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Chalcone isomerase (CHI) is a key enzyme in flavonoid biosynthesis. In plants, CHIs occur in multigene families, and they are divided into four types, types I–IV. Type I and II CHIs are bona fide CHIs with CHI activity, and type III and IV CHIs are non-catalytic members with different functions. Rice contains seven CHI family genes (OsCHIs). Molecular analysis suggested that OsCHI3 is a type I CHI, and the other OsCHIs were classified into types III and IV. To elucidate their biochemical functions, OsCHI1, OsCHI3, OsCHI6, and OsCHI7 were expressed in Escherichia coli, and the recombinant OsCHI proteins were purified. An activity assay of recombinant OsCHIs showed that OsCHI3 catalyzed the isomerization of naringenin chalcone and isoliquiritigenin, whereas the other recombinant OsCHIs had no CHI activity. OsCHI3 also exhibited a strong preference to naringenin chalcone compared to isoliquiritigenin, which agrees well with the catalytic properties of type I CHIs. These results ascertain OsCHI3 to be a bona fide CHI in rice. OsCHI3 and the other OsCHIs were expressed constitutively throughout the rice growth period and different tissues. OsCHI3 expression was induced immediately in response to ultra-violet (UV) stress, suggesting its involvement in the biosynthesis of sakuranetin, a flavonoid phytoalexin in rice.

Methylation is a common structural modification that can alter and improve the biological activities of natural compounds. O-Methyltransferases (OMTs) catalyze the methylation of a wide array of secondary metabolites, including flavonoids, and are potentially useful tools for the biotechnological production of valuable natural products. An OMT gene (PfOMT3) was isolated from perilla leaves as a putative flavonoid OMT (FOMT). Phylogenetic analysis and sequence comparisons showed that PfOMT3 is a class II OMT. Recombinant PfOMT3 catalyzed the methylation of flavonoid substrates, whereas no methylated product was detected in PfOMT3 reactions with phenylpropanoid substrates. Structural analyses of the methylation products revealed that PfOMT3 regiospecifically transfers a methyl group to the 7-OH of flavonoids. These results indicate that PfOMT3 is an FOMT that catalyzes the 7-O-methylation of flavonoids. PfOMT3 methylated diverse flavonoids regardless of their backbone structure. Chrysin, naringenin and apigenin were found to be the preferred substrates of PfOMT3. Recombinant PfOMT3 showed moderate OMT activity toward eriodictyol, luteolin and kaempferol. To assess the biotechnological potential of PfOMT3, the biotransformation of flavonoids was performed using PfOMT3-transformed Escherichia coli. Naringenin and kaempferol were successfully bioconverted to the 7-methylated products sakuranetin and rhamnocitrin, respectively, by E. coli harboring PfOMT3.

Detection, purification and characterization of proteins are essential procedures in the field of biochemistry. Epitope tag systems are commonly used to characterize unknown proteins. The Deinococcus radiodurans bacterial phytochrome (DrBphP) protein has been used as an antigen to generate anti-DrBphP mouse monoclonal antibodies and to identify their specific epitopes. Among these antibodies, the 2B8 monoclonal antibody recognizes an epitope of 9 amino acids (RDPLPFFPP). The 2B8 epitope does not match the amino acid sequence for any known protein. On Western blot analysis, the 2B8 antibody showed strong and highly specific interactions with the 2B8 epitope. These results suggest that the 2B8 epitope-antibody is a useful epitope tag system for protein characterization. In addition, we generated a modified epitope (RDPLPAFPP) via point mutation in a previous study. This modified epitope showed significantly increased reactivity with the 2B8 antibody. In this study, we developed a protein purification system using the 2B8 epitope tag and antibody. 2B8 antibodies were bound to protein G-agarose beads as affinity ligands. Recombinant DrBphP proteins were then exposed to 2B8 antibody-bound protein G-agarose beads. Bound DrBphP proteins were then eluted by competition with the original or modified 2B8 epitope peptides. DrBphP proteins were successfully purified via an affinity chromatography system using a 2B8 original peptide and even better purified using the 2B8 modified peptide. These findings indicate that the 2B8 epitope tag system is a better tool for protein detection and purification.

Chalcone synthase (CHS) is a key enzyme in the flavonoid pathway, participating in the production of phenolic phytoalexins. The rice genome contains 31 CHS family genes (OsCHSs). The molecular characterization of OsCHSs suggests that OsCHS8 and OsCHS24 belong in the bona fide CHSs, while the other members are categorized in the non-CHS group of type III polyketide synthases (PKSs). Biochemical analyses of recombinant OsCHSs also showed that OsCHS24 and OsCHS8 catalyze the formation of naringenin chalcone from p-coumaroyl-CoA and malonyl-CoA, while the other OsCHSs had no detectable CHS activity. OsCHS24 is kinetically more efficient than OsCHS8. Of the OsCHSs, OsCHS24 also showed the highest expression levels in different tissues and developmental stages, suggesting that it is the major CHS isoform in rice. In oschs24 mutant leaves, sakuranetin content decreased to 64.6% and 80.2% of those in wild-type leaves at 2 and 4 days after UV irradiation, respectively, even though OsCHS24 expression was mostly suppressed. Instead, the OsCHS8 expression was markedly increased in the oschs24 mutant under UV stress conditions compared to that in the wild-type, which likely supports the UV-induced production of sakuranetin in oschs24. These results suggest that OsCHS24 acts as the main CHS isozyme and OsCHS8 redundantly contributes to the UV-induced production of sakuranetin in rice leaves.

Cinnamyl alcohol dehydrogenase (CAD) is involved in the final step of the phenylpropanod pathway, catalyzing the NADPH-dependent reduction of hydroxy-cinnamaldehydes into the corresponding alcohols. The rice genome contains twelve CAD and CAD-like genes, collectively called OsCADs. To elucidate the biochemical function of the OsCADs, OsCAD1, 2, 6, and 7, which are highly expressed in rice, were cloned from rice tissues. The cloned OsCADs were heterologously expressed in Escherichia coli as His-tag fusion proteins. The activity assay of the recombinant OsCADs showed that OsCAD2, 6, and 7 have CAD activity toward hydroxycinnamaldehydes, but OsCAD1 has no detectable catalytic activity. The kinetic parameters of the enzyme reactions demonstrated that OsCAD2 has the highest catalytic activity among the examined enzymes. This result agrees well with the finding that the Zn binding and NADPH binding motifs and the residues constituting the substrate binding pocket in bona fide plant CADs were fully conserved in OsCAD2. Although they have large variations in the residue for the substrate binding pocket, OsCAD6 and 7 catalyzed the reduction of hydroxycinnamaldehydes with a similar efficiency. Alignment of amino acid sequences showed that OsCAD1 lacks the GxxxxP motif for NADPH binding and has mismatches in residues important in the reduction process, which could be responsible for the loss of catalytic activity. OsCAD2 belongs to CAD Class I with bona fide CADs from other plant species and is constitutively expressed throughout the developmental stages of rice, with preferential expression in actively lignifying tissues such as the root, stem, and panicle, suggesting that it is mainly involved in developmental lignification in rice. The expression of OsCAD2 was also induced by biotic and abiotic stresses such as Xanthomonas oryzae pv. oryzae (Xoo) infection and UV-irradiation, suggesting that it plays a role in the defense response of rice, in addition to a bona fide role in developmental lignification. OsCAD6 and 7 belong in CAD Class II. Their expression is relatively lower than that of OsCAD2 and is confined to certain tissues, such as the leaf sheath, stem, and panicle. The expression of OsCAD6 was stimulated by Xoo infection and UV-irradiation. Thus OsCAD6 appears to be an inducible OsCAD that is likely involved in the defense response of rice against biotic and abiotic stresses.

