Showing papers in "Plant and Cell Physiology in 2000"

Role of Organic Acids in Detoxification of Aluminum in Higher Plants

Abstract: Phytotoxicity of aluminum ion (Al3+) is a serious problem limiting crop production on acid soils. Organic acids with Al-chelating ability play an important role in the detoxification of Al both externally and internally. Al is detoxified externally by the secretion of organic acids such as citric, oxalic, and/or malic acids from the roots. The secretion of organic acids is highly specific to Al and the site of secretion is localized to the root apex. The kind of organic acids secreted as well as secretion pattern differ among plant species. There are two patterns of Al-induced secretion of organic acids: In pattern I, there is no discernible delay between the addition of Al and the onset of the release of organic acids. Activation of the anion channel seems to be involved in this pattern; In pattern II, there is a marked lag phase between the addition of Al and the onset of organic acid release. The action of genes related to the metabolism and secretion of organic acids seems to be involved in this pattern. Internal detoxification of Al in Al-accumulating plants is achieved by the formation of Al-organic acid complex. For instance, the complex of Al-citrate (1:1) in hydrangea and Al-oxalate (1:3) in buckwheat has been identified.

Stress tolerance in transgenic tobacco seedlings that overexpress glutathione S-transferase/glutathione peroxidase.

Abstract: Overexpression of a tobacco glutathione S-transferase with glutathione peroxidase activity (GST/GPX) in transgenic tobacco (Nicotiana tabacum L.) enhanced seedling growth under a variety of stressful conditions. In addition to increased GST and GPX activity, transgenic GST/GPX-expressing (GST+) seedlings had elevated levels of monodehydroascorbate reductase activity. GST+ seedlings also contained higher levels of glutathione and ascorbate than wild-type seedlings and the glutathione pools were more oxidized. Thermal or salt-stress treatments that inhibited the growth of wild-type seedlings also caused increased levels of lipid peroxidation. These treatments had less effect on the growth of GST+ seedling growth and did not lead to increased lipid peroxidation. Stress-induced damage resulted in reduced metabolic activity in wild-type seedlings while GST+ seedlings maintained metabolic activity levels comparable to seedlings grown under control conditions. These results indicate that overexpression of GST/GPX in transgenic tobacco seedlings provides increased glutathione-dependent peroxide scavenging and alterations in glutathione and ascorbate metabolism that lead to reduced oxidative damage. We conclude that this protective effect is primarily responsible for the ability of GST+ seedlings to maintain growth under stressful conditions.

Circadian waves of expression of the APRR1/TOC1 family of pseudo-response regulators in Arabidopsis thaliana: insight into the plant circadian clock.

Abstract: The Arabidopsis pseudo-response regulator, APRR1, has a unique structural design containing a pseudo-receiver domain and a C-terminal CONSTANS motif. This protein was originally characterized as a presumed component of the His-to-Asp phosphorelay systems in Arabidopsis thaliana. Recently, it was reported that APRR1 is identical to the TOC1 gene product, a mutational lesion of which affects the periods of many circadian rhythms in Arabidopsis plants. TOC1 is believed to be a component of the presumed circadian clock (or central oscillator). Based on these facts, in this study four more genes, each encoding a member of the APRR1/TOC1 family of pseudo-response regulators were identified and characterized with special reference to circadian rhythms. It was found that all these members of the APRR1/TOC1 family (APRR1, APRR3, APRR5, APRR7, and APRR9) are subjected to a circadian rhythm at the level of transcription. Furthermore, in a given 24 h period, the APRR-mRNAs started accumulating sequentially after dawn with 2-3 h intervals in the order of APRR9-->APRR7-->APRR5-->APRR3-->APRR1. These sequential events of transcription, termed 'circadian waves of APRR1/TOCI', were not significantly affected by the photoperiod conditions, if any (e.g. both long and short days), and the expression of APRR9 was first boosted always after dawn. Among these APRRs, in fact, only the expression of APRR9 was rapidly and transiently induced also by white light, whereas such light responses of others were very dull, if any. These results collectively support the view that these members of the APRR1/TOC1 family are together all involved in an as yet unknown mechanism underlying the Arabidopsis circadian clock. Here we propose that the circadian waves of the APRR1/TOC1 family members are most likely a molecular basis of such a biological clock in higher plants.

