The ABC transporter AtPDR8 is a cadmium extrusion pump conferring heavy metal resistance

Cadmium (Cd) and lead (Pb) are widespread pollutants that are toxic to plant growth. The expression of AtPDR8 was upregulated in cadmium- or lead-treated Arabidopsis thaliana. To test whether AtPDR8 is involved in heavy metal resistance, we examined transgenic Arabidopsis that over-expressed AtPDR8 and RNAi plants that exhibited a severely reduced AtPDR8 transcript level, as well as T-DNA insertion mutants of this ABC transporter. AtPDR8-over-expressing plants were more resistant to Cd(2+) or Pb(2+) than the wild-type and had lower Cd contents. In contrast, AtPDR8 RNAi transgenic plants and T-DNA insertion lines were more sensitive to Cd(2+) or Pb(2+) compared to wild-type plants and had higher Cd contents. The GFP-AtPDR8 protein was targeted to the plasma membrane, and GUS activity was present in most cells but strongest in the root hair and epidermal cells. Cd extrusion was higher in the AtPDR8-over-expressing plants in a flux assay using isolated protoplasts and radioactive (109)Cd, and was lower in the RNAi transgenic plants than in the wild-type. Together, these data strongly support a role for AtPDR8 as an efflux pump of Cd(2+) or Cd conjugates at the plasma membrane of Arabidopsis cells. As AtPDR8 has been suggested to be involved in the pathogen response and in the transport of chemicals that mediate pathogen resistance, this ABC protein is likely to transport a very broad range of substrates.

Differential regulation of EIN3 stability by glucose and ethylene signalling in plants

Glucose is a global regulator of growth and metabolism that is evolutionarily conserved from unicellular microorganisms to multicellular animals and plants. In photosynthetic plants, glucose shows hormone-like activities and modulates many essential processes, including embryogenesis, germination, seedling development, vegetative growth, reproduction and senescence. Genetic and phenotypic analyses of Arabidopsis mutants with glucose-insensitive (gin) and glucose-oversensitive (glo) phenotypes have identified an unexpected antagonistic interaction between glucose and the plant stress hormone ethylene. The ethylene-insensitive etr1 and ein2 mutants have glo phenotypes, whereas the constitutive ethylene signalling mutant ctr1 is allelic to gin4 (refs 4, 5). The precise molecular mechanisms underlying the complex signalling network that governs plant growth and development in response to nutrients and plant hormones are mostly unknown. Here we show that glucose enhances the degradation of ETHYLENE-INSENSITIVE3 (EIN3), a key transcriptional regulator in ethylene signalling, through the plant glucose sensor hexokinase. Ethylene, by contrast, enhances the stability of EIN3. The ein3 mutant has a glo phenotype, and overexpression of EIN3 in transgenic Arabidopsis decreases glucose sensitivity.

Arabidopsis root K+-efflux conductance activated by hydroxyl radicals: single-channel properties, genetic basis and involvement in stress-induced cell death

Reactive oxygen species (ROS) are central to plant stress response, signalling, development and a multitude of other processes. In this study, the plasma-membrane hydroxyl radical (HR)-activated K(+) channel responsible for K(+) efflux from root cells during stress accompanied by ROS generation is characterised. The channel showed 16-pS unitary conductance and was sensitive to Ca(2+), tetraethylammonium, Ba(2+), Cs(+) and free-radical scavengers. The channel was not found in the gork1-1 mutant, which lacks a major plasma-membrane outwardly rectifying K(+) channel. In intact Arabidopsis roots, both HRs and stress induced a dramatic K(+) efflux that was much smaller in gork1-1 plants. Tests with electron paramagnetic resonance spectroscopy showed that NaCl can stimulate HR generation in roots and this might lead to K(+)-channel activation. In animals, activation of K(+)-efflux channels by HRs can trigger programmed cell death (PCD). PCD symptoms in Arabidopsis roots developed much more slowly in gork1-1 and wild-type plants treated with K(+)-channel blockers or HR scavengers. Therefore, similar to animal counterparts, plant HR-activated K(+) channels are also involved in PCD. Overall, this study provides new insight into the regulation of plant cation transport by ROS and demonstrates possible physiological properties of plant HR-activated K(+) channels.

