Protection against heat stress-induced oxidative damage in Arabidopsis involves calcium, abscisic acid, ethylene, and salicylic acid

StCaM2, a calcium binding protein, alleviates negative effects of salinity and drought stress in tobacco

KEY MESSAGE

Overexpression of StCaM2 in tobacco promotes plant growth and confers increased salinity and drought tolerance by enhancing the photosynthetic efficiency, ROS scavenging, and recovery from membrane injury. Calmodulins (CaMs) are important Ca2+ sensors that interact with effector proteins and drive a network of signal transduction pathways involved in regulating the growth and developmental pattern of plants under stress. Herein, using in silico analysis, we identified 17 CaM isoforms (StCaM) in potato. Expression profiling revealed different temporal and spatial expression patterns of these genes, which were modulated under abiotic stress. Among the identified StCaM genes, StCaM2 was found to have the largest number of abiotic stress responsive promoter elements. In addition, StCaM2 was upregulated in response to some of the selected abiotic stress in potato tissues. Overexpression of StCaM2 in transgenic tobacco plants enhanced their tolerance to salinity and drought stress. Accumulation of reactive oxygen species was remarkably decreased in transgenic lines compared to that in wild type plants. Chlorophyll a fluorescence analysis suggested better performance of photosystem II in transgenic plants under stress compared to that in wild type plants. The increase in salinity stress tolerance in StCaM2-overexpressing plants was also associated with a favorable K+/Na+ ratio. The enhanced tolerance to abiotic stresses correlated with the increase in the activities of anti-oxidative enzymes in transgenic tobacco plants. Overall, our results suggest that StCaM2 can be a novel candidate for conferring salt and drought tolerance in plants.

Cell Cycle Regulation in the Plant Response to Stress

As sessile organisms, plants face a variety of environmental challenges. Their reproduction and survival depend on their ability to adapt to these stressors, which include water, heat stress, high salinity, and pathogen infection. Failure to adapt to these stressors results in programmed cell death and decreased viability, as well as reduced productivity in the case of crop plants. The growth and development of plants are maintained by meiosis and mitosis as well as endoreduplication, during which DNA replicates without cytokinesis, leading to polyploidy. As in other eukaryotes, the cell cycle in plants consists of four stages (G1, S, G2, and M) with two major check points, namely, the G1/S check point and G2/M check point, that ensure normal cell division. Progression through these checkpoints involves the activity of cyclin-dependent kinases and their regulatory subunits known as cyclins. In order for plants to survive, cell cycle control must be balanced with adaption to dynamic environmental conditions. In this review, we summarize recent advances in our understanding of cell cycle regulation in plants, with a focus on the molecular interactions of cell cycle machinery in the context of stress tolerance.

Overexpression of CsCaM3 Improves High Temperature Tolerance in Cucumber

High temperature (HT) stress affects the growth and production of cucumbers, but genetic resources with high heat tolerance are very scarce in this crop. Calmodulin (CaM) has been confirmed to be related to the regulation of HT stress resistance in plants. CsCaM3, a CaM gene, was isolated from cucumber inbred line "02-8." Its expression was characterized in the present study. CsCaM3 transcripts differed among the organs and tissues of cucumber plants and could be induced by HTs or abscisic acid, but not by salicylic acid. CsCaM3 transcripts exhibited subcellular localization to the cytoplasm and nuclei of cells. Overexpression of CsCaM3 in cucumber plants has the potential to improve their heat tolerance and protect against oxidative damage and photosynthesis system damage by regulating the expression of HT-responsive genes in plants, including chlorophyll catabolism-related genes under HT stress. Taken together, our results provide useful insights into stress tolerance in cucumber.

