Apical Dome Research Articles

Relocalization of the calcium gradient and a dihydropyridine receptor is involved in upward bending by bulging of Chara protonemata, but not in downward bending by bowing of Chara rhizoids.

The localization of cytoplasmic free calcium and a dihydropyridine (DHP) receptor, a putative calcium channel, was recorded during the opposite graviresponses of tip-growing Chara rhizoids and Chara protonemata by using the calcium indicator Calcium Crimson and a fluorescently labeled dihydropyridine (FL-DHP). In upward (negatively gravitropically) growing protonemata and downward (positively gravitropically) growing rhizoids, a steep Ca2+ gradient and DHP receptors were found to be symmetrically localized in the tip. However, the localization of the Ca2+ gradient and DHP receptors differed considerably during the gravitropic responses upon horizontal positioning of the two cell types. During the graviresponse of rhizoids, a continuous bowing downward by differential flank growth, the Ca2+ gradient and DHP receptors remained symmetrically localized in the tip at the centre of growth. However, after tilting protonemata into a horizontal position, there was a drastic displacement of the Ca2+ gradient and FL-DHP to the upper flank of the apical dome. This displacement occurred after the apical intrusion and sedimentation of the statoliths but clearly before the change in the growth direction became evident. In protonemata, the reorientation of the growth direction started with the appearance of a bulge on that site of the upper flank which was predicted by the asymmetrically displaced Ca2+ gradient. With the upward shift of the cell tip, which is suggested to result from a statolith-induced displacement of the growth centre, the Ca2+ gradient and DHP receptors became symmetrically relocalized in the apical dome. No major asymmetrical rearrangement was observed during the following phase of gravitropic curvature which is characterized by slower rates of bending. Labeling with FL-DHP was completely inhibited by a non-fluorescently labeled dihydropyridine. From these results it is suggested that FL-DHP labels calcium channels in rhizoids and protonemata. In rhizoids, positive gravitropic curvature is caused by differential growth limited to the opposite subapical flanks of the apical dome, a process which does not involve displacement of the growth centre, the calcium gradient or calcium channels. In protonemata, however, it is proposed that a statolith-induced asymmetrical relocalization of calcium channels and the Ca2+ gradient precedes, and might mediate, the rearrangement of the centre of growth, most likely by the displacement of the Spitzenkorper, to the upper flank, which results in the negative gravitropic reorientation of the growth direction.

Formation of Pseudowhorls inPeperomia verticillata(L.) A. Dietr. Shoots Exhibiting Various Phyllotactic Patterns

Pseudowhorls are composed of leaves attached at almost equal levels and separated by single fully elongated internodes. InPeperomia verticillata, pseudowhorls form regularly in shoots exhibiting both spiral and truly whorled patterns of phyllotaxis. In spiral systems, they are composed of successive leaves positioned on the ontogenetic helix. In whorled phyllotaxis, leaves of two adjacent whorls occur at almost the same level and this way form a pseudowhorl. The number of leaves per pseudowhorl depends on the type of phyllotactic pattern and also the system of primordia packing. In all the shoots, regardless of the type of phyllotaxis, the number of leaves per pseudowhorl equals the number of leaf primordia in physical contact with the apical dome. It is the same as the higher number in contact parastichy pairs in spiral patterns or the number of orthostichies in whorled phyllotaxis. The pseudowhorled pattern is already manifested in the arrangement of leaf primordia. In spiral and whorled phyllotaxis the plastochron ratio calculated for primordia or whorls belonging to adjacent pseudowhorls is always higher than that calculated for members of one pseudowhorl. Moreover, angular distances between primordia of one pseudowhorl in spiral patterns are more uniform than expected in Fibonacci phyllotaxis. These observations were made on plants both growing in pots and culturedin vitro. 6-Benzylaminopurine, a synthetic cytokinin, added to the medium increases the mean number of leaves per pseudowhorl. It seems that this effect is indirect: phyllotaxis changes first rather than the destiny of a particular internode in a process of selective elongation.

