Microbial responses to salt-induced osmotic stress

Summary The construction and assembly of a model root region is described. The model was used to manipulate the soil matrix, soil microorganisms, and to simulate release of root exudates. The design of the apparatus facilitated long-term, direct microscopic observations of microbial activity in soil and on artificial roots. Preliminary studies indicate that microbial responses to osmotic stress and to changes in components of exudate solutions are easily monitored.     

The salt stress relief and growth promotion effect of Rs-5 on cotton

The effect of the Rs-5 bacteria strain, identified as Klebsiella oxytoca and isolated with ACC as the sole nitrogen source, on salt stressed cotton seedling growth was studied. It was demonstrated that Rs-5 could obviously relieve salt stress and promote cotton seedling growth. After treatment with Rs-5, the individual plant height and dry weight of cotton increased by 14.9 and 26.9%, respectively, compared to the control. Further analysis found that Rs-5 exhibited the ability to increase the cotton’s absorption of the N, P, K, and Ca elements and decrease the absorbability of the Na element under salt stress. In addition, Rs-5 itself could produce phytohormone-auxin, and was capable of dissolving phosphorus (P). The ratio of the dissolved P diameter to the colony diameter was 1.86. The dissolved P was 81.6 mg·l⁻¹ in media after four days of incubation. Responsible Editor: Petra Marschner.     

Cardiolipin and the osmotic stress responses of bacteria

Cells control their own hydration by accumulating solutes when they are exposed to high osmolality media and releasing solutes in response to osmotic down-shocks. Osmosensory transporters mediate solute accumulation and mechanosensitive channels mediate solute release. Escherichia coli serves as a paradigm for studies of cellular osmoregulation. Growth in media of high salinity alters the phospholipid headgroup and fatty acid compositions of bacterial cytoplasmic membranes, in many cases increasing the ratio of anionic to zwitterionic lipid. In E. coli, the proportion of cardiolipin (CL) increases as the proportion of phosphatidylethanolamine (PE) decreases when osmotic stress is imposed with an electrolyte or a non-electrolyte. Osmotic induction of the gene encoding CL synthase (cls) contributes to these changes. The proportion of phosphatidylglycerol (PG) increases at the expense of PE in cls⁻ bacteria and, in Bacillus subtilis, the genes encoding CL and PG synthases (clsA and pgsA) are both osmotically regulated. CL is concentrated at the poles of diverse bacterial cells. A FlAsH-tagged variant of osmosensory transporter ProP is also concentrated at E. coli cell poles. Polar concentration of ProP is CL-dependent whereas polar concentration of its paralogue LacY, a H⁺-lactose symporter, is not. The proportion of anionic lipids (CL and PG) modulates the function of ProP in vivo and in vitro. These effects suggest that the osmotic induction of CL synthesis and co-localization of ProP with CL at the cell poles adjust the osmolality range over which ProP activity is controlled by placing it in a CL-rich membrane environment. In contrast, a GFP-tagged variant of mechanosensitive channel MscL is not concentrated at the cell poles but anionic lipids bind to a specific site on each subunit of MscL and influence its function in vitro. The sub-cellular locations and lipid dependencies of other osmosensory systems are not known. Varying CL content is a key element of osmotic adaptation by bacteria but much remains to be learned about its roles in the localization and function of osmoregulatory proteins.     

Hypermethylation of tobacco heterochromatic loci in response to osmotic stress

 Plants have to cope with a number of envi-ronmental stresses which may potentially induce genetic and epigenetic changes and thus contribute to genome variability. In the present study we inspected the DNA methylation status of two heterochromatic loci (defined with repetitive DNA sequences HRS60 and GRS) in a tobacco cell culture exposed to osmotic stress. Investigations were performed on a TBY-2 cell suspension culture, and the stress was elicited with NaCl or D-mannitol. Using the restriction enzymes MspI/HpaII and MboI/Sau3AI in combination with Southern hydridization we observed a reversible hypermethylation of the external cytosine at the CpCpG trinucleotides in cells grown under mild osmotic stress equal to a NaCl concentration of 10 g/l. There were no changes in the methylation of the internal cytosine as the CpG dinucleotides within the CCGG motifs (HpaII sites) appeared to be fully methylated in tobacco DNA repetitive sequences under normal physiological conditions. The data suggest epigenetic changes in the plant genome based on de novo methylation of DNA in response to environmental stress. Received: 26 November 1996/Accepted: 20 December 1996     

