Plant and Cell Physiology - current issue

The Mechanism Selecting the Guide Strand from Small RNA Duplexes is Different Among Argonaute Proteins

Takeda, A., Iwasaki, S., Watanabe, T., Utsumi, M., Watanabe, Y. — 2008-04-14

Double-stranded RNA induces RNA silencing and is cleaved into 21–24 nt small RNA duplexes by Dicer enzyme. A strand of Dicer-generated small RNA duplex (called the guide strand) is then selected by a thermodynamic mechanism to associate with Argonaute (AGO) protein. This AGO–small RNA complex functions to cleave mRNA, repress translation or modify chromatin structure in a sequence-specific manner. Although a model plant, Arabidopsis thaliana, contains 10 AGO genes, their roles and molecular mechanisms remain obscure. In this study, we analyzed the roles of Arabidopsis AGO2 and AGO5. Interestingly, the 5' nucleotide of small RNAs that associated with AGO2 was mainly adenine (85.7%) and that with AGO5 was mainly cytosine (83.5%). Small RNAs that were abundantly cloned from the AGO2 immunoprecipitation fraction (miR163-LL, which is derived from the Lower Left of mature miR163 in pre-miR163, and miR390) and from the AGO5 immunoprecipitation fraction (miR163-UL, which is derived from the Upper Left of mature miR163 in pre-miR163, and miR390^*) are derived from the single small RNA duplexes, miR163-LL/miR163-UL and miR390/miR390^*. Each strand of the miR163-LL/miR163-UL duplex is selectively sorted to associate with AGO2 or AGO5 in a 5' nucleotide-dependent manner rather than in a thermodynamic stability-dependent manner. Furthermore, we showed that both AGO2 and AGO5 have the ability to bind cucumber mosaic virus-derived small RNAs. These results clearly indicate that the mechanism selecting the guide strand is different among AGO proteins and that multiple AGO genes are involved in anti-virus defense in plants.

Phytochrome-Regulated PIL1 Derepression is Developmentally Modulated

Hwang, Y.-s., Quail, P. H. — 2008-04-14

We define the photoresponsiveness, during seedling de-etiolation, of PHYTOCHROME-INTERACTING FACTOR 3-LIKE 1 (PIL1), initially identified by microarray analysis as an early-response gene that is robustly repressed by first exposure to light. We show that PIL1 mRNA abundance declines rapidly, with a half-time of 15 min, to a new steady-state level, 10-fold below the initial dark level, within 45 min of first exposure to red light. Analysis of phy-null mutants indicates that multiple phytochromes, including phyA and phyB, impose this repression. Conversely, PIL1 expression is rapidly derepressed by subsequent far-red irradiation of previously red light-exposed seedlings. However, the magnitude of this derepression is modulated over time, in a biphasic manner, in response to increasing duration of pre-exposure to continuous red light: (i) an early phase (up to about 6 h) of relatively rapidly increasing effectiveness of far-red reversal of repression, as declining phyA levels relieve initial very low fluence suppression of this response; and (ii) a second phase (beyond 6 h) of gradually declining effectiveness of far-red reversal, to only 20% of maximal derepression, within 36 h of continuous red light exposure, with no evidence of circadian modulation of this responsiveness, an observation in striking contrast to a previous report for entrained, green seedlings exposed to vegetative shade. These data, together with analysis of phytochrome signaling mutants and overexpressors with aberrant de-etiolation phenotypes, suggest that the second-phase decline in robustness of PIL1 derepression is an indirect consequence of the global developmental transition from the etiolated to the de-etiolated state, and that circadian coupling of derepression requires entrainment.