The floral development of Pleuropetalum darwinii, an anomalous member of the Amaranthaceae

Summary The genus Pleuropetalum differs from the Amaranthaceae in its higher stamen and carpel number, coupled with numerous ovules. The floral development of Pleuropetalum darwinii is investigated with the SEM and is compared with the Amaranthaceae and other taxa of the Caryophyllales. Inflorescences are cymose with reduced partial inflorescences. Bracteoles are closely associated with the flower only separated by a short pedicel. Sepal initiation follows a 2/5 sequence but there is an important lag between the first two and the remaining sepals. The androecium arises on a circular primordium; the first four stamens are initiated as pairs opposite the two outer sepals, and are followed by paired or single stamens opposite the inner sepals. However, the number of stamens can fluctuate, or their sequence may be variable. The gynoecium starts its growth as a ring primordium with a number of apices enclosing a flattened apical dome. In early stages septal tissue can sometimes be observed. The central dome becomes completely subdivided in different sectors on which a large number of coiled ovules arise centrifugally. The development of flower and androecium shows similarities with other octandrous Caryophyllales, especially Phytolacca. The shared development is interpreted as a primitive feature. A comparison with other Amaranthaceae on the one hand, and with the Caryophyllales on the other shows evidence that a pluristaminate androecium with outer stamen pairs is plesiomorphic in the Amaranthaceae.

Biolistic-mediated transient gene expression in shoot apical meristems of the prickly-pear (Opuntia ficus-indica)

We have demonstrated the transient expression of the GUS gene in cells of the meristematic apical dome of Opuntia ficus-indica. DNA delivery into the cells was achieved using a biolistic PDS-1000He instrument from Bio-Rad Laboratories. The transforming DNA was coated in tungsten particles with diameter of 1.3 m m and the distance between the flying disk and the target tissue was 7.5cm and the shooting pressure was adjusted to 1200 psi. This is the first demonstration that the biolistic transformation system can be used to express a transgene in a member of the Cactaceae.

Reproductive and vegetative bud differentiation in Olea europaea L.

SummaryThe apical meristem of cv. Nocellara Etnea buds is described in the course of its anatomical progression, from July until bud break, the following spring. Differences in the apex of potentially vegetative and flower buds are already visible in the summer. In leaf buds the apex is a round dome, with two buttresses determined by leaf primordia. The cells of the outer layers are arranged in concentric rings, distinguished by different conditions of the nucleic acids. In flower buds the apical dome is less prominent, and the outer cell layers are histologically quite uniform. These characteristics remain unchanged in flower buds, and are little modified in leaf buds, until December; in January flower bud differentiation becomes more active, and both bud types start developing with increasing intensity. Our observations postulate a very early anatomical separation of the two bud types, on which further environmental factors might determine the final differentiation process.

Activation of latent origins of DNA replication in florally determined shoot meristems of long-day and short-day plants: Silene coeli-rosa and Pharbitis nil

Replicon spacing was measured during the S-phase of the cell cycle in shoot meristems of Silene coeli-rosa L., a long-day (LD) plant, and Pharbitis nil Chois, a short-day (SD) plant to examine the hypothesis that activation of latent origins of DNA replication is a feature of floral determination. Silene coeli-rosa was germinated and grown in SD for 28 d and then exposed to either a florally inductive combination of 7 LD + 2 SD, the last day of which coincides with determination of the sepal and stamen whorls, or was germinated and grown in 37 non-inductive SD. Pharbitis nil was germinated and grown in continuous light (CL) for 5 d and then given either 48 h of inductive darkness followed by 1 d of CL, the last day of which coincides with determination of the sepal, petal and stamen whorls, or given one of two independent non-inductive treatments: 48 h dark interrupted by red light (R) + 1 d of CL, or 8 d of CL. Following these treatments, each batch of plants was exposed to tritiated [methyl-3H]thymidine for 30, 60, 90 or 120 min. Apical domes were dissected, nuclei lysed and prepared as fibre autoradiographs from which replicon size was recorded. In S. coeli-rosa, replicon size was in the range 10–15 μm in SD (non-inductive) and 0–5 μm in LD (inductive) while in P. nil it was 10–15 μm in the 48 h dark interrupted by R, 5–10 μm in CL (both non-inductive) but was reduced to 0–5 μm in the 48 h dark treatment (inductive). Therefore, the recruitment of additional initiation points for DNA replication occurred in both a LD and a SD plant immediately before the appearance of floral organs. The data are consistent in showing that a shortening of S-phase, which is a characteristic feature of florally determined shoot meristems for both species, is brought about by the activation of latent origins of DNA replication.

Hypergravity can reduce but not enhance the gravitropic response of Chara globularis protonemata.