Hormonal regulation of reproductive growth under normal and heat-stress conditions in legume and other model crop species

Legume crops are grown throughout the world and provide an excellent food source of digestible protein and starch, as well as dietary fibre, vitamins, minerals, and flavonoids. Fruit and seeds from legumes are also an important source of vegetables for a well-balanced diet. A trend in elevated temperature as a result of climate change increases the risk of a heat stress-induced reduction in legume crop yield. High temperatures during the crop reproductive development phase are particularly detrimental to fruit/seed production because the growth and development of the reproductive tissues are sensitive to small changes in temperature. Hormones are signalling molecules that play important roles in a plant's ability to integrate different environmental inputs and modify their developmental processes to optimize growth, survival, and reproduction. This review focuses on the hormonal regulation of reproductive development and heat stress-induced alteration of this regulation during (i) pollination, (ii) early fruit set, and (iii) seed development that affects fruit/seed yield in legume and other model crops. Further understanding of hormone-regulated reproductive growth under non-stress and heat-stress conditions can aid in trait selection and the development of gene modification strategies and cultural practices to improve heat tolerance in legume crops contributing to improved food security.

Integrated physiological and hormonal profile of heat-induced thermotolerance in Pinus radiata

Despite great interest, not only from the economic point of view but also in terms of basic science, research on heat stress tolerance in conifers remains scarce. To fill this gap, a time-course experiment using expected temperature increase was performed aiming to identify physiological and biochemical traits that allow the characterization of heat-induced thermotolerance and recovery in Pinus radiata D. Don plants. Several physiological parameters were assessed during heat exposure and after recovery, and multiple phytohormones-abscisic acid (ABA), indole-3-acetic acid (IAA), cytokinins (CKs), gibberellins, jasmonic acid, salicylic acid (SA) and brassinosteroids-were quantified by ultra-performance liquid chromatography-mass spectrometry from unique sample. Furthermore, tissue specific stress-signaling was monitored by IAA and ABA immunolocalization. Multivariate statistical analysis of the data enabled clustering of the shorter- and longer-term effects of heat stress exposure. Two sequential physiological responses were identified: an immediate and a delayed response, essentially determined by specific phytohormones, proline, malondialdehyde and total soluble sugar patterns. Results showed that ABA and SA play a crucial role in the first stage of response to heat stress, probably due to the plant's urgent need to regulate stomatal closure and counteract the increase in oxidative membrane damage demonstrated in shorter-term exposures. However, in longer exposures and recovery, proline, total sugars, IAA and CKs seem to be more relevant. This integrated approach pinpointed some basic mechanisms of P. radiata physiological responses underlying thermotolerance processes and after recovery.

Role of redox homeostasis in thermo-tolerance under a climate change scenario

BACKGROUND

Climate change predictions indicate a progressive increase in average temperatures and an increase in the frequency of heatwaves, which will have a negative impact on crop productivity. Over the last decade, a number of studies have addressed the question of how model plants or specific crops modify their metabolism when exposed to heat stress.

SCOPE

This review provides an overview of the redox pathways that contribute to how plants cope with heat stress. The focus is on the role of reactive oxygen species (ROS), redox metabolites and enzymes in the signalling pathways leading to the activation of defence responses. Additional attention is paid to the regulating mechanisms that lead to an increase in specific ROS-scavenging systems during heat stress, which have been studied in different model systems. Finally, increasing thermo-tolerance in model and crop plants by exposing them to heat acclimation or to exogenous treatments is discussed.

CONCLUSIONS

Although there is clear evidence that several strategies are specifically activated according to the intensity and the duration of heat stress, as well as the capacity of the different species or genotypes to overcome stress, an alteration in redox homeostasis seems to be a common event. Different mechanisms that act to enhance redox systems enable crops to overcome heat stress more effectively. Knowledge of thermo-tolerance within agronomic biodiversity is thus of key importance to enable researchers to identify new strategies for overcoming the impacts of climate change, and for decision-makers in planning for an uncertain future with new choices and options open to them.

