Abscisic acid-induced heat tolerance in Bromus inermis Leyss cell-suspension cultures. Heat-stable, abscisic acid-responsive polypeptides in combination with sucrose confer enhanced thermostability

Subsurface aeration mitigates organic material mulching-induced anaerobic stress via regulating hormone signaling in Phyllostachys praecox roots

Organic material mulching has been used extensively to allow Phyllostachys praecox to promote growth and development of shoots. However, the bamboo forest always showed a significant degradation, probably due to anaerobic damage caused by the mulching after several years. Therefore, we have innovatively proposed an improvement measure to aerate the underground pipes for the first time. We investigated the role of subsurface pipe aeration in regulating root hypoxia to reduce the stress and to identify the degradation mechanism. Results showed that aeration increased oxygen concentration, shoot yield and root growth compared with mulching, and the aeration enhanced the concentration of indole-3-acetic acid (IAA) and the expression of Aux/IAAs (Aux1, Aux2, Aux3, and Aux4). Aeration reduced gibberellin (GA), ethylene (ETH), and abscisic acid (ABA) contents as well as anaerobic enzyme activities (alanine transaminase, AlaAT; alcohol dehydrogenase, ADH; pyruvate decarboxylase, PDC; and lactate dehydrogenase, LDH), which alleviated root damage in anoxic conditions. Furthermore, correlation showed that the activities of ADH, LDH, PDC, and AlaAT showed significant linear correlations with soil oxygen levels. RDA analyses showed that ABA, IAA, and ETH were found as the key driving hormones of Aux/IAAs in the root of the forest mulched with organic material. Here we show that subsurface aeration increases soil oxygen concentration, shoot yield, root growth and regulates phytohormone concentrations and Aux/IAAs expression, which reduces anaerobic enzyme activities. Consequently, subsurface pipe aeration is an effective measure to mitigate the degradation of bamboo forests caused by soil hypoxia that results from organic material mulching.

Physiological mechanisms of ABA-induced salinity tolerance in leaves and roots of rice

Abscisic acid (ABA) plays a crucial role in response to abiotic stress as important small molecules in regulating metabolism. This study aimed to evaluate the ability of foliar spraying ABA to regulate growth quality at rice seedling stage under salt stress. Results demonstrated that salt stress strongly reduced all the growth parameters of two rice seedlings ('Chaoyouqianhao' and 'Huanghuazhan'), caused prominent decrease in the levels of photosynthetic pigments (mainly in Huanghuazhan), photosynthesis and fluorescence parameters. Salinity treatment increased the concentration of malondialdehyde (MDA) and hydrogen peroxide (H2O2) in roots, whereas significant decreased H2O2 was found in leaves of Huanghuazhan. Additionally, salinity triggered high Na+ content particularly in leaves and enhanced catalase (CAT) activities, ascorbate peroxidase (APX) and peroxidase (POD) activities of the two rice seedlings. Nevertheless, salinity-induced increased root ascorbic acid (AsA) and glutathione (GSH) levels while decreased in leaves, which depended on treatment time. Conversely, ABA application partially or completely mitigated salinity toxicity on the seedlings. ABA could reverse most of the changed physiological parameters triggered by salt stress. Specially, ABA treatment improved antioxidant enzyme levels and significantly reduced the Na+ content of two varieties as well as increased the K+, Mg2+ and Ca2+ content in leaves and roots. ABA treatment increased the hormone contents of 1-aminocclopropane carboxylic acid (ACC), trans-zeatin (TZ), N6-isopentyladenosine (IPA), Indole-3-acetic acid (IAA), and ABA in leaves of two rice varieties under salt stress. It is suggested that ABA was beneficial to protect membrane lipid peroxidation, the modulation of antioxidant defense systems and endogenous hormonal balance with imposition to salt stress.

Nitric Oxide and Abscisic Acid Mediate Heat Stress Tolerance through Regulation of Osmolytes and Antioxidants to Protect Photosynthesis and Growth in Wheat Plants

Nitric oxide (NO) and abscisic acid (ABA) play a significant role to combat abiotic stress. Application of 100 µM sodium nitroprusside (SNP, NO donor) or ABA alleviated heat stress effects on photosynthesis and growth of wheat (Triticum aestivum L.) plants exposed to 40 °C for 6 h every day for 15 days. We have shown that ABA and NO synergistically interact to reduce the heat stress effects on photosynthesis and growth via reducing the content of H2O2 and thiobarbituric acid reactive substances (TBARS), as well as maximizing osmolytes production and the activity and expression of antioxidant enzymes. The inhibition of NO and ABA using c-PTIO (2-4 carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) and fluridone (Flu), respectively, reduced the osmolyte and antioxidant metabolism and heat stress tolerance. The inhibition of NO significantly reduced the ABA-induced osmolytes and antioxidant metabolism, exhibiting that the function of ABA in the alleviation of heat stress was NO dependent and can be enhanced with NO supplementation.Thus, regulating the activity and expression of antioxidant enzymes together with osmolytes production could act as a possible strategy for heat tolerance.

Identification and expression profile of the soil moisture and Ralstonia solanacearum response CYPome in ginger (Zingiber officinale)

Background

Cytochrome P450s play crucial roles in various biosynthetic reactions. Ginger (Zingiber officinale), which is often threatened by Ralstonia solanacearum, is the most economically important crop in the family Zingiberaceae. Whether the cytochrome P450 complement (CYPome) significantly responds to this pathogen has remained unclear.

Methods

Transcriptomic responses to R. solanacearum and soil moisture were analyzed in ginger, and expression profiles of the CYPome were determined based on transcriptome data.

Results

A total of 821 P450 unigenes with ORFs ≥ 300 bp were identified. Forty percent soil moisture suppressed several key P450 unigenes involved in the biosynthesis of flavonoids, gingerols, and jasmonates, including unigenes encoding flavonoid 3'-hydroxylase, flavonoid 3',5'-hydroxylase, steroid 22-alpha-hydroxylase, cytochrome P450 family 724 subfamily B polypeptide 1, and allene oxide synthase. Conversely, the expression of P450 unigenes involved in gibberellin biosynthesis and abscisic acid catabolism, encoding ent-kaurene oxidase and abscisic acid 8'-hydroxylase, respectively, were promoted by 40% soil moisture. Under R. solanacearum infection, the expression of P450 unigenes involved in the biosynthesis of the above secondary metabolites were changed, but divergent expression patterns were observed under different soil moisture treatments. High moisture repressed expression of genes involved in flavonoid, brassinosteroid, gingerol, and jasmonate biosynthesis, but promoted expression of genes involved in GA anabolism and ABA catabolism. These results suggest possible mechanisms for how high moisture causes elevated susceptibility to R. solanacearum infection.

