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1.
Biochim Biophys Acta ; 1076(3): 395-400, 1991 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-2001387

RESUMO

In the biosynthesis of proteins, each amino acid passes from the aminoacyl adenylate to become an amino acid ester and finally a 2' (3') peptidyl ester of the AMP residue at the end of a tRNA. Consequently, the chemistry of protein synthesis is the chemistry of aminoacyl and peptidyl AMP. Our data has revealed properties of 5'-AMP and its esters which should allow the preferential catalytic synthesis of L-amino acid peptides via a bis(2', 3'-aminoacyl) ester intermediate. Results in this paper concern one step in the proposed process and show that preexisting Ac-L-Phe monoester reacts about 2.5-times faster to form diester than preexisting Ac-D-Phe monoester.


Assuntos
Monofosfato de Adenosina , Monofosfato de Adenosina/análogos & derivados , Peptídeos/síntese química , Fenilalanina/análogos & derivados , Monofosfato de Adenosina/síntese química , Ésteres , Indicadores e Reagentes , Isomerismo , Cinética , Espectroscopia de Ressonância Magnética
2.
Bioscience ; 46(5): 365-9, 1996 May.
Artigo em Inglês | MEDLINE | ID: mdl-11539194

RESUMO

NASA: The Krogh principle refers to the use of a large number of animals to study the large number of physiological problems, rather than limiting study to a particular organism for all problems. There may be organisms that are more suited to study of a particular problem than others. This same principle applies to plants. The authors are concerned with the recent trend in plant biology of using Arabidopsis thaliana as the "organism of choice." Arabidopsis is an excellent organism for molecular genetic research, but other plants are superior models for other research areas of plant biology. The authors present examples of the successful use of the Krogh principle in plant cell biology research, emphasizing the particular characteristics of the selected research organisms that make them the appropriate choice.^ieng


Assuntos
Arabidopsis/fisiologia , Botânica/métodos , Células Vegetais , Plantas/metabolismo , Acetabularia , Animais , Arabidopsis/citologia , Arabidopsis/genética , Arabidopsis/metabolismo , Botânica/tendências , Respiração Celular , Chlorella , Modelos Biológicos , Neurospora , Fotossíntese/fisiologia , Fenômenos Fisiológicos Vegetais , Plantas/genética , Projetos de Pesquisa , Zea mays
3.
Braz J Med Biol Res ; 22(8): 931-44, 1989.
Artigo em Inglês | MEDLINE | ID: mdl-2699261

RESUMO

1. Comparisons were made of the results of searches within and among different species of organisms for sequence matches between transfer RNAs and ribosomal RNAs. The purpose was to determine whether the matching sequences might result from selection acting on the two RNAs within a common cellular environment. 2. The results indicate that most matches do not reflect such selection. The matches described were more frequent than those found in searches among randomized sequences and the frequency of intraspecific matches was not significantly higher than that of interspecific matches. 3. The matches are thought to identify conserved vestiges of a molecule or molecules ancestral to both classes of RNAs (Bloch, D.P., McArthur, B. and Mirrop, S. (1985). BioSystems, 17: 209-225). The matching sequences are interpreted as homologies.


Assuntos
Sequência de Bases , Escherichia coli/genética , RNA Ribossômico/genética , RNA de Transferência/genética , Saccharomyces cerevisiae/genética , Homologia de Sequência do Ácido Nucleico , Anticódon/genética , Código Genético , Halobacterium/genética , Mitocôndrias/análise , Probabilidade
4.
Gravit Space Biol Bull ; 10(2): 57-64, 1997 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-11540121

RESUMO

The statolith hypothesis for gravity sensing is the most accepted model of gravity sensing in plants. However, the lukewarm acceptance of the statolith hypothesis among plant physiologists is most likely due to inertia and not due to any one experiment done over the past one hundred years that unambiguously pitted the statolith hypothesis against any of the competing hypotheses. Here we emphasize the weaknesses of the statolith hypothesis and suggest that the gravitational pressure hypothesis is superior in describing gravisensing in all plant cells.