Sugar is appreciated to function as a signaling molecule by controlling gene expression, thereby regulating various developmental processes in plants. We screened sucrose-insensitive (sin) mutants in Arabidopsis that showed normal germination and seedling development under high concentration (438 mM) of sucrose. Compared to wild-type and other sugar mutants, sin mutants seedlings exhibited resistance to high glucose, high salt, and paclobutrazol, inhibitor of gibberellin biosynthesis. We discuss the unique characteristics of these sin mutants in this report.

Cinnamoyl-CoA reductase (CCR) is the first committed enzyme in the monolignol pathway for lignin biosynthesis and catalyzes the conversion of hydroxycinnamoyl-CoAs into hydroxycinnamaldehydes. In the rice genome, 33 genes are annotated as CCR and CCR-like genes, collectively called OsCCRs. To elucidate the functions of OsCCRs, their phylogenetic relationships, expression patterns at the transcription levels and biochemical characteristics were thoroughly analyzed. Of the 33 OsCCRs, 24 of them encoded polypeptides of lengths similar to those of previously identified plant CCRs. The other nine OsCCRs had much shorter peptide lengths. Phylogenetic tree and sequence similarities suggested OsCCR4, 5, 17, 18, 19, 20, and 21 as likely candidates for functional CCRs in rice. To elucidate biochemical functions, OsCCR1, 5, 17, 19, 20, 21, and 26 were heterologously expressed in Escherichia coli and the resulting recombinant OsCCRs were purified to apparent homogeneity. Activity assays of the recombinant OsCCRs with hydroxycinnamoyl-CoAs revealed that OsCCR17, 19, 20, and 21 were biochemically active CCRs, in which the NAD(P)-binding and NADP-specificity motifs as well as the CCR signature motif were fully conserved. The kinetic parameters of enzyme reactions revealed that feruloyl-CoA, a precursor for the guaiacyl (G)-unit of lignin, is the most preferred substrate of OsCCR20 and 21. This result is consistent with a high content (about 70%) of G-units in rice lignins. Phylogenetic analysis revealed that OsCCR19 and 20 were grouped with other plant CCRs involved in developmental lignification, whereas OsCCR17 and 21 were closely related to stress-responsible CCRs identified from other plant species. In agreement with the phylogenetic analysis, expression analysis demonstrated that OsCCR20 was constitutively expressed throughout the developmental stages of rice, showing particularly high expression levels in actively lignifying tissues, such as roots and stems. These results suggest that OsCCR20 is primarily involved in developmental deposition of lignins in secondary cell walls. As expected, the expressions of OsCCR17 and 21 were induced in response to biotic and abiotic stresses, such as Magnaporthe grisea and Xanthomonas oryzae pv. oryzae (Xoo) infections, UV-irradiation and high salinity, suggesting that these genes play a role in defense-related processes in rice.

Although they are less abundant in nature, methoxyflavonoids have distinct physicochemical and pharmacological properties compared to common nonmethylated flavonoids. Thus, enzymatic conversion and biotransformation using genetically engineered microorganisms of flavonoids have been attempted for the efficient production of methoxyflavonoids. Because of their regiospecificity, more than two flavonoid O-methyltransferases (FOMTs) and enzyme reactions are required to biosynthesize di(or poly)-methoxyflavonoids. For the one-step biotechnological production of bioactive di-O-methylflavonoids, we generated a multifunctional FOMT fusing a 3'-OMT (SlOMT3) and a 7-OMT (OsNOMT). The SlOMT3/OsNOMT fusion enzyme possessed both 3'- and 7-OMT activities to diverse flavonoid substrates, which were comparable to those of individual SlOMT3 and OsNOMT. The SlOMT3/OsNOMT enzyme also showed 3'- and 7-OMT activity for 7- or 3'-O-methylflavonoids, respectively, suggesting that the fusion enzyme can sequentially methylate flavonoids into di-O-methylflavonoids. The biotransformation of the flavonoids quercetin, luteolin, eriodictyol, and taxifolin using SlOMT3/OsNOMT-transformed Escherichia coli generated corresponding di-O-methylflavonoids, rhamnazin, velutin, 3',7-di-O-methyleriodictyol, and 3',7-di-O-methyltaxifolin, respectively. These results indicate that dimethoxyflavonoids may be efficiently produced from nonmethylated flavonoid precursors through a one-step biotransformation using the engineered E. coli harboring the SlOMT3/OsNOMT fusion gene.

Bacteriophytochromes (BphP) are phytochrome-like light sensing proteins in bacteria, which use biliverdin as a chromophore. In order to study the biochemical properties of the DrBphP protein, five (2B8, 2C11, 3B2, 3D2, and 3H7) anti-DrBphP monoclonal antibodies were produced through the immunization of mice with purified full-length DrBphP and DrBphN (1–321 amino acid) proteins, and epitope mapping was then carried out. Among the five antibodies, 2B8 and 2C11 preferentially recognized the N-terminal region of BphP whereas 3B2, 3D2, and 3H7 showed preference for the C-terminal region. We performed further epitope mapping using recombinant truncated BphP proteins to narrow down their target sequences. The results demonstrated that each of the five monoclonal antibodies recognized different regions on the DrBphP protein. Additionally, epitopes of 2B8 and 3H7 antibodies were discovered to be shorter than 10 amino acids (2B8: RDPLPFFPP, 3H7: PGEIEEA). These two antibodies with such specific recognition epitopes could be especially valuable for developing new peptide tags for protein detection and purification.

In plants, pyrophosphate: fructose-6-phosphate 1-phosphotransferase (PFP) is a regulatory enzyme that participates in glycolysis and gluconeogenesis. Arabidopsis contains two PFPα subunit genes (PFPα1 and PFPα2) and two PFPβ subunit genes (PFPβ1 and PFPβ2). The single-knockout mutants of the PFP subunit genes were isolated, and double and quadruple pfp mutants were generated by crossing the single mutants. To elucidate the role of PFP in stress tolerance, the responses of the double and quadruple pfp knockout mutants to stress conditions, including osmotic and salt stresses, were examined. The seedling growth of the pfpα1/α2 and pfpβ1/β2 double mutants and the pfpα1/α2/β1/β2 quadruple mutant was severely retarded under salt and osmotic stress conditions compared with that of the wild type. The expression of PFP subunit genes increased in response to salt and osmotic stresses. In contrast, the vegetative growth of the wild type and pfp mutants after the seedling stage was similarly affected by salt and osmotic stresses. These findings suggest that PFP plays a role in the adaptation of Arabidopsis seedlings to salt and osmotic stresses.

