membrane via MDA. Whereas, to cope the membranous damage due to excessive cellular generation of ROS, wild grapevines triggered the enhanced activities of SOD to dismutase the free oxygen radicals into H₂O₂, then CAT enzyme convert the H₂O₂ into water molecules. Higher accumulation of ROS in commercial scion varieties were also coped by wild grapevines Dakh and Fatati through the upregulation of POD and APX enzymes activities. Based on these enzymatic and non-enzymatic indices, biplot and cluster analysis classified the wild grapevines as rootstock into three distinct categories comprises on relatively tolerant i.e., Dakh (Vitis vinifera subsp. sylvestris) and Fatati (Vitis vinifera subsp. sativa), moderate tolerant i.e., Toran (Vitis vinifera subsp. sylvestris) and relatively susceptible category i.e., Zarishk (Vitis vinifera subsp. sylvestris).

Transgenic poplar trees overexpressing AtGolS2, a stress-responsive galactinol synthase gene derived from Arabidopsis thaliana, improved drought tolerance in a confined field

Drought is an abiotic stress that limits plant growth and productivity, and the development of trees with improved drought tolerance is expected to expand potential plantation areas and to promote sustainable development. Previously we reported that transgenic poplars (Populus tremula × P. tremuloides, T89) harboring the stress-responsive galactinol synthase gene, AtGolS2, derived from Arabidopsis thaliana were developed and showed improved drought stress tolerance in laboratory conditions. Herein we report a field trial evaluation of the AtGolS2-transgenic poplars. The rainfall-restricted treatments on the poplars started in late May 2020, 18 months after transplanting to the field, and were performed for 100 days. During these treatments, the leaf injury levels were observed by measuring photosynthetic quantum yields twice a week. Observed leaf injury levels varied in response to soil moisture fluctuation and showed a large difference between transgenic and non-transgenic poplars during the last month. Comparison of the leaf injury levels against three stress classes clustered by the machine learning approach revealed that the transgenic poplars exhibited significant alleviation of leaf injuries in the most severe stress class. The transgenes and transcript levels were stable in the transgenic poplars cultivated in the field conditions. These results indicated that the overexpression of AtGolS2 significantly improved the drought stress tolerance of transgenic poplars not only in the laboratory but also in the field. In future studies, molecular breeding using AtGolS2 will be an effective method for developing practical drought-tolerant forest trees.

Effects of extraction methods for a new source of biostimulant from Sargassum horneri on the growth of economically important red algae, Neopyropia yezoensis

Sargassum horneri is a major bloom forming species in Korea and China. It is important to find a way to utilize the huge biomass of Sargassum horneri in the region. Seaweed-derived biostimulants are primarily derived from brown algae and are known to improve terrestrial crop growth and tolerance to abiotic stresses. Neopyropia yezoensis is the most important seaweed cultured species in Korea, and research is required to increase heat resistance as a solution against climate change. In this study, various extraction methods were used to obtain Sargassum horneri extract, and it was applied to Neopyropia yezoensis to evaluate the effect on physiological activity. Metabolites of Sargassum horneri were extracted by using four different methods: boiling (SBE), soaking (SSE), autoclaving (SAE) and ethanol (SEE). The SBE, SSE and SAE derived extracts showed increased tolerance to high-temperature stress that had inhibited the growth of Neopyropia yezoensis, and show improved growth compared to the control group. The SBE and SSE extraction methods improved the content of phycobiliprotein, but also the SBE increased superoxide dismutase (SOD) activity. Based on the results of this study, the boiling extraction method appears to be the most suitable method for the extraction of plants stimulants from Sargassum horneri.

Alterations of Oxidative Stress Indicators, Antioxidant Enzymes, Soluble Sugars, and Amino Acids in Mustard [Brassica juncea (L.) Czern and Coss.] in Response to Varying Sowing Time, and Field Temperature

