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PubMed2001

The protein storage vacuole: a unique compound organelle.

Storage proteins are deposited into protein storage vacuoles (PSVs) during plant seed development and maturation and stably accumulate to high levels; subsequently, during germination the storage proteins are rapidly degraded to provide nutrients for use by the embryo. Here, we show that a PSV has within it a membrane-bound compartment containing crystals of phytic acid and proteins that are characteristic of a lytic vacuole. This compound organization, a vacuole within a vacuole whereby storage functions are separated from lytic functions, has not been described previously for organelles within the secretory pathway of eukaryotic cells. The partitioning of storage and lytic functions within the same vacuole may reflect the need to keep the functions separate during seed development and maturation and yet provide a ready source of digestive enzymes to initiate degradative processes early in germination.

Aquaporins↗
PubMed1999

Cloning and characterization of a trypsin inhibitor cDNA from amaranth (Amaranthus hypochondriacus) seeds.

We previously isolated and sequenced the major trypsin inhibitor from Amaranthus hypochonidriacus seeds. This amaranth trypsin inhibitor (AmTI) is a 69 amino acid protein with high homology to members of the potato-1 inhibitor family. This paper describes the cloning and expression of a cDNA encoding this trypsin inhibitor in various vegetative tissues of the amaranth plant during seed development and imbibition, and investigates the possible induction of AmTI expression by wounding. We obtained a 393 bp cDNA sequence with an open reading frame corresponding to a polypeptide with 76 amino acid residues. With the exception of one residue (Ser-41), the polypeptide agrees with the amino acid sequence previously reported, plus 7 more residues at the N-terminus. These N-terminal residues are thought to be part of the signal used for intracellular sorting. The organ specificity of AmTI gene expression was investigated by northern analysis, showing that mRNA corresponding to AmTI genes was present in stems of plants growing under normal conditions. The kinetics of accumulation of the AmTI-mRNA, protein, and inhibitory activity during seed development and imbibition was determined. AmTI-mRNA accumulation reached a maximum at 14 days after anthesis (daa) and then gradually decreased, being barely detectable 36 daa. The AmTI protein accumulation followed the same profile as the inhibitory activity, both were delayed with respect to the mRNA. The maximum level was observed 22 daa, and then gradually decreased until a steady state was reached as seed maturation proceeded. Upon imbibition, a gradual decrease in AmTI protein and inhibitory activity was shown; however, an AmTI transcript was detected 24 h after imbibition. In contrast to representative members of the potato I family, this inhibitor was not inducible by wounding of leaves.

Amino Acid Sequence↗
PubMed2003

Characterization of cyclopropane fatty-acid synthase from Sterculia foetida.

Cyclopropane synthase from Sterculia foetida developing seeds catalyzes the addition of a methylene group from S-adenosylmethionine to the cis double bond of oleic acid (Bao, X., Katz, S., Pollard, M., and Ohlrogge, J. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 7172-7177). To understand this enzyme better, differential expression in leaf and seed tissues, protein properties, and substrate preferences of plant cyclopropane synthase were investigated. Immunoblot analysis with antibodies raised to recombinant S. foetida cyclopropane synthase (SfCPA-FAS) revealed that SfCPA-FAS is expressed in S. foetida seeds, but not in leaves, and is a membrane protein localized to microsomal fractions. Transformed tobacco cells expressing SfCPA-FAS were labeled in vivo with L-[methyl-(14)C]methionine and assayed in vitro with S-adenosyl-L-[methyl-(14)C]methionine. These kinetic experiments demonstrated that dihydrosterculate was synthesized from oleic acid esterified at the sn-1 position of phosphatidylcholine (PC). Furthermore, analysis of acyl chains at sn-1 and sn-2 positions that accumulated in PC from S. foetida developing seeds and from tobacco cells expressing SfCPA-FAS also demonstrated that greater than 90% of dihydrosterculate was esterified to the sn-1 position. Thus, we conclude that SfCPA-FAS is a microsomal localized membrane protein that catalyzes the addition of methylene groups derived from S-adenosyl-L-methionine across the double bond of oleic acid esterified to the sn-1 position of PC. A survey of plant and bacterial genomes for sequences related to SfCPA-FAS indicated that a peptide domain with a putative flavin-binding site is either fused to the methyltransferase domain of the plant protein or is often found encoded by a gene adjacent to a bacterial cyclopropane synthase gene.

Cell Fractionation↗
PubMed1994

Regulation of an Arabidopsis oleosin gene promoter in transgenic Brassica napus.

