Elimination of flowering and most cytological changes after selective long-day exposure of the shoot tip of Sinapis alba

Entire plants of Sinapis. alba exposed to a single long day were induced to flower. However, if only the shoot tip was exposed to the long, day, no flowering ensued. In the apical meristem of plants with only the shoot tip exposed to the long day, none of the ultra structural changes normally observed in the meristem of induced plants were detected, except for a marked increase in the number of mitochondria per cell. We conclude that the great majority of ultra structural changes normally occurring in the shoot meristem during floral transition are not direct effects of day length on the tip but are caused by signal(s) generated in induced leaves.     

Survival of Hop Stunt Viroid in the Hop Garden1)

A lot of 105 specimens from 25 families including weeds or wild plants which had grown naturally in the severely infested hop garden were tested for detecting reservoir plants for hop stunt viroid (HSV). HSV was detected in hop plants only. Susceptibility tests with various cultivated plants including 14 families indicated that hop and Humulus japonicus developed visible symptoms, while tomato was symptomless. When infected hop plant residues, leaves and cones, were left to be weather-beaten, infectivity of HSV was completely lost within 3 months. No transmission through the pollen or the ovule was demonstrated. HSV could survice in root systems of hop plants during the winter months. Based on these results, the route of HSV survival in the hop garden was discussed.     

Developmental Morphology of Hop Stunt Viroid-infected Hop Plants and Analysis of Their Cone Yield1)

Infection of hop plants with hop stunt viroid (HSV) results in the retardation of the growth rate except for the rate of leaf emergence and the disappearance of the fold-like structure over the epidermal cell. Mature cones from HSV-infected hop plants remained small-sized and the content of α-acid was half to one third of that of HSV–free hop cones. In HSV-infected hop cones, the lupulin glands are distributed most abundantly on the bracteoles and the perianths and their numbers are reduced by at least 60% of that in the HSV-free control. Scanning electron micrographs confirm that most of the lupulin glands on bracteoles from HSV-infected hop cones shrivel severely, but not those from HSV-free hop cones. They also reveal that the lupulin glands on the perianths from both, HSV-free and HSV–infected hop cones become withered. Moreover, spherical granules (1.2 to 1.9μm in diameter) were not observed on the surface of the lupulin glands from HSV-infected hop cones.     

Hop line-pattern virus in relation to the etiology and distribution of nettlehead disease

Hop line-pattern virus (HLPV) was transmissible by mechanical inoculation to hop plants; it induced characteristic severe symptoms in Humulus lupulus L. var. neo-mexicanus Nels. & Cockerell and the commercial derivatives College Cluster and Keyworth's Midseason, but none in the traditional English varieties of H. lupulus (e.g. Fuggle).
Mechanical transmission of hop nettlehead virus (HNV) was facilitated by the presence of HLPV in the test plants; hop seedlings and clonal plants escaped infection by sap inoculum that infected plants of two varieties already infected with HLPV. HNV was also transferred by stem contact and by knife cuts to plants carrying HLPV.
Infection with HLPV was latent in twelve nettlehead-diseased Fuggle plants from different fields, and in diseased and symptomless plants in a nettlehead outbreak in W.G.V., a variety that previously had escaped infection. It is suggested either that HLPV predisposes hop plants to infection with HNV or that nettlehead disease is caused by dual infection with both viruses.
Localized and scattered patterns of nettlehead spread were observed in hop plantations; these two types are usually attributed to different modes of spread which would be compatible with a complex etiology of the disease.     

Evaluation of 515 expressed sequence tags obtained from guard cells of Brassica campestris

The ear shoot of maize (Zea mays L.) consists of the peduncle and reproductive tissues (ear). Genetic mosaics induced by the unstable allele of thech1 locus were used for cell lineage analysis of the ear shoot. The unstablech1-m1 allele, caused by the insertion of a transposable element, gives rise to yellow-green seedlings with many small revertant green stripes. Rare plants with large revertant sectors comprising 30–50% of the plant were selected. Nineteen plants showing large sectors on the main stem were subjected to sector boundary analysis. Sectoring was recorded for the main stem, leaf subtending the ear shoot, peduncle, prophyll and ear. The reproductive part of the ear shoot, the ear, was scored after removal of the husks and subsequent exposure to light. In 18 cases the ear was non-sectored yellow-green or green. In an additional four cases, peduncle cell lineages entered the ear, but only in the proximal part, while the tip of the ear was non-sectored. Two additional ears showed longitudinal sectors which reached the tip of the ear. These observations indicate that in the lateral meristem of the ear shoot two types of cellular clone exist. One will generate the peduncle, the other will found the ear. Sector boundary analysis indicates that for the vegetative part of the ear shoot the number of meristem founder cells is high, whereas only a few initials are recruited for the formation of the ear. The presence of ear sectors not starting in the peduncle and reaching the ear tip, and the finding that the ear is frequently non-sectored, suggest that this organ derives from an apical type of growth.     

