盐、碱胁迫下小冰麦体内的pH及离子平衡

通过混合两种中性盐（NaCl和Na₂SO₄）和两种碱性盐（NaHCO₃和Na₂CO₃）分别模拟出不同强度的盐、碱胁迫条件,对小冰麦苗进行12 d胁迫处理,测定茎叶组织液的pH值及Na⁺、K⁺、Ca²⁺、Cl^-、SO₄^2-、NO₃^-、H₂PO₄-和有机酸等溶质的浓度,以探讨盐、碱两种胁迫下小冰麦体内的pH及离子平衡特点.结果表明：盐、碱胁迫下小冰麦茎叶内的pH值均稳定不变;随胁迫强度的增加,盐胁迫下小冰麦茎叶内有机酸浓度没有明显变化,Cl^-浓度大幅度增加,而碱胁迫下有机酸浓度大幅度增加,Cl^-浓度没有明显变化.盐、碱胁迫下小冰麦茎叶中的阳离子均以Na⁺和K⁺为主,但阴离子的来源明显不同.盐胁迫下无机阴离子对负电荷的贡献起主导作用,其贡献率达61.3%～66.7%;而碱胁迫下,随胁迫强度的增大,有机酸对负离子的贡献率从38.35%上升到61.60%,逐渐成为主导成分.实验结果表明,有机酸积累是小冰麦在碱胁迫下保持体内离子平衡和pH稳定的关键生理响应.     

Modeling and mechanism of the adsorption of proton onto natural bamboo sawdust

Natural bamboo sawdust with cellulose-lignin polymeric structure was used as a raw adsorbent to remove heavy metal and dyes in water. The analysis of surface properties showed that high proton affinity sites were mainly composed of phenolic and alcohol hydroxyl, while low proton affinity sites mainly consisted of carboxylic acid, silicon hydroxyl, aluminum hydroxyl and some low densities of sulfhydryl and phosphoryl groups. The results fitted by NEM surface complexation model showed: three-sites model fitted acid-base titration data better than two-sites and one-site model. Acid-base titration experiments showed the amount of acid and base consumed was not uniform between acid and alkali side. This was probably due to the swelling of cellulose in acid side, but the possibility that different reactions occurred in acid and alkali side cannot be completely excluded. Pznpc of sawdust was between pH 5.2 and 5.5.     

The effect of a genetically reduced plasma membrane protonmotive force on vegetative growth of Arabidopsis

The plasma membrane proton gradient is an essential feature of plant cells. In Arabidopsis (Arabidopsis thaliana), this gradient is generated by the plasma membrane proton pump encoded by a family of 11 genes (abbreviated as AHA, for Arabidopsis H(+)-ATPase), of which AHA1 and AHA2 are the two most predominantly expressed in seedlings and adult plants. Although double knockdown mutant plants containing T-DNA insertions in both genes are embryonic lethal, under ideal laboratory growth conditions, single knockdown mutant plants with a 50% reduction in proton pump concentration complete their life cycle without any observable growth alteration. However, when grown under conditions that induce stress on the plasma membrane protonmotive force (PMF), such as high external potassium to reduce the electrical gradient or high external pH to reduce the proton chemical gradient, aha2 mutant plants show a growth retardation compared with wild-type plants. In this report, we describe the results of studies that examine in greater detail AHA2's specific role in maintaining the PMF during seedling growth. By comparing the wild type and aha2 mutants, we have measured the effects of a reduced PMF on root and hypocotyl growth, ATP-induced skewed root growth, and rapid cytoplasmic calcium spiking. In addition, genome-wide gene expression profiling revealed the up-regulation of potassium transporters in aha2 mutants, indicating, as predicted, a close link between the PMF and potassium uptake at the plasma membrane. Overall, this characterization of aha2 mutants provides an experimental and theoretical framework for investigating growth and signaling processes that are mediated by PMF-coupled energetics at the cell membrane.     

