Apparent respiratory discrimination is correlated with growth rate in the shoot apex of sunflower (Helianthus annuus)

The literature offers no consensus as to whether the delta(13)C of respired CO(2) is identical to that of the respiratory substrate, perhaps because of differences in measurement technique and growth conditions. To address this issue, the delta(13)C of respired CO(2) from growing sunflower shoot apices was measured and compared with that of soluble carbohydrates extracted from the respiring tissues. Shoot apices were studied because any influence of growth and biosynthesis was expected to be maximally expressed in these rapidly growing tissues. The two most probable substrates, starch and soluble sugars, were similar in delta(13)C (P=0.46). The delta(13)C of respired CO(2) was enriched in (13)C compared with these putative substrates (P<0.0001). This apparent enrichment ranged from 2.2 per thousand-5.7 per thousand, and decreased with relative growth rate (P<0.0001). The respiratory enrichment was counterbalanced by a depletion in the tissue constructed from the residual carbohydrates. The depletion varied from 2.2 per thousand to 3.0 per thousand relative to soluble carbohydrates (P<0.05), as predicted from mass-balance arguments. These results support the idea that respired CO(2) is enriched relative to its substrates. Variation in growth rates may help to explain the variable amounts of respiratory discrimination described in the literature.     

Kinetin action on the development of microbody enzymes in sunflower cotyledons in the dark

Summary Removal of the roots from etiolated sunflower seedlings (Helianthus annuus L.) at various stages of development resulted in a premature or enhanced decline of the activities of catalase (E.C. 1.11.1.6) and isocitrate lyase (E.C. 4.1.3.1) (glyoxysomes), and hydroxypyruvate reductase (E.C. 1.1.1.26) and glycolate oxidase (E.C. 1.1.3.1) (leaf peroxisomes) in the cotyledons. Treatment of the cuttings with kinetin in the dark inhibited the loss of glyoxysomal enzyme activities and, at the same time, induced a three to fivefold increase in leaf-peroxisomal enzyme activities. The decline of glyoxysomal enzyme activities was also suppressed after application of both kinetin and cycloheximide (50 g/ml). The kinetin-mediated rise of leaf-peroxisomal enzyme activities was strongly curtailed in the presence of cycloheximide. The dose effect curves of kinetin action on the development of glyoxysomal and of leaf-peroxisomal enzyme activities were different, supporting the hypothesis that the mechanisms of kinetin action on the two microbody enzyme systems are different. Nitrogen nutrition of intact seedling effected the development of microbody enzyme activities in a pattern closely resembling that of kinetin action. Presumably endogenous cytokinins produced by the roots are involved in the regulation of microbody enzyme activities in cotyledons of dark-grown sunflower seedlings.     

Developmental control of cell division patterns in the shoot apex

The shoot apical meristem is a group of rapidly dividing cells that generate all aerial parts of the plant. It is a highly organised structure, which can be divided into functionally distinct domains, characterised by specific proliferation rates of the individual cells. Genetic studies have enabled the identification of regulators of meristem function. These factors are involved in the formation and maintenance of the meristem, as well as in the formation of the primordia. Somehow, they must also govern cell proliferation rates within the shoot apex. Possible links between meristem regulators and the cell cycle machinery will be discussed. In order to analyse the role of cell proliferation in development, cell cycle gene expression has been perturbed using transgenic approaches and mutation. The effect of these alterations on growth and development at the shoot apex will be presented. Together, these studies give a first insight into the regulatory networks controlling the cell cycle and into the significance of cell proliferation in plant development.     

Steroid hormone effects on growth and apical dominance of sunflower

External application of estradiol-17β increased shoot growth but decreased root growth of sunflower seedlings. It completely inhibited cotyledonary axillary bud development in decapitated plants at the concentration of 1 μg/plant. Concentrations lower than this promoted cotyledonary axillary bud formation. Testosterone on the other hand inhibited both shoot and root growth and promoted cotyledonary axillary bud formation at all the concentrations used. Progesterone at high (0.25,μg/plant) concentration promoted shoot growth but inhibited root growth. A low concentration (0.1 μg/plant) of progesterone produced the opposite effect.     