Gibberellin Biosynthesis: Its Regulation by Endogenous and Environmental Signals

Abstract: The hormone gibberellin (GA) plays an essential role in many aspects of plant growth and development, such as seed germination, stem elongation and flower development. In recent years, exciting progress has been made in understanding how the biosynthesis of this hormone is regulated by endogenous and environmental factors. This has resulted from isolation of genes encoding enzymes involved in GA biosynthesis and metabolism, which also enabled us to manipulate the pathway by modifying the expression of these genes in transgenic plants. In addition, new GA response mutants provided information about how signaling components are involved in feedback regulation of the GA biosynthetic pathway.

A Null Mutation in a bZIP Factor Confers ABA-Insensitivity in Arabidopsis thaliana

Abstract: We have used a modification of the classical ABA-insensitive screen (Koornneef et al. 1984) to isolate novel mutations in the ABA signal transduction pathway of Arabidopsis thaliana. In our screen, mutants were recovered on the basis of their growth-insensitivity to ABA (GIA) rather than germination-insensitivity. Here we present the isolation of the gia1 mutant as well as the identification of the gia1 gene by positional cloning and complementation studies. GIA1 is predicted to code for a bZIP transcription factor with high homology to previously characterized plant bZIP transcription factors (DPBF1, ABFs and TRAB1) known for their ability to bind ABA-responsive DNA elements. Our results provide in vivo evidence that a bZIP factor may indeed be involved in ABA signaling. Since GIA1 turned out to be identical to ABI5, we designated GIA1 as ABI5 in the present paper.

Involvement of Jasmonate- and Salicylate-Related Signaling Pathways for the Production of Specific Herbivore-Induced Volatiles in Plants

Abstract: We compared volatiles from lima bean leaves (Phaseolus lunatus) infested by either beet armyworm (Spodoptera exigua), common armyworm [Mythimna (Pseudaletia) separata], or two-spotted spider mite (Tetranychus urticae). We also analyzed volatiles from the leaves treated with jasmonic acid (JA) and/or methyl salicylate (MeSA). The volatiles induced by aqueous JA treatment were qualitatively and quantitatively similar to those induced by S. exigua or M. separata damage. Furthermore, both S. exigua and aqueous JA treatment induced the expression of the same basic PR genes. In contrast, gaseous MeSA treatment, and aqueous JA treatment followed by gaseous MeSA treatment, induced volatiles that was qualitatively and quantitatively more similar to the T. urticae-induced volatiles than those induced by aqueous JA treatment. In addition, T. urticae damage resulted in the expression of the acidic and basic PR genes that were induced by gaseous MeSA treatment and by aqueous JA treatment, respectively. Based on these data, we suggest that in lima bean leaves, the JA-related signaling pathway is involved in the production of caterpillar-induced volatiles, while both the SA-related signaling pathway and the JA-related signaling pathway are involved in the production of T. urticae-induced volatiles.

Genes encoding pseudo-response regulators: insight into His-to-Asp phosphorelay and circadian rhythm in Arabidopsis thaliana.

Abstract: In the higher plant, Arabidopsis thaliana, results from recent intensive studies suggested that His-to-Asp phosphorelay mechanisms are involved presumably in propagation of environmental stimuli, such as phytohormones (e.g. ethylene and cytokinin). Here we identified and characterized a set of novel Arabidopsis genes whose products considerably resemble the authentic response regulators (ARR-series) of Arabidopsis in the sense that they have a phospho-accepting receiver-like domain. However, they should be discriminated from the classical ones in the strict sense that they lack the invariant phospho-accepting aspartate site. They were thus named APRRs (Arabidopsis pseudo-response regulators). Two such representatives, APRR1 and APRR2, were characterized extensively through cloning of the corresponding cDNAs, in terms of their structural designs, biochemical properties, subcellular localization in plant cells, and expression profiles at the transcriptional level. The result of in vitro phosphorylation experiment with the Arabidopsis AHP phosphotransmitter suggested that the pseudo-receivers have no ability to undergo phosphorylation. The result of transient expression assay with onion epidermal cells showed that the GFPAPRR1 fusion protein has an ability to enter into the nuclei. The C-terminal domain of APRR1, termed CONSTANS-motif, appears to be responsible for the nuclearlocalization. The most intriguing result was that the accumulation of APRR1 transcript is subjected to a circadian rhythm. The APRR1 protein is identical to the one that was recently suggested to interact with the ABI3 (ABISCISIC ACID INSENSITIVE3) protein. These are discussed with special reference to the His-to-Asp phosphorelay signal transduction and circadian rhythm in Arabidopsis thaliana.