Progressive deletions of the 5'-flanking sequences of an Arabidopsis oleosin gene were fused to beta-glucuronidase (GUS) and introduced into Brassica napus plants using Agrobacterium-mediated transformation. The effect of these deletions on the quantitative level of gene expression, organ specificity and developmental regulation was assessed. In addition, the influence of abscisic acid (ABA), jasmonic acid (JA), sorbitol and a combined ABA/sorbitol treatment on gene expression was investigated. Sequences that positively regulate quantitative levels of gene expression are present between -1100 to -600 and -400 to -200 of the promoter. In addition, sequences present between -600 and -400 down-regulate quantitative levels of expression. In transgenic B. napus plants, the oleosin promoter directs seed-specific expression of GUS which is present at early stages of seed development and increases throughout seed maturation. Sequences present between -2500 and -1100 of the promoter are involved in modulating the levels of expression at early stages of embryo development. Histochemical staining of embryos demonstrated that expression is uniform throughout the tissues of the embryo. Sequences involved in the response to ABA and sorbitol are present between -400 and -200. The induction of GUS activity by a combined ABA/sorbitol treatment is additive suggesting that ABA is not the sole mediator of osmotically induced oleosin gene expression. A response to JA was only observed when the oleosin promoter was truncated to -600 suggesting that the reported effect of JA on oleosin gene expression may be at a post-transcriptional level.

Abscisic Acid↗
PubMed1999

Isolation of a gene encoding Arabidopsis membrane-associated acyl-CoA binding protein and immunolocalization of its gene product.

Until recently, only cytosolic acyl-CoA binding proteins (ACBPs) have been characterized. The isolation of an Arabidopsis thaliana cDNA encoding a novel membrane-associated ACBP that accumulates in developing seeds, designated ACBP1, has provided evidence for the existence of membrane-associated forms of ACBPs (Chye, 1998, Plant Mol. Biol. 38, 827-838). We now report on the isolation of its corresponding gene from an A. thaliana Columbia genomic library using the ACBP1 cDNA as a hybridization probe. Nucleotide sequence analysis of Arabidopsis ACBP1 showed that its promoter lacks a TATA box, resembling the promoters of rat, Drosophila and human genes encoding cytosolic ACBP and suggesting that it is a housekeeping gene. We show by Western blot analysis that ACBP1 expression in developing seeds coincides with lipid deposition and that homologues of membrane-associated ACBP1 exist in other plants. Using light microscopy, we show that ACBP1 is strongly expressed in the embryo at the cotyledons, hypocotyl, procambium of the axis and in most peripheral cells of the cotyledons and hypocotyl. Immunogold labelling localized ACBP1 to vesicles, to the plasma membrane especially at epidermal cells of heart, torpedo and cotyledonary stage embryos, and to the cell wall of the outer integument cells at the seed coat. Our results suggest that ACBP1 is involved in intermembrane lipid transport from the ER via vesicles to the plasma membrane where it could maintain a membrane-associated acyl pool; its immunolocalization to the cell wall of outer integument cells at the seed coat suggests a role in cuticle and cutin formation.

Amino Acid Sequence↗
PubMed2005

Genomic imprinting in plants: the epigenetic version of an Oedipus complex.

Genomic imprinting is a mitotically stable epigenetic modification that results in the functional non-equivalency of both parental genomes following fertilization. In flowering plants, studies of parent-of-origin effects have mostly identified genes that are only transcribed from a maternally inherited allele. In Arabidopsis, the Polycomb group protein MEDEA regulates seed development through the expression of the MADS-box gene PHERES1. Activation of the maternal MEDEA allele requires the function of DEMETER, a plant DNA glycosylase that also controls the transcriptional activity of the maternally inherited allele of the late-flowering gene FWA. Current studies of parent-of-origin effects have mostly identified genes that are only transcribed from a maternally inherited allele. Our current understanding of parent-of-origin effects could represent a new form of an Oedipus complex in which flowering plants prefer to rely transcriptionally on their maternal rather than their paternal chromosomes to ensure normal initiation of seed development.

Gene Expression Regulation, Plant↗
PubMed1994

3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Activity in the Endosperm of Maize vivipary Mutants.

During seed maturation the levels of the rate-limiting enzyme of isoprenoid biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) decrease while the levels of the isoprenoid compund abscisic acid (ABA) increase. In the present study, we demonstrate that HMGR specific activity is inversely correlated with endogenous ABA levels in Zea mays endosperm during seed development. HMGR specific activity and ABA levels were measured in the endosperm of the maize vivipary mutuants vp2, vp5, and vp7, which are defective in ABA biosynthesis, and vp1, which is defective in an ABA response element. Reduced ABA levels were observed in the endosperm of vp2, vp5, and vp7, whereas HMGR activity levels were higher compared with wild-type sibling endosperm activity. HMGR activities were increased by as much as 37% (vp2), 45% (vp5), and 58% (vp7) in the mutants. Endosperm HMGR activity in the vp1 mutant was also increased (41%) relative to wild-type siblings, even though vp1 does not have reduced ABA levels. In addition, exogenous ABA inhibits HMGR activity 34 to 50% in maize roots. These results sugggest the HMGR activity levels during seed development are regulated via a Vp1-dependent signal transduction pathway that is affected by the reduced ABA content of vp2, vp5, and vp7 endosperm.