The role of the apex in normal and tropic growth of sunflower hypocotyls

Regional growth in vertical and horizontal etiolated sunflower hypocotyls from which the apical hook tissue had been either partly or wholly excised, was measured 24 h later, the regions having been demarcated with resin beads. Removal of the cotyledons (an excision which included the distal end of the shoot apex) had little effect on growth during this period but excision of the apical hook significantly reduced growth. In vertically orientated seedlings, removal of half of the hook severely reduced growth in all other growing regions and removal of the entire hook totally inhibited growth. This inhibition of growth was not a consequence of the removal of the region of growth but a consequence of the removal of a region on which growth was dependent. In horizontal seedlings, the situation was more complex inasmuch as a horizontal orientation itself induced growth in previously non-growing regions. This new growth was localised in its extent and was not as severely affected by progressive excision of the hook as was growth in vertical seedlings. The results are discussed in terms of overall growth co-ordination in the hypocotyl.     

Effect of darkening the cotyledons on the growth and curvature of the sunflower hypocotyl: evidence of hydraulic signalling

When both cotyledons of light-grown sunflower seedlings (Helianthusannuus L.) were darkened by covering them with aluminium foil,the resulting increase in the rate of elongation of the hypocotylwas closely proportional to the associated reduction in seedlingtranspiration and to the increase in xylem water potential (_PX.Covering only one cotyledon induced curvature away from thatside. This response was associated with a higher _PX in the vascularbundles connected directly with the covered cotyledon than inthose connected with the illuminated cotyledon. The water contentof peripheral tissues of the hypocotyl below the covered cotyledonwas also higher than that of similar samples from below theilluminated cotyledon. These results are consistent with thehypothesis that the effect of darkening the cotyledons on thegrowth and curvature of the hypocotyl is mediated by hydraulicsignalling, characterized by transmission to the hypocotyl ofan increase in _PX resulting from the reduction in cotyledontranspiration. Key words: Cotyledons, Helianthus annuus, hydraulic signalling, hypocotyl, transpiration, water potential     

Flower primordium formation at the Arabidopsis shoot apex: quantitative analysis of surface geometry and growth

Geometry changes, especially surface expansion, accompanying flower primordium formation are investigated at the reproductive shoot apex of Arabidopsis with the aid of a non-invasive replica method and a 3-D reconstruction algorithm. The observed changes are characteristic enough to differentiate the early development of flower primordium in Arabidopsis into distinct stages. Primordium formation starts from the fast and anisotropic growth at the periphery of the shoot apical meristem, with the maximum extension in the meridional direction. Surprisingly, the primordium first becomes a shallow crease, and it is only later that this shape changes into a bulge. The bulge is formed from the shallow crease due to slower and less anisotropic growth than at the onset of primordium formation. It is proposed that the shallow crease is the first axil, i.e. the axil of a putative rudimentary bract subtending the flower primordium proper, while the flower primordium proper is the bulge formed at the bottom of this axil. At the adaxial side of the bulge, the second axil (a narrow and deep crease) is formed setting the boundary between the flower primordium proper and the shoot apical meristem. Surface growth, leading to the formation of the second axil, is slow and anisotropic. This is similar to the previously described growth pattern at the boundary of the leaf primordium in Anagallis.     

Inhibition of growth and synchronised cell division in the shoot apex in relation to flowering in Silene

When plants of Silene coeli-rosa (L.) Godron were induced by seven long days, then exposed to darkness for 48 h before being returned to short days, they went on to initiate flowers with a delay of about 2 d. The synchronisation of cell division which normally occurs before flower initiation was suppressed, showing that it is not essential for flowering. Periods of darkness of up to 240 h inhibited apical growth and leaf initiation but did not prevent eventual flowering in short days. The commitment of the apex to flower was therefore maintained while apical growth was inhibited.Abbreviations SD short day(s) - LD long day(s)     

Surface growth at the reproductive shoot apex of Arabidopsis thaliana pin-formed 1 and wild type

With the aid of a non-destructive replica method and computational protocol, surface geometry and expansion at the reproductive shoot apex are analysed for pin-formed 1 (pin1) Arabidopsis thaliana and compared with the wild type. The observed complexity of geometry and expansion at the pin1 apex indicates that both components of shoot apex growth, i.e. the meristem self-perpetuation and initiation of lateral organs, are realized by the pin1 apex. The realization of the latter component, however, is only occasionally completed. The pin1 apex is generally dome-shaped, but its curvature is not uniform, especially later during apex ontogeny, when bulges and saddle-shaped regions appear on its periphery. The only saddle-shaped regions at the wild-type shoot apex are creases separating flower primordia from the meristem. Surface expansion at the pin1 apex is faster than at the wild type. In both pin1 and wild type the apex surface is differentiated into regions of various areal strain rates. In the pin1 apex, but not in the wild type, these regions correspond to the geometrically distinguished central and peripheral zones. Expansion of the central zone of the pin1 apex is nearly isotropic and slower than in the peripheral zone. The peripheral zone is differentiated into ring-shaped portions of different expansion anisotropy. The distal portion of this zone expands anisotropically, similar to regions of the wild-type apex periphery, which contact older flower primordia. The proximal portion expands nearly isotropically, like sites of flower initiation in the wild type. The peripheral zone in pin1 is surrounded by a 'basal zone', a sui generis zone, where areal strain rates are low and expansion is anisotropic. The possible relationships between the observed regions of different expansion and the various gene expression patterns in the pin1 apex known from the literature are discussed.     

