SPIRE-a software tool for bicontinuous phase recognition: application for plastid cubic membranes

Bicontinuous membranes in cell organelles epitomize nature's ability to create complex functional nanostructures. Like their synthetic counterparts, these membranes are characterized by continuous membrane sheets draped onto topologically complex saddle-shaped surfaces with a periodic network-like structure. Their structure sizes, (around 50-500 nm), and fluid nature make transmission electron microscopy (TEM) the analysis method of choice to decipher their nanostructural features. Here we present a tool, Surface Projection Image Recognition Environment (SPIRE), to identify bicontinuous structures from TEM sections through interactive identification by comparison to mathematical "nodal surface" models. The prolamellar body (PLB) of plant etioplasts is a bicontinuous membrane structure with a key physiological role in chloroplast biogenesis. However, the determination of its spatial structural features has been held back by the lack of tools enabling the identification and quantitative analysis of symmetric membrane conformations. Using our SPIRE tool, we achieved a robust identification of the bicontinuous diamond surface as the dominant PLB geometry in angiosperm etioplasts in contrast to earlier long-standing assertions in the literature. Our data also provide insights into membrane storage capacities of PLBs with different volume proportions and hint at the limited role of a plastid ribosome localization directly inside the PLB grid for its proper functioning. This represents an important step in understanding their as yet elusive structure-function relationship.

Comparative Time-Course Physiological Responses and Proteomic Analysis of Melatonin Priming on Promoting Germination in Aged Oat (Avena sativa L.) Seeds

Melatonin priming is an effective strategy to improve the germination of aged oat (Avena sativa L.) seeds, but the mechanism involved in its time-course responses has remained largely unknown. In the present study, the phenotypic differences, ultrastructural changes, physiological characteristics, and proteomic profiles were examined in aged and melatonin-primed seed (with 10 μM melatonin treatment for 12, 24, and 36 h). Thus, 36 h priming (T36) had a better remediation effect on aged seeds, reflecting in the improved germinability and seedlings, relatively intact cell ultrastructures, and enhanced antioxidant capacity. Proteomic analysis revealed 201 differentially abundant proteins between aged and T36 seeds, of which 96 were up-accumulated. In melatonin-primed seeds, the restoration of membrane integrity by improved antioxidant capacity, which was affected by the stimulation of jasmonic acid synthesis via up-accumulation of 12-oxo-phytodienoic acid reductase, might be a candidate mechanism. Moreover, the relatively intact ultrastructures enabled amino acid metabolism and phenylpropanoid biosynthesis, which were closely associated with energy generation through intermediates of pyruvate, phosphoenolpyruvate, fumarate, and α-ketoglutarate, thus providing energy, active amino acids, and secondary metabolites necessary for germination improvement of aged seeds. These findings clarify the time-course related pathways associated with melatonin priming on promoting the germination of aged oat seeds.

Effects of silicon nanoparticles on molecular, chemical, structural and ultrastructural characteristics of oat (Avena sativa L.)

Nowadays, nanoparticles are extensively being utilized in medicine, industry, and agriculture thus distributed into the environment. Hence, it is essential to examine exactly the impact of these materials on a variety of organisms, including various species of plants. Therefore, in the current study, we compared the effects of sodium silicate and nano silicon (SiNP) (both at two concentrations of 5 and 10 mM) separately on lignification of the xylem cell wall, antioxidant enzyme activities, ultrastructure of leaf and root cells, expression of silicon transporter (Lsi1) and phenylalanine ammonia lyase (PAL), and also evaluated the protein content and chemical content of oat plants (Avena sativa L.) cultured hydroponically. The results indicated that SiNPs didn't have a toxic effect on the oat plants, and in many cases they enhanced plant growth. The effect of SiNPs on the chemical content of the treated plants was almost identical with silicate. The silicon transporter (Lsi1) gene was expressed in plants exposed to SiNPs, however, at lower levels (∼37% in roots) than those exposed to silicate treatments. The SiNPs increased PAL expression and lignification in leaves and roots, however, at lower levels those of silicate (˃50% in fourth leaves of 10 mM concentration). They were aggregated in the roots (268-366 nm) and deposited in nano size on the cell walls of leaves. In general, their effects in the plants were identical to silicate but differed in intensity.