Journal Article↗
PubMed1980

Changes in Activities of Enzymes of Nitrogen Metabolism in Seedcoats and Cotyledons during Embryo Development in Pea Seeds.

In the seedcoats of developing pea seeds, the maximal activities of asparaginase (EC 3.5.1.1) and aspartate: alpha-ketoglutarate aminotransferase (EC 2.6.1.1) are attained early in development, before the embryo has expanded to fill the embryo sac. These two enzyme activities could account for the early absence of asparagine and aspartate from the fluid secreted by the seedcoats into the embryo sac.CHANGES IN THE ACTIVITIES OF ALANINE: alpha-ketoglutarate aminotransferase (EC 2.6.1.2), glutamate dehydrogenase (EC 1.4.1.3), glutamine synthetase (EC 6.3.1.2), and glutamate synthase (EC 1.4.1.13) have also been measured, in cotyledons as well as seedcoats. On a fresh weight basis, the highest activities of asparaginase and both aminotransferases developed in the seedcoats, whereas the highest activities of the remaining enzymes developed in the cotyledons.The data indicate that the amide groups of imported asparagine and glutamine are metabolized differently, largely by asparaginase and glutamate synthase, respectively. The NH(4) (+) released by the action of asparaginase is evidently reassimilated in cotyledon cells by the joint action of glutamate dehydrogenase, glutamine synthetase, and glutamate synthase. The data emphasize the central importance of alpha-ketoglutarate-glutamate cycling in the redistribution of amino groups associated with the net synthesis of amino acids and reserve proteins.

Journal Article↗
PubMed1995

Biochemical and molecular characterization of a barley seed beta-glucosidase.

A 60-kDa beta-glucosidase (BGQ60) was purified and characterized from seeds of barley (Hordeum vulgare L.). BGQ60 catalytic activity was restricted to the cleavage of short-chain oligosaccharides composed of (1-2)-, (1-3)-, and/or (1-4)-beta-linked glucose or mannose units. These oligosaccharides are the primary products of endosperm cell wall polysaccharide hydrolysis by other enzymes. In keeping with this, complete hydrolysis of the major polysaccharide of barley starchy endosperm cell wall, (1-3,1-4)-beta-glucan, to free glucose was shown to require the concerted action of endo-(1-3,1-4)-beta-glucanase and BGQ60. The complete amino acid sequence of BGQ60 was determined by protein sequencing combined with the deduced sequence of the corresponding cDNA and genomic clones. The BGQ60 primary structure exhibits extensive homology to members of glycosyl hydrolase family 1 (EC 3.2.1.21). Southern and Northern blot analysis with the cDNA as probe indicated that BGQ60 is encoded by a single gene, and that BGQ60 mRNA only accumulates in the starch endosperm tissue of late developing seeds. The bgq60 structural gene of approximately 5 kilobases contains an open reading frame encoding 485 amino acids interrupted by 9 introns. The complete nucleotide sequence of the bgq60 structural gene represents the first characterized plant gene encoding a beta-glucosidase. The barley BGQ60 is a novel plant beta-glucosidase with a hitherto undescribed specific enzymatic activity. The possible biological functions of BGQ60 during barley seed development and germination are discussed.

Amino Acid Sequence↗
PubMed1983

Regulation of Legumin Levels in Developing Pea Seeds under Conditions of Sulfur Deficiency: Rates of Legumin Synthesis and Levels of Legumin mRNA.

It was shown previously that when peas (Pisum sativum L.) are grown with suboptimal sulfur supply the level of legumin (the more S-rich of the two major seed storage proteins) in the mature seed is selectively reduced (Randall, Thomson, Schroeder, 1979 Aust J Plant Physiol 6: 11-24). This paper reports a study of the cellular mechanisms involved in regulating legumin synthesis under these conditions. Pulse and pulse-chase labeling experiments were carried out with excised, immature cotyledons from normal and S-deficient plants. Legumin was isolated from cotyledon extracts by immunochromatography, and the proportion of legumin synthesis relative to total protein synthesis was determined. Results showed that reduced legumin accumulation could largely be accounted for by a greatly reduced level of legumin synthesis (80-88% reduction) rather than by a major increase in legumin breakdown.Legumin mRNA levels were assayed by two methods. In vitro translation of polysomal RNA from cotyledons of normal and S-deficient plants indicated a reduction of 60 to 70% in synthesis of legumin-related products by preparations from S-deficient plants. A legumin cDNA clone was constructed, characterized, and used to measure the levels of legumin mRNA in polysomal and total RNA preparations from developing cotyledons. Legumin mRNA levels were reduced by 90% in preparations from S-deficient plants.When restored to an adequate S supply, S-deficient plants (or pods taken from such plants) recovered normal levels of legumin synthesis (in vivo and in vitro) and of legumin mRNA. These results indicate that reduced legumin accumulation under conditions of S deficiency is primarily a consequence of reduced levels of legumin mRNA.