The simulation model of growth and cell divisions for the root apex with an apical cell in application to Azolla pinnata

In contrast to seed plants, the roots of most ferns have a single apical cell which is the ultimate source of all cells in the root. The apical cell has a tetrahedral shape and divides asymmetrically. The root cap derives from the distal division face, while merophytes derived from three proximal division faces contribute to the root proper. The merophytes are produced sequentially forming three sectors along a helix around the root axis. During development, they divide and differentiate in a predictable pattern. Such growth causes cell pattern of the root apex to be remarkably regular and self-perpetuating. The nature of this regularity remains unknown. This paper shows the 2D simulation model for growth of the root apex with the apical cell in application to Azolla pinnata. The field of growth rates of the organ, prescribed by the model, is of a tensor type (symplastic growth) and cells divide taking principal growth directions into account. The simulations show how the cell pattern in a longitudinal section of the apex develops in time. The virtual root apex grows realistically and its cell pattern is similar to that observed in anatomical sections. The simulations indicate that the cell pattern regularity results from cell divisions which are oriented with respect to principal growth directions. Such divisions are essential for maintenance of peri-anticlinal arrangement of cell walls and coordinated growth of merophytes during the development. The highly specific division program that takes place in merophytes prior to differentiation seems to be regulated at the cellular level.     

Cell polarity and the control of apical growth in Streptomyces

Streptomyces cells grow by building cell wall at one pole-the hyphal tip. Although analogous to hyphal growth in fungi, this is achieved in a prokaryote, without any of the well-known eukaryotic cell polarity proteins, and it is also unique among bacterial cases of cell polarity. Further, polar growth of Streptomyces and the related mycobacteria and corynebacteria is independent of the MreB cytoskeleton and involves a number of coiled-coil proteins, including the polarity determinant DivIVA. Recent progress sheds light on targeting of DivIVA to hyphal tips and highlight protein phosphorylation in the regulation of actinobacterial growth. Furthermore, cell polarity affects not only cell envelope biogenesis in Streptomyces, but apparently also assembly of fimbriae, conjugation and migration of nucleoids.     

Hide and seek: uncloaking the vegetative shoot apex of Arabidopsis thaliana

Leaf primordia are iteratively formed on the flanks of the shoot apical meristem (SAM) at the vegetative shoot apex of Arabidopsis thaliana. The youngest leaf primordia and the SAM are extensively covered by older proliferating leaves, making it difficult to obtain accurate volumetric data from these structures. Combination of serial histological sections combined with 3D reconstruction software allowed us to acquire such data. Here, we compared the SAMs of wild‐type plants of the Columbia‐0 and Landsberg erecta ecotypes with those of clavata3‐2 (clv3‐2) mutants, which produce an enlarged SAM. In addition, the SAM size and morphology of plants over‐expressing the gibberellin‐20 oxidase (GA20OX) gene was examined, and the effect of mild osmotic stress on primordium size was measured. Efficient 3D visualization of gene expression patterns is also possible with this method, as illustrated by the analysis of SHOOTMERISTEMLESS:GUS and WUSCHEL:GUS reporter lines.     

Effects of Pluronic F-68 on shoot regeneration from cultured jute cotyledons and on growth of transformed roots

The effects have been studied of the non-ionic surfactant, Pluronic F-68, on the growth in culture of jute (Corchorus capsularis L.) cotyledons with attached petioles, cotyledon explants and transformed roots. Supplementation of culture medium with 0.001–0.5% (w/v) of either commercial grade Pluronic F-68 or a purified fraction prepared by passage through silica gel, stimulated shoot production from the petioles of C. capsularis var. D154 and C134 cotyledons. This effect was most marked in C134, because of the failure of control cotyledons to produce shoots in the absence of Pluronic. Plants regenerated from Pluronic-treated cotyledons were morphologically normal. Growth of transformed roots of C. capsularis var. D154 was stimulated in medium supplemented with commercial grade or purified Pluronic F-68, with maximum increases in both fresh and dry weights with 0.1% (w/v) of the surfactant. Roots cultured in the presence of Pluronic F-68 could be maintained without sub-culture for up to 70 days, whereas roots cultured in the absence of Pluronic required subculture every 7 days, to prevent necrosis. Transformed roots also produced callus in the presence of 0.001–1.0% (w/v) of either commercial grade or purified Pluronic. The biotechnological implications of these results are discussed in relation to the potential value of non-ionic surfactants as growth-stimulating additives to plant culture media.Abbreviations NAA -naphthaleneacetic acid - BA 6-benzyladenine - IAA indole-3-acetic acid - MS Murashige & Skoog (1962)     