Assuntos
Gravitação , Sensação Gravitacional/fisiologia , Modelos Biológicos , Fenômenos Fisiológicos Vegetais , Fenômenos Biofísicos , Biofísica , Canais de Cálcio , Membrana Celular/fisiologia , Clorófitas , Força Compressiva , Citoplasma/fisiologia , Transferência de Energia , Matriz Extracelular/fisiologia , Plastídeos/fisiologia
5.
Z Naturforsch C J Biosci ; 44(3-4): 296-306, 1989.
Artigo em Inglês | MEDLINE | ID: mdl-2742690

RESUMO

As part of a continuing study on the origin of genetic coding and the process of protein synthesis, we have compared sequences of a large number of transfer RNAs and several 5S ribosomal RNAs using automated routines. Transfer RNAs were found to exhibit a high degree of matching with 5S rRNAs. These matches are considered to be indicative of sequence homology, reflecting common ancestry of the two molecules. Other possible explanations for the matches (convergence, transfection) are discussed and found to be highly implausible. Matches are also found between 5S rRNA and the introns of yeast precursor tRNAs. Many matches extend from before the 3' end to after the 5' end of circularized 5S rRNA sequences. Data is presented which indicates a tandemly duplicated or circular molecule could have served as the precursor to both 5S rRNA and tRNA. A derivative of this molecule may have functioned as a universal translator before the evolution of the highly specialized tRNAs.


Assuntos
Evolução Biológica , RNA Ribossômico 5S/genética , RNA Ribossômico/genética , RNA de Transferência/genética , Animais , Íntrons , Precursores de RNA/genética , Homologia de Sequência do Ácido Nucleico , Especificidade da Espécie
6.
Z Naturforsch C J Biosci ; 43(5-6): 476-8, 1988.
Artigo em Inglês | MEDLINE | ID: mdl-3048007

RESUMO

Corresponding sites of the Euglena chloroplast and yeast small subunit ribosomal RNAs (rRNAs) show only an insignificant match with each other but show extensive matches with Euglena chloroplast tRNA(arg). The match with the tRNA extends farther toward the 5' end of the Euglena rRNA and toward the 3' end of the yeast rRNA. The expected number of such configurations given the number of RNAs searched is about 1 in 100,000. Comparison of two sequences with a third sequence frequently reveals relationships where pairwise comparisons fail to do so.


Assuntos
Evolução Biológica , RNA Ribossômico/genética , RNA de Transferência/genética , Animais , Sequência de Bases , Cloroplastos/análise , Euglena/genética , Dados de Sequência Molecular , Saccharomyces cerevisiae/genética , Homologia de Sequência do Ácido Nucleico
7.
Z Naturforsch C J Biosci ; 42(1-2): 129-33, 1987.
Artigo em Inglês | MEDLINE | ID: mdl-2953129

RESUMO

Two E. coli tRNA(Ile) sequences were compared against those of 36 other E. coli tRNAs. tRNA(Ile) 1 was found to bear high similarity with tRNA(Val) 1 (E = 1.11 X 10(-18] while tRNA(Ile) 2 had the greatest match (E = 3.40 X 10(-19] with tRNA(Lysl) (E is the expected number of such matches, per search, based on coincidence). These matches, which we consider to represent homologies, extend from base 7 to base 67 in the former and base 7 to the end (76) in the latter pair. These results coupled with others on the lower activity of isoleucine in reactions postulated to be important in primitive protein synthesis (i.e., esterification reactions and non-enzymatic activation by ATP [1-3]) lead us to propose that isoleucine was included among the proteinaceous amino acids, and received its anticodonic assignment, relatively late in evolution through mutation of tRNAs previously employed for other amino acids.


Assuntos
Evolução Biológica , Escherichia coli/genética , Aminoacil-RNA de Transferência/genética , RNA de Transferência/genética , Sequência de Bases , Códon
9.
Planta ; 203(Suppl 1): S79-84, 1997 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-11540332

RESUMO

Since the nineteenth century, the merits of two alternate models for explaining the mechanism of plant gravity perception have been discussed. The gravitational pressure model states that plant cells perceive gravity by sensing their relative buoyancy to that of the surrounding medium, whereas the more popular starch-statolith model states that intracellular sedimenting particles act as gravity sensors. Vertically-oriented Chara internodal cells exhibit a gravity dependent polarity of cytoplasmic streaming such that the downwardly-directed stream moves ca. 10% faster than the upwardly-directed stream. This polarity of cytoplasmic streaming is not simply a consequence of gravity acting directly on the cytoplasm but is rather under physiological control. When Chara internodal cells are placed in a medium more dense than themselves, the gravity-induced polarity of cytoplasmic streaming is reversed. This phenomenon cannot be explained by a model which relies on intracellular sedimenting particles as gravity sensors but is consistent with the gravitational pressure model for gravity sensing. We propose that gravity causes the internodal cells to settle within the confines of the extracellular matrix resulting in a tension between the plasma membrane and the extracellular matrix at the top of the cell and a compression between the plasma membrane and the extracellular matrix at the bottom of the cell. These stresses are proposed to act upon peptides which span the plasma membrane/extracellular matrix interface at the ends of the cells and which subsequently activate Ca2+ channels which in turn may induce a polarity of cytoplasmic streaming.


