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.     

Anisocotyly and meristem initiation in an unorthodox plant, <Emphasis Type="Italic">Streptocarpus rexii</Emphasis> (Gesneriaceae)

In common with most Old World Gesneriaceae; Streptocarpus Lindl. shows anisocotylous growth, i.e., the continuous growth of one cotyledon after germination. Linked to this phenomenon is an unorthodox behaviour of the shoot apical meristem (SAM) that determines the growth pattern of acaulescent species (subgenus Streptocarpus). In contrast caulescent species develop a conventional central post-embryonic SAM (mainly subgenus Streptocarpella). We used S. rexii Lindl. as a model to investigate anisocotyly and meristem initiation in Streptocarpus by using histological techniques and analyses of the expression pattern of the meristematic marker SrSTM1 during ontogeny. In contrast to Arabidopsis thaliana (L.) Heynh., S. rexii does not establish a SAM during embryogenesis, and the first evidence of a SAM-like structure occurs during post-embryonic development on the axis (the petiolode) between the two cotyledons. The expression pattern of SrSTM1 suggests a function in maintaining cell division activity in the cotyledons before becoming localized in the basal meristem, initially at the proximal ends of both cotyledons, later at the base of the continuously growing macrocotyledon, and the groove meristem on the petiolode. The latter is equivalent to a displaced SAM seemingly originating de novo under the influence of endogenous factors. Applied cytokinin retains SrSTM1expression in the small cotyledon, thus promoting isocotyly and re-establishment of a central post-embryonic SAM. Hormone-dependent delocalization of the process of meristem development could underlie anisocotyly and the unorthodox SAM formation in Streptocarpus. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Developmental analyses of the phyllomorph formation in the rosulate species Streptocarpus rexii (Gesneriaceae)

Although some species of Streptocarpus (Gesneriaceae) do not possess a layered shoot apical meristem (SAM), but three individual meristems, the basal meristem (BM), the petiolode meristem (PM) and the groove meristem (GM) on the petiolode from which additional phyllomorphs are formed. To gain insights into the processes involved, we examined the development of seedlings from germination to the formation of the primary phyllomorph in S. rexii, a rosulate species. Our specific focus was to examine the relationship between the functional activity of the GM and meristematic activity, which was assessed by a combined analysis of toluidine blue staining of histological sections and the incorporation of BrdU into meristematic tissues. The results were integrated into 3-D graphics, which suggests a complex spatial and temporal interaction within the GM. The significance of our observations is discussed and compared to the SAM observed in most other angiosperms.     

GA2 and GA20-oxidase expressions are associated with the meristem position in Streptocarpus rexii (Gesneriaceae)

We examined genes involved in the regulatory pathway of gibberellin (GA) in meristems of Streptocarpus rexii. The plants do not possess a typical shoot apical meristem (SAM) and form unique meristems: the basal meristem extends the lamina area of one cotyledon to produce anisocotylous seedlings; the groove meristem forms new leaves at the base of the macrocotyledon. Exogenous application of GA significantly suppresses the basal meristem activity in developing cotyledons and the seedlings remain isocotyl. To examine the role of endogenous GA on these meristems in vivo, we isolated homologs of GA2-oxidase responsible for degrading active GAs (SrGA2ox), and GA20-oxidase regulating the rate limiting step of active GA synthesis (SrGA20ox). During embryogenesis, while first partly overlapping, the expression of SrGA2ox and SrGA20ox became more differentiated and mutually exclusive, ending with SrGA2ox being expressed solely in the adaxial–proximal domain of the embryo in regions with meristem activity, whereas SrGA20ox was restricted to the fork between the two cotyledons. The latter may be responsible for suppressing the formation of an embryonic SAM in S. rexii. In developing seedlings, SrGA2ox expression also followed the centers of meristem activity, where SrGA20ox expression was excluded. Our results suggest that low levels of GA are required in S. rexii meristems for their establishment and maintenance. Thus, the meristems in S. rexii share similar regulatory pathways suggested for the SAM in model plants, but that in S. rexii evolutionary modifications involving a lateral transfer of function, from shoot to leaves, is implicated in attaining the unusual morphology of the plants.     

