Maize genes encoding the small subunit of ADP-glucose pyrophosphorylase

Plant ADP-glucose pyrophosphorylase (AGP) is a heterotetrameric enzyme composed of two large and two small subunits. Here, we report the structures of the maize (Zea mays) genes encoding AGP small subunits of leaf and endosperm. Excluding exon 1, protein-encoding sequences of the two genes are nearly identical. Exon 1 coding sequences, however, possess no similarity. Introns are placed in identical positions and exhibit obvious sequence similarity. Size differences are primarily due to insertions and duplications, hallmarks of transposable element visitation. Comparison of the maize genes with other plant AGP small subunit genes leads to a number of noteworthy inferences concerning the evolution of these genes. The small subunit gene can be divided into two modules. One module, encompassing all coding information except that derived from exon 1, displays striking similarity among all genes. It is surprising that members from eudicots form one group, whereas those from cereals form a second group. This implies that the duplications giving rise to family members occurred at least twice and after the separation of eudicots and monocot cereals. One intron within this module may have had a transposon origin. A different evolutionary history is suggested for exon 1. These sequences define three distinct groups, two of which come from cereal seeds. This distinction likely has functional significance because cereal endosperm AGPs are cytosolic, whereas all other forms appear to be plastid localized. Finally, whereas barley (Hordeum vulgare) reportedly employs only one gene to encode the small subunit of the seed and leaf, maize utilizes the two genes described here.

Cyclophilin a binds to peroxiredoxins and activates its peroxidase activity

Six distinct peroxiredoxin (Prx) proteins (Prx I-VI) from distinct genes have been identified in mammalian tissues. Prxs are members of a group of peroxidases that have conserved reactive cysteine residue(s) in the active site(s). An immediate physiological electron donor for the peroxidase catalysis for five Prx proteins (Prx I-V) has been identified as thioredoxin (Trx), but that for Prx VI (1-Cys Prx) is still unclear. To identify an immediate electron donor and a binding protein for Prx VI, we performed a Prx VI protein overlay assay. A 20-kDa binding protein was identified by the Prx VI protein overlay assay with flow-through fractions from a High-Q column with rat lung crude extracts. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) and MS-Fit, we identified the 20-kDa Prx VI-binding protein as a cyclophilin A (CyP-A). The binding of recombinant human CyP-A (hCyP-A) to Prx VI was confirmed by using the hCyP-A protein overlay assay and Western immunoblot analysis with hCyP-A-specific antibodies. hCyP-A enhanced the antioxidant activity of Prx VI, as well as the other known mammalian Prx isotypes. hCyP-A supported antioxidant activity of Prx II and Prx VI both against thiol (dithiothreitol)-containing metal-catalyzed oxidation (MCO) systems and ascorbate-containing MCO systems. Prx II was reduced by hCyP-A without help from any other reductant, and the reduction was cyclosporin A-independent. These results strongly suggest that CyP-A not only binds to Prx proteins but also supports its peroxidase activity as an immediate electron donor. In addition, Cys(115) and Cys(161) of hCyP-A were found to be involved in the activation and the reduction of Prx.

FLP/FRT-mediated restoration of normal phenotypes and clonal sectors formation in rolC transgenic tobacco

Site-specific recombination systems have been shown to excise transgene DNA sequences positioned between their cognate target sites, and thus be used to generate clonal sectors in transgenic plants. Here we characterized clonal sectors derived from genetic reversion of rolC (A. rhizogenes)--induced vegetative and reproductive phenotypes, mediated by FLP recombinase from S. cerevisiae, in tobacco. The constitutive expression of rolC induces pleiotropic effects including reduced apical dominance and plant height, lanceolate and pale green leaves and small, male-sterile flowers. Two transgenic male-sterile tobacco lines (N. tabacum, Samsun NN) expressing a 35sP-rolC gene construct flanked by two FRT (FLP recombinase target) sites, were cross-pollinated with pollen from a constitutive 35sP-FLP expressing line. Three main phenotypes were generated in result of recombinase-mediated excision of the 35sP-rolC locus in the F1 (FLP x FRT-35sP-rolC-FRT) hybrid progenies: (a) restoration of male fertility, associated with reversion to normal leaf phenotypes prior to flower bud formation, (b) development of normal and fertile lateral shoot sectors on the background of rolC-type plants, (c) restoration of partially fertile flowers, associated with display of peripheral normal leaf sectors surrounding rolC-type inner-leaf tissues, consistent with periclinal chimeras. These results, supported by DNA molecular analysis, indicate that site-specific recombination might be used as a relatively efficient tool for generation of transgenic periclinal chimeric plants.

Stability of barley aleurone transcripts: Dependence on protein synthesis, influence of the starchy endosperm and destabilization by GA3

We have studied the stability of Barley aleurone and embryo expressed (Balem) transcripts in aleurone layers. The Per1, Ole1 and Ole2 transcripts are abundant during desiccation and in dry resting seeds, while B12D and B22E transcripts are expressed mainly during seed maturation and germination. From 21 to 40 days post anthesis (DPA) incubation of aleurone layers resulted in a substantial, but differential reduction in the levels of these transcripts. In contrast, Balem transcript levels in aleurone layers of incubated embryoless grains were (except for B22E) similar to those of freshly dissected layers. Cycloheximide lowered transcript levels significantly. This indicates that a protein-synthesis-dependent mRNA-stabilizing mechanism is active in the aleurone cells when attached to the starchy endosperm. At the onset of seed desiccation (40 DPA), half-lives of transcripts to be stored in the dry seed were up to several days longer than the half-life of B22E, which decreases during seed maturation. While the Per1, Ole1 and Ole2 transcript levels decline rapidly in the aleurone layers of mature, germinating seeds, the genes are actively transcribed and their transcripts highly stable in the aleurone of incubated embryoless seeds. The expression of Ole1 and Ole2, as well as Per1, can be repressed 100-1 000-fold by gibberellic acid (GA3) in a dose-dependent manner. Abscisic acid can counteract the GA3 repression. Incubations with transcriptional and translational inhibitors indicate that GA3 inhibits the transcription of these genes and at the same time induces a protein-synthesis-dependent mechanism destabilizing their mRNA molecules present.

ENDOSPERM DEVELOPMENT: Cellularization and Cell Fate Specification

The endosperm develops from the central cell of the megagametophyte after introduction of the second male gamete into the diploid central cell. Of the three forms of endosperm in angiosperms, the nuclear type is prevalent in economically important species, including the cereals. Landmarks in nuclear endosperm development are the coenocytic, cellularization, differentiation, and maturation stages. The differentiated endosperm contains four major cell types: starchy endosperm, aleurone, transfer cells, and the cells of the embryo surrounding region. Recent research has demonstrated that the first two phases of endosperm occur via mechanisms that are conserved among all groups of angiosperms, involving directed nuclear migration during the coenocytic stage and anticlinal cell wall deposition by cytoplasmic phragmoplasts formed in interzones between radial microtubular systems emanating from nuclear membranes. Complete cellularization of the endosperm coenocyte is achieved through centripetal growth of cell files, extending to the center of the endosperm cavity. Key points in cell cycle control and control of the MT (microtubular) cytoskeletal apparatus central to endosperm development are discussed. Specification of cell fates in the cereal endosperm appears to occur via positional signaling; cells in peripheral positions, except over the main vascular tissues, assume aleurone cell fate. Cells over the main vascular tissue become transfer cells and all interior cells become starchy endosperm cells. Studies in maize have implicated Crinkly4, a protein receptor kinase-like molecule, in aleurone cell fate specification.

