Evaluation of the acute toxic response induced by triazophos to the non-target green algae Chlorella pyrenoidosa.

Residues of triazophos in aquatic ecosystems due to extensive use for controlling pests in agriculture has became worldwide concern, while the toxic response of triazophos on the non-target green algae in aquatic environment is not well studied. Therefore, the acute (96 h) toxic effects of 1 and 10 mg/L triazophos on green algae Chlorella pyrenoidosa were evaluated in present study. The results showed that the growth was notably inhibited when treated with triazophos and the 96 h-EC50 (median inhibition concentration) were 12.79 mg/L. The content of photosynthetic pigments (including chl a, chl b, total-chl and carotinoids) clearly decreased under two treatments after 48 h and 96 h with exception for the values at 48 h exposure in 1 mg/L treatment. In addition, the transcript abundance of photosynthesis-related genes (psbA, psbC and rbcL) showed obvious decrease in above two treatments after exposure 96 h to triazophos. In response to 10 mg/L triazophos treatment, the morphology of thylakoid chloroplast of algal cells were obviously damaged. It was also found that starch granules increased with down-regulation of atpB gene expression in 10 mg/L treatment, which suggests that triazophos may inhibit the energy metabolism of C. pyrenoidosa. Moreover, the algal growth inhibition was along with the increase of intracellular reactive oxygen species (ROS), activity of antioxidant enzymes and malondialdehyde content indicating oxidative damage and lipid peroxidation in the algal cells. Our findings reveal that triazophos has potential toxicity and environmental risks to one of the primary producers green algae.

Acute toxicity of triflumizole to freshwater green algae Chlorella vulgaris.

Triflumizole is one of imidazole fungicides that works by inhibiting ergosterol biosynthesis, and is widely used for the control of powdery mildew and scabs on various fruits and crops. Triflumizole residue has been frequently detected in soil and aquatic ecosystems. While many studies have focused on its toxic effect on terrestrial and aquatic animals, little attention has been paid to aquatic algae, the primary producers of aquatic environments. Therefore, we evaluated the acute (96 h) toxicity effects of triflumizole on the freshwater algae Chlorella vulgaris, by examining growth, cell morphology, photosynthesis, and oxidative stress. The results showed that the 96 h median inhibition concentration (96 h-EC50) was 0.82 mg/L (95% confidential interval 0.70-0.98 mg/L).The growth of algal cells was conspicuously inhibited by triflumizole exposure, and the cell surfaces appeared to be shrunkThe chlorophyll content (including Chl-a, Chl-b and T-Chl) dramatically decreased at triflumizole concentrations of 0.2 and 1.0 mg/L. In addition, the transcript abundance of photosynthesis-related genes (psaB, psbC and rbcL) showed obvious decreases in above treatments after 96 h of exposure to triflumizole. Moreover, the algal growth inhibition was accompanied by an increase in intracellular reactive oxygen species and malondialdehyde content, as well as increased activity of antioxidant enzymes such as superoxide dismutase and peroxidase, indicating oxidative stress and lipid peroxidation. Our findings reveal that triflumizole has potential toxicity to the primary producers (freshwater algae) in aquatic ecosystems.

An antimicrobial peptide essential for bacterial survival in the nitrogen-fixing symbiosis.

In the nitrogen-fixing symbiosis between legume hosts and rhizobia, the bacteria are engulfed by a plant cell membrane to become intracellular organelles. In the model legume Medicago truncatula, internalization and differentiation of Sinorhizobium (also known as Ensifer) meliloti is a prerequisite for nitrogen fixation. The host mechanisms that ensure the long-term survival of differentiating intracellular bacteria (bacteroids) in this unusual association are unclear. The M. truncatula defective nitrogen fixation4 (dnf4) mutant is unable to form a productive symbiosis, even though late symbiotic marker genes are expressed in mutant nodules. We discovered that in the dnf4 mutant, bacteroids can apparently differentiate, but they fail to persist within host cells in the process. We found that the DNF4 gene encodes NCR211, a member of the family of nodule-specific cysteine-rich (NCR) peptides. The phenotype of dnf4 suggests that NCR211 acts to promote the intracellular survival of differentiating bacteroids. The greatest expression of DNF4 was observed in the nodule interzone II-III, where bacteroids undergo differentiation. A translational fusion of DNF4 with GFP localizes to the peribacteroid space, and synthetic NCR211 prevents free-living S. meliloti from forming colonies, in contrast to mock controls, suggesting that DNF4 may interact with bacteroids directly or indirectly for its function. Our findings indicate that a successful symbiosis requires host effectors that not only induce bacterial differentiation, but also that maintain intracellular bacteroids during the host-symbiont interaction. The discovery of NCR211 peptides that maintain bacterial survival inside host cells has important implications for improving legume crops.

