Effect of sub-optimal root zone temperatures at varied nutrient supply and shoot meristem temperature on growth and nutrient concentrations in maize seedlings (Zea mays L.)

Maize seedlings were grown for 10 to 20 days in either nutrient solution or in soils with or without fertilizer supply. Air temperature was kept uniform for all treatments, while root zone temperature (RZT) was varied between 12 and 24°C. In some treatments the basal part of the shoot (with apical shoot meristem and zone of leaf elongation) was lifted up to separate the indirect effects of root zone temperature on shoot growth from the direct effects of temperature on the shoot meristem.Shoot and root growth were decreased by low RZT to a similar extent irrespective of the growth medium (i.e. nutrient solution, fertilized or unfertilized soil). In all culture media Ca concentration was similar or even higher in plants grown at 12 as compared to 24°. At lower RZT concentrations of N, P and K in the shoot dry matter decreased in unfertilized soil, whereas in nutrient solution and fertilized soil only the K concentration decreased.When direct temperature effects on the shoot meristem were reduced by lifting the basal part of the shoot above the temperature-controlled root zone, shoot growth at low RZT was significantly increased in nutrient solution and fertilized soil, but not in unfertilized soil. In fertilized soil and nutrient solution at low RZT the uptake of K increased to a similar extent as plant growth, and thus shoot K concentration was not reduced by increasing shoot growth rates. In contrast, uptake of N and P was not increased, resulting in significantly decreased shoot concentrations.It is concluded that shoot growth at suboptimal RZT was limited both by a direct temperature effect on shoot activity and by a reduced nutrient supply through the roots. Nutrient concentrations in the shoot tissue at low RZT were not only influenced by availability in the substrate and dilution by growth, but also by the internal demand for growth.     

Photoautotrophic in vitro Multiplication of the Orchid Dendrobium under CO2 Enrichment

An attempt to reduce the production cost on tissue cultured plants, photoautotrophic culture of a high value orchid Dendrobium was established under CO2-enriched conditions. The shoot length and the number of leaves were almost equal in plantlets grown on medium with 2 % sucrose or without sucrose and under normal or enhanced (40 g m-3) CO2 concentration, whereas the fresh and dry masses were higher in cultures grown in sucrose containing media or under CO2 enrichment. Development of roots was observed only on media without sucrose, but CO2 enrichment did not have significant effects on in vitro rootings. This revised version was published online in July 2006 with corrections to the Cover Date.     

Virus elimination and pathogen-free plantlets regeneration in Cucurbita pepo L.

An efficient in vitro protocol was established for developing pathogen-free plantlets in Cucurbita pepo through meristem culture. Meristems of about 0.3–0.5 mm in size were isolated from shoot tips of 25–30 day old in vitro grown plants. For primary establishment of isolated apical meristem, MS liquid medium supplemented with 2.0 mgl KIN and 0.5 mg/l GA₃ was found to be most effective in both cultivars. MS semisolid medium containing 2.0 mg/l BAP were found to be most effective for shoot development from primarily established meristem in both cultivars. A good number of shoots were not concomitant with good rooting. The best root induction was found in media having 1.0 mg/l IBA in cv. Bulum. It was found that cv. Bulum was better than cv. Rumbo in all stages of meristem culture. The presence of virus in plantlets was achieved by DAS-ELISA test, where 68–81% plantlets have been proved to be virus free among the studied viruses. Healthy growth and vigour was observed in meristem derived plants over their source plants after cultivation under natural conditions.     

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.     

A simple procedure of Gossypium meristem shoot tip culture

In order to develop transgenic plants via the biolistic gun method regenerable embryogenic tissues are required. Meristem shoot tips of 19 cultivars of cotton were cultured on several media formulations and assessed for shoot and root development. The best shoot development was observed on media containing 0.46 mM kinetin while rooting was observed on media containing 2.68 mM NAA and 0.46 mM kinetin. No intervarietal variability was observed. A complete protocol was developed from meristem tip culture to field transfer. This methodology is simple and replaces the existing protocols for meristem tip culture of cotton. This revised version was published online in June 2006 with corrections to the Cover Date.     

