Embryonic shoot apical meristem formation in higher plants

The shoot apical meristem (SAM) is essential for organ formation in higher plants. How the SAM is formed during plant development is poorly understood, however. In this review, we focus on several recent studies that provide new insights into the mechanism of SAM formation during embryogenesis. Recently, positive and negative regulators of the class I KNOX genes, which are thought to be necessary for SAM formation, have been identified; the Arabidopsis CUP-SHAPED COTYLEDON (CUC) genes are required for the expression of a class I KNOX gene, SHOOT MERISSTEMLES (STM) during embryogenesis, and the Arabidopsis ASYMMETRIC LEAVES1 (AS1), AS2, and several other genes negatively regulate KNOX gene expression in cotyledon primordia. Also, several genes that are involved in the formation of the adaxial–abaxial axis of cotyledons seem to regulate embryonic SAM formation. Electronic Publication     

Embryology of Ceratopteris richardii (Pteridaceae, tribe Ceratopterideae), with emphasis on placental development

This comprehensive study of early embryology in Ceratopteris richardii combines light microscopy with the first ultrastructural evaluation of any pteridophyte embryo. Emphasis is placed on ontogeny of the foot and placental transfer cells. The embryology of C. richardii shares many similarities with that of other polypodiacious ferns while exhibiting distinctive division patterns. Formative embryonic stages have been reconstructed into three-dimensional models for ease of interpretation. The zygote divides perpendicular to the gametophyte plane and anterioposterior axis. This division establishes a prone embryological habit that maximizes rapid independent establishment of a leaf-root axis in a cordate gametophyte. After the formation of a globular eight-celled stage, initials of the first leaf, and root and shoot apical meristems are defined early by discrete formative divisions. Concomitantly, the foot expands and differentiates to transport nutrients from the gametophyte for the developing embryonic organs. Transfer cell wall ingrowth deposition begins in the gametophyte placental cells before the adjacent sporophyte cells just after the eight-celled stage. These observations provide an anatomical framework for future comparative developmental genetic studies of embryogenesis in free-sporing plants.     

Plant stem cells and their regulations in shoot apical meristems

Stem cells in plants, established during embryogenesis, are located in the centers of the shoot apical meristem (SAM) and the root apical meristem (RAM). Stem cells in SAM have a capacity to renew themselves and to produce new organs and tissues indefinitely. Although fully differentiated organs such as leaves do not contain stem cells, cells in such organs do have the capacity to re-establish new stem cells, especially under the induction of phytohormones in vitro. Cytokinin and auxin are critical in creating position signals in the SAM to maintain the stem cell organizing center and to position the new organ primordia, respectively. This review addresses the distinct features of plant stem cells and focuses on how stem cell renewal and differentiation are regulated in SAMs.     

Endogenous hormonal levels and growth of dark-incubated shoots of Catasetum fimbriatum

Apical shoots and Lateral buds of the epiphytic orchid Catasetum fimbriatum give rise to rootless etiolated stolons, when cultured in the presence of light and then transferred to the dark. The stolons are characterized by fast and continuous apical longitudinal growth. Measurements of four endogenous cytokinin, indole-3-acetic acid (IAA) and abscisic acid (ABA) levels in etiolated shoots and light-grown plants were low. However, after transfer of green plants to the dark, cytokinin Levels increased 3- and 7-fold by 10 and 30 days of incubation, respectively. IAA levels also increased significantly, but the increase was not as great as for cytokinins. A similar trend was observed in the roots. A close relationship seems to exist between both cytokinin accumulation and the formation of etiolated stolons. Variations in ABA levels were practically inconspicuous. The presence of paclobutrazol in the medium, a potent inhibitor of gibberellin synthesis, strongly inhibited etiolated and non-etiolated longitudinal shoot growth, although no apparent effect was observed on apical meristem activity.     

Transgenic plants from shoot apical meristems of <Emphasis Type="Italic">Vitis vinifera</Emphasis> L. “Thompson Seedless” via <Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated transformation

Shoot apical meristem explants of Vitis vinifera “Thompson Seedless” were used for Agrobacterium-mediated genetic transformation. It was determined that the meristems had to be subjected to a dark growth phase then wounded to obtain transgenic plants. Morphological and histological studies illustrated the role of wounding to expose apical meristem cells for transformation. A bifunctional egfp/nptII fusion gene was used to select kanamycin resistant plants that expressed green fluorescent protein (GFP). Kanamycin at a concentration of 16 mg L⁻¹ in selection medium resulted in recovery of non-chimeric transgenic plants that uniformly expressed GFP, whereas 8 mg L⁻¹ kanamycin allowed non-transgenic and/or chimeric plants to develop. Polymerase chain reaction (PCR) and Southern blot analyses confirmed the presence of transgenes and their stable integration into the genome of regenerated plants. Up to 1% of shoot tips produced stable transgenic cultures within 6 weeks of treatment, resulting in a total of 18 independent lines.     

Developmental anatomy of the reproductive shoot in <Emphasis Type="Italic">Hydrobryum japonicum</Emphasis> (Podostemaceae)

Podostemaceae are unusual aquatic angiosperms adapting to extreme habitats, i.e., rapids and waterfalls, and have unique morphologies. We investigated the developmental anatomy of reproductive shoots scattered on crustose roots of Hydrobryum japonicum by scanning electron microscopy and using semi-thin serial sections. Two developmental patterns were observed: bracts arise either continuously from an area of meristematic cells that has produced leaves, or within differentiated root ground tissue beneath, and internal to, leaf base scars after an interruption. In both patterns, the bract primordia arise endogenously at the base of youngest bracts in the absence of shoot apical meristem, involving vacuolated-cell detachment to each bract separately. The different transition patterns of reproductive shoot development may be caused by different stages of parental vegetative shoots. The floral meristem arises between the two youngest bracts, and is similarly accompanied by cell degeneration. In contrast, the floral organs, including the spathella, arise exogenously from the meristem. Bract development, like vegetative leaf development, is unique to this podostemad, while floral-organ development is conserved.     

