Mechanisms of water transport mediated by PIP aquaporins and their regulation via phosphorylation events under salinity stress in barley roots

Water homeostasis is crucial to the growth and survival of plants under water-related stress. Plasma membrane intrinsic proteins (PIPs) have been shown to be primary channels mediating water uptake in plant cells. Here we report the water transport activity and mechanisms for the regulation of barley (Hordeum vulgare) PIP aquaporins. HvPIP2 but not HvPIP1 channels were found to show robust water transport activity when expressed alone in Xenopus laevis oocytes. However, the co-expression of HvPIP1 with HvPIP2 in oocytes resulted in significant increases in activity compared with the expression of HvPIP2 alone, suggesting the participation of HvPIP1 in water transport together with HvPIP2 presumably through heteromerization. Severe salinity stress (200 mM NaCl) significantly reduced root hydraulic conductivity (Lp(r)) and the accumulation of six of 10 HvPIP mRNAs. However, under relatively mild stress (100 mM NaCl), only a moderate reduction in Lp(r) with no significant difference in HvPIP mRNA levels was observed. Sorbitol-mediated osmotic stress equivalent to 100 and 200 mM NaCl induced nearly identical Lp(r) reductions in barley roots. Furthermore, the water transport activity in intact barley roots was suggested to require phosphorylation that is sensitive to a kinase inhibitor, staurosporine. HvPIP2s also showed water efflux activity in Xenopus oocytes, suggesting a potential ability to mediate water loss from cells under hypertonic conditions. Water transport via HvPIP aquaporins and the significance of reductions of Lp(r) in barley plants during salinity stress are discussed.     

Effect of low root temperature on hydraulic conductivity of rice plants and the possible role of aquaporins

The role of root temperature T(R) in regulating the water-uptake capability of rice roots and the possible relationship with aquaporins were investigated. The root hydraulic conductivity Lp(r) decreased with decreasing T(R) in a measured temperature range between 10 degrees C and 35 degrees C. A single break point (T(RC) = 15 degrees C) was detected in the Arrhenius plot for steady-state Lp(r). The temperature dependency of Lp(r) represented by activation energy was low (28 kJ mol(-1)) above T(RC), but the value is slightly higher than that for the water viscosity. Addition of an aquaporin inhibitor, HgCl(2), into root medium reduced osmotic exudation by 97% at 25 degrees C, signifying that aquaporins play a major role in regulating water uptake. Below T(RC), Lp(r) declined precipitously with decreasing T(R) (E(a) = 204 kJ mol(-1)). When T(R) is higher than T(RC), the transient time for reaching the steady-state of Lp(r) after the immediate change in T(R) (from 25 degrees C) was estimated as 10 min, while it was prolonged up to 2-3 h when T(R) < T(RC). The Lp(r) was completely recovered to the initial levels when T(R) was returned back to 25 degrees C. Immunoblot analysis using specific antibodies for the major aquaporin members of PIPs and TIPs in rice roots revealed that there were no significant changes in the abundance of aquaporins during 5 h of low temperature treatment. Considering this result and the significant inhibition of water-uptake by the aquaporin inhibitor, we hypothesize that the decrease in Lp(r) when T(R) < T(RC) was regulated by the activity of aquaporins rather than their abundance.     

大丽轮枝菌毒素与拟南芥互作反应中SA、NO与H2O2信号的相关性

本文研究了大丽轮枝菌毒素（VD-toxin)与拟南芥互作反应中外源SA、NO供体、NO合酶抑制剂等对拟南芥幼苗H2O2含量的影响,并对H2O2的积累部位进行了DAB组化染色检测。大丽轮枝菌毒素、外源SA、NO供体处理拟南芥幼苗均能诱导H2O2的积累,NO供体的诱导作用最强;NO合酶抑制剂处理则未表现出H2O2含量的增强;H2O2的积累部位主要在叶片的表皮毛和维管束组织。结果表明,在大丽轮枝菌毒素与拟南芥互作反应中,H2O2可能作为信号分子参与了SA和NO调控的拟南芥防卫反应,NO信号与H2O2信号间的关系可能更密切。     

Proteomic identification of glyceraldehyde 3-phosphate dehydrogenase as an inhibitory target of hydrogen peroxide in Arabidopsis.

