Studies on Metamastophora (Corallinaceae,Rhodophyta). I. M. flabellata (Sonder) Setchell: Morphology and anatomy

A morphological-anatomical study of Australian populations of Metamastophora flabellata (Sonder) Setchell, the type species of Metamastophora (Corallinaceae, Rhodophyta), has revealed that the primarily erect or ascending non-geniculate thallus possesses a dorsi-ventral organization of tissues. All conceptacles are uniporate and arise dorsally. Two distinct vegetative meristems occur: an apical primary meristem from which hypothallial cells are produced basipetally and a sub-epithallial secondary meristem which generates perithallial cells basipetally and secondary epithallial cells acropetally. Primary epithallial cells arise from divisions of subapical hypothallial cells. In younger parts, tissues are produced only dorsal to the hypothallium; in veins and stipes, tissue production occurs both dorsal and ventral to the hypothallium. Mature tetrasporic conceptacles contain peripheral tetrasporangia with zonately divided contents and a central sterile columella. Gametic conceptacles produce fertile tissue across the entire conceptacle chamber floor. After fertilization, the zygotic nucleus or a derivative is transferred (presumably) to an auxiliary cell through cells of the carpogonial branch; no tubular transfer siphon develops. Mature fusion cells are composed of the amalgamated supporting cells of carpogonial branches and are initiated from a single supporting cell which functions as an auxiliary cell. Unbranched 3–4 celled gonimoblast filaments arise from the fusion cell, do not become connected to other cells, and produce terminal carposporangia. Results from this study have led to a redefinition of hypothallium and perithallium in relation to meristems rather than substrate. In addition, carposporophyte ontogeny in the Corallinaceae is considered in terms of the presumed mode of transfer of the zygotic nucleus to the fusion cell, the extent of fusion cell development, and gonimoblast filament production in relation to auxiliary cells and fusion cells.     

New records of Peyssonnelia armorica and Peyssonnelia harveyana (Rhodophyta, Gigartinales) from Japan

Two species of the crustose red algal genus Peyssonnelia (Gigartinales, Peyssonneliaceae) are reported from Japanese waters for the first time. These species share the following combination of vegetative and reproductive features: thalli with appressed margins, perithallial filaments arising from the whole upper surface of each hypothallial cell (the Peyssonnelia rubra‐type anatomy), unicellular rhizoids, hypobasal calcification and spermatangia that are produced in double chains (the Peyssonnelia harveyana‐type spermatangial filament). However, they differ obviously from each other in the hypothallus orientation as seen from below, the perithallus structure in relation to the consistency of the crust, the origin of gonimoblasts and the elevation of the nemathecia. Peyssonnelia armorica is characterized by: (i) hypothallial filaments comprising a polyflabellate layer; (ii) easily separable perithallial filaments in a gelatinous matrix; (iii) gonimoblasts originating exclusively from the auxiliary cell; and (iv) semi‐immersed (slightly elevated) nemathecia. Peyssonnelia harveyana is characterized by: (i) hypothallial filaments arranged in parallel rows; (ii) closely packed perithallial filaments in a firm matrix; (iii) gonimoblsts originating from both the auxiliary cell and the connecting filament; and (iv) conspicuously elevated nemathecia.     

Characterization of the crustose red alga Peyssonnelia japonica (Rhodophyta,Gigartinales) and its taxonomic relationship with Peyssonnelia boudouresquei based on morphological and molecular data

The vegetative and reproductive morphology of the crustose red alga Peyssonnelia japonica (Segawa) Yoneshigue was re‐examined based on the holotype specimen and recent collections from various localities in Japan, including the type locality, and Hawaii. This species is characterized by the following features: thallus with appressed margins, perithallial filaments arising from the entire upper surface of each hypothallial cell (the Peyssonnelia rubra‐type), easily separable perithallial filaments in a gelatinous matrix, hypothallial filaments arranged in parallel rows, unicellular rhizoids, hypobasal calcification, gonimoblasts derived mainly from connecting filaments, and spermatangia produced in a series of whorls comprised of one to four paired spermatangia surrounding each central cell (the Peyssonnelia dubyi‐type). In addition to these features, the dimensions of the vegetative and reproductive structures of Peyssonnelia boudouresquei Yoneshigue described from Brazil were consistent with those of P. japonica. Molecular phylogenetic analyses using partial 26S rDNA, rbcL, and cox2‐3 spacer DNA sequences also supported the monophyly of P. japonica (from 16 localities in Japan and one locality in Hawaii) and P. boudouresquei (from two localities in Brazil). Therefore, P. boudouresquei may be a taxonomic synonym of P. japonica. However, considering the relatively high sequence divergences between the two taxa (2.1–2.5% in partial 26S rDNA, 5.9–6.7% in rbcL, and 5.8–6.7% in cox2‐3 spacer), and the relatively limited geographic sampling ranges, we reserve the taxonomic conclusion until further morphological and genetic data of the specimens from other geographic areas connecting Japan and Brazil become available.     