Enzymes that scavenge reactive oxygen species are down-regulated prior to gibberellic acid-induced programmed cell death in barley aleurone

Gibberellins (GAs) initiate a series of events that culminate in programmed cell death, whereas abscisic acid (ABA) prevents this process. Reactive oxygen species (ROS) are key elements in aleurone programmed cell death. Incubation of barley (Hordeum vulgare) aleurone layers in H2O2 causes rapid death of all cells in GA- but not ABA-treated layers. Sensitivity to H2O2 in GA-treated aleurone cells results from a decreased ability to metabolize ROS. The amounts and activities of ROS scavenging enzymes, including catalase (CAT), ascorbate peroxidase, and superoxide dismutase are strongly down-regulated in aleurone layers treated with GA. CAT activity, protein, and Cat2 mRNA decline rapidly following exposure of aleurone layers to GA. In ABA-treated layers, on the other hand, the amount and activity of CAT and Cat2 mRNA increases. Incubation in ABA maintains high amounts of ascorbate peroxidase and superoxide dismutase, whereas GA brings about a rapid reduction in the amounts of these enzymes. These data imply that GA-treated cells loose their ability to scavenge ROS and that this loss ultimately results in oxidative damage and cell death. ABA-treated cells, on the other hand, maintain their ability to scavenge ROS and remain viable.

Disulfide bond assignment, lipid transfer activity and secondary structure of a 7-kDa plant lipid transfer protein, LTP2

The 7-kDa lipid transfer proteins, LTP2s, share some amino-acid sequence similarities with the 9-kDa isoforms, LTP1s. Both proteins display an identical cysteine motif and, in this regard, LTP2s have been classified as lipid transfer proteins. However, in contrast with LTP1s, no data are available on their structure, cysteine pairings, lipid transfer and lipid binding properties. We reported on the isolation of two isoforms of 7-kDa lipid transfer protein, LTP2, from wheat seeds and showed for the first time that they indeed display lipid transfer activity. Trypsin and chymotrypsin digestions of the native LTP2 afforded the sequence of both isoforms and assignment of disulfide bonds. The cysteine pairings, Cys10--Cys24, Cys25--Cys60, Cys2--Cys34, Cys36--Cys67, revealed a mismatch at the Cys34-X-Cys36 motif of LTP2 compared to LTP1. Moreover, the secondary structure as determined by circular dichroism suggested an identical proportion of alpha helices, beta sheets and random coils. By analogy with the structure of the LTP1, we discussed what structural changes are required to accommodate the LTP2 disulfide pattern.

Accumulation of soluble and wall-bound indolic metabolites in Arabidopsis thaliana leaves infected with virulent or avirulent Pseudomonas syringae pathovar tomato strains

The chemical structures and accumulation kinetics of several major soluble as well as wall-bound, alkali-hydrolyzable compounds induced upon infection of Arabidopsis thaliana leaves with Pseudomonas syringae pathovar tomato were established. All identified accumulating products were structurally related to tryptophan. Most prominent among the soluble substances were tryptophan, beta-d-glucopyranosyl indole-3-carboxylic acid, 6-hydroxyindole-3-carboxylic acid 6-O-beta-d-glucopyranoside, and the indolic phytoalexin camalexin. The single major accumulating wall component detectable under these conditions was indole-3-carboxylic acid. All of these compounds increased more rapidly, and camalexin as well as indole-3-carboxylic acid reached much higher levels, in the incompatible than in the compatible P. syringae/A. thaliana interaction. The only three prominent phenylpropanoid derivatives present in the soluble extract behaved differently. Two kaempferol glycosides remained largely unaffected, and sinapoyl malate decreased strongly upon bacterial infection with a time course inversely correlated with that of the accumulating tryptophan-related products. The accumulation patterns of both soluble and wall-bound compounds, as well as the disease resistance phenotypes, were essentially the same for infected wild-type and tt4 (no kaempferol glycosides) or fah1 (no sinapoyl malate) mutant plants. Largely different product combinations accumulated in wounded or senescing A. thaliana leaves. It seems unlikely that any one of the infection-induced compounds identified so far has a decisive role in the resistance response to P. syringae.

Rice 1Cys-peroxiredoxin over-expressed in transgenic tobacco does not maintain dormancy but enhances antioxidant activity

Possible functions that have been proposed for the plant 1Cys-peroxiredoxin, include activity as a dormancy regulator and as an antioxidant. The transcript level of rice 1Cys-peroxiredoxin (R1C-Prx) rapidly decreased after imbibition of rice seeds, but the protein was detected for 15 days after imbibition. To investigate the function of this protein, we generated transgenic tobacco plants constitutively expressing the R1C-Prx gene. The transgenic R1C-Prx plants showed a germination frequency similar to control plants. However, the transgenic lines exhibited higher resistance against oxidative stress, suggesting that antioxidant activity may be its primary function.

Presence of multiple cDNAs encoding an isoform of ADP-glucose pyrophosphorylase large subunit from sweet potato and characterization of expression levels

Three cDNAs (iAGPLI-1, iAGPLI-2, and iAGPLI-3) encoding an isoform of AGPase large subunit were isolated from a sweet potato cDNA library constructed from tuberous root tissue. iAGPLI-1 was 2,161 bp in length and contained an open reading frame of 517 amino acids with a calculated molecular mass of 57,689 Da. iAGPLI-2 and iAGPLI-3 were 1,804 and 1,524 bp in length, respectively, and contained partial open reading frames of 490 and 385 amino acids. Deduced amino acid sequence comparison analysis showed that iAGPLI-1 has sequence identity with iAGPLI-2 (97.9) and iAGPLI-3 (98.7%) while iAGPLI-2 and iAGPLI-3 have 96.8% sequence identity. iAGPLI-1 had the highest sequence identity of 77.8% with potato AGPase (sAGPL1). Steady-state levels of iAGPLI-1 transcripts were expressed predominantly in the stem, and moderately in the tuberous root, but not in either the roots or leaves. However, AGPase activity was present in all tissues. The expression level in the stem declined dramatically after a 12 h incubation in the dark to nearly 3% of the value under light, although the activity under a dark condition remained at half the levels in light. The activity levels were not correlated with the transcript levels. iAGPL transcripts in leaves were induced by sucrose feeding but not by glucose or fructose. Therefore, the expression of iAGPLI-1 is regulated in stem tissue preferentially and by sucrose. Southern blot analysis showed that the sweet potato genome contained several copies of iAGPLI gene probably due to polyploidy.