Overexpression of Mitochondrial Citrate Synthase in Arabidopsis thaliana Improved Growth on a Phosphorus-Limited Soil

Abstract: The gene for mitochondrial citrate synthase (CS) was isolated from Daucus carota (DcCS) and introduced into Arabidopsis thaliana (strain WS) using Agrobacterium tumefaciens-mediated transformation. Characteristics of citrate excretion were compared between T3 transgenic plants, which were derived from the initial transgenic plants by self-fertilization and homozygous for DcCS, and the control plants that had no DcCS. The highest CS activity 0.78 micromol protein min(-1) exhibited by the transgenic plants was about threefold greater than that found in the control plants (0.23-0.28 micromol protein min(-1)). Western analysis of the transgenic plants showed two CS signals corresponding to signals obtained from both D. carota and A. thaliana. Thus, it appears that the CS polypeptides by ectopic expression of DcCS were processed into the mature form and localized in the mitochondria of A. thaliana. The signal corresponding to the mature form of DcCS were greater in the transgenic plants having higher levels of CS activity. When the transgenic plants were grown in Al-phosphate media, a correlation between the levels of CS activity and the amounts of citrate excreted into the medium. The highest value (5.1 nmol per plant) was about 2.5-fold greater than that from control plants (1.9 nmol per plant). Both growth and P accumulation were greater in transgenic plants with high CS activity than that in control plants when they were grown on an acid soil where the availability of phosphate was low due to the formation of Al-phosphate. It appears that the overexpression of CS in A. thaliana improves the growth in phosphorous limited soil as a result of enhanced citrate excretion from the roots.

Spatially and temporally regulated expression of rice MADS box genes with similarity to Arabidopsis class A, B and C genes.

Abstract: The floral morphology of grass species is distinct from that of typical dicot plants. In order to achieve a better understanding of the molecular basis for this diversion, we isolated RAP1A, RAP1B and RAG, putative rice orthologs for the Arabidopsis class A gene APETALA1 (AP1) and class C gene AGAMOUS (AG). The expression patterns of RAP1A, RAG and OsMADS2, a rice ortholog of the class B gene, were analyzed by in situ hybridization. RAP1A mRNA was expressed in the apical region of the floral meristem at an early stage of spikelet development, and then its expression was localized in developing lemma, palea and lodicules. The OsMADS2 transcript was first observed in the region where stamen primordia are formed. Soon after, OsMADS2 mRNA appeared in the lodicule primordia as well as the stamen primordia, and this RNA accumulation pattern persisted until late stages of floral development. The expression of RAG was observed in stamens and pistils of wild-type young spikelets. These RNA accumulation patterns are mostly similar to those of Arabidopsis class A, B, C genes, supporting the notion that the ABC model may be extended to rice.

Transport processes of solutes across the vacuolar membrane of higher plants.

Abstract: The central vacuole is the largest compartment of a mature plant cell and may occupy more than 80% of the total cell volume. However, recent results indicate that beside the large central vacuole, several small vacuoles may exist in a plant cell. These vacuoles often belong to different classes and can be distinguished either by their contents in soluble proteins or by different types of a major vacuolar membrane protein, the aquaporins. Two vacuolar proton pumps, an ATPase and a PPase energize vacuolar uptake of most solutes. The electrochemical gradient generated by these pumps can be utilized to accumulate cations by a proton antiport mechanism or anions due to the membrane potential difference. Uptake can be catalyzed by channels or by transporters. Growing evidence shows that for most ions more than one transporter/channel exist at the vacuolar membrane. Furthermore, plant secondary products may be accumulated by proton antiport mechanisms. The transport of some solutes such as sucrose is energized in some plants but occurs by facilitated diffusion in others. A new class of transporters has been discovered recently: the ABC type transporters are directly energized by MgATP and do not depend on the electrochemical force. Their substrates are organic anions formed by conjugation, e.g. to glutathione. In this review we discuss the different transport processes occurring at the vacuolar membrane and focus on some new results obtained in this field.

Jasmonate induction of putrescine N-methyltransferase genes in the root of Nicotiana sylvestris.