The impact of elevated temperature at the reproductive stage of a crop is one of the critical limitations that influence crop growth and productivity globally. This study was aimed to reveal how sowing time and changing field temperature influence on the regulation of oxidative stress indicators, antioxidant enzymes activity, soluble sugars (SS), and amino acids (AA) in Indian Mustard. The current study was carried out during the rabi 2017-2018 and 2018-2019 where, five varieties of mustard viz. Pusa Mustard 25 (PM-25) (V1), PM-26 (V2), BPR-541-4 (V3), RH-406 (V4), and Urvashi (V5) were grown under the field conditions on October 30 (normal sowing; S1), November 18 (late sowing; S2) and November 30 (very late sowing; S3) situations. The S1 and S3 plants, at mid-flowering stage, showed a significant variation in accumulation of SS (8.5 and 17.3%), free AA (235.4 and 224.6%), and proline content (118.1 and 133%), respectively, and played a crucial role in the osmotic adjustment under stress. The results showed that S3 sowing, exhibited a significant induction of the hydrogen peroxide (H₂O₂) (110.2 and 86.6%) and malondialdehyde (23.5 and 47.5%) concentrations, respectively, which indicated the sign of oxidative stress in plants. Interestingly, the polyphenol oxidase, peroxidase, superoxide dismutase, and catalase enzyme activities were also significantly increased in S3 plants compared to S1 plants, indicating their significant roles in ameliorating the oxidative stress. Furthermore, the concentration of fatty acid levels such as palmitic, stearic, oleic, and linoleic acids level also significantly increased in S3 plants, which influenced the seed and oil quality. The study suggests that the late sowing significantly impaired the biochemical mechanisms in Indian mustard. Further, the mustard variety V4 (RH-406) was found to be effective for cultivation as well as environmental stress adoption in Indian soils, and it could be highly useful in breeding for developing heat-tolerant genotypes for ensuring the food security.

Citrus rootstocks modify scion antioxidant system under drought and heat stress combination

The activation of the antioxidant system under stress combination is a transmissible trait from the rootstock to the scion. Therefore, rootstock selection is key to improve crop performance and a sustainable production under changing climate conditions. Climate change is altering weather conditions such as mean temperatures and precipitation patterns. Rising temperatures, especially in certain regions, accelerates soil water depletion and increases drought risk, which affects agriculture yield. Previously, our research demonstrated that the citrus rootstock Carrizo citrange (Citrus sinensis × Poncirus trifoliata) is more tolerant than Cleopatra mandarin (C. reshni) to drought and heat stress combination, in part, due to a higher activation of the antioxidant system that alleviated damage produced by oxidative stress. Here, by using reciprocal grafts of both genotypes, we studied the importance of the rootstock on scion performance and antioxidant response under this stress combination. Carrizo rootstock, under stress combination, positively influenced Cleopatra scion by reducing H₂O₂ accumulation, increasing superoxide dismutase (SOD) and ascorbate peroxidase (APX) enzymatic activities and inducing SOD1, APX2 and catalase (CAT) protein accumulations. On the contrary, Cleopatra rootstock induced decreases in APX2 expression, CAT activity and SOD1, APX2 and CAT contents on Carrizo scion. Taken together, our findings indicate that the activation of the antioxidant system under stress combination is a transmissible trait from the rootstock to the scion and highlight the importance of the rootstock selection to improve crop performance and maintain citrus yield under the current scenario of climate change.

The efficient physiological strategy of a novel tomato genotype to adapt to chronic combined water and heat stress

Climate change is increasing the frequency of high temperature shocks and water shortages, pointing to the need to develop novel tolerant varieties and to understand the mechanisms employed to withstand combined abiotic stresses. Two tomato genotypes, a heat-tolerant Solanum lycopersicum accession (LA3120) and a novel genotype (E42), previously selected as a stable yielding genotype under high temperatures, were exposed to single and combined water and heat stress. Plant functional traits, pollen viability and physiological (leaf gas exchange and chlorophyll a fluorescence emission measurements) and biochemical (antioxidant content and antioxidant enzyme activity) measurements were carried out. A Reduced Representation Sequencing approach allowed exploration of the genetic variability of both genotypes to identify candidate genes that could regulate stress responses. Both abiotic stresses had a severe impact on plant growth parameters and on the reproductive phase of development. Growth parameters and leaf gas exchange measurements revealed that the two genotypes used different physiological strategies to overcome individual and combined stresses, with E42 having a more efficient capacity to utilize the limiting water resources. Activation of antioxidant defence mechanisms seemed to be critical for both genotypes to counteract combined abiotic stresses. Candidate genes were identified that could explain the different physiological responses to stress observed in E42 compared with LA3120. Results here obtained have shown how new tomato genetic resources can be a valuable source of traits for adaptation to combined abiotic stresses and should be used in breeding programmes to improve stress tolerance in commercial varieties.