Hydrogen peroxide (H2O2) is now recognised as a key signalling molecule in eukaryotes. In plants, H2O2 is involved in regulating stomatal closure, gravitropic responses, gene expression and programmed cell death. Although several kinases, such as oxidative signal-inducible 1 (OXI1) kinase and mitogen-activated protein kinases are known to be activated by exogenous H2O2, little is known about the proteins that directly react with H2O2. Here, we utilised a proteomic approach, using iodoacetamide-based fluorescence tagging of proteins in conjunction with mass spectrometric analysis, to identify several proteins that might be potential targets of H2O2 in the cytosolic fraction of Arabidopsis thaliana, the most prominent of which was cytosolic glyceraldehyde 3-phosphate dehydrogenase (cGAPDH; EC 1.2.1.12). cGAPDH from Arabidopsis is inactivated by H2O2 in vitro, and this inhibition is reversible by the subsequent addition of reductants such as reduced glutathione (GSH). It has been suggested recently that Arabidopsis GAPDH has roles outside of its catalysis as part of glycolysis, while in other systems this includes that of mediating reactive oxygen species (ROS) signalling. Here, we suggest that cGAPDH in Arabidopsis might also have such a role in mediating ROS signalling in plants.     

Systemic Responses in Arabidopsis thaliana Infected and Challenged with Pseudomonas syringae pv syringae

Attack of plants by necrotizing pathogens leads to acquired resistance to the same or other pathogens in tissues adjacent to or remotely located from the site of initial attack. We have used Arabidopsis thaliana inoculated with the incompatible pathogen Pseudomonas syringae pv syringae on the lower leaves to test the induction of systemic reactions. When plants were challenged with Pseudomonas syringae pv syringae in the upper leaves, bacterial titers remained stable in those preinfected on the lower leaves. However, there was a distinct decrease in symptoms that correlated with a local and systemic increase in salicylic acid (SA) and in chitinase activity. Peroxidase activity only increased at the site of infection. No changes in catalase activity were observed, either at the local or at the systemic level. No inhibition of catalase could be detected in tissue in which the endogenous levels of SA were elevated either naturally (after infection) or artificially (after feeding SA to the roots). The activity of catalase in homogenates of A. thaliana leaves could not be inhibited in vitro by SA. SA accumulation was induced by H2O2 in leaves, suggesting a link between H2O2 from the oxidative burst commonly observed during the hypersensitive reaction and the induction of a putative signaling molecule leading to system acquired resistance.     

Redox-regulatory mechanisms induced by oxidative stress in Brassica juncea roots monitored by 2-DE proteomics

ROS, including hydrogen peroxide (H(2)O(2)), can serve as cellular signaling molecules following oxidative stress. Analysis of the redox state of proteins in Brassica juncea roots by 2-DE proteomics following treatment with either exogenous H(2)O(2) or buthionine sulfoximine, which depletes glutathione to cause accumulation of endogenous H(2)O(2), led to the identification of different sets of proteins. These data suggest that exogenous and endogenous oxidative stresses trigger specialized responses.     

Influence of salicylic acid on H2O2 production, oxidative stress, and H2O2-metabolizing enzymes. Salicylic acid-mediated oxidative damage requires H2O2.