The relationship between the position of the statoliths and the direction and rate of tip growth in negatively gravitropic protonemata of Chara globularis was studied with a centrifuge video microscope. Cells placed perpendicularly to the acceleration vector (stimulation angle 90 degrees) showed a gradual reduction of the gravitropic curvature with increasing accelerations from 1 g to 8 g despite complete sedimentation of all statoliths on the centrifugal cell flank. It is argued that the increased weight of the statoliths in hypergravity impairs their acropetal transport which is induced when the cell axis deviates from the normal upright orientation. When the statoliths were centrifuged deep into the apical dome at 6 g and a stimulation angle of 170 degrees the gravitropic curvature after 1 h was identical to that determined for the same cells at 1 g and the same stimulation angle. This indicates that gravitropism in Chara protonemata is either independent of the pressure exerted by the statoliths on an underlying structure or is already saturated at 1 g. When the statoliths were moved along the apical cell wall at 8 g and the stimulation angle was gradually increased from 170 degrees to 220 degrees the gravitropic curvature reverted sharply when the cluster of statoliths passed over the cell pole. This experiment supports the hypothesis that in Chara protonemata asymmetrically distributed statoliths inside the apical dome displace the Spitzenkorper and thus the centre of growth, resulting in gravitropic bending. In contrast to the positively gravitropic Chara rhizoids, no modifications either in the transport of statoliths during basipetal acceleration (6 g, stimulation angle 0 degree, 5 h) or in the subsequent gravitropic response could be detected in the protonemata. The different effects of centrifugation on the positioning of statoliths in Chara protonemata and rhizoids indicate subtle differences in the function of the cytoskeleton in both types of cells.

Distribution and dynamics of the cytoskeleton in graviresponding protonemata and rhizoids of characean algae: exclusion of microtubules and a convergence of actin filaments in the apex suggest an actin-mediated gravitropism.

The organization of the microtubule (MT) and actin microfilament (MF) cytoskeleton of tip-growing rhizoids and protonemata of characean green algae was examined by confocal laser scanning microscopy. This analysis included microinjection of fluorescent tubulin and phallotoxins into living cells, as well as immunofluorescence labeling of fixed material and fluorescent phallotoxin labeling of unfixed material. Although the morphologically very similar positively gravitropic (downward growing) rhizoids and negatively gravitropic (upward growing) protonemata show opposite gravitropic responses, no differences were detected in the extensive three-dimensional distribution of actin MFs and MTs in both cell types. Tubulin microinjection revealed that in contrast to internodal cells, fluorescent tubulin incorporated very slowly into the MT arrays of rhizoids, suggesting that MT dynamics are very different in tip-growing and diffusely expanding cells. Microtubules assembled from multiple sites at the plasma membrane in the basal zone, and a dense subapical array emerged from a diffuse nucleation centre on the basal side of the nuclear envelope. Immunofluorescence confirmed these distribution patterns but revealed more extensive MT arrays. In the basal zone, short branching clusters of MTs form two cortical hemicylinders. Subapical, axially oriented MTs are distributed in equal density throughout the peripheral and inner cytoplasm and are closely associated with subapical organelles. Microtubules, however, are completely absent from the apical zones of rhizoids and protonemata. Actin MFs were found in all zones of rhizoids and protonemata including the apex. Two files of axially oriented bundles of subcortical actin MFs and ring-like actin structures in the streaming endoplasm of rhizoids were detected in the basal zones by microinjection or rhodamine-phalloidin labeling. The subapical zone contains a dense array of mainly axially oriented actin MFs that co-distribute with the subapical MT array. In the apex, actin MFs form thicker bundles that converge into a remarkably distinct actin patch in the apical dome, whose position coincides with the position of the endoplasmic reticulum aggregate in the centre of the Spitzenkörper. Actin MFs radiate from the actin patch towards the apical membrane. Together with results from previous inhibitor studies (Braun and Sievers, 1994, Eur J Cell Biol 63: 289-298), these results suggest that MTs have a stabilizing function in maintaining the polar cytoplasmic and cytoskeletal organization. The motile processes, however, are mediated by actin. In particular, the actin cytoskeleton appears to be involved in the structural and functional organization of the Spitzenkörper and thus is responsible for controlling cell shape and growth direction. Despite the similar structural arrangements of the actin cytoskeleton, major differences in the function of actin MFs have been observed in rhizoids and protonemata. Since actin MFs are more directly involved in the gravitropic response of protonemata than of rhizoids, the opposite gravitropsim in the two cell types seems to be based mainly on different properties and activities of the actin cytoskeleton.

