Acides aminés libres et protéiques des divers types de rameaux du Periploca graeca: Rôle dans la circumnutation des tiges volubiles et compartimentation intracellulaire

Free and Bound Amino Acids in the Different Shoot Types of Periploca graeca: Role in the Circumnutation of Twining Shoots, and Cellular Compartmentation. A study has been made of amino acids of wall proteins, cytoplasmic TCA insoluble proteins and proteins of organelles in growing tissues from three types of shoots: short upright, twining, and creeping, all carried by the same specimen of Periploca graeca L. Each shoot type presents a specific pattern. The twining shoots are very rich in cytoplasmic TCA-insoluble proteins. The upright shoots distinguish themselves by a high level of proteins in the organelles and in the cell-wall. However, the composition of wallproteins is almost the same in the three types of shoots. This result excludes the existence of particular wall proteins (extensin, for example) in the twining and rapidly growing shoots. In these shoots the high level of free prolin is not a consequence of low incorporation in the proteins. Distribution of prolin, aspartic acid and glutamic acid between the cellular compartments (wall, hyaloplasm and organelles) has a specific pattern in each shoot type.     

Cellulose orientation at the surface of the Arabidopsis seedling. Implications for the biomechanics in plant development

In diffuse growing cells the orientation of cellulose fibrils determines mechanical anisotropy in the cell wall and hence also the direction of plant and organ growth. This paper reports on the mean or net orientation of cellulose fibrils in the outer epidermal wall of the whole Arabidopsis plant. This outer epidermal wall is considered as the growth-limiting boundary between plant and environment. In the root a net transverse orientation of the cellulose fibrils occurs in the elongation zone, while net random and longitudinal orientations are found in subsequent older parts of the differentiation zone. The position and the size of the transverse zone is related with root growth rate. In the shoot the net orientation of cellulose fibrils is transverse in the elongating apical part of the hypocotyl, and longitudinal in the fully elongated basal part. Leaf primordia and very young leaves have a transverse orientation. Throughout further development the leaf epidermis builds a very complex pattern of cells with a random orientation and cells with a transverse or a longitudinal orientation of the cellulose fibrils. The patterns of net cellulose orientation correlate well with the cylindrical growth of roots and shoots and with the typical planar growth of the leaf blade. On both the shoot and the root surface very specific patterns of cellulose orientation occur at sites of specific cell differentiation: trichome-socket cells complexes on the shoot and root hairs on the root.     

Circumnutation and distribution of phytohormones in <Emphasis Type="Italic">Vigna angularis</Emphasis> epicotyls

Circumnutation is a plant growth movement in which the tips of axial organs draw a circular orbit. Although it has been studied since the nineteenth century, its mechanism and significance are still unclear. Greened adzuki bean (Vigna angularis) epicotyls exhibited a clockwise circumnutation in the top view with a constant period of 60 min under continuous white light. The bending zone of circumnutation on the epicotyls was always located in the region 1–3 cm below the tip, and its basal end was almost identical to the apical end of the region where the epicotyl had completely elongated. Therefore, epidermal cells that construct the bending zone are constantly turning over with their elongation growth. Since exogenously applied auxin transport inhibitors and indole-3-acetic acid (IAA) impaired circumnutation without any effect on the elongation rate of epicotyls, we attempted to identify the distribution pattern of endogenous auxin. Taking advantage of its large size, we separated the bending zone of epicotyls into two halves along the longitudinal axis, either convex/concave pairs in the plane of curvature of circumnutation or pre-convex/pre-concave pairs perpendicular to the plane. By liquid chromatography–mass spectrometry, we found, for the first time, that IAA and gibberellin A₁ were asymmetrically distributed in the pre-convex part in the region 1–2 cm below the tip. This region of epicotyl sections exhibited the highest responsiveness to exogenously applied hormones, and the latent period between the hormone application and the detection of a significant enhancement in elongation was 15 min. Our results suggest that circumnutation in adzuki bean epicotyls with a 60 min period is maintained by differential growth in the bending zone, which reflects the hormonal status 15 min before and which is shifting sequentially in a circumferential direction. Cortical microtubules do not seem to be involved in this regulation.     