Verosphacela silvae sp. nov. (Onslowiaceae, Phaeophyceae) from the Mediterranean Sea

We describe Verosphacela silvae sp. nov., from the Mediterranean Sea. It consists of horizontal filaments living on the lower face of the red alga Peyssonnelia rubra (Greville) J. Agardh, from which erect filaments up to 1.5 mm high rise and grow upright after passing through the thallus of the supporting species. There are both horizontal and erect filaments growing by apical cells. In the subapical cells, 1–2 longitudinal divisions occur (more frequently in the erect filaments) but no secondary transverse divisions occur. Erect filaments bear lateral propagules on a stalk of one to three (rarely more) cells. Propagules, with neither apical cells nor arms, consist of seven cells. Zoidangia are borne at the apex of erect laterals. The new species differs from V. ebrachia Henry mainly in habit, propagules and zoidangia. In addition, distinct from V. ebrachia, filaments of V. silvae never penetrate between the cuticle and the cell wall of the supporting alga. Moreover, propagules of V. silvae consist of seven cells, whereas those of V. ebrachia consist of 9–13 cells, and zoidangia are terminal on laterals in V. silvae, whereas in V. ebrachia they are sessile on both axes and laterals.     

A new crustose red alga Peyssonnelia rumoiana (Rhodophyta, Gigartinales) from Japan

The marine red alga Peyssonnelia rumoiana Kato et Masuda, sp. nov. (Peyssonneliaceae, Gigartinales) is described from warm‐ and cold‐temperate regions in Japan. It is principally characterized by having hypo‐thallial filaments comprising a polyflabelate layer, proximal perithallial cells arising from the whole upper surface of each hypothallial cell (Peyssonnelia rubra‐type anatomy) and closely packed in a firm matrix, the production of two filaments from the proximal perithallial cell, unicellular rhizoids, appressed crust margins and hypobasal calcification. The alga is distinguished from related species by: (i) its conspicuously elevated cystocarpic (100–150 μm high) and tetrasporangial (80–110 μm high) nemathecia; (ii) tetrasporangia with or without a unicellular pedicel; and (iii) large (25–45 μm in diameter by 70–115 μm in length) tetrasporangia and (iv) the production of double chains of spermatangia (Peyssonnelia harveyana‐type spermatangial development).     

Metapeyssonnelia tangerina (Peyssonneliaceae,Rhodophyta), a new species associated with coral reef habitats in Puerto Rico,Caribbean Sea

A new Metapeyssonnelia species that comprises up to 7% bottom cover at shallow‐water reef habitats in southwest Puerto Rico is described herein. It forms conspicuous orange encrustations on hard substrata and does not grow on living coral as does its two Caribbean congeners. The new species possesses conspicuous, to 30 cm in extent, tightly adherent crusts up to 950 μm thick, only hypobasal calcification, hypothallial cells arranged in broad flabellules and superficial (raised) tetrasporangial and carposporangial nemathecia. Tetrasporangia are pedicellate, borne laterally from cup‐like cells that are derived from basal paraphysal cells. Tetrasporangia measure up to 120 μm long and individual carposporangia to 80 μm long. The new species differs from other Metapeyssonnelia species developmentally in that perithallial cells at mid thallus height will divide laterally to form a new hypothallium. Small subunit gene sequences relate the new species to the two Metapeyssonnelia species that are previously known from Puerto Rico.     