Assuntos
Polaridade Celular , Clorófitas/citologia , Corrente Citoplasmática/fisiologia , Sensação Gravitacional/fisiologia , Protoplastos/fisiologia , Cálcio/metabolismo , Clorófitas/metabolismo , Clorófitas/fisiologia , Corrente Citoplasmática/efeitos dos fármacos , Pressão Hidrostática , Oligopeptídeos/farmacologia , Proteínas de Plantas/metabolismo , Proteínas de Plantas/fisiologia , Receptores de Peptídeos/metabolismo , Receptores de Peptídeos/fisiologia , Estrôncio/metabolismo
10.
Plant Cell Physiol ; 32(8): 1137-44, 1991.
Artigo em Inglês | MEDLINE | ID: mdl-11537171

RESUMO

We measured the densities of the cell sap, endoplasm and cell wall of Nitellopsis obtusa and Chara corallina using interference microscopy, refractometry, immersion refractometry, equilibrium sedimentation and chemical microanalysis techniques. These values are important for the determination of many rheological properties of the cytoplasm as well as for understanding buoyancy regulation, dispersal mechanisms and how cells respond to gravity. The average densities of the cell sap, endoplasm and cell wall are 1,006.9, 1,016.7 and 1,371 kg m-3 for Nitellopsis and 1,005.0, 1,013.9, and 1,355.3 kg m-3 for Chara.


Assuntos
Clorófitas/química , Clorófitas/citologia , Citoplasma/química , Parede Celular/química , Parede Celular/ultraestrutura , Clorófitas/fisiologia , Citoplasma/ultraestrutura , Gravitropismo/fisiologia , Sensação Gravitacional/fisiologia , Microscopia de Interferência , Organelas , Refratometria , Reologia
11.
Orig Life Evol Biosph ; 20(3-4): 303-8, 1990.
Artigo em Inglês | MEDLINE | ID: mdl-2290687

RESUMO

It is generally true that evolving systems begin simply and become more complex in the evolutionary process. For those who try to understand the origin of a biochemical system, what is required is the development of an idea as to what simpler system preceeded the present one. Here we present an hypothesis that a universal tRNA molecule, capable of reading many codons may have preceeded the appearance of individual tRNAs. Evidence seems to suggest that this molecule may have been derived from a common ancestor of the contemporary 5S rRNAs and tRNAs.


Assuntos
Evolução Biológica , Origem da Vida , RNA de Transferência/genética , Modelos Biológicos , Biossíntese de Proteínas , RNA Ribossômico 5S/genética
12.
Physiol Plant ; 98(4): 917-21, 1996 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-11539338

RESUMO

NASA: The physiology of gravity perception in plants is examined and a model of gravitational pressure is explained and compared to the statolith model. The gravitational pressure model is based on studies of tension and compression of the plasma membrane against the extracellular matrix. Further studies examine the role of peptides or enzymes that inhibit a compression receptor and calcium channels.^ieng


Assuntos
Clorófitas/citologia , Sensação Gravitacional/fisiologia , Plastídeos/fisiologia , Protoplastos/fisiologia , Fenômenos Biofísicos , Biofísica , Cálcio/fisiologia , Membrana Celular/fisiologia , Clorófitas/crescimento & desenvolvimento , Clorófitas/fisiologia , Corrente Citoplasmática/fisiologia , Matriz Extracelular/fisiologia , Amido/deficiência , Amido/fisiologia
13.
Protoplasma ; 155: 43-57, 1990.
Artigo em Inglês | MEDLINE | ID: mdl-11540068