Cell Elongation and Revolving Movement in Phaseolus vulgaris L. Twining Shoots

In Phaseolus vulgaris L., the shoot displays a revolving movementthat occurs rhythmically in a highly regular manner. Previousdata led to think that revolving movement is driven by turgorand volume changes in the epidermal cells of the bending zone.To document this hypothesis, the time course of in situ celllength variations in the bending zone was measured during themovement of the shoot and related to the phase of the revolvingmovement. Each ten minutes, a photograph of cells was takenand the revolving movement was simultaneously recorded usingtime-lapse microphotography and video-monitoring. In the movingpart of the shoot, epidermal cells displayed partly reversiblelength variations during their growth. Data were processed byFourier analysis to determine whether or not a periodicity exists.Rhythm in cell length variations was evidenced only when initialcell lengths were ranged between 60 and 120 µM. In thiscase, the period corresponds to that of the revolving movement. Thus, revolving movement is related to partly reversible lengthvariations in the cells of the bending zone. These results agreewith the hypothesis of an involvement of turgor mediated volumechanges in the revolving movement (Received September 16, 1997; Accepted June 18, 1998)     

The role of actin filaments in the gravitropic response of snapdragon flowering shoots

The involvement of the actin and the microtubule cytoskeleton networks in the gravitropic response of snapdragon ( Antirrhinum majus L.) flowering shoots was studied using various specific cytoskeleton modulators. The microtubule-depolymerizing drugs tested had no effect on gravitropic bending. In contrast, the actin-modulating drugs, cytochalasin D (CD), cytochalasin B (CB) and latrunculin B (Lat B) significantly inhibited the gravitropic response. CB completely inhibited shoot bending via inhibiting general growth, whereas CD completely inhibited bending via specific inhibition of the differential flank growth in the shoot bending zone. Surprisingly, Lat B had only a partial inhibitory effect on shoot bending as compared to CD. This probably resulted from the different effects of these two drugs on the actin cytoskeleton, as was seen in cortical cells. CD caused fragmentation of the actin cytoskeleton and delayed amyloplast displacement following gravistimulation. In contrast, Lat B caused a complete depolymerization of the actin filaments in the shoot bending zone, but only slightly reduced the amyloplast sedimentation rate following gravistimulation. Taken together, our results suggest that the actin cytoskeleton is involved in the gravitropic response of snapdragon shoots. The actin cytoskeleton within the shoot cells is necessary for normal amyloplast displacement upon gravistimulation, which leads to the gravitropic bending.     

Characterization of the asymmetric growth of gravistimulated snapdragon spikes by stem and cell dimension analyses

Growth patterns of detached spikes of gravistimulated snapdragon (Antirrhinum majus L.) were analyzed in detail. The length increment of 5-mm marked subsections in the upper and lower flanks of the stem-bending zone was measured during gravistimulation using time-lapse photographs. At the onset of bending, a negative relative growth rate of the upper flank was detected, followed by increased relative growth rate in both lower and upper flanks. Consequently, a differential stem growth pattern was obtained during gravistimulation, which was significantly and specifically abolished by calcium antagonists reported previously to inhibit stem curvature of snapdragon. The differential growth patterns resulted from dynamic modifications of the cell dimensions in the epidermal and cortical stem layers. Bending started with both shrinking and widening of the epidermal cells and a parallel decrease in length and height of cortical cells at the upper stem flank. These changes were accompanied with a concomitant increase in length and height of the cortical cells on the lower stem flank, followed by a growth increase of epidermal cells. Our results suggest that both the epidermal and cortical cells play an important role in gravitropic shoot bending of snapdragon.     

Spatial distribution of growth rates and of epidermal cell lengths in the elongation zone during leaf development in Lolium perenne L.