Mitochondrial structural and antioxidant system responses to aging in oat (Avena sativa L.) seeds with different moisture contents

We observed the relationship between lifespan and mitochondria, including antioxidant systems, ultrastructure, and the hydrogen peroxide and malondialdehyde contents in 4 h imbibed oat (Avena sativa L.) seeds that were aged with different moisture contents (4%, 10% and 16%) for 0 (the control), 8, 16, 24, 32 and 40 d at 45 °C. The results showed that the decline in the oat seed vigor and in the integrity of the mitochondrial ultrastructure occurred during the aging process, and that these changes were enhanced by higher moisture contents. Mitochondrial antioxidants in imbibed oat seeds aged with a 4% moisture content were maintained at higher levels than imbibed oat seeds aged with a 10% and 16% moisture content. These results indicated that the levels of mitochondrial antioxidants and malondialdehyde after imbibition were related to the integrity of the mitochondrial membrane in aged oat seeds. The scavenging role of mitochondrial superoxide dismutase was inhibited in imbibed oat seeds aged at the early stage. Monodehydroascorbate reductase and dehydroascorbate reductase played more important roles than glutathione reductase in ascorbate regeneration in aged oat seeds during imbibition.

Histological analysis and 3D reconstruction of winter cereal crowns recovering from freezing: a unique response in oat (Avena sativa L.)

The crown is the below ground portion of the stem of a grass which contains meristematic cells that give rise to new shoots and roots following winter. To better understand mechanisms of survival from freezing, a histological analysis was performed on rye, wheat, barley and oat plants that had been frozen, thawed and allowed to resume growth under controlled conditions. Extensive tissue disruption and abnormal cell structure was noticed in the center of the crown of all 4 species with relatively normal cells on the outside edge of the crown. A unique visual response was found in oat in the shape of a ring of cells that stained red with Safranin. A tetrazolium analysis indicated that tissues immediately inside this ring were dead and those outside were alive. Fluorescence microscopy revealed that the barrier fluoresced with excitation between 405 and 445 nm. Three dimensional reconstruction of a cross sectional series of images indicated that the red staining cells took on a somewhat spherical shape with regions of no staining where roots entered the crown. Characterizing changes in plants recovering from freezing will help determine the genetic basis for mechanisms involved in this important aspect of winter hardiness.

Integrated approach to assessing the effects of soils polluted with heavy metals on a plant population

This study addresses the effects of soil polluted with more than one heavy metal in a grass species. A 16-week bioassay with Avena sativa L. was conducted in microcosms using soils from two abandoned mines in central Spain that contained levels above World Health Organization (WHO) reference limits for soils of more than three heavy metals. Pollution effects were examined at cell, tissue, organ, plant and population levels. For this purpose, dry weight, maximum height and number of leaves were determined; leaf tissues were observed by low temperature scanning electron microscopy; the metal contents of roots and shoots were determined by plasma emission spectroscopy and their distribution in different tissues was analyzed by X-ray microanalysis using an environmental scanning electron microscope. The results explain the accumulation and translocation of soil metals by this plant species; their effects in cells, tissues and growth of plants; and allow inference on population effects. The discussion of the methodological approach leads us to propose a valid protocol to assess the effects of a set of heavy metals present in the topsoil of polluted sites on a plant population. We recommend its use for an ecotoxicological diagnosis and risk analysis of similarly polluted sites.

Individual maize chromosomes in the C(3) plant oat can increase bundle sheath cell size and vein density

C(4) photosynthesis has evolved in at least 66 lineages within the angiosperms and involves alterations to the biochemistry, cell biology, and development of leaves. The characteristic "Kranz" anatomy of most C(4) leaves was discovered in the 1890s, but the genetic basis of these traits remains poorly defined. Oat × maize addition lines allow the effects of individual maize (Zea mays; C(4)) chromosomes to be investigated in an oat (Avena sativa; C(3)) genetic background. Here, we have determined the extent to which maize chromosomes can introduce C(4) characteristics into oat and have associated any C(4)-like changes with specific maize chromosomes. While there is no indication of a simultaneous change to C(4) biochemistry, leaf anatomy, and ultrastructure in any of the oat × maize addition lines, the C(3) oat leaf can be modified at multiple levels. Maize genes encoding phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, and the 2'-oxoglutarate/malate transporter are expressed in oat and generate transcripts of the correct size. Three maize chromosomes independently cause increases in vein density, and maize chromosome 3 results in larger bundle sheath cells with increased cell wall lipid deposition in oat leaves. These data provide proof of principle that aspects of C(4) biology could be integrated into leaves of C(3) crops.