Transcriptomic analysis of short-term heat stress response in Pinellia ternata provided novel insights into the improved thermotolerance by spermidine and melatonin

Heat stress has been a major environmental factor limiting the growth and development of Pinellia ternata which is an important Chinese traditional medicine. It has been reported that spermidine (SPD) and melatonin (MLT) play pivotal roles in modulating heat stress response (HSR). However, the roles of SPD and MLT in HSR of P. ternata, and the potential mechanism is still unknown. Here, exogenous SPD and MLT treatments alleviated heat-induced damages in P. ternata, which was supported by the increased chlorophyll content, OJIP curve, and relative water content, and the decreased malondialdehyde and electrolyte leakage. Then, RNA sequencing between CK (control) and Heat (1 h of heat treatment) was conducted to analyze how genes were in response to short-term heat stress in P. ternata. A total of 14,243 (7870 up- and 6373 down-regulated) unigenes were differentially expressed after 1 h of heat treatment. Bioinformatics analysis revealed heat-responsive genes mainly included heat shock proteins (HSPs), ribosomal proteins, ROS-scavenging enzymes, genes involved in calcium signaling, hormone signaling transduction, photosynthesis, pathogen resistance, and transcription factors such as heat stress transcription factors (HSFs), NACs, WRKYs, and bZIPs. Among them, PtABI5, PtNAC042, PtZIP17, PtSOD1, PtHSF30, PtHSFB2b, PtERF095, PtWRKY75, PtGST1, PtHSP23.2, PtHSP70, and PtLHC1 were significantly regulated by SPD or MLT treatment with same or different trends under heat stress condition, indicating that exogenous application of MLT and SPD might enhance heat tolerance in P. ternata through regulating these genes but may with different regulatory patterns. These findings contributed to the identification of potential genes involved in short-term HSR and the improved thermotolerance by MLT and SPD in P. ternata, which provided important clues for improving thermotolerance of P. ternata.

SMRT and Illumina RNA sequencing reveal novel insights into the heat stress response and crosstalk with leaf senescence in tall fescue

BACKGROUND

As a cool-season grass species, tall fescue (Festuca arundinacea) is challenged by increasing temperatures. Heat acclimation or activation of leaf senescence, are two main strategies when tall fescue is exposed to heat stress (HS). However, lacking a genome sequence, the complexity of hexaploidy nature, and the short read of second-generation sequencing hinder a comprehensive understanding of the mechanism. This study aims to characterize the molecular mechanism of heat adaptation and heat-induced senescence at transcriptional and post-transcriptional levels.

RESULTS

Transcriptome of heat-treated (1 h and 72 h) and senescent leaves of tall fescue were generated by combining single-molecular real-time and Illumina sequencing. In total, 4076; 6917, and 11,918 differentially expressed genes (DEGs) were induced by short- and long-term heat stress (HS), and senescence, respectively. Venn and bioinformatics analyses of DEGs showed that short-term HS strongly activated heat shock proteins (Hsps) and heat shock factors (Hsfs), as well as specifically activated FK506-binding proteins (FKBPs), calcium signaling genes, glutathione S-transferase genes, photosynthesis-related genes, and phytohormone signaling genes. By contrast, long-term HS shared most of DEGs with senescence, including the up-regulated chlorophyll catabolic genes, phytohormone synthesis/degradation genes, stress-related genes, and NACs, and the down-regulated photosynthesis-related genes, FKBPs, and catalases. Subsequently, transient overexpression in tobacco showed that FaHsfA2a (up-regulated specifically by short-term HS) reduced cell membrane damages caused by HS, but FaNAC029 and FaNAM-B1 (up-regulated by long-term HS and senescence) increased the damages. Besides, alternative splicing was widely observed in HS and senescence responsive genes, including Hsps, Hsfs, and phytohormone signaling/synthesis genes.

CONCLUSIONS

The short-term HS can stimulate gene responses and improve thermotolerance, but long-term HS is a damage and may accelerate leaf senescence. These results contribute to our understanding of the molecular mechanism underlying heat adaptation and heat-induced senescence.

Abscisic Acid Negatively Modulates Heat Tolerance in Rolled Leaf Rice by Increasing Leaf Temperature and Regulating Energy Homeostasis

BACKGROUND

Abscisic acid (ABA) acts as a signaling hormone in plants against abiotic stress, but its function in energy homeostasis under heat stress is unclear.

RESULTS

Two rice genotypes, Nipponbare (wild-type, WT) with flat leaves and its mutant high temperature susceptibility (hts) plant with semi-rolled leaves, were subjected to heat stress. We found significantly higher tissue temperature, respiration rate, and ABA and H2O2 contents in leaves as well as a lower transpiration rate and stomatal conductance in hts than WT plants. Additionally, increased expression of HSP71.1 and HSP24.1 as well as greater increases in carbohydrate content, ATP, NAD (H), and dry matter weight, were detected in WT than hts plants under heat stress. More importantly, exogenous ABA significantly decreased heat tolerance of hts plants, but clearly enhanced heat resistance of WT plants. The increases in carbohydrates, ATP, NAD (H), and heat shock proteins in WT plants were enhanced by ABA under heat stress, whereas these increases were reduced in hts plants.

CONCLUSION

It was concluded that ABA is a negative regulator of heat tolerance in hts plants with semi-rolled leaves by modulating energy homeostasis.