Apical Morphology in the Hop (Humulus lupulus) during Flower Initiation

The diameter and shape of the apical dome of hop shoots weremeasured in the perennial hop during four growing seasons inthe field as well as under controlled conditions. After 10 to12 weeks' growth with almost constant apical diameter, thereis a marked decrease in diameter in June. Since this takes placeafter the attainment of ‘ripeness to flower’ itmay be interpreted as the first sign of flower initiation. Holdingplants in non-inductive daylengths prevents this decrease. Thereis also a progressive change in the ratio of diameter to heightof the apical dome which occurs throughout the entire growthperiod and this appears to be independent of flower induction. The period between the change in apical diameter and the microscopicappearance of floral organs is analogous to the prefloral stagedescribed for other species.     

Apical Growth Cessation and Shoot Tip Abscission in Salix

Time course of apical shoot growth and shoot tip abortion in northern ecotypes (lat. 69°39′N, long. 18°37′E) of Salix pentandra and S. caprea have been investigated. In trees more than 15 years old growing under natural climatic conditions apical growth cessation and shoot tip abortion normally occurred in June-July when the day length still was 24 h. Application of GA₃, in spring to the apex effectively delayed growth cessation and shoot tip abortion. Application of kinetin was without effect. First-year seedlings of both species grew continuously at temperatue of 9 to 24°C in 24 h photoperiod. Short days induced apical growth cessation, but two to four (S. pentandra) or three to five (S. caprea) weeks of 12 h photoperiod were required to stop the elongation growth. The results indicated that the critical photoperiod for apical growth cessation in the used ecotype of S. pentandra was 16 to 18 h at 18°C. Short days had a minor effect only on the formation of apical leaf primordia in small seedlings. Development of axillary buds and radial growth were stimulated by short days when compared with long days. Small seedlings of both species (3 to 8 cm high at the start) formed terminal buds in short days, but in large seedlings (more than about 15 cm high) apical growth cessation was accompanied by shoot tip abortion. Abscisic acid applied to the apex or through a leaf did not induce growth cessation in S. pentandra seedlings grown in continuous light. The growth retardants CCC, B-9 and Phosphon D reduced growth rate under continuous light and induced shoot tip abortion in some plants. The effect of CCC was counteracted by GA₃. Apical growth cessation in short days was significantly delayed by a single GA₁ application.     

Independent control of organogenesis and shoot tip abortion are key factors to developmental plasticity in kiwifruit (Actinidia)

BACKGROUND AND AIMS: In kiwifruit (Actinidia), the number of nodes per shoot is highly variable and is influenced by genotype and environmental conditions. To understand this developmental plasticity, three key processes were studied: organogenesis by the shoot apical meristem during shoot growth; expansion of phytomers; and shoot tip abortion. METHODS: Studies were made of organogenesis and shoot tip abortion using light and scanning electron microscopy. The effect of temperature on shoot growth cessation was investigated using temperature indices over the budbreak period, and patterns of shoot tip abortion were quantified using stochastic modelling. KEY RESULTS: All growing buds began organogenesis before budbreak. During shoot development, the number of phytomers initiated by the shoot apical meristem is correlated with the number of expanding phytomers and the mean internode length. Shoot tip abortion is preceded by growth cessation and is not brought about by the death of the shoot apical meristem, but occurs by tissue necrosis in the sub-apical zone. For most genotypes studied, the probability of shoot tip abortion is higher during expansion of the preformed part of the shoot. Lower temperatures during early growth result in a higher probability of shoot tip abortion. CONCLUSIONS: Organogenesis and shoot tip abortion are controlled independently. All buds have the potential to become long shoots. Conditions that increase early growth rate postpone shoot tip abortion.     

A shoot meristem-like organ in animals; monopodial and sympodial growth in Hydrozoa

Thecate Hydrozoa produce stems from which polyps branch off. Similar to plants these stems form in two ways, either in a sympodial or in a monopodial type of growth. In the latter group a terminal organ develops which has similarities to a shoot apical meristem of higher plants: it elongates without a further differentiation. Similar to leaf formation in plants, thecate Hydrozoa produce polyps in a repetitive manner. This process continues during the whole life of the animal and has not yet been found to be limited by internal mechanisms. We studied the monopodially growing thecate Hydrozoon Dynamena pumila and suggest that the stem tip, the apical shoot meristem-like organ, is a polyp primordium hindered to develop into a polyp by the laterally developing polyps.     