Accumulation of extra-chloroplastic triacylglycerols in Arabidopsis seedlings during heat acclimation

Heat acclimation enables plants to tolerate and survive short-term heat stress on hot days. In Arabidopsis thaliana, a genetically programmed heat shock response can be rapidly triggered in the temperature range of 32-38°C through activation of heat shock transcription factors (HSF). The heat shock response leads to heat acclimation and confers short-term protection against temperatures above 40°C. However, little is known about metabolic adjustments during heat acclimation.Untargeted metabolite analyses of A. thaliana seedlings revealed that levels of polyunsaturated triacylglycerols (TG) rapidly and dramatically increase during heat acclimation. TG accumulation was found to be temperature dependent in a temperature range of 32-50°C (optimum at 42°C) and reversible after a return from 37°C to normal growth temperatures. Heat-induced TGs accumulated in extra-chloroplastic compartments and increased in both roots and shoots to a similar extent. Analysis of mutants deficient in all four HSFA1 master regulator genes or the HSFA2 gene revealed that TG accumulation was not dependent on HSFs. Moreover, the TG response was not limited to heat stress because drought and salt stress also triggered an accumulation of TGs, but not short-term osmotic, cold, and high light stress. Lipid analysis revealed that heat-induced accumulation of TGs was not due to massive de novo fatty acid synthesis. It is hypothesized that TGs serve as transient stores for fatty acids that may be required for membrane remodelling during heat acclimation.

β-cyclocitral upregulates salicylic acid signalling to enhance excess light acclimation in Arabidopsis

β-cyclocitral (β-CC), a volatile oxidized derivative of β-carotene, can upregulate the expression of defence genes to enhance excess light (EL) acclimation. However, the signalling cascades underlying this process remain unclear. In this study, salicylic acid (SA) is involved in alleviating damage to promote β-CC-enhanced EL acclimation. In early stages of EL illumination, β-CC pretreatment induced SA accumulation and impeded reactive oxygen species (ROS) production in the chloroplast. A comparative analysis of two SA synthesis pathways in Arabidopsis revealed that SA concentration mainly increased via the isochorismate synthase 1 (ICS1)-mediated isochorismate pathway, which depended on essential regulative function of enhanced disease susceptibility 1 (EDS1). Further results showed that, in the process of β-CC-enhanced EL acclimation, nuclear localization of nonexpressor of pathogenesis-related genes 1 (NPR1) was regulated by SA accumulation and NPR1 induced subsequent transcriptional reprogramming of gluthathione-S-transferase 5 (GST5) and GST13 implicated in detoxification. In summary, β-CC-induced SA synthesis contributes to EL acclimation response by decreasing ROS production in the chloroplast, promoting nuclear localization of NPR1, and upregulating GST transcriptional expression. This process is a possible molecular regulative mechanism of β-CC-enhanced EL acclimation.

Improvement of Polyunsaturated Fatty Acid Production in Echium acanthocarpum Transformed Hairy Root Cultures by Application of Different Abiotic Stress Conditions

Fatty acids are of great nutritional, therapeutic, and physiological importance, especially the polyunsaturated n-3 fatty acids, possessing larger carbon chains and abundant double bonds or their immediate precursors. A few higher plant species are able to accumulate these compounds, like those belonging to the Echium genus. Here, the novel E. acanthocarpum hairy root system, which is able to accumulate many fatty acids, including stearidonic and α-linolenic acids, was optimized for a better production. The application of abiotic stress resulted in larger yields of stearidonic and α-linolenic acids, 60 and 35%, respectively, with a decrease in linoleic acid, when grown in a nutrient medium consisting of B5 basal salts, sucrose or glucose, and, more importantly, at a temperature of 15°C. The application of osmotic stress employing sorbitol showed no positive influence on the fatty acid yields; furthermore, the combination of a lower culture temperature and glucose did not show a cumulative boosting effect on the yield, although this carbon source was similarly attractive. The abiotic stress also influenced the lipid profile of the cultures, significantly increasing the phosphatidylglycerol fraction but not the total lipid neither their biomass, proving the appropriateness of applying various abiotic stress in this culture to achieve larger yields.

Salicylic acid beyond defence: its role in plant growth and development

In recent years salicylic acid (SA) has been the focus of intensive research due to its function as an endogenous signal mediating local and systemic plant defence responses against pathogens. It has also been found that SA plays a role during the plant response to abiotic stresses such as drought, chilling, heavy metal toxicity, heat, and osmotic stress. In this sense, SA appears to be, just like in mammals, an 'effective therapeutic agent' for plants. Besides this function during biotic and abiotic stress, SA plays a crucial role in the regulation of physiological and biochemical processes during the entire lifespan of the plant. The discovery of its targets and the understanding of its molecular modes of action in physiological processes could help in the dissection of the complex SA signalling network, confirming its important role in both plant health and disease. Here, the evidence that supports the role of SA during plant growth and development is reviewed by comparing experiments performed by exogenous application of SA with analysis of genotypes affected by SA levels and/or perception.