Programmed cell death during endosperm development

The endosperm of cereals functions as a storage tissue in which the majority of starch and seed storage proteins are synthesized. During its development, cereal endosperm initiates a cell death program that eventually affects the entire tissue with the exception of the outermost cells, which differentiate into the aleurone layer and remain living in the mature seed. To date, the cell death program has been described for maize and wheat endosperm, which exhibits common and unique elements for each species. The progression of endosperm programmed cell death (PCD) in both species is accompanied by an increase in nuclease activity and the internucleosomal degradation of nuclear DNA, hallmarks of apoptosis in animals. Moreover, ethylene and abscisic acid are key to mediating PCD in cereal endosperm. The progression of the cell death program in developing maize endosperm follows a highly organized pattern whereas in wheat endosperm, PCD initiates stochastically. Although the essential characteristics of cereal endosperm PCD are now known, the molecular mechanisms responsible for its execution remain to be identified.

Analysis by two-dimensional electrophoresis of the effect of salt stress on the polypeptide patterns in roots of a salt-tolerant and a salt-sensitive cultivar of wheat

The effect of salt stress on the polypeptide levels in roots of two wheat (Triticum durum) cultivars with different sensitivity to NaCl (cv. Ben Bachir, sensitive; cv. Chili, tolerant), was examined by two-dimensional polyacrylamide gel electrophoresis. Blue-stained gels were analyzed by visual inspection to identify changes that resulted when seedlings were grown in the presence of 200 mM NaCl for four days. Although the protein patterns for control and salt-stressed seedlings were qualitatively similar, the net synthesis of a 26 kDa polypeptide was significantly changed. This observation was mainly noticeable in the more tolerant cultivar. With the intention of identifying its function, the NH2-terminal of this polypeptide was sequenced. A 20 amino acid sequence was obtained and compared to sequences available in different databases. Possible roles of this polypeptide, depending on the homologies of its amino acid sequence with known proteins, in salinity tolerance are discussed.

PRELI, the human homologue of the avian px19, is expressed by germinal center B lymphocytes

We report the identification of a human cDNA encoding a 25 kDa protein of relevant evolutionary and lymphoid interest (PRELI). PRELI was cloned by screening a B lymphocyte-specific cDNA library with a probe generated by mRNA differential display. PRELI amino acid sequence is 85% similar to the avian px19 protein, expressed within the blood islands and in the liver during avian embryo development. PRELI and px19 contain tandem repeats (A/TAEKAK) of the late embryogenesis abundant (LEA) motif, characteristic of a group of survival molecules and originally thought to be present only in plant proteins. Interestingly, PRELI expression is high in the fetal liver, a major site for B cell lymphopoiesis, while the mRNA levels in other fetal tissues such as the brain, lung, and kidney are comparatively low. At the adult stage, PRELI expression is drastically reduced in the liver but exhibits high mRNA levels in the spleen, brain, lung and kidney tissues, suggesting that PRELI expression may be important for the development of vital and immunocompetent organs. Moreover, PRELI is also highly expressed in the adult lymph nodes and peripheral blood leukocytes, further stressing that at the adult stage, PRELI expression may be important during secondary immune responses. Consistent with this hypothesis, the expression of PRELI is predominant within germinal centers (GC), a stage in which B lymphocytes are under a stressful selection pressure. Taken together these data: (i) strongly support the notion that the conserved LEA motif represents a phylogenetic link between plants and animals, (ii) reveal a novel molecule whose expression may play a role in the maturation of distinct human tissues, and (iii) suggest that PRELI expression may be important for GC B lymphocytes.

FePer 1, a gene encoding an evolutionarily conserved 1-Cys peroxiredoxin in buckwheat (Fagopyrum esculentum Moench), is expressed in a seed-specific manner and induced during seed germination

A cDNA corresponding to 1-Cys peroxiredoxin, an evolutionarily conserved thiol-specific antioxidant enzyme, was isolated from buckwheat (Fagopyrum esculentum Moench), a dicotyledonous plant species belonging to the Polygonaceae family. The cDNA, which we have designated as FePer1, contains a major open reading frame capable of encoding a polypeptide of 219 residues with a predicted molecular mass of 24.3kDa. The deduced primary structure of FePer1 polypeptide shows a high level (about 70%) of sequence homology to other recently identified plant 1-Cys peroxiredoxins. FePer1 also exhibits a significant level of sequence similarity to non-plant 1-Cys peroxiredoxins, sharing 52 and 42% identities with mammalian and fungal 1-Cys peroxiredoxins, respectively. As for all 1-Cys peroxiredoxins identified from various organisms, the amino acid sequence proposed to constitute the active site of the enzyme is highly conserved in FePer1 polypeptide. The gene corresponding to FePer1 cDNA is a single-copy gene in the buckwheat genome. Its expression is regulated in a seed-specific and temporal manner during seed development. FePer1 gene is induced transiently for a short period immediately after seed imbibition.

Microbial relatives of the seed storage proteins of higher plants: conservation of structure and diversification of function during evolution of the cupin superfamily

This review summarizes the recent discovery of the cupin superfamily (from the Latin term "cupa," a small barrel) of functionally diverse proteins that initially were limited to several higher plant proteins such as seed storage proteins, germin (an oxalate oxidase), germin-like proteins, and auxin-binding protein. Knowledge of the three-dimensional structure of two vicilins, seed proteins with a characteristic beta-barrel core, led to the identification of a small number of conserved residues and thence to the discovery of several microbial proteins which share these key amino acids. In particular, there is a highly conserved pattern of two histidine-containing motifs with a varied intermotif spacing. This cupin signature is found as a central component of many microbial proteins including certain types of phosphomannose isomerase, polyketide synthase, epimerase, and dioxygenase. In addition, the signature has been identified within the N-terminal effector domain in a subgroup of bacterial AraC transcription factors. As well as these single-domain cupins, this survey has identified other classes of two-domain bicupins including bacterial gentisate 1, 2-dioxygenases and 1-hydroxy-2-naphthoate dioxygenases, fungal oxalate decarboxylases, and legume sucrose-binding proteins. Cupin evolution is discussed from the perspective of the structure-function relationships, using data from the genomes of several prokaryotes, especially Bacillus subtilis. Many of these functions involve aspects of sugar metabolism and cell wall synthesis and are concerned with responses to abiotic stress such as heat, desiccation, or starvation. Particular emphasis is also given to the oxalate-degrading enzymes from microbes, their biological significance, and their value in a range of medical and other applications.