Bacteriophytochromes are composed of N-terminal region for chromophore binding and C-terminal domain for transmission of a light signal. In this study, the possibility of using the chromophore-binding domain from a Deinococcus radiodurans bacteriophytochrome as a protein tag to enable a rapid and simple detection method for the production of recombinant proteins were evaluated.

Cholera toxin B subunit (CTB) has the potential to be an effective adjuvant for mucosal vaccines because of its ability to increase antigen uptake and presentation by antigen-presenting cells through GM1-ganglioside binding. CTB has been produced using different recombinant protein expression systems. This study used the geminiviral replicon system to transiently express CTB in Nicotiana benthamiana. The plant-optimized CTB gene was cloned into a geminiviral vector and infiltrated into N. benthamiana leaves. The highest CTB protein level was observed on day 4 with approximately 4 μg/g fresh weight. The Western blot analysis using anti-CTB suggests assembly of CTB into oligomers. Based on the GM1-ELISA results, this CTB transiently expressed in plants showed biological activity for binding the intestinal epithelial cell membrane glycolipid receptor, GM1-glanglioside, which implies its potential as an adjuvant for mucosal vaccines.

Flavonoids are ubiquitous secondary metabolites in plants and possess a wide range of biological activities such as anti-oxidant, anti-microbial, and anti-cancer activities. As a food ingredient, flavonoids are also thought to have health-promoting and disease-preventing properties. Structural modifications of flavonoids can improve their biological activities. Methylation is a common flavonoid modification that is catalyzed by O-methyltransferases (OMTs). We isolated four putative OMTs from the miniature tomato cultivar Micro-Tom. One OMT (SlOMT3) was successfully expressed in E. coli and purified. Recombinant SlOMT3 was found to be a regiospecific flavonoid 3′/5′-O-methyltransferase. Biochemical parameters show that SlOMT3 has higher binding affinity and catalytic efficiency for quercetin and luteolin than for eriodictyol. This suggests that flavonols and flavones are preferable substrates for SlOMT3. Biotransformation is considered a promising method to modify flavonoid structures. Transgenic E. coli expressing SlOMT3 was used to modify flavonoid substrates and was found to efficiently convert quercetin, luteolin, and eriodictyol to isorhamnetin, chrysoeriol and homoeriodictyol, respectively.

Leaf starch synthesized during the day for transient storage of photoassimilated carbon is degraded the following night to support respiration and growth in plants. Maltose is a major product of starch degradation, and is exported to the cytosol through the maltose transporter (MEX1). The Arabidopsis mex1 mutant displays growth retardation and an exceptional chlorotic phenotype that is not observed in other mutants demonstrating defective starch synthesis or degradation. Consistent with the chlorotic phenotype, proteomic analysis revealed degeneration of the photosynthetic machinery in mex1, and the down-regulation of essential components for photosynthesis was also observed. The chlorosis observed in mex1 occurs during vegetative growth period under normal growth conditions, which is distinct from general senescence-induced chlorosis. No up-regulation of senescence-related genes was found in the proteomic analysis of mex1, suggesting that the chlorotic process occurring in mex1 is likely distinct from senescence-dependent processes. On the other hand, cellular processes needed to survive stress situations caused by the blocking of maltose export are induced in mex1 by up-regulation of stress-related proteins, such as a germin-like protein and glutathione S-transferase. The increased abundance of heat shock protein 93-V participating in chloroplast biogenesis and rubisco activase, a regulatory protein of photosynthesis, likely reflects an attempt by the mex1 mutant to maintain chloroplast function to survive stress conditions.

Nascent polypeptide associated complex (NAC) proteins bind to newly synthesized polypeptide chains from ribosome and are involved in various physiological processes. An in vitro pulldown assay demonstrated that the previously identified NAC protein interacts with phytochromes. Phytochrome-interacting NAC protein (PNAC) was expressed in the cytosol, and translocation of the PNAC protein into nucleus by irradiation was not observed. These findings suggest that PNAC is a possible cytosolic partner protein to phytochromes. Histochemical analysis of PNAC promoter-β-glucuronidase (GUS) transgenic plants showed that PNAC expression was increased by light treatment, especially red light, indicating that phytochromes likely modulate PNAC gene expression.

The ankyrin domain is one of the most common protein motifs in eukaryotic proteins. Repeated ankyrin domains are ubiquitous and their mediation of protein-protein interactions is involved in a number of physiological and developmental responses such as the cell cycle, signal transduction and cell differentiation. A novel putative phytochrome-interacting ankyrin repeat protein 2 (PIA2) containing three repeated ankyrin domains was identified in Arabidopsis. An in vitro pull-down and phosphorylation assay revealed that PIA2 is phosphorylated and interacts directly with oat phytochrome A. The N-terminal domain of PIA2 was specifically phosphorylated, whereas interactions between the domains of PIA2 and phytochrome A had no Pr/Pfr preference. PIA2 was ubiquitously expressed in most tissues and was localized in both the nucleus and the cytoplasm independent of treatment with light of specific wavelengths. Anthocyanin accumulation in seedlings grown under far-red light, a typical phenotype of wild-type plants, was reduced in a loss-of-function mutant of PIA2 (pia2), whereas anthocyanin accumulation was increased in an overexpressing plant (PIA2-OX). The gene expression of UDP-flavonoid-3'-glucosyl-transferase (UF3GT), a major enzyme in the anthocyanin biosynthesis processes, was decreased in pia2 knockout plants suggesting that decreased anthocyanin was because of the decreased expression of UF3GT. Our results suggest that PIA2 plays a role in the anthocyanin biosynthesis during seedling development as a novel phytochrome-interacting protein.

Summary Filoviruses (Ebola and Marburg viruses) cause severe and often fatal haemorrhagic fever in humans and non-human primates. The US Centers for Disease Control identifies Ebola and Marburg viruses as ‘category A’ pathogens (defined as posing a risk to national security as bioterrorism agents), which has lead to a search for vaccines that could prevent the disease. Because the use of such vaccines would be in the service of public health, the cost of production is an important component of their development. The use of plant biotechnology is one possible way to cost-effectively produce subunit vaccines. In this work, a geminiviral replicon system was used to produce an Ebola immune complex (EIC) in Nicotiana benthamiana. Ebola glycoprotein (GP1) was fused at the C-terminus of the heavy chain of humanized 6D8 IgG monoclonal antibody, which specifically binds to a linear epitope on GP1. Co-expression of the GP1-heavy chain fusion and the 6D8 light chain using a geminiviral vector in leaves of N. benthamiana produced assembled immunoglobulin, which was purified by ammonium sulphate precipitation and protein G affinity chromatography. Immune complex formation was confirmed by assays to show that the recombinant protein bound the complement factor C1q. Size measurements of purified recombinant protein by dynamic light scattering and size-exclusion chromatography also indicated complex formation. Subcutaneous immunization of BALB/C mice with purified EIC resulted in anti-Ebola virus antibody production at levels comparable to those obtained with a GP1 virus-like particle. These results show excellent potential for a plant-expressed EIC as a human vaccine.