Characterization of anisocotylous leaf formation in Streptocarpus wendlandii (Gesneriaceae): significance of plant growth regulators

BACKGROUND AND AIMS: Unifoliate species of Gesneriaceae are unique, as they bear only one leaf throughout their life history. The development of this leaf (termed a macrocotyledon) derived from one of two cotyledons is intriguing. The other cotyledon does not develop further and is termed a microcotyledon. This process of unequal cotyledon development is termed anisocotyly. In this study the process of macrocotyeldon formation was studied and the effects of plant hormones on the macrocotyledon development were investigated. METHODS: Streptocarpus wendlandii was chosen as the main subject material, as it was found to be suitable for experimental studies in laboratory conditions. Morphological analyses were carried out with light and scanning electron microscopy. Plant hormones were applied exogenously. KEY RESULTS: The macrocotyledon of S. wendlandii is produced through cell division activity in the basal meristem of the enlarging cotyledon. The newly developed region in the macrocotyledon displayed distinct morphological changes, including the formation of long, needle-shaped trichomes. The newly formed region was surrounded by lateral veins. No such change was observed in the microcotyledon. Furthermore, it was shown that development of anisocotyly is suppressed by the application of cytokinin, resulting in the formation of two nearly equal-sized cotyledons. Both cotyledons displayed macrocotyledon characteristics. This observation in S. wendlandii was confirmed using Monophyllaea glabra, another unifoliate species in the same family. CONCLUSIONS: It is proposed that developmental changes of the macrocotyledon have characteristics of a developmental phase-change, and cytokinins may be involved in its formation. These results are discussed in the light of current knowledge of phase-change transitions in plant vegetative development.     

PHYSIOLOGICAL INVESTIGATIONS ON ALARIA ESCULENTA (LAMINARIALES,PHAEOPHYCEAE). III. EXUDATION BY THE BLADE1,2,3

Comparison of exudation rate of medullary conducting cells in the midrib of Alaria esculenta (L.) Grev. showed lowest rates for the blade meristem and highest rates in the non-growing region, 300–500 mm from the meristem. Holding plants under continuous darkness or severing the wings from the midrib reduced exudation rate by 26 and 37%, respectively. Osmotic pressure of exudate in sink (meristem) and source (non-growing region, up to 500 mm from the meristem) were similar (34.6–36.1 · 10²kpa). Pressure flow mechanism of translocation is evaluated in Alaria.     

Ontogenetic Anatomy of Streptocarpus grandis(Gesneriaceae) with Implications for Evolution of Monophylly

The monophylly of Streptocarpus grandis was examined ontogeneticallyand anatomically. When the seed is shed, the embryo is composedof a hypocotyl and two equal-sized cotyledons, lacking rootand shoot apices. During germination, cell division and subsequentcell enlargement occur in the hypocotyl and cotyledons. Thehypocotyl soon produces a primary root from its distal tip;this involves surface and subsurface cells at the point of attachmentof the suspensor remnant. In the cotyledons, cell enlargementand differentiation occur basipetally, leaving small meristematiccells at the bases. These small cells give rise to the basalmeristem in one of the two cotyledons, which contributes toan accrescent cotyledon. The groove meristem, which later differentiatesinto an inflorescence, arises in place of shoot apices whenthe cotyledons become visibly unequal in size. It later exhibitsa tunica-corpus like configuration and differentiates directlyinto an inflorescence meristem. The evolution of this uniquegrowth of one-leaved Streptocarpus is discussed with regardto morphogenetic data.Copyright 2000 Annals of Botany Company Anisocotyly, developmental anatomy, evolution, Gesneriaceae, one-leaf plant, ontogeny, Streptocarpus grandis     

<Emphasis Type="Italic">WUS</Emphasis> and <Emphasis Type="Italic">STM</Emphasis> homologs are linked to the expression of lateral dominance in the acaulescent <Emphasis Type="Italic">Streptocarpus rexii</Emphasis> (Gesneriaceae)