We investigated how salicylic acid (SA) enhances H2O2 and the relative significance of SA-enhanced H2O2 in Arabidopsis thaliana. SA treatments enhanced H2O2 production, lipid peroxidation, and oxidative damage to proteins, and resulted in the formation of chlorophyll and carotene isomers. SA-enhanced H2O2 levels were related to increased activities of Cu,Zn-superoxide dismutase and were independent of changes in catalase and ascorbate peroxidase activities. Prolonging SA treatments inactivated catalase and ascorbate peroxidase and resulted in phytotoxic symptoms, suggesting that inactivation of H2O2-degrading enzymes serves as an indicator of hypersensitive cell death. Treatment of leaves with H2O2 alone failed to invoke SA-mediated events. Although leaves treated with H2O2 accumulated in vivo H2O2 by 2-fold compared with leaves treated with SA, the damage to membranes and proteins was significantly less, indicating that SA can cause greater damage than H2O2. However, pretreatment of leaves with dimethylthiourea, a trap for H2O2, reduced SA-induced lipid peroxidation, indicating that SA requires H2O2 to initiate oxidative damage. The relative significance of the interaction among SA, H2O2, and H2O2-metabolizing enzymes with oxidative damage and cell death is discussed.     

The response of Arabidopsis root water transport to a challenging environment implicates reactive oxygen species- and phosphorylation-dependent internalization of aquaporins