In the past 10 years, a lot of plant circadian rhythm researches have published in molecular biology and biochemistry. We discussed with published molecular studies of circadian clock and rhythmic genes in Arabidopsis, rice and algae. However past this studies are not sufficient to explain the whole rhythmic metabolism. Recently many researchers have concerned post-transcriptional, translational and post-translational modification of rhythmic proteins. From the view point of the high-throughput study, we could suggest the proteomic analysis with 2-DE gel electrophoresis and MS/MS techniques for the identification of modified proteins.

The composition and distribution of seed storage proteins are important factors for eating quality such as flavor and texture in rice. The levels of rice protein disulfide isomerase and rice chaperone binding protein proteins were examined in rice cultivars which have different eating qualities. Results showed that their levels have no direct correlation with eating quality.

To analyze the surface properties of bacteriophytochrome (BphP), five (2B8, 2C11, 3B2, 3D2, 3H7) anti-BphP monoclonal antibodies were produced by using full-length of BphP of Deinococcus radiodurnas. 2B8 and 2C11 preferentially recognized the epitopes at N-terminal region of BphP, whereas 3B2, 3D2 and 3H7 showed preferential affinities to the epitopes of C-terminal region of BphP.

In Arabidopsis, the compartmentation of sugars into vacuoles is known to be facilitated by sugar transporters. However, vacuolar sugar transporters have not been studied in detail in other plant species. To characterize the rice (Oryza sativa) tonoplast monosaccharide transporters, OsTMT1 and OsTMT2, we analysed their subcellular localization using green fluorescent protein (GFP) and expression patterns using reverse-transcription polymerase chain reaction (RT-PCR), performed histochemical beta-glucuronidase (GUS) assay and in situ hybridization analysis, and assessed sugar transport ability using isolated vacuoles. Expression of OsTMT-GFP fusion protein in rice and Arabidopsis revealed that the OsTMTs localize at the tonoplast. Analyses of OsTMT promoter-GUS transgenic rice indicated that OsTMT1 and OsTMT2 are highly expressed in bundle sheath cells, and in vascular parenchyma and companion cells in leaves, respectively. Both genes were found to be preferentially expressed in the vascular tissues of roots, the palea/lemma of spikelets, and in the main vascular tissues and nucellar projections on the dorsal side of the seed coats. Glucose uptake studies using vacuoles isolated from transgenic mutant Arabidopsis (tmt1-2-3) expressing OsTMT1 demonstrated that OsTMTs are capable of transporting glucose into vacuoles. Based on expression analysis and functional characterization, our present findings suggest that the OsTMTs play a role in vacuolar glucose storage in rice.

Rho-related GTPase of plants (ROP) plays an important role in plant growth and development as a signaling protein. Plant RopGEFs are recently identified ROP activator proteins in Arabidopsis. In this study, we cloned 14 RopGEFs in Arabidopsis and characterized their expression patterns in response to abiotic stresses. Fourteen RopGEF genes were categorized into three groups based on their amino acid homologies and molecular sizes. Most RopGEFs were expressed predominantly in flower but some RopGEFs displayed a tissue-specific expression pattern. RopGEF1, 4, 5, and 11 were expressed in all tissues including root and leaves whereas RopGEF7, 8, 9, and 13 were expressed only in flowers. The transcript levels of 14 RopGEFs were changed significantly depending upon abiotic stresses such as cold, heat, drought and salts. RopGEF5 transcription was up-regulated by salt and drought treatment but down-regulated by heat. RopGEF14 transcript level was also increased by salt but decreased by heat stress. The transcript levels of RopGEF1, 7, 9, and 12 were enhanced in response to heat stress but showed no changes in response to cold stresses. Drought stress activated Group 3 RopGEFs such as RopGEF5 and 7. Taken together, 14 RopGEFs are responding to the abiotic stresses individually or as a group.

It was known that long after far-red light 1 (LAF1), a positive phytochrome A (phyA) signal transducer, is ubiquitinated by constitutive photomorphogenic 1 (COP1) in vitro and stabilizes long hypocotyl in far-red 1 (HFR1), another phyA signal transducer, through direct interaction. Our results imply that LAF1 stabilizes phyA by inhibiting or blocking COP1 activity, or by directly interacting with phyA, resulting in the stimulation of phyA signaling.

During photosynthesis, triose-phosphates (trioseP) ex- ported from the chloroplast to the cytosol are converted to sucrose via cytosolic fructose-1,6-bisphosphatase (cFBPase). Expression analysis in rice suggests that OscFBP1 plays a major role in the cytosolic conversion of trioseP to sucrose in leaves during the day. The isolated OscFBP1 mutants exhibited markedly decreased photo- synthetic rates and severe growth retardation with reduced chlorophyll content, which results in plant death. Analysis of primary carbon metabolites revealed both significantly reduced levels of sucrose, glucose, fructose and starch in leaves of these mutants, and a high accumulation of sucrose to starch in leaves of rice plants. In the oscfbp1 mutants, products of glycolysis and the TCA cycle were significantly increased.A partitioning experiment of 14 C-labelled photo- assimilates revealed altered carbon distributions including a slight increase in the insoluble fraction representing transi- tory starch, a significant decrease in the neutral fraction corresponding to soluble sugars and a high accumulation of phosphorylated intermediates and carboxylic acid fractions in the oscfbp1 mutants. These results indicate that the impaired synthesis of sucrose in rice cannot be sufficiently compensated for by the transitory starch-mediated path- ways that have been found to facilitate plant growth in the equivalent Arabidopsis mutants.

The Arabidopsis (Arabidopsis thaliana) hexokinase 1 (AtHXK1) is recognized as an important glucose (Glc) sensor. However, the function of hexokinases as Glc sensors has not been clearly demonstrated in other plant species, including rice (Oryza sativa). To investigate the functions of rice hexokinase isoforms, we characterized OsHXK5 and OsHXK6, which are evolutionarily related to AtHXK1. Transient expression analyses using GFP fusion constructs revealed that OsHXK5 and OsHXK6 are associated with mitochondria. Interestingly, the OsHXK5ΔmTP-GFP and OsHXK6ΔmTP-GFP fusion proteins, which lack N-terminal mitochondrial targeting peptides, were present mainly in the nucleus with a small amount of the proteins seen in the cytosol. In addition, the OsHXK5NLS-GFP and OsHXK6NLS-GFP fusion proteins harboring nuclear localization signals were targeted predominantly in the nucleus, suggesting that these OsHXKs retain a dual-targeting ability to mitochondria and nuclei. In transient expression assays using promoter∷luciferase fusion constructs, these two OsHXKs and their catalytically inactive alleles dramatically enhanced the Glc-dependent repression of the maize (Zea mays) Rubisco small subunit (RbcS) and rice α-amylase genes in mesophyll protoplasts of maize and rice. Notably, the expression of OsHXK5, OsHXK6, or their mutant alleles complemented the Arabidopsis glucose insensitive2-1 mutant, thereby resulting in wild-type characteristics in seedling development, Glc-dependent gene expression, and plant growth. Furthermore, transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited hypersensitive plant growth retardation and enhanced repression of the photosynthetic gene RbcS in response to Glc treatment. These results provide evidence that rice OsHXK5 and OsHXK6 can function as Glc sensors.