The Arabidopsis transcription factor MYB77 modulates auxin signal transduction

Auxin is a key plant hormone that regulates plant development, apical dominance, and growth-related tropisms, such as phototropism and gravitropism. In this study, we report a new Arabidopsis thaliana transcription factor, MYB77, that is involved in auxin response. In MYB77 knockout plants, we found that auxin-responsive gene expression was greatly attenuated. Lateral root density in the MYB77 knockout was lower than the wild type at low concentrations of indole-3-acetic acid (IAA) and also under low nutrient conditions. MYB77 interacts with auxin response factors (ARFs) in vitro through the C terminus (domains III and IV) of ARFs and the activation domain of MYB77. A synergistic genetic interaction was demonstrated between MYB77 and ARF7 that resulted in a strong reduction in lateral root numbers. Experiments with protoplasts confirmed that the coexpression of MYB77 and an ARF C terminus enhance reporter gene expression. R2R3 MYB transcription factors have not been previously implicated in regulating the expression of auxin-inducible genes. Also it was previously unknown that ARFs interact with proteins other than those in the Aux/IAA family via conserved domains. The interaction between MYB77 and ARFs defines a new type of combinatorial transcriptional control in plants. This newly defined transcription factor interaction is part of the plant cells' repertoire for modulating response to auxin, thereby controlling lateral root growth and development under changing environmental conditions.

Changing patterns of localization of the tobacco mosaic virus movement protein and replicase to the endoplasmic reticulum and microtubules during infection

Tobacco mosaic virus (TMV) derivatives that encode movement protein (MP) as a fusion to the green fluorescent protein (MP:GFP) were used in combination with antibody staining to identify host cell components to which MP and replicase accumulate in cells of infected Nicotiana benthamiana leaves and in infected BY-2 protoplasts. MP:GFP and replicase colocalized to the endoplasmic reticulum (ER; especially the cortical ER) and were present in large, irregularly shaped, ER-derived structures that may represent "viral factories." The ER-derived structures required an intact cytoskeleton, and microtubules appeared to redistribute MP:GFP from these sites during late stages of infection. In leaves, MP:GFP accumulated in plasmodesmata, whereas in protoplasts, the MP:GFP was targeted to distinct, punctate sites near the plasma membrane. Treating protoplasts with cytochalasin D and brefeldin A at the time of inoculation prevented the accumulation of MP:GFP at these sites. It is proposed that the punctate sites anchor the cortical ER to plasma membrane and are related to sites at which plasmodesmata form in walled cells. Hairlike structures containing MP:GFP appeared on the surface of some of the infected protoplasts and are reminiscent of similar structures induced by other plant viruses. We present a model that postulates the role of the ER and cytoskeleton in targeting the MP and viral ribonucleoprotein from sites of virus synthesis to the plasmodesmata through which infection is spread.

Molecular identification and physiological characterization of a novel monosaccharide transporter from Arabidopsis involved in vacuolar sugar transport

The tonoplast monosaccharide transporter (TMT) family comprises three isoforms in Arabidopsis thaliana, and TMT-green fluorescent protein fusion proteins are targeted to the vacuolar membrane. TMT promoter-beta-glucuronidase plants revealed that the TONOPLAST MONOSACCHARIDE TRANSPORTER1 (TMT1) and TMT2 genes exhibit a tissue- and cell type-specific expression pattern, whereas TMT3 is only weakly expressed. TMT1 and TMT2 expression is induced by drought, salt, and cold treatments and by sugar. During cold adaptation, tmt knockout lines accumulated less glucose and fructose compared with wild-type plants, whereas no differences were observed for sucrose. Cold adaptation of wild-type plants substantially promoted glucose uptake into isolated leaf mesophyll vacuoles. Glucose uptake into isolated vacuoles was inhibited by NH(4)(+), fructose, and phlorizin, indicating that transport is energy-dependent and that both glucose and fructose were taken up by the same carrier. Glucose import into vacuoles from two cold-induced tmt1 knockout lines or from triple knockout plants was substantially lower than into corresponding wild-type vacuoles. Monosaccharide feeding into leaf discs revealed the strongest response to sugar in tmt1 knockout lines compared with wild-type plants, suggesting that TMT1 is required for cytosolic glucose homeostasis. Our results indicate that TMT1 is involved in vacuolar monosaccharide transport and plays a major role during stress responses.