Aquaporins, which facilitate the diffusion of water across biological membranes, are key molecules for the regulation of water transport at the cell and organ levels. We recently reported that hydrogen peroxide (H₂O₂) acts as an intermediate in the regulation of Arabidopsis root water transport and aquaporins in response to NaCl and salicylic acid (SA).¹ Its action involves signaling pathways and an internalization of aquaporins from the cell surface. The present addendum connects these findings to another recent work which describes multiple phosphorylations in the C-terminus of aquaporins expressed in the Arabidopsis root plasma membrane.² A novel role for phosphorylation in the process of salt-induced relocalization of AtPIP2;1, one of the most abundant root aquaporins, was unraveled. Altogether, the data delineate reactive oxygen species (ROS)-dependent signaling mechanisms which, in response to a variety of abiotic and biotic stresses, can trigger phosphorylation-dependent PIP aquaporin intracellular trafficking and root water transport downregulation.Key words: reactive oxygen species, aquaporin, phosphorylation, cell signaling, stress, protein relocalization, root water transportPlants can regulate their water uptake capacity i.e. their root hydraulic conductivity (Lp_r) on a short term (minutes to hour) basis through regulation of plasma membrane (PM) aquaporins of the Plasma membrane Intrinsic Protein (PIP) subfamily.³ It has been known for a long time that salt stress (NaCl), as many other abiotic stresses such as cold, anoxia or nutrient deprivation, induces an inhibition of Lp_r in many plant species.³ In the recent study by Boursiac et al. (2008),¹ we identified SA as a new inhibitory increased the accumulation of ROS in roots, it was hypothesized that H₂O₂ or other ROS may have a central role in the regulation of root water transport in response to various biotic or abiotic stimuli. When Arabidopsis roots were treated with mM concentrations of exogenous H₂O₂, Lp_r was inhibited within minutes by up to 90%. These findings are consistent with previous reports showing that ROS can downregulate water transport in cucumber and maize roots or in the algae Chara corallina.^4–7 H₂O₂ and possibly other derived ROS may modulate the Lp_r through signaling mechanisms or by a direct oxidative gating of aquaporins. The latter hypothesis, which has been favored in previous studies by Steudle and colleagues,^6,7 was investigated by Boursiac et al., by functionally expressing aquaporins in Xenopus oocytes and by testing their sensitivity to external H₂O₂. The results show that Arabidopsis aquaporins are insensitive to direct oxidation by H₂O₂ or hydroxyl radicals. Thus, these and complementary pharmacological analyses on excised roots rather support a role for H₂O₂ as a second messenger that connects environmental stimulus perception to water transport regulation in plant roots. The additional finding that H₂O₂ can be transported by aquaporins^8,9 opens the possibility of intricate loop mechanisms whereby these proteins may interfere with their own regulation. For example, active PIP aquaporins could facilitate the diffusion within the cell of NADPH-oxidase derived apoplastic H₂O₂, which in turn would activate signaling pathways acting on PIP activity and/or subcellular localization.In a previous study, we monitored the subcellular localization of AtPIP1;2 and AtPIP2;1, two of the most abundant PIPs in roots, by expression in transgenic Arabidopsis of fusions with the green fluorescent protein (GFP).¹⁰ We observed that a 100 mM NaCl treatment induced in 2–4 hours an increased intracellular labeling which was interpreted as an intracellular relocalization of the two aquaporins.¹⁰ In our more recent study, both a 150 mM NaCl and a 0.5 mM SA treatments induced an intracellular labeling by GFP-PIP1;2 and PIP2;1-GFP fusions, with a “fuzzy” pattern or at the level of spherical bodies. Preventing the NaCl- or SA-dependent accumulation of ROS with exogenous catalase was able to almost completely counteract the effects of the two stimuli on the localization pattern of the PIP2;1-GFP fusion. In addition, the inhibition of Lp_r by SA was also counteracted at 33% by the catalase treatment. Altogether, the data stress the importance of an ROS-induced relocalization of aquaporins in the regulation of root water transport. Yet, we still miss quantitative data and complementary pharmacological evidence to determine the exact contribution of aquaporin relocalization with respect to other aquaporin regulatory mechanisms.Another recent work by our group has, however, provided deeper insights into the mechanisms of stress-induced relocalization of aquaporins in plants.² Our group identified by mass spectrometry multiple adjacent phosphorylation sites (up to 4 in the case of AtPIP2;4) in the C-terminus of aquaporins expressed at the root plasma membrane.