Acaulescent species of Streptocarpus Lindl. show unusual patterns of growth, characterized by anisocotyly (i.e. the unequal growth of cotyledons after germination) and lack of a conventional embryonic shoot apical meristem (SAM). A SAM-like structure appears during post-embryonic development on the axis of the continuously growing cotyledon. Since we have shown previously that KNOX genes are involved in this unusual morphology of Streptocarpus rexii, here we investigated the expression pattern of WUSCHEL (WUS), which is also required for the indeterminacy of the SAM, but is expressed independently from KNOX in Arabidopsis thaliana. In A. thaliana WUSCHEL is involved in the maintenance of the stem cell fate in the organizing centre. The expression pattern of the WUS ortholog in S. rexii (SrWUS) strongly deviates from that of the model plant, suggesting a fundamentally different spatial and temporal regulation of signalling involved in meristem initiation and maintenance. In S. rexii, exogenous application of growth regulators, i.e. gibberellin (GA₃), cytokinin (CK) and a gibberellin biosynthesis inhibitor (PAC), prevents anisocotyly and relocates meristematic cells to a position of conventional SAMs; this coincides with a re-localization of the two main pathways controlling meristem formation, the SrWUS and the KNOX pathways. Our results suggest that the establishment of a hormone imbalance in the seedlings is the basis of anisocotyly, causing a lateral dominance of the macrocotyledon over the microcotyledon. The peculiar morphogenetic program in S. rexii is linked to this delicate hormone balance and is the result of crosstalk between endogenous hormones and regulatory genes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Regeneration from callus of Undaria pinnatifida (Harvey) Suringar (Laminariales,Phaeophyta)

Calli were formed on the explants of midrib, meristem and immature stipe parts from freshly collected Undaria pinnatifida sporophytes. Each part was sterilized by Betadine and ethanol, and was cut into explants. The explants were incubated on an agar medium at 10 hours light and 14 hours dark photoperiod under a photon flux density of 80 µmol m^–2 s^–1. Callus was formed best on the explants of meristem parts at a temperature of 13 °C on PESI medium. Calli were cut off from the explants and were transferred into a sterile liquid PESI medium in flasks. Callus was dark brown in colour and was composed of well-pigmented cells. The cells were loosely bound and were separated by low power sonication, and were easy to attach to vinylon strings. From the calli formed on the explants of meristem parts, entire fronds were regenerated, but from the calli formed on the explants of midrib parts, only thin layered laminae were regenerated. The calli formed on the explants of immature stipe parts did not exhibit any regeneration at all.     

INFLORESCENCE AND FLOWER DEVELOPMENT IN THE PIPERACEAE. I. PEPEROMIA

Flowers of Peperomia species are the simplest structurally of any of the members of the Piperaceae. The spicate inflorescences form terminally and in axillary position; in each, the apex first is zonate in configuration with a two-layered tunica while 3-4 leaves are initiated. Later, when the inflorescence apical meristem begins bract initiation, the biseriate tunica persists, but zonal distinctions diminish and the apex can be described in terms of a simple tunicacorpus configuration. The inflorescence apex aborts after producing 30-40 bracts in acropetal succession an abscission layer forms across the base of the apex, and the meristem dries and drops off. Bracts are produced by periclinal divisions in T₂ (and occasionally also in the third layer as well); the later-formed floral apices arise by periclinal divisions in T₂ and the third layer. Each floral apex is at first a long transverse ridge in the axil, perpendicular to the long axis of the inflorescence. This establishes bilateral symmetry in the flower, which persists throughout subsequent growth. The floral meristem becomes saddle-shaped, and two stamen primordia are delimited, one at either end and lower than the central floral apex. A solitary carpel is initiated abaxially, and soon forms a circular rim which heightens as a tube with an apical pore. Within the open carpel, a solitary ovule is initiated from the entire remains of the floral apical meristem; it, hence, is terminal in the flower, and its placentation is basal. Carpellary closure in P. metallica results from accelerated growth of the abaxial lip, and the two margins become appressed. Species differ greatly as to whether the abaxial or the adaxial lobe predominates in late stages of carpel development. In P. metallica, the receptive portion of the stigma forms from the shorter lobe which is overtopped. Stigmatoid tissue forms internal to the receptive stigma. The prevailing bilateral floral symmetry, absence of a perianth, and the spicate inflorescence are features which distinguish Peperomia (and Piperaceae) from the magnolialian line of angiosperms.     