The diversity of lectin-detectable sugar residues on root hair tips of selected legumes correlates with the diversity of their host ranges for rhizobia

Summary. Four legumes and two nonlegumes were investigated for the presence of sugars at the tips of their root hairs, using commercially available lectins which have specific affinities for certain sugars. It was found that while only one lectin (RCA-I, which binds to [3D-galactose) bound to narrow-host-range legumes and one nonlegume, five out of ten lectins tested bound to the root hair tips of the broad-host-range legume siratro (Macroptilium atropurpureum). None of the lectins tested bound to any part of Arabidopsis roots. Binding of lectins (and therefore the presence of sugars) only at the tips of growing root hairs has led us to deduce that the sugars are the glyco moiety of membrane-bound glycoproteins that are recycled at the base of the tip apical dome, along with excess plasma membrane that is known to be recycled there. As many kinds of signal transduction molecules are membrane-bound glycoproteins, we suggest that these sugars may be involved in early interactions with rhizobia, and that the broad-host-range legume siratro has more kinds of sugars to cope with the wide range of rhizobia it is able to accept for symbiotic interactions. As far as we know, this is the first report of multiple sugars at the same surface area of a tip-growing plant cell.

Cytoskeletal control of polar growth in plant cells

There are two quite different modes of polar cell expansion in plant cells, namely, diffuse growth and tip growth. The direction of diffuse growth is determined by the orientation of cellulose microfibrils in the cell wall, which in turn are aligned by microtubules in the cell cortex. The orientation of the cortical microtubule array changes in response to developmental and environmental signals, and recent evidence indicates that microtubule disassembly/reassembly and microtubule translocation participate in reorientation of the array. Tip growth, in contrast, is governed mainly by F-actin, which has several putative forms and functions in elongating cells. Longitudinal cables are involved in vesicle transport to the expanding apical dome and, in some tip growers, a subapical ring of F-actin may participate in wall-membrane adhesions. The structure and function of F-actin within the apical dome may be variable, ranging from a dense meshwork to sparse single filaments. The presence of multiple F-actin structures in elongating tips suggests extensive regulation of this cytoskeletal array.

In vitro morphogenesis of Sorghum bicolor (L.) moench: Efficient plant regeneration from shoot apices

Summary An efficient and reproducible plant regeneration system has been developed from shoot apices of aseptically germinated seedlings of sorghum. After four passages of culture at two-week intervals on MS basal medium containing 2 or 4 mg/L BA, the shoot apices produced multiple shoot clumps, each capable of regenerating more than 100 shoots, via an intensive differentiation of both axillary and adventitious buds. When the shoot apices were cultured on MS medium supplemented with 0.5 mg/L 2,4-D and 2 or 4 mg/ L BA, a highly efficient differentiation of adventitious buds was initiated from enlarged apical domes and thickened leaf bases of the shoot apices six weeks after inoculation. Over 200 shoots were induced from a shoot apex eight weeks after culture. Somatic embryos were produced direcdy from the enlarged and organized apical domes of primary and secondary shoots without apparent callus formation after subsequent subcultures of the multiple shoot clumps on MS medium with 0.5 mg/L 2,4-D and 2 or 4 mg/L BA. All 18 sorghum genotypes tested followed a similar differentiation and regeneration pattern at a high frequency. Most of the multiple shoots and somatic embryos developed into fertile plants after rooting on IBA-containing MS medium and transfer to greenhouse.

Cellular Origins of Kinocilia, Stereocilia, and Microvilli on Tentacles of Sea Anemones of the Genus Calliactis (Cnidaria: Anthozoa)