Eco-physiological responses of linseed (Linum usitatissimum) to salt and alkali stresses

AimsEffects of salt and alkali stresses (NaCl-Na₂SO₄ and NaHCO₃-Na₂CO₃) were compared on growth, photosynthesis characters, ionic balance and osmotic adjustment of linseed (Linum usitatissimum), to elucidate the mechanisms of salt and alkali stress (high pH value) damage to plants, and their physiological adaptive mechanisms to the stresses. MethodsThe experiment was carried out in an artificial greenhouse. Plants grew at approximately 700 mmol·m^-2·s^-1 photosynthetic photon flux density (PPFD) in greenhouse under photoperiod of 15 h in light and 9 h in dark. In each plastic pot (17 cm diameter) which contained 2.5 kg of washed sand, 20 linseed seeds were sown. The seedlings were exposed to stresses lasting 14 days after 2 months.Important findingsThe inhibitory effects of alkali stress on linseed growth were more remarkable than those of salt stress, indicating that alkali and salt represent two distinct forms of stress. The alkali stress increased the Na⁺ content in shoots, damaged the photosynthetic system, and highly reduced the net photosynthetic rate and C assimilation capacity. Under salinity stress, the Na⁺ content increased, the K⁺ content decreased with increasing stress. Greater changes were observed under alkali than under salt stress. Alkali stress caused the massive influx of Na⁺, which probably explained that the harmful of alkali stress on plants was stronger than that of salt stress. Under alkali stress, Ca²⁺ and Mg²⁺ decreased in roots, showing that high pH value around roots hindered the absorption of them. Fe²⁺ and Zn²⁺ had little effects on the osmotic adjustment, mainly because of they had a low ion content. Under salt stress, anion increased in order to balance the sharp increase of Na⁺. However, alkali stress made severe deficit of negative charge, broke the intracellular ionic balance and pH homeostasis, and caused a series of strain response. Our results showed that linseed enhanced the synthesis of soluble sugars to balance massive influx of Na⁺ under salt stress, but linseed enhanced the synthesis of organic acids to compensate for the shortage of inorganic anions, which might be a key pathway for the pH adjustment. In conclusion, the alkali stress (high pH value) clearly inhibited the growth, element absorption, ion homeostasis reconstruction of plants. Organic acid concentration is possibly a key adaptive factor for linseed to maintain intracellular ion balance and regulate high pH value under alkali stress.     

亚麻响应盐、碱胁迫的生理特征

利用中性盐NaCl、Na₂SO₄和碱性盐NaHCO₃、Na₂CO₃混合模拟不同强度的盐、碱胁迫条件, 对亚麻(Linum usitatissimum)进行14天胁迫处理, 测定其地上部分和根生长速率、光合特征、离子平衡及有机渗透调节物质积累, 以探讨亚麻对盐、碱两种胁迫的生理响应特点。研究表明: 亚麻生长对盐、碱胁迫的响应存在差异, 在相同盐浓度下, 碱胁迫对亚麻的伤害大于盐胁迫。碱胁迫使地上部分中Na⁺浓度急剧增高, 造成叶绿体破坏、光合色素含量下降, 光合能力及碳同化能力也急剧下降。亚麻中Na⁺含量随着胁迫强度的增加而升高, 而K⁺含量呈下降趋势, 碱胁迫下的变化明显大于盐胁迫。因此, 碱胁迫导致Na⁺过度积累可能是碱胁迫对植物伤害大于盐胁迫的最主要原因。碱胁迫下Ca²⁺和Mg²⁺在根中下降明显, 可见高pH值阻碍根对Ca²⁺和Mg²⁺的吸收。Fe²⁺和Zn²⁺对渗透调节的影响不大, 因为它们的离子含量较低。盐胁迫促进阴离子(Cl^-、H₂PO₄^-和SO₄^2-)的积累来平衡大量涌入的Na⁺, 但是碱胁迫明显减少无机阴离子含量, 可能造成严重营养胁迫(如P和S不足)。亚麻在盐胁迫下积累大量可溶性糖来平衡大量的Na⁺, 但碱胁迫下积累大量有机酸来维持细胞内离子平衡和pH值稳定, 碱胁迫大量积累的有机酸也可能被分泌到根外调节根外的pH值, 这说明亚麻对两种不同胁迫的响应方式不同。研究证明高pH值会直接影响植物根系的生长发育, 影响植物矿质元素的吸收, 阻碍离子稳态重建, 有机酸代谢是亚麻碱胁迫下的关键适应机制。     

碱胁迫对羊草和向日葵的影响

以抗盐碱性较强的单子叶植物羊草和双子叶植物向日葵为材料,对其进行中性盐、碱性盐和各种中碱性混合盐等胁迫处理,以日相对生长率(RGR)为主要胁变指标分析各种胁迫的特点及其相互关系.结果表明,碱性盐胁迫与中性盐胁迫实际上是既相关又有本质区别的两种不同胁迫,应该将碱性盐胁迫定义为碱胁迫,而将中性盐胁迫定义为盐胁迫.碱胁迫区别于盐胁迫的关键是高pH值.以缓冲量作为碱胁迫的胁强指标,而以盐度作为盐胁迫的胁强指标较为理想.盐碱混合胁迫时,两种胁迫表现出协同效应.     