Morphological and physiological responses of the halophyte, Odyssea paucinervis (Staph) (Poaceae), to salinity

In this study, salt tolerance was investigated in Odyssea paucinervis Staph, an ecologically important C₄ grass that is widely distributed in saline and arid areas of southern Africa. Plants were subjected to 0.2%, 10%, 20%, 40%, 60% and 80% sea water dilutions (or 0.076, 3.8, 7.6, 15.2, 22.8, and 30.4 parts per thousand) for 11 weeks. Increase in salinity from 0.2% to 20% sea water had no effect on total dry biomass accumulation, while further increase in salinity to 80% sea water significantly decreased biomass by over 50%. Morphological changes induced by salinity included reductions in the number of culms, leaves and internodes as well as decreases in internode length and leaf length:leaf width ratios. Carbon dioxide exchange, leaf conductance and transpiration decreased at salinities of 40% and higher, while quantum yield of photosystem II (PSII), electron transport rate (ETR) through PSII and intrinsic photosynthetic efficiency generally decreased at salinities of 60% and higher compared to 0.2% sea water. Concentrations of Na⁺ and Cl⁻ increased significantly with salinity increase in both roots and shoots. Na⁺/K⁺ ratios in the roots and shoots ranged from 0.66 to 3.28 and increased with increase in substrate salinity. The maximal rate of secretion at 80% sea water was 415 nmol cm⁻² d⁻¹ for Na⁺ and 763 nmol m⁻² d⁻¹ for Cl⁻ with high selectivity for these two ions. Predawn and midday ψ decreased with increase in salinity and were more negative than those of the treatment solutions. The concentration of proline increased with increase in salinity in both roots and shoots. The data clearly indicated that O. paucinervis is a highly salt-tolerant species that is morphologically and physiologically adapted to a saline environment.     

Induction of water-stress proteins in cyanobacteria exposed to matric- or osmotic-water stress

Abstract: Specific stress polypeptides were detected in three drought-resistant cyanobacteria ( Phormidium autumnale , LPP₄ and Chroococcidiopsis sp.) subjected to matric- and osmotic-water stress. Drought stress caused the induction of at least 2–3 new polypeptides of apparent molecular masses of 30 kDa and 40–45 kDa. The polypeptide of 30 kDa was located in the thylakoid membranes, and the 45-kDa polypeptide in the cytoplasm. When these cyanobacteria were exposed to salt stress polypeptides of similar size appeared.     

The impact of arbuscular mycorrhizal fungi in mitigating salt-induced adverse effects in sweet basil (Ocimum basilicum L.)

Salinity is one of the serious abiotic stresses adversely affecting the majority of arable lands worldwide, limiting the crop productivity of most of the economically important crops. Sweet basil (Osmium basilicum) plants were grown in a non-saline soil (EC = 0.64 dS m⁻¹), in low saline soil (EC = 5 dS m⁻¹), and in a high saline soil (EC = 10 dS m⁻¹). There were differences between arbuscular mycorrhizal (Glomus deserticola) colonized plants (+AMF) and non-colonized plants (−AMF). Mycorrhiza mitigated the reduction of K, P and Ca uptake due to salinity. The balance between K/Na and between Ca/Na was improved in +AMF plants. Growth enhancement by mycorrhiza was independent from plant phosphorus content under high salinity levels. Different growth parameters, salt stress tolerance and accumulation of proline content were investigated, these results showed that the use of mycorrhizal inoculum (AMF) was able to enhance the productivity of sweet basil plants under salinity conditions. Mycorrhizal inoculation significantly increased chlorophyll content and water use efficiency under salinity stress. The sweet basil plants appeared to have high dependency on AMF which improved plant growth, photosynthetic efficiency, gas exchange and water use efficiency under salinity stress. In this study, there was evidence that colonization with AMF can alleviate the detrimental salinity stress influence on the growth and productivity of sweet basil plants.     