Meristem as a Self-Renewing System: Maintenance and Cessation of Cell Proliferation (A Review)

The mechanisms of the maintenance of long-term cell proliferation and its cessation in the meristem of the growing root were analyzed. Quiescent center (QC) remains in the meristem for a long time, whereas all other cells leave the meristem after several mitotic cycles. The question arises as to what extent such organization of proliferation corresponds to the concept of stem cells elaborated for animals. The definition of animal stem cells is met by the QC cells rather than by actively dividing cells that boundary it. However, QC is not a self-maintaining population of cells originated during early stages of embryogenesis; it is formed from dividing cells in the main or lateral root. After root decapitation, the QC can arise from the cells that normally would leave the meristem before long. There is a zone of the meristem whose cells are capable of remaining and forming QC after the removal of the apical part of the root. Maintenance of the size of the meristem depends on the interaction between QC, initial cells located at its surface, and the actively dividing cells. Apparently, the life span of cells in the meristem determines the time when the meristematic cell will begin the elongation. The number of cells starting the elongation depends on proliferation rate and on the changes in life span of meristematic cells which determine their initial number. The life span of the cells in the meristem for most actively dividing cells does not depend on the cell divisions, and remains unchanged in the presence of various inhibitors. As a result of inhibited proliferation in the main part of the meristem, cell divisions in the QC are activated and newly formed cells may proceed to rapid divisions. Thus, the size of the meristem is maintained by the operation of several mechanisms, and individual processes may be, on the one hand, relatively independent and, on the other hand, regulated either by feedback or directly. As a result, the root growth becomes resistant to various external events.     

The role of endogenous auxin in root initiation

This paper describes the process of the formation of adventitious roots. There appears to be good agreement that this consists of four stages, defifferentiation coupled with the formation of a meristematic locus, cell division to form a radially symmetrical cluster of cells, further divisions coupled with organisation into a bilaterally symmetrical meristem and finally growth of cells in the basal part of the meristem which causes its protursion through the epidermis. Evidence for the involvement of auxins in these various stages is reviewed and the extent to which rooting of easy- and hard-to-root species can be accounted for in terms of auxin content discussed. Peaks of IAA occur soon after excision of cuttings in some species and there is some evidence suggesting that this is correlated with changes in peroxidase activity. The possible involvement of cytokinins with auxins is briefly considered.     

Histogenesis of Xylem of Eucommia ulmoides Cultured in Vitro

Following a complete ringing of the main stem of Eucommia ulmoides Oliv. regeneration of normal new bark has been observed as a subsequence of repeated partial differentiation of the immature xylem into cambium. The immature xylem, when cultured in vitro, showed production of callus tissue from the ray cells. After 15–30 days of continuous culture, meristematic tissue appeared as a discontinuous and more or less regularly arranged.zone was visible within the callus tissue. However, the meristematic cells, unlike the elongated fusif0rm initial cells, are isometric in shape. This meristem differentiated into tracheids centripetally. Nevertheless, no centrifugal differentiation into phloem was evidenced. The vascular tissues were ultimately degenerated after 6 months cultured in vitro.     

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.     

Thidiazuron-induced morphogenesis in tamarind seedlings

Summary Germination of tamarind seeds in medium containing thidiazuron (TDZ) resulted in induction of nodular protrusions in and around the cotyledonary node meristem. The structures developed radially in well-defined circles and subsequently spread towards the cotyledonary bridge and also in the proximal part of the hypocotyl. The structures developed into shoots on transfer to medium devoid of growth regulators. Histological studies revealed that the protrusions initiated from the nodal meristem and extended to the non-meristematic region between the two meristems and also in the proximal part of the hypocotyl in seedlings germinated in 9.08 μM TDZ. Newly formed cell layers and less-differentiated meristematic protrusions were also seen. With the increase in the distance from the meristem, the buds were less differentiated; in the proximal part of the hypocotyl only the multiple layers of meristematic cells were noted. With extension of the period of incubation, the TDZ-induced meristematic activity extended laterally in circles towards the neighboring region. The radial spread of the meristematic activity from the center of the nodal meristem was also evident at 18.16 μM TDZ. From the pattern of the morphogenic development and the histological studies it may be hypothesized that in tamarind, TDZ influences the existing meristems specifically. Subsequently de novo organogenesis is triggered in the neighboring cells.     