Meristem aging is not responsible for age-related changes in growth and abscisic acid levels in the Mediterranean shrub, Cistus clusii

To obtain new insights into the mechanisms underlying aging in perennials, we measured abscisic acid levels, growth and other stress indicators in leaves of Cistus clusii Dunal plants of different ages grown under Mediterranean field conditions. Recently emerged leaves from 9-year-old plants were compared to those of 1-year-old plants (obtained from cuttings from 9-year-old plants) to evaluate the effects of meristem aging on plant aging. Rooting and successful establishment of the cuttings allowed us to compare the physiology of plants with old meristems, but of different size. Plants obtained from cuttings were rejuvenated, with new leaves displaying a higher leaf area and chlorophyll content, but smaller leaf mass per unit area ratios and endogenous abscisic acid levels than those of 9-year-old plants. A comparative study in 1-, 4- and 9-year-old plants revealed that abscisic acid levels increase during the early stages of plant life (with increases of 90% between 1- and 4-year-old plants), but then remain constant at advanced developmental stages (between 4- and 9-year-old plants). Although leaf biomass was 53% smaller in 9-year-old compared to 4-year-old plants, the dry matter produced per shoot apical meristem was equivalent in both plant groups due to an increased number of leaves per shoot in the former. It is concluded that (i) C. clusii plants maintain the capacity to rejuvenate for several years; (ii) newly emerged leaves accumulate higher amounts of abscisic acid during early stages of plant life, but the levels of this phytohormone later remain constant; and (iii) although plant aging leads to the production of smaller leaves, the amount of biomass produced per shoot apical meristem remains constant at advanced developmental stages.     

Shoot apical meristem: In vitro regeneration and morphogenesis in wheat (Triticum aestivum L.)

Summary We report a less genotype-dependent in vitro regeneration system capable of producing multiple shoot clumps and whole plants in four different wheat genotypes. Shool apical meristems from 7-d-old-seedlings produced axillary and adventitious shoots and somatic embryos on media containing N⁶-benzyladenine (BA) and 2,4-dichlorophenoxyacetic acid (2,4-D). All four genotypes responded positively to shoot multiplication depending upon media composition. Scanning electron microscopies of cultures showed a proliferating budding state that gave rise to adventitious shoots and somatic embryos on further multiplication. The percentage of relative shoot apical meristem multiplication was 80–90%, and the average number of shoot meristems per multiplied shoot was 40–50 in all genotypes. Among different concentrations of phytohormones, 2 and 4 mgl⁻¹ BA (8.8 and 17.7 μM) in combination with 0.5 mg l⁻¹ 2,4-D (2.26 μM) gave the best results. Actively multiplying shoot clumps were recovered with high frequency among 3-mo.-old cultures. These shoot clumps regenerated normally and produced fertile plants containing viable seeds. This in vitro system might prove useful for the production of transgenic plants of wheat in a relatively genotype-independent manner.     

Xanthone compounds in shoot cultures of <Emphasis Type="Italic">Gentianella bulgarica</Emphasis>

Shoot cultures of Gentianella bulgarica established from seedling epicotyls were maintained on MS medium supplemented with BA 0.2 mg l⁻¹ + NAA 0.1 mg l⁻¹. Cultures were prone to precocious flowering requiring the use of small shoot buds for multiplication purposes. The contents of three xanthone compounds identified as DGL, BGL, and DMB, in different plant material were determined by HPLC. The analysis revealed that the production of xanthones was affected by different concentrations of BA in medium. Shoot cultures grown at higher BA concentrations contained more DGL than material grown in nature. The concentrations of other two xanthones were lower in shoot cultures than in plants from nature. The radical scavenging activity of plant extracts and xanthones was investigated by DPPH test. Samples from plants grown in nature showed the highest activity (IC₅₀ = 0.26 mg ml⁻¹), while the extracts of shoot cultures grown in media with higher concentrations of BA showed moderate activities (IC₅₀ from 1.6 to 4.4 mg ml⁻¹).     