Abstract: Nicotine alkaloids are synthesized in the root of Nicotiana species, and their synthesis increases after insect attack, wounding and jasmonate treatment of the leaf. Putrescine N-methyltransferase (PMT) catalyzes the first committed step in nicotine biosynthesis. The expression patterns of the three Nicotiana sylvestris PMT genes (NsPMT1, NsPMT2, and NsPMT3) are reported in this study. Transcripts of the NsPMT genes were detected only in the root, and were up-regulated by methyl jasmonate treatment. When the 5'-flanking regions of NsPMT1, NsPMT2, and NsPMT3 were fused independently to beta-glucuronidase reporter gene and introduced into N. sylvestris by Agrobacterium-mediated transformation, all introduced transgenes were expressed in the cortex, endodermis, and xylem in the root, as well as upregulated by methyl jasmonate treatment. These qualitatively similar patterns of expression for the NsPMT genes are achieved with only 0.25 kb of their conserved 5'-flanking regions, which contained no known jasmonate-responsive elements.

Integrating cellular and organismic aspects of vascular differentiation.

Abstract: Vascular differentiation can be studied at two levels, and they should complement one another: as an aspect of integrated plant development and as cellular processes. The differentiation of organized strands that connect between organs is induced by polar auxin flow, towards the roots. Anatomy, therefore, can be a complementary method of observing polarity and its changes. As expected for a self-correcting and essential system, vascular patterning mutations are relatively rare and have pleiotropic effects, including modifications of responses to auxin and its transport. Tissue polarity both expresses and depends on auxin transport, a feedback that could account for the determined nature of polarity as well as the gradual canalization of differentiation to vascular strands. This predicts that the molecules responsible for polarity will be localized gradually as differentiation proceeds. Further, a modified location of these molecules can be expected to precede anatomical expressions of a new, regenerated, polarity. Tracheary differentiation is probably the best studied example of cell differentiation. Within the plant, however, this differentiation is coupled to oriented cell growth either along or at right angles to the axis of auxin flow, depending on tissue competence. Differentiation is also coupled to the differentiation of the other components of the vascular system. There are, presumably, early joint stages to these differentiation processes, but what they are remains an intriguing problem.

Evaluation of the Defense System in Chloroplasts to Photooxidative Stress Caused by Paraquat Using Transgenic Tobacco Plants Expressing Catalase from Escherichia coli

Abstract: We evaluated the defense system in chloroplasts to photooxidative stress imposed by paraquat treatment under illumination in transgenic tobacco plants with increased tolerance to drought stress at a high light intensity produced by catalase from Escherichia coli targeted to chloroplasts [Shikanai et al. (1998) FEBS Lett. 428: 47]. At 24 h after the paraquat application, Chl was destroyed in the wild-type plants, but not in transgenic plants. Photosynthetic activities monitored by CO2 fixation and Chl fluorescence were much less affected by the paraquat treatment in transgenic lines. The activities of chloroplastic ascorbate peroxidase (APX) isoenzymes decreased in parallel with the depletion of ascorbate (AsA) in leaves in both lines. Paraquat treatment had no effect on the transcript level of chloroplastic APX isoenzymes, while it significantly lowered the level of their proteins. These data suggest that the depletion of AsA in chloroplasts under severe stress conditions inactivates and degrades chloroplastic APX isoenzymes.

The YELLOW VARIEGATED (VAR2) locus encodes a homologue of FtsH, an ATP-dependent protease in Arabidopsis.

Abstract: Variegated leaves are often caused by a nuclear recessive mutation in higher plants. Characterization of the gene responsible for variegation has shown to provide several pathways involved in plastid differentiation. Here we describe an Arabidopsis variegated mutant isolated by T-DNA tagging. The mutant displayed green and yellow sectors in all green tissues except for cotyledons. Cells in the yellow sector of the mutant contained both normal-appearing and mutant chloroplasts. The isolated mutant was shown to be an allele of the previously reported mutant, yellow variegated (var2). Cloning and molecular characterization of the VAR2 locus revealed that it potentially encodes a chloroplastic homologue of FtsH, an ATP-dependent metalloprotease that belongs to a large protein family involved in various cellular functions. ftsH-like genes appear to comprise a small gene family in Arabidopsis genome, since at least six homologues were found in addition to VAR2. Dispensability of VAR2 was therefore explained by the redundancy of genes coding for FstHs. In the yellow regions of the mutant leaves, accumulation of photosynthetic protein components in the thylakoid membrane appeared to be impaired. Based on the role of FtsH in a protein degradation pathway in plastids, we propose a possibility that VAR2 is required for plastid differentiation by avoiding partial photooxidation of developing chloroplasts.