Microbial responses to environmental arsenic

Microorganisms have evolved dynamic mechanisms for facing the toxicity of arsenic in the environment. In this sense, arsenic speciation and mobility is also affected by the microbial metabolism that participates in the biogeochemical cycle of the element. The ars operon constitutes the most ubiquitous and important scheme of arsenic tolerance in bacteria. This system mediates the extrusion of arsenite out of the cells. There are also other microbial activities that alter the chemical characteristics of arsenic: some strains are able to oxidize arsenite or reduce arsenate as part of their respiratory processes. These type of microorganisms require membrane associated proteins that transfer electrons from or to arsenic (AoxAB and ArrAB, respectively). Other enzymatic transformations, such as methylation-demethylation reactions, exchange inorganic arsenic into organic forms contributing to its complex environmental turnover. This short review highlights recent studies in ecology, biochemistry and molecular biology of these processes in bacteria, and also provides some examples of genetic engineering for enhanced arsenic accumulation based on phytochelatins or metallothionein-like proteins.     

Perspectives of reducing the deleterious effect of de-icing salt upon vegetation

Summary The protective effect of a nutrient solution upon young forest plants, seriously stressed by de-icing salt (NaCl) were investigated. The results show, that the adsorbed salt ions in the soil are already exchanged at low concentrations (2 bars), whilst plant survival and growth showed only at higher concentrations (10 bars) a significant optimum.     

Physiological responses of Zygosaccharomyces rouxii to osmotic stress

When cell suspensions of Zygosaccharomyces rouxii were subjected to osmotic shock with NaCl, the cell volume decreased sharply and plasmolysis was observed. The cell subsequently recovered and volumes similar to those of cells growing at the respective water activity (a_w) values were found. Cycloheximide prevented cell recovery, indicating the involvement of protein synthesis in the recovery process. The intracellular glycerol concentration of Z. rouxii incubated in the presence of [¹⁴C]glycerol increased from 13 to 96 mmol/l during the initial 20 min after an upshock from 0.998 a_w to 0.96 a_w. All the intracellular glycerol was labelled and therefore derived from the medium. Labelled glycerol was subsequently utilized and replaced by unlabelled glycerol produced by the cell within 90 min. The initial increase in glycerol concentration following the upshock was confirmed by ¹³C-nuclear magnetic resonance (NMR) spectroscopic studies of cell extracts. The combined dihydroxyacetone and dihydroxyacetone phosphate concentrations fluctuated during this period, whereas glycerol-3-phosphate initially increased and then remained constant. This indicates that the production of glycerol is regulated. Decreases in ATP and polyphosphate levels were observed following osmotic upshock and may reflect a greater demand for ATP during the period of adjustment to decreased a_w. The changes in cell volume and in ATP concentration following osmotic upshock may serve as osmoregulatory signals in Z. rouxii, as suggested previously for other microorganisms.Correspondence to: S. G. Kilian     

Escherichia coli and Lactobacillus plantarum responses to osmotic stress

Escherichia coli and Lactobacillus plantarum were subjected to final water potentials of −5.6 MPa and −11.5 MPa with three solutes: glycerol, sorbitol and NaCl. The water potential decrease was realized either rapidly (osmotic shock) or slowly (20 min) and a difference in cell viability between these conditions was only observed when the solute was NaCl. The cell mortality during osmotic shocks induced by NaCl cannot be explained by a critical volume decrease or by the intensity of the water flow across the cell membrane. When the osmotic stress is realized with NaCl as the solute, in a medium in which osmoregulation cannot take place, the application of a slow decrease in water potential resulted in the significant maintenance of cell viability (about 70–90%) with regard to the corresponding viability observed after a sudden step change to same final water potential (14–40%). This viability difference can be explained by the existence of a critical internal free Na⁺ concentration. Received: 20 May 1998 / Received revision: 31 July 1998 / Accepted: 31 July 1998     

Photosynthesis under osmotic stress

The reversibility of the inhibition of photosynthetic reactions by water stress was examined with four systems of increasing complexity—stromal enzymes, intact chloroplasts, mesophyll protoplasts, and leaf slices. The inhibition of soluble chloroplast enzymes by high solute concentrations was instantly relieved when solutes were properly diluted. In contrast, photosynthesis was not restored but actually more inhibited when isolated chloroplasts exposed to hypertonic stress were transferred to conditions optimal for photosynthesis of unstressed chloroplasts. Upon transfer, chloroplast volumes increased beyond the volumes of unstressed chloroplasts, and partial envelope rupture occurred. In protoplasts and leaf slices, considerable and rapid, but incomplete restoration of photosynthesis was observed during transfer from hypertonic to isotonic conditions. Chloroplast envelopes did not rupture in situ during water uptake. It is concluded that inhibition of photosynthesis by severe water stress is at the biochemical level brought about in part by reversible inhibition of chloroplast enzymes and in part by membrane damage which requires repair mechanisms for reversibility. Both soluble enzymes and membranes appear to be affected by the increased concentration of internal solutes, which is caused by dehydration.     

