Novel mutant phenotypes of a dark-germinating mutant<Emphasis Type="Italic"> dkg1</Emphasis> in the fern<Emphasis Type="Italic"> Ceratopteris richardii</Emphasis>

The mutant dark-germinating 1 (dkg1) of the fern Ceratopteris richardii was originally characterized by two phenotypes, germination in the dark and inhibition of germination by light. In this work, we examined whether other phenotypes are present in the gametophytic generation of the dkg1 mutant. Although dkg1 prothalli grown in darkness were elongated as in the case of the wild type, some developmental processes were found to proceed even in complete darkness: (1) the apical and subapical zones developed largely by forming a lateral meristem; (2) asymmetric cell division for rhizoid differentiation occurred in the subapical elongation zone; (3) an archegonium was formed in the proximity of the meristem; and (4) chloroplast relocation could occur without de novo protein synthesis. Furthermore, these processes were shown to be under the control of phytochrome in the wild-type gametophytes on the basis of red/far-red reversibility. These results indicate that the DKG1 gene is pleiotropic and is involved in several phytochrome-mediated responses in the gametophyte development of C. richardii.     

Comparative development of heavily asymmetric-cordate gametophytes of <Emphasis Type="Italic">Anemia phyllitidis</Emphasis> (Anemiaceae) focusing on meristem behavior

Development of heavily asymmetric cordate gametophytes of Anemia phyllitidis (Anemiaceae), one of the schizaeoid ferns, was examined using a sequential observation technique; epi-illuminated light micrographs of the same growing gametophytes were taken approximately every 24 h. The apical cell-like wedge-shaped cell was produced once from the terminal cell of a germ filament, but it stopped dividing soon after production of one or two derivative cells. Without a functional apical cell, the gametophyte developed by intercalary growth until the early stage of wing formation, and then the multicellular (pluricellular) meristem arose from the lower lateral side of the gametophyte. This was in sharp contrast to the observation that the multicellular meristem forms in place of the apical cell in typical cordate gametophytes. Loss of the functional apical cell probably caused a site-shift in the multicellular meristem of the Anemia phyllitidis gametophyte during evolution from apical to lateral. The results suggest that apical cell-based and multicellular meristems are primarily independent of each other. The multicellular meristem produced cells equally in the distal and proximal directions to form wings in both directions but proximally produced cells divided much less frequently. As a result, a heavily asymmetric gametophyte was formed.     

Effect of temperature and dark pretreatment on the germination of three species of Jamesonia (Pteridaceae,Polypodiopsida)

To study the influence of temperature on the germination ability of three species of Jamesonia (Jamesonia imbricata, Jamesonia scammaniae and Jamesonia rotundifolia), spores were cultured at 10°C, 15°C and 20°C. A temperature of 15°C was selected as representative of the natural annual average temperature of the paramo environment that Jamesonia species inhabit. In addition, a dark pretreatment of 2 days was tested to verify if germination was enhanced. The results indicated that germination of Jamesonia, considering the three species as a whole, is affected by temperature, but is independent of the dark treatment. All species showed higher and faster germination at 20°C, and exhibited a threshold minimum temperature around 10°C, below which germination is avoided or extremely low and delayed. This could suggest that spore germination in Jamesonia is adapted to establish gametophyte populations during frost‐free periods.     

Gametophyte development and reproduction of <Emphasis Type="Italic">Argyrochosma nivea</Emphasis> (Pteridaceae)

The gametophyte of Argyroschosma nivea was studied, mainly focusing in its morphological development, and in the apogamous production of sporophytes. Some observations on the spores were also made. As far as it is known, this is the second species of the genus whose gametophytes are studied. The germination pattern followed the Vittaria type. The subsequent developmental processes followed the Ceratopteris type. Some of the gametophytes reached an adult stage with a cordate, symmetric shape, but most of them developed as irregular, lobed prothalli. The sporophyte emerged from the anterior part of the prothallus, without formation of gametangia. First, a cell became active and originated a proliferating area of small cells. From this area, long glandular hairs were formed followed by a projected conical cluster of cells. The cluster elongated into a sporophytic structure and its apex became progressively spatulate and finally trilobulate, with marginal, glandular hairs, stomata and tracheids continuously produced. This sporophyte secreted granules of white farina from its beginnings. The production of farina in the sporophyte but not in the gametophyte could help to support the idea of the segregation of this species from its traditional location in Notholaena to Argyrochosma, as farinose gametophytes seem to be a synapomorphy of the notholenoids, group that includes Notholaena but not Argyroschoma.     