Plant growth-regulating molecules as thermoprotectants: functional relevance and prospects for improving heat tolerance in food crops

Among various abiotic stresses, heat stress is one of the most damaging, threatening plant productivity and survival all over the world. Warmer temperatures due to climatic anomalies above optimum growing temperatures have detrimental impacts on crop yield potential as well as plant distribution patterns. Heat stress affects overall plant metabolism in terms of physiology, biochemistry, and gene expression. Membrane damage, protein degradation, enzyme inactivation, and the accumulation of reactive oxygen species are some of the harmful effects of heat stress that cause injury to various cellular compartments. Although plants are equipped with various defense strategies to counteract these adversities, their defensive means are not sufficient to defend against the ever-rising temperatures. Hence, substantial yield losses have been observed in all crop species under heat stress. Here, we describe the involvement of various plant growth-regulators (PGRs) (hormones, polyamines, osmoprotectants, antioxidants, and other signaling molecules) in thermotolerance, through diverse cellular mechanisms that protect cells under heat stress. Several studies involving the exogenous application of PGRs to heat-stressed plants have demonstrated their role in imparting tolerance, suggesting the strong potential of these molecules in improving the performance of food crops grown under high temperature.

Label-free quantitative proteomic analysis of Panax ginseng leaves upon exposure to heat stress

Background

Ginseng is one of the well-known medicinal plants, exhibiting diverse medicinal effects. Its roots possess anticancer and antiaging properties and are being used in the medical systems of East Asian countries. It is grown in low-light and low-temperature conditions, and its growth is strongly inhibited at temperatures above 25°C. However, the molecular responses of ginseng to heat stress are currently poorly understood, especially at the protein level.

Methods

We used a shotgun proteomics approach to investigate the effect of heat stress on ginseng leaves. We monitored their photosynthetic efficiency to confirm physiological responses to a high-temperature stress.

Results

The results showed a reduction in photosynthetic efficiency on heat treatment (35°C) starting at 48 h. Label-free quantitative proteome analysis led to the identification of 3,332 proteins, of which 847 were differentially modulated in response to heat stress. The MapMan analysis showed that the proteins with increased abundance were mainly associated with antioxidant and translation-regulating activities, whereas the proteins related to the receptor and structural-binding activities exhibited decreased abundance. Several other proteins including chaperones, G-proteins, calcium-signaling proteins, transcription factors, and transfer/carrier proteins were specifically downregulated.

Conclusion

These results increase our understanding of heat stress responses in the leaves of ginseng at the protein level, for the first time providing a resource for the scientific community.

To grow or not to grow: a stressful decision for plants

Progress in improving abiotic stress tolerance of crop plants using classic breeding and selection approaches has been slow. This has generally been blamed on the lack of reliable traits and phenotyping methods for stress tolerance. In crops, abiotic stress tolerance is most often measured in terms of yield-capacity under adverse weather conditions. "Yield" is a complex trait and is determined by growth and developmental processes which are controlled by environmental signals throughout the life cycle of the plant. The use of model systems has allowed us to gradually unravel how plants grow and develop, but our understanding of the flexibility and opportunistic nature of plant development and its capacity to adapt growth to environmental cues is still evolving. There is genetic variability for the capacity to maintain yield and productivity under abiotic stress conditions in crop plants such as cereals. Technological progress in various domains has made it increasingly possible to mine that genetic variability and develop a better understanding about the basic mechanism of plant growth and abiotic stress tolerance. The aim of this paper is not to give a detailed account of all current research progress, but instead to highlight some of the current research trends that may ultimately lead to strategies for stress-proofing crop species. The focus will be on abiotic stresses that are most often associated with climate change (drought, heat and cold) and those crops that are most important for human nutrition, the cereals.

Abscisic acid induced freezing tolerance in chilling-sensitive suspension cultures and seedlings of rice

BACKGROUND

The role of abscisic acid (ABA) as a possible activator of cold acclimation process was postulated since endogenous levels of ABA increase temporarily or constitutively during cold-hardening. Exogenous application of ABA has been known to induce freezing tolerance at ambient temperatures in in vitro systems derived from cold hardy plants. Yet, some cell cultures acquired much greater freezing tolerance by ABA than by cold whilst maintaining active growth. This raises questions about the relationships among ABA, cold acclimation and growth cessation. To address this question, we attempted to 1) determine whether exogenous ABA can confer freezing tolerance in chilling-sensitive rice suspension cells and seedlings, which obviously lack the mechanisms to acquire freezing tolerance in response to cold; 2) characterize this phenomenon by optimizing the conditions and compare with the case of cold hardy bromegrass cells.

RESULTS

Non-embryogenic suspension cells of rice suffered serious chilling injury when exposed to 4°C. When incubated with ABA at the optimal conditions (0.5-1 g cell inoculum, 75 μM ABA, 25-30°C, 7-10 days), they survived slow freezing (2°C/h) to -9.0 ~ -9.3°C (LT50: 50% killing temperature) while control cells were mostly injured at -3°C (LT50: -0.5 ~ -1.5°C). Ice-inoculation of the cell suspension at -3°C and survival determination by regrowth confirmed that ABA-treated rice cells survived extracellular freezing at -9°C. ABA-induced freezing tolerance did not require any exposure to cold and was best achieved at 25-30°C where the rice cells maintained high growth even in the presence of ABA. ABA treatment also increased tolerance to heat (43°C) as determined by regrowth. ABA-treated cells tended to have more augmented cytoplasm and/or reduced vacuole sizes compared to control cultures with a concomitant increase in osmolarity and a decrease in water content. ABA-treated (2-7 days) in vitro grown seedlings and their leaves survived slow freezing to -3°C with only marginal injury (LT50: -4°C) whereas untreated seedlings were killed at -3°C (LT50: -2°C).

CONCLUSIONS

The results indicate that exogenous ABA can induce some levels of freezing tolerance in chilling-sensitive rice cells and seedlings, probably by eliciting mechanisms different from low temperature-induced cold acclimation.