Differential stress responses of early salt-stress responding genes in common wheat

Four early salt-stress responding genes (WESR1-4) in common wheat (Triticum aestivum L.) were analyzed for their temporal accumulation of mRNA during salt stress, osmotic stress and abscisic acid (ABA) treatment. All genes showed transient stimulation by 0.15 M NaCl treatment. WESR1 and WESR2 were induced by both osmotic stress and exogenous ABA treatment. WESR3 responded to exogenous ABA, but not to osmotic stress. WESR4 did not show significant response to either osmotic stress or exogenous ABA treatment. These results suggest that wheat has at least two salt stress signal transduction pathways, an ABA-dependent and ABA-independent pathway.     

Antioxidant responses of shoots and roots of lentil to NaCl-salinity stress

The effect of salt stress (100 mM and 200 mM NaCl) on antioxidant responses in shoots and roots of 14-day-old lentil (Lens culinaris M.) seedlings was investigated. Salt stress caused a significant decrease in length, wet-dry weight and an increase in proline content of both shoot and root tissues. In leaf tissues, high salinity treatment resulted in a 4.4 fold increase in H₂O₂ content which was accompanied by a significant level of lipid peroxidation and an increase in electrolyte leakage. Root tissues were less affected with respect to these parameters. Leaf tissue extracts exhibited four activity bands, of which two were identified as Cu/Zn-SOD and others as Fe-SOD and Mn-SOD. Fe-SOD activity was missing in root extracts. In both tissues Cu/Zn-SOD activity comprised 70–75% of total SOD activity. Salt stress did not cause a significant increase in total SOD activity of leaf tissues but a significant enhancement (88%) was observed in roots mainly due to an enhancement in Cu/ZnSOD isoforms. Compared to leaf tissues a significantly higher constitutive ascorbate peroxidase (APX) and glutathion reductase (GR) activity was observed in root tissues. Upon salt stress no significant change in the activity of APX, catalase (CAT) and GR was observed in root tissues but a higher APX activity was present when compared to leaf tissues. On the other hand, in leaf tissues, with the exception of CAT, salt stress caused significant enhancement in the activity of other antioxidant enzymes. These results suggested that, root tissues of lentil are protected better from NaCl stress induced oxidative damage due to enhanced total SOD activity together with a higher level of APX activity under salinity stress. To our knowledge this is the first report describing antioxidant enzyme activities in lentil.     

Microbial responses to nitrogen addition in three contrasting grassland ecosystems

The effects of global N enrichment on soil processes in grassland ecosystems have received relatively little study. We assessed microbial community response to experimental increases in N availability by measuring extracellular enzyme activity (EEA) in soils from three grasslands with contrasting edaphic and climatic characteristics: a semiarid grassland at the Sevilleta National Wildlife Refuge, New Mexico, USA (SEV), and mesic grasslands at Konza Prairie, Kansas, USA (KNZ) and Ukulinga Research Farm, KwaZulu-Natal, South Africa (SAF). We hypothesized that, with N enrichment, soil microbial communities would increase C and P acquisition activity, decrease N acquisition activity, and reduce oxidative enzyme production (leading to recalcitrant soil organic matter [SOM] accumulation), and that the magnitude of response would decrease with soil age (due to higher stabilization of enzyme pools and P limitation of response). Cellulolytic activities followed the pattern predicted, increasing 35–52% in the youngest soil (SEV), 10–14% in the intermediate soil (KNZ) and remaining constant in the oldest soil (SAF). The magnitude of phosphatase response did not vary among sites. N acquisition activity response was driven by the enzyme closest to its pH optimum in each soil: i.e., leucine aminopeptidase in alkaline soil, β-N-acetylglucosaminidase in acidic soil. Oxidative enzyme activity varied widely across ecosystems, but did not decrease with N amendment at any site. Likewise, SOM and %C pools did not respond to N enrichment. Between-site variation in both soil properties and EEA exceeded any treatment response, and a large portion of EEA variability (leucine aminopeptidase and oxidative enzymes), 68% as shown by principal components analysis, was strongly related to soil pH (r = 0.91, P < 0.001). In these grassland ecosystems, soil microbial responses appear constrained by a molecular-scale (pH) edaphic factor, making potential breakdown rates of SOM resistant to N enrichment.