Relative elemental growth rates (REGR) and lengths of epidermal cells along the elongation zone of Lolium perenne L. leaves were determined at four developmental stages ranging from shortly after emergence of the leaf tip to shortly before cessation of leaf growth. Plants were grown at constant light and temperature. At all developmental stages the length of epidermal cells in the elongation zone of both the blade and sheath increased from 12 m at the leaf base to about 550 m at the distal end of the elongation zone, whereas the length of epidermal cells within the joint region only increased from 12 to 40 m. Throughout the developmental stages elongation was confined to the basal 20 to 30 mm of the leaf with maximum REGR occurring near the center of the elongation zone. Leaf elongation rate (LER) and the spatial distributions of REGR and epidermal cell lengths were steady to a first approximation between emergence of the leaf tip and transition from blade to sheath growth. Elongation of epidermal cells in the sheath started immediately after the onset of elongation of the most proximal blade epidermal cells. During transition from blade to sheath growth the length of the blade and sheath portion of the elongation zone decreased and increased, respectively, with the total length of the elongation zone and the spatial distribution of REGR staying near constant, with exception of the joint region which elongated little during displacement through the elongation zone. Leaf elongation rate decreased rapidly during the phase when only the sheath was growing. This was associated with decreasing REGR and only a small decrease in the length of the elongation zone. Data on the spatial distributions of growth rates and of epidermal cell lengths during blade elongation were used to derive the temporal pattern of epidermal cell elongation. These data demonstrate that the elongation rate of an epidermal cell increased for days and that cessation of epidermal cell elongation was an abrupt event with cell elongation rate declining from maximum to zero within less than 10 h.Abbreviations LER leaf elongation rate - REGR relative elemental growth rates     

Contributions of the surface of the root tip to the growth of Zea roots in soil

The development of the epidermal layer of roots of Zea is traced from the quiescent centre to the zone where root hairs develop. In the zone of cell division a three layered coat forms on the outside of the epidermal cells consisting of the outer epidermal walls, overlaid by a two-layered pellicle composed of a thick fibrillar inner layer of polysaccharide, and a thin fibrillar outer layer of protein. The epidermal cells divide several times in the same longitudinal file but rarely across a radius to give a new longitudinal file. Thus, the radial walls become much thicker than all but the original transverse walls, and packets of up to 32 daughter cells derived from a single initial may be distinguished. The pellicle develops during these divisions as a continuum over the outer walls of the daughter cells. It is proposed that the pellicle provides a stiffening to the forward end of the root which permits it to penetrate soil without bending. Support for this hypothesis is shown by the Zea mays mutant Ageotropic in which the pellicle is absent, the epidermal surface is disorganized, and which grows crookedly through soil. In the zone of extension growth of normal roots of two Zea species the pellicle thins and disappears. Circumferential strips of the pellicle were peeled off the young epidermal cells and could be stretched to twice their length. This deformation is partly the result of the pellicle stretching and breaking above the attachments of the radial walls. After normal thinning of the pellicle, detachment of the radial walls at their outer ends produces a corrugated surface in the proximal zone of the root tips. In dicotyledons (e.g., soybean), there is no similar pellicle, but a stiff root tip is produced by a long multi-layered root cap, the proximal portion of which covers the elongating epidermal surface.     

Epidermal Patterning Genes Impose Non-cell Autonomous Cell Size Determination and have Additional Roles in Root Meristem Size Control

The regulation of cellular growth is of vital importance for embryonic and postembryonic patterning. Growth regulation in the epidermis has importance for organ growth rates in roots and shoots, proposing epidermal cells as an interesting model for cellular growth regulation. Here we assessed whether the root epidermis is a suitable model system to address cell size determination. In Arabidopsis thaliana L., root epidermal cells are regularly spaced in neighbouring tricho- (root hair) and atrichoblast (non-hair) cells, showing already distinct cell size regulation in the root meristem. We determined cell sizes in the root meristem and at the onset of cellular elongation, revealing that not only division rates but also cellular shape is distinct in tricho- and atrichoblasts. Intriguingly, epidermal-patterning mutants, failing to define differential vacuolization in neighbouring epidermal cell files, also display non-differential growth. Using these epidermal-patterning mutants, we show that polarized growth behaviour of epidermal tricho- and atrichoblast is interdependent, suggesting non-cell autonomous signals to integrate tissue expansion. Besides the interweaved cell-type-dependent growth mechanism, we reveal an additional role for epidermal patterning genes in root meristem size and organ growth regulation. We conclude that epidermal cells represent a suitable model system to study cell size determination and interdependent tissue growth.     

Inhibition of the gravitropic bending response of flowering shoots by salicylic acid.