Anatomy and development of the fern sporophyte

Recent research on the developmental anatomy and morphology of the fern sporophyte is reviewed. Detailed histological and experimental studies of the organization of the fern shoot apical meristem have reconfirmed the recently controversial role of the shoot apical cell as the single apical initial of the meristem. The shoot apical meristem is nevertheless an anatomically and functionally complex structure with a strongly zoned cytohistological organization. Fern shoot apex organization can be compared with that of seed plants. The control of leaf initiation and phyllotaxy remains poorly understood. Studies differ as to whether leaf initiation in ferns involves one leaf mother cell or a multicellular region of the shoot apex. The concept of non-appendicular fronds is refuted for living ferns. The later developmental changes in the determinate leaf apical and marginal meristems of the leaf primordium form an area that is still largely unexplored but could be investigated by methods similar to those used to study shoot and root apices. Branching in ferns is morphologiclaly and developmentally diverse. There is apparently more than one developmental mode of dichotomous branching, and several modes of lateral bud formation have been described, including the phyllogenous initiation of branches at the base of leaf primordia. Developmental changes in bud meristems related to apical dominance, inhibition, and bud activation is another major area for continued study. The traditional concept of the role of the root apical cell has been reestablished by studies similar to those made of the shoot apex. Detailed ultrastructural investigations of the root ofAzolla have given a sophisticated new picture of developmental processes in that organ. Fern roots show remarkably precise patterns of histogenesis in relation to apical segmentation. The formation of secondary vascular tissue inBotrychium suggests that the Ophioglossales may be related to the seed plants. The causal relationship of leaf (and branch and root) formation and the initiation of vascular tissue in the shoot needs more study. Although still poorly understood, protoxylem systems in ferns are variable and may have morphological and systematic significance. Recent investigations of hydraulic conductance in fern stems have found possible correlations of conductance levels with growth forms. The anatomical diversity of ferns makes comparative functional anatomy a promising field for future study.     

Endogenous Hormonal Profiles in Hop Development

Changes in gibberellins (GAs), indole-3-acetic acid (IAA), and cytokinins associated with the transition from vegetative growth to reproductive growth in Humulus lupulus L. buds and leaves harvested at fortnight intervals were studied. During vegetative growth, GA₁ increased gradually and the lowest content was observed during flower development. Both GA₃ and GA₄ showed a dramatic increase in the samples taken from the apical part of axillary branches from plants 4–5 m high, which corresponds to the maximum vegetative development prior to macroscopically visible inflorescences. Notable increases in the cytokinins trans-zeatin (t-Z), isopentenyladenine (iP), and the riboside and ribotide forms of iP were also obtained. The auxin, indole-3-acetic acid, was the most abundant plant hormone, and its content was highest during vegetative growth. These results show for the first time a relationship between endogenous hormone profiles and both vegetative and reproductive development in hop plants, which may be relevant for future research on the control of the flowering by exogenous hormone applications.     

Coordination of cell proliferation and cell fate decisions in the angiosperm shoot apical meristem.

A unique feature of flowering plants is their ability to produce organs continuously, for hundreds of years in some species, from actively growing tips called apical meristems. All plants possess at least one form of apical meristem, whose cells are functionally analogous to animal stem cells because they can generate specialized organs and tissues. The shoot apical meristem of angiosperm plants acts as a continuous source of pluripotent stem cells, whose descendents become incorporated into organ primordia and acquire different fates. Recent studies are unveiling some of the molecular pathways that specify stem cell fate in the center of the shoot apical meristem, that confer organ founder cell fate on the periphery, and that connect meristem patterning elements with events at the cellular level. The results are providing important insights into the mechanisms through which shoot apical meristems integrate cell fate decisions with cellular proliferation and global regulation of growth and development.     

Rates of Cell Division in the Shoot Apical Meristem of Pisum

The relative rates of cell division in different regions ofthe pea shoot apical meristem were obtained by measuring theincrease in the numbers of metaphases following applicationof colchicine to the plants. Absolute values for the rates ofcell division could be calculated since the average rate ofcell division for the whole apex was known. Measurements ofthe rates of cell division were obtained at defined intervalsduring the course of a single plastochron. Within each regionof the apex the rate of cell division did not change more thanabout two-fold throughout the plastochron. There was very littleor no increase in the rate of cell division associated withleaf initiation. The formation of a leaf primordium and thesubsequent growth of the apical dome apparently result fromchanges in the direction of growth rather than changes in therates of growth. Three main regions were discernible withinthe apical meristem: a region with a slow rate of cell divisionin the apical dome, a region of a faster rate of cell divisionat the base of the apical dome and at the site of initiationof procambial strands, and a region of an intermediate rateof cell division in the newly initiated leaf primordium andthe adjacent part of the shoot axis.     