RESUMO

The internodal cells of the characean alga Nitellopsis obtusa were chosen to investigate the effect of gravity on cytoplasmic streaming. Horizontal cells exhibit streaming with equal velocities in both directions, whereas in vertically oriented cells, the downward-streaming cytoplasm flows ca. 10% faster than the upward-streaming cytoplasm. These results are independent of the orientation of the morphological top and bottom of the cell. We define the ratio of the velocity of the downward- to the upward-streaming cytoplasm as the polar ratio (PR). The normal polarity of a cell can be reversed (PR < 1) by treatment with neutral red (NR). The NR effect may be the result of membrane hyperpolarization, caused by the opening of K+ channels. The K+ channel blocker TEA Cl- inhibits the NR effect. External Ca2+ is required for normal graviresponsiveness. The [Ca2+] of the medium determines the polarity of cytoplasmic streaming. Less than 1 micromole Ca2+ resulted in a PR < 1 while greater than 1 micromole Ca2+ resulted in the normal gravity response. The voltage-dependent Ca(2+)-channel blocker, nifedipine, inhibited the gravity response in a reversible manner, while treatment with LaCl3 resulted in a PR < 1, indicating the presence of two types of Ca2+ channels. A new model for graviperception is presented in which the whole cell acts as the gravity sensor, and the plasma membrane acts as the gravireceptor. This is supported by ligation and UV irradiation experiments which indicate that the membranes at both ends of the cell are required for graviperception. The density of the external medium also affects the PR of Nitellopsis. Calculations are presented that indicate that the weight of the protoplasm may provide enough potential energy to open ion channels.


Assuntos
Polaridade Celular/fisiologia , Clorófitas/fisiologia , Corrente Citoplasmática/fisiologia , Gravitação , Sensação Gravitacional/fisiologia , Cálcio/farmacologia , Membrana Celular/fisiologia , Canais de Cloreto/fisiologia , Clorófitas/citologia , Clorófitas/efeitos dos fármacos , Corrente Citoplasmática/efeitos dos fármacos , Gravitropismo/efeitos dos fármacos , Gravitropismo/fisiologia , Potenciais da Membrana/fisiologia , Vermelho Neutro/farmacologia , Nifedipino/farmacologia , Canais de Potássio/fisiologia , Transdução de Sinais/fisiologia , Tetraetilamônio , Compostos de Tetraetilamônio/farmacologia
14.
Orig Life Evol Biosph ; 18(1-2): 97-105, 1988.
Artigo em Inglês | MEDLINE | ID: mdl-3285285

RESUMO

Earlier studies (1) have shown there are direct correlations between the hydrophobicity ranking of most amino acids and their anticodonic nucleotides. However, four anticodonic assignments, i.e. those for Trp, Tyr, Ile and the XGA anticodons for Ser, did not correlate. It was our proposal that this failure to correlate was due to the fact that these assignments were made late, relative to the bulk of the assignments, in evolution through the mutation of existing tRNAs. We have shown (2) that E. coli tRNA(Ile 1) and tRNA(Ile 2) were likely derived from tRNA(Val 1) and tRNA(Lys) respectively and E. coli tRNA(Tyr) was possibly derived from E. coli 5s rRNA or a common precursor with 5s rRNA (3). The fact that quite high homologies were observed in these comparisons is consistent with the late evolution of the tRNAs in question. We now examine the evolution of E. coli tRNA(Trp) by comparing its homology with other E. coli tRNAs. The data suggest a possible evolutionary relationship with E. coli tRNA(Gly) or tRNA(Arg). The data support the idea of the late assignment of anticodons to Trp.


Assuntos
Evolução Biológica , Escherichia coli/genética , RNA de Transferência Aminoácido-Específico/genética , RNA de Transferência de Triptofano/genética , Anticódon , Sequência de Bases , Dados de Sequência Molecular , RNA Ribossômico 5S/genética
15.
J Cell Sci ; 101 ( Pt 3): 611-23, 1992 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-1522145

RESUMO

The cell-extracellular matrix junction, which includes the cell wall and the outer surface of the plasma membrane, may be an essential region for the perception of gravity by the internodal cells of Chara corallina. Typically, when an internodal cell is oriented vertically, the downwardly directed cytoplasmic stream travels at a velocity that is 10% faster than that of the upwardly directed stream. However when the cells are treated with impermeant hydrolytic enzymes that partially digest cellulose or hemicellulose, the cells lose their ability to respond to gravity even though streaming continues. By contrast, enzymes that digest pectins have no effect on the gravity-induced polarity of cytoplasmic streaming. Furthermore, gravisensing is sensitive to protease treatment; Proteinase K, thermolysin and collagenase but not trypsin, alpha-chymotrypsin or carboxypeptidase B, inhibit gravisensing. These findings indicate that proteins in the cell-extracellular matrix junction may be required for gravisensing. Moreover, the tetrapeptide Arg-Gly-Asp-Ser (RGDS) inhibits gravisensing in a concentration-dependent manner, indicating that the gravireceptor may be an integrin-like protein. The macromolecules necessary for gravisensing have been localized to the cell ends. As a consequence of the exoplasmic site of action of the enzymes and the tetrapeptides, we interpret the results to mean that they are acting on the gravireceptor, although we cannot eliminate the possibility that they are acting on the signal transduction chain. On the whole, our observations indicate that the cell-extracellular matrix junction is a sine qua non for graviperception in statolith-free Chara internodal cells and we suggest that the gravireceptor is located in this region.