Sea anemones feed by orchestrated movements of tentacles covered with ciliary cones overlying both nematocytes and sensory neurons and mediating nematocyst discharge into prey. The purpose of this study was to investigate the functional morphology of tentacle sensory receptors on Calliactis parasitica and C. tricolor with a combination of scanning electron microscopy (SEM) of surface features and transmission electron microscopy (TEM) of internal cellular details. We found that the ciliary cones of two types of nematocysts differed: mastigophore-containing nematocytes had an inner ring of large-diameter stereocilia, and basitrich-containing nematocytes had small-diameter stereocilia. Two types of sensory neurons were identified: one had a ciliary cone with an inner ring of large-diameter stereocilia, and the other had a long kinocilium surrounded by a basal ring of short, stubby microvilli. Varying lengths of stereocilia on peripheral supportive cells contributed to both ciliary cones of nematocytes and sensory neurons. Both the supportive epithelial cells with stereocilia and the nonsupportive epithelial cells without stereocilia had a single long kinocilium with a basal body, an accessory centriole, and a thick striated rootlet. Gland cells had a short cilium surrounded by even shorter microvilli. Spirocysts formed apical domes on spirocytes, which had two peripheral rings of short microvilli but no cilium. We believe this combined SEM and TEM study of sensory hairs on tentacles of two species of Calliactis adds to our knowledge of ciliary structures and their cellular associations in sea anemones. Additional key words: ciliary cones, nematocytes, sensory cells, gland cells Sea anemones have feeding tentacles covered with ciliary cones which overlay nematocysts and sensory neurons (Pantin 1942). The ciliary cones have in common a kinocilium central to several rings of stereocilia of varying lengths derived from adjacent supportive cells (Mariscal 1974a,b; Peteya 1975; Mariscal & Bigger 1976). The kinocilium is characterized by a typical 9x2 + 2 pattern of microtubules, whereas the stereocilia have filamentous cores of actin similar to those of vertebrate hair cells (Watson & Roberts 1995). Ciliary cones of sea anemones have been compared to vertebrate hair bundles both morphologically and physiologically (Mariscal 1974a; Watson & Roberts 1995; Mire & Watson 1997). A directional displacement of a vertebrate hair bundle that stretches tip links between adjacent stereocilia and opens transduction channels causes depolarization of the hair cell (Osborne et al. 1984; Markin & Hudspeth 1995). Bundle deflection of sensory neuron ciliary cones having tip links between adjacent stereocilia induces ion channels a Author for correspondence. E-mail: westfall@vet.ksu.edu to open, resulting in depolarization of supportive cells on one side of the complex (Mire & Watson 1997). Ciliary cones of nematocytes and of sensory neurons in sea anemones appear similar. At first, it was believed that chemoreception of N-acetylated sugars by supportive cells caused nematocyte ciliary cones to elongate and frequency tune to movements of swimming prey, leading to discharge of nematocysts (Watson & Hessinger 1989, 1991). This view supports the independent effector hypothesis of nematocyst discharge (Pantin 1942). More recent work, using better optics, has demonstrated that the sensory neuron/supporting cell complexes respond to N-acetylated sugars and elongate their hair bundles to tune to movements of swimming prey (Mire-Thibodeaux & Watson 1994; Watson & Roberts 1995). Those studies suggested that the nervous system plays a role in nematocyst discharge. In the present study, we have used a combination of SEM and TEM to distinguish ciliary cones of sensory neurons from those of the nematocytes and to further differentiate between various kinds of nemaThis content downloaded from 157.55.39.144 on Wed, 21 Sep 2016 05:31:38 UTC All use subject to http://about.jstor.org/terms Sensory structures on tentacles of sea anemones tocytes in the genus Calliactis. We have also investigated the cellular origins of other sensory structures such as kinocilia, stereocilia, and microvilli.

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.

Comparative study of the development of zygotic and somatic embryos of chickpea ( Cicer arietinum L.)

The early stages of development of zygotic and somatic embryos (globular, heart and early cotyledonary stage) were studied in chickpea ( Cicer arietinum L.). Immature pods were collected 3–15 days after pollination (DAP) and used for the isolation of zygotic embryos. Immature cotyledonary segments and mature embryo axes of cultivar PG12 were cultured on Murashige and Skoog's (MS) basal medium supplemented with 3.0 mg l −1 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), for the induction of somatic embryos. The suspensor of zygotic embryo of chickpea was found to consist of six tiers of biseriate cells. By contrast somatic embryos were raised on a multiseriate suspensor-like stalk. The size of a globular-stage somatic embryo was much larger than zygotic embryo at a similar stage of development. The initiation of leaf primordia from the shoot apical meristem was also studied. The cells that composed the apical dome differed, in that the cells of zygotic embryo were flat in comparison to the curved cells of somatic embryo. Germinated seedlings and converted somatic embryo showed similarity in organization, differing only in the size of cotyledons.