SodERF3, a Novel Sugarcane Ethylene Responsive Factor (ERF), Enhances Salt and Drought Tolerance when Overexpressed in Tobacco Plants

2008-04-14

The molecular signals and pathways that govern biotic and abiotic stress responses in sugarcane are poorly understood. Here we describe SodERF3, a sugarcane (Saccharum officinarum L. cv Ja60-5) cDNA that encodes a 201-amino acid DNA-binding protein that acts as a transcriptional regulator of the ethylene responsive factor (ERF) superfamily. Like other ERF transcription factors, the SodERF3 protein binds to the GCC box, and its deduced amino acid sequence contains an N-terminal putative nuclear localization signal (NLS). In addition, a C-terminal short hydrophobic region that is highly homologous to an ERF-associated amphiphilic repression-like motif, typical for class II ERFs, was found. Northern and Western blot analysis showed that SodERF3 is induced by ethylene. In addition, SodERF3 is induced by ABA, salt stress and wounding. Greenhouse-grown transgenic tobacco plants (Nicotiana tabacum L. cv. SR1) expressing SodERF3 were found to display increased tolerance to drought and osmotic stress.

Proteomic Analysis of Highly Purified Peroxisomes from Etiolated Soybean Cotyledons

Arai, Y., Hayashi, M., Nishimura, M. — 2008-04-14

To identify previously unknown peroxisomal proteins, we established an optimized method for isolating highly purified peroxisomes from etiolated soybean cotyledons using Percoll density gradient centrifugation followed by iodixanol density gradient centrifugation. Proteins in highly purified peroxisomes were separated by two-dimensional PAGE. We performed peptide mass fingerprinting of proteins separated in the gel with matrix-assisted laser desorption ionization time-of-flight mass spectrometry and used the peptide mass fingerprints to search a non-redundant soybean expressed sequence tag database. We succeeded in assigning 92 proteins to 70 sequences in the database. Among them, proteins encoded by 30 sequences were judged to be located in peroxisomes. These included enzymes for fatty acid β-oxidation, the glyoxylate cycle, photorespiratory glycolate metabolism, stress response and metabolite transport. We also show experimental evidence that plant peroxisomes contain a short-chain dehydrogenase/reductase family protein, enoyl-CoA hydratase/isomerase family protein, 3-hydroxyacyl-CoA dehydrogenase-like protein and a voltage-dependent anion-selective channel protein.

Characterization of Cd Translocation and Identification of the Cd Form in Xylem Sap of the Cd-Hyperaccumulator Arabidopsis halleri

Ueno, D., Iwashita, T., Zhao, F.-J., Ma, J. F. — 2008-04-14

Arabidopsis halleri is a Cd hyperaccumulator; however, the mechanisms involved in the root to shoot translocation of Cd are not well understood. In this study, we characterized Cd transfer from the root medium to xylem in this species. Arabidopsis halleri accumulated 1,500 mg kg^–1 Cd in the shoot without growth inhibition. A time-course experiment showed that the release of Cd into the xylem was very rapid; by 2 h exposure to Cd, Cd concentration in the xylem sap was 5-fold higher than that in the external solution. The concentration of Cd in the xylem sap increased linearly with increasing Cd concentration in the external solution. Cd transfer to the xylem was completely inhibited by the metabolic inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP). Cd concentration in the xylem sap was decreased by increasing the concentration of external Zn, but enhanced by Fe deficiency treatment. Analysis with ¹¹³Cd-nuclear magnetic resonance (NMR) showed that the chemical shift of ¹¹³Cd in the xylem sap was the same as that of Cd(NO₃)₂. Metal speciation with Geochem-PC also showed that Cd occurred mainly in the free ionic form in the xylem sap. These results suggest that Cd transfer from the root medium to the xylem in A. halleri is an energy-dependent process that is partly shared with Zn and/or Fe transport. Furthermore, Cd is translocated from roots to shoots in inorganic forms.