Fusion of oil bodies in endosperm of oat grains

Few microscopical studies have been made on lipid storage in oat grains, with variable results as to the extent of lipid accumulation in the starchy endosperm. Grains of medium- and high-lipid oat (Avena sativa L.) were studied at two developmental stages and at maturity, by light microscopy using different staining methods, and by scanning and transmission electron microscopy. Discrete oil bodies occurred in the aleurone layer, scutellum and embryo. In contrast, oil bodies in the starchy endosperm often had diffuse boundaries and fused with each other and with protein vacuoles during grain development, forming a continuous oil matrix between the protein and starch components. The different microscopical methods were confirmative to each other regarding the coalescence of oil bodies, a phenomenon probably correlated with the reduced amount of oil-body associated proteins in the endosperm. This was supported experimentally by SDS-PAGE separation of oil-body proteins and immunoblotting and immunolocalization with antibodies against a 16 kD oil-body protein. Much more oil-body proteins per amount of oil occurred in the embryo and scutellum than in the endosperm. Immunolocalization of 14 and 16 kD oil-body associated proteins on sectioned grains resulted in more heavy labeling of the embryo, scutellum and aleurone layer than the rest of the endosperm. Observations on the appearance of oil bodies at an early stage of development pertain to the prevailing hypotheses of oil-body biogenesis.

Sad3 and sad4 are required for saponin biosynthesis and root development in oat

Avenacins are antimicrobial triterpene glycosides that are produced by oat (Avena) roots. These compounds confer broad-spectrum resistance to soil pathogens. Avenacin A-1, the major avenacin produced by oats, is strongly UV fluorescent and accumulates in root epidermal cells. We previously defined nine loci required for avenacin synthesis, eight of which are clustered. Mutants affected at seven of these (including Saponin-deficient1 [Sad1], the gene for the first committed enzyme in the pathway) have normal root morphology but reduced root fluorescence. In this study, we focus on mutations at the other two loci, Sad3 (also within the gene cluster) and Sad4 (unlinked), which result in stunted root growth, membrane trafficking defects in the root epidermis, and root hair deficiency. While sad3 and sad4 mutants both accumulate the same intermediate, monodeglucosyl avenacin A-1, the effect on avenacin A-1 glucosylation in sad4 mutants is only partial. sad1/sad1 sad3/sad3 and sad1/sad1 sad4/sad4 double mutants have normal root morphology, implying that the accumulation of incompletely glucosylated avenacin A-1 disrupts membrane trafficking and causes degeneration of the epidermis, with consequential effects on root hair formation. Various lines of evidence indicate that these effects are dosage-dependent. The significance of these data for the evolution and maintenance of the avenacin gene cluster is discussed.

Lipids in grain tissues of oat (Avena sativa): differences in content, time of deposition, and fatty acid composition

Oat (Avena sativa) is unusual in comparison with other cereals since there are varieties with up to 18% oil content. The lipid content and fatty acid composition in different parts of the grain during seed development were characterized in cultivars Freja (6% oil) and Matilda (10% oil), using thin-layer and gas chromatography, and light and electron microscopy. The majority of lipids (86-90%) were found in the endosperm. Ninety-five per cent of the higher oil content of cv. Matilda compared with cv. Freja was due to increased oil content of the endosperm. Up to 84% of the lipids were deposited during the first half of seed development, when seeds where still green with a milky endosperm. Microscopy studies revealed that whereas oil bodies of the embryo and scutellum still contained a discrete shape upon grain maturation, oil bodies of the endosperms fused upon maturation and formed smears of oil.

Apoptotic cell death is a common response to pathogen attack in oats

We have examined the characteristics of cell death induced by pathogen infection in oats with respect to following hallmark apoptotic features: DNA laddering, chromatin condensation, and electron microscopic-terminal deoxynucleotidyl transferase-mediated UTP end labeling positive response. A wide range of plant pathogens representing different levels of parasitism in susceptible and resistant interactions were used for the inocula, which include (i) an obligate parasite, Puccinia coronata f. sp. avenae (the crown rust fungus); (ii) a facultative biotroph parasite, Magnaporthe grisea (the blast fungus); (iii) pathogenic bacteria, Pseudomonas syringae pv. atropurpurea and P. syringae pv. coronafaciens (the halo or stripe blights of oats); and (iv) Ryegrass mottle virus. Surprisingly, any of the pathogens used induced most of the apoptotic features in oat cells at and around the infection sites, indicating that apoptotic cell death is a common phenomenon in oats during pathogen attack. The localization and the timing of apoptotic cell death during a course of infection were, however, quite different depending on the interactions (compatible or incompatible) and the pathogens (fungi, bacteria, or viruses). Possible roles of apoptotic cell death in the susceptible and resistant interactions are discussed.