Abscisic acid and hydrogen peroxide induce modification of plasma membrane H(+)-ATPase from Cucumis sativus L. roots under heat shock

We examined the effect of heat shock (HS), for 2 h at 48°C, on plasma membrane H(+)-ATPase (PM-H(+)-ATPase) measured as the hydrolytic and H(+)-pumping activity. Some of the plants were transferred after 2 h HS to control temperature for another 24 h, as post-stressed (PS) plants. A significant increase of PM-H(+)-ATPase in plants subjected to HS was observed. The stimulation of PM-H(+)-ATPase was higher in PS plants. Estimation of transcript levels of cucumber PM-H(+)-ATPase in roots indicated that the action of HS affected gene expression levels. Transcript levels of two isoforms, CsHA4 and CsHA8, in PS plants were elevated. The expression of PM-H(+)-ATPase genes was not affected in plants treated for 2 h with HS. HS elevated the endogenous level of abscisic acid (ABA) both in plants treated for 2 h with HS and in PS plants. Moreover, in PS plants, a distinctly higher level of H(2)O(2) was observed. It was also demonstrated that transcript levels of PM-H(+)-ATPase were elevated in cucumber roots after 24-h treatment of plants with ABA or H(2)O(2). Both of these compounds seem to play an important role in increasing ATPase activity during heat stress, because the use of the inhibitors tungstate and DPI restrained stimulation of PM-H(+)-ATPase activity by heat. Moreover, protein blot analysis with an antibody against phosphothreonine and 14-3-3 protein indicated that increased activity of PM-H(+)-ATPase under HS resulted from phosphorylation of the enzyme. Taken together, the data presented here suggest that, under post-heat stress conditions, abscisic acid and hydrogen peroxide are involved in PM-ATPase modification, through stimulation of gene expression of that PM proton pump. Moreover, heat treatment of cucumber plants results in increased phosphorylation of PM-ATPase and thus fast post-translational modification, leading to activation of the enzyme protein.

Abiotic stress and control of grain number in cereals

Grain number is the only yield component that is directly associated with increased grain yield in important cereal crops like wheat. Historical yield studies show that increases in grain yield are always accompanied by an increase in grain number. Adverse weather conditions can cause severe fluctuations in grain yield and substantial yield losses in cereal crops. The problem is global and despite its impact on world food production breeding and selection approaches have only met with limited success. A specific period during early reproductive development, the young microspore stage of pollen development, is extremely vulnerable to abiotic stress in self-fertilising cereals (wheat, rice, barley, sorghum). A better understanding of the physiological and molecular processes that lead to stress-induced pollen abortion may provide us with the key to finding solutions for maintaining grain number under abiotic stress conditions. Due to the complexity of the problem, stress-proofing our main cereal crops will be a challenging task and will require joint input from different research disciplines.

Control of abscisic acid catabolism and abscisic acid homeostasis is important for reproductive stage stress tolerance in cereals

Drought stress at the reproductive stage causes pollen sterility and grain loss in wheat (Triticum aestivum). Drought stress induces abscisic acid (ABA) biosynthesis genes in anthers and ABA accumulation in spikes of drought-sensitive wheat varieties. In contrast, drought-tolerant wheat accumulates lower ABA levels, which correlates with lower ABA biosynthesis and higher ABA catabolic gene expression (ABA 8'-hydroxylase). Wheat TaABA8'OH1 deletion lines accumulate higher spike ABA levels and are more drought sensitive. ABA treatment of the spike mimics the effect of drought, causing high levels of sterility. ABA treatment represses the anther cell wall invertase gene TaIVR1, and drought-tolerant lines appeared to be more sensitive to the effect of ABA. Drought-induced sterility shows similarity to cold-induced sterility in rice (Oryza sativa). In cold-stressed rice, the rate of ABA accumulation was similar in cold-sensitive and cold-tolerant lines during the first 8 h of cold treatment, but in the tolerant line, ABA catabolism reduced ABA levels between 8 and 16 h of cold treatment. The ABA biosynthesis gene encoding 9-cis-epoxycarotenoid dioxygenase in anthers is mainly expressed in parenchyma cells surrounding the vascular bundle of the anther. Transgenic rice lines expressing the wheat TaABA8'OH1 gene under the control of the OsG6B tapetum-specific promoter resulted in reduced anther ABA levels under cold conditions. The transgenic lines showed that anther sink strength (OsINV4) was maintained under cold conditions and that this correlated with improved cold stress tolerance. Our data indicate that ABA and ABA 8'-hydroxylase play an important role in controlling anther ABA homeostasis and reproductive stage abiotic stress tolerance in cereals.

Abscisic acid pretreatment enhances salt tolerance of rice seedlings: proteomic evidence

Enhanced salt tolerance of rice seedlings by abscisic acid (ABA) pretreatment was observed from phenotypic and physiological analyses. Total proteins from rice roots treated with ABA plus subsequent salt stress were analyzed by using proteomics method. Results showed that, 40 protein spots were uniquely upregulated in the seedlings under the condition of ABA pretreatment plus subsequent salt stress, whereas only 16 under the condition of salt treatment. About 78% (31 spots) of the 40 protein spots were only upregulated in the presence of the subsequent salt stress, indicating that plants might have an economical strategy to prevent energy loss under a false alarm. The results also showed that more enzymes involved in energy metabolism, defense, primary metabolism, etc. were upregulated uniquely in ABA-pretreated rice seedlings, suggesting more abundant energy supply, more active anabolism (nitrogen, nucleotide acid, carbohydrate, etc), and more comprehensive defense systems in ABA-pretreated seedlings than in salt stressed ones.

Identification of an Arabidopsis Nodulin-related protein in heat stress

We identified a Nodulin-related protein 1 (NRP1) encoded by At2g03440, which was previously reported to be RPS2 interacting protein in yeast-two-hybrid assay. Northern blotting showed that AtNRP1 expression was suppressed by heat stress (42 degrees C) and induced by low temperature (4 degrees C) treatment. Strong GUS staining was observed in the sites of meristematic tissues of pAtNRP1:: GUS transgenic plants, such as shoot apex and root tips, young leaf veins, stamens and stigmas of flowers, and abscission layers of young siliques. To study AtNRP1 biological functions, we have characterized both loss-of-function T-DNA insertion and transgenic overexpression plants for AtNRP1 in Arabidopsis. The T-DNA insertion mutants displayed no obvious difference as compared to wild-type Arabidopsis under heat stress, but the significant enhanced susceptibility to heat stress was revealed in two independent AtNRP1-overexpressing transgenic lines. Further study found that the decreased thermotolerance in AtNRP1-overexpressing lines accompanied significantly decreased accumulation of ABA after heat treatment, which was probably due to AtNRP1 playing a role in negative-feedback regulation of the ABA synthesis pathway. These results support the viewpoint that the application of ABA inhibits nodulation and nodulin-related gene expression and threaten adverse ambient temperature can impact the nodulin-related gene expression.