Herbivory could unlock mutations sequestered in stratified shoot apices of genetic mosaics

Many higher plants have shoot apical meristems that possess discrete cell layers, only one of which normally gives rise to gametes following the transition from vegetative meristem to floral meristem. Consequently, when mutations occur in the meristems of sexually reproducing plants, they may or may not have an evolutionary impact, depending on the apical layer in which they reside. In order to determine whether developmentally sequestered mutations could be released by herbivory (i.e., meristem destruction), a characterized genetic mosaic was subjected to simulated herbivory. Many plants develop two shoot meristems in the leaf axils of some nodes, here referred to as the primary and secondary axillary meristems. Destruction of the terminal and primary axillary meristems led to the outgrowth of secondary axillary meristems. Seed derived from secondary axillary meristems was not always descended from the second apical cell layer of the terminal shoot meristem as is expected for terminal and primary shoot meristems. Vegetative and reproductive analysis indicated that secondary meristems did not maintain the same order of cell layers present in the terminal shoot meristem. In secondary meristems reproductively sequestered cell layers possessing mutant cells can be repositioned into gamete-forming cell layers, thereby adding mutant genes into the gene pool. Herbivores feeding on shoot tips may influence plant evolution by causing the outgrowth of secondary axillary meristems.     

Formation and maintenance of the shoot apical meristem

Development in higher plants is characterized by the reiterative formation of lateral organs from the flanks of shoot apical meristems. Because organs are produced continuously throughout the life cycle, the shoot apical meristem must maintain a pluripotent stem cell population. These two tasks are accomplished within separate functional domains of the apical meristem. These functional domains develop gradually during embryogenesis. Subsequently, communication among cells within the shoot apical meristem and between the shoot apical meristem and the incipient lateral organs is needed to maintain the functional domains within the shoot apical meristem.     

Studies on plant growth-regulating substances: Interactions between auxins and inactive analogues

Applications of streptomycin sulphate or protectant fungicides reduced numbers of secondary infections in severely infected hop plantings. When primary infected shoots treated with streptomycin were left on plants for 3 weeks no significant increase in secondary disease resulted. Streptomycin, which was absorbed rapidly by hop plants, could not be detected in expressed shoot sap by early June and had no effect on crop yield or quality.     

Surface-viewed shoot apex ofAngiopteris lygodiifolia Ros. (Marattiaceae)

Marattian ferns are thought to be an exception to the rule that a single apical cell is always present in the shoot apex of ferns; the occurrence of plural apical initials has been generally accepted for these ferns. However, a contradicting conclusion was reached in this study which examined the apical organization of the shoot ofAngiopteris lygodiifolia Ros., using fresh materials which had not been fixed. Shoot apices were hand-sectioned transversely into thin sections, including the surface layer of the shoot apex, which were observed by differential interference contrast microscopy without staining. In contrast with the generally accepted view, the shoot apex ofA. lygodiifolia was found to usually possess a single apical cell with three cutting faces. The segments cut off from the apical cell are regularly arranged in a helical sequence. The apical cell seems to actually function as an initial cell of the whole shoot apex. The shoot apices, particularly those of plants cultivated in a greenhouse, sometimes show somewhat irregular organization. In extreme cases, no apical cell is recognizable. However, even in these exceptional cases of such apparently irregular shoot apices, plural apical initials are not found.     

Effects of boron starvation on boron compartmentation, and possibly hormone-mediated elongation growth and apical dominance of pea (Pisum sativum) plants

Two experiments were carried out to study the effects of boron (B) deficiency on 7-day-old pea plants for 6 or 9 days under controlled growth chamber conditions. Growth and apical dominance (AD) of the plants and their B concentration and compartmentation were followed throughout the starvation period. Additionally, auxin (indoleacetic acid, IAA) concentration in the shoot apex and polar transport from it were measured along with the cytokinin (CK) concentration in the shoot apex and the roots. The results demonstrate that during a 6-day B-deficiency period, B concentration in the water-insoluble residue of the roots was very stable and could not easily be reduced. In contrast, B concentration in the cell sap fraction was very sensitive to external B supply. Twelve hours after transferring the plants from B-sufficient to B-deficient solutions, the B concentration in root cell sap declined to half the concentration of the control plants. In addition, B concentration in the new aerial plant parts, which developed after the onset of the B-deficiency treatment, was extremely low. A decline in elongation growth could be observed as soon as about 4 days after the imposition of B deficiency. This preceded the first measurable growth of lateral buds (release from AD). Before the onset of these morphological changes, there was a considerable decline in CK concentration, accompanied by a dramatic decrease in IAA export out of the shoot apex, a decline in IAA concentration in the shoot apex and the roots and a reduced capacity for polar IAA-transport. These changes are discussed as possible reasons for the observed reduction in elongation growth and AD. These hormonal changes themselves are possibly the result of the decreased symplasmic B concentration, which in turn may be responsible for the reduced concentration in apical CKs. A sequence of events, which may be causally related, is suggested to explain the effects of B deficiency on the growth and AD of pea plants.