Nascent Peptide-Mediated Translation Elongation Arrest of Arabidopsis thaliana CGS1 mRNA Occurs Autonomously

2008-04-14

The Arabidopsis thaliana CGS1 gene encodes cystathionine -synthase, the first committed enzyme of methionine biosynthesis in higher plants. Expression of CGS1 is feedback-regulated at the step of mRNA degradation in response to S-adenosyl-l-methionine (AdoMet). A short stretch of amino acid sequence, termed the MTO1 region, encoded within the first exon of CGS1 itself acts in cis in the regulation. In vitro analyses using wheat germ extract (WGE) revealed that AdoMet induces temporal translation arrest of CGS1 mRNA prior to mRNA degradation. This translational pausing occurs immediately downstream of the MTO1 region and is mediated by the nascent MTO1 peptide. In order to elucidate further the nature of this unique regulatory mechanism, we have examined whether a non-plant system also contains the post-transcriptional regulation activity. Despite the fact that mammals do not carry cystathionine -synthase, AdoMet was able to induce the MTO1 sequence-dependent translation elongation arrest in rabbit reticulocyte lysate (RRL) in a similar manner to that observed in WGE. This result suggests that MTO1 peptide-mediated translation arrest does not require a plant-specific factor and rather most probably occurs via a direct interaction between the nascent MTO1 peptide and the ribosome that has translated it. In contrast, decay intermediates of CGS1 mRNA normally observed upon induction of CGS1 mRNA decay in plant systems were not detected in RRL, raising the possibility that CGS1 mRNA degradation involves a plant-specific mechanism.

Comparative Mutant Analysis of Arabidopsis ABCC-Type ABC Transporters: AtMRP2 Contributes to Detoxification, Vacuolar Organic Anion Transport and Chlorophyll Degradation

2008-04-14

The enormous metabolic plasticity of plants allows detoxification of many harmful compounds that are generated during biosynthetic processes or are present as biotic or abiotic toxins in their environment. Derivatives of toxic compounds such as glutathione conjugates are moved into the central vacuole via ATP-binding cassette (ABC)-type transporters of the multidrug resistance-associated protein (MRP) subfamily. The Arabidopsis genome contains 15 AtMRP isogenes, four of which (AtMRP1, 2, 11 and 12) cluster together in one of two major phylogenetic clades. We isolated T-DNA knockout alleles in all four highly homologous AtMRP genes of this clade and subjected them to physiological analysis to assess the function of each AtMRP of this group. None of the single atmrp mutants displayed visible phenotypes under control conditions. In spite of the fact that AtMRP1 and AtMRP2 had been described as efficient ATP-dependent organic anion transporters in heterologous expression experiments, the contribution of three of the AtMRP genes (1, 11 and 12) to detoxification is marginal. Only knockouts in AtMRP2 exhibited a reduced sensitivity towards 1-chloro-2,4-dinitrobenzene, but not towards other herbicides. AtMRP2 but not AtMRP1, 11 and 12 is involved in chlorophyll degradation since ethylene-treated rosettes of atmrp2 showed reduced senescence, and AtMRP2 expression is induced during senescence. This suggests that AtMRP2 is involved in vacuolar transport of chlorophyll catabolites. Vacuolar uptake studies demonstrated that transport of typical MRP substrates was reduced in atmrp2. We conclude that within clade I, only AtMRP2 contributes significantly to overall organic anion pump activity in vivo.

Effects of Conditional IPT-Dependent Cytokinin Overproduction on Root Architecture of Arabidopsis Seedlings

2008-04-14

Cytokinin (CK) has been known to inhibit primary root elongation and suggested to act as an auxin antagonist in the regulation of lateral root (LR) formation. While the role of auxin in root development has been thoroughly studied, the detailed and overall description of CK effects on root system morphology, particularly that of developing lateral root primordia (LRPs), and hence its role in organogenesis is still in progress. Here we examine the effects of conditional endogenous CK overproduction on root architecture and consider its temporal aspect during the early development of Arabidopsis thaliana. We employed the pOp/LhGR system to induce ectopic ipt overexpression with a glucocorticoid dexamethasone at designated developmental points. The transient CaMV 35S>GR>ipt transactivation greatly enhanced levels of biologically active CKs of zeatin (Z)-type and identified a distinct developmental interval during which primary root elongation is susceptible to increases in endogenous CK production. Long-term CK overproduction inhibited primary root elongation by reducing quantitative parameters of primary root meristem, disturbed a characteristic graded distribution pattern of auxin response in LRPs and impaired their development. Our findings indicate the impact of perturbed endogenous CK on the regulation of asymmetric auxin distribution during LRP development and imply that there is cross-talk between auxin and CK during organogenesis in A. thaliana.