Role of the ZWILLE gene in the regulation of central shoot meristem cell fate during Arabidopsis embryogenesis.

Postembryonic development in higher plants is marked by repetitive organ formation via a self-perpetuating stem cell system, the shoot meristem. Organs are initiated at the shoot meristem periphery, while a central zone harbors the stem cells. Here we show by genetic and molecular analyses that the ZWILLE (ZLL) gene is specifically required to establish the central-peripheral organization of the embryo apex and that this step is critical for shoot meristem self-perpetuation. zll mutants correctly initiate expression of the shoot meristem-specific gene SHOOT MERISTEMLESS in early embryos, but fail to regulate its spatial expression pattern at later embryo stages and initiate differentiated structures in place of stem cells. We isolated the ZLL gene by map-based cloning. It encodes a novel protein, and related sequences are highly conserved in multicellular plants and animals but are absent from bacteria and yeast. We propose that ZLL relays positional information required to maintain stem cells of the developing shoot meristem in an undifferentiated state during the transition from embryonic development to repetitive postembryonic organ formation.     

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.     

Role of liquid versus agar-gelled media in mass propagation and ex vitro survival in bananas

Shoot cultures from meristem explants of three cultivars of banana, ‘Cavendish’, ‘Bluggoe’ and ‘Silk’ were established on Murashige and Skoog medium with 8.9 μM benzyladenine and 0.98 μM indolebutyric acid. Different types of media, such as agar-gelled, agitated liquid and static liquid, were assessed for their ability to support shoot multiplication and ex vitro survival. Liquid media were found better for shoot multiplication whereas agar-gelled medium supported maximum ex vitro survival. For maximum plantlet production, growing in static liquid medium followed by a brief culture on agar-gelled medium has been suggested.     

The ULTRAPETALA gene controls shoot and floral meristem size in Arabidopsis

The regulation of proper shoot and floral meristem size during plant development is mediated by a complex interaction of stem cell promoting and restricting factors. The phenotypic effects of mutations in the ULTRAPETALA gene, which is required to control shoot and floral meristem cell accumulation in Arabidopsis thaliana, are described. ultrapetala flowers contain more floral organs and whorls than wild-type plants, phenotypes that correlate with an increase in floral meristem size preceding organ initiation. ultrapetala plants also produce more floral meristems than wild-type plants, correlating with an increase in inflorescence meristem size without visible fasciation. Expression analysis indicates that ULTRAPETALA controls meristem cell accumulation partly by limiting the domain of CLAVATA1 expression. Genetic studies show that ULTRAPETALA acts independently of ERA1, but has overlapping functions with PERIANTHIA and the CLAVATA signal transduction pathway in controlling shoot and floral meristem size and meristem determinacy. Thus ULTRAPETALA defines a novel locus that restricts meristem cell accumulation in Arabidopsis shoot and floral meristems.     

APETALA2 regulates the stem cell niche in the Arabidopsis shoot meristem

Postembryonic organ formation in higher plants relies on the activity of stem cell niches in shoot and root meristems where differentiation of the resident cells is repressed by signals from surrounding cells. We searched for mutations affecting stem cell maintenance and isolated the semidominant l28 mutant, which displays premature termination of the shoot meristem and differentiation of the stem cells. Allele competition experiments suggest that l28 is a dominant-negative allele of the APETALA2 (AP2) gene, which previously has been implicated in floral patterning and seed development. Expression of both WUSCHEL (WUS) and CLAVATA3 (CLV3) genes, which regulate stem cell maintenance in the wild type, were disrupted in l28 shoot apices from early stages on. Unlike in floral patterning, AP2 mRNA is active in the center of the shoot meristem and acts via a mechanism independent of AGAMOUS, which is a repressor of WUS and stem cell maintenance in the floral meristem. Genetic analysis shows that termination of the primary shoot meristem in l28 mutants requires an active CLV signaling pathway, indicating that AP2 functions in stem cell maintenance by modifying the WUS-CLV3 feedback loop.     