GAMETOPHYTES AND YOUNG SPOROPHYTES OF OPHIOGLOSSUM CROTALOPHOROIDES WALT.

The lens-shaped, dorsiventral gametophytes of Ophioglossum crotalophoroides are superficially different from the cylindrical gametophytes of other species of Ophioglossum. However, they have the same features as other Ophioglossum gametophytes (fundamentally axial organization, radially symmetrical apical meristem, radial distribution of gametangia) except that the cylindrical axis is reduced in length. Young sporophyte development is unique in the genus: all primary organs clearly arise from the embryo and develop simultaneously. The length of the life cycle of O. crotalophoroicles is considerably shorter than that of some other species in the Ophioglossaceae. A timetable for gametophyte and young sporophyte development is postulated. Spores germinate soon after they are released in the spring, and mature gametophytes develop by the next growing season. Fertilization occurs approximately one year after spore dispersal, and after two years, the photosynthetic first leaf of the young sporophyte emerges.     

DEVELOPMENTAL STUDIES IN SMILAX (LILIACEAE). I. ORGANOGRAPHY AND THE SHOOT APEX

The ontogeny of the shoot and the tendril-bearing leaf was studied in four species of Smilax which differ considerably in growth habit. Growth is indeterminate in S. bona-nox, S. rotundifolia, S. laurifolia and determinate in S. pumila. Shoot apical organization in all four species is consistent with the tunica-corpus concept. The number of tunica layers varies from 2 in S. bona-nox, S. pumila, and S. rotundifolia to 3 (2–4) in S. laurifolia. A primary thickening meristem was observed in 5. laurifolia. Shoot tip abortion involving a cessation of vegetative growth resembling that found in many woody plants occurs in S. pumila. Even though there are distinct differences in growth habit, vigor, and anatomy, the four species share many developmental similarities. Hence, subtle changes in apical ontogeny may be associated with marked differences in habit.     

Studies on morphology and metabolism of prothalli during GA3-induced formation of antheridia in Anemia phyllitidis

In Schizaeaceae ferns, including Anemia phyllitidis, formation of antheridia is known to be induced by exogenously applied gibberellic acid. Also present studies show that GA₃ (10⁻⁵ mol·dm⁻³) modifies the development of gametophytes of Anemia phyllitidis. Simultaneously with formation of antheridia, they exhibit lower number of cells but only slightly lowered profile areas and lengths of prothalli. Growth in size of individual cells compensates for lowered division frequency. Cytophotometric measurements reveal no essential changes in the DNA content in vegetative cells of the control and GA₃-stimulated gametophytes. It remains at haploid level and therefore it is assumed that cell cycle is blocked at G1 phase. Application of GA₃ increases the total amount of proteins. CZE (Capillary Zone Electrophoresis) separation of peptides extracted from control and GA₃-treated prothalli indicates the differences in the ratio of their particular forms. In GA₃-treated gametophytes the activities of acid and basic phosphatases, contents of carbohydrates (glucose, starch), chlorophyll, the number of chloroplasts and dry mass of prothalli are increased. GA₃-intensified metabolism, evidenced in gametophytes of A. phyllitidis, may be interpreted as a stimulatory mechanism which influences metabolic pathways involved in forming, developing and maturing of male sex organs.     