Hormone- and light-mediated regulation of heat-induced differential petiole growth in Arabidopsis

Plants react quickly and profoundly to changes in their environment. A sudden increase in temperature, for example, induces differential petiole growth-driven upward leaf movement (hyponastic growth) in Arabidopsis (Arabidopsis thaliana). We show that accessions that face the strongest fluctuations in diurnal temperature in their natural habitat are least sensitive for heat-induced hyponastic growth. This indicates that heat-induced hyponastic growth is a trait subject to natural selection. The response is induced with kinetics remarkably similar to ethylene- and low light-induced hyponasty in several accessions. Using pharmacological assays, transcript analysis, and mutant analyses, we demonstrate that ethylene and the photoreceptor protein phytochrome B are negative regulators of heat-induced hyponastic growth and that low light, phytochrome A, auxin, polar auxin transport, and abscisic acid are positive regulators of heat-induced hyponastic growth. Furthermore, auxin, auxin polar transport, phytochrome A, phytochrome B, and cryptochromes are required for a fast induction of heat-induced hyponastic growth.

High humidity induces abscisic acid 8'-hydroxylase in stomata and vasculature to regulate local and systemic abscisic acid responses in Arabidopsis

Levels of endogenous abscisic acid (ABA) are changed dynamically in response to environmental conditions. The ABA 8'-hydroxylase is a key enzyme in ABA catabolism and is encoded by CYP707A genes. In this study, we examined physiological roles of Arabidopsis (Arabidopsis thaliana) CYP707As in the plant's response to changes in humidity. The cyp707a1 and cyp707a3 mutants displayed lower stomatal conductance under turgid conditions (relative humidity 60%) than the wild type. When wild-type plants were transferred to high-humidity conditions (relative humidity 90%), CYP707A1 and CYP707A3 transcript levels increased, followed by the reduction of ABA levels. The cyp707a3 mutant exhibited high ABA levels even after transferring to high-humidity conditions, whereas, under similar conditions, the cyp707a1 mutant exhibited low ABA levels comparable to the wild type. Analysis of spatial expression patterns by using transgenic plants harboring a promoterbeta-glucuronidase gene indicated that high-humidity-induced expression of CYP707A1 and CYP707A3 occurred primarily in guard cells and vascular tissues, respectively. Furthermore, stomatal closure of the cyp707a1 mutant, but not cyp707a3 mutant, was ABA hypersensitive when epidermal peel was treated with exogenous ABA, suggesting that CYP707A1 is essential for ABA catabolism inside the guard cells. These results implicate that CYP707A3 reduces the amount of mobile ABA in vascular tissues in response to high humidity, whereas CYP707A1 inactivates local ABA pools inside the guard cells. Taken together, ABA catabolism in both vascular tissues and guard cells participates in the systemic ABA action that controls stomatal movement in response to high humidity.

Heat stress-responsive transcriptome analysis in heat susceptible and tolerant wheat (Triticum aestivum L.) by using Wheat Genome Array

Background

Wheat is a major crop in the world, and the high temperature stress can reduce the yield of wheat by as much as 15%. The molecular changes in response to heat stress are poorly understood. Using GeneChip^® Wheat Genome Array, we analyzed genome-wide gene expression profiles in the leaves of two wheat genotypes, namely, heat susceptible 'Chinese Spring' (CS) and heat tolerant 'TAM107' (TAM).

Results

A total of 6560 (~10.7%) probe sets displayed 2-fold or more changes in expression in at least one heat treatment (false discovery rate, FDR, α = 0.001). Except for heat shock protein (HSP) and heat shock factor (HSF) genes, these putative heat responsive genes encode transcription factors and proteins involved in phytohormone biosynthesis/signaling, calcium and sugar signal pathways, RNA metabolism, ribosomal proteins, primary and secondary metabolisms, as well as proteins related to other stresses. A total of 313 probe sets were differentially expressed between the two genotypes, which could be responsible for the difference in heat tolerance of the two genotypes. Moreover, 1314 were differentially expressed between the heat treatments with and without pre-acclimation, and 4533 were differentially expressed between short and prolonged heat treatments.

Conclusion

The differences in heat tolerance in different wheat genotypes may be associated with multiple processes and mechanisms involving HSPs, transcription factors, and other stress related genes. Heat acclimation has little effects on gene expression under prolonged treatments but affects gene expression in wheat under short-term heat stress. The heat stress responsive genes identified in this study will facilitate our understanding of molecular basis for heat tolerance in different wheat genotypes and future improvement of heat tolerance in wheat and other cereals.

Over-expression of the Arabidopsis DRE/CRT-binding transcription factor DREB2C enhances thermotolerance

The dehydration responsive element binding protein 2 (DREB2) subgroup belongs to the plant-specific APETALA2/ethylene-responsive element binding factor (AP2/ERF) family of transcription factors. We have characterized cDNA encoding Arabidopsis thaliana DREB2C, which is induced by mild heat stress. Both an electrophoretic mobility shift assay (EMSA) and a yeast one-hybrid assay revealed that DREB2C(145-528) was able to form a complex with the dehydration responsive element/C-repeat (DRE/CRT; A/GCCGAC) motif. A trans-activating ability test in yeast demonstrated that DREB2C could effectively function as a trans-activator. Constitutive expression of DREB2C under the control of the cauliflower mosaic virus (CaMV) 35S promoter led to enhanced thermotolerance in transgenic lines of Arabidopsis. Microarray and RT-PCR analyses of transgenic plants revealed that DREB2C regulates expression of several heat stress-inducible genes that contain DRE/CRT elements in their promoters. From these data, we deduced that DREB2C is a regulator of heat stress tolerance in Arabidopsis.