Cadmium-inducible expression of the ABC-type transporter AtABCC3 increases phytochelatin-mediated cadmium tolerance in Arabidopsis

The heavy metal cadmium (Cd) is a widespread environmental contaminant with harmful effects on living cells. In plants, phytochelatin (PC)-dependent Cd detoxification requires that PC-Cd complexes are transported into vacuoles. Here, it is shown that Arabidopsis thaliana seedlings defective in the ABCC transporter AtABCC3 (abcc3) have an increased sensitivity to different Cd concentrations, and that seedlings overexpressing AtABCC3 (AtABCC3ox) have an increased Cd tolerance. The cellular distribution of Cd was analysed in protoplasts from abcc3 mutants and AtABCC3 overexpressors grown in the presence of Cd, by means of the Cd-specific fluorochromes 5-nitrobenzothiazole coumarin (BTC-5N) and Leadmium™ Green AM dye. This analysis revealed that Cd is mostly localized in the cytosol of abcc3 mutant protoplasts whereas there is an increase in vacuolar Cd in protoplasts from AtABCC3ox plants. Overexpression of AtABCC3 in cad1-3 mutant seedlings defective in PC production and in plants treated with l-buthionine sulphoximine (BSO), an inhibitor of PC biosynthesis, had no effect on Cd tolerance, suggesting that AtABCC3 acts via PCs. In addition, overexpression of AtABCC3 in atabcc1 atabcc2 mutant seedlings defective in the Cd transporters AtABCC1 and AtABCC2 complements the Cd sensitivity of double mutants, but not in the presence of BSO. Accordingly, the level of AtABCC3 transcript in wild type seedlings was lower than that of AtABCC1 and AtABCC2 in the absence of Cd but higher after Cd exposure, and even higher in atabcc1 atabcc2 mutants. The results point to AtABCC3 as a transporter of PC-Cd complexes, and suggest that its activity is regulated by Cd and is co-ordinated with the activity of AtABCC1/AtABCC2.

Aquaporins Contribute to ABA-Triggered Stomatal Closure through OST1-Mediated Phosphorylation

Stomatal movements in response to environmental stimuli critically control the plant water status. Although these movements are governed by osmotically driven changes in guard cell volume, the role of membrane water channels (aquaporins) has remained hypothetical. Assays in epidermal peels showed that knockout Arabidopsis thaliana plants lacking the Plasma membrane Intrinsic Protein 2;1 (PIP2;1) aquaporin have a defect in stomatal closure, specifically in response to abscisic acid (ABA). ABA induced a 2-fold increase in osmotic water permeability (Pf) of guard cell protoplasts and an accumulation of reactive oxygen species in guard cells, which were both abrogated in pip2;1 plants. Open stomata 1 (OST1)/Snf1-related protein kinase 2.6 (SnRK2.6), a protein kinase involved in guard cell ABA signaling, was able to phosphorylate a cytosolic PIP2;1 peptide at Ser-121. OST1 enhanced PIP2;1 water transport activity when coexpressed in Xenopus laevis oocytes. Upon expression in pip2;1 plants, a phosphomimetic form (Ser121Asp) but not a phosphodeficient form (Ser121Ala) of PIP2;1 constitutively enhanced the Pf of guard cell protoplasts while suppressing its ABA-dependent activation and was able to restore ABA-dependent stomatal closure in pip2;1. This work supports a model whereby ABA-triggered stomatal closure requires an increase in guard cell permeability to water and possibly hydrogen peroxide, through OST1-dependent phosphorylation of PIP2;1 at Ser-121.