Life History,Reproductive Morphology and Development of the Antarctic Brown Alga Desmarestia menziesii J. Agardh*

The life history, reproduction and development of Desmarestia menziesii J. Agardh from Antarctica is described. Unilocular sporangia occur singly or in small groups in the outermost cortical layer of the sporophyte. They are formed by periclinal division of cortex cells into a stalk cell and the sporangium initial. Meiospores germinate into dioecious microscopic filamentous gametophytes. As in other perennial Antarctic species of the Desmarestiales, gametangia are formed in culture under short-day conditions or in darkness. In nature, juvenile sporophytes should therefore be formed in winter. They develop only attached to the oogonium. At first they are uniseriate and elongate by means of an intercalary meristem located in their middle part. Laterals are formed predominantly in this region, and they subsequently give rise to secondary laterals. The branching pattern is opposite to alternate in both young and adult plants. Cortication of the main axis is initiated by filaments growing out from the lowermost cells of the primary laterals. In sporophytes of this developmental stage the meristem of the main axis is confined to a small region where cortication starts and above. Lateral branches elongate and become corticated in the same way as the main axis. In mature plants, cells of the inner cortex can become meristematic again and form a meristoderm which contributes to axis thickness by periclinal and anticlinal divisions. The observations are discussed in relation to possible evolutionary relationships in the genus Desmarestia and in the order Desmarestiales.     

Adventitious shoot formation in decapitated dicotyledonous seedlings starts with regeneration of abnormal leaves from cells not located in a shoot apical meristem

Regeneration of new shoots in plant tissue culture is often associated with appearance of abnormally shaped leaves. We used the adventitious shoot regeneration response induced by decapitation (removal of all preformed shoot apical meristems, leaving a single cotyledon) of greenhouse-grown cotyledon-stage seedlings to test the hypothesis that such abnormal leaf formation is a normal regeneration progression following wounding and is not conditioned by tissue culture. To understand why shoot regeneration starts with defective organogenesis, the regeneration response was characterized by morphology and scanning electron and light microscopy in decapitated cotyledon-stage Cucurbita pepo seedlings. Several leaf primordia were observed to regenerate prior to differentiation of a de novo shoot apical meristem from dividing cells on the wound surface. Early regenerating primordia have a greatly distorted structure with dramatically altered dorsoventrality. Aberrant leaf morphogenesis in C. pepo gradually disappears as leaves eventually originate from a de novo adventitious shoot apical meristem, recovering normal phyllotaxis. Similarly, following comparable decapitation of seedlings from a number of families (Chenopodiaceae, Compositae, Convolvulaceae, Cucurbitaceae, Cruciferae, Fabaceae, Malvaceae, Papaveraceae, and Solanaceae) of several dicotyledonous clades (Ranunculales, Caryophyllales, Asterids, and Rosids), stems are regenerated bearing abnormal leaves; the normal leaf shape is gradually recovered. Some of the transient leaf developmental defects observed are similar to responses to mutations in leaf shape or shoot apical meristem function. Many species temporarily express this leaf development pathway, which is manifest in exceptional circumstances such as during recovery from excision of all preformed shoot meristems of a seedling.     

Developmental anatomy of the shoot apical cell,rhizophore and root ofSelaginella uncinata

The developmental anatomy of the shoot apex, rhizophore and root ofSelaginella uncinata was examined by the semi-thin section method. The shoot apex has a single, lens-shaped apical cell with two cutting faces. Rhizophore primordia are initiated exogenously at the branching point of the second youngest lateral shoot. The rhizophore apex has a tetrahedral apical cell with three cutting faces. A pair of root primordia is initiated endogenously from inner cells of the rhizophore apex, after the rhizophore apical cell becomes unidentifiable losing its activity, and subsequently a root cap is formed from the distal face of the root apical cell. During the course of successive root branching the apical cell in an original root apical meristem becomes unidentifiable and then a new apical cell is initiated in each of the bifurcated root apical meristems. The root branching mode seems to be equivalent to the described dichotomous branching mode of fern shoots. Our results demonstrate a distinct morphogenetical difference between the rhizophore and the root, and confirm the exogenous origin of the rhizophore, as described for other species ofSelaginella. This evidence indicates that the rhizophore is not an aerial root but a leafless, root-producing axial organ.     