The effect of heat shock on paclitaxel production in Taxus yunnanensis cell suspension cultures: role of abscisic acid pretreatment

The heat shock (HS) response is a conserved cellular defense mechanism to elevated temperatures, observed in cells from bacteria to human. It is characterized by the increased accumulation of HS proteins. This work examines the effect of HS on the secondary metabolite biosynthesis of cultured plant cells. Suspension cultures of Taxus yunnanensis cells, which produce the anticancer diterpenoid paclitaxel (Taxol), were heat shocked at 35-50 degrees C for 30-60 min. The results show that HS reduced cell viability and growth but significantly induced paclitaxel production. The HS-induced paclitaxel production depended on the intensity of HS and the physiological state of the cells. Abscisic acid (ABA)-pretreatment not only increased cell viability and growth upon HS but also improved HS-induced paclitaxel yield. The best culture phase to apply the HS was the late-exponential growth phase. Under the optimized condition, HS enhanced paclitaxel yield by sixfold to 6.8 mg/L. In addition, a prior mild-HS treatment also significantly increased HS-induced paclitaxel production. Furthermore, HS induced oxidative burst, the early event of plant defense response to pathogen attack and other stress challenge; the addition of putative inhibitors of lipoxygenase, a key enzyme for jasmonic acid biosynthesis, significantly inhibited HS-induced pacliatxel accumulation. The stimulation of secondary metabolite production by HS may be a result of HS-induced plant cell defense response.

A novel ABA-hypersensitive mutant in Arabidopsis defines a genetic locus that confers tolerance to xerothermic stress

Hot and dry air (harmattan or xerothermic climate) greatly inhibits plant growth, particularly flowering and seed setting of crops. Little is known about the mechanism of plant response to this extreme environmental stress due to the lack of valuable genetic resource. Here, we report the isolation and characteristics of a unique Arabidopsis mutant, hat1 (harmattan tolerant 1), which shows high tolerance to hot and dry air. Under normal growth conditions, the mutant does not differ in morphology and soil drought tolerance compared to the wild type. When subjected to high temperature (42 degrees C) and low humidity (10-15%), however, it could survive up to 6 days, while the wild type (Col-0) died after 24 h. The hat1 mutant also exhibits enhanced tolerance to soil drought, but only under xerothermic conditions. Mutant plants tightly close their stomata to retain water under xerothermic stress, and are more tolerant to high salinity at all developmental stages, accumulating less Na+ and more K+ than wild-type plants during NaCl treatment. Interestingly, hat1 plants are also ABA-hypersensitive. Genetic analysis revealed that the hat1 phenotype is caused by a dominant mutation at a single nuclear locus. Mapping studies indicate that Hat1 is located at an interval of 168 kb on chromosome 5 in which 21 genes are known to be regulated by diverse abiotic stresses. A mutant of this kind, to our knowledge, has not been previously reported. Thus, this report serves as a starting point in the genetic dissection of the plant response to xerothermic stress, and provides physiological and genetic evidence of the existence of a novel abiotic stress response pathway that is also ABA-dependent.

Proteome profiling of Populus euphratica Oliv. upon heat stress

BACKGROUND AND AIMS

Populus euphratica is a light-demanding species ecologically characterized as a pioneer. It grows in shelter belts along riversides, being part of the natural desert forest ecosystems in China and Middle Eastern countries. It is able to survive extreme temperatures, drought and salt stress, marking itself out as an important plant species to study the mechanisms responsible for survival of woody plants under heat stress.

METHODS

Heat effects were evaluated through electrolyte leakage on leaf discs, and LT(50) was determined to occur above 50 degrees C. Protein accumulation profiles of leaves from young plants submitted to 42/37 degrees C for 3 d in a phytotron were determined through 2D-PAGE, and a total of 45 % of up- and downregulated proteins were detected. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF)/TOF analysis, combined with searches in different databases, enabled the identification of 82 % of the selected spots.

KEY RESULTS

Short-term upregulated proteins are related to membrane destabilization and cytoskeleton restructuring, sulfur assimilation, thiamine and hydrophobic amino acid biosynthesis, and protein stability. Long-term upregulated proteins are involved in redox homeostasis and photosynthesis. Late downregulated proteins are involved mainly in carbon metabolism.

CONCLUSIONS

Moderate heat response involves proteins related to lipid biogenesis, cytoskeleton structure, sulfate assimilation, thiamine and hydrophobic amino acid biosynthesis, and nuclear transport. Photostasis is achieved through carbon metabolism adjustment, a decrease of photosystem II (PSII) abundance and an increase of PSI contribution to photosynthetic linear electron flow. Thioredoxin h may have a special role in this process in P. euphratica upon moderate heat exposure.

Gene expression profiles during heat acclimation in Arabidopsis thaliana suspension-culture cells

Thermotolerance is induced by moderated heat acclimation. Suspension cultures of heat-acclimated Arabidopsis thaliana L. (Heynh.), ecotype Columbia, show thermotolerance against lethal heat shock (9 min, 50 degrees C), as evidenced by a chlorophyll assay and fluorescein diacetate staining. To monitor the genome-wide transcriptome changes induced by heat acclimation at 37 degrees C, we constructed an A. thaliana cDNA microarray containing 7,989 unique genes, and applied it to A. thaliana suspension-culture cells harvested at various times (0.5, 1, 2.5, 6, and 16 h) during heat acclimation. Data analysis revealed 165 differentially expressed genes that were grouped into ten clusters. We compared these genes with published and publicly available microarray heat-stress-related data sets in AtGenExpress. Heat-shock proteins were strongly expressed, as previously reported, and we found several of the up-regulated genes encoded detoxification and regulatory proteins. Moreover, the transcriptional induction of DREB2 (dehydration responsive element-binding factor 2) subfamily genes and COR47/rd17 under heat stress suggested cross-talk between the signaling pathways for heat and dehydration responses.