Isolation and characterization of polymorphic cDNAs partially encoding ADP-glucose pyrophosphorylase (AGPase) large subunit from sweet potato

cDNA clones encoding sweet potato AGPase large subunit (iAGPLI) from the cDNA library constructed from the tuberous root were isolated. Two clones were characterized and named iAGPLI-a and iAGPLI-b. They were 1,661 bp and 1,277 bp in length and contained partial open reading frames of 450 and 306 amino acids, respectively. Both nucleic acid and amino acid sequence identities between iAGPLI-a and iAGPLI-b were 83.8% and 97.3%, respectively. Based on the amino acid sequence analysis, iAGPLI-a and iAGPLI-b share the highest sequence identity (81%) with potato AGPase large subunit. The iAGPLI-a and iAGPLI-b genes were expressed predominantly in the stem and weakly in the tuberous root, and no transcript was expressed in other tissues. The sweet potato genome contains several copies of the iAGPLI gene.

Purification and partial characterization of a second cysteine proteinase inhibitor from ungerminated barley (Hordeum vulgare L.)

It was previously shown that ungerminated barley contains inhibitors that suppress the activities of green malt cysteine proteinases. This paper reports the purification and partial characterization of a second barley cysteine endoproteinase inhibitor, a protein called lipid transfer protein 2 (LTP2). The chromatographically purified inhibitor had a molecular mass of 7112. The amino acid composition and sequence data of the purified inhibitor indicated that it was a protein whose gene, but not the protein itself, was isolated earlier from barley aleurone tissue. The purified protein inhibited the activities of electrophoretically separated green malt cysteine proteinases but not the activities of the serine- or metalloproteinases. The purified LTP2 inhibited the same proteases as the LTP1 that was characterized previously but was present in the mature seed in much smaller amounts. Neither LTP1 nor LTP2 has been proven to transport lipids in vivo, and it seems possible that both serve to keep cysteine endoproteinases that are synthesized during barley seed development inactive until the plant needs them. The small amount of LTP2 in the seed made it impossible to determine whether it, like LTP1, is involved in beer foam formation. Because of its proteinase-inhibiting ability and its resistance to heat inactivation, some of the LTP2 may persist in beer.

Is leaf ADP-glucose pyrophosphorylase an allosteric enzyme?

Barley leaf ADP-glucose pyrophosphorylase (AGPase), a key enzyme of starch synthesis in the chloroplast stroma, was analysed, in both directions of the reaction, with respect to details of its regulation by 3-phosphoglycerate (PGA) and inorganic phosphate (Pi) which serve as activator and inhibitor, respectively. AGPase was found to catalyse a close-to-equilibrium reaction, with the K(eq) value of approximately 0.5, i.e. slightly favouring the pyrophosphorolytic direction. When the enzyme was analysed by substrate kinetics, PGA acted either as a linear (hyperbolic response) 'non-competitive' activator (forward reaction) or a linear near-'competitive' activator (reverse reaction). When the activation and inhibition patterns with PGA and Pi, respectively, were studied in detail by Dixon plots, the response curves to effectors also followed hyperbolic kinetics, with the experimentally determined K(a) and K(i) values on the order of micromolar. The results suggest that the regulation of AGPase proceeds via a non-cooperative mechanism, where neither of the effectors, when considered separately, induces any allosteric response. The evidence, discussed in terms of an overall kinetic mechanism/regulation of leaf AGPase, prompts caution in classifying the protein as an 'allosteric enzyme'.

Active Fe-containing superoxide dismutase and abundant sodF mRNA in Nostoc commune (Cyanobacteria) after years of desiccation

Active Fe-superoxide dismutase (SodF) was the third most abundant soluble protein in cells of Nostoc commune CHEN/1986 after prolonged (13 years) storage in the desiccated state. Upon rehydration, Fe-containing superoxide disumutase (Fe-SOD) was released and the activity was distributed between rehydrating cells and the extracellular fluid. The 21-kDa Fe-SOD polypeptide was purified, the N terminus was sequenced, and the data were used to isolate sodF from the clonal isolate N. commune DRH1. sodF encodes an open reading frame of 200 codons and is expressed as a monocistronic transcript (of approximately 750 bases) from a region of the genome which includes genes involved in nucleic acid synthesis and repair, including dipyrimidine photolyase (phr) and cytidylate monophosphate kinase (panC). sodF mRNA was abundant and stable in cells after long-term desiccation. Upon rehydration of desiccated cells, there was a turnover of sodF mRNA within 15 min and then a rise in the mRNA pool to control levels (quantity of sodF mRNA in cells in late logarithmic phase of growth) over approximately 24 h. The extensive extracellular polysaccharide (glycan) of N. commune DRH1 generated superoxide radicals upon exposure to UV-A or -B irradiation, and these were scavenged by SOD. Despite demonstrated roles for the glycan in the desiccation tolerance of N. commune, it may in fact be a significant source of damaging free radicals in vivo. It is proposed that the high levels of SodF in N. commune, and release of the enzyme from dried cells upon rehydration, counter the effects of oxidative stress imposed by multiple cycles of desiccation and rehydration during UV-A or -B irradiation in situ.

Vacuolar H(+)-ATPase is expressed in response to gibberellin during tomato seed germination

Completion of germination (radicle emergence) by gibberellin (GA)-deficient (gib-1) mutant tomato (Lycopersicon esculentum Mill.) seeds is dependent upon exogenous GA, because weakening of the endosperm tissue enclosing the radicle tip requires GA. To investigate genes that may be involved in endosperm weakening or embryo growth, differential cDNA display was used to identify mRNAs differentially expressed in gib-1 seeds imbibed in the presence or absence of GA(4+7). Among these was a GA-responsive mRNA encoding the 16-kD hydrophobic subunit c of the V(0) membrane sector of vacuolar H(+)-translocating ATPases (V-ATPase), which we termed LVA-P1. LVA-P1 mRNA expression in gib-1 seeds was dependent on GA and was particularly abundant in the micropylar region prior to radicle emergence. Both GA dependence and tissue localization of LVA-P1 mRNA expression were confirmed directly in individual gib-1 seeds using tissue printing. LVA-P1 mRNA was also expressed in wild-type seeds during development and germination, independent of exogenous GA. Specific antisera detected protein subunits A and B of the cytoplasmic V(1) sector of the V-ATPase holoenzyme complex in gib-1 seeds only in the presence of GA, and expression was localized to the micropylar region. The results suggest that V-ATPase plays a role in GA-regulated germination of tomato seeds.