Amphotericin B-induced oxidative damage and killing of Candida albicans

Amphotericin B (AmB) is known to bind to ergosterol in fungal cell membranes, but the precise mechanism of its toxicity to cells is as yet poorly understood. AmB autooxidizes, and it is possible that its antifungal effects could result from oxidative damage. Exposure of protoplasts of Candida albicans to AmB under hypoxic conditions reduced protoplast lysis by as much as 80% compared with incubations in air. Protoplasts were protected from AmB-induced lysis by exogenous catalase and/or superoxide dismutase (SOD). Whole cells of C. albicans were protected by exogenous catalase from AmB-induced leakage of [3H]leucine and from killing by AmB. Cells grown on medium inducing high levels of endogenous catalase were resistant to AmB-induced growth inhibition. In contrast, AmB-induced K+ leakage was not hindered under hypoxic conditions or in the presence of catalase or SOD. Thus the lethal and lytic effects of AmB on C. albicans cells and protoplasts, but not prelethal AmB-induced K+ leakage, are mediated by oxidative damage.

Fumonisin B1-induced cell death in arabidopsis protoplasts requires jasmonate-, ethylene-, and salicylate-dependent signaling pathways

We have established an Arabidopsis protoplast model system to study plant cell death signaling. The fungal toxin fumonisin B1 (FB1) induces apoptosis-like programmed cell death (PCD) in wild-type protoplasts. FB1, however, only marginally affects the viability of protoplasts isolated from transgenic NahG plants, in which salicylic acid (SA) is metabolically degraded; from pad4-1 mutant plants, in which an SA amplification mechanism is thought to be impaired; or from jar1-1 or etr1-1 mutant plants, which are insensitive to jasmonate (JA) or ethylene (ET), respectively. FB1 susceptibility of wild-type protoplasts decreases in the dark, as does the cellular content of phenylalanine ammonia-lyase, a light-inducible enzyme involved in SA biosynthesis. Interestingly, however, FB1-induced PCD does not require the SA signal transmitter NPR1, given that npr1-1 protoplasts display wild-type FB1 susceptibility. Arabidopsis cpr1-1, cpr6-1, and acd2-2 protoplasts, in which the SA signaling pathway is constitutively activated, exhibit increased susceptibility to FB1. The cpr6-1 and acd2-2 mutants also constitutively express the JA and ET signaling pathways, but only the acd2-2 protoplasts undergo PCD in the absence of FB1. These results demonstrate that FB1 killing of Arabidopsis is light dependent and requires SA-, JA-, and ET-mediated signaling pathways as well as one or more unidentified factors activated by FB1 and the acd2-2 mutation.

Sodium fluxes through nonselective cation channels in the plasma membrane of protoplasts from Arabidopsis roots

The aim of the present work was to characterize Na(+) currents through nonselective cation channels (NSCCs) in protoplasts derived from root cells of Arabidopsis. The procedure of the protoplast isolation was modified to increase the stability of Arabidopsis root protoplasts in low external Ca(2+) by digesting tissue in elevated Ca(2+). Experiments in whole-cell and outside-out modes were carried out. We found that Na(+) currents in Arabidopsis root protoplasts were mediated by cation channels that were insensitive to externally applied tetraethylammonium(+) and verapamil, had no time-dependent activation (permanently opened or completely activated within 1-2 ms), were voltage independent, and were weakly selective for monovalent cations. The selectivity sequence was as follows: K(+) (1.49) > NH(4)(+) (1.24) > Rb(+) (1.15) approximately equal to Cs(+) (1.10) approximately equal to Na(+) (1.00) > Li(+) (0.73) > tetraethylammonium(+) (0.47). Arabidopsis root NSCCs were blocked by H(+) (pK approximately equal to 6.0), Ca(2+) (K(1/2) approximately equal to 0.1 mM), Ba(2+), Zn(2+), La(3+), Gd(3+), quinine, and the His modifier diethylpyrocarbonate. They were insensitive to most organic blockers (nifedipine, verapamil, flufenamate, and amiloride) and to the SH-group modifier p-chloromercuriphenyl sulfonic acid. Voltage-insensitive, Ca(2+)-sensitive single channels were also resolved. Properties of Arabidopsis root NSCCs are discussed and compared with characteristics of similar conductances studied previously in plants and animals. It is suggested that NSCCs present a distinct group of plant ion channels, mediating toxic Na(+) influx to the cell and probably having other important roles in physiological processes of plants.