Mitochondrial oxidative burst involved in apoptotic response in oats

Apoptotic cell response in oats is induced by victorin, a host-selective toxin secreted by Cochliobolus victoriae and thought to exert toxicity by inhibiting mitochondrial glycine decarboxylase (GDC) in Pc-2/Vb oats. We examined the role of mitochondria, especially the organelle-derived production of reactive oxygen species (ROS), in the induction of apoptotic cell death. Cytofluorimetric analysis showed that victorin caused mitochondrial deltaPsim breakdown and mitochondrial oxidative burst. Ultrastructural analysis using a cytochemical assay based on the reaction of H2O2 with CeCl3 detected H2O2 eruption at permeability transition pore-like sites on the mitochondrial membrane in oat cells treated with victorin. ROS generation preceded the apoptotic cell responses seen in chromatin condensation and DNA laddering. Both aminoacetonitrile (a specific GDC inhibitor) and antimycin A (a mitochondrial complex III inhibitor) also induced mitochondrial H2O2 eruption, and led to the apoptotic response in oat cells. ROS scavengers such as N-acetyl-l-cysteine and catalase suppressed the mitochondrial oxidative burst and delayed chromatin condensation and DNA laddering in the victorin- or antimycin A-treated leaves. These findings indicate possible involvement of mitochondria, especially mitochondrial-derived ROS generation, as an important regulator in controlling apoptotic cell death in oats.

Isolation and characterization of two novel retrotransposons of the Ty1-copia group in oat genomes

Two repetitive sequences, As32 and As22, of 826 and 742 bp, respectively, were isolated from Avena strigosa (As genome). Databank searches revealed their high homology to different segments of the family of Ty1-copia retrotransposons. Southern hybridization showed them to be present in diploid and polyploid oat species. Polymerase chain reaction with primers designed to amplify the segment between them showed that As32 and As22 sequences are composed of two different Ty1-copia retrotransposons. The segment amplified from the pAs32 insert was 2,264 bp long and contained the entire GAG and AP domains, and more than half of the IN domain. This new element has been designated TAS-1 (transposon, A. strigosa, 1) and appears to contain a long open reading frame that encodes a polypeptide of 625 amino acids. Slot-blot and fluorescence in situ hybridization analyses revealed it to be a component of both A- and D-genome chromosomes. Further, the chromosomes involved in one C-A intergenomic translocation in A. murphyi (AC genomes), one C-D intergenomic translocation in A. byzantina cv. Kanota (ACD genomes), and two C-D intergenomic translocations in A. sativa cv. Extra Klock, were identified. Based on its physical distribution and Southern hybridization pattern, a parental retro-transposon represented by TAS-1 appears to have been active at least twice during the evolution of the genomes in species of Avena.

Membrane geometry of "open" prolamellar bodies

The "open" type of prolamellar body in etiplasts was examined by electron microscopy to characterise its three-dimensional organisation. As in more compact forms of prolamellar body, its basic geometric unit is a tetrahedrally branched tubule. In the "open" type, these lie smoothly confluent with one another at the vertices of 5- and 6-membered rings which circumscribe the faces of three kinds of polyhedra: pentagonal dodecahedra (with 12 pentagonal faces), 14-hedra (2 opposite hexagonal faces joined by two circlets of six pentagonal faces), and 15-hedra (3 hexagonal and 12 pentagonal faces). These polyhedra join confluently in their turn, sharing faces with one another in at least one recognisable super-structure which accounts for the appearance of "open" prolamellar bodies in many ultrathin sections. In this organisation, columns of pentagonal dodecahedra are arranged at 120 degrees to one another in the x-y-plane of the lattice. They do not fill the plane but intersect so as to delimit voids in the form of hexagonally arranged 14-hedra (with hexagonal rings in the x-y-plane). Strata of this type alternate with strata made of face-sharing 15-hedra (with their hexagonal rings normal to x-y), which also delimit 14-hedra. The 14-hedra thus lie in register in the z-axis in hexagonally arranged columns, normal to the alternating strata. Other possible organisations cannot be excluded and local variations and dislocations certainly occur, but many micrographs that display elements of symmetry in "open" prolamellar bodies can be matched to thin slices through such a model. Its geometry is like that of the cages of water molecules in type IV (sensu Jeffrey = type III sensu O'Keeffe) clathrate-hydrates, point group P6/mmm, but about two orders of magnitude larger.