The upward gravitropic bending of cut snapdragon, lupinus and anemone flowering shoots was inhibited by salicylic acid (SA) applied at 0.5 mM and above. This effect was probably not due to acidification of the cytoplasm, since other weak acids did not inhibit bending of snapdragon shoots. In order to study its mode of inhibitory action, we have examined in cut snapdragon shoots the effect of SA on three processes of the gravity-signaling pathway, including: amyloplast sedimentation, formation of ethylene gradient across the stem, and differential growth response. The results show that 1 mM SA inhibited differential ethylene production rates across the horizontal stem and the gravity-induced growth, without significantly inhibiting vertical growth or amyloplast sedimentation following horizontal placement. However, 5 mM SA inhibited all three gravity-induced processes, as well as the growth of vertical shoots, while increasing flower wilting. It may, therefore, be concluded that SA inhibits bending of various cut flowering shoots in a concentration-dependent manner. Thus, at a low concentration SA exerts its effect in snapdragon shoots by inhibiting processes operating downstream to stimulus sensing exerted by amyloplast sedimentation. At a higher concentration SA inhibits bending probably by exerting general negative effects on various cellular processes.     

Effects of shoot bending on lateral fate and hydraulics: invariant and changing traits across five apple genotypes

The aim of this work was to study the variability of physiological responses to bending and the relationship with hydraulic conductance of the sap pathway to the laterals for five apple genotypes. The study focuses on the fate of the laterals. The genetic variability of bending can have two sources: a genetic variability of stem geometry which can lead to differences in mechanical state; and a genetic variability of sensitivity to bending. Since the aim was to check if some genetic variability of sensitivity to bending exists, the genetic variability of shoot geometry was taken into account. To do so, bending was controlled by imposing different bending intensities using guides of different curvature conferring a similar level of deformation to the five genotypes. Bending was done either in the proximal zone or in the distal zone of shoots, in June and in the following winter, respectively. A Principal Component Analysis comparing upright and bent shoots revealed that bending in the proximal zone stimulated vegetative growth of buds which would otherwise stay latent. A second Principal Component Analysis restricted to bent shoots revealed that bending increased the abortion of laterals in the lower face of the shoots. The abortion phenomenon was to the detriment of sylleptic laterals or of inflorescence, depending on the genotype. There was a strong effect of position around the shoot on within-shoot hydraulics. Hydraulic conductance was significantly decreased in the lower face of the shoot bent in winter. This result suggested a causal relationship between this phenomenon and lateral abortion.     

Rhythmic Circumnutation Movement of the Shoots in Phaseolus Vulgaris L

The apex of growing stems in twining plants describes a rhythmic movement in space called circumnutation. By the method of orthogonal projections, the position in space of the apex can be determined constantly. The mathematical analysis of data allowed us to determine that far Phaseolus vulgaris L. (cv. Mangetout Blanc de Juillet) grown under constant illumination and temperature (25°C) the period is about 100 min. This movement has been related to rhythmic changes in the osmotic potential of the cells located in the bending zone of the stem for a constant period. These variations are longitudinally and laterally coordinated. Treatment with a solution of LiCI at 7 × 10 'M supplied to the root system induces a lengthening of the period. The effect of the treatment is reversed by K⁺ ions. From these results we deduce that rhythmic changes of the membranes are implicated in the circumnutation movements of twining plants.     

Cell growth analysis during steady and non-steady growth in leaves of perennial ryegrass (Lolium perenne L.) subject to defoliation

The effect of defoliation on leaf elongation rate (LER) and on the spatial distribution of epidermal cell lengths in the leaf growth zone was studied in vegetative main tillers of perennial ryegrass (Lolium perenne L. cv Modus) grown in a controlled environment. A new material approach was used to analyse the responses of epidermal cell expansion and production during the initial, non‐steady growth phase following defoliation. The analysis involved assigning an identity to individual expanding cells, assessing the displacement and estimating the expansion of cells with assigned identity during day 1 and day 2 after defoliation. LER decreased by 34% during the first 2 d after defoliation and did not recover to the pre‐defoliation rate within the 14 day regrowth period. Decreased LER on day 1 and day 2 after defoliation was associated with (i) a decrease in the length of the leaf growth zone; (ii) a decrease in the length at which epidermal cells stopped expanding; (iii) a reduced expansion of cells at intermediate growth stages; and (iv) a reduction in cell production (i.e. division) and an associated decrease in the number of expanding cells in the growth zone. However, defoliation had no effect on the expansion of cells located in the proximal part of the growth zone. Reduced LER at 14 d after defoliation was associated with a reduced cell production rate (27% lower than the pre‐defoliation rate) and decreased final cell size ( ? 28%).     