Adaptive acid tolerance response in Listeria monocytogenes: isolation of an acid-tolerant mutant which demonstrates increased virulence.

The ability of Listeria monocytogenes to tolerate low-pH environments is of particular importance because the pathogen encounters such environments in vivo, both during passage through the stomach and within the macrophage phagosome. In our study, L. monocytogenes was shown to exhibit a significant adaptive acid tolerance response following a 1-h exposure to mild acid (pH 5.5), which is capable of protecting cells from severe acid stress (pH 3.5). Susceptibility to pH 3.5 acid is growth phase dependent. Stationary-phase Listeria cultures are naturally resistant to the challenge pH (pH 3.5), while exponential-phase cultures require adaptation at pH 5.5 to induce acid tolerance. Adaptation requires protein synthesis, since treatment with chloramphenicol prevents the development of acid tolerance. Induction of the acid tolerance response also protects L. monocytogenes against the effect of other environmental stresses. Acid-adapted cells demonstrate increased tolerance toward thermal stress, osmotic stress, crystal violet, and ethanol. Following prolonged exposure of L. monocytogenes to pH 3.5, we isolated mutants which constitutively demonstrate increased acid tolerance at all stages of the growth cycle. These mutants do not display full acid tolerance, but their resistance to low pH can be further increased following adaptation to mild-acid conditions. The mutants demonstrated increased lethality for mice relative to that of the wild type when inoculated by the intraperitoneal route. When administered as lower inocula, the mutants reached higher levels in the spleens of infected mice than did the wild type. The data suggest that low-pH conditions may have the potential to select for L. monocytogenes mutants with increased natural acid tolerance and increased virulence.     

Transport of lactate and other short-chain monocarboxylates in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae IGC4072 grown in lactic acid medium transported lactate by an accumulative electroneutral proton-lactate symport with a proton-lactate stoichiometry of 1:1. The accumulation ratio measured with propionate increased with decreasing pH from ca. 24-fold at pH 6.0 to ca. 1,400-fold at pH 3.0. The symport accepted the following monocarboxylates (Km values at 25 degrees C and pH 5.5): D-lactate (0.13 mM), L-lactate (0.13 mM), pyruvate (0.34 mM), propionate (0.09 mM), and acetate (0.05 mM), whereas apparently a different proton symport accepted formate (0.13 mM). The lactate system was inducible and was subject to glucose repression. Undissociated lactic acid entered the cells by simple diffusion. The permeability of the plasma membrane for undissociated lactic acid increased exponentially with pH, and the diffusion constant increased 40-fold when the pH was increased from 3.0 to 6.0.     

Transport of lactate and other short-chain monocarboxylates in the yeast Saccharomyces cerevisiae.

Saccharomyces cerevisiae IGC4072 grown in lactic acid medium transported lactate by an accumulative electroneutral proton-lactate symport with a proton-lactate stoichiometry of 1:1. The accumulation ratio measured with propionate increased with decreasing pH from ca. 24-fold at pH 6.0 to ca. 1,400-fold at pH 3.0. The symport accepted the following monocarboxylates (Km values at 25 degrees C and pH 5.5): D-lactate (0.13 mM), L-lactate (0.13 mM), pyruvate (0.34 mM), propionate (0.09 mM), and acetate (0.05 mM), whereas apparently a different proton symport accepted formate (0.13 mM). The lactate system was inducible and was subject to glucose repression. Undissociated lactic acid entered the cells by simple diffusion. The permeability of the plasma membrane for undissociated lactic acid increased exponentially with pH, and the diffusion constant increased 40-fold when the pH was increased from 3.0 to 6.0.     

pH regulates genes for flagellar motility, catabolism, and oxidative stress in Escherichia coli K-12