LIGNOTUBER ONTOGENY IN THE CORK-OAK (QUERCUS SUBER; FAGACEAE) I. LATE EMBRYO

Changes at the cotyledonary node of the cork-oak (Quercus suber L.) were examined during the embryo maturation phase using light microscopy and scanning electron microscopy techniques. During the maturation phase the embryo axis elongates by diffuse growth, the apical meristem forms the first leaf primordia, and the radicle meristem remains inactive. The primary axis of the embryo bears, axillary to the cotyledons, in the range of five to seven pairs of lateral buds at differing stages of development. Two or three pairs of these buds are visible, occurring on the upper unfused portion of the embryonic axis, while the remaining buds are hidden by the fused cotyledonary tissues. Lateral buds develop from clusters of cells in the peripheral meristem forming a shell zone delimiting the bud meristem. Lateral buds do not undergo much development until germination begins. The results are discussed with reference to the possible role of the cotyledonary node as the source of the lignotuber in the cork-oak.     

Consequences of transforming narrow leafed lupin (Lupinus angustifolius [L.]) with an ipt gene under control of a flower-specific promoter

Phenotypes of five transgenic lines of narrow-leafed lupin (Lupinus angustifolius [L] cv Merrit) stably transformed with the isopentenyl pyrophosphate transferase (ipt) gene from Agrobacterium tumefaciens coupled to a flower-specific promoter (TP12) from Nicotiana tabacum [L.] are described. Expression of the transgene was detected in floral tissues and in shoot apical meristems on all orders of inflorescence. In each transgenic line there was significant axillary bud outgrowth at all nodes on the main stem with pronounced branch development from the more basal nodes in three of the lines. The lowest basal branches developed in a manner similar to the upper stem axillary branches on cv Merrit and bore fruits, which, in two lines, contained a significant yield of filled seeds at maturity. Senescence of the cotyledons was delayed in all lines with green cotyledons persisting beyond anthesis in one case. IPT expression increased cytokinin (CK) levels in flowers, meristem tissues and phloem exudates in a form specific manner, which was suggestive of localized flower and meristem production with significant long-distance re-distribution in phloem. The total number of fruits formed (pod set) on some transgenic lines was increased compared to cv Merrit. Grain size compared to cv Merrit was not significantly altered in transgenic lines.     

UFO in the Arabidopsis inflorescence apex is required for floral-meristem identity and bract suppression

The UNUSUAL FLORAL ORGANS (UFO) gene of Arabidopsis encodes an F-box protein required for the determination of floral-organ and floral-meristem identity. Mutation of UFO leads to dramatic changes in floral-organ type which are well-characterized whereas inflorescence defects are more subtle and less understood. These defects include an increase in the number of secondary inflorescences, nodes that alternate between forming flowers and secondary inflorescences, and nodes in which a single flower is subtended by a bract. Here, we show how inflorescence defects correlate with the abnormal development of floral primordia and establish a temporal requirement for UFO in this process. At the inflorescence apex of ufo mutants, newly formed primordia are initially bract-like. Expression of the floral-meristem identity genes LFY and AP1 are confined to a relatively small adaxial region of these primordia with expression of the bract-identity marker FIL observed in cells that comprise the balance of the primordia. Proliferation of cells in the adaxial region of these early primordia is delayed by several nodes such that primordia appear “chimeric” at several nodes, having visible floral and bract components. However, by late stage 2 of floral development, growth of the bract generally ceases and is overtaken by development of the floral primordium. This abnormal pattern of floral meristem development is not rescued by expression of UFO from the AP1 promoter, indicating that UFO is required prior to AP1 activation for normal development of floral primordia. We propose that UFO and LFY are jointly required in the inflorescence meristem to both promote floral meristem development and inhibit, in a non-cell autonomous manner, growth of the bract.Shelley R. Hepworth and Jennifer E. Klenz contributed equally to this work.     