Light signal transduction in plants involves an intricate series of pathways which is finely regulated by interactions between specific signalling proteins, as well as by protein modifications such as phosphorylation and ubiquitination. The identification of novel phytochrome-interacting proteins and the precise signalling mechanisms that they mediate is still ongoing. In our present study, we show that the newly identified putative phytochrome-associated protein, PAPP2C (phytochrome-associated protein phosphatase type 2C), interacts in the nucleus with phyA (phytochrome A) and phyB, both in vitro and in vivo. Moreover, the phosphatase activity of PAPP2C and its association with phytochromes were found to be enhanced by red light, indicating that it plays a role in mediating phytochrome signalling. In particular, PAPP2C specifically binds to the N-terminal PHY domain of the phytochromes. We thus speculate that this interaction reflects a unique regulatory function of this phosphatase toward established phytochrome-associated proteins. We also show that PAPP2C effectively dephosphorylates phytochromes in vitro. Interestingly, PAPP2C indirectly mediates the dephosphorylation of PIF3 (phytochrome-interacting factor 3) in vitro. Taken together, we suggest that PAPP2C functions as a regulator of PIF3 by dephosphorylating phytochromes in the nucleus.

Rubisco is a major photosynthetic plant enzyme in the chloroplasts, catalyzing a photosynthetic reaction through carboxylation and oxygenation in the leaves. Despite its biological importance, its high abundance causes difficulties in the proper separation of protein mixtures during 2-dimensional gel electrophoresis (2-DE). Here, we resolved those plant soluble proteins by efficiently removing Rubisco. This resulted in a high quality and resolution of 2-DE gels. Rubisco removal was achieved through aggregation in the presence of a high DTT concentration, which subsequently increased the visualization of less abundant proteins and reduced horizontal streaking. This simple method may provide a means for finding more biologically important protein targets via plant proteomics.

Cytosolic fructose-1,6-bisphosphatase (cFBPase) in plants is a key regulatory enzyme in the photosynthetic sucrose biosynthesis. Plant cFBPases, like the mammalian FBPases, are inhibited by adenosine 5'-monophosphate (AMP) and fructose-2,6-bisphosphate (Fru-2,6-). In the mammalian FBPases, Lys112 and Tyr113 play important roles in the AMP binding. To understand roles of the corresponding residues, Lys115 and Tyr116, in pea cFBPase, the mutant cFBPases were generated by site-directed mutagenesis. The alterations of Lys115 to Gin and Tyr116 to Phe displayed small changes in and for Fru-2,6-, indicating that the mutation causes minor effects on the enzyme catalysis and Fru-2,6- binding, whereas resulted in higher than 500-fold increase of compared with that of the wild-type enzyme. Results indicate the residues Lys115 and Tyr116 play important roles in the binding of AMP to the allosteric site of the pea cFBPase.

ADP-glucose pyrophosphorylase (AGP) catalyzes the first committed step of starch biosynthesis in higher plants. To identify AGP isoforms essential for this biosynthetic process in sink and source tissues of rice plants, we analyzed the rice AGP gene family which consists of two genes, OsAGPS1 and OsAGPS2, encoding small subunits (SSU) and four genes, OsAGPL1, OsAGPL2, OsAGPL3 and OsAGPL4, encoding large subunits (LSU) of this enzyme heterotetrameric complex. Subcellular localization studies using green fluorescent protein (GFP) fusion constructs indicate that OsAGPS2a, the product of the leaf-preferential transcript of OsAGPS2, and OsAGPS1, OsAGPL1, OsAGPL3, and OsAGPL4 are plastid-targeted isoforms. In contrast, two isoforms, SSU OsAGPS2b which is a product of a seed-specific transcript of OsAGPS2, and LSU OsAGPL2, are localized in the cytosol. Analysis of osagps2 and osagpl2 mutants revealed that a lesion of one of the two cytosolic isoforms, OsAGPL2 and OsAGPS2b, causes a shrunken endosperm due to a remarkable reduction in starch synthesis. In leaves, however, only the osagps2 mutant appears to severely reduce the transitory starch content. Interestingly, the osagps2 mutant was indistinguishable from wild type during vegetative plant growth. Western blot analysis of the osagp mutants and wild type plants demonstrated that OsAGPS2a is an SSU isoform mainly present in leaves, and that OsAGPS2b and OsAGPL2 are the major SSU and LSU isoforms, respectively, in the endosperm. Finally, we propose a spatiotemporal complex model of OsAGP SSU and LSU isoforms in leaves and in developing endosperm of rice plants.

To elucidate the role of SSIIIa during starch synthesis in rice (Oryza sativa L.) endosperm, we characterized null mutants of this gene, generated by T-DNA insertions. Scanning electron microscope (SEM) analysis revealed that the starch granules in these mutants are smaller and rounder compared with the wild type controls, and that the mutant endosperm is characterized by a loosely packed central portion exhibiting a floury-like phenotype. Hence, the OsSSIIIa (Oryza sativa SSIIIa) mutations are referred to as white-core floury endosperm 5-1 (flo5-1) and flo5-2. Based upon their X-ray diffraction patterns, the crystallinity of the starch in the flo5 mutant endosperm is decreased compared with wild type. Through determination of the chain-length distribution of the mutant endosperm starch, we found that flo5-1 and flo5-2 mutants have reduced the content of long chains with degree of polymerization (DP) 30 or greater compared with the controls. This suggests that OsSSIIIa/Flo5 plays an important role in generating relatively long chains in rice endosperm. In addition, DP 6 to 8 and DP 16 to 20 appeared to be reduced in endosperm starch of flo5-1 and flo5-2, whereas DP 9 to 15 and DP 22 to 29 were increased in these mutants. By the use of differential scanning calorimetry (DSC), the gelatinization temperatures of endosperm starch were found to be 1-5 degrees C lower than those of the control. We propose a distinct role for OsSSIIIa/Flo5 and the coordinated action of other SS isoforms during starch synthesis in the seed endosperm of rice.

Fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase (F2KP) is a bifunctional enzyme involved in regulating photosynthetic carbon metabolism by modulating the cellular level of fructose-2,6-bisphosphate (Fru-2,6-P2) in plants. Rice appears to have two F2KP genes — OsF2KP1 and OsF2KP2 — on chromosomes 5 and 3, respectively. RT-PCR analysis showed that both genes are primarily expressed in the leaves, with transcripts somewhat less detectable in the flowers, roots, and seeds. To elucidate the functions of these OsF2KPs, we generated a Tos17 insertion mutant of OsF2KP1 and an RNAi mutant of OsF2KP2. In mutant plants, expression of the corresponding OsF2KPs was inhibited mainly in the leaves. At the end of day, sucrose contents had increased in the leaves of mutant plants while both fructose-6-phosphate 2-kinase activities and Fru-2,6-P2 levels had declined. Our results suggest that the two OsF2KPs are involved in the regulation of sucrose synthesis by controlling the levels of Fru-2,6-P2 during the daytime.