Salicylic acid deficiency in NahG transgenic lines and sid2 mutants increases seed yield in the annual plant Arabidopsis thaliana

Salicylic acid-deficient NahG transgenic lines and sid2 mutants were used to evaluate the role of this compound in the development of the short-lived, annual plant Arabidopsis thaliana, with a particular focus on the interplay between salicylic acid and other phytohormones. Low salicylic acid levels led to increased growth, as well as to smaller abscisic acid levels and reduced damage to PSII (as indicated by F(v)/F(m) ratios) during the reproductive stages in rosette leaves of NahG transgenic lines and sid2 mutants, compared with wild-type plants. Furthermore, salicylic acid deficiency highly influenced seed yield and composition. Seed production increased by 4.4-fold and 3.5-fold in NahG transgenic lines and sid2 mutants, respectively, compared to the wild type. Salicylic acid deficiency also improved seed composition in terms of antioxidant vitamin concentrations, seeds of salicylic acid-deficient plants showing higher levels of alpha- and gamma-tocopherol (vitamin E) and beta-carotene (pro-vitamin A) than seeds of wild-type plants. Seeds of salicylic acid-deficient plants also showed higher nitrogen concentrations than seeds of wild-type plants. It is concluded that (i) the sid2 gene, which encodes for isochorismate synthase, plays a central role in salicylic acid biosynthesis during plant development in A. thaliana, (ii) salicylic acid plays a role in the regulation of growth, senescence, and seed production, (iii) there is a cross-talk between salicylic acid and other phytohormones during plant development, and (iv) the concentrations of antioxidant vitamins in seeds may be influenced by the endogenous levels of salicylic acid in plants.

Are ABA, ethylene or their interaction involved in the response of leaf growth to soil water deficit? An analysis using naturally occurring variation or genetic transformation of ABA production in maize

The role of abscisic acid (ABA) and its possible interaction with ethylene in mediating leaf elongation response to soil water deficit are a matter of controversy. To address this question, we used a set of maize genotypes with various levels of ABA either due to natural variability or to genetic transformation targeted on NCED/VP14, a key enzyme of ABA synthesis. The transgenic lines yielded less strong phenotypes than available mutants, making it possible to use them under normal growing conditions. We focused on leaf elongation during night periods in order to avoid the confounding effect of ABA on leaf water status. Our results suggest that over a wide range, internal ABA level (measured in both leaf extracts or xylem sap) has no clear effect on leaf elongation response to soil water deficit, except in the case of an antisense line presenting the strongest reduction in ABA accumulation that showed a slight maintenance of leaf elongation during water deficit. Leaf ethylene production rate was variable and not related to water deficit except in the ABA-deficient transgenic lines where it was increased by water deficit on average but not systematically. Moreover, variability in ethylene production rate was not linked to variability in elongation rate. Our results thus suggest that neither ABA nor ethylene seems to play a major role in the control of leaf elongation response to soil water deficit.

Involvement of the small GTPase Rac in the defense responses of tobacco to pathogens