Complex structure of knobs and centromeric regions in maize chromosomes

The recovery of maize (Zea mays L.) chromosome addition lines of oat (Avena sativa L.) from oat x maize crosses enables us to analyze the structure and composition of individual maize chromosomes via the isolation and characterization of chromosome-specific cosmid clones. Restriction fragment fingerprinting, sequencing, and in situ hybridization were applied to discover a new family of knob associated tandem repeats, the TR1, which are capable of forming fold-back DNA segments, as well as a new family of centromeric tandem repeats, CentC. Analysis of knob and centromeric DNA segments revealed a complex organization in which blocks of tandemly arranged repeating units are interrupted by insertions of other repeated DNA sequences, mostly represented by individual full size copies of retrotransposable elements. There is an obvious preference for the integration/association of certain retrotransposable elements into knobs or centromere regions as well as for integration of retrotransposable elements into certain sites (hot spots) of the 180-bp repeat. DNA hybridization to a blot panel of eight individual maize chromosome addition lines revealed that CentC, TR1, and 180-bp tandem repeats are found in each of these maize chromosomes, but the copy number of each can vary significantly from about 100 to 25,000. In situ hybridization revealed variation among the maize chromosomes in the size of centromeric tandem repeats as well as in the size and composition of knob regions. It was found that knobs may be composed of either 180-bp or TR1, or both repeats, and in addition to large knobs these repeated elements may form micro clusters which are detectable only with the help of in situ hybridization. The association of the fold-back elements with knobs, knob polymorphism and complex structure suggest that maize knob may be consider as megatransposable elements. The discovery of the interspersion of retrotransposable elements among blocks of tandem repeats in maize and some other organisms suggests that this pattern may be basic to heterochromatin organization for eukaryotes.

Magnetophoretic induction of curvature in coleoptiles and hypocotyls

Coleoptiles of barley (Hordeum vulgare) were positioned in a high gradient magnetic field (HGMF, dynamic factor gradient of H(2)/2 of 10(9)-10(10) Oe2 cm-1), generated by a ferromagnetic wedge in a uniform magnetic field and rotated on a 1 rpm clinostat. After 4 h 90% of coleoptiles had curved toward the HGMF. The cells affected by HGMF showed clear intracellular displacement of amyloplasts. Coleoptiles in a magnetic field next to a non-ferromagnetic wedge showed no preferential curvature. The small size of the area of nonuniformity of the HGMF allowed mapping of the sensitivity of the coleoptiles by varying the initial position of the wedge relative to the coleoptile apex. When the ferromagnetic wedge was placed 1 mm below the coleoptile tip only 58% of the coleoptiles curved toward the wedge indicating that the cells most sensitive to intracellular displacement of amyloplasts and thus gravity sensing are confined to the top 1 mm portion of barley coleoptiles. Similar experiments with tomato hypocotyls (Lycopersicum esculentum) also resulted in curvature toward the HGMF. The data strongly support the amyloplast-based gravity-sensing system in higher plants and the usefulness of HGMF to substitute gravity in shoots.

Magnetophoretic induction of curvature in coleoptiles and hypocotyls

Coleoptiles of barley (Hordeum vulgare) were positioned in a high gradient magnetic field (HGMF, dynamic factor gradient of H(2)/2 of 10(9)-10(10) Oe2 cm-1), generated by a ferromagnetic wedge in a uniform magnetic field and rotated on a 1 rpm clinostat. After 4 h 90% of coleoptiles had curved toward the HGMF. The cells affected by HGMF showed clear intracellular displacement of amyloplasts. Coleoptiles in a magnetic field next to a non-ferromagnetic wedge showed no preferential curvature. The small size of the area of nonuniformity of the HGMF allowed mapping of the sensitivity of the coleoptiles by varying the initial position of the wedge relative to the coleoptile apex. When the ferromagnetic wedge was placed 1 mm below the coleoptile tip only 58% of the coleoptiles curved toward the wedge indicating that the cells most sensitive to intracellular displacement of amyloplasts and thus gravity sensing are confined to the top 1 mm portion of barley coleoptiles. Similar experiments with tomato hypocotyls (Lycopersicum esculentum) also resulted in curvature toward the HGMF. The data strongly support the amyloplast-based gravity-sensing system in higher plants and the usefulness of HGMF to substitute gravity in shoots.