Arabidopsis thaliana AtHAL3: a flavoprotein related to salt and osmotic tolerance and plant growth

We have isolated two Arabidopsis thaliana genes, AtHAL3a and AtHAL3b, showing homology with HAL3, a yeast protein which regulates the cell cycle and tolerance to salt stress through inhibition of the PPZ1 type-1 protein phosphatase. Expression of AtHAL3a in yeast hal3 mutants partially complements their LiCl sensitivity, suggesting possible conserved functions between both proteins. AtHAL3a and AtHAL3b are induced by salt stress and AtHAL3a is the most expressed in non-stressed plants, particularly in seeds. In situ hybridization demonstrates enrichment of AtHAL3a mRNA in seed embryos and in the vascular phloem of different plant tissues. AtHAL3 proteins show striking homology with a group of proteins found in fungi, plants and animals and some homology with a large family of prokaryotic flavoproteins. Recombinant AtHAL3a protein purified from Escherichia coli was yellow because it contained a non-covalently bound chromophore revealed as flavin mononucleotide. Trans- genic Arabidopsis plants, with gain of AtHAL3a function, show altered growth rates and improved tolerance to salt and osmotic stress.

The Allosterically Unregulated Isoform of ADP-Glucose Pyrophosphorylase from Barley Endosperm Is the Most Likely Source of ADP-Glucose Incorporated into Endosperm Starch

We present the results of studies of an unmodified version of the recombinant major barley (Hordeum vulgare) endosperm ADP-glucose pyrophoshorylase (AGPase) expressed in insect cells, which corroborate previous data that this isoform of the enzyme acts independently of the allosteric regulators 3-phosphoglycerate and inorganic phosphate. We also present a characterization of the individual subunits expressed separately in insect cells, showing that the SS AGPase is active in the presence of 3-phosphoglycerate and is inhibited by inorganic phosphate. As a step toward the elucidation of the role of the two AGPase isoforms in barley, the temporal and spatial expression profile of the four barley AGPase transcripts encoding these isoforms were studied. The results show that the steady-state level of beps and bepl, the transcripts encoding the major endosperm isoform, correlated positively with the rate of endosperm starch accumulation. In contrast, blps and blpl, the transcripts encoding the major leaf isoform, were constitutively expressed at a very low steady-state level throughout the barley plant. The implications of these findings for the evolution of plant AGPases are discussed.

In vivo characterization of a thioredoxin h target protein defines a new peroxiredoxin family

Disruption of the two thioredoxin genes in yeast dramatically affects cell viability and growth. Expression of Arabidopsis thioredoxin AtTRX3 in the Saccharomyces thioredoxin Delta strain EMY63 restores a wild-type cell cycle, the ability to grow on methionine sulfoxide, and H2O2 tolerance. In order to isolate thioredoxin targets related to these phenotypes, we prepared a C35S (Escherichia coli numbering) thioredoxin mutant to stabilize the intermediate disulfide bridged complex and we added a polyhistidine N-terminal extension in order to purify the complex rapidly. Expression of this mutant thioredoxin in the wild-type yeast induces a reduced tolerance to H2O2, but only limited change in the cell cycle and no change in methionine sulfoxide utilization. Expression in the Delta thioredoxin strain EMY63 allowed us to isolate a complex of the thioredoxin with YLR109, an abundant yeast protein related to PMP20, a peroxisomal protein of Candida. No function has so far been attributed to this protein or to the other numerous homologues described in plants, animals, fungi, and prokaryotes. On the basis of the complementation and of low similarity with peroxiredoxins, we produced YLR109 and one of its Arabidopsis homologues in E. coli to test their peroxiredoxins activity. We demonstrate that both recombinant proteins present a thioredoxin-dependent peroxidase activity in vitro. The possible functions of this new peroxiredoxin family are discussed.

The dormancy-related peroxiredoxin anti-oxidant, PER1, is localized to the nucleus of barley embryo and aleurone cells

Protection against desiccation-induced injury, including damage by reactive oxygen species (ROS), is a necessary component of the genetic programmes active during late seed development. Likewise, protection against ROS respiration by-products is required during seed imbibition and germination. Late embryogenesis abundant (LEA) proteins are proposed to protect seed tissues against desiccation-induced damage. Specifically, the atypical Lea gene Per1 in barley (Hordeum vulgare L.) has been proposed to play a protective role in embryo and aleurone cells against free-radical damage during late seed development and early imbibition. PER1 represents a subgroup of the peroxiredoxin family of thiol-requiring anti-oxidants with one conserved cysteine residue (1-Cys), and displays in vitro anti-oxidant activity. In this work, we use antiserum generated against PER1 to study protein accumulation patterns as well as localization at the tissue, cellular and subcellular level. While previous studies have shown the Per1 transcript to be dormancy-related, we show here that the protein level is maintained in imbibed dormant seeds, but not in non-dormant seeds. Our data identify the location of this seed-specific peroxiredoxin as the nucleus of immature embryos and aleurone layers. Highest levels of protein are detected in nucleoli. In contrast, in mature imbibed dormant seeds, cytosolic levels are comparable to that of the nucleus. A putative nuclear localization signal (NLS) of bipartite nature was identified in the C-terminal end of the PER1 sequence. Protective roles for PER1 in seeds are discussed.

Cloning and expression of a new isotype of the peroxiredoxin gene of Chinese cabbage and its comparison to 2Cys-peroxiredoxin isolated from the same plant

A cDNA encoding a newly identified isotype of peroxiredoxin (Prx) was isolated from a Chinese cabbage flower bud cDNA library and designated CPrxII. Database searches using the predicted CPrxII amino acid sequence revealed no substantial homology to other proteins with the exception of the yeast type II Prx with which CPrxII shares 27.8% sequence identity. Recombinant CPrxII expressed in Escherichia coli was able to protect glutamine synthetase from inactivation in a metal-catalyzed oxidation system and to reduce H2O2 with electrons provided by thioredoxin. This specific antioxidant activity of CPrxII was about 6-fold higher than that of 2Cys-Prx of the same plant. In contrast to 2Cys-Prx, which is predominantly expressed in leaf tissue of cabbage seedlings, CPrxII is highly expressed in root tissue as revealed by Northern and Western blot analyses. The CPrxII gene exists as a small multigene family in the cabbage genome.

Peroxiredoxin antioxidants in seed physiology

Peroxiredoxins are thiol–requiring antioxidants found in organisms ranging from bacteria to humans. They can be divided into two subgroups with either one or two conserved cysteine residues. In plants, 1–Cys peroxiredoxins have been identified in a number of grasses and cereals, and in the dicotyledonous speciesArabidopsis thaliana. In contrast to other antioxidants, the 1–Cys peroxiredoxin genes are expressed solely in seeds, and only in the parts of the seeds surviving desiccation, i.e. the embryo and the aleurone layer. The expression pattern is characteristic of late embryogenesis–abundant genes. The PER1 protein of barley is present in high concentrations in the nucleus at the onset of desiccation. 1–Cys genes are expressed in a dormancy–related manner in mature seeds, in that transcript levels are high in imbibed dormant seeds, but disappear upon germination of their non–dormant counterparts. 1–Cys transcript levels can be up–regulated by ABA and osmotic stresses and suppressed by gibberellic acid. Two hypotheses have been put forward on the function of 1–Cys peroxiredoxins in seed physiology. First, these proteins might protect macromolecules of embryo and aleurone cells against damaging reactive oxygen species during seed desiccation and early imbibition. And second, seed peroxiredoxins might play a role in the maintenance of dormancy. These hypotheses are discussed, taking into account present knowledge of the biochemistry and molecular biology of peroxiredoxins.