Bactericidal properties of human and murine groups I, II, V, X, and XII secreted phospholipases A(2)

Group IIA secreted phospholipase A(2) (sPLA2) is known to display potent Gram-positive bactericidal activity in vitro and in vivo. We have analyzed the bactericidal activity of the full set of recombinant murine and human groups I, II, V, X, and XII sPLA2s on Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli. The rank order potency among human sPLA2s against Gram-positive bacteria is group IIA > X > V > XII > IIE > IB, IIF (for murine sPLA2s: IIA > IID > V > IIE > IIC, X > IB, IIF), and only human group XII displays detectable bactericidal activity against the Gram-negative bacterium E. coli. These studies show that highly basic sPLA2s display potent bactericidal activity with the exception of the ability of the acidic human group X sPLA2 to kill Gram-positive bacteria. By studying the Bacillus subtilis and S. aureus bactericidal potencies of a large panel of human group IIA mutants in which basic residues were mutated to acidic residues, it was found that: 1) the overall positive charge of the sPLA2 is the dominant factor in dictating bactericidal potency; 2) basic residues on the putative membrane binding surface of the sPLA2 are modestly more important for bactericidal activity than are other basic residues; 3) relative bactericidal potency tracks well with the ability of these mutants to degrade phospholipids in the bacterial membrane; and 4) exposure of the bacterial membrane of Gram-positive bacteria by disruption of the cell wall dramatically reduces the negative effect of charge reversal mutagenesis on bactericidal potency.

Characterization of genes involved in cytokinin signaling and metabolism from rice

Two-component signaling elements play important roles in plants, including a central role in cytokinin signaling. We characterized two-component elements from the monocot rice (Oryza sativa) using several complementary approaches. Phylogenetic analysis reveals relatively simple orthologous relationships among the histidine kinases in rice and Arabidopsis (Arabidopsis thaliana). In contrast, the histidine-containing phosphotransfer proteins (OsHPs) and response regulators (OsRRs) display a higher degree of lineage-specific expansion. The intracellular localizations of several OsHPs and OsRRs were examined in rice and generally found to correspond to the localizations of their dicot counterparts. The functionality of rice type-B OsRRs was tested in Arabidopsis; one from a clade composed of both monocot and dicot type-B OsRRs complemented an Arabidopsis type-B response regulator mutant, but a type-B OsRR from a monocot-specific subfamily generally did not. The expression of genes encoding two-component elements and proteins involved in cytokinin biosynthesis and degradation was analyzed in rice roots and shoots and in response to phytohormones. Nearly all type-A OsRRs and OsHK4 were up-regulated in response to cytokinin, but other cytokinin signaling elements were not appreciably affected. Furthermore, multiple cytokinin oxidase (OsCKX) genes were up-regulated by cytokinin. Abscisic acid treatment decreased the expression of several genes involved in cytokinin biosynthesis and degradation. Auxin affected the expression of a few genes; brassinosteroid and gibberellin had only modest effects. Our results support a shared role for two-component elements in mediating cytokinin signaling in monocots and dicots and reveal how phytohormones can impact cytokinin function through modulating gene expression.