苏铁(Cycas revoluta Thunb.)种子的胚与胚乳解剖结构研究

对苏铁(Cycas revoluta Thunb.)种子的胚和胚乳组织进行了解剖研究。结果表明:苏铁种子为有胚乳种子,兼有胚乳和外胚乳,成熟时具直立型胚。胚乳的表层细胞含有角蜡质,胞核大,不含淀粉粒;中层细胞胞核明显;内层细胞胞核不明显,富含淀粉颗粒,淀粉粒单脐点明显。胚孔端的胚乳内陷成一凹槽,似贮藏窖。成熟的子叶胚为倒生胚胎,位于胚乳细胞解体后形成的囊腔中,子叶胚长度在胚乳中占到种子的1/3至2/3,已达到生理成熟阶段。双子叶直立,半合生。胚状体基部呈喙状突起,喙状突起下端连着一根肠叠着的丝状吸器,吸器基部连着一个小气囊。胚芽由顶端分生组织和数枚真叶组成,此时真叶已具羽状叶原基和绒毛原始体。在胚状体中发现有长管细胞及螺纹加厚的导管,在子叶中脉有数条并列的螺环纹导管。     

The SHOOT MERISTEMLESS gene is required for maintenance of undifferentiated cells in Arabidopsis shoot and floral meristems and acts at a different regulatory level than the meristem genes WUSCHEL and ZWILLE

The function of the SHOOT MERISTEMLESS (STM) gene in shoot and floral meristems throughout Arabidopsis development has been analyzed. The results show that STM plays a major role in maintaining shoot and floral meristems. In an allelic series of stm mutants the shoot meristem was either reduced or completely absent in mature embryos and mutant seedling cotyledons showed partial fusion, indicating that the STM gene affects embryonic shoot meristem development and spacing of cotyledons. Postembryonically, stm mutants initiated adventitious shoot development at a position corresponding to the shoot meristem in wild-type. Repetitively initiated defective mutant shoot and floral meristems were consumed during primordia formation and typically terminated prematurely in fused ectopic primordia, indicating that STM is required for continuous shoot and floral meristem function. Analogous defects were observed in stm embryonic and postembryonic development suggesting that similar mechanisms are employed in embryonic and postembryonic organ primordia initiation. Allelic combinations suggest different thresholds for STM requirement during plant development. STM requirement could not be bypassed by standard growth factor regimes or by shoot regeneration from calli. The results suggest that STM functions by preventing incorporation of cells in the meristem center into differentiating organ primordia and that this role can completely account for all defects observed in stm mutants. Mutations in the WUSCHEL (WUS) and ZWILLE (ZLL) genes result in defective organization and premature termination of shoot meristems. Genetic interactions between STM, WUS and ZLL were analyzed and the results indicate that STM acts upstream of WUS and ZLL. Therefore, while STM appears to function in keeping central meristem cells undifferentiated, WUS and ZLL seem to be subsequently required for proper function of these cells.     

Study of aberrant morphologies and lack of conversion of somatic embryos of chickpea (Cicer arietinum L.)

Summary Somatic embryos which originated from mature embryo axes of the chickpea (Cicer arietinum L.) showed varied morphologies. Embryos were classified based on shape of the embryo and number of cotyledons. “Normal” (zygotic-like) embryos were bipolar structures with two cotyledons and a well-developed shoot and root apical meristem, whereas “aberrant” embryos were horn-shaped, had single and multiple cotyledons, and were fasciated. Histological examination revealed the absence of a shoot apical meristem in horn-shaped embryos. Fasciated embryos showed diaxial fusion of two embryos. Secondary embryogenesis was also observed, in which the embryos emerged from the hypocotyl and cotyledonary region of the primary somatic embryo. This report documents the absence of an apical meristem as a vital factor in the lack of conversion of aberrant somatic embryos.