Expression of barley ADP-glucose pyrophosphorylase in Escherichia coli: processing and regulatory considerations

Full length cDNAs for barley ADP-glucose pyrophosphorylase (AGPase) coding for the large subunits of the endosperm and leaf homologues of the enzyme (AGPase-S1 and -S2, respectively) and for the small subunit protein from endosperm (AGPase-B1), have been expressed in Escherichia coli. The cDNAs for AGPase-S1 and -S2 required different induction conditions for their maximal expression and they encoded immunologically distinct proteins. The AGPase-S1 that was produced by E. coli had the same M(r) (58 kDa) as the corresponding protein in barley crude endosperm extracts, whereas the bacteria-produced AGPase-S2 (55 kDa) was larger than its counterpart from barley leaf preparations (53 kDa). An enzymatically active AGPase expressed in E. coli from a double construct containing cDNAs for AGPase-S1 and -B1 subunits was insensitive to the activation by 3-phosphoglycerate and to inhibition by inorganic phosphate, similarly to the enzyme in barley endosperm. Neither AGPase-S1 nor -B1 were active when expressed alone in the bacteria. The data are discussed with respect to possible mechanisms of intracellular targeting of immature AGPase-S proteins in barley tissues and regarding previous data on effector regulation of the barley enzyme.

Sugars and light/dark exposure trigger differential regulation of ADP-glucose pyrophosphorylase genes in Arabidopsis thaliana (thale cress)

Expression of four Arabidopsis (thale cress) genes corresponding to the small (ApS) and large subunits (ApL1, ApL2, ApL3) of ADP-glucose pyrophosphorylase (AGPase), a key enzyme of starch biosynthesis, was found to be profoundly and differentially regulated by sugar and light/dark exposures. Transcript levels of both ApL2 and ApL3, and to a lesser extent ApS, increased severalfold upon feeding sucrose or glucose to the detached leaves in the dark, whereas the mRNA content for ApL1 decreased under the same conditions. Glucose was, in general, less effective than sucrose in inducing regulation of AGPase genes, possibly due to observed limitations in its uptake when compared with sucrose uptake by detached leaves. Osmotic agents [sorbitol, poly(ethylene glycol)] had no effect on ApS, ApL2 and ApL3 transcript level, but they did mimic the effect of sucrose on ApL1 gene, suggesting that the latter is regulated by osmotic pressure rather than any particular sugar. For all the genes the sugar effect was closely mimicked by an exposure of the dark-pre-adapted leaves to the light. Under both dark and light conditions, sucrose fed to the detached leaves was found to be rapidly metabolized to hexoses and, to some extent, starch. Starch production reflected most probably an increase in substrate availability for AGPase reaction rather than being due to changes in AGPase protein content, since both the sugar feeding and light exposure had little or no effect on the activity of AGPase or on the levels of its small and large subunit proteins in leaf extracts. The data suggest tight translational or post-translational control, but they may also reflect spatial control of AGPase gene expression within a leaf. The sugar/light-dependent regulation of AGPase gene expression may represent a part of a general cellular response to the availability/allocation of carbohydrates during photosynthesis.

Identification of a 28 kDa secretory protein from rat olfactory epithelium as a thiol-specific antioxidant

The 28 kDa secretory protein is one of the abundant water-soluble proteins in olfactory epithelium of mammals. Analysis of partial amino acid sequence of the 28 kDa protein strongly suggested that it belongs to a new family of highly conserved antioxidant proteins requiring thiol for their antioxidant activity (TSA/AhpC family). In the present study, we found the 28 kDa protein to have thiol-dependent antioxidant activity, thereby protecting radical-sensitive proteins such as glutamine synthetase and hemoglobin from oxidative modification caused by thiol-dependent metal ion-catalyzed oxidation system. The purified 28 kDa protein did not possess catalase or glutathione peroxidase activities, and required thiols to exhibit its antioxidant activity. The 28 kDa protein is the first member of the family of thiol-specific antioxidants identified in olfactory epithelium and the first secretory protein shown to be thiol-specific antioxidant.

PCR cloning and characterization of multiple ADP-glucose pyrophosphorylase cDNAs from tomato

Four ADP-glucose pyrophosphorylase (AGP) cDNAs were cloned from tomato fruit and leaves by the PCR techniques. Three of them (agp S1, agp S2, and agp S3) encode the large subunit of AGP, the fourth one (agp B) encodes the small subunit. The deduced amino acid sequences of the cDNAs show very high identities (96-98%) to the corresponding potato AGP isoforms, although there are major differences in tissue expression profiles. All four tomato AGP transcripts were detected in fruit and leaves; the predominant ones in fruit are agp B and agp S1, whereas in leaves they are agp B and agp S3. Genomic southern analysis suggests that the four AGP transcripts are encoded by distinct genes.

Characterization of a gene encoding an abscisic acid-inducible type-2 lipid transfer protein from rice

The cloning and sequence analysis of a novel gene that encodes a type 2 non-specific lipid transfer-like protein (LTP) from rice is reported. Sequence analysis revealed an ORF encoding a protein showing characteristics of the LTP proteins. However, rice LTP2 is more similar to heterologous LTPs than to rice LTP1, supporting the existence of two distinct families of plant LTPs. Ltp2 mRNA is accumulated only in mature seeds. In vegetative tissues, mRNA was only detected after treatment with abscisic acid (ABA), mannitol or NaCl. Transient expression experiments that the 61 nucleotides upstream of the TATA box, containing two ACGT boxes and the motif I, are sufficient for ABA responsiveness of the Ltp gene.

The expression of a peroxiredoxin antioxidant gene, AtPer1, in Arabidopsis thaliana is seed-specific and related to dormancy

We have isolated a gene, AtPer1, from the dicotyledon Arabidopsis thaliana, which shows similarity to the 1-cysteine (1-Cys) peroxiredoxin family of antioxidants. In higher plants, members of this group of antioxidants have previously only been isolated from monocotyledons. It has been suggested that seed peroxiredoxins protect tissues from reactive oxygen species during desiccation and early imbibition and/or are involved in the maintenance of/protection during dormancy. AtPer1 expression is restricted to seeds. Despite differences in seed development between monocots and dicots, AtPer1 shows an expression pattern during seed development and germination similar to the dormancy-related transcript Per1 in barley. In situ hybridization identifies AtPer1 as the first aleurone-expressed transcript characterized in developing Arabidopsis seeds. The transcript is also expressed in the embryo. AtPer1 expression in seeds is unaltered in an ABA-deficient mutant (aba-1) during seed development, while expression in seeds of an ABA-insensitive mutant (abi3-1) is reduced. The transcript is not induced in vegetative tissue in response to stress by ABA or drought. AtPer1 transcript levels are correlated to germination frequencies of wildtype seeds, but AtPer1 transcript abundance is not sufficient for expression of dormancy in non-dormant mutants. Hypotheses on peroxiredoxin function are discussed in view of the results presented here.