Medicago truncatula esn1 defines a genetic locus involved in nodule senescence and symbiotic nitrogen fixation.

Symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti results in the formation on the host roots of new organs, nodules, in which biological nitrogen fixation takes place. In infected cells, rhizobia enclosed in a plant-derived membrane, the symbiosome membrane, differentiate to nitrogen-fixing bacteroids. The symbiosome membrane serves as an interface for metabolite and signal exchanges between the host cells and endosymbionts. At some point during symbiosis, symbiosomes and symbiotic cells are disintegrated, resulting in nodule senescence. The regulatory mechanisms that underlie nodule senescence are not fully understood. Using a forward genetics approach, we have uncovered the early senescent nodule 1 (esn1) mutant from an M. truncatula fast neutron-induced mutant collection. Nodules on esn1 roots are spherically shaped, ineffective in nitrogen fixation, and senesce early. Atypical among fixation defective mutants isolated thus far, bacteroid differentiation and expression of nifH, Leghemoglobin, and DNF1 genes are not affected in esn1 nodules, supporting the idea that a process downstream of bacteroid differentiation and nitrogenase gene expression is affected in the esn1 mutant. Expression analysis shows that marker genes involved in senescence, macronutrient degradation, and remobilization are greatly upregulated during nodule development in the esn1 mutant, consistent with a role of ESN1 in nodule senescence and symbiotic nitrogen fixation.

Sodium-Related Adaptations to Drought: New Insights From the Xerophyte Plant Zygophyllum xanthoxylum.

Understanding the unusual physiological mechanisms that enable drought tolerance in xerophytes will be of considerable benefit because of the potential to identify novel and key genetic elements for future crop improvements. These plants are interesting because they are well-adapted for life in arid zones; Zygophyllum xanthoxylum, for example, is a typical xerophytic shrub that inhabits central Asian deserts, accumulating substantial levels of sodium (Na+) in its succulent leaves while growing in soils that contain very low levels of this ion. The physiological importance of this unusual trait to drought adaptations remains poorly understood, however. Thus, 2-week-old Z. xanthoxylum plants were treated with 50 mM NaCl (Na) for 7 days in this study in order to investigate their drought tolerance, leaf osmotic potential (Ψs) related parameters, anatomical characteristics, and transpiration traits. The results demonstrated that NaCl treatment significantly enhanced both the survivability and durability of Z. xanthoxylum plants under extreme drought conditions. The bulk of the Na+ ions encapsulated in plants was overwhelmingly allocated to leaves rather than roots or stems under drought conditions; thus, compared to the control, significantly more Na+ compared to other solutes such as K+, Ca2+, Cl-, sugars, and proline accumulated in the leaves of NaCl-treated plants and led to a marked decrease (31%) in leaf Ψs. In addition, the accumulation of Na+ ions also resulted in mesophyll cell enlargement and leaf succulence, enabling the additional storage of water; Na+ ions also reduced the rate of water loss by decreasing stomatal density and down-regulating stomatal aperture size. The results of this study demonstrate that Z. xanthoxylum has evolved a notable ability to utilize Na+ ions to lower Ψs, swell its leaves, and decrease stomatal aperture sizes, in order to enable the additional uptake and storage of water and mitigate losses. These distinctive drought adaption characteristics mean that the xerophytic plant Z. xanthoxylum presents a fascinating case study for the potential identification of important and novel genetic elements that could improve crops. This report provides insights on the eco-physiological role of sodium accumulation in xerophytes adapted to extremely arid habitats.

Hydrogen sulfide - cysteine cycle system enhances cadmium tolerance through alleviating cadmium-induced oxidative stress and ion toxicity in Arabidopsis roots.