Effect of Root and Shoot Meristem Temperature on Shoot to Root Dry Matter Partitioning and the Internal Concentrations of Nitrogen and Carbohydrates in Maize and Wheat

Maize (Zea mays L.) and spring wheat (Triticum aestivum L.)were grown in nutrient solution at uniformly high air temperature(20 °C), but different root zone temperatures (RZT 20, 16,12 °C). To manipulate the ratio of shoot activity to rootactivity, the plants were grown with their shoot base includingthe apical meristem either above (i.e. at 20 °C) or withinthe nutrient solution (i.e. at 20, 16 or 12 °C). In wheat, the ratio of shoot:root dry matter partitioning decreasedat low RZT, whereas the opposite was true for maize. In bothspecies, dry matter partitioning to the shoot was one-sidedlyincreased when the shoot base temperature, and thus shoot activity,were increased at low RZT. The concentrations of non-structuralcarbohydrates (NSC) in the shoots and roots were higher at lowin comparison to high RZT in both species, irrespective of theshoot base temperature. The concentrations of nitrogen (N) inthe shoot and root fresh matter also increased at low RZT withthe exception of maize grown at 12 °C RZT and 20 °Cshoot base temperature. The ratio of NSC:N was increased inboth species at low RZT. However this ratio was negatively correlatedwith the ratio of shoot:root dry matter partitioning in wheat,but positively correlated in maize. It is suggested that dry matter partitioning between shoot androots at low RZT is not causally related to the internal nitrogenor carbohydrate status of the plants. Furthermore, balancedactivity between shoot and roots is maintained by adaptationsin specific shoot and root activity, rather than by an alteredratio of biomass allocation between shoot and roots.Copyright1994, 1999 Academic Press Wheat, Triticum aestivum, maize, Zea mays, root temperature, shoot meristem temperature, biomass allocation, shoot:root ratio, carbohydrate status, nitrogen status, functional equilibrium     

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.     

Morphogenesis in Selaginella. III. Meristem determination and cell differentiation.

Selaginella willdenovii Baker is a prostrate vascular cryptogam with a dorsiventral stem. At each major branching of the stem tip a dorsal and a ventral angle meristem are formed. The ventral meristem becomes determined as a root and the dorsal meristem as a shoot. Indoleacetic acid (IAA) is transported basipetally in the stem and has been found to be the regulatory agent for meristem determination both in vitro and in vivo.Growth measurements of intact plants indicated that the sequence of development for each stem unit is frond expansion, internodal elongation, ventral meristem growth as a root, and dorsal meristem growth as a shoot. The principal experimental findings of this study are as follows. Triiodobenzoic acid (TIBA), an inhibitor of auxin transport alters the normal pattern of development in intact plants, causing ventral meristems to develop as shoots and dorsal meristems to develop precociously. Dorsal meristems grown in sterile culture on an auxin-free medium develop as shoots, but in the presence of IAA develop as roots. Meristems transferred after excision from auxin-free to plus-auxin medium on successive days showed an increasing tendency to develop as shoots, with more than 50% doing so by day 5. The mitotic index is low at the time of excision of the meristem, rises to a peak on day 5 and then declines.     

Shoot meristem formation and maintenance

The shoot apical meristem of higher plants is a self-maintaining stem cell system which gives rise to the entire above-ground part of a plant. In the past year, genetic and molecular studies have provided increasing insight into the processes of shoot meristem formation and maintenance, as well as into the relation between the apical meristem and its products.