Biochemical Changes and Antioxidant Variations in Date Palm (Phoenix dactylifera L.) Varieties during Flower Induction and Development

The present investigation was carried out to explore the biochemical changes and antioxidant variations, including non-enzymatic and enzymatic antioxidant variations, in the leaves of different varieties of date palm (Phoenix dactylifera L.) belonging to the early, mid-, and late-flowering categories in the United Arab Emirates. The changes in the protein and phenol concentration; the ascorbic acid, reduced glutathione, and α-tocopherol contents; and the activity of peroxidase and polyphenol oxidase were studied in the leaves during the preflowering, flowering, and postflowering stages of the date palms. Two varieties each from the early (Shaham, Khanezi), mid- (Barhee, Nabthasaif), and late- (Khasab, Fardh) flowering types were used in this study. The protein content in the leaves was higher in the early flowering varieties during the preflowering stage but lower in the other two varieties. The phenol content showed an opposite trend to the protein. There was significant variation in the ascorbic acid content and a reduction in glutathione and α-tocopherol between the leaves of different varieties. Similarly, the activity of the antioxidant enzyme ascorbate peroxidase in the leaves was higher during the preflowering stage in all varieties. The superoxide dismutase (SOD), polyphenol oxidase (PPO), and catalase (CAT) activity was highest in the Bharhee leaves for all the stages. The peroxidase activity (POD) was highest in the Fardh variety of date palm, whereas the Khanezi variety exhibited the lowest activity. This study can be used as a baseline for developing more protocols for understanding the possible roles of biochemicals, antioxidants, antioxidant enzymes, and their interactions in the regulation of flower development in different date palm varieties.

Drought and Elevated CO2 Impacts Photosynthesis and Biochemicals of Basil (Ocimum basilicum L.)

Drought-induced reduction in crop growth and productivity can be compensated by increasing atmospheric carbon dioxide (CO2), a significant contributor to climate change. Drought stress (DS) affects crops worldwide due to dwindling water resources and irregular rainfall patterns. The experiment was set up under a randomized complete block design within a three-by-two factorial arrangement. Six SPAR chambers represent three blocks (10 replications each), where each chamber has 30 pots in three rows. Each chamber was maintained with 30/22 (day/night) °C temperature, with either ambient (aCO2; 420 ppm) or elevated CO2 (eCO2; 720 ppm) concentrations. This experiment was designed to address the impact of DS on the physiological and biochemical attributes and study how the eCO2 helps alleviate the adversity of DS in basil. The study demonstrated that DS + eCO2 application highly accelerated the decrease in all forms of carotene and xanthophylls. eCO2 positively influenced and increased anthocyanin (Antho) and chlorophyll (LChl). eCO2 supplementation increased LChl content in basil under DS. Furthermore, DS significantly impeded the photosynthetic system in plants by decreasing CO2 availability and causing stomatal closure. Although eCO2 did not increase net photosynthesis (Pn) activity, it decreased stomatal conductance (gs) and leaf transpiration rate (E) under DS, showing that eCO2 can improve plant water use efficiency by lowering E and gs. Peroxidase and ascorbate activity were higher due to the eCO2 supply to acclimate the basil under the DS condition. This study suggests that the combination of eCO2 during DS positively impacts basil’s photosynthetic parameters and biochemical traits than aCO2.

Use of Thermal Imaging to Assess Water Status in Citrus Plants in Greenhouses

The direct examination of plant canopy temperature can assist in optimizing citrus irrigation management in greenhouses. This study aimed to develop a method to measure canopy temperature using thermal imaging in one-year-old citrus plants in a greenhouse to identify plants with water stress and verify its potential to be used as a tool to assess citrus water status. The experiment was conducted for 48 days (27 November 2019 to 13 January 2020). We evaluated the influence of five levels of irrigation on two citrus species (‘Red Ruby’ grapefruit (Citrus paradisi) and ‘Valencia’ sweet orange (Citrus sinensis (L.) Osbeck)). Images were taken using a portable thermal camera and analyzed using open-source software. We determined canopy temperature, leaf photosynthesis and transpiration, and plant biomass. The results indicated a positive relationship between the amount of water applied and the temperature response of plants exposed to different water levels. Grapefruit and sweet orange plants that received less water and were submitted to water restrictions showed higher canopy temperatures than the air (up to 6 °C). The thermal images easily identified water-stressed plants. Our proof-of-concept study allowed quickly obtaining the canopy temperature using readily available equipment and can be used as a tool to assess citrus water status in one-year-old citrus plants in greenhouses and perhaps in commercial operations with mature trees in the field after specific experimentation. This technique, coupled with an automated system, can be used for irrigation scheduling. Thus, setting up a limit temperature is necessary to start the irrigation system and set the irrigation time based on the soil water content. To use this process on a large scale, it is necessary to apply an automation routine to process the thermal images in real time and remove the weeds from the background to determine the canopy temperature.