Light signals recognized by phytochromes are transduced through interactions between down-stream signaling components. Phytochrome-interacting ankyrin repeat protein 2 (PIA2) was found to interact with phytochrome interacting factor 3 (PIF3), a well-known repressor of plant photomorphogenesis in response to phytochrome-mediated light signalling. Both PIA2 and PIF3 are known to be positive regulators of anthocyanin accumulation in Arabidopsis seedlings under far-red conditions. Thus, we investigated the functional relationship between PIA2 and PIF3 in light signalling. We found that PIA2 suppressed PIF3 phosphorylation by phyA. To elucidate how PIA2 modulates phyA-mediated PIF3 phosphorylation, we generated non-phosphorylation mutants and N-terminal α-helix breaking mutants of PIA2. PIF3 phosphorylation by phyA was not suppressed by α-helix breaking PIA2 mutants. The α-helix breaking mutations also resulted in remarkably decreased interactions between PIA2 and PIF3. However, the non-phosphorylation mutants exhibited no effect on phyA-mediated PIF3 phosphorylation. In addition, decreased anthocyanin accumulation in pia2 knockout plant seedlings was not rescued by overexpression of the α-helix breaking mutant in transgenic plants under far-red conditions. These results suggest that PIA2 modulates phyA-mediated PIF3 phosphorylation by physical interaction with PIF3 and that the secondary structure of the PIA2 N-terminus is important in this modulation.

Bacteriophytochromes are phytochrome-like light-sensing photoreceptors that use biliverdin as a chromophore. To study the biochemical properties of the Deinococcus radiodurans bacteriophytochrome (DrBphP) protein, two anti-DrBphP mouse monoclonal antibodies (2B8 and 3H7) were generated. Their specific epitopes were identified in our previous report. We present here fine epitope mapping of these two antibodies by using truncation and substitution of original epitope sequences in order to identify minimized epitope peptides. The previously reported original epitope sequences for 2B8 and 3H7 were truncated from both sides. Our analysis showed that the minimal peptide sequence lengths for 2B8 and 3H7 antibodies were nine amino acids (RDPLPFFPP) and six amino acids (PGEIEE), respectively. We further characterized these peptides in order to investigate their reactivity after single deletion and single substitution of the original peptides. We found that single-substituted 2B8 epitope (RDPLPAFPP) and dual-substituted 3H7 epitope (PGEIAD) showed significantly increased reactivity. These two antibodies with high reactivity for the short modified peptide sequences are valueble for developing new peptide tags for protein research.

Phytochrome A (phyA) is a light labile phytochrome that mediates plant development under red/far-red light condition. Degradation of phyA is initiated by red light-induced phyA-ubiquitin conjugation through the 26S proteasome pathway. The N-terminal of phyA is known to be important in phyA degradation. To determine the specific lysine residues in the N-terminal domain of phyA involved in light-induced ubiquitination and protein degradation, we aligned the amino acid sequence of the N-terminal domain of Arabidopsis phyA with those of phyA from other plant species. Based on the alignment results, phytochrome over-expressing Arabidopsis plants were generated. In particular, wild-type and mutant (substitutions of conserved lysines by arginines) phytochromes fused with GFP were expressed in phyA(-)211 Arabidopsis plants. Degradation kinetics of over-expressed phyA proteins revealed that degradation of the K206R phyA mutant protein was delayed. Delayed phyA degradation of the K206R phyA mutant protein resulted in reduction of red-light-induced phyA-ubiquitin conjugation. Furthermore, seedlings expressing the K206R phyA mutant protein showed an enhanced phyA response under far-red light, resulting in inhibition of hypocotyl elongation as well as cotyledon opening. Together, these results suggest that lysine 206 is the main lysine for rapid ubiquitination and protein degradation of Arabidopsis phytochrome A.

Hexokinase (HXK) is a dual-function enzyme that both phosphorylates hexose to form hexose 6-phosphate and plays an important role in sugar sensing and signaling. To investigate the roles of hexokinases in rice growth and development, we analyzed rice sequence databases and isolated ten rice hexokinase cDNAs, OsHXK1 (Oryza sativa Hexokinase 1) through OsHXK10. With the exception of the single-exon gene OsHXK1, the OsHXKs all have a highly conserved genomic structure consisting of nine exons and eight introns. Gene expression profiling revealed that OsHXK2 through OsHXK9 are expressed ubiquitously in various organs, whereas OsHXK10 expression is pollen-specific. Sugars induced the expression of three OsHXKs, OsHXK2, OsHXK5, and OsHXK6, in excised leaves, while suppressing OsHXK7 expression in excised leaves and immature seeds. The hexokinase activity of the OsHXKs was confirmed by functional complementation of the hexokinase-deficient yeast strain YSH7.4-3C (hxk1, hxk2, glk1). OsHXK4 was able to complement this mutant only after the chloroplast-transit peptide was removed. The subcellular localization of OsHXK4 and OsHXK7, observed with green fluorescent protein (GFP) fusion constructs, indicated that OsHXK4 is a plastid-stroma-targeted hexokinase while OsHXK7 localizes to the cytosol.

Phytochromes are plant photoreceptors that regulate the growth and development of plants in response to light. Phytochromes exist as dimers and dimerization is thought to be important for phytochrome function. Phytochromes contain two major domains, the N-terminal domain responsible for chromophore ligation and photosensory specificity and the C-terminal domain responsible for dimerization and regulatory functions. We have investigated the dimerization motifs by means of the yeast two-hybrid assays and size exclusion chromatography using purified recombinant phytochromes. From dimerization analyses using internal deletion mutants, site-specific mutants, and C-terminal fragments of the pea phytochrome A, the primary contact region for dimerization was localized to the region between Val730 and Gly821. Further analysis using purified full-length phytochrome mutants and Per–Arnt–Sim 1 (PAS1) and PAS2 fragments revealed that the PAS2 domain is required for dimerization, but that the PAS1 domain is not.

To further elucidate the mechanisms underlying carbon-partitioning in plants, we established an experimental system by generating transgenicArabidopsis lines that overexpress both the fructose 6-phosphate, 2-kinase (F6P,2-K) and the fructose 2,6-bisphosphatase (F26BPase) domains. We also produced knockout transgenic plants for these domains via RNAi and T-DNA tagging. In F6P,2-K overexpressing transgenics, F6P,2-K activity increased slightly and Fru-2,6-P2 levels were elevated by 80%, compared with the wild type (WT). F26BPase activity was similar between the WT and transgenic plants. However, when that domain was overexpressed, F26BPase activity was increased by 70% compared with the WT, whereas F6P,2-K activity was reduced to 85% of the WT level. In knockout and RNAi mutant lines that showed reduced F6P,2-K and F26BPase activities, levels of Fru-2,6-P2 were only between 3 to 7% of those for the WT. In F6P,2-K overexpressing transgenic lines, the levels of starch, hexose, and triose phosphates slightly increased, while sucrose content was marginally reduced. In F26BPase overexpressing plants, however, the levels of soluble sugars and hexose phosphates were slightly increased, but starch and triose phosphate contents declined. Furthermore, compared with the WT, the levels of soluble sugars rose while starch and hexose phosphate quantities decreased in 2-kinase/fructose-2,6-bisphophatase knockout mutants. Therefore, our data reaffirms that Fru-2,6-P2 contributes to the regulation of photosynthetic carbon-partitioning between starch and sucrose inArabidopsis leaves by limiting sucrose synthesis.

