Characterizing the stress/defense transcriptome of <Emphasis Type="Italic">Arabidopsis</Emphasis>

Background  

To understand the gene networks that underlie plant stress and defense responses, it is necessary to identify and characterize the genes that respond both initially and as the physiological response to the stress or pathogen develops. We used PCR-based suppression subtractive hybridization to identify Arabidopsis genes that are differentially expressed in response to ozone, bacterial and oomycete pathogens and the signaling molecules salicylic acid (SA) and jasmonic acid.     

Endophytic infection modifies organic acid and mineral element accumulation by rice under Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> stress

Background and aims

Saline and alkali soils severely impact plant growth. Endophyte and plant associations are known to significantly modify plant metabolism. This study reports the effects of a type of endophyte on organic acid (OA) accumulation and ionic balance in rice under Na₂CO₃ stress.

Methods

Rice seedlings with (E+) and without (E-) endophytic infection were subjected to different levels of Na₂CO₃ stress (0, 5, 10, 15, and 20 mM) for two weeks. Organic acids and mineral elements in the leaves and roots were determined.

Results

Seedlings with endophytic infection accumulated mainly citrate and fumarate, with some malate and succinate in the leaves. In the roots, accumulation of malate and fumarate was enhanced significantly by endophytic infection, while less citrate and succinate was accumulated under Na₂CO₃ stress, which suggested that leaves and roots use different mechanisms to control OA metabolism. Endophytes reduced the total Na and Na:K ratios, but increased ST values, the percent changes of other measured nutrients, Chl content, and dry weight per plant under Na₂CO₃ stress.

Conclusions

Endophytic infection plays a key role in maintaining plant growth by improving nutrient uptake and adjusting OA accumulation under Na₂CO₃ stress. The application of endophytes can enhance the resistance of rice to salinity.

     

Plant growth and responses of antioxidants of <Emphasis Type="Italic">Chenopodium album</Emphasis> to long-term NaCl and KCl stress

The effects of long-term NaCl and KCl treatment on plant growth and antioxidative responses were investigated in Chenopodium album, a salt-resistant species widely distributed in semi-arid and light-saline areas of Xinjiang, China. Growth parameters [plant height, branch number, leaf morphology and chlorophyll (Chl) content], the level of oxidative stress [superoxide anion radical (O₂ ⁻), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) concentrations], activity of antioxidant enzymes [superoxide dismutase (SOD), catalase (CAT), peroxidase (POX)], the contents of non-enzymatic antioxidants [carotenoids (Car) and ascorbic acid (AsA)] and expression of selected genes were investigated. Plants were grown in the presence of 0, 50, and 300 mM NaCl or KCl for 2 months. Growth was stimulated by 50 mM NaCl or KCl, maintained stable at 300 mM NaCl, but was inhibited by 300 mM KCl. Three hundred mM NaCl did not affect O₂ ⁻, H₂O₂, MDA, Car and AsA, but increased the activities of SOD, CAT and POX compared to the controls. RT-PCR analysis suggested that expression of some genes encoding antioxidant enzymes could be induced during long-term salt stress, which was consistent with the enzyme activities. Treatment with 300 mM KCl was associated with elevated oxidative stress, and significantly decreased Car and AsA contents. These results suggest that an efficient antioxidant machinery is important for overcoming oxidative stress induced by treatment with high NaCl concentrations in C. album. Other strategies of ion regulation may also contribute to the differential tolerance to Na and K at higher concentrations.     

Elevated CO<Subscript>2</Subscript> ameliorated oxidative stress induced by elevated O<Subscript>3</Subscript> in <Emphasis Type="Italic">Quercus mongolica</Emphasis>

Using open top chambers, the effects of elevated O₃ (80 nmol mol⁻¹) and elevated CO₂ (700 μmol mol⁻¹), alone and in combination, were studied on young trees of Quercus mongolica. The results showed that elevated O₃ increased malondialdehyde content and decreased photosynthetic rate after 45 days of exposure, and prolonged exposure (105 days) induced significant increase in electrolyte leakage and reduction of chlorophyll content. All these changes were alleviated by elevated CO₂, indicating that oxidative stress on cell membrane and photosynthesis was ameliorated. After 45 days of exposure, elevated O₃ stimulated activities of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (APX, EC 1.11.1.11), but the stimulation was dampened under elevated CO₂ exposure. Furthermore, ascorbate (AsA) and total phenolics contents were not higher in the combined gas treatment than those in elevated O₃ treatment. It indicates that the protective effect of elevated CO₂ against O₃ stress was achieved hardly by enhancing ROS scavenging ability after 45 days of exposure. After 105 days of exposure, elevated O₃ significantly decreased activities of SOD, catalase (CAT, EC 1.11.1.6) and APX and AsA content. Elevated CO₂ suppressed the O₃-induced decrease, which could ameliorate the oxidative stress in some extent. In addition, elevated CO₂ increased total phenolics content in the leaves both under ambient O₃ and elevated O₃ exposure, which might contribute to the protection against O₃-induced oxidative stress as well.     