Antimicrobial activity of cecropins

The lytic peptides, cecropins, were originally isolated from the haemolymph of the giant silk moth, Hyalophora cecropia and possess antibacterial and anticancer activity in vitro. This study investigated the antimicrobial activity of these peptides against human pathogens using standardised assay techniques, and the activity of cecropin B on outer and inner bacterial membranes. From a panel of 15 organisms, Gram-negative bacteria were generally more sensitive to cecropins than Gram-positive organisms, especially the lipopolysaccharide defective mutant, Escherichia coli BUE55. Cecropins B and P1 shared similar MIC values whereas Shiva-1, a cecropin B analogue, was less active. Through combination studies with hydrophobic antibiotics and electron microscopy, cecropin B was shown to disrupt the bacterial outer membrane. Protoplasts of Staphylococcus aureus and Staphylococcus epidermidis were resistant to cecropin B, suggesting that the cytoplasmic membranes of Gram-positive organisms were inherently more resistant to the peptide.

The Role of auxin, pH, and stress in the activation of embryogenic cell division in leaf protoplast-derived cells of alfalfa

Culturing leaf protoplast-derived cells of the embryogenic alfalfa (Medicago sativa subsp. varia A2) genotype in the presence of low (1 microM) or high (10 microM) 2, 4-dichlorophenoxyacetic acid (2,4-D) concentrations results in different cell types. Cells exposed to high 2,4-D concentration remain small with dense cytoplasm and can develop into proembryogenic cell clusters, whereas protoplasts cultured at low auxin concentration elongate and subsequently die or form undifferentiated cell colonies. Fe stress applied at nonlethal concentrations (1 mM) in the presence of 1 microM 2,4-D also resulted in the development of the embryogenic cell type. Although cytoplasmic alkalinization was detected during cell activation of both types, embryogenic cells could be characterized by earlier cell division, a more alkalic vacuolar pH, and nonfunctional chloroplasts as compared with the elongated, nonembryogenic cells. Buffering of the 10 microM 2,4-D-containing culture medium by 10 mM 2-(N-morpholino)ethanesulfonic acid delayed cell division and resulted in nonembryogenic cell-type formation. The level of endogenous indoleacetic acid (IAA) increased transiently in all protoplast cultures during the first 4 to 5 d, but an earlier peak of IAA accumulation correlated with the earlier activation of the division cycle in embryogenic-type cells. However, this IAA peak could also be delayed by buffering of the medium pH by 2-(N-morpholino)ethanesulfonic acid. Based on the above data, we propose the involvement of stress responses, endogenous auxin synthesis, and the establishment of cellular pH gradients in the formation of the embryogenic cell type.

Enhanced reactive oxygen species scavenging by overproduction of superoxide dismutase and catalase delays postharvest physiological deterioration of cassava storage roots

Postharvest physiological deterioration (PPD) of cassava (Manihot esculenta) storage roots is the result of a rapid oxidative burst, which leads to discoloration of the vascular tissues due to the oxidation of phenolic compounds. In this study, coexpression of the reactive oxygen species (ROS)-scavenging enzymes copper/zinc superoxide dismutase (MeCu/ZnSOD) and catalase (MeCAT1) in transgenic cassava was used to explore the intrinsic relationship between ROS scavenging and PPD occurrence. Transgenic cassava plants integrated with the expression cassette p54::MeCu/ZnSOD-35S::MeCAT1 were confirmed by Southern-blot analysis. The expression of MeCu/ZnSOD and MeCAT1 was verified by quantitative reverse transcription-polymerase chain reaction and enzymatic activity analysis both in the leaves and storage roots. Under exposure to the ROS-generating reagent methyl viologen or to hydrogen peroxide (H2O2), the transgenic plants showed higher enzymatic activities of SOD and CAT than the wild-type plants. Levels of malondialdehyde, chlorophyll degradation, lipid peroxidation, and H2O2 accumulation were dramatically reduced in the transgenic lines compared with the wild type. After harvest, the storage roots of transgenic cassava lines show a delay in their PPD response of at least 10 d, accompanied by less mitochondrial oxidation and H2O2 accumulation, compared with those of the wild type. We hypothesize that this is due to the combined ectopic expression of Cu/ZnSOD and CAT leading to an improved synergistic ROS-scavenging capacity of the roots. Our study not only sheds light on the mechanism of the PPD process but also develops an effective approach for delaying the occurrence of PPD in cassava.