Heat stress phenotypes of Arabidopsis mutants implicate multiple signaling pathways in the acquisition of thermotolerance

To investigate the importance of different processes to heat stress tolerance, 45 Arabidopsis (Arabidopsis thaliana) mutants and one transgenic line were tested for basal and acquired thermotolerance at different stages of growth. Plants tested were defective in signaling pathways (abscisic acid, salicylic acid, ethylene, and oxidative burst signaling) and in reactive oxygen metabolism (ascorbic acid or glutathione production, catalase) or had previously been found to have temperature-related phenotypes (e.g. fatty acid desaturase mutants, uvh6). Mutants were assessed for thermotolerance defects in seed germination, hypocotyl elongation, root growth, and seedling survival. To assess oxidative damage and alterations in the heat shock response, thiobarbituric acid reactive substances, heat shock protein 101, and small heat shock protein levels were determined. Fifteen mutants showed significant phenotypes. Abscisic acid (ABA) signaling mutants (abi1 and abi2) and the UV-sensitive mutant, uvh6, showed the strongest defects in acquired thermotolerance of root growth and seedling survival. Mutations in nicotinamide adenine dinucleotide phosphate oxidase homolog genes (atrbohB and D), ABA biosynthesis mutants (aba1, aba2, and aba3), and NahG transgenic lines (salicylic acid deficient) showed weaker defects. Ethylene signaling mutants (ein2 and etr1) and reactive oxygen metabolism mutants (vtc1, vtc2, npq1, and cad2) were more defective in basal than acquired thermotolerance, especially under high light. All mutants accumulated wild-type levels of heat shock protein 101 and small heat shock proteins. These data indicate that, separate from heat shock protein induction, ABA, active oxygen species, and salicylic acid pathways are involved in acquired thermotolerance and that UVH6 plays a significant role in temperature responses in addition to its role in UV stress.

Generation of active pools of abscisic acid revealed by in vivo imaging of water-stressed Arabidopsis

A noninvasive, cell-autonomous reporter system was developed to monitor the generation and distribution of physiologically active pools of abscisic acid (ABA). ABA response (abi1-1) and biosynthesis (aba2-1) mutants of Arabidopsis (Arabidopsis thaliana) were used to validate the system in the presence and absence of water stress. In the absence of water stress, low levels of ABA-dependent reporter activation were observed in the columella cells and quiescent center of the root as well as in the vascular tissues and stomata of cotyledons, suggesting a nonstress-related role for ABA in these cell types. Exposure of seedlings to exogenous ABA resulted in a uniform pattern of reporter expression. In marked contrast, reporter expression in response to drought stress was predominantly confined to the vasculature and stomata. Surprisingly, water stress applied to the root system resulted in the generation of ABA pools in the shoot but not in the root. The analysis of the response dynamics revealed a spread of physiologically active ABA from the vascular tissue into the areoles of the cotyledons. Later, ABA preferentially activated gene expression in guard cells. The primary sites of ABA action identified by in planta imaging corresponded to the sites of ABA biosynthesis, i.e. guard cells and cells associated with vascular veins. Hence, water stress recognized by the root system predominantly results in shoot-localized ABA action that culminates in a focused response in guard cells.

ABF2, an ABRE-binding bZIP factor, is an essential component of glucose signaling and its overexpression affects multiple stress tolerance

Phytohormone abscisic acid (ABA) regulates stress-responsive gene expression during vegetative growth, which is mediated largely by cis-elements sharing the ACGTGGC consensus. Although many transcription factors are known to bind the elements in vitro, only a few have been demonstrated to have in vivo functions and their specific roles in ABA/stress responses are mostly unknown. Here, we report that ABF2, an ABF subfamily member of bZIP proteins interacting with the ABA-responsive elements, is involved in ABA/stress responses. Its overexpression altered ABA sensitivity, dehydration tolerance, and the expression levels of ABA/stress-regulated genes. Furthermore, ABF2 overexpression promoted glucose-induced inhibition of seedling development, whereas its mutation impaired glucose response. The reduced sugar sensitivity was not observed with mutants of two other ABF family members, ABF3 and ABF4. Instead, these mutants displayed defects in ABA, salt, and dehydration responses, which were not observed with the abf2 mutant. Our data indicate distinct roles of ABF family members: whereas ABF3 and ABF4 play essential roles in ABA/stress responses, ABF2 is required for normal glucose response. We also show that ABF2 overexpression affects multiple stress tolerance.

Thermotolerance and antioxidant systems in Agrostis stolonifera: involvement of salicylic acid, abscisic acid, calcium, hydrogen peroxide, and ethylene

This study investigated whether pre-treating plants with specific putative signaling components and heat acclimation would induce tolerance of a cool-season grass, creeping bentgrass (Agrostis stolonifera var. palustris), to subsequent heat stress and whether thermotolerance induction of those pretreatments was associated with the regulation of antioxidant regenerating enzymes. The treatments included foliar application of salicylic acid (SA), abscisic acid (ABA), calcium chloride (CaCl2), hydrogen peroxide (H2O2), 1-aminocyclopropane-1-carboxylic acid (ACC, a precursor of ethylene prior to the exposure of plants to heat stress (35 degrees C) in a growth chamber. Physiological measurements including turf quality, leaf photosynthetic rate, and levels of oxidative damage demonstrated that all treatments increased heat tolerance. The better heat tolerance for pre-treated plants as compared to controls was related to the protection of oxidative damage under heat stress. APX activity increased over the first 2 days and 5 days of heating for ACC and CaCl2 respectively, but for only 12 h for H2O2. SA and ABA pre-treatments had no effects on APX activity earlier, but maintained APX activity at a significantly higher level than in controls after 24 h of heating. SA and ABA pre-treatments had no effects on POX activity. ACC treatment significantly increased POX activity. Pre-treatment with CaCl2, H2O2, and HA reduced POX activity, particularly during the later phase of heating. Plants treated with SA, CaCl2, H2O2 and HA had lower CAT activity than their control plants prior to heating and within 48 h of heat stress. ABA and ACC pre-treatments maintained higher CAT activity than the controls after 48 h of heating. ACC, CaCl2, or HA pre-treatments increased SOD activity only before 5 days of heat stress. SA and ABA pre-treatments had less effect on APX activity earlier under heat stress. These results suggest that specific groups of potential signaling molecules may induce tolerance of creeping bentgrass to heat stress by reducing oxidative damage.