Cell-wall invertase (CIN) catalyzes the hydrolysis of sucrose into glucose and fructose for the supply of carbohydrates to sink organs via an apoplastic pathway. To study the CIN genes in rice (Oryza sativa L.), we isolated cDNA clones showing amino acid similarity to the plant cell wall invertase proteins from a search of rice sequence databases. Profile analyses revealed that the cloned genes are expressed in unique patterns in various organs. For example, transcripts of OsCIN1, OsCIN2, OsCIN4, and OsCIN7 were detected in immature seeds whereas OsCIN3 gene expression was flower-specific. Further transcript analysis of these genes expressed in developing seeds indicated that OsCIN1, OsCIN2, and OsCIN7 might play an important role involving sucrose partitioning to the embryo and endosperm. Sucrose, a substrate of CINs, induced the accumulation of OsCIN1 transcripts in excised leaves and OsCIN2 in immature seeds, while the level of OsCIN5 was significantly down-regulated in excised leaves treated with sucrose. Infecting the tissues with rice blast (Magnaporthe grisea) as a biotic stressor increased the expression of OsCIN1, OsCIN4, and OsCIN5, suggesting that these genes may participate in a switch in metabolism to resist pathogen invasion. These results demonstrate that OsCIN genes play diverse roles involving the regulation of metabolism, growth, development, and stress responses.

To investigate the effects of sucrose-phosphate synthase (SPS) on carbon partitioning, transgenicArabidopsis plants transformed withSynechocystis SPS were constructed. The integration, copy number and expression level were confirmed by Southern and Northern blot analyses. SPS activity in leaves from the transgenic and wild type plants was not significantly different. The level of sucrose and starch in the leaves of transgenic plant were slightly decreased compared to wild type. The glucose and fructose contents were increased up to two-fold compared to wild type during the light period. It is our speculation that the decreased sucrose level of the transgenic plant might be caused by the high acid invertase.

Genipin, a hydrolysate of geniposide from gardenia fruits, produces blue pigments on reaction with amino acids. The colorimetric detection of amino acids using this genipin reaction was evaluated and compared with the well-known ninhydrin reaction. The molar absorptivities of the blue pigments, the reaction products of genipin with various amino acids, were greater than those of the respective ninhydrin reaction products. When asparagine was reacted with genipin, the molar absorptivity was about 14 times higher than with ninhydrin. The absorbance of the genipin–amino acids increased linearly with increase of amino acid concentration, indicating that genipin could be a useful reagent for quantitation of amino acids. Thin layer chromatographic analysis showed that the genipin reaction produces clear and stable colored spots. The blue ninhydrin reaction spots were usually bleached in 24 h at room temperature, while the genipin reaction spots remained unchanged for several months. The addition of 0.1 mM Cu2+ and Fe3+ decreased the absorbance of Gly–ninhydrin pigment by 50% and 98%, respectively, but those metal ions did not affect the absorbance of the Gly–genipin pigment.

Rice produces a wide array of phytoalexins in response to pathogen attacks and UV-irradiation. Except for the flavonoid sakuranetin, most phytoalexins identified in rice are diterpenoid compounds. Analysis of phenolic-enriched fractions from UV-treated rice leaves showed that several phenolic compounds in addition to sakuranetin accumulated remarkably in rice leaves. We isolated two compounds from UV-treated rice leaves using silica gel column chromatography and preparative HPLC. The isolated phenolic compounds were identified as phenylamide compounds: N-trans-cinnamoyltryptamine and N-p-coumaroylserotonin. Expression analysis of biosynthetic genes demonstrated that genes for arylamine biosynthesis were upregulated by UV irradiation. This result suggested that phenylamide biosynthetic pathways are activated in rice leaves by UV treatment. To unravel the role of UV-induced phenylamides as phytoalexins, we examined their antimicrobial activity against rice fungal and bacterial pathogens. N-trans-Cinnamoyltryptamine inhibited the growth of rice brown spot fungus (Bipolaris oryzae). In addition to the known antifungal activity to the blast fungus, sakuranetin had antimicrobial activity toward B. oryzae and Rhizoctonia solani (rice sheath blight fungus). UV-induced phenylamides and sakuranetin also had antimicrobial activity against rice bacterial pathogens for grain rot (Burkholderia glumae), blight (Xanthomonas oryzae pv. oryzae) and leaf streak (X. oryzae pv. oryzicola) diseases. These findings suggested that the UV-induced phenylamides in rice are phytoalexins against a diverse array of pathogens.

Phytochromes comprise a principal family of red/far-red light sensors in plants. Although phytochromes were thought originally to be confined to photosynthetic organisms, we have recently detected phytochrome-like proteins in two heterotrophic eubacteria, Deinococcus radiodurans and Pseudomonas aeruginosa. Here we show that these form part of a widespread family of bacteriophytochromes (BphPs) with homology to two-component sensor histidine kinases. Whereas plant phytochromes use phytochromobilin as the chromophore, BphPs assemble with biliverdin, an immediate breakdown product of haem, to generate photochromic kinases that are modulated by red and far-red light. In some cases, a unique haem oxygenase responsible for the synthesis of biliverdin is part of the BphP operon. Co-expression of this oxygenase with a BphP apoprotein and a haem source is sufficient to assemble holo-BphP in vivo. Both their presence in many diverse bacteria and their simplified assembly with biliverdin suggest that BphPs are the progenitors of phytochrome-type photoreceptors.

The committed step in the biosynthesis of the phytochrome chromophore phytochromobilin involves the oxidative cleavage of heme by a heme oxygenase (HO) to form biliverdin IXα. Through positional cloning of the photomorphogenic mutant hy1, the Arabidopsis HO (designated AtHO1) responsible for much of phytochromobilin synthesis recently was identified. Using theAtHO1 sequence, we identified families ofHO genes in a number of plants that cluster into two subfamilies (HO1- and HO2-like). The tomato (Lycopersicon esculentum) yg-2 andNicotiana plumbaginifolia pew1 photomorphogenic mutants are defective in specific HO genes. Phenotypic analysis of a T-DNA insertion mutant of Arabidopsis HO2 revealed that the second HO subfamily also contributes to phytochromobilin synthesis. Homozygous ho2-1 plants show decreased chlorophyll accumulation, reduced growth rate, accelerated flowering time, and reduced de-etiolation. A mixture of apo- and holo-phyA was detected in etiolated ho2-1 seedlings, suggesting that phytochromobilin is limiting in this mutant, even in the presence of functional AtHO1. The patterns of ArabidopsisHO1 and HO2 expression suggest that the products of both genes overlap temporally and spatially. Taken together, the family of HOs is important for phytochrome-mediated development in a number of plants and that each family member may uniquely contribute to the phytochromobilin pool needed to assemble holo-phytochromes.