Changes in the ultrastructure of cell walls, cellulose synthesis, and glucan synthase activity from gravistimulated pulvini of oat (Avena sativa)

Ultrastructural analyses of the cell walls from top and bottom halves of gravistimulated pulvini from oat leaves show a decrease in the density of material within the cell walls from the lower halves of pulvini after 24 h of gravistimulation. Assays of cellulose synthesis with a 14C-sucrose pulse-chase experiment indicate no difference in the amount of new cellulose synthesized in top compared with bottom halves of gravistimulated pulvini. The highest rate of cellulose synthesis occurs with 12-24 h of gravistimulation. Treatment of graviresponding pulvini with 2,6-dichlorobenzonitrile (DCBN) had only a minor effect on segment gravitropic curvature. We also found that there is no difference in the activities of either glucan synthase I or glucan synthase II in top halves as compared with bottom halves of gravistimulated pulvini. We conclude that the graviresponse in oat stems is not driven by new cell wall synthesis but, rather, by changes in cell wall plasticity and osmotic potential.

Polyamine binding to proteins in oat and Petunia protoplasts

Previous work (A Apelbaum et al. [1988] Plant Physiol 88: 996-998) has demonstrated binding of labeled spermidine (Spd) to a developmentally regulated 18 kilodalton protein in tobacco tissue cultures derived from thin surface layer explants. To assess the general importance of such Spd-protein complexes, we attempted bulk isolation from protoplasts of Petunia and oat (Avena sativa). In Petunia, as in tobacco, fed radioactive Spd is bound to protein, but in oat, Spd is first converted to 1,3,-diaminopropane (DAP), probably by polyamine oxidase action. In oat, binding of DAP to protein depends on age of donor leaf and conditions of illumination and temperature, and the extraction of the DAP-protein complex depends upon buffer and pH. The yield of the DAP-protein complex was maximized by extraction of frozen-thawed protoplasts with a pH 8.8 carbonate buffer containing SDS. Its molecular size, based on Sephacryl column fractionation of ammonium sulfate precipitated material, exceeded 45 kilodaltons. Bound Spd or DAP can be released from their complexes by the action of Pronase, but not DNAse, RNAse, or strong salt solutions, indicating covalent attachment to protein.

Cytological and ultrastructural studies on root tissues

The anatomy and fine structure of roots from oat and mung bean seedlings, grown under microgravity conditions for 8 days aboard the Space Shuttle, was examined and compared to that of roots from ground control plants grown under similar conditions. Roots from both sets of oat seedlings exhibited characteristic monocotyledonous tissue organization and normal ultrastructural features, except for cortex cell mitochondria, which exhibited a 'swollen' morphology. Various stages of cell division were observed in the meristematic tissues of oat roots. Ground control and flight-grown mung bean roots also showed normal tissue organization, but root cap cells in the flight-grown roots were collapsed and degraded in appearance, especially at the cap periphery. At the ultrastructural level, these cells exhibited a loss of organelle integrity and a highly-condensed cytoplasm. This latter observation perhaps suggests a differing tissue sensitivity for the two species to growth conditions employed in space flight. The basis for abnormal root cap cell development is not understood, but the loss of these putative gravity-sensing cells holds potential significance for long term plant growth orientation during space flight.

Calcium movements and the cellular basis of gravitropism

An early gravity-transduction event in oat coleoptiles which precedes any noticeable bending is the accumulation of calcium on their prospective slower-growing side. Sub-cellular calcium localization studies indicate that the gravity-stimulated redistribution of calcium results in an increased concentration of calcium in the walls of responding cells. Since calcium can inhibit the extension growth of plant cell walls, this selective accumulation of calcium in walls may play a role in inducing the asymmetry of growth which characterizes gravitropism. The active transport of calcium from cells into walls is performed by a calcium-dependent ATPase localized in the plasma membrane. Evidence is presented in support of the hypothesis that this calcium pump is regulated by a feed-back mechanism which includes the participation of calmodulin.