A developmental study of the epidermis of young roots ofZea mays L.

Summary In the apical meristems of main and young lateral roots of corn the uniseriate epidermis is clearly continuous with the most distal cell tier of the quiescent centre. These cells are characterized by the presence on their outer periclinal walls of material which forms the thin root cap junction layer over the apical pole and which thickens appreciably over the flanks of the meristem to form a distinctive extracellular deposit on the young epidermal cells. This material is polysaccharide in nature as indicated by strong periodic acid Schiff's positivity but its autofluorescence also suggests the presence of phenolic compounds.During their development the epidermal cells undergo marked shape change from periclinally flattened, polygonal at the root pole, through columnar on the meristem flank to tabular in the root hair zone. The mucigel thins markedly as cells become tabular but initiation of a root hair is characterized by deposition of polysaccharide on the inside of the periclinal wall where the hair will develop.     

THE PRIMARY ROOT EPIDERMIS OF PANICUM VIRGATUM L. I. ONTOGENY AND FINE STRUCTURE OF THE EPIDERMAL CYTOPLASMIC INCLUSIONS

The ontogeny of large, globular, epidermal cytoplasmic inclusions (ECI) in P. virgatum roots was studied at the ultrastructural level. These ECI were seen to originate in meristematic cells as small electron translucent vesicles. Subsequently, the ECI, which appeared to be temporary storage sites, were seen to enlarge and increase in density by accumulating masses of a granular matrix as well as some small vesicular inclusions. In the zone of elongation, as the epidermal cells matured, the ECI within each cell gradually fused and the contents were lost. The pattern of the ontogeny of the ECI in the growing epidermal cells was consistent with the presence of cells of different physiologies in the zone of cell elongation of these roots.     

Spatial and temporal quantitative analysis of cell division and elongation rate in growing wheat leaves under saline conditions

Leaf growth in grasses is determined by the cell division and elongation rates, with the duration of cell elongation being one of the processes that is the most sensitive to salinity. Our objective was to investigate the distribution profiles of cell production, cell length and the duration of cell elongation in the growing zone of the wheat leaf during the steady growth phase. Plants were grown in loamy soil with or without 120 mmol/L NaCl in a growth chamber, and harvested at day 3 after leaf 4 emerged. Results show that the elongation rate of leaf 4 was reduced by 120 mmol/L NaCl during the steady growth phase. The distribution profile of the lengths of abaxial epidermal cells of leaf 4 during the steady growth stage shows a sigmoidal pattern along the leaf axis for both treatments. Although salinity did not affect or even increased the length of the epidermal cells in some locations in the growth zone compared to the control treatment, the final length of the epidermal cells was reduced by 14% at 120 mmol/L NaCl. Thus, we concluded that the observed reduction in the leaf elongation rate derived in part from the reduced cell division rate and either the shortened cell elongation zone or shortened duration of cell elongation. This suggests that more attention should be paid to the effects of salinity on those properties of cell production and the period of cell maturation that are related to the properties of cell wall.     

Role of the SPIRAL1 gene family in anisotropic growth of Arabidopsis thaliana

Arabidopsis spiral1 (spr1) mutants show a right-handed helical growth phenotype in roots and etiolated hypocotyls due to impaired directional growth of rapidly expanding cells. SPR1 encodes a small protein with as yet unknown biochemical functions, though its localization to cortical microtubules (MTs) suggests that SPR1 maintains directional cell expansion by regulating cortical MT functions. The Arabidopsis genome contains five SPR1-LIKE (SP1L) genes that share high sequence identity in N- and C-terminal regions. Overexpression of SP1Ls rescued the helical growth phenotype of spr1, indicating that SPR1 and SP1L proteins share the same biochemical functions. Expression analyses revealed that SPR1 and SP1L genes are transcribed in partially overlapping tissues. A combination of spr1 and sp1l mutations resulted in randomly oriented cortical MT arrays and isotropic expansion of epidermal cells. These observations suggest that SPR1 and SP1Ls act redundantly in maintaining the cortical MT organization essential for anisotropic cell growth, and that the helical growth phenotype of spr1 results from a partially compromised state of cortical MTs. Additionally, inflorescence stems of spr1 sp1l multiple mutants showed a right-handed tendril-like twining growth, indicating that a directional winding response may be conferred to the non-directional nutational movement by modulating the expression of SPR1 homologs.     