Journal Article↗
PubMed1968

Physical forces in dormancy and germination of xanthium seeds.

The germination of seeds of Xanthium pensylvanicum Wallr. occurs in 2 phases, an initial passive phase of water uptake followed by an active phase of growth. These 2 phases have been separated experimentally, and shown to occur similarly in isolated cotyledons and embryonic axes. Measurements of the physical thrust generated by the entire seed and its separate components of cotyledon and axis reveal that non-dormant Xanthium seeds develop more than twice the thrust of dormant seeds, and that this difference develops principally in the second phase of enlargement of the axis. Measurement of the forces required for piercing the testa of these seeds establishes that whereas the thrust developed by non-dormant seed is adequate to cause testa rupture, that developed by dormant seeds is not. It is concluded that the dormancy of Xanthium involves an inadequacy in the embryo for rupture of the testa.

Journal Article↗
PubMed1994

Tobacco cytochrome b5: cDNA isolation, expression analysis and in vitro protein targeting.

A full-length clone encoding cytochrome b5 has been isolated from a tobacco leaf cDNA library in lambda gt11 by PCR using degenerate primers. This cDNA encodes a protein of 139 residues which exhibits a high degree of homology to other cytochrome b5s, the message for which is expressed predominantly in developing seeds and in pigmented flower tissue. In the developing tobacco seed the mRNA is abundant at very early stages (< 10 days after flowering). Southern analysis indicated that more than one gene encodes cytochrome b5 in the tobacco genome. In vitro transcription and translation studies of the cDNA indicated that the protein inserts into the ER membrane by a non-SRP-mediated pathway and that the C-terminus of the protein is required for targeting and insertion.

Amino Acid Sequence↗
PubMed1996

Phosphatidate phosphatases of mammals, yeast, and higher plants.

Phosphatidate phosphatase (EC 3.1.3.4) catalyzes the hydrolysis of phosphatidate to yield sn-1,2-diacylglycerol and inorganic phosphate. In mammalian systems, forms of phosphatidate phosphatase involved in glycerolipid synthesis and signal transduction have been identified. Forms of the enzyme involved in signal transduction have been purified and partially characterized. In yeast, phosphatidate phosphatases associated with the endoplasmic reticulum and mitochondria have also been purified and partially characterized. Information on phosphatidate phosphatases from mammals and yeast is useful in characterizing the enzyme from plant systems. This review examines progress on the characterization of phosphatidate phosphatases from mammals, yeast, and higher plants. The purification and characterization of the phosphatidate phosphatase involved in glycerolipid synthesis in developing oilseeds may lead to the identification of the encoding gene. Increasing our understanding of the enzymes of lipid synthesis in developing seeds will aid in the development of biotechnological strategies for seed oil modification.

Animals↗
PubMed1999

Auxin redistributes upwards in graviresponding gynophores of the peanut plant.

The peanut (Arachis hypogaea L.) produces flowers aerially, but buries the recently fertilized ovules into the soil, where fruit and seed development occur. The young seeds are carried down into the soil at the tip of a specialized organ called the gynophore. Although the gynophore has a typical shoot anatomy, it responds positively to gravity like a root. In this study, we explore the role of the plant growth regulator indole-3-acetic acid (IAA) in the growth and the gravitropic response of the peanut gynophore. With an immunolocalization technique using an IAA monoclonal antibody, we localized IAA within the tissues of vertically oriented and gravistimulated gynophores. We found that in vertically oriented gynophores, IAA labeling occurs in the periphery of the gynophore, in the entire cortex and epidermis. Within 20 min of horizontal reorientation, the IAA signal gradually increases in the upper cortex/ epidermis and diminishes in the lower cortex/epidermis. At 1.5 h after gravistimulation, all of the IAA immunolocalization signal is detected in the upper cortex and epidermis--none is detected in the lower side. Growth rate measurements also indicate that after 1-2 h of reorientation, the growth rate maximum on the upper side corresponds temporally and spatially to the growth rate minimum on the lower side. Experiments using radioactively labeled IAA corroborate an upper-side redistribution of this hormone upon horizontal reorientation. These results are analyzed with respect to the current theories of plant gravitropic response, and a model for a possible gravity-induced IAA redistribution from the lower to the upper side of the peanut gynophore is proposed.

Arachis↗