Novel Putative Photoreceptor and Regulatory Genes Required for the Positive Phototactic Movement of the Unicellular Motile Cyanobacterium Synechocystis sp. PCC 6803

Abstract: Synechocystis: sp. PCC 6803 is a unicellular motile cyanobacterium, which shows positive or negative phototaxis on agar plates under lateral illumination. By gene disruption in a substrain showing of positive phototaxis, it was demonstrated that mutants defective in sll0038, sll0039, sll0041, sll0042 or sll0043 lost positive phototaxis but showed negative phototaxis away from the light source. Mutants of sll0040, which is located within the cluster of these genes, retained the capacity of positive phototaxis but to a lesser extent than the parent cells. These genes are homologous to che genes, which are involved in flagellar switching for bacterial chemotaxis. Interestingly, sll0041 (designated pisJ1) is predicted to have a chromophore-binding motif of phytochrome-like proteins and a signaling motif of chemoreceptors for bacterial chemotaxis. It is strongly suggested that the positive phototactic response was mediated by a phytochrome-like photoreceptor and CheA/CheY-type signal transduction system.

Effects of ethylene precursor and inhibitors for ethylene biosynthesis and perception on nodulation in Lotus japonicus and Macroptilium atropurpureum.

Abstract: ;Inhibitors of ethylene synthesis or its physiological function enhanced nodulation in Lotus japonicus and Macroptilium atropurpureum. In contrast, the application of 1aminocyclopropane-1-carboxylic acid, a precursor of ethylene biosynthesis, reduced the nodule number in these legumes. These results suggest that an ethylene-mediated signaling pathway is involved in the nodulation process even in the determinate nodulators.

Determination of the Rate of Photoreduction of O2 in the Water-Water Cycle in Watermelon Leaves and Enhancement of the Rate by Limitation of Photosynthesis

Abstract: A study was performed to determine how the electron fluxes for the photosynthetic carbon reduction (PCR) and the photorespiratory carbon oxidation (PCO) cycles affect the photoreduction of O2 at PSI, which is the limiting step in the water-water cycle. Simultaneous measurements were made of CO2-gas exchange, transpiration and quantum yield of PSII [

Reversal of Blue Light-Stimulated Stomatal Opening by Green Light

Abstract: Blue light-stimulated stomatal opening in detached epidermis of Vicia faba is reversed by green light. A 30 s green light pulse eliminated the transient opening stimulated by an immediately preceding blue light pulse. Opening was restored by a subsequent blue light pulse. An initial green light pulse did not alter the response to a subsequent blue light pulse. Reversal also occurred under continuous illumination, with or without a saturating red light background. The magnitude of the green light reversal depended on fluence rate, with full reversal observed at a green light fluence rate twice that of the blue light. Continuous green light given alone stimulated a slight stomatal opening, and had no effect on red light-stimulated opening. An action spectrum for the green light effect showed a maximum at 540 nm and minor peaks at 490 and 580 nm. This spectrum is similar to the action spectrum for blue light-stimulated stomatal opening, red-shifted by about 90 nm. The carotenoid zeaxanthin has been implicated as a photoreceptor for the stomatal blue light response. Blue/ green reversibility might be explained by a pair of interconvertible zeaxanthin isomers, one absorbing in the blue and the other in the green, with the green absorbing form being the physiologically active one.

Molecular characterization of cDNA encoding oxygen evolving enhancer protein 1 increased by salt treatment in the mangrove Bruguiera gymnorrhiza