² Phosphorylation of AtPIP2;1, which shows a simpler profile with only two sites at Ser280 and Ser283, was studied in closer detail by site-directed mutagenesis and expression in transgenic Arabidopsis of GFP-PIP2;1 fusions. A Ser283Ala mutation, which mimics a constitutively dephosphorylated Ser283, induced a marked intracellular accumulation of GFP-PIP2;1 in resting conditions. Because no phenotype was observed after a Ser280Ala mutation, the data suggest a specific role for Ser283 phosphorylation in the proper targeting of the protein. When plants were treated by 100 mM NaCl for 2 to 4 hours, the wild type (WT) and Ser280Ala mutant forms of GFP-PIP2;1 showed similar intracellular staining, in both “fuzzy” structures or spherical bodies. On the contrary, the Ser283Ala mutant did not label any spherical body. Interestingly, a Ser283Asp mutation that mimics a constitutively phosphorylated Ser283 resulted in a salt-induced labeling of spherical bodies similar to the one observed with WT GFP-PIP2;1 whereas no “fuzzy” staining was observed. Therefore, the phosphorylation status of Ser283 seems to determine the redistribution of AtPIP2;1 towards fuzzy structures (non-phosphorylated Ser283) or spherical bodies (phosphorylated Ser283). Although the nature of these intracellular structures remains to be identified, we now consider the possibility that the spherical bodies correspond to the late endosome/prevacuolar compartment that orientates aquaporins towards a degradation pathway whereas the fuzzy structures may act as a storage compartment for subsequent relocalization of PIP aquaporins to the PM, and rapid recovery of the PM water permeability. Although we favor the idea that the intracellular labeling shown by GFP-PIP2;1 in response to salt originates from aquaporins relocalized from the PM, newly synthesized proteins may also contribute to this pattern.Prak et al., also developed an absolute quantification method to show that the phosphorylation profile of AtPIP2;1 at the root plasma membrane was altered upon 100 mM NaCl and 2 mM H₂O₂ treatments. Whereas NaCl decreased the abundance of phosphorylated Ser283, H₂O₂ enhanced the overall phosphorylation of the AtPIP2;1 C-terminus. These observations add another level of complexity to the mechanisms of stimulus-induced and phosphorylation- dependent relocalisation of plant aquaporins uncovered in our group. Although one of the primary effects of NaCl is undoubtedly an accumulation of ROS, the difference in phosphorylation patterns observed in response to H₂O₂ and NaCl treatments may come from quantitative and kinetic differences in ROS patterns between the two treatments or from additional regulations activated by salt.We note that phosphorylation of PIP aquaporins had already been investigated in detail.^11–13 In particular, studies with spinach SoPIP2;1 has pointed to two phosphorylation sites, Ser115 in the first cytoplasmic loop (loop B) and Ser274 at the C-terminus, as important for modulating the water transport activity of this aquaporin after expression in Xenopus oocytes. A role for these two sites in aquaporin gating was also deduced from the atomic structure of SoPIP2;1.¹⁴ Whereas Ser280 in AtPIP2;1 corresponds to Ser274 in SoPIP2;1, the functional role of sites equivalent to Ser283 in AtPIP2;1 had not been considered previously in any other PIP. To our knowledge, the study by Prak et al., provides the first evidence in plants for a role of phosphorylation on the relocalization of aquaporins and highlights the importance of multiple phosphorylations sites in the C-terminus of aquaporins, as has been recently shown in human Aquaporin-2.^15,16Overall, the advance provided by our two recent studies delineates a working model (Fig. 1), whereby multiple abiotic and biotic stresses, which all induce an accumulation of ROS, activate common signaling pathways to downregulate root water transport. We have provided evidence that some of these pathways are calcium- and/ or protein kinase-dependent. One regulatory mechanism triggered by these pathways is the relocalization of aquaporins into intracellular “fuzzy” structures or bigger spherical bodies. For AtPIP2;1, the sorting between these structures is determined in part by the phosphorylation status of Ser283, which ultimately may control the cellular fate of the protein for degradation or remobilization to the PM. A coming challenge will be to determine how this and other cellular mechanisms quantitatively contribute to the integrated regulation of water transport at the cell and tissue (whole root) levels. Another avenue for future research will be to identify the molecular components involved in upstream ROS-dependent cell signaling and aquaporin phosphorylation. These studies will tell us how the regulation of root water uptake in parallel to the regulation of transpiration allows the plant to preserve its water status when it is continuously challenged by multiple stresses.Open in a separate windowFigure 1Tentative model of regulation of root hydraulic conductivity (Lp_r) through reactive oxygen species (ROS) signaling. Multiple biotic and abiotic stimuli such as NaCl or salicylic acid can induce an intra- and/or extracellular accumulation of ROS by acting on their production, degradation or transport. The stimulus-induced ROS in turn activate signaling pathways involving protein kinases and cytosolic calcium. These events result in changes in the phosphorylation and subcellular localization patterns of plasma membrane (PM) aquaporins (PIPs). In particular, endocytosis can direct PIPs towards various intracellular compartments for subsequent recycling at the PM or degradation. Phosphorylation can interfere with this routing process, but also determines the intrinsic water transport activity (gating) of PM localized PIPs. The possibility exists that signaling components directly act on PIP gating, recycling or degradation through phosphorylation- and endocytosis-independent pathways (not shown). In addition, transport of H₂O₂ by PIP aquaporins may provide retroactive effects of aquaporins on upstream signaling events. Aquaporin activity at the PM determines root cell water permeability, which contributes to most of Lp_r in Arabidopsis. The overall scheme shows how stress-induced ROS signaling results in an inhibition of PIP aquaporin activity and, as a consequence, in an overall downregulation of Lp_r.     