Studies on the behavior of organelles and their nucleoids in the root apical meristem of Arabidopsis thaliana (L.) Col.

The behavior of cell nuclei, mitochondrial nucleoids (mt-nucleoids) and plastid nucleoids (ptnucleoids) was studied in the root apical meristem of Arabidopsis thaliana. Samples were embedded in Technovit 7100 resin, cut into thin sections and stained with 4′-6-diamidino-2-phenylindole for light-microscopic autoradiography and microphotometry. Synthesis of cell nuclear DNA and cell division were both active in the root apical meristem between 0 μm and 300 μm from the central cells. It is estimated that the cells generated in the lower part of the root apical meristem enter the elongation zone after at least four divisions. Throughout the entire meristematic zone, individual cells had mitochondria which contained 1–5 mt-nucleoids. The number of mitochondria increased gradually from 65 to 200 in the meristem of the central cylinder. Therefore, throughout the meristem, individual mitochondria divided either once or twice per mitotic cycle. By contrast, based on the incorporation of [³H]thymidine into organelle nucleoids, syntheses of mitochondrial DNA (mtDNA) and plastid DNA (ptDNA) occurred independently of the mitotic cycle and mainly in a restricted region (i.e., the lower part of the root apical meristem). Fluorimetry, using a videointensified microscope photon-counting system, revealed that the amount of mtDNA per mt-nucleoid in the cells in the lower part of the meristem, where mtDNA synthesis was active, corresponded to more than 1 Mbp. By contrast, in the meristematic cells just below the elongation zone of the root tip, the amount of mtDNA per mt-nucleoid fell to approximately 170 kbp. These findings strongly indicate that the amount of mtDNA per mitochondrion, which has been synthesized in the lower part of the meristem, is gradually reduced as a result of continual mitochondrial divisions during low levels of mtDNA synthesis. This phenomenon would explain why differentiated cells in the elongation zone have mitochondria that contain only extremely small amounts of mtDNA. This work was supported by a Grant-in Aid (T.K.) for Special Research on Priority Areas (Project No. 02242102, Cellular and Molecular Basis for Reproduction Processes in Plants) from the Ministry of Education, Science and Culture of Japan and by a Grant-in Aid (T.K.) for Original and Creative Research Project on Biotechnology from the Research Council, Ministry of Agriculture, Forestry and Fisheries of Japan.     

THE GAMETOPHYTE OF ANEMIA COLIMENSIS

Anemia colimensis, the most primitive known species of the genus, has been investigated with respect to gametophyte development and morphology. In the early stages the thalli are in general similar to those of species of Anemia which are charcterized by the development of an initially lateral marginal meristem. One significant difference was noted in the frequent appearance of meristematic zones on both sides of the young plate. One of these becomes the marginal meristem; the other eventually ceases activity. The mature gametophyte is a spherical cushion of complex organization, the result of extreme ruffling of the wings of the thallus and the remarkable abundance of filamentous outgrowths and deep lobing of the wings. This form of gametophyte is unique among the investigated anemias. It is proposed that the formation of a lateral meristem, the “Ceratopteris type” of prothallial development, is primitive in Anemia and that the “Adiantum type,” at least in the Schizaeaceae, is derived. This suggestion is discussed in relation to other possible interpretations.     

Development of the intercalary meristem in Chorda filum (Laminariales, Phaeophyceae) and other primitive Laminariales

Development of the intercalary meristem in the terete laminarialean species Chorda filum (L.) Stackhouse was studied in culture using light and transmission electron microscopy as well as by tracing elongation and cell divisions in various parts of the sporophyte. Growth of C. filum sporophytes could be classified into three developmental stages: (i) diffuse growth; (ii) basal meristematic growth; and (iii) intercalary meristematic growth. In the diffuse growth stage, elongation and cell division frequency were almost the same in each cell. In the basal meristematic growth stage, elongation and division of cells became localized in the tissues derived from the meristematic initial cell. Cells of the basal meristematic region contained smaller chloroplasts and many small opaque vesicles. In the intercalary meristematic growth stage, there was further elongation and differentiation of cells originating from the meristematic region, and this became more active in adjacent regions below the meristem than in regions above the meristem, causing the relative position of the intercalary meristem to shift towards the tip of the sporophyte. Meristematic cells of C. filum contained well-developed Golgi vesicles around the nucleus (perinuclear Golgi), many secretion vesicles and many small disk-shaped chloroplasts whose thylakoids were not well developed. Sporophytes of three other terete members of Laminariales, Chorda tomentosa Lyngbye, Pseudochorda nagaii (Tokida) Kawai et Kurogi, and Pseudochorda gracilis Kawai et Nabata, show diffuse growth and basal meristematic growth, but no intercalary meristematic growth. This suggests that the common ancestor of the Pseudochordaceae and Chordaceae had basal meristematic growth, and intercalary meristematic growth evolved more recently in C. filum.     