Developmental morphology of the Asian one-leaf plant, Monophyllaea glabra (Gesneriaceae) with emphasis on inflorescence morphology

We examined the developmental morphology of the tropical Asian one-leaf plant Monophyllaea glabra, which is believed to have diverged first in the phylogenetic tree of the genus. The embryo within the seed consists of two cotyledons and a hypocotyl with no shoot or root apical meristems. The endogenous root meristem is formed nearer the hypocotyl end than in other examined Monophyllaea species. One of the cotyledons grows to form the macrocotyledon by means of the basal meristem. The groove meristem arises between the anisocotyledons, shifts toward the macrocotyledon, and is transformed to the inflorescence apex, which produces inflorescence axes in the axils of all ventral bracts of two rows, and secondary inflorescences in the axils of the lower dorsal bracts of the other two rows. The macrocotyledon may act as a ventral bract for the first inflorescence axis at the reproductive stage. This organization suggests that a common ancestor of Monophyllaea and Whytockia with decussate inflorescences diverged in one direction to become Monophyllaea and in another to become Whytockia.     

Sperm ultrastructure in two species of the polychaete genusHarmothoe (Polynoidae)

The structure of spermatozoa is described for two species of polynoid polychaete,Harmothoe imbricata andHarmothoe impar, from material fixed and examined by both scanning and transmission electron microscopy. The two species undergo spermiogenesis within discrete testes. The testis ofH. imbricata is shown to have a layer of epithelial cells which possess an outer cuticular layer and a microvillous inner surface. Spermatocytes of both species are spherical but there are marked differences in the shape and size of the spermatozoa of the two species.H. impar has a classical primitive spermatozoon with a rounded head (2 μm long) and a button-shaped acrosome. Fully differentiated spermatozoa ofH. imbricata are modified from the primitive form by having a long head (10 μm length) with a pointed acrosome about 6 μm in length. Spermatozoa ofH. imbricata have a ring of up to fourteen mitochondria around a centrally inserted flagellum at the posterior whereasH. impar has a ring of four or five spherical mitochondria. Spermiogenesis is well synchronised inH. imbricata but all developmental stages can be found simultaneously in the testis ofH. impar. The differences in sperm structure of the two species may be related to differences in breeding biology which are hitherto unknown.     

Mechanisms of primordium formation during adventitious root development from walnut cotyledon explants

 In walnut (Juglans regia L.), an otherwise difficult-to-root species, explants of cotyledons have been shown to generate complete roots in the absence of exogenous growth regulators. In the present study, this process of root formation was shown to follow a pattern of adventitious, rather than primary or lateral, ontogeny: (i) the arrangement of vascular bundles in the region of root formation was of the petiole type; (ii) a typical root primordium was formed at the side of the procambium within a meristematic ring of actively dividing cells located around each vascular bundle; (iii) the developing root apical meristem was connected in a lateral way with the vascular bundle of the petiole. This adventitious root formation occurred in three main stages of cell division, primordium formation and organization of apical meristem. These stages were characterized by expression of LATERAL ROOT PRIMORDIUM-1 and CHALCONE SYNTHASE genes, which were found to be sequentially expressed during the formation of the primordium. Activation of genes related to root cell differentiation started at the early stage of primordium formation prior to organization of the root apical meristem. The systematic development of adventitious root primordia at a precise site gave indications on the positional and biochemical cues that are necessary for adventitious root formation. Received: 30 July 1999 / Accepted: 16 February 2000     

Developmental morphology of strap-shaped gametophytes of Colysis decurrens: a new look at meristem development and function in fern gametophytes

Background and Aims

The gametophytes of most homosporous ferns are cordate–thalloid in shape. Some are strap- or ribbon-shaped and have been assumed to have evolved from terrestrial cordate shapes as an adaptation to epiphytic habitats. The aim of the present study was to clarify the morphological evolution of the strap-shaped gametophyte of microsoroids (Polypodiaceae) by precise analysis of their development.