The protistan parasite Perkinsus marinus is resistant to selected reactive oxygen species

The parasite Perkinsus marinus has devastated natural and farmed oyster populations along the Atlantic and Gulf coasts of North America. When viable P. marinus trophozoites are engulfed by oyster hemocytes, the typical accumulation of reactive oxygen species (ROS) normally associated with phagocyte activity is not observed. One hypothesis to explain this is that the parasite rapidly removes ROS. A manifestation of efficient ROS removal should be a high level of resistance to exogenous ROS. We investigated the in vitro susceptibility of P. marinus to ROS as compared to the estuarine bacterium Vibrio splendidus. We find that P. marinus is markedly less susceptible than V. splendidus to superoxide and hydrogen peroxide (H(2)O(2)), but equally sensitive to hypochlorite. Viable P. marinus trophozoites degrade H(2)O(2) in vitro, but lack detectable catalase activity. However, extracts contain an ascorbate dependent peroxidase activity that may contribute to H(2)O(2) removal in vitro and in vivo.     

Salicylic and jasmonic acids in regulation of the proantioxidant state in wheat leaves infected by Septoria nodorum Berk

Influence of mediators of the signal systems of salicylic (SA) and jasmonic (JA) acids and their mixture on reactive oxygen species' (ROS) (superoxide radical O2*- and H2O2) generation and activity of oxidoreductases (oxalate oxidase, peroxidase and catalase) in leaves of wheat Triticum aestivum L. infected by Septoria leaf blotch pathogen Septoria nodorum Berk. has been studied. Presowing treatment of seeds by SA and JA decreased the development rate of fungus on wheat leaves. SA provided earlier inductive effect on production of O2*- and H2O2 compared with JA. The protective effect of the salicylic and jasmonic acids against Septoria leaf blotch pathogen was caused by activation of oxalate oxidase, induction of anion and cation peroxidases, and decrease of catalase activity. Ability of compounds to stimulate ROS in the plant tissues can be used as criteria for evaluation of immune-modulating activity of new substances for protection of the plants.     

H2O2参与水杨酸诱导丹参培养细胞中丹酚酸B合成的信号转导

过氧化氢(Hydrogen peroxide,H2O2)为活性氧(Reactive oxygen species,ROS)的一种,存在于许多生物体系中并介导植物中多种生理和生化过程。为了探讨H2O2作为信号分子在水杨酸(Salicylic acid,SA)诱导丹参培养细胞合成丹酚酸B(Salvianolic acid B,Sal B)过程中的作用,分别考察了SA和H2O2、过氧化氢酶(Catalase,CAT)、二甲基硫脲(2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide,DMTU)及咪唑(Imidazole,IMD)对苯丙氨酸解氨酶(Phenylalanine ammonia-lyase,PAL)和酪氨酸氨基转移酶(Tyrosine aminotransferase,TAT)的活性及Sal B含量的影响。结果表明,SA处理可有效地诱导丹参培养细胞中H2O2产生、PAL和TAT活性升高以及Sal B合成积累量的增加;外源施加10～30 mmol/L H2O2也可以有效促进PAL、TAT活性升高和Sal B合成积累量的增加;用H2O2的清除剂CAT处理发现,CAT对SA或外源H2O2诱导的Sal B合成积累具有消除作用,说明H2O2可能作为SA诱导Sal B积累过程中的上游信号分子起作用;用H2O2淬灭剂DMTU处理,可以有效抑制SA对Sal B合成的促进作用;用质膜烟酰胺腺嘌呤二核苷酸(Nicotinamide vadenine dinucleotide phosphate,NADPH)氧化酶(H2O2来源的主要酶)抑制剂IMD处理,可以抑制Sal B的合成,但这种抑制效果可以部分被外源施加的SA削弱,说明通过HADPH氧化酶产生的H2O2受阻时,SA诱导的Sal B合成积累也会受到抑制。表明H2O2是介导SA诱导丹参培养细胞中Sal B合成积累的信号分子。     

Specific plasma membrane aquaporins of the PIP1 subfamily are expressed in sieve elements and guard cells