Cadmium (Cd2+) is a common toxic heavy metal ion. We investigated the roles of hydrogen sulfide (H2S) and cysteine (Cys) in plant responses to Cd2+ stress. The expression of H2S synthetic genes LCD and DES1 were induced by Cd2+ within 3 h, and endogenous H2S was then rapidly released. H2S promoted the expression of Cys synthesis-related genes SAT1 and OASA1, which led to endogenous Cys accumulation. The H2S and Cys cycle system was stimulated by Cd2+ stress, and it maintained high levels in plant cells. H2S inhibited the ROS burst by inducing alternative respiration capacity (AP) and antioxidase activity. H2S weakened Cd2+ toxicity by inducing the metallothionein (MTs) genes expression. Cys promoted GSH accumulation and inhibited the ROS burst, and GSH induced the expression of phytochelatin (PCs) genes, counteracting Cd2+ toxicity. In summary, the H2S and Cys cycle system played a key role in plant responses to Cd2+ stress. The Cd2+ tolerance was weakened when the cycle system was blocked in lcddes1-1 and oasa1 mutants. This paper is the first to describe the role of the H2S and Cys cycle system in Cd2+ stress and to explore the relevant and specificity mechanisms of H2S and Cys in mediating Cd2+ stress.

Co-expression of xerophyte Zygophyllum xanthoxylum ZxNHX and ZxVP1-1 enhances salt and drought tolerance in transgenic Lotus corniculatus by increasing cations accumulation.

Lotus corniculatus L. is an important legume for forage, but is sensitive to salinity and drought. To develop salt- and drought-resistant L. corniculatus, ZxNHX and ZxVP1-1 genes encoding tonoplast Na+/H+ antiporter and H+-pyrophosphatase (H+-PPase) from a succulent xerophyte Zygophyllum xanthoxylum L., which is well adapted to arid environments through accumulating Na+ in its leaves, were transferred into this forage. We obtained the transgenic lines co-expressing ZxNHX and ZxVP1-1 genes (VX) as well as expressing ZxVP1-1 gene alone (VP). Compared with wild-type, both VX and VP transgenic lines grew better at 200mM NaCl, and also exhibited higher tolerance and faster recovery from water-deficit stress: these performances were associated with more Na+, K+ and Ca2+ accumulation in their leaves and roots, which caused lower leaf solute potential and thus retained more water. Moreover, the transgenic lines maintained lower relative membrane permeability and higher net photosynthesis rate under salt or water-deficit stress. These results indicate that expression of tonoplast Na+/H+ antiporter and H+-PPase genes from xerophyte enhanced salt and drought tolerance of L. corniculatus. Furthermore, compared with VP, VX showed higher shoot biomass, more cations accumulation, higher water retention, lesser cell membrane damage and higher photosynthesis capacity under salt or water-deficit condition, suggesting that co-expression of ZxVP1-1 and ZxNHX confers even greater performance to transgenic L. corniculatus than expression of the single ZxVP1-1.

The ZxNHX gene encoding tonoplast Na(+)/H(+) antiporter from the xerophyte Zygophyllum xanthoxylum plays important roles in response to salt and drought.

Sodium (Na(+)) has been found to play important roles in the adaptation of xerophytic species to drought conditions. The tonoplast Na(+)/H(+) antiporter (NHX) proved to be involved in the compartmentalization of Na(+) into vacuoles from the cytosol. In this study, a gene (ZxNHX) encoding tonoplast Na(+)/H(+) antiporter was isolated and characterized in Zygophyllum xanthoxylum, a succulent xerophyte growing in desert areas of northwest China. The results revealed that ZxNHX consisted of 532 amino acid residues with a conserved binding domain ((78)LFFIYLLPPI(87)) for amiloride and shared high similarity (73-81%) with the identified tonoplast Na(+)/H(+) antiporters in other plant species. Semi-quantitative RT-PCR analysis showed that the mRNA level of ZxNHX was significantly higher in the leaf than in stem or root. The transcript abundance of ZxNHX in Z. xanthoxylum subjected to salt (5-150 mM NaCl) or drought (50-15% of field water capacity (FWC)) was 1.4-8.4 times or 2.3-4.4 times that of plants grown in the absence of NaCl or 70% of FWC, respectively. Leaf Na(+) concentration in plants exposed to salt or drought was 1.7-5.2 times or 1.5-2.2 times that of corresponding control plants, respectively. It is clear that there is a positive correlation between up-regulation of ZxNHX and accumulation of Na(+) in Z. xanthoxylum exposed to salt or drought. Furthermore, Z. xanthoxylum accumulated larger amounts of Na(+) than K(+) in the leaf under drought conditions, even in low salt soil. In summary, our results suggest that ZxNHX encodes a tonoplast Na(+)/H(+) antiporter and plays important roles in Na(+) accumulation and homeostasis of Z. xanthoxylum under salt and drought conditions.