Small chloroplast-targeted DnaJ proteins are involved in optimization of photosynthetic reactions in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Background  

DnaJ proteins participate in many metabolic pathways through dynamic interactions with various components of these processes. The role of three small chloroplast-targeted DnaJ proteins, AtJ8 (At1 g80920), AtJ11 (At4 g36040) and AtJ20 (At4 g13830), was investigated here using knock-out mutants of Arabidopsis thaliana. Photochemical efficiency, capacity of CO₂ assimilation, stabilization of Photosystem (PS) II dimers and supercomplexes under high light illumination, energy distribution between PSI and PSII and phosphorylation of PSII-LHCII proteins, global gene expression profiles and oxidative stress responses of these DnaJ mutants were analyzed.     

Natural diversity of potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis>) invertases

Background  

Invertases are ubiquitous enzymes that irreversibly cleave sucrose into fructose and glucose. Plant invertases play important roles in carbohydrate metabolism, plant development, and biotic and abiotic stress responses. In potato (Solanum tuberosum), invertases are involved in 'cold-induced sweetening' of tubers, an adaptive response to cold stress, which negatively affects the quality of potato chips and French fries. Linkage and association studies have identified quantitative trait loci (QTL) for tuber sugar content and chip quality that colocalize with three independent potato invertase loci, which together encode five invertase genes. The role of natural allelic variation of these genes in controlling the variation of tuber sugar content in different genotypes is unknown.     

Plant-like substitutions in the large-subunit carboxy terminus of <Emphasis Type="Italic">Chlamydomonas</Emphasis> Rubisco increase CO<Subscript>2</Subscript>/O<Subscript>2</Subscript> Specificity

Background  

Ribulose-1,5-bisphosphate is the rate-limiting enzyme in photosynthesis. The catalytic large subunit of the green-algal enzyme from Chlamydomonas reinhardtii is ~90% identical to the flowering-plant sequences, although they confer diverse kinetic properties. To identify the regions that may account for species variation in kinetic properties, directed mutagenesis and chloroplast transformation were used to create four amino-acid substitutions in the carboxy terminus of the Chlamydomonas large subunit to mimic the sequence of higher-specificity plant enzymes.     

Overexpression of Organellar and Cytosolic <Emphasis Type="Italic">AtHSP90</Emphasis> in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> Impairs Plant Tolerance to Oxidative Stress

Three AtHSP90 isoforms, cytosol-localized AtHSP90.2, chloroplast-localized AtHSP90.5, and endoplasmic reticulum (ER)-localized AtHSP90.7 genes, were constitutively overexpressed in Arabidopsis thaliana to study their functional mechanisms under oxidative stress. Overexpression of AtHSP90 genes reduced germination of transgenic seeds under oxidative stress. When exposed to 10 mM H₂O₂, AtHSP90 transgenic seedlings displayed lower activities of superoxide dismutase, catalase, and peroxidase; higher content of malondialdehyde; and higher levels of protein damage than detected in the wild type. This indicated that overexpression of AtHSP90.2, AtHSP90.5, and AtHSP90.7 in Arabidopsis impaired plant tolerance to oxidative stress. Moreover, overexpression of chloroplast- and ER-localized AtHSP90 resulted in lower resistance to oxidative stress than that of cytosolic AtHSP90. This suggested that HSP90.2, HSP90.5, and HSP90.7 localized in different cellular compartments were involved in different functional mechanisms during oxidative stress.     

Enhancement of artemisinin content and biomass in <Emphasis Type="Italic">Artemisia annua</Emphasis> by exogenous GA<Subscript>3</Subscript> treatment

Artemisinin is a promising and potent antimalarial drug naturally produced by the plant Artemisia annua L. but in very low yield. Its artemisinin content is known to be greatly affected by both genotype and environmental factors. In this study, the production of artemisinin and leaf biomass in Artemisia annua L. was significantly increased by exogenous GA₃ treatment. The effect of GA₃ application on expression of proposed key enzymes involved in artemisinin yield was examined in both wild type (007) and FPS-overexpression (253-2) lines of A. annua. In the wild type (007) at 6 h post GA₃ application there was an abrupt rise in FPS, ADS and CYP71AV1 expression and at 24 h a temporary and significant peak in artemisinin (1.45-fold higher than the control). After GA₃ application in line 253-2, there was a dramatic rise in expression of FPS at 3 h, CYP71AV1 at 9 h and ADS at 72 h and accumulation of artemisinin after 7 days, which was a delay when compared with the wild type plant. Thus, increased artemisinin content from exogenous GA₃ treatment was associated with increased expression of key enzymes in the artemisinin biosynthesis pathway. Interestingly, exogenous GA₃ continuously enhanced artemisinin content from the vegetative stage to flower initiation in both plant lines and gave significantly higher leaf biomass than in control plants. Consequently, the artemisinin yield in GA₃-treated plants was much higher than in control plants. Although the maximum artemisinin content was found at the full blooming stage [2.1% dry weight (DW) in 007 and 2.4% DW in 253-2], the highest artemisinin yield in GA₃-treated plants was obtained during the flower initiation stage (2.4 mg/plant in 007 and 2.3 mg/plant in 235-2). This was 26.3 and 27.8% higher, respectively, than in non-treated plants 007 and 253-2. This study showed that exogenous GA₃ treatment enhanced artemisinin production in pot experiments and should be suitable for field application.     