The Role of Electron Transport in Determining the Temperature Dependence of the Photosynthetic Rate in Spinach Leaves Grown at Contrasting Temperatures

Yamori, W., Noguchi, K., Kashino, Y., Terashima, I. — 2008-04-14

The temperature response of the uncoupled whole-chain electron transport rate (ETR) in thylakoid membranes differs depending on the growth temperature. However, the steps that limit whole-chain ETR are still unclear and the question of whether the temperature dependence of whole-chain ETR reflects that of the photosynthetic rate remains unresolved. Here, we determined the whole-chain, PSI and PSII ETR in thylakoid membranes isolated from spinach leaves grown at 30°C [high temperature (HT)] and 15°C [low temperature (LT)]. We measured temperature dependencies of the light-saturated photosynthetic rate at 360 µl l^–1 CO₂ (A360) in HT and LT leaves. Both of the temperature dependences of whole-chain ETR and of A360 were different depending on the growth temperature. Whole-chain ETR was less than the rates of PSI ETR and PSII ETR in the broad temperature range, indicating that the process was limited by diffusion processes between the PSI and PSII. However, at high temperatures, whole-chain ETR appeared to be limited by not only the diffusion processes but also PSII ETR. The C₃ photosynthesis model was used to evaluate the limitations of A360 by whole-chain ETR (Pr) and ribulose bisphosphate carboxylation (Pc). In HT leaves, A360 was co-limited by Pc and Pr at low temperatures, whereas at high temperatures, A360 was limited by Pc. On the other hand, in LT leaves, A360 was solely limited by Pc over the entire temperature range. The optimum temperature for A360 was determined by Pc in both HT and LT leaves. Thus, this study showed that, at low temperatures, the limiting step of A360 was different depending on the growth temperature, but was limited by Pc at high temperatures regardless of the growth temperatures.

Influence of Chloroplastic Photo-Oxidative Stress on Mitochondrial Alternative Oxidase Capacity and Respiratory Properties: A Case Study with Arabidopsis yellow variegated 2

Yoshida, K., Watanabe, C., Kato, Y., Sakamoto, W., Noguchi, K. — 2008-04-14

Mitochondrial alternative oxidase (AOX), the unique respiratory terminal oxidase in plants, catalyzes the energy-wasteful cyanide (CN)-resistant respiration. Although it has been demonstrated that leaf AOX is up-regulated under high-light (HL) conditions, the in vivo mechanism of AOX up-regulation by light is still unknown. In the present study, we examined whether the photo-oxidative stress in the chloroplast modulates mitochondrial respiratory properties, especially the AOX capacity, using Arabidopsis leaf-variegated mutant yellow variegated 2 (var2) and exposing plants to HL. var2 mutants lack FtsH2 metalloprotease required for the repair of damaged PSII. Indeed, var2-1 suffered from photo-oxidative stress even before the HL treatments. While the activities of tricarboxylic acid cycle enzymes and cytochrome c oxidase in var2-1 were almost identical to those in the wild type, the amount of AOX protein and the CN-resistant respiration rate were higher in var2-1. Real-time PCR analysis revealed that HL treatment induced the expression of some energy-dissipating respiratory genes, including AOX1a, NDB2 and UCP5, more strongly in var2-1. Western blotting using var2-1 leaf extracts specific to green or white sectors, containing functional or non-functional photosynthetic apparatus, respectively, revealed that more AOX protein was induced in the green sectors by the HL treatment. These results indicate that photo-oxidative stress by excess light is involved in the regulation of respiratory gene expression and the modulation of respiratory properties, especially the AOX up-regulation.