Unequal distribution of osmiophilic particles in the epidermal periplasmic space of upper and lower flanks of gravi-responding rye coleoptiles

In various studies, auxin (IAA)-induced coleoptile growth has been reported to be closely correlated with an increased occurrence of osmiophilic particles (OPs) at the inner surface of the outer, growth-limiting epidermal cell wall, indicating a possible function related to the mechanism of IAA-induced wall loosening. In order to test whether changes in cell elongation rates of upper and lower flanks (UFs, LFs, respectively) during graviresponsive growth are reflected in appropriate changes in the occurrence of OPs, rye (Secale cereale L.) coleoptiles either as segments or as part of intact seedlings, were gravitropically stimulated by positioning them horizontally for 2 h. Ultrastructural analyses within the UFs and LFs of the upward-bending coleoptiles revealed a distinct imbalance in the occurrence of OPs. The number of OPs per transverse epidermal cell section of the elongation-inhibited UF on average amounted to twice the number of OPs counted in epidermal cell sections of the faster-growing LF. As a hypothesis, the results lead us to suggest that OPs are involved in the mechanism of wall loosening and that temporary growth inhibition of epidermal cells of the UF during upward bending is mediated by inhibition of OP entry into the cell walls. Thereby, more OPs accumulate near the inner surface of the outer wall of epidermal cells of the UF compared with the LF.     

Does salinity reduce growth in maize root epidermal cells by inhibiting their capacity for cell wall acidification?

The reduction in growth of maize (Zea mays L.) seedling primary roots induced by salinization of the nutrient medium with 100 millimolar NaCl was accompanied by reductions in the length of the root tip elongation zone, the length of fully elongated epidermal cells, and the apparent rate of cell production: Each was partially restored when calcium levels in the salinized growth medium were increased from 0.5 to 10.0 millimolar. We investigated the possibility that the inhibition of elongation growth by salinity might be associated with an inhibition of cell wall acidification, such as that which occurs when root growth is inhibited by IAA. A qualitative assay of root surface acidification, using bromocresol purple pH indicator in agar, showed that salinized roots, with and without extra calcium, produced a zone of surface acidification which was similar to that produced by control roots. The zone of acidification began 1 to 2 millimeters behind the tip and coincided with the zone of cell elongation. The remainder of the root alkalinized its surface. Kinetics of surface acidification were assayed quantitatively by placing a flat tipped pH electrode in contact with the elongation zone. The pH at the epidermal surfaces of roots grown either with 100 millimolar NaCl (growth inhibitory), or with 10 millimolar calcium ± NaCl (little growth inhibition), declined from 6.0 to 5.1 over 30 minutes. We conclude that NaCl did not inhibit growth by reducing the capacity of epidermal cells to acidify their walls.     

Microtubule reorientation in shoots precedes bending during the gravitropic response of cut snapdragon spikes

The microtubule reorientation during the gravitropic bending of cut snapdragon (Antirrhinum majus L.) spikes was investigated. Using indirect immunofluorescence methods, we examined changes in microtubule orientation in the cortex, endodermis and pith tissues of the shoot bending zone, in response to gravistimulation. Our results show that dense microtubule arrays were visible throughout the cortical, endodermal and pith shoot tissues, and that the transverse orientation of the microtubules (perpendicular to the growth axis) was specifically associated with the shoot growing bending zone. Microtubules showed gravity-induced kinetics of changes in their orientation, which occurred only in the upper stem flank and preceded shoot bending. While this observation, that the gravity-induced microtubule orientation precedes bending, was previously reported only in special above-ground organs such as coleoptiles and hypocotyls, our present study is the first to show that such patterns of change occur in mature flowering shoots. These changes were exhibited first in the upper flank of the cortex and then in the upper flank of the endodermis. No changes in microtubule orientation were observed in the cortex or endodermis tissues of the lower flanks or in the pith, suggesting that these tissues continue to grow during shoot gravistimulation. Our results imply that microtubules may be involved in growth cessation of the upper shoot flank occurring during the gravitropic bending of snapdragon cut spikes.