Regulation of plant cytosolic glyceraldehyde 3-phosphate dehydrogenase isoforms by thiol modifications

Cytosolic NAD-dependent glyceraldehyde 3-P dehydrogenase (GAPDH; GapC; EC 1.2.1.12) catalyzes the oxidation of triose phosphates during glycolysis in all organisms, but additional functions of the protein has been put forward. Because of its reactive cysteine residue in the active site, it is susceptible to protein modification and oxidation. The addition of GSSG, and much more efficiently of S-nitrosoglutathione, was shown to inactivate the enzymes from Arabidopsis thaliana (isoforms GapC1 and 2), spinach, yeast and rabbit muscle. Inactivation was fully or at least partially reversible upon addition of DTT. The incorporation of glutathione upon formation of a mixed disulfide could be shown using biotinylated glutathione ethyl ester. Furthermore, using the biotin-switch assay, nitrosylated thiol groups could be shown to occur after treatment with nitric oxide donors. Using mass spectrometry and mutant proteins with one cysteine lacking, both cysteines (Cys-155 and Cys-159) were found to occur as glutathionylated and as nitrosylated forms. In preliminary experiments, it was shown that both GapC1 and GapC2 can bind to a partial gene sequence of the NADP-dependent malate dehydrogenase (EC 1.2.1.37; At5g58330). Transiently expressed GapC-green fluorescent protein fusion proteins were localized to the nucleus in A. thaliana protoplasts. As nuclear localization and DNA binding of GAPDH had been shown in numerous systems to occur upon stress, we assume that such mechanism might be part of the signaling pathway to induce increased malate-valve capacity and possibly other protective systems upon overreduction and initial formation of reactive oxygen and nitrogen species as well as to decrease and protect metabolism at the same time by modification of essential cysteine residues.

Arabidopsis annexin1 mediates the radical-activated plasma membrane Ca²+- and K+-permeable conductance in root cells

Plant cell growth and stress signaling require Ca²⁺ influx through plasma membrane transport proteins that are regulated by reactive oxygen species. In root cell growth, adaptation to salinity stress, and stomatal closure, such proteins operate downstream of the plasma membrane NADPH oxidases that produce extracellular superoxide anion, a reactive oxygen species that is readily converted to extracellular hydrogen peroxide and hydroxyl radicals, OH•. In root cells, extracellular OH• activates a plasma membrane Ca²⁺-permeable conductance that permits Ca²⁺ influx. In Arabidopsis thaliana, distribution of this conductance resembles that of annexin1 (ANN1). Annexins are membrane binding proteins that can form Ca²⁺-permeable conductances in vitro. Here, the Arabidopsis loss-of-function mutant for annexin1 (Atann1) was found to lack the root hair and epidermal OH•-activated Ca²⁺- and K⁺-permeable conductance. This manifests in both impaired root cell growth and ability to elevate root cell cytosolic free Ca²⁺ in response to OH•. An OH•-activated Ca²⁺ conductance is reconstituted by recombinant ANN1 in planar lipid bilayers. ANN1 therefore presents as a novel Ca²⁺-permeable transporter providing a molecular link between reactive oxygen species and cytosolic Ca²⁺ in plants.

Abscisic acid signal transduction in the barley aleurone is mediated by phospholipase D activity