Genetic Evidence for the Role of Isopentenyl Diphosphate Isomerases in the Mevalonate Pathway and Plant Development in Arabidopsis

2008-04-14

Isopentenyl/dimethylallyl diphosphate isomerase (IPI) catalyzes the interconversion of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), which are the universal C₅ units of isoprenoids. In plants, IPP and DMAPP are synthesized via the cytosolic mevalonate (MVA) and plastidic methylerythritol phosphate (MEP) pathways, respectively. However, the role of IPI in each pathway and in plant development is unknown due to a lack of genetic studies using IPI-defective mutants. Here, we show that the atipi1atipi2 double mutant, which is defective in two Arabidopsis IPI isozymes, exhibits dwarfism and male sterility under long-day conditions and decreased pigmentation under continuous light, whereas the atipi1 and atipi2 single mutants are phenotypically normal. We also show that the sterol and ubiquinone levels in the double mutant are <50% of those in wild-type plants, and that the male-sterile phenotype is chemically complemented by squalene, a sterol precursor. In vivo isotope labeling experiments using the atipi1atipi2 double mutant revealed a decrease in the incorporation of MVA (in its lactone form) into sterols, with no decrease in the incorporation of MEP pathway intermediates into tocopherol. These results demonstrate a critical role for IPI in isoprenoid biosynthesis via the MVA pathway, and they imply that IPI is essential for the maintenance of appropriate levels of IPP and DMAPP in different subcellular compartments in plants.

The Signaling Role of Extracellular ATP and its Dependence on Ca2+ Flux in Elicitation of Salvia miltiorrhiza Hairy Root Cultures

Wu, S.-J., Liu, Y.-S., Wu, J.-Y. — 2008-04-14

The application of a polysaccharide elicitor from yeast extract, YE, to Salvia miltiorrhiza hairy root cultures induced transient release of ATP from the roots to the medium, leading to a dose-dependent increase in the extracellular ATP (eATP) level. The eATP level rose to a peak (about 6.5 nM with 100 mg l^–1 YE) at about 10 h after YE treatment, but dropped to the control level 6 h later. The elicitor-induced ATP release was dependent on membrane Ca²⁺ influx, and abolished by the Ca²⁺ chelator EGTA or the channel blocker La³⁺. The YE-induced H₂O₂ production was strongly inhibited by reactive blue (RB), a specific inhibitor of membrane purinoceptors. On the other hand, the application of exogenous ATP at 10–100 µM to the cultures also induced rapid and dose-dependent increases in H₂O₂ production and medium pH, both of which were effectively blocked by RB and EGTA. The non-hydrolyzable ATP analog ATPS was as effective as ATP, but the hydrolyzed derivatives ADP or AMP were not so effective in inducing the pH and H₂O₂ increases. Our results suggest that ATP release is an early event and that eATP plays a signaling role in the elicitation of plant cell responses; Ca²⁺ is required for activation of the elicitor-induced ATP release and the eATP signal transduction. This is the first report on ATP release induced by a fungal elicitor and its involvement in the elicitor-induced responses in plant cells.

Expression of Exogenous Genes Under the Control of Endogenous HSP70 and CAB Promoters in the Closterium peracerosum-strigosum-littorale complex

Abe, J., Hiwatashi, Y., Ito, M., Hasebe, M., Sekimoto, H. — 2008-04-14

A unicellular charophyte alga, Closterium peracerosum–strigosum–littorale complex (C. psl. complex), has been studied in order to obtain basic information regarding sexual reproduction in plants. Systems for gene introduction and transient expression were developed for endogenous genes using phleomycin resistance (ble) and Chlamydomonas green fluorescent protein (cgfp) genes as selection markers. These genes have codon usage similar to that of genes in the C. psl. complex. To drive these genes strongly into C. psl. complex cells, two native promoters of the C. psl. complex genome—CpHSP70 and CpCAB1—were linked to a ble::cgfp fusion gene and introduced into the cells by particle bombardment. Following 2 d of incubation, we found 500 cells expressing GFP under the control of the CpHSP70 promoter, which were identified following heat shock treatment at 42°C, and 100 cells expressing GFP under the control of the CpCAB1 promoter, which were observed in lit conditions. In contrast, the GFP signal was only detected in two cells when ble::cgfp under control of the cauliflower mosaic virus 35S promoter was introduced. The ble::cgfp fusion protein was detected in the nucleus, whereas the single cgfp protein was detected in the cytoplasm. Our results indicate that the newly isolated native promoters of CpHSP70 and CpCAB1 are useful tools for inducing exogenous gene expression under heat shock and lit conditions, respectively. In addition, this strategy can be used for transient assays, such as the intracellular localization of unknown gene products in the C. psl. complex.