Mitotic activity in wheat shoot apical meristems: effect of dissection to expose the apical dome

An efficient method, less laborious than histological procedures, is described to screen relatively large numbers of shoot apices for mitotic activity. Mitotic activity of shoot apices of Triticum aestivum L. was observed by differential interference contrast (DIC) microscopy of apices infiltrated with a clearing fluid (chloral hydrate/phenol/lactic acid/dibutylphthalate/benzyl benzoate). Serial optical sections were viewed through entire vegetative apical domes and floral primordia. In vegetative shoots, mitotic cells were observed throughout the light and dark cycles of plants maintained in either 8 or 16 h photoperiods. Mitotic activity was lower in the dark phase and increased through the light cycle in both photoperiods. Cells in L1 and L2 layers at the summit of the apex were mitotically active and contributed to the developing shoot and floral structures. Thus, cells in L2 at the summit of vegetative apices are valid targets for transformation leading subsequently to modified germ line cells. Dissections to expose apices for DNA delivery inhibited mitotic activity; recovery periods greater than 48 h would be needed for restoration of normal activity. This suggests that a period of recovery from dissection would be beneficial for attempts at integrative transformation of apical cells.     

Peltate trichomes in the dormant shoot apex of Metrodorea nigra,a Rutaceae species with rhythmic growth

• In Metrodorea nigra, a Rutaceae species with rhythmic growth, the shoot apex in the dormant stage is enclosed by modified stipules. The young organs are fully covered with peltate secretory trichomes, and these structures remain immersed in a hyaline exudate within a hood-shaped structure. Our study focused on the morpho-functional characterization of the peltate trichomes and cytological events associated with secretion.
• Shoot apices were collected during both dormant and active stages and processed for anatomical, cytochemical and ultrastructural studies.
• Trichomes initiate secretion early on, remain active throughout leaf development, but collapse as the leaves expand; at which time secretory cavities start differentiation in the mesophyll and secretion increases as the leaf reaches full expansion. The subcellular apparatus of the trichome head cells is consistent with hydrophilic and lipophilic secretion. Secretion involves two vesicle types: the smaller vesicles are PATAg-positive (periodic acid/thiocarbohydrazide/silver proteinate) for carbohydrates and the larger ones are PATAg-negative. In the first phase of secretory activity, the vesicles containing polysaccharides discharge their contents through exocytosis with the secretion accumulating beneath the cuticle, which detaches from the cell wall. Later, a massive discharge of lipophilic substances (lipids and terpenes/phenols) results in their accumulation between the wall and cuticle. Release of the secretions occurs throughout the cuticular microchannels.
• Continued protection of the leaves throughout shoot development is ensured by replacement of the collapsed secretory trichomes by oil-secreting cavities. Our findings provide new perspectives for understanding secretion regulation in shoot apices of woody species with rhythmic growth.

     

Improvements in shoot apex regeneration of two fiber crops: cotton and kenaf

Cotton (Gossypium hirsutum L.) and kenaf ( Hibiscus cannabinus L.) belong to the Malvaceae family, and both are used as sources of fibers. Shoot apices from vigorous seedlings aseptically germinated from 3 different cultivars of both cotton and kenaf were used in this study. The cotton and kenaf shoot apex size was between 2–3 mm containing the meristem, unexpanded leaves, and a small portion of the cotyledon. Shoot apices were placed on 18 different media comparing full and 1/2 strength Murashige and Skoog (1962) plus vitamins, and combinations of 0, 0.1, and 1 mg l^-1 of naphthaleneacetic acid and 6-benzyladenine (BA). The shoot apices of both crops developed successfully without intervening callus formation, and no significant differences among cultivars were found. An average of 58% of the cotton shoot apices initiated shoot and rooted in full strength Murashige and Skoog (1962) plus vitamins in 6 weeks. For kenaf, an average of 92% of shoot apices initiated shoot and rooted in full strength Murashige and Skoog plus vitamins and 0.1 mg l^-1 BA in 3 weeks. All regenerated plants of both crops were phenotypically normal and set seeds. This revised version was published online in June 2006 with corrections to the Cover Date.