Assuntos
Proteínas da Matriz Extracelular/fisiologia , Matriz Extracelular/fisiologia , Gravitação , Proteínas de Plantas , Sequência de Aminoácidos , Metabolismo dos Carboidratos , Parede Celular/fisiologia , Clorófitas/fisiologia , Hidrólise , Metabolismo dos Lipídeos , Proteínas de Membrana/fisiologia , Dados de Sequência Molecular , Ácidos Nucleicos/metabolismo , Oligopeptídeos/fisiologia , Proteínas de Plantas/metabolismo
16.
Protoplasma ; 168(3-4): 141-52, 1992.
Artigo em Inglês | MEDLINE | ID: mdl-11540076

RESUMO

Hydrostatic pressure applied to one end of a horizontal Chara cell induces a polarity of cytoplasmic streaming, thus mimicking the effect of gravity. A positive hydrostatic pressure induces a more rapid streaming away from the applied pressure and a slower streaming toward the applied pressure. In contrast, a negative pressure induces a more rapid streaming toward and a slower streaming away from the applied pressure. Both the hydrostatic pressure-induced and gravity-induced polarity of cytoplasmic streaming respond identically to cell ligation, UV microbeam irradiation, external Ca2+ concentrations, osmotic pressure, neutral red, TEA Cl-, and the Ca2+ channel blockers nifedipine and LaCl3. In addition, hydrostatic pressure applied to the bottom of a vertically-oriented cell can abolish and even reverse the gravity-induced polarity of cytoplasmic streaming. These data indicate that both gravity and hydrostatic pressure act at the same point of the signal transduction chain leading to the induction of a polarity of cytoplasmic streaming and support the hypothesis that characean cells respond to gravity by sensing a gravity-induced pressure differential between the cell ends.


Assuntos
Clorófitas/fisiologia , Corrente Citoplasmática/fisiologia , Gravitação , Sensação Gravitacional/fisiologia , Transdução de Sinais/fisiologia , Cálcio/farmacologia , Bloqueadores dos Canais de Cálcio/farmacologia , Polaridade Celular , Clorófitas/citologia , Clorófitas/efeitos dos fármacos , Corrente Citoplasmática/efeitos dos fármacos , Sensação Gravitacional/efeitos dos fármacos , Pressão Hidrostática , Lantânio/farmacologia , Luz , Vermelho Neutro/farmacologia , Nifedipino/farmacologia , Pressão Osmótica , Bloqueadores dos Canais de Potássio , Transdução de Sinais/efeitos dos fármacos , Tetraetilamônio , Compostos de Tetraetilamônio/farmacologia
17.
J Mol Evol ; 31(3): 244-8, 1990 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-1698989

RESUMO

This minireview is a summary of the basic concepts and pieces of experimental evidence supporting a hypothesis that suggests a mechanism whereby purine monoribonucleotides having D-ribose may be able to preferentially catalyze the synthesis of L-amino acid peptides. The proposed mechanism involves a 2'-3' diaminoacyl intermediate and the preference accrues from several factors that favor the L-isomer, principally for hydrophobic amino acids. Although the hypothesis has not been fully tested, some crucial evidence has been published. Other pieces of evidence are now being submitted or are in press for publication and still other experiments, principally on the step of peptide bond formation, are in the process of being carried out. The purpose of a review at this point is to present the hypothesis to the scientific community in hopes of generating discussion, suggestions, and evaluation by other workers. Should the hypothesis prove correct, it may represent the most primitive and fundamental relationship between the nucleic acid and protein systems. In addition, it would represent another important example of the catalytic ability of RNA.