Bacteriophytochromes (BphP) are phytochrome-like light sensing proteins in bacteria, which use biliverdin as a chromophore. In order to study the biochemical properties of the DrBphP protein, five (2B8, 2C11, 3B2, 3D2, and 3H7) anti-DrBphP monoclonal antibodies were produced through the immunization of mice with purified full-length DrBphP and DrBphN (1–321 amino acid) proteins, and epitope mapping was then carried out. Among the five antibodies, 2B8 and 2C11 preferentially recognized the N-terminal region of BphP whereas 3B2, 3D2, and 3H7 showed preference for the C-terminal region. We performed further epitope mapping using recombinant truncated BphP proteins to narrow down their target sequences. The results demonstrated that each of the five monoclonal antibodies recognized different regions on the DrBphP protein. Additionally, epitopes of 2B8 and 3H7 antibodies were discovered to be shorter than 10 amino acids (2B8: RDPLPFFPP, 3H7: PGEIEEA). These two antibodies with such specific recognition epitopes could be especially valuable for developing new peptide tags for protein detection and purification.

Physiological functions of sucrose (Suc) transporters (SUTs) localized to the tonoplast in higher plants are poorly understood. We here report the isolation and characterization of a mutation in the rice (Oryza sativa) OsSUT2 gene. Expression of OsSUT2-green fluorescent protein in rice revealed that OsSUT2 localizes to the tonoplast. Analysis of the OsSUT2 promoter::β-glucuronidase transgenic rice indicated that this gene is highly expressed in leaf mesophyll cells, emerging lateral roots, pedicels of fertilized spikelets, and cross cell layers of seed coats. Results of Suc transport assays in yeast were consistent with a H+-Suc symport mechanism, suggesting that OsSUT2 functions in Suc uptake from the vacuole. The ossut2 mutant exhibited a growth retardation phenotype with a significant reduction in tiller number, plant height, 1,000-grain weight, and root dry weight compared with the controls, the wild type, and complemented transgenic lines. Analysis of primary carbon metabolites revealed that ossut2 accumulated more Suc, glucose, and fructose in the leaves than the controls. Further sugar export analysis of detached leaves indicated that ossut2 had a significantly decreased sugar export ability compared with the controls. These results suggest that OsSUT2 is involved in Suc transport across the tonoplast from the vacuole lumen to the cytosol in rice, playing an essential role in sugar export from the source leaves to sink organs.

Summary •In higher plants, the plastidic glucose translocator (pGlcT) is assumed to play a role in the export of starch degradation products, but this has not yet been studied in detail. •To elucidate the role of pGlcT in the leaves of Arabidopsis thaliana, we generated single and double mutants lacking three plastidic sugar transporters, pGlcT, the triose-phosphate/phosphate translocator (TPT), and the maltose transporter (MEX1), and analyzed their growth phenotypes, photosynthetic properties and metabolite contents. •In contrast to the pglct-1 and pglct-2 single mutants lacking a visible growth phenotype, the double mutants pglct-1/mex1 and tpt-2/mex1 displayed markedly inhibited plant growth. Notably, pglct-1/mex1 exhibited more severe growth retardation than that seen for the other mutants. In parallel, the most severe reductions in sucrose content and starch turnover were observed in the pglct-1/mex1 mutant. The concurrent loss of pGlcT and MEX1 also resulted in severely reduced photosynthetic activities and extreme chloroplast abnormalities. •These findings suggest that pGlcT, together with MEX1, contributes significantly to the export of starch degradation products from chloroplasts in A. thaliana leaves, and that this starch-mediated pathway for photoassimilate export via pGlcT and MEX1 is essential for the growth and development of A. thaliana.

Many aspects of plant metabolism that are involved in plant growth and development are influenced by light-regulated diurnal rhythms as well as endogenous clock-regulated circadian rhythms. To identify the rhythmic proteins in rice, periodically grown (12h light/12h dark cycle) seedlings were harvested for three days at six-hour intervals. Continuous dark-adapted plants were also harvested for two days. Among approximately 3000 reproducible protein spots on each gel, proteomic analysis ascertained 354 spots (~12%) as light-regulated rhythmic proteins, in which 53 spots showed prolonged rhythm under continuous dark conditions. Of these 354 ascertained rhythmic protein spots, 74 diurnal spots and 10 prolonged rhythmic spots under continuous dark were identified by MALDI-TOF MS analysis. The rhythmic proteins were functionally classified into photosynthesis, central metabolism, protein synthesis, nitrogen metabolism, stress resistance, signal transduction and unknown. Comparative analysis of our proteomic data with the public microarray database (the Plant DIURNAL Project) and RT-PCR analysis of rhythmic proteins showed differences in rhythmic expression phases between mRNA and protein, suggesting that the clock-regulated proteins in rice are modulated by not only transcriptional but also post-transcriptional, translational, and/or post-translational processes.

Phytochromes enable plants to sense light information and regulate developmental responses. Phytochromes interact with partner proteins to transmit light signals to downstream components for plant development. PIRF1 (phytochrome-interacting ROP guanine-nucleotide exchange factor (RopGEF 1)) functions as a light-signaling switch regulating root development through the activation of ROPs (Rho-like GTPase of plant) in the cytoplasm. In vitro pulldown and yeast two-hybrid assays confirmed the interaction between PIRF1 and phytochromes. PIRF1 interacted with the N-terminal domain of phytochromes through its conserved PRONE (plant-specific ROP nucleotide exchanger) region. PIRF1 also interacted with ROPs and activated them in a phytochrome-dependent manner. The Pr form of phytochrome A enhanced the RopGEF activity of PIRF1, whereas the Pfr form inhibited it. A bimolecular fluorescence complementation analysis demonstrated that PIRF1 was localized in the cytoplasm and bound to the phytochromes in darkness but not in light. PIRF1 loss of function mutants (pirf1) of Arabidopsis thaliana showed a longer root phenotype in the dark. In addition, both PIRF1 overexpression mutants (PIRF1-OX) and phytochrome-null mutants (phyA-211 and phyB-9) showed retarded root elongation and irregular root hair formation, suggesting that PIRF1 is a negative regulator of phytochrome-mediated primary root development. We propose that phytochrome and ROP signaling are interconnected through PIRF1 in regulating the root growth and development in Arabidopsis.

In an attempt to understand the complex regulatory mechanisms underlying sucrose-induced flavonoid biosynthesis, we examined several Arabidopsis mutants with altered anthocyanin accumulation. We determined that disruption of ethylene signaling results in a dramatic increase in sucrose-induced anthocyanin accumulation. Furthermore, we investigated why the ein2-1 (ethylene insensitive) Arabidopsis mutant accumulates higher levels of anthocyanin in response to sucrose than wild-type Arabidopsis. An increased level of PAP1 transcript in the ein2-1 mutant appears to be the main factor responsible for the increased accumulation of anthocyanin in response to sucrose. Therefore, our results indicate that the ethylene signaling pathway plays a negative role in sucrose-induced anthocyanin accumulation. We believe that the explanation for this observation may be related to the initiation of the senescence program in plants.