Abstract: Young plants of the common Okinawa mangrove species Bruguiera gymnorrhiza were transferred from freshwater to a medium with seawater salt level (500 mM NaCl). Two-dimensional gel electrophoresis revealed in the leaf extract of the plant a 33 kDa protein with pI 5.2, whose quantity increased as a result of NaCl treatment. The N-terminal amino acids sequence of this protein had a significant homology with mature region of oxygen evolving enhancer protein 1 (OEE1) precursor. The cloning of OEE1 precursor cDNA fragment was carried out by means of reverse transcription-PCR (RT-PCR) using degenerated primers. Both 3'- and 5'-regions were isolated by rapid amplification of cDNA ends (RACE) method. The deduced amino acid sequence consisted of 322 amino acids and was 87% identical to that of Nicotiana tabacum. In B. gymnorrhiza, the predicted amino acid sequence of the mature protein starts at the residue number 85 of the open reading frame. The first 84-amino acid residues correspond to a typical transit sequence for the signal directing OEE1 to its appropriate compartment of chloroplast. The expression of OEE1 was analyzed together with other OEE subunits and D1 protein of photosystem II. The transcript levels of all the three OEEs were enhanced by NaCl treatment, but the significant increase of D1 protein was not observed.

Induction of Chalcone Synthase Expression in White Spruce by Wounding and Jasmonate

Abstract: The phenylpropanoid pathway has important functions in angiospermous plants following exposure to environmental stresses, such as wounding and pathogen attack, that lead to production of compounds including lignin, flavonoids and phytoalexins. Chalcone synthase (CHS) is a key enzyme in this pathway, catalyzing the first step in flavonoid biosynthesis, whose expression can be induced in response to environmental stress. To explore the response of conifers to environmental stress, expression of spruce CHS and its inducibility were investigated. A partial spruce CHS cDNA clone was isolated using PCR. Examination of the expression patterns of the CHS gene family in white spruce revealed accumulation of CHS mRNA in needle tissue following mechanical wounding, or application of signal molecules like jasmonic acid or methyl jasmonate. Repeated mechanical wounding or jasmonate applications had an enhancing effect on transcript accumulation in needles. A systemic accumulation of CHS mRNAs following wounding was also observed. Conifers thus appear to possess a general wound response similar to that found for angiosperms, which includes CHS induction as well as its inducibility by jasmonic acid and airborne methyl jasmonate.

Characterization of Organelles in the Vacuolar-Sorting Pathway by Visualization with GFP in Tobacco BY-2 Cells

Abstract: We have shown the localization and mobilization of modified green fluorescent proteins (GFPs) with various signals in different compartments in a vacuolar-sorting system of tobacco BY-2 cells. In contrast to the efficient secretion of GFP from the transformed cells expressing SP-GFP composed of a signal peptide and GFP, accumulation of GFP in the vacuoles was observed in the cells expressing SP-GFP fused with the C-terminal peptide of pumpkin 2S albumin. This indicated that this peptide is sufficient for vacuolar targeting. Interestingly, the fluorescence in the vacuoles disappeared sharply at 7 d after inoculation of the cells, but it appeared again after re-inoculation into a new culture medium. When SP-GFP was fused with the region, termed PV72C, including a transmembrane domain and a cytosolic tail of a vacuolar-sorting receptor PV72, GFP-PV72C was detected in the Golgi-complex-like small particles. Prolonged culture showed that GFP-PV72C that reached the prevacuolar compartments was cleaved off the PV72C region to produce GFP, that arrived at the vacuoles to be diffused. These findings suggested that the vacuolar-sorting receptor might be recycled between the Golgi complex and prevacuolar compartments.

Responses of wild watermelon to drought stress: accumulation of an ArgE homologue and citrulline in leaves during water deficits.

Abstract: Wild watermelon from the Botswana desert had an ability to survive under severe drought conditions by maintaining its water status (water content and water potential) In the analysis by two-dimensional electrophoresis of leaf proteins, seven spots were newly induced after watering stopped One with the molecular mass of 40 kilodaltons of the spots was accumulated abundantly The cDNA encoding for the protein was cloned based on its amino-terminal sequence and the amino acid sequence deduced from the determined nucleotide sequences of the cDNA exhibited homology to the enzymes belong to the ArgE/DapE/Acy1/Cpg2/YscS protein family (including acetylornithine deacetylase, carboxypeptidase and aminoacylase-1) This suggests that the protein is involved in the release of free amino acid by hydrolyzing a peptidic bond As the drought stress progressed, citrulline became one of the major components in the total free amino acids Eight days after withholding watering, although the lower leaves wilted significantly, the upper leaves still maintained their water status and the content of citrulline reached about 50% in the total free amino acids The accumulation of citrulline during the drought stress in wild watermelon is an unique phenomenon in C3-plants These results suggest that the drought tolerance of wild watermelon is related to (1) the maintenance of the water status and (2) a metabolic change to accumulate citrulline

Involvement of CUP-SHAPED COTYLEDON genes in gynoecium and ovule development in Arabidopsis thaliana.