Assuntos
Aminoácidos/biossíntese , RNA/metabolismo , Catálise , Estrutura Molecular , RNA/química
18.
Biochemistry ; 30(11): 2768-72, 1991 Mar 19.
Artigo em Inglês | MEDLINE | ID: mdl-2007115

RESUMO

Biological systems synthesize proteins with an almost exclusive use of L-amino acids and virtually none of the D isomer. There has been no satisfactory explanation for the origin of this use of the L isomer. Research presented here shows that at pH 5, transfer of phenylalanine from the adenylate anhydride to ester occurs and is 95-97% efficient for the L isomer and only about 50% efficient for the D isomer. The origin of the use of the L isomer, given D-ribose nucleotides, may be based in part on this stereoselectivity.


Assuntos
Monofosfato de Adenosina/análogos & derivados , Aminoácidos/metabolismo , Fenilalanina , Monofosfato de Adenosina/síntese química , Monofosfato de Adenosina/química , Cromatografia Líquida de Alta Pressão , Indicadores e Reagentes , Cinética , Modelos Moleculares , Conformação Molecular , Estereoisomerismo
19.
Am J Bot ; 84(11): 1516-21, 1997 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-11541058

RESUMO

Gravity induces a polarity of cytoplasmic streaming in vertical internodal cells of Chara such that the downwardly directed stream moves faster than the upwardly directed stream. In order to determine whether the statolith theory (in which intracellular sedimenting particles are responsible for gravity sensing) or the gravitational pressure theory (in which the entire protoplast acts as the gravity sensor) best explain the gravity response in Chara internodal cells, we controlled the physical properties of the external medium, including density and osmolarity, with impermeant solutes and examined the effect on the polarity of cytoplasmic streaming. As the density of the external medium is increased, the polarity of cytoplasmic streaming decreases and finally disappears when the density of the external medium is equal to that of the cell (1015 kg/m3). A further increase in the density of the external medium causes a reversal of the gravity response. These results are consistent with the gravitational pressure theory of gravity sensing since the buoyancy of the protoplast is dependent on the difference between the density of the protoplast and the external medium, and are inconsistent with the statolith theory since the buoyancy of intracellular particles are unaffected by changes in the external medium.


Assuntos
Clorófitas/fisiologia , Meios de Cultura/farmacologia , Corrente Citoplasmática/efeitos dos fármacos , Gravitação , Sensação Gravitacional/efeitos dos fármacos , Protoplastos/fisiologia , Fenômenos Biofísicos , Biofísica , Polaridade Celular/efeitos dos fármacos , Polaridade Celular/fisiologia , Clorófitas/citologia , Clorófitas/efeitos dos fármacos , Clorófitas/crescimento & desenvolvimento , Meios de Cultura/química , Corrente Citoplasmática/fisiologia , Matriz Extracelular/química , Matriz Extracelular/fisiologia , Sensação Gravitacional/fisiologia , Pressão Hidrostática , Concentração Osmolar , Plastídeos/fisiologia , Soluções
20.
Am J Bot ; 84(11): 1522-9, 1997 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-11541059

RESUMO

The roots of rice seedlings, growing in artificial pond water, exhibit robust gravitropic curvature when placed perpendicular to the vector of gravity. To determine whether the statolith theory (in which intracellular sedimenting particles are responsible for gravity sensing) or the gravitational pressure theory (in which the entire protoplast acts as the gravity sensor) best accounts for gravity sensing in rice roots, we changed the physical properties of the external medium with impermeant solutes and examined the effect on gravitropism. As the density of the external medium is increased, the rate of gravitropic curvature decreases. The decrease in the rate of gravicurvature cannot be attributed to an inhibition of growth, since rice roots grown in 100 Osm/m3 (0.248 MPa) solutions of different densities all support the same root growth rate but inhibit gravicurvature increasingly with increasing density. By contrast, the sedimentation rate of amyloplasts in the columella cells is unaffected by the external density. These results are consistent with the gravitational pressure theory of gravity sensing, but cannot be explained by the statolith theory.


Assuntos
Meios de Cultura/farmacologia , Gravitropismo/efeitos dos fármacos , Sensação Gravitacional/efeitos dos fármacos , Oryza/crescimento & desenvolvimento , Raízes de Plantas/crescimento & desenvolvimento , Fenômenos Biofísicos , Biofísica , Polaridade Celular , Meios de Cultura/química , Gravitação , Gravitropismo/fisiologia , Sensação Gravitacional/fisiologia , Pressão Hidrostática , Oryza/efeitos dos fármacos , Oryza/fisiologia , Concentração Osmolar , Raízes de Plantas/efeitos dos fármacos , Plastídeos/efeitos dos fármacos , Plastídeos/fisiologia , Soluções
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