Gene expression profiles of Escherichia coli K-12 W3110 were compared as a function of steady-state external pH. Cultures were grown to an optical density at 600 nm of 0.3 in potassium-modified Luria-Bertani medium buffered at pH 5.0, 7.0, and 8.7. For each of the three pH conditions, cDNA from RNA of five independent cultures was hybridized to Affymetrix E. coli arrays. Analysis of variance with an alpha level of 0.001 resulted in 98% power to detect genes showing a twofold difference in expression. Normalized expression indices were calculated for each gene and intergenic region (IG). Differential expression among the three pH classes was observed for 763 genes and 353 IGs. Hierarchical clustering yielded six well-defined clusters of pH profiles, designated Acid High (highest expression at pH 5.0), Acid Low (lowest expression at pH 5.0), Base High (highest at pH 8.7), Base Low (lowest at pH 8.7), Neutral High (highest at pH 7.0, lower in acid or base), and Neutral Low (lowest at pH 7.0, higher at both pH extremes). Flagellar and chemotaxis genes were repressed at pH 8.7 (Base Low cluster), where the cell's transmembrane proton potential is diminished by the maintenance of an inverted pH gradient. High pH also repressed the proton pumps cytochrome o (cyo) and NADH dehydrogenases I and II. By contrast, the proton-importing ATP synthase F1Fo and the microaerophilic cytochrome d (cyd), which minimizes proton export, were induced at pH 8.7. These observations are consistent with a model in which high pH represses synthesis of flagella, which expend proton motive force, while stepping up electron transport and ATPase components that keep protons inside the cell. Acid-induced genes, on the other hand, were coinduced by conditions associated with increased metabolic rate, such as oxidative stress. All six pH-dependent clusters included envelope and periplasmic proteins, which directly experience external pH. Overall, this study showed that (i) low pH accelerates acid consumption and proton export, while coinducing oxidative stress and heat shock regulons; (ii) high pH accelerates proton import, while repressing the energy-expensive flagellar and chemotaxis regulons; and (iii) pH differentially regulates a large number of periplasmic and envelope proteins.     

Do pH changes in the leaf apoplast contribute to rapid inhibition of leaf elongation rate by water stress? Comparison of stress responses induced by polyethylene glycol and down‐regulation of root hydraulic conductivity

We have dissected the influences of apoplastic pH and cell turgor on short-term responses of leaf growth to plant water status, by using a combination of a double-barrelled pH-selective microelectrodes and a cell pressure probe. These techniques were used, together with continuous measurements of leaf elongation rate (LER), in the (hidden) elongating zone of the leaves of intact maize plants while exposing roots to various treatments. Polyethylene glycol (PEG) reduced water availability to roots, while acid load and anoxia decreased root hydraulic conductivity. During the first 30 min, acid load and anoxia induced moderate reductions in leaf growth and turgor, with no effect on leaf apoplastic pH. PEG stopped leaf growth, while turgor was only partially reduced. Rapid alkalinization of the apoplast, from pH 4.9 ± 0.3 to pH 5.8 ± 0.2 within 30 min, may have participated to this rapid growth reduction. After 60 min, leaf growth inhibition correlated well with turgor reduction across all treatments, supporting a growth limitation by hydraulics. We conclude that apoplastic alkalinization may transiently impair the control of leaf growth by cell turgor upon abrupt water stress, whereas direct hydraulic control of growth predominates under moderate conditions and after a 30-60 min delay following imposition of water stress.     

Influence of the protein environment on the properties of a tyrosyl radical in reaction centers from Rhodobacter sphaeroides

The influence of the local environment on the formation of a tyrosyl radical was investigated in modified photosynthetic reaction centers from Rhodobacter sphaeroides. The reaction centers contain a tyrosine residue placed approximately 10 A from a highly oxidizing bacteriochlorophyll dimer. Measurements by both optical and electron paramagnetic resonance spectroscopy revealed spectral features that are assigned as arising primarily from an oxidized bacteriochlorophyll dimer at low pH values and from a tyrosyl radical at high pH values, with a well-defined transition that occurred with a pK(a) of 6.9. A model based on the wild-type structure indicated that the Tyr at M164 is likely to form a hydrogen bond with His M193 and to interact weakly with Glu M173. Substitution of Tyr or Glu for His at M193 increased the pK(a) for the transition from 6.9 to 8.9, while substitution of Gln for His M193 resulted in a higher pK(a) value. Substitution of Glu M173 with Gln resulted in loss of the partial formation of the tyrosyl that occurs in the other mutants at low pH values. The results are interpreted in terms of the ability of the residues to act as proton acceptors for the oxidized tyrosine, with the pK(a) values reflecting those of either the putative proton acceptor or the tyrosine, in accord with general models of amino acid radicals.     