During the hypersensitive response (HR), plants accumulate reactive oxygen species (ROS) that are likely generated at least in part by an NADPH oxidase similar to that found in mammalian neutrophils. An essential regulator of mammalian NADPH oxidase is the small GTP-binding protein Rac. To investigate whether Rac also regulates the pathogen-induced oxidative burst in plants, a dominant negative form of the rice OsRac1 gene was overexpressed in tobacco carrying the N resistance gene. Following infection with Tobacco mosaic virus (TMV), DN-OsRacl plants developed smaller lesions than wild-type plants, accumulated lower levels of lipid peroxidation products, and failed to activate expression of antioxidant genes. These results, combined with the demonstration that superoxide and hydrogen peroxide levels were reduced in DN-OsRacl tobacco developing a synchronous HR triggered by transient expression of the TMV p50 helicase domain or the Pto and AvrPto proteins, suggest that ROS production is impaired. The dominant negative effect of DN-OsRacl could be rescued by transiently overexpressing the wild-type OsRac1 protein. TMV-induced salicylic acid accumulation also was compromised in DN-OsRacl tobacco. Interestingly, while systemic acquired resistance to TMV was not impaired, nonhost resistance to Pseudomonas syringae pv. maculicola ES4326 was suppressed. Thus, the effect DN-OsRac1 expression exerts on the resistance signaling pathway appears to vary depending on the identity of the inoculated pathogen.

Plant-specific calmodulin-binding proteins

Calmodulin CaM is the most prominent Ca2+ transducer in eukaryotic cells, regulating the activity of numerous proteins with diverse cellular functions. Many features of CaM and its downstream targets are similar in plants and other eukaryotes. However, plants possess a unique set of CaM-related proteins, and several unique CaM target proteins. This review discusses recent progress in identifying plant-specific CaM-binding proteins and their roles in response to biotic and abiotic stresses and development. The review also addresses aspects emerging from recent structural studies of CaM interactions with target proteins relevant to plants.

Analysis of natural allelic variation of Arabidopsis seed germination and seed longevity traits between the accessions Landsberg erecta and Shakdara, using a new recombinant inbred line population

Quantitative trait loci (QTL) mapping was used to identify loci controlling various aspects of seed longevity during storage and germination. Similar locations for QTLs controlling different traits might be an indication for a common genetic control of such traits. For this analysis we used a new recombinant inbred line population derived from a cross between the accessions Landsberg erecta (Ler) and Shakdara (Sha). A set of 114 F9 recombinant inbred lines was genotyped with 65 polymerase chain reaction-based markers and the phenotypic marker erecta. The traits analyzed were dormancy, speed of germination, seed sugar content, seed germination after a controlled deterioration test, hydrogen peroxide (H2O2) treatment, and on abscisic acid. Furthermore, the effects of heat stress, salt (NaCl) stress, osmotic (mannitol) stress, and natural aging were analyzed. For all traits one or more QTLs were identified, with some QTLs for different traits colocating. The relevance of colocation for mechanisms underlying the various traits is discussed.

Roles of the reactive oxygen species-generating peroxidase reactions in plant defense and growth induction

Extracellularly secreted plant peroxidases (POXs) are considered to catalyze the generation of reactive oxygen species (ROS) coupled to oxidation of plant hormone indole-3-acetic acid (IAA) and defense-related compounds salicylic acid (SA), aromatic monoamines (AMAs) and chitooligosaccharides (COSs). This review article consists of two parts, which describe H(2)O(2)-dependent and H(2)O(2)-independent mechanisms for ROS generation, respectively. Recent studies have shown that plant POXs oxidize SA, AMAs and COSs in the presence of H(2)O(2) via a conventional POX cycle, yielding the corresponding radical species, such as SA free radicals. These radical species may react with oxygen, and superoxide (O(2)(.-)) is produced. Through the series of reactions 2 moles of O(2)(.-) can be formed from 1 moles of H(2)O(2), thus leading to oxidative burst. It has been revealed that the ROS induced by SA, AMAs and COSs triggers the increase in cytosolic Ca(2+) concentration. Actually POXs transduce the extracellular signals into the redox signals that eventually stimulate the intracellular Ca(2+) signaling required for induction of defense responses. On the other hand, IAA can react with oxygen and plant POXs in the absence of H(2)O(2), by forming the ternary complex enzyme-IAA-O(2), which readily dissociates into enzyme, IAA radicals and O(2)(.-). This article covers the recent reports showing that extracellularly produced hydroxy radicals derived from O(2)(.-) mediate the IAA-induced cell elongation. Here a novel model for IAA signaling pathway mediated by extracellular ROS produced by cell-wall POXs is proposed. In addition, possible controls of the IAA-POX reactions by a fungal alkaloid are discussed.