The Multiple Roles of Ascorbate in the Abiotic Stress Response of Plants: Antioxidant, Cofactor, and Regulator

Ascorbate (ASC) plays a critical role in plant stress response. The antioxidant role of ASC has been well-studied, but there are still several confusing questions about the function of ASC in plant abiotic stress response. ASC can scavenge reactive oxygen species (ROS) and should be helpful for plant stress tolerance. But in some cases, increasing ASC content impairs plant abiotic stress tolerance, whereas, inhibiting ASC synthesis or regeneration enhances plant stress tolerance. This confusing phenomenon indicates that ASC may have multiple roles in plant abiotic stress response not just as an antioxidant, though many studies more or less ignored other roles of ASC in plant. In fact, ACS also can act as the cofactor of some enzymes, which are involved in the synthesis, metabolism, and modification of a variety of substances, which has important effects on plant stress response. In addition, ASC can monitor and effectively regulate cell redox status. Therefore, we believe that ASC has atleast triple roles in plant abiotic stress response: as the antioxidant to scavenge accumulated ROS, as the cofactor to involve in plant metabolism, or as the regulator to coordinate the actions of various signal pathways under abiotic stress. The role of ASC in plant abiotic stress response is important and complex. The detail role of ASC in plant abiotic stress response should be analyzed according to specific physiological process in specific organ. In this review, we discuss the versatile roles of ASC in the response of plants to abiotic stresses.

High temperatures modify plant responses to abiotic stress conditions

Climate change is altering environments in which plants and different crops grow and survive. We already experienced an increase in worldwide average earth surface temperatures, as well as frequency and extent of damaging heat waves. These conditions collide in the field with other abiotic stresses such as water deficit, high salinity, increased light irradiation, and so on, generating complex harmful conditions that destabilize agricultural systems. The conditions generated during these episodes of stress combination greatly differ from those occurring in the field when different stress factors occur individually; conditions that have been the focus of study for decades. Fortunately, knowledge of physiological and molecular responses to stress combinations and the cost they inflict on plant growth and yield has been exponentially increasing in the past several years. Understanding plant performance under multiple stress combinations will allow breeding crops capable of maintaining yield production under the new climatic conditions. Here, after reviewing recent data on physiological, hormonal and transcriptional responses to different stress combinations, we highlight the importance of photodamage avoidance, abscisic and jasmonic acid signaling, and the upregulation of genes involved in oxidation-reduction processes, photosynthesis and protein metabolism, for plant acclimation to conditions of high temperatures, in combination with other common abiotic stress factors such as drought or salinity. Finally, we propose new approaches to investigate the response of plants to stress combinations and discuss strategies for improving crop resilience to stress combination.

Enhancing antioxidant systems by preharvest treatments with methyl jasmonate and salicylic acid leads to maintain lemon quality during cold storage

The effects of preharvest treatments with 0.1 mM methyl jasmonate (MeJA) and 0.5 mM salicylic acid (SA) on quality parameters of lemon fruit and their relationship with antioxidant systems, gene expression and bioactive compounds at harvest and during cold storage were evaluated. Results showed that total antioxidant activity, total phenolic content and the major individual phenolics (hesperidin and eriocitrin) were always higher in treated fruit than in controls. The activity of the antioxidant enzymes catalase, peroxidase and ascorbate peroxidase was also increased at harvest by SA and MeJA treatments, especially the last enzyme, for which the expression of its codifying gene was also enhanced. In addition, treated fruit had lower weight and firmness losses, respiration rate and production of ethylene than controls. Moreover, sugars and organic acids were maintained at higher concentration in flavedo and juice as a consequence of preharvest SA and MeJA treatments, showing an effect on maintaining fruit quality properties.

The Role of the Plant Antioxidant System in Drought Tolerance

Water deficiency compromises plant performance and yield in many habitats and in agriculture. In addition to survival of the acute drought stress period which depends on plant-genotype-specific characteristics, stress intensity and duration, also the speed and efficiency of recovery determine plant performance. Drought-induced deregulation of metabolism enhances generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) which in turn affect the redox regulatory state of the cell. Strong correlative and analytical evidence assigns a major role in drought tolerance to the redox regulatory and antioxidant system. This review compiles current knowledge on the response and function of superoxide, hydrogen peroxide and nitric oxide under drought stress in various species and drought stress regimes. The meta-analysis of reported changes in transcript and protein amounts, and activities of components of the antioxidant and redox network support the tentative conclusion that drought tolerance is more tightly linked to up-regulated ascorbate-dependent antioxidant activity than to the response of the thiol-redox regulatory network. The significance of the antioxidant system in surviving severe phases of dehydration is further supported by the strong antioxidant system usually encountered in resurrection plants.