Auxin: A major regulator of organogenesis

Plant development is characterized by the continuous initiation of tissues and organs. The meristems, which are small stem cell populations, are involved in this process. The shoot apical meristem produces lateral organs at its flanks and generates the growing stem. These lateral organs are arranged in a stereotyped pattern called phyllotaxis. Organ initiation in the peripheral zone of the meristem involves accumulation of the plant hormone auxin. Auxin is transported in a polar way by influx and efflux carriers located at cell membranes. Polar localization of the PIN1 efflux carrier in meristematic cells generates auxin concentration gradients and PIN1 localization depends, in turn, on auxin gradients: this feedback loop generates a dynamic auxin distribution which controls phyllotaxis. Furthermore, PIN-dependent local auxin gradients represent a common module for organ initiation, in the shoot and in the root.     

The structure of the apical meristem of Isoetes engelmannii, I. riparia and I. macrospora (Isoetales)

The shoot apex in three species of Isoetes (I. engelmannii, I. macrospora and I. riparia ) is differentiated into a cap-like initiation layer of somewhat columnar meristematic cells which divide most frequently anticlinally and a subadjacent central core of cells which divide irregularly, but with periclinal divisions predominating. The conflicting views of apical organization in Isoetes are discussed.     

A comparative analysis of the <Emphasis Type="Italic">Arabidopsis</Emphasis> mutant <Emphasis Type="Italic">amp1-1</Emphasis> and a novel weak <Emphasis Type="Italic">amp1</Emphasis> allele reveals new functions of the AMP1 protein

Ethylene and gibberellins have a synergistic stimulatory effect on hypocotyl elongation of light-grown Arabidopsis thaliana (L.) Heynh. seedlings. A screen for mutants with decreased response to these hormones led to the isolation of a novel allele (amp1-7) of the ALTERED MERISTEM PROGRAM (AMP) 1 locus. The amp1-7 allele contains a missense mutation causing a phenotype, which is weaker than that of the amp1-1 mutant that carries a nonsense mutation. The mutant phenotype prompted the hypothesis that AMP1 is involved in ethylene and GA signalling pathways or in a parallel pathway-controlling cell and hypocotyl elongation and cellular organization. Amp1 mutants contain higher zeatin concentrations causing enlargement of the apical meristem, which was confirmed by cytokinin application to wild type seedlings. Light grown amp1 seedlings have shorter hypocotyls than wild type; however, application of cytokinins promotes hypocotyl elongation of both Col-0 and amp1. We suggest that in amp1 mutants either zeatin overproduction or its action is strictly localized. Nelson J. M. Saibo and Wim H. Vriezen contributed equally to this work.     

Meristem maintenance and compound-leaf patterning utilize common genetic mechanisms in tomato

Balancing shoot apical meristem (SAM) maintenance and organ formation from its flanks is essential for proper plant growth and development and for the flexibility of organ production in response to internal and external cues. Leaves are formed at the SAM flanks and display a wide variability in size and form. Tomato (Solanum lycopersicum) leaves are compound with lobed margins. We exploited 18 recessive tomato mutants, representing four distinct phenotypic classes and six complementation groups, to track the genetic mechanisms involved in meristem function and compound-leaf patterning in tomato. In goblet (gob) mutants, the SAM terminates following cotyledon production, but occasionally partially recovers and produces simple leaves. expelled shoot (exp) meristems terminate after the production of several leaves, and these leaves show a reduced level of compoundness. short pedicel (spd) mutants are bushy, with impaired meristem structure, compact inflorescences, short pedicels and less compound leaves. In multi drop (mud) mutants, the leaves are more compound and the SAM tends to divide into two active meristems after the production of a few leaves. The range of leaf-compoundness phenotypes observed in these mutants suggests that compound-leaf patterning involves an array of genetic factors, which act successively to elaborate leaf shape. Furthermore, the results indicate that similar mechanisms underlie SAM activity and compound-leaf patterning in tomato.     

<Emphasis Type="Italic">ATHB23</Emphasis>, an <Emphasis Type="Italic">Arabidopsis</Emphasis> class I homeodomain-leucine zipper gene,is expressed in the adaxial region of young leaves

Homeobox genes are essential regulators of plant development. ATHB23, a class I homeodomain leucine zipper gene of Arabidopsis, was found to be induced by treatment with the phytohormone gibberellin (GA). In order to clarify its role in development, we performed a histochemical analysis of transgenic plants containing a construct with a GUS::GFP reporter under the control of the 1.5 kb upstream region of ATHB23. The construct was mainly expressed in young leaves and the styles of flowers but not in mature leaves. Microscopic examination of young leaves revealed that it was expressed in the adaxial domain of leaf primordia and the rib meristem. Expression of ATHB23, like that of GA5 encoding GA 20-oxidase, was reduced in mutants related to adaxial-abaxial leaf polarity (phb-1d, se-2, and kan1 kan2). Reduced expression of the GUS::GFP reporter gene was also observed in an se-2 background. These results indicate that ATHB23 is under the control of GA and other activators such as PHB, and is involved in establishing polarity during leaf development.