Methods

Spores of Colysis decurrens collected in Kagoshima, Japan, were cultured and observed microscopically. Epi-illuminated micrographs of growing gametophytes were captured every 24 h, allowing analysis of the cell lineage of meristems. Light microscopy of resin-sections and scanning electron microscopy were also used.

Key Results

Contrary to previous assumptions that strap-shaped Colysis gametophytes have no organized meristem, three different types of meristems are formed during development: (1) apical-cell based – responsible for early growth; (2) marginal – further growth, including gametophyte branching; and (3) multicellular – formation of cushions with archegonia. The cushion is two or three layers thick and intermittent. The apical-cell and multicellular meristems are similar to those of cordate gametophytes of other ferns, but the marginal meristem is unique to the strap-shaped gametophyte of this fern.

Conclusions

The strap-shaped gametophytes of C. decurrens may have evolved from ancestors with a cordate shape by insertion of the marginal meristem phase between the first apical-cell-based meristem and subsequent multicellular meristem phases. Repeated retrieval of the marginal meristem at the multicellular meristem phase would result in indefinite prolongation of gametophyte growth, an ecological adaptation to epiphytic habitats.     

Effect of mutations in the PINHEAD gene of Arabidopsis on the formation of shoot apical meristems

The primary shoot apical meristem of angiosperm plants is formed during embryogenesis. Lateral shoot apical meristems arise postembryonically in the axils of leaves. Recessive mutations at the PINHEAD locus of Arabidopsis interfere with the ability of both the primary shoot apical meristem as well as lateral shoot apical meristems to form. However, adventitious shoot apical meristems can form in pinhead mutant seedlings from the axils of the cotyledons and also from cultred root explants. In this report, the phenotype of pinhead mutants is described, and a hypothesis for the role of the wild-type PINHEAD gene product in shoot meristem initiation is presented. © 1995 Wiley-Liss, Inc.     

COMPARISON OF SHOOT APEX DYNAMICS AND EARLY LEAF ONTOGENY IN TWO SPECIES OF SARACA (LEGUMINOSAE)

Shoot apices of Saraca indica produce adult leaves that have 4 to 6 pairs of leaflets, whereas those of S. bijuga usually have only 2 pairs. In both species one leaflet pair is found during the juvenile phase. Juvenility lasts many plastochrons in S. bijuga but is restricted to a few in S. indica. The shoot apical meristems of these two taxa are similar in structure, cell number, and cell size; however, the shoot apex of Saraca bijuga is slightly more stratified, having 2–3 tunica layers as opposed to 1–2 in S. indica. For most of the plastochron the apical meristem in both species is situated laterally at the base of the most recently formed leaf. A newly forming primordium and its internode shift the apical meristem upward unilaterally; the meristem passes through a brief apical dome stage and becomes positioned 180° from its origin at the beginning of the plastochron. Hence, there is a true pendulum meristem in both species. In S. bijuga the maximum length of the youngest leaf primordium, just prior to the formation of its successor, is twice that of S. indica. The internodes immediately below the shoot apex and the axillary buds develop more rapidly in S. bijuga than in S. indica. It is suggested that the bijugate leaf of S. bijuga represents a case of neoteny in plants.     

ORGANOGRAPHY,BRANCHING, AND THE PROBLEM OF LEAF VERSUS BUD DIFFERENTIATION IN THE VINING EPIPHYTIC FERN GENUS MICROGRAMMA