Centrosome and microtubule dynamics in apical cells ofSphacelaria rigidula (Phaeophyceae) treated with nocodazole

Treatment of young thalli ofSphacelaria rigidula with 0.04 μg of nocodazole (Nz) per ml for up to 36 h affects microtubules (Mts) only slightly, but blocks a large number of mitotic cells in metaphase, without disruption of the metaphase plate. Higher concentrations of Nz (0.1 μg/ml) depolymerize interphase Mts. Only a few perinuclear and some short Mts resist and remain associated with the centrosomes. Fragmented Mts or groups of Mts sometimes remain in the apical dome. After treatment with 0.1 μg of Nz per ml, prometaphase cells are blocked at metaphase, while post-metaphase cells become binuclear, due to the failure of cytokinesis. With anticentrin immunofluorescence, a positive centrin signal is always observed in the centrosome area. Centrosome duplication is not affected by Nz, but separation is disturbed. After recovering for 2–4 h, most of the blocked metaphases proceed normally. In such cells duplicated centrosomes are seen in different stages of separation. In some cells independent aster-like microtubule configurations appear in the apical dome, occasionally displaying centrin at their centre. During recovery various configurations of bimitosis or multipolar mitosis were found. The multipolar spindles may share common centrosomes. Up to four centrosomes may accompany each nucleus. In some 24 h treated cells, as well as in cells recovering for 2 h, the centrin-positive structure is rod-like, extending in opposite directions from the usual position to the poles. Electron microscopical examination of thin sections revealed that the growth pattern of the apical cells is disrupted after Nz treatment. The observations show that: (a) the Mt cytoskeleton is involved in maintaining the polarity and growth pattern of apical cells, (b) mitosis is blocked by low concentrations of Nz without significant depolymerization of Mts, (c) the centrosome cycle is independent of the nuclear cycle, (d) centrosome separation and differentiation are disturbed by Nz treatment, (e) during recovery from Nz treatment, centrosomal material that may have separated from the centrosomes, as well as Mt fragments that resisted depolymerization, may operate as Mt nucleation centres.

Accelerated Early Growth of Rice at Elevated CO2 (Is It Related to Developmental Changes in the Shoot Apex?).

The influence of elevated CO2 on the development of the shoot apex and on subsequent vegetative growth and grain yield was investigated using rice (Oryza sativa L. cv Jarrah) grown in flooded soil at either 350 or 700 [mu]L CO2 L-1. At 8 d after planting (DAP), elevated CO2 increased the height and diameter of the apical dome and lengths of leaf primordia and tiller buds but had no effect on their numbers. By 16 DAP, there were five tiller buds in the apex at 700 [mu]L CO2 L-1 compared with only three tiller buds at 350 [mu]L CO2 L-1. These changes in development of the shoot apex at high CO2 were forerunners to faster development of the vegetative shoot at elevated CO2 between 11 and 26 DAP as evidenced by increases in the relative growth rates of the shoot and tillers. Accelerated development at high CO2 was responsible for the 42% increase in tiller number at the maximum tillering stage and the 57% enhancement of grain yield at the final harvest. The link between high CO2 effects on development during the first 15 DAP and final tiller number and grain yield was demonstrated by delaying exposure of plants to high CO2 for 15 d. The delay totally inhibited the tillering response to high CO2, and the increase in grain yield of 20% arose from a greater number of grains per panicle. Consequently, it can be concluded that accelerated development in the shoot apex early in development is crucial for obtaining maximum increases in grain yield at elevated atmospheric CO2 concentrations.

Gravitropism in tip-growing cells.

Unicellular tip-growing cells are excellent experimental systems in which to study gravitropism because cell extension, gravity sensing and the gravity response are all confined to the apical dome. Thus various approaches can be used to determine the distinct steps of the short gravitropic signal-transduction chain, which lacks a signal-transmission phase between the gravity-sensing cells and the competent responding target cells. Single-cell systems readily allow in-vivo observation of cellular processes during gravistimulation at 1 g, centrifugation, clinostatting and in microgravity, as well as permitting fluorescence labeling. Such diverse studies have revealed fascinating information on the mechanism of gravitropic tip growth, especially on the important role of the cytoskeleton in the positioning of the statoliths and in organizing and adjusting the Spitzenkorper. A hypothesis explaining the negative and positive gravitropism of Chara rhizoids and Chara protonemata has been put forward, which emphasizes the role of the actin cytoskeleton in the process of gravitropic tip-growth. Differences in the gravitropic responses of single-cell systems, however, reflect a diversity of gravitropic mechanisms, and represent an example of parallel evolution.