High temperatures change the perspective: Integrating hormonal responses in citrus plants under co-occurring abiotic stress conditions

Plants growing in the field are subjected to multiple stress factors acting simultaneously. Abnormally high temperatures are expected to affect wild plants and crops in the next years due to global warming. In this work, we have studied physiological, hormonal and molecular responses of the citrus rootstock, Carrizo citrange (Poncirus trifoliata L. Raf. × Citrus sinensis L. Osb.) subjected to wounding or high salinity occurring individually or in combination with heat stress. According to our results, combination of high salinity and heat stress aggravated the negative effects of salt intoxication in Carrizo. The high transpiration rate caused by high temperatures counteracted physiological responses of plants to salt stress and increased Cl^- intake in leaves. In addition, 12-oxo-phytodienoic acid accumulated specifically under combination of wounding and heat stress, whereas at low temperatures, wounded plants accumulated jasmonic acid (JA) and JA-isoleucine (JA-Ile). Moreover, an antagonism between salicylic acid (SA) and JA was observed, and wounded plants subjected to high temperatures did not accumulate JA nor JA-Ile whereas SA levels increased (via isochorismate synthase biosynthetic pathway). Wounded plants did not accumulate abscisic acid (ABA) but its catabolite phaseic acid. This could act as a signal for the upregulation of (ABA)-RESPONSIVE ELEMENT (ABRE)-BINDING TRANSCRIPTION FACTOR 2 (CsAREB2) and RESPONSIVE TO DISSECATION 22 (CsRD22) in an ABA-independent way. This work uncovers some mechanisms that explain Carrizo citrange tolerance to high temperatures together with different hormonal signals in response to specific stresses. It is suggested that co-occurring abiotic stress conditions can modify (either enhance or reduce) the hormonal response to modulate specific responses.

Mitochondrial alternative cyanide-resistant oxidase is involved in an increase of heat stress tolerance in spring wheat

The aim of this study was to determine the influence of different heat treatments on the alternative cyanide-resistant oxidase (AOX) capacity and establish a relation between the heat stress tolerance of spring wheat (Triticum aestivum L.), content of water-soluble carbohydrates in leaves and the alternative respiratory pathway (AP) capacity. We identified a positive relation between these studied parameters. Heat exposure at 39 °C for 24 h increased the heat stress tolerance of seedlings, content of water-soluble carbohydrates and AOX capacity, and the AOX capacity was also high after the subsequent influence of heat shock (50 °C for 3 h). The increased AOX capacity correlated with an increased level of water-soluble carbohydrates in leaves. The content of the AOX protein increased after heat exposure at 39 °C (for 3 h and 24 h) and after the subsequent influence of heat shock (50 °C for 1 and 3 h) at 39 °C for 24 h. We also detected that the content of AOX protein isoforms depends on the duration and intensity of heat treatment. It was concluded that AOX plays an important role in the acclimation of plants to high temperatures.

Genome-Wide Identification, Classification and Expression Analysis of the HSP Gene Superfamily in Tea Plant (Camellia sinensis)

Heat shock proteins (HSPs) function as molecular chaperones. These proteins are encoded by a multigene family whose members play crucial roles in plant growth, development and stress response. However, little is known about the HSP gene superfamily in tea plant. In this study, a total of 47 CsHSP genes were identified, including 7 CsHSP90, 18 CsHSP70, and 22 CssHSP genes. Phylogenetic and composition analyses showed that CsHSP proteins in the same subfamily have similar gene structures and conserved motifs, but significant differences exist in the different subfamilies. In addition, expression analysis revealed that almost all CsHSP genes were specifically expressed in one or more tissues, and significantly induced under heat and drought stress, implying that CsHSP genes play important roles in tea plant growth, development, and response to heat and drought stress. Furthermore, a potential interaction network dominated by CsHSPs, including HSP70/HSP90 organizing protein (HOP) and heat shock transcription factor (HSF), is closely related to the abovementioned processes. These results increase our understanding of CsHSP genes and their roles in tea plant, and thus, this study could contribute to the cloning and functional analysis of CsHSP genes and their encoded proteins in the future.