Enhanced low oxygen survival in Arabidopsis through increased metabolic flux in the fermentative pathway

We manipulated the enzyme activity levels of the alcohol fermentation pathway, pyruvate decarboxylase (PDC), and alcohol dehydrogenase (ADH) in Arabidopsis using sense and antisense overexpression of the corresponding genes (PDC1, PDC2, and ADH1). Transgenic plants were analyzed for levels of fermentation and evaluated for changes in hypoxic survival. Overexpression of either Arabidopsis PDC1 or PDC2 resulted in improved plant survival. In contrast, overexpression of Arabidopsis ADH1 had no effect on flooding survival. These results support the role of PDC as the control step in ethanol fermentation. Although ADH1 null mutants had decreased hypoxic survival, attempts to reduce the level of PDC activity enough to see an effect on plant survival met with limited success. The combination of flooding survival data and metabolite analysis allows identification of critical metabolic flux points. This information can be used to design transgenic strategies to improve hypoxic tolerance in plants.

Purification of native dehydrin from Glycine Max cv., Pisum sativum, and Rosmarinum officinalis by affinity chromatography

An improved method for the purification of dehydrin from soy (glycine max) is described. Acidic extraction of soy whey was followed by a three step chromatographic process: capture on copper charged Chelating Sepharose Big Beads, intermediate hydrophobic interaction chromatography on Source 15 PHE, and a polishing step on blue Sepharose. The 32-kDa native soy dehydrin was purified to a purity of greater than 98.5% with an overall recovery of 63%. When compared to a previously published purification procedure, recovery, time requirements, and sample preparation steps were improved. The developed method is readily scaleable. Preliminary results show that the process can be used for dehydrins from rosemary (Rosmarinum officinalis) and pea (Pisum sativum).

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

Plants, in common with all organisms, have evolved mechanisms to cope with the problems caused by high temperatures. We examined specifically the involvement of calcium, abscisic acid (ABA), ethylene, and salicylic acid (SA) in the protection against heat-induced oxidative damage in Arabidopsis. Heat caused increased thiobarbituric acid reactive substance levels (an indicator of oxidative damage to membranes) and reduced survival. Both effects required light and were reduced in plants that had acquired thermotolerance through a mild heat pretreatment. Calcium channel blockers and calmodulin inhibitors increased these effects of heating and added calcium reversed them, implying that protection against heat-induced oxidative damage in Arabidopsis requires calcium and calmodulin. Similar to calcium, SA, 1-aminocyclopropane-1-carboxylic acid (a precursor to ethylene), and ABA added to plants protected them from heat-induced oxidative damage. In addition, the ethylene-insensitive mutant etr-1, the ABA-insensitive mutant abi-1, and a transgenic line expressing nahG (consequently inhibited in SA production) showed increased susceptibility to heat. These data suggest that protection against heat-induced oxidative damage in Arabidopsis also involves ethylene, ABA, and SA. Real time measurements of cytosolic calcium levels during heating in Arabidopsis detected no increases in response to heat per se, but showed transient elevations in response to recovery from heating. The magnitude of these calcium peaks was greater in thermotolerant plants, implying that these calcium signals might play a role in mediating the effects of acquired thermotolerance. Calcium channel blockers and calmodulin inhibitors added solely during the recovery phase suggest that this role for calcium is in protecting against oxidative damage specifically during/after recovery.

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

Plants, in common with all organisms, have evolved mechanisms to cope with the problems caused by high temperatures. We examined specifically the involvement of calcium, abscisic acid (ABA), ethylene, and salicylic acid (SA) in the protection against heat-induced oxidative damage in Arabidopsis. Heat caused increased thiobarbituric acid reactive substance levels (an indicator of oxidative damage to membranes) and reduced survival. Both effects required light and were reduced in plants that had acquired thermotolerance through a mild heat pretreatment. Calcium channel blockers and calmodulin inhibitors increased these effects of heating and added calcium reversed them, implying that protection against heat-induced oxidative damage in Arabidopsis requires calcium and calmodulin. Similar to calcium, SA, 1-aminocyclopropane-1-carboxylic acid (a precursor to ethylene), and ABA added to plants protected them from heat-induced oxidative damage. In addition, the ethylene-insensitive mutant etr-1, the ABA-insensitive mutant abi-1, and a transgenic line expressing nahG (consequently inhibited in SA production) showed increased susceptibility to heat. These data suggest that protection against heat-induced oxidative damage in Arabidopsis also involves ethylene, ABA, and SA. Real time measurements of cytosolic calcium levels during heating in Arabidopsis detected no increases in response to heat per se, but showed transient elevations in response to recovery from heating. The magnitude of these calcium peaks was greater in thermotolerant plants, implying that these calcium signals might play a role in mediating the effects of acquired thermotolerance. Calcium channel blockers and calmodulin inhibitors added solely during the recovery phase suggest that this role for calcium is in protecting against oxidative damage specifically during/after recovery.

The aspartyl replacement of the active site histidine in histidine-containing protein, HPr, of the Escherichia coli Phosphoenolpyruvate:Sugar phosphotransferase system can accept and donate a phosphoryl group. Spontaneous dephosphorylation of acyl-phosphate autocatalyzes an internal cyclization

The active site residue, His(15), in histidine-containing protein, HPr, can be replaced by aspartate and still act as a phosphoacceptor and phosphodonor with enzyme I and enzyme IIA(glucose), respectively. Other substitutions, including cysteine, glutamate, serine, threonine, and tyrosine, failed to show any activity. Enzyme I K(m) for His(15) --> Asp HPr is increased 10-fold and V(max) is decreased 1000-fold compared with wild type HPr. The phosphorylation of Asp(15) led to a spontaneous internal rearrangement involving the loss of the phosphoryl group and a water molecule, which was confirmed by mass spectrometry. The protein species formed had a higher pI than His(15) --> Asp HPr, which could arise from the formation of a succinimide or an isoimide. Hydrolysis of the isolated high pI form gave only aspartic acid at residue 15, and no isoaspartic acid was detected. This indicates that an isoimide rather than a succinimide is formed. In the absence of phosphorylation, no formation of the high pI form could be found, indicating that phosphorylation catalyzed the formation of the cyclization. The possible involvement of Asn(12) in an internal cyclization with Asp(15) was eliminated by the Asn(12) --> Ala mutation in His(15) --> AspHPr. Asn(12) substitutions of alanine, aspartate, serine, and threonine in wild type HPr indicated a general requirement for residues capable of forming a hydrogen bond with the Nepsilon(2) atom of His(15), but elimination of the hydrogen bond has only a 4-fold decrease in k(cat)/K(m).