Characterization of cDNAs encoding small and large subunits of ADP-glucose pyrophosphorylases from watermelon (Citrullus vulgaris S.)

Three cDNA clones encoding ADP-glucose pyrophosphorylases were isolated from a full red fruit cDNA library of watermelon (Citrullus vulgaris S.). Sequence analyses indicated that one clone, wms1, corresponds to the small subunit, and two clones, wml1 and wml2 (a partial gene), are the large subunits of AGPase. The presumed AGPase proteins encoded by wms1, wml1, and wml2 have 526, 526, and 481 amino acids, respectively. The protein sequences have the conserved amino acids important for the substrate or regulator binding site, with some variation. Developmental changes in the amounts of wms1, wml1, and wml2 transcripts in fruits were measured by northern blot analysis. Their expression levels decreased from the small green to medium green stages, then increased in accordance with fruit ripening, which was different from those of tomato and oriental melon.

Molecular cloning and organ-specific expression of three isoforms of tomato ADP-glucose pyrophosphorylase gene

We isolated three cDNAs encoding different isoforms of ADP-glucose pyrophosphorylase (AGP) large submits from tomato plants. Three clones, designated AgpL1, AgpL2, and AgpL3 were 2019, 2105, and 1850bp, respectively. The clones had a long, uninterrupted open reading frame with a start codon at the 5' region and different copies of polyadenylation signal (AATAAA) at the 3' region, deriving 57-58kDa polypeptides. Sequence comparison and phylogenetic analysis revealed that the three isoforms represented different types of AGP large subunits, AgpL1 was strongly expressed in stems and weakly in roots. Accumulation of AgpL1 transcripts was found even in unpollinated ovaries and sustained at the early stages of fruit development. ApgL2 was expressed in roots and fruits. AgpL3 was exclusively expressed in leaves. The present study suggests that the three isoforms of tomato AGP large subunits are organ-specific in their expressions.

ADP-glucose pyrophosphorylase is localized to both the cytoplasm and plastids in developing pericarp of tomato fruit

The intracellular location of ADP-glucose pyrophosphorylase (AGP) in developing pericarp of tomato (Lycopersicon esculentum Mill) has been investigated by immunolocalization. With the use of a highly specific anti-tomato fruit AGP antibody, the enzyme was localized in cytoplasm as well as plastids at both the light and electron microscope levels. The immunogold particles in plastids were localized in the stroma and at the surface of the starch granule, whereas those in the cytoplasm occurred in cluster-like patterns. Contrary to the fruit, the labeling in tomato leaf cells occurred exclusively in the chloroplasts. These data demonstrate that AGP is localized to both the cytoplasm and plastids in developing pericarp cells of tomato.

Thioredoxins: structure and function in plant cells

Thioredoxins are ubiquitous small-molecular-weight proteins (typically 100-120 amino-acid residues) containing an extremely reactive disulphide bridge with a highly conserved sequence -Cys-Gly(Ala/Pro)-Pro-Cys-. In bacteria and animal cells, thioredoxins participate in multiple reactions which require reduction of disulphide bonds on selected target proteins/ enzymes. There is now ample biochemical evidence that thioredoxins exert very specific functions in plants, the best documented being the redox regulation of chloroplast enzymes. Another area in which thioredoxins are believed to play a prominent role is in reserve protein mobilization during the process of germination. It has been discovered that thioredoxins constitute a large multigene family in plants with different-subcellular localizations, a unique feature in living cells so far. Evolutionary studies based on these molecules will be discussed, as well as the available biochemical and genetic evidence related to their functions in plant cells. Eukaryotic photosynthetic plant cells are also unique in that they possess two different reducing systems, one extrachloroplastic dependent on NADPH as an electron donor, and the other one chloroplastic, dependent on photoreduced ferredoxin. This review will examine in detail the latest progresses in the area of thioredoxin structural biology in plants, this protein being an excellent model for this purpose. The structural features of the reducing enzymes ferredoxin thioredoxin reductase and NADPH thioredoxin reductase will also be described. The properties of the target enzymes known so far in plants will be detailed with special emphasis on the structural features which make them redox regulatory. Based on sequence analysis, evidence will be presented that redox regulation of enzymes of the biosynthetic pathways first appeared in cyanobacteria possibly as a way to cope with the oxidants produced by oxygenic photosynthesis. It became more elaborate in the chloroplasts of higher plants where a co-ordinated functioning of the chloroplastic and extra chloroplastic metabolisms is required. CONTENTS Summary 543 I. Introduction 544 II. Thioredoxins from photosynthetic organisms as a structural model 545 III. Physiological functions 552 IV. The thioredoxin reduction systems 556 V. Structural aspects of target enzymes 558 VI. Concluding remarks 563 Acknowledgements 564 References 564.

THE SYNTHESIS OF THE STARCH GRANULE

This review describes and discusses the implications of recent discoveries about how starch polymers are synthesized and organized to form a starch granule. Three issues are highlighted. 1. The role and importance of ADPglucose pyrophosphorylase in the generation of ADPglucose as the substrate for polymer synthesis. 2. The contributions of isoforms of starch-branching enzyme, starch synthase, and debranching enzyme to the synthesis and ordered packing of amylopectin molecules. 3. The requirements for and regulation of the synthesis of amylose.

Molecular cloning and expression of the large subunit of ADP-glucose pyrophosphorylase from barley (Hordeum vulgare) leaves

A cDNA clone, blpl14, corresponding to the large subunit of ADP-glucose pyrophosphorylase (AGPase), has been isolated from a cDNA library prepared from leaves of barley (Hordeum vulgare L.). An open reading frame encodes a protein of 503 aa, with a calculated molecular weight of 54815. The derived aa sequence contains a putative transit peptide sequence, required for targeting to plastids, and has a highly conserved positioning of critical Lys residues that are believed to be involved in effector binding. The derived aa sequence shows 97% identity with the corresponding protein from wheat, but only 36% identity with AGPase from E. coli. The blpl14 gene is expressed predominantly in leaves and to a lesser degree in seed endosperm, but not roots, of barley.

Excision of the maize transposable element Ac in periclinal chimeric leaves of 35S-Ac-rolC transgenic aspen-Populus

The transposable element Ac from maize, in combination with the phenotypic selectable marker rolC, was employed in transformation experiments of a hybrid aspen clone. A number of transgenic clones exhibited light-green sectors on green leaves. In vitro regeneration from leaves showing a high number of light-green spots resulted in R2 plants, which also showed light-green sectored leaves. However, only one out of 385 regenerated plants obtained showed green leaves. Both PCR and northern analysis indicated Ac excision and restoration of rolC expression. In Southern blot analysis of this green plant additional bands were observed as compared to the original R1 plant. The occurrence of these bands and a suggested Ac excision in the non-green L1-epidermal layer leading to periclinal chimerism of this plant is discussed.