The plant hormones abscisic acid (ABA) and gibberellic acid (GA) are important regulators of the dormancy and germination of seeds. In cereals, GA enhances the synthesis and secretion of enzymes (principally alpha-amylases) in the aleurone cells of the endosperm, which then mobilize the storage reserves that fuel germination. ABA inhibits this enhanced secretory activity and delays germination. Despite the central role of ABA in regulating germination, the signal transduction events leading to altered gene expression and cellular activity are essentially unknown. We report that the application of ABA to aleurone protoplasts increased the activity of the enzyme phospholipase D (PLD) 10 min after treatment. The product of PLD activity, phosphatidic acid (PPA), also increased transiently at this time. The application of PPA to aleurone protoplasts led to an ABA-like inhibition of alpha-amylase production, and induction of the ABA up-regulated proteins ASI (amylase subtilisin inhibitor) and RAB (responsive to ABA). Inhibition of PLD activity by 0.1% 1-butanol during the initial 20 min of ABA treatment resulted in inhibition of ABA-regulated processes. This inhibition coincided with the timing of PLD activation by ABA and was overcome by simultaneous addition of PPA. These results suggest that ABA activates the enzyme PLD to produce PPA that is involved in triggering the subsequent ABA responses of the aleurone cell.

Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action

Efforts to reduce the global burden of bacterial disease and contend with escalating bacterial resistance are spurring innovation in antibacterial drug and biocide development and related technologies such as photodynamic therapy and photochemical disinfection. Elucidation of the mechanism of action of these new agents and processes can greatly facilitate their development, but it is a complex endeavour. One strategy that has been popular for many years, and which is garnering increasing interest due to recent technological advances in microscopy and a deeper understanding of the molecular events involved, is the examination of treated bacteria for changes to their morphology and ultrastructure. In this review, we take a critical look at this approach. Variables affecting antibacterial-induced alterations are discussed first. These include characteristics of the test organism (e.g. cell wall structure) and incubation conditions (e.g. growth medium osmolarity). The main body of the review then describes the different alterations that can occur. Micrographs depicting these alterations are presented, together with information on agents that induce the change, and the sequence of molecular events that lead to the change. We close by highlighting those morphological and ultrastructural changes which are consistently induced by agents sharing the same mechanism (e.g. spheroplast formation by peptidoglycan synthesis inhibitors) and explaining how changes that are induced by multiple antibacterial classes (e.g. filamentation by DNA synthesis inhibitors, FtsZ disruptors, and other types of agent) can still yield useful mechanistic information. Lastly, recommendations are made regarding future study design and execution.

Arabidopsis thaliana root non-selective cation channels mediate calcium uptake and are involved in growth

Calcium is a critical structural and regulatory nutrient in plants. However, mechanisms of its uptake by root cells are poorly understood. We have found that Ca2+ influx in Arabidopsis root epidermal protoplasts is mediated by voltage-independent rapidly activating Ca2+-permeable non-selective cation channels (NSCCs). NSCCs showed the following permeability (P) sequence: PCa (1.00) = PBa (0.93) > PZn (0.51), PCa/PNa = 0.19, PCa/PK = 0.14. They were inhibited by quinine, Gd3+, La3+ and the His modifier diethylpyrocarbonate, but not by the Ca2+ or K+ channel antagonists, verapamil and tetraethylammonium (TEA+). Single channel conductance measured in 20 mm external Ca2+ was 5.9 pS. Calcium-permeable NSCCs co-existed with hyperpolarisation-activated Ca2+ channels (HACCs), which activated 40-60 min after forming the whole-cell configuration. HACCs activated at voltages <-130 to -150 mV, showed slow activation kinetics and were regulated by cytosolic Ca2+ ([Ca2+]cyt). Using aequorin-expressing plants, a linear relationship between membrane potential (Vm) and resting [Ca2+]cyt was observed, indicating the involvement of NSCCs. Intact root 45Ca2+ influx was reduced by Gd3+ (NSCC blocker) but was verapamil and TEA+ insensitive. In the root elongation zone, both root net Ca2+ influx (measured by Ca2+-selective vibrating microelectrode) and NSCC activity were increased compared to the mature epidermis, suggesting the involvement of NSCC in growth. A Ca2+ acquisition system based on NSCC and HACC co-existence is proposed. In mature epidermal cells, NSCC-mediated Ca2+ influx dominates whereas in specialised root cells (root hairs and elongation zone cells) where elevated [Ca2+]cyt activates HACCs, HACC-mediated Ca2+ influx predominates.