Abstract: When mutations in CUP-SHAPED COTYLEDON1 (CUC1) and CUC2 are combined, severe defects involving fusion of sepals and of stamens occur in Arabidopsis flowers. In addition, septa of gynoecia do not fuse along the length of the ovaries and many ovules have their growth arrested. CUC2 is expressed at the tips of septal primordia during gynoecium development and at the boundary between nucellus and chalaza during ovule development. These expression patterns are partially consistent with the phenotype of the mutant gynoecium. CUC2 mRNA is also shown to be expressed at the boundaries between meristems and organ primordia during both the vegetative and reproductive phases. This expression pattern indicates that CUC2 is generally involved in organ separation in shoot and floral meristems.

Regulation of ethylene-induced transcription of defense genes.

Abstract: Ethylene-induced gene expression has been studied in systems in which the biosynthesis of ethylene is stimulated during developmental process such as ripening of fruit, senescence of flower petals, or during pathogen infection. Functional analysis of the promoters of these genes revealed that the ethylene-responsive cis-elements of fruit ripening genes and senescence genes differed from that of defense genes whose expression is induced by ethylene in response to pathogen infection. The ethylene-responsive element identified as the GCC box (AGCCGCC) is commonly found in the promoter region of the ethylene-inducible defense genes. The ethylene responsive element binding factors that interact with the GCC box were demonstrated to be the transcription factors, which respond to extracellular signals to modulate GCC box-mediated gene expression positively or negatively.

Mechanism of the Carotenoid-to-Bacteriochlorophyll Energy Transfer via the S1 State in the LH2 Complexes from Purple Bacteria

Abstract: This investigation was motivated by a desire to get a deeper insight into the mechanism of carotenoiod-to-bacteriochlorophyll (Car-to-BChl) energy transfer proceeding via the carotenoid S1 state. (Here, we call the 2Ag- and 1Bu+ states “the S1 and S2 states” according to the notation presently accepted.) To systematically examine the effect of the conjugation length of carotenoid on the rate and efficiency of the Car(S1)-to-BChl(Qy) energy transfer, we performed the following experiments. (1) Subpicosecond time-resolved absorption spectroscopy was employed to measure the S1-state lifetimes of lycopene (number of conjugated CC bonds, n = 11), spheroidene (n = 10), and neurosporene (n = 9), both free in n-hexane and bound to the LH2 complexes from Rhodospirillum molischianum (Rs. molischianum), Rhodobactor sphaeroides (Rb. sphaeroides) 2.4.1, and Rb. sphaeroides G1C, respectively. The lifetime of each free (bound) carotenoid was determined to be 4.7(3.4) ps for lycopene, 9.3(1.7) ps for spheroidene, and 21....

An Arabidopsis Gene Encoding a Ca2+-Binding Protein is Induced by Abscisic Acid during Dehydration

Abstract: An Arabidopsis thaliana RD20 cDNA, which was isolated as one of drought-inducible genes, encodes a putative protein with a conserved EF-hand Ca2+-binding domain. The recombinant RD20 protein was shown to bind Ca2+. The transcription of RD20 gene was induced not only by drought but also by ABA and high salinity.

Blue light-induced chloroplast relocation in Arabidopsis thaliana as analyzed by microbeam irradiation.

Abstract: Chloroplast relocation in mesophyll cells of Arabidopsis thaliana was observed microscopically and analyzed by microbeam irradiation. Chloroplasts located along the anticlinal walls in dark-adapted cells. When part of a cell was irradiated with a microbeam of high fluence rate blue light (B) simultaneously with background red light (R) on the whole cell, the chloroplasts moved towards the B-irradiated area, but did not enter the beam. The background R illumination activated cytoplasmic motility as well as chloroplast movement. Without R illumination, there was little chloroplast relocation. In light-adapted cells in which the chloroplasts were spread over the cell surface perpendicular to the incident light, R-illumination had the same effect. Under background R, the chloroplasts moved out of the area irradiated with a B microbeam of 8 or 30 W m(-2) (avoidance response), but chloroplasts outside the beam moved towards the area irradiated with the B microbeam (accumulation response). These results suggest that the signals for accumulation and avoidance responses were generated in a single cell by high fluence rate B. cry1cry2, npq1 and nph1 mutants showed B-induced chloroplast relocation. Both the accumulation and avoidance responses were observed in all the mutants, although in the nph1 mutant, the sensitivity of accumulation movement was slightly lower than that of the wild type. We discuss the possible photoreceptor for B-induced chloroplast relocation.