Isolation, Characterization, and in Situ Hybridization Studies of the Abundantly Transcribed Potato (Solanum tuberosum `Superior') Homeobox cDNA POTH1

Homeobox genes contain sequences coding for DNA-binding motifs. These sequences are highly conserved across both the animal and plant kingdoms. Members of this gene family code for transcription factors that are key regulators of developmental organization. In an attempt to further elucidate the developmental process of tuberization in the potato plant, a full-length homeobox cDNA has been isolated via sequence homology from an early tuberization stage cDNA library constructed from 4-day axillary bud tubers. This cDNA, POTH1, has been sequenced and characterized by Southern blotting, northern analysis, sequence comparison, and in situ hybridization. POTH1 is shown to be a class I homeobox gene with 45% overall similarity to Kn-1 of maize and 73% match in the homeobox region. Messenger RNA accumulation studies indicate that POTH1 mRNA, unlike most homeobox transcripts, is not limited to a particular organ or developmental stage. Instead, POTH1 mRNA accumulates in rapidly growing cells of the potato plant: the apical meristems, the vascular cambium, the edges of young leaves, axillary buds, and root tips. In situ studies indicate accumulation of POTH1 mRNA in the tunica and corpus layers of the apical dome of the shoot apex and the stolon apex. In the stolon, growth and proliferation of the parenchymal cells associated with the vascular cambium contribute to swelling during early stages of tuberization, and this tissue accumulates POTH1 mRNA. It is possible that POTH1 may be posttranscriptionally regulated in a particular organ or stage of growth, or that it is involved in a wider range of growth processes than most plant homeobox genes.

Early inflorescence and floral development in Zea mays land race Chapalote (Poaceae)

Tassel and ear primordia were collected from greenhouse‐grown specimens of the Mexican maize landrace Chapalote and prepared for scanning electron microscopic (SEM) examination. Measurements of inflorescence apices and spikelet pair primordia (spp) were made from SEM micrographs. Correlation of inflorescence apex diameter with number of spikelet ranks showed no significant difference between tassels and ears, except at the two‐rank level where the ear apical meristem had a significantly smaller diameter than corresponding two‐ranked tassels. Within individual inflorescences, spp in different ranks enlarged at comparable rates, although the rates from one ear to the next along the stem differed. In both tassels and ears, spp divide to form paired sessile and pedicellate spikelet primordia when the spp is 150 μm wide; ear axes are significantly thicker than tassel axes at the time of bifurcation. The similarities in growth between ear and tassel primordia lend further support to the hypothesis that both the maize tassel and ear are derived from a common inflorescence pattern, a pattern shared with teosinte. Inflorescence primordial growth also suggests that a key character difference between teosinte and maize, distichous vs. polystichous arrangement of spikelets, may be related to size of the apical dome and/or rate of primordium production by the apical meristem. There appears to be more than a single morphological event in the shift from vegetative to reproductive growth. The evocation of axillary buds (ears) is independent of, and temporally separated from, the transition to flowering at the primary shoot apex (tassel).

Serial Development of Foliar Gemmae in Tortula(Pottiales, Musci), an Ultrastructural Study

The development and liberation mechanism of foliar gemmae have been studied by electron microscopy in two mosses, Tortula latifoliaBruch and Tortula papillosaWils. The gemmae develop on the adaxial surface of mature leaves from single initial cells on both the lamina and costa in T. latifoliabut only on the costa in T. papillosa. Elongation of the initial cell is associated with the deposition of a highly extensible new wall whilst the old wall and cuticle in the apical dome rupture. The first division is transverse and separates a short basal cell embedded in the foliar tissue and a distal cell, or gemma primordium, protruding from the leaf surface. Subsequent divisions of the gemma primordium give rise to a six-to-eight-celled globose gemma with mucilaginous outer walls. During gemma development the basal cell produces a new wall and elongates again whilst the common wall with the gemma splits apart centripetally along the boundary between the old and new wall in the basal cell; plasmodesmal connections are gradually severed and eventually the young gemma remains connected to the basal cell only by mucilage. After separation of the first-formed gemma, the basal cell may expand and produce a second gemma by the same mechanism. The whole process may be repeated several times resulting in the formation of a chain of gemmae stuck together by mucilage and which are liberated only when the leaves are fully hydrated. Accumulation of abundant lipid deposits in the gemmae after symplasmic isolation reflects considerable photosynthetic autonomy.