DCW11, Down-Regulated Gene 11 in CW-Type Cytoplasmic Male Sterile Rice, Encoding Mitochondrial Protein Phosphatase 2C is Related to Cytoplasmic Male Sterility

Fujii, S., Toriyama, K. — 2008-04-14

Causes of cytoplasmic male sterility (CMS) in plants have been studied for two decades, and mitochondrial chimeric genes have been predicted to induce CMS. However, it is unclear what happens after CMS-associated proteins accumulate in mitochondria. In our previous study of microarray analysis, we found that 140 genes are aberrantly regulated in anthers of CW-type CMS of rice (Oryza sativa L.). In the present study, we investigated DCW11, one of the down-regulated genes in CW-CMS encoding a protein phosphatase 2C (PP2C). DCW11 mRNA was preferentially expressed in anthers, with the highest expression in mature pollen. As predicted by the N-terminal sequence, DCW11 signal peptide–green fluorescent protein (GFP) fusion protein was localized in mitochondria. Knockdown of DCW11 in wild-type rice by RNA interference caused a major loss of seed-set fertility, without visible defect in pollen development. Since this knockdown phenotype resembled that of CW-CMS, we concluded that the down-regulation of DCW11 is correlated with CW-CMS. This idea was supported by the up-regulation of alternative oxidase 1a (AOX1a), which is known to be regulated by mitochondrial retrograde signaling, in DCW11 knockdown lines. Down-regulation of DCW11 and up-regulation of AOX1a were also observed in two other types of rice CMS. Our result indicates that DCW11 could play a role as a mitochondrial signal transduction mediator in pollen germination.

Heat Stress Stimulates Nitric Oxide Production in Symbiodinium microadriaticum: A Possible Linkage between Nitric Oxide and the Coral Bleaching Phenomenon

Bouchard, J. N., Yamasaki, H. — 2008-04-14

Nitric oxide (NO) is a gas displaying multiple physiological functions in plants, animals and bacteria. The enzymes nitrate reductase and NO synthase have been suggested to be involved in the production of NO in plants and algae, but the implication of those enzymes in NO production under physiological conditions remains obscure. Symbiodinium microadriaticum, commonly referred to as zooxanthellae, is a marine microalga commonly found in symbiotic association with a cnidarian host including reef-building corals. Here we demonstrate NO production in zooxanthellae upon supplementation of either sodium nitrite or l-arginine as a substrate. The nitrite-dependent NO production was detected electrochemically and confirmed by the application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger. Cells stained with the diaminofluorescein, DAF-2 DA, an NO fluorescent probe, showed an increase in fluorescence intensity upon supplementation of both sodium nitrite and l-arginine. Microscopic observations of DAF-stained cells verified that NO was produced inside the cells. NO production in S. microadriaticum was found to increase upon exposure of cells to an acute heat stress which also caused a decline in the photosynthetic efficiency of PSII (F_v/F_m). This study provides substantial evidence to confirm that zooxanthellae can synthesize NO even when they are not in a symbiotic association with a coral host. The increase in NO production at high temperatures suggests that heat stress stimulates the microalgal NO production in a temperature-dependent manner. The implications of these findings are discussed in the light of the coral bleaching phenomenon which is associated with elevated sea surface temperature due to global warming.