BACKGROUND INFORMATION: Transmembrane water flow is aided by water-specific channel proteins, aquaporins. Plant genomes code for approx. 35 expressed and functional aquaporin isoforms. Plant aquaporins fall into four different subfamilies of which the PIPs (plasma membrane intrinsic proteins) constitute the largest and evolutionarily most conserved subfamily with 13 members in Arabidopsis and maize. Furthermore, the PIPs can be divided into two phylogenetic groups, PIP1 and PIP2, of which the PIP1 isoforms are most tightly conserved, sharing >90% amino acid sequence identity. As the nomenclature implies, the majority of PIPs have been shown to be localized at the plasma membrane. Recently, two highly abundant plasma membrane aquaporins, SoPIP2;1 and SoPIP1;2, have been purified and structurally characterized. RESULTS: We report the cloning of a cDNA encoding SoPIP1;2 and show that there are at least five additional sequences homologous with SoPIP2;1 and SoPIP1;2 in the spinach genome. To understand their role in planta, we have investigated the cellular localization of the aquaporin homologues SoPIP1;2 and SoPIP1;1. By Western- and Northern-blot analyses and by immunocytochemical detection at the light and electron microscopic levels, we show that SoPIP1;2 is highly expressed in phloem sieve elements of leaves, roots and petioles and that SoPIP1;1 is present in stomatal guard cells. CONCLUSIONS: Localization of the two abundant aquaporin isoforms suggests roles for specific PIPs of the PIP1 subgroup in phloem loading, transport and unloading, and in stomatal movements.     

氮磷亏缺对玉米根系水流导度的影响

在人工气候室水培条件下,从单根和整株根系两个层次研究了N、P营养与玉米(Zea mays L.)根系水流导度(root hydraulic conductivity,Lpr)间的关系。结果表明：表型抗旱的杂交种F1代户单4号和母本天四的单根水导和整株根系水导均高于不抗旱的父本478,其中天四的单根水导最高,而户单4号的整株根系水导最高。N、P亏缺均使玉米单根水导和整株根系水导降低,但与N亏块相比,P亏缺的植株具有较高的整株根系水导和较低的单根水导。整株根系的水导更能反映植物根系的输水性能。     

Oxidative gating of water channels (aquaporins) in corn roots

An oxidative gating of water channels (aquaporins: AQPs) was observed in roots of corn seedlings as already found for the green alga Chara corallina. In the presence of 35 mM hydrogen peroxide (H2O2)--a precursor of hydroxyl radicals (*OH)--half times of water flow (as measured with the aid of pressure probes) increased at the level of both entire roots and individual cortical cells by factors of three and nine, respectively. This indicated decreases in the hydrostatic hydraulic conductivity of roots (Lp(hr)) and of cells (Lp(h)) by the same factors. Unlike other stresses, the plant hormone abscisic acid (ABA) had no ameliorative effect either on root LP(hr) or on cell Lp(h) when AQPs were inhibited by oxidative stress. Closure of AQPs reduced the permeability of acetone by factors of two in roots and 1.5 in cells. This indicated that AQPs were not ideally selective for water but allowed the passage of the organic solute acetone. In the presence of H2O2, channel closure caused anomalous (negative) osmosis at both the root and the cell level. This was interpreted by the fact that in the case of the rapidly permeating solute acetone, channel closure caused the solute to move faster than the water and the reflection coefficient (sigma s) reversed its sign. When H2O2 was removed from the medium, the effects were reversible, again at both the root and the cell level. The results provide evidence of oxidative gating of AQPs, which leads on to inhibition of water uptake by the roots. Possible mechanisms of the oxidative gating of AQPs induced by H2O2 (*OH) are discussed.     

水杨酸调节决明根系铝诱导的氧化胁迫

水杨酸(Salicylicacid,SA)在调节生物和非生物胁迫,诱导植物氧化胁迫中起着重要的作用,但对铝诱导的氧化胁迫的调节作用尚不清楚。本文研究了SA对决明(CassiatoraL.)根系铝诱导的H2O2和O2-含量变化,包括抗氧化酶活性以及细胞质膜过氧化胁迫变化的影响。介质中20mmol/L铝处理增加质膜透性,导致MDA含量上升及根尖细胞Evansblue染色加重(测定细胞死亡),而外源供给5mmol/LSA能缓解铝诱导的氧化胁迫。SA处理能明显降低根尖H2O2和O2-的含量,但两者含量与CAT、APX和GR的活性变化没有相关性,而与POD活性增加有关。水杨酸诱导H2O2含量的下降与抑制O2-积累和SOD活性有关。结果表明,SA可能激活一条由H2O2介导的、依赖于POD的抗氧化机制来缓解脂质的过氧化作用。