New potential markers of in vitro tomato morphogenesis identified by mRNA differential display

The identification of plant genes involved in early phases of in vitro morphogenesis can not only contribute to our understanding of the processes underlying growth regulator-controlled determination, but also provide novel markers for evaluating the outcome of in vitro regeneration experiments. To search for such genes and to monitor changes in gene expression accompanying in vitro regeneration, we have adapted the mRNA differential display technique to the comparative analysis of a model system of tomato cotyledons that can be driven selectively toward either shoot or callus formation by means of previously determined growth regulator supplementations. Hormone-independent transcriptional modulation (mainly down-regulation) has been found to be the most common event, indicating that a non-specific reprogramming of gene expression quantitatively predominates during the early phases of in vitro culture. However, cDNA fragments representative of genes that are either down-regulated or induced in a programme-specific manner could also be identified, and two of them (G35, G36) were further characterized. One of these cDNA fragments, G35, corresponds to an mRNA that is down-regulated much earlier in callus- (day 2) than in shoot-determined explants (day 6). The other, G36, identifies an mRNA that is transiently expressed in shoot-determined explants only, well before any macroscopic signs of differentiation become apparent, and thus exhibits typical features of a morphogenetic marker.

A peroxiredoxin antioxidant is encoded by a dormancy-related gene, Per1, expressed during late development in the aleurone and embryo of barley grains

Antioxidants can remove damaging reactive oxygen species produced as by-products of desiccation and respiration during late embryogenesis, imbibition of dormant seeds and germination. We have expressed a protein, PER1, encoded by the Balem (barley aleurone and embryo) transcript previously called B15C, and show it to reduce oxidative damage in vitro. PER1 shares high similarity to a novel group of thiol-requiring antioxidants, named peroxiredoxins, and represents a subgroup with only one conserved cysteine residue (1-Cys). PER1 is the first antioxidant belonging to the 1-Cys subgroup shown to be functionally active, and the first peroxiredoxin of any kind to be functionally described in plants. The steady state level of the transcript, Per1, homologous to a dormancy-related transcript (pBS128) from bromegrass (Bromus secalinus), increases considerably in imbibed embryos from dormant barley (Hordeum vulgare L.) grains. Our investigations also indicate that Per1 transcript levels are dormancy-related in the aleurone layer of whole grains. In contrast to most seed-expressed antioxidants Per1 disappears in germinating embryos, and in the mature aleurone the transcript is down-regulated by the germinating embryo or by gibberellic acid (GA). Our data show that the barley seed peroxiredoxin is encoded by a single Per1 gene. Possible roles of the PER1 peroxiredoxin in barley grains during desiccation, dormancy and imbibition are discussed.

Isolation of molecular markers from the barley endosperm coenocyte and the surrounding nucellus cell layers

The cereal endosperm develops from a coenocyte to a cellular storage organ through formation of nucleo-cytoplasmic domains and cell wall deposition in the interzones between these domains. During its early stages, the endosperm develops in close contact with nucellus, the sporophytic tissue which gives rise to the megagametophyte. Owing to the positioning of the two tissues deeply within the ovary, neither cell types have been easily accessible for molecular studies. In this paper we report for the first time the cloning of molecular markers for the barley endosperm coenocyte and the nucellus. The novel END1 and NUC1 cDNAs were isolated by differential screening of a cDNA library from 5 DAP (days after pollination) ovaries using a positive probe from hand-dissected embryo sacs with adhering nucellus and testa cell layers, and a negative probe from pericarp. In situ and northern blot hybridization data show that END1 transcripts are asymmetrically distributed in the endosperm coenocyte limited to an area over the nucellar projection. In the cellular endosperm, END1 transcripts are present in modified aleurone cells and a few layers of ventral starchy endosperm cells. The second clone, NUC1, hybridizes to transcripts in the nucellus before fertilization and in autolyzing nucellus cells after fertilization. At later stages, after the disappearance of nucellus, NUC1 transcripts are present in the nucellar epidermis and in the lateral cells of the nucellar projection. This work provide tools for future elucidation of the genes specifying endosperm histogenesis.

Primary structure and expression of plant homologues of animal and fungal thioredoxin-dependent peroxide reductases and bacterial alkyl hydroperoxide reductases

Higher plants express genes encoding peroxiredoxins of the two-cysteine type. This is concluded from the isolation of cDNAs from spinach (Spinacia oleracea) and barley (Hordeum vulgare cv. Gerbel) which are homologous to animal, fungal, and bacterial two-cysteine peroxiredoxins. Northern blot analysis indicated the presence of at least one corresponding gene in all angiosperms analyzed suggesting that bas1 is a member of an ubiquitous gene family encoding a protein of fundamental importance in oxidative stress defense also in plants. In barley, expression increased upon application of methyl viologen but was not affected by ozone. mRNA levels increased during deetiolation in the light. Maximal abundance of bas1 transcripts was observed in young developing shoot segments where cell division and elongation take place. Expression was insignificant in roots. The amount of bas1 protein was high in the leaf blade, particularly in etiolated plants, and did not respond to oxidative stress. bas1 protein was not detected in roots. From our data, we suggest that bas1 is an antioxidant enzyme particularly important in the developing shoot and photosynthesizing leaf.

LIPID-TRANSFER PROTEINS IN PLANTS

Lipid-transfer proteins (LTP) are basic, 9-kDa proteins present in high amounts (as much as 4% of the total soluble proteinss) in higher plants. LTPs can enhance the in vitro transfer of phospholipids between membranes and can bind acyl chains. On the basis of these properties, LTPs were thought to participate in membrane biogenesis and regulation of the intracellular fatty acid pools. However, the isolation of several cDNAs and genes revealed the presence of a signal peptide indicating that LTPs could enter the secretory pathway. They were found to be secreted and located in the cell wall. Thus, novel roles were suggested for plant LTPs: participation in cutin formation, embryogenesis, defense reactions against phytopathogens, symbiosis, and the adaptation of plants to various environmental conditions. The validity of these suggestions needs to be determined, in the hope that they will elucidate the role of this puzzling family of plant proteins.

Two new oleosin isoforms with altered expression patterns in seeds of the Arabidopsis mutant fus3

Oleosins are proteins associated with lipid bodies mainly synthesised during seed development. Using a subtractive hybridisation approach two new members of the oleosin gene family of Arabidopsis thaliana have been isolated. The quantitative and temporal expression patterns of both genes are found to be affected in the fus3 mutant defective in late embryogenesis. This pattern is interpreted as a molecular marker for a mutant specific developmental change from a seed maturation to a germination pathway.