Adenosine thiamine triphosphate accumulates in <Emphasis Type="Italic">Escherichia coli</Emphasis> cells in response to specific conditions of metabolic stress

Background  

E. coli cells are rich in thiamine, most of it in the form of the cofactor thiamine diphosphate (ThDP). Free ThDP is the precursor for two triphosphorylated derivatives, thiamine triphosphate (ThTP) and the newly discovered adenosine thiamine triphosphate (AThTP). While, ThTP accumulation requires oxidation of a carbon source, AThTP slowly accumulates in response to carbon starvation, reaching ~15% of total thiamine. Here, we address the question whether AThTP accumulation in E. coli is triggered by the absence of a carbon source in the medium, the resulting drop in energy charge or other forms of metabolic stress.     

Exogenous H<Subscript>2</Subscript>O<Subscript>2</Subscript> improves the chilling tolerance of manilagrass and mascarenegrass by activating the antioxidative system

The effect of foliar pretreatment by hydrogen peroxide (H₂O₂) at low concentrations of 0, 5, 10, and 15 mM on the chilling tolerance of two Zoysia cultivars, manilagrass (Zoysia matrella) and mascarenegrass (Zoysia tenuifolia), was studied. The optimal concentration for H₂O₂ pretreatment was 10 mM, as demonstrated by the lowest malondialdehyde (MDA) content and electrolyte leakage (EL) levels and higher protein content under chilling stress (7°C/2°C, day/night). Prior to initiation of chilling, exogenous 10 mM H₂O₂ significantly increased catalase (CAT), ascorbate peroxidase (APX), glutathione-dependent peroxidases (GPX), and glutathione-S-transferase (GST) activities in manilagrass, and guaiacol peroxidase (POD), APX, and glutathione reductase (GR) activities in mascarenegrass, suggesting that H₂O₂ may act as a signaling molecule, inducing protective metabolic responses against further oxidative damage due to chilling. Under further stress, optimal pretreatments alleviated the increase of H₂O₂ level and the decrease of turfgrass quality, and improved CAT, POD, APX, GR, and GPX activities, with especially significant enhancement of APX and GPX activities from the initiation to end of chilling. These antioxidative enzymes were likely the important factors for acquisition of tolerance to chilling stress in the two Zoysia cultivars. Our results showed that pretreatment with H₂O₂ at appropriate concentration may improve the tolerance of warm-season Zoysia grasses to chilling stress, and that manilagrass had better tolerance to chilling, as evaluated by lower MDA and EL, and better turfgrass quality, regardless of the pretreatment applied.     

Changes in antioxidant gene expression and induction of oxidative stress in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis> L.) under Al stress

Aluminium toxicity has been recognized as a primary growth-limiting factor in acid soil, resulting in a decrease in plant growth and production. In this experiment we have studied the induction of oxidative stress and changes in antioxidant gene expression in pea (var. ALASKA) under aluminium (Al) stress. We have found that Al treatment affected the growth of pea plant and induced oxidative stress with a change in antioxidant gene expression profile. While the expression of glutathione-s-transferase (GST) and catalase (CAT) was more in root, cytosolic Ascorbate peroxidase (cAPX) expression increased in shoot under aluminium stress. Copper- Zinc Superoxide dismutase (Cu-Zn SOD) gene expression was higher after 24 h but decreased after 48 h along with elevated expression of manganese superoxide dismutase (MnSOD) and iron-superoxide dismutase (FeSOD) at higher aluminium contentrations after 24 and 48 h. Aluminium stress elevated hydrogen peroxide (H₂O₂) level and affected the growth. The proline content did not change significantly, whereas glutathione content increased with a decreased ascorbate content under Al stress. The present study indicates that aluminium treatment affected the antioxidant gene expression and induced oxidative stress in pea plant.     

Transfer RNA modifications and genes for modifying enzymes in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Background  

In all domains of life, transfer RNA (tRNA) molecules contain modified nucleosides. Modifications to tRNAs affect their coding capacity and influence codon-anticodon interactions. Nucleoside modification deficiencies have a diverse range of effects, from decreased virulence in bacteria, neural system disease in human, and gene expression and stress response changes in plants. The purpose of this study was to identify genes involved in tRNA modification in the model plant Arabidopsis thaliana, to understand the function of nucleoside modifications in plant growth and development.