Isolation and characterization of Zygosaccharomyces rouxii mutants defective in proton pumpout activity and salt tolerance

Two mutants defective in salt tolerance were identified among hygromycin B (HygB)-resistant mutants of Zygosaccharomyces rouxii. These mutants showed different phenotypes in terms of sensitivity towards high concentrations of glucose and KCl. Recovery of salt tolerance by the addition of KCl and CaCl₂ or by lowering pH (pH 4.0) was different for the two mutants. Moreover, both mutants showed lowered plasma membrane (PM-) ATPase activity and proton pumpout activity. They exhibited neither growth nor proton pumpout activity in a medium containing 5% NaCl. The proton pumpout activity was inhibited by vanadate, an inhibitor of PM-ATPase, only when cells were incubated in the presence of more than 1% NaCl. Damage of the proton pumpout activity seems to be the reason for the salt sensitivity of both mutants. We showed that it was essential for Z. rouxii cells to pump out protons under a high salt environment using mutants defective in this ability.     

Physiological and metabolomic responses of bermudagrass (Cynodon dactylon) to alkali stress

Bermudagrass (Cynodon dactylon) is a widely used warm‐season turfgrass species with superior stress tolerance except for cold. In this study, a comparative analysis of the responses to alkali stress in bermudagrass at the physiological and metabolomic levels were performed. Mild alkali with relatively low pH slightly inhibited growth of bermudagrass as evidenced by lower electrolyte leakage, more rapid growth and higher survival rate when compared to moderate and severe alkali treatments. Moreover, the amount of 37 metabolites including amino acids, organic acids, sugars and sugar alcohols were modulated by the alkali treatments. Among them, 15 metabolites were involved in carbon and amino acid metabolic pathways. Under mild alkali stress, bermudagrass possibly slowed down metabolisms to maintain basic growth. However, moderate and severe alkali‐stressed plants accumulated significantly higher amount of carbohydrates which might result in carbon starvation. Taken together, alkali stress had severely inhibitory effect partially due to combined ionic stress and high pH stress. These results suggested that bermudagrass employed different strategies in response to alkali stresses with different pH and ionic values.     

Role of the PhoP-PhoQ system in the virulence of Erwinia chrysanthemi strain 3937: involvement in sensitivity to plant antimicrobial peptides, survival at acid Hh, and regulation of pectolytic enzymes

Erwinia chrysanthemi is a phytopathogenic bacterium that causes soft-rot diseases in a broad number of crops. The PhoP-PhoQ system is a key factor in pathogenicity of several bacteria and is involved in the bacterial resistance to different factors, including acid stress. Since E. chrysanthemi is confronted by acid pH during pathogenesis, we have studied the role of this system in the virulence of this bacterium. In this work, we have isolated and characterized the phoP and phoQ mutants of E. chrysanthemi strain 3937. It was found that: (i) they were not altered in their growth at acid pH; (ii) the phoQ mutant showed diminished ability to survive at acid pH; (iii) susceptibility to the antimicrobial peptide thionin was increased; (iv) the virulence of the phoQ mutant was diminished at low and high magnesium concentrations, whereas the virulence of the phoP was diminished only at low magnesium concentrations; (v) in planta Pel activity of both mutant strains was drastically reduced; and (vi) both mutants lagged behind the wild type in their capacity to change the apoplastic pH. These results suggest that the PhoP-PhoQ system plays a role in the virulence of this bacterium in plant tissues, although it does not contribute to bacterial growth at acid pH.     

Characterization of AtCHX17, a member of the cation/H+ exchangers, CHX family, from Arabidopsis thaliana suggests a role in K+ homeostasis

The Arabidopsis genome contains many sequences annotated as encoding H(+)-coupled cotransporters. Among those are the members of the cation:proton antiporter-2 (CPA2) family (or CHX family), predicted to encode Na(+),K(+)/H(+) antiporters. AtCHX17, a member of the CPA2 family, was selected for expression studies, and phenotypic analysis of knockout mutants was performed. AtCHX17 expression was only detected in roots. The gene was strongly induced by salt stress, potassium starvation, abscisic acid (ABA) and external acidic pH. Using the beta-glucuronidase reporter gene strategy and in situ RT-PCR experiments, we have found that AtCHX17 was expressed preferentially in epidermal and cortical cells of the mature root zones. Knockout mutants accumulated less K(+) in roots in response to salt stress and potassium starvation compared with the wild type. These data support the hypothesis that AtCHX17 is involved in K(+) acquisition and homeostasis.