Investigation of the development and organography of the shoot systems of Microgramma vacciniifolia and M. squamulosa was undertaken for the purpose of determining: (1) the features of shoot growth that are responsible for the distinctive vining character of these epiphytic ferns; and (2) the mode of origin of branches and their contrast with leaf initiation. Shoots of both species are dorsiventral and plagiotropic (i.e., parallel to the substrate) in habit. Since the shoot apical meristem is radial in transectional symmetry, shoot dorsiventrality in Microgramma is a postgenital or secondary developmental event, and its inception is related to the initiation of lateral appendages. Leaves and buds arise in a distichous phyllotaxis and occupy opposite and alternating positions on the dorsal surfaces and flanks of the rhizome. Endogenous roots are initiated in two rows from the ventral surface of the stem, in the vicinity of the rhizome meristem; however, they do not emerge from the rhizome until some distance behind the tip and do not elongate until the region of substrate contact. We conclude that the vining nature of this fern rhizome is a result of precocious internodal elongation and the concomitant delay of leaf and bud expansion in the region of stem elongation. In addition, observation of branch origin confirms previous suggestions that branching in Microgramma is strictly lateral and extra-axillary and not a dichotomous derivative as proposed by some workers. Leaf and bud primordia differ not only in the nature of their respective vascular supplies but also in their actual course of initiation. In the case of the leaf, the primordium is precociously emergent and exhibits a lenticular apical cell at its summit when it is only one plastochron removed from the flanks of the apical meristem. By contrast, initials of the bud primordium divide less actively and remain in a sunken position for at least 5–6 plastochrons; only when the bud apex becomes expanded and emergent does a tetrahedral apical cell become recognizable at the tip of the bud promeristem. Because of the distinctive pattern of branch and leaf origin, as well as the lack of adventitious and phyllogenous origin of branch primordia, we suggest that the shoot of Microgramma is a useful test organism for the re-examination of the problem of leaf and bud determination in the ferns.     

FASCIATION AND DICHOTOMOUS BRANCHING IN ECHINOCEREUS (CACTACEAE)

In Echinocereus reichenbachii dichotomous branching and fasciation (cresting) are rare events. Both were found together in only a few of many populations investigated and are interpreted as variants of a single phenomenon. They may occur at any stage of shoot development, but crest meristems arise most commonly on young branches among clusters of normal shoots. Sometimes they appear on unbranched young plants or seedlings, very rarely on older shoots. Dichotomy results from the division of an apical meristem into equal parts each of which functions independently, producing a forked shoot. Fasciation involves the extension of a single meristem into an apical ridge. The product is a flabellate shoot that becomes undulate if growth along the summit continues. In longisection linear meristems appear similar to radial sections of normal shoots; in median sagittal section they have a much extended central mother cell zone within which the cell pattern resembles a rib meristem. Although crest meristems become sluggish or even inactive with age, localized renewed growth may occur spontaneously or be induced by injury. In this species the random production of normal shoots from crest meristems (defasciation) was not observed, but if much or all of such a meristem is removed, branches may arise from lateral areoles, and these are always normal. It seems, therefore, that whatever induces fasciation in E. reichenbachii originates in and is restricted to the apical meristem and its immediate vicinity.     

Cytokinins induce photomorphogenic development in dark-grown gametophytes of Ceratopteris richardii

The cytokinins benzylaminopurine, kinetin and isopentenyladenine induce photomorphogenesis in dark-grown gametophytes of the fern Ceratopteris richardii. At sub-nanomolar concentrations each altered the rate and pattern of cell division, elongation and differentiation, mimicking aspects of the light-mediated transition from filamentous to prothallial growth. Untreated dark-grown gametophytes grow as narrow, elongate, asexual filaments with an apical meristem. Cytokinin treatments as low as 10(-12) M reduced the length-to-width ratio through decreased cell elongation, increased periclinal cell division and induced the formation of rhizoid initials in the cells immediately below the apical meristem. Higher concentrations (10(-9)-10(-8) M) induced conversion of the meristem from apical to notch morphology. Cytokinins induced both red- and blue-light-mediated photomorphogenic events, suggesting stimulation of both phytochrome and cryptochrome signaling; however, cytokinin treatment only partially substituted for light in that it did not induce hermaphroditic sexual development or spore germination in the dark. Additionally, cytokinins did not increase chlorophyll synthesis in dark-grown gametophytes, which unlike angiosperms are able to produce mature chloroplasts in the dark. Cytokinin treatment had only slight effects on light-grown gametophytes. These results suggest evolutionary conservation between angiosperms and pteridophytes in the role of cytokinins in regulating photomorphogenesis.