Wound-induced ethylene synthesis and expression and formation of 1-aminocyclopropane-1-carboxylate (ACC) synthase, ACC oxidase, phenylalanine ammonia-lyase, and peroxidase in wounded mesocarp tissue of Cucurbita maxima.

Abstract: 1-Aminocyclopropane-l-carboxylate (ACC) synthase was rapidly induced in mesocarp tissue of Cucurbita maxima after wounding in the cut surface layer in 1 mm thickness (ca. 9 cells) (first layer) in both the enzyme activity and the levels of transcript. This led to a rapid accumulation of ACC and hence ethylene production. In the inside tissue (1-2 mm) (second layer), no significant induction of ACC synthase was observed, which resulted in a low level of ACC, although ethylene was evolved at a much lower rate than the first one. In contrast to ACC synthase, ACC oxidase was induced markedly in both the first and second layers and the development of its activity and the levels of mRNA remained high until later stages. It was considered that wound ethylene was closely associated with the development of ACC oxidase, since 2,5-norbornadiene (NBD), an inhibitor of ethylene action, substantially suppressed it. Phenylalanine ammonia-lyase (PAL) greatly increased in activity after wounding similarly to that of ACC synthase, in which increase in PAL activity occurred predominantly in the first layer. Induction of peroxidase activity after wounding had a close correlation in profile with that of ACC oxidase in that marked increases in the activity were observed in both the first and second layers and were strongly suppressed by NBD application. Four peroxidase isozymes were found by PAGE, among which a fraction was newly detected after wounding.

Transgenic rice plants expressing a Bacillus subtilis protoporphyrinogen oxidase gene are resistant to diphenyl ether herbicide oxyfluorfen.

Abstract: Protoporphyrinogen oxidase (Protox), the penultimate step enzyme of the branch point for the biosynthetic pathway of Chi and hemes, is the target site of action of diphenyl ether (DPE) herbicides. However, Bacillus subtilis Protox is known to be resistant to the herbicides. In order to develop the herbicide-resistant plants, the transgenic rice plants were generated via expression of B. subtilis Protox gene under ubiquitin promoter targeted to the cytoplasm or to the plastid using Agrobacterium-mediated gene transformation. The integration and expression of the transgene were investigated at To generation by DNA and RNA blots. Most transgenic rice plants revealed one copy transgene insertion into the rice genome, but some with 3 copies. The expression levels of B. subtilis Protox mRNA appeared to correlate with the copy number. Furthermore, the plastidal transgenic lines exhibited much higher expression of the Protox mRNA than the cytoplasmic transgenic lines. The transgenic plants expressing the B. subtilis Protox gene at T o generation were found to be resistant to oxyfluorfen when judged by cellular damage with respect to cellular leakage, Chi loss, and lipid peroxidation. The transgenic rice plants targeted to the plastid exhibited higher resistance to the herbicide than the transgenic plants targeted to the cytoplasm. In addition, possible resistance mechanisms in the transgenic plants to DPE herbicides are discussed.

A Secreted Peptide Growth Factor, Phytosulfokine, Acting as a Stimulatory Factor of Carrot Somatic Embryo Formation

Abstract: Somatic embryogenesis of the carrot (Daucus carota L.) depends on a set of factors, some of which accumulate in culture medium (conditioned medium, CM). When embryogenic cell clusters were transferred to an embryo-inducing medium, addition of CM derived from somatic embryo culture markedly stimulated somatic embryo formation. The active principles were purified using a simple bioassay system and identified to be phytosulfokines (PSKs), sulfated oligopeptide growth factors originally isolated from a CM derived from asparagus {Asparagus officinalis L.) mesophyll culture. Quantification studies using a competition ELISA system employing an anti-PSK-a polyclonal antibody showed that PSK production might be related to growth of cells, rather than development of somatic embryos. Thus the stimulatory effect of PSK on somatic embryo formation might be due to promotion of cell proliferation.