Constitutive Components and Induced Gene Expression are Involved in the Desiccation Tolerance of Selaginella tamariscina

Liu, M.-S., Chien, C.-T., Lin, T.-P. — 2008-04-14

Selaginella tamariscina, one of the most primitive vascular plants, can remain alive in a desiccated state and resurrect when water becomes available. To evaluate the nature of desiccation tolerance in this plant, we compared the composition of soluble sugars and saturation ratios of phospholipids (PLs) between hydrated and desiccated tissues of S. tamariscina using gas chromatography. In this study, differences in gene expression and ABA contents were also analyzed during dehydration. The results revealed that trehalose (at >130 mg g^–1 DW) was the major soluble sugar, and low saturated fatty acid content in PLs (0.31) was maintained in both hydrated and desiccated tissues. In addition, the ABA content of S. tamariscina increased 3-fold, and genes involved in ABA signaling and cellular protection were up-regulated while photosystem-related genes were down-regulated during dehydration. The biochemical and molecular findings suggest that both constitutive and inducible protective molecules contribute to desiccation tolerance of S. tamariscina.

Effects of Blue Light Deficiency on Acclimation of Light Energy Partitioning in PSII and CO2 Assimilation Capacity to High Irradiance in Spinach Leaves

Matsuda, R., Ohashi-Kaneko, K., Fujiwara, K., Kurata, K. — 2008-04-14

Blue light effects on the acclimation of energy partitioning characteristics in PSII and CO₂ assimilation capacity in spinach to high growth irradiance were investigated. Plants were grown hydroponically in different light treatments that were a combination of two light qualities and two irradiances, i.e. white light and blue-deficient light at photosynthetic photon flux densities (PPFDs) of 100 and 500 µmol m^–2 s^–1. The CO₂ assimilation rate, the quantum efficiency of PSII (PSII) and thermal dissipation activity $${F}_{\mathrm{v}}^{\mathrm{\prime }}$$ / $${F}_{\mathrm{m}}^{\mathrm{\prime }}$$ in young, fully expanded leaves were measured under 1,600 µmol m^–2 s^–1 white light. The CO₂ assimilation rate and PSII were higher, while $${F}_{\mathrm{v}}^{\mathrm{\prime }}$$ / $${F}_{\mathrm{m}}^{\mathrm{\prime }}$$ was lower in plants grown under high irradiance than in plants grown under low irradiance. These responses were observed irrespective of the presence or absence of blue light during growth. The extent of the increase in the CO₂ assimilation rate and PSII and the decrease in $${F}_{\mathrm{v}}^{\mathrm{\prime }}$$ / $${F}_{\mathrm{m}}^{\mathrm{\prime }}$$ by high growth irradiance was smaller under blue light-deficient conditions. These results indicate that blue light helps to boost the acclimation responses of energy partitioning in PSII and CO₂ assimilation to high irradiance. Similarly, leaf N, Cyt f and Chl contents per unit leaf area increased by high growth irradiance, and the extent of the increment in leaf N, Cyt f and Chl was smaller under blue light-deficient conditions. Regression analysis showed that the differences in energy partitioning in PSII and CO₂ assimilation between plants grown under high white light and high blue-deficient light were closely related to the difference in leaf N.

Plant Catalase is Imported into Peroxisomes by Pex5p but is Distinct from Typical PTS1 Import

2008-04-14

We have previously demonstrated that the targeting signal of pumpkin catalase, Cat1, is an internal PTS1 (peroxisomal targeting signal 1)-like sequence, QKL, located at –13 to –11 from the C-terminus, which is different from the typical PTS1 SKL motif located in the C-terminus. Here we show that Cat1 import into peroxisome is dependent on the cytosolic PTS receptor, Pex5p, in Arabidopsis, similar to typical PTS1 import, and that other components for transport of peroxisomal matrix proteins such as Pex14p, Pex13p, Pex12p and Pex10p also contribute to the import of Cat1. Interestingly, however, we found that Cat1 interacts with the N-terminal domain of Pex5p, but not the C-terminal domain for interaction with the typical PTS1, revealing that Pex5p recognizes Cat1 in a manner distinct from typical PTS1.

Arabidopsis VPS35, a Retromer Component, is Required for Vacuolar Protein Sorting and Involved in Plant Growth and Leaf Senescence

2008-04-14