Meristem clusters in cortical layer of thallus cells of the cultured red alga Palmaria palmata

For the first time somatic spotlike formations of 1.5 mm in diameter and height up to 0.5 mm were revealed in thalli of the red alga Palmaria palmata reared in an aerated stirred culture. It was determined that some cortical cells of the thallus are able to divide in the periclinal and anticlinal directions forming spots. Deep freezing and subsequent thawing of thalli showed that the cortical cells of spots were meristematic cells. In certain conditions they enabled the formation of prolifications, which germinated plantlets. These clusters of meristem tissue in the cortical layer of cells of the thallus of P. palmata, which were formed in the culture as in nature, function as growth cells facilitating growth of thalli in thick and natural “planting material” formed upon thalli fragmenting after their freezing in the winter season.     

New methods of obtaining plantlets and tetraspores from fragments and cell aggregates of meristematic and submeristematic tissue of the red alga Palmaria palmata

Isolation of meristematic tissue of the red alga Palmaria palmata by a freezing-thawing method and further maintenance of the tissue in culture showed the existence of groups of meristematic cells in superficial cortical layers of thallus forming wart-like outgrowths. For the first time, proliferations (plantlets) were obtained from meristematic tissue of sporophytic and male gametophytic fronds and tetraspores from submeristematic tissue of sporophytic fronds within a short period (6 weeks). Tissue fragments (1 × 1 mm²) from upper margins of fresh thalli and cell aggregates (10−100,000 cells) from marginal meristem and meristematic warts of fresh thalli and thalli after the freezing-thawing procedure were cultured for getting plantlets. Tissue fragments (TF) and cell aggregates (CA) from submeristematic tissue of fresh thalli were cultured for obtaining tetraspores. For mass getting proliferations (plantlets) and tetraspores we recommend to use CA from marginal tissue of fresh fronds because of fast growth, high numbers of proliferations and simple techniques of the method. The freezing-thawing method allows also to identify meristematic tissue and to obtain plantlets of red algae with apical meristem (e.g., Gelidium spp.).     

The shoot meristem as a site of continuous organogenesis

In higher plants, organ formation occurs throughout life. This remarkable process occurs at a collection of stem cells termed the shoot meristem. The shoot meristem originates during embryogenesis and is later responsible for generating the above-ground portion of the plant. The shoot meristem can be thought of as having two zones, a central zone containing meristematic cells in an undifferentiated state, and a surrounding peripheral zone where cells enter a specific developmental pathway toward a differentiated state. Recent advances have revealed several genes that specifically regulate meristem development inArabidopsis. The function of these genes and their genetic interactions are described.     

Dependence of Root Cell Growth and Division on Root Diameter

Primary roots of 98 species from different families of monocotyledonous and dicotyledonous plants and adventitious roots obtained from bulbs and rhizomes of 24 monocot species were studied. Root growth rate, root diameter, length of the meristem and elongation zones, number of meristematic cells in a file of cortical cells, and length of fully elongated cells were evaluated in each species after the onset of steady growth. The mitotic cycle duration and relative cell elongation rate were calculated. In all species, the meristem length was approximately equal to two root diameters. When comparing different species, the rate of root growth increased with a larger root diameter. This was due to an increase in the number of meristematic cells in a row and, to a lesser degree, to a greater length of fully elongated cells. The duration of the mitotic cycle and the relative cell elongation rate did not correlate with the root diameter. It is suggested that the meristem size depends on the level of nutrient inflow from upper tissues, and is thereby controlled during further growth.