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.     

Resistance to freezing in liquid nitrogen of carnation (Dianthus caryophyllus L. var Eolo) apical and axillary shoot tips excised from different aged in vitro plantlets

The ability of shoot tips from carnation (Dianthus caryophyllus L., var. Eolo) cultured in vitro to develop resistance to freezing in liquid nitrogen depends on the physiological state of the cell material and the pretreatment conditions. Regrowth rates close to 100% have been obtained with apical shoot tips isolated from 2 month-old stems, precultured on medium supplemented with sucrose (0.75M) and treated with dimethylsulfoxide (5% or more). Resistance of axillary shoot tips decreased progressively as a funtion of their distance from the apical shoot tip. During the development of the stem from axillary buds (obtained by cutting), progressive increases in the regrowth rate of frozen apices were noted, from 30% before cutting (axillary buds) to 98% after 3 weeks of culture.Abbreviations DMSO dimethylsulfoxide - LN liquid nitrogen     

Intrapetiolar inflorescence buds in Salacca (Palmae): development and significance

The inflorescence in all species of Salacca is enclosed in a chamber within the leaf base and is exserted through a slit on the abaxial surface of the leaf base. The inflorescence bud is interpreted ds an axillary meristem that becomes radially displaced by adaxial growth of the leaf primordium. A fine channel is produced from the leaf axil to the base of the inflorescence and persists at maturity. The channel and the bud chamber enlarge as the leaf elongates. They are lined by an epidermal layer. There is no cellular breakdown until the collapse and tearing of tissues of the leaf during inflorescence enlargement late in ontogeny. The vegetative bud is positioned about 130⁰ from the axil of its subtending leaf and lies directly below the abaxial inflorescence slit of the leaf above. Vegetative bud development was not observed, hut there is a suggestion of relatively late initiation. The separation of. Eleiodoxa from Salacca is supported by differences in the development of inflorescence and vegetative buds.     

Dysfunctional mitochondria regulate the size of root apical meristem and leaf development in Arabidopsis

Mitochondria play an important role in maintaining metabolic and energy homeostasis in the plant cell. Thus, perturbation of mitochondrial structure and function will affect plant growth and development. Arabidopsis slow growth3 (slo3) is defective in At3g61360 that encodes a pentatricopeptide repeat (PPR) protein. Analysis of slo3 mitochondrial RNA metabolism revealed that the splicing of nad7 intron 2 is impaired, which leads to a dramatic reduction in complex I activity. So the SLO3 PPR protein is a splicing factor that is required for the removal of nad7 intron 2 in Arabidopsis. The slo3 mutant plants have obvious phenotypes with severe growth retardation and delayed development. The size of root apical meristem (RAM) is reduced and the production of meristem cells is decreased in slo3. Furthermore, the rosette leaves of slo3 are curled or crinkled, which may be derived from uneven growth of the leaf surface. The underlying mechanisms by which dysfunctional mitochondria affect these growth and developmental phenotypes have yet to be established. Nonetheless, plant hormone auxin is known to play an important role in orchestrating the development of RAM and leaf shape. It is possible that dysfunctional mitochondria may interact with auxin signaling pathways to regulate the boundary of RAM and the cell division arrest front during leaf growth in Arabidopsis.     

Auxin transport inhibitors act through ethylene to regulate dichotomous branching of lateral root meristems in pine

Many soil fungi colonize the roots of pines to form symbiotic organs known as ectomycorrhizas. Dichotomous branching of short lateral roots and the formation of coralloid organs are diagnostic of ectomycorrhizas in many pine species, although the regulation of these changes in root morphology is not well understood. We used axenic root cultures of six pine species to examine the role of auxin, cytokinin, ethylene and nutrients in the regulation of root architecture. Surprisingly, extensive dichotomous and coralloid branching of lateral roots occurred spontaneously in Pinus taeda , P. halepensis and P. muricata . In P. sylvestris , P. ponderosa and P. nigra , treatment with auxin transport inhibitors (ATIs), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or the ethylene-releasing compound 2-chloroethylphosphonic acid (CEPA or ethephon) induced extensive dichotomous branching and coralloid organ formation. Formation of both spontaneous and ATI-induced coralloid structures was blocked by treatment with an ethylene synthesis inhibitor L-α-(2-aminoethoxyvinyl)glycine; this inhibition was reversed by either ACC or CEPA. In addition, the induction of this unique morphogenetic pattern in pine root cultures was regulated by nutrient levels. The morphology and anatomical organization of the chemically induced dichotomous and coralloid structures, as well as the regulation of their formation by nutrient levels, show a striking similarity to those of ectomycorrhizas.     

Effects of temperature and elevated CO2 on cell division in shoot meristems: differential responses of two natural populations of Dactylis glomerata L.

The aim was to establish whether temperature and/or elevated [CO₂] (-700 μmol mol^?1) affects the cell doubling time (cdt) in the different zones of the shoot apex of two natural populations of Dactylis glomerata originating in Portugal (38° S3′ N) and in Sweden (63° 09′ N). In the Portuguese population at ambient [CO₂], only the pith rib meristem (PRM) exhibited a significant shortening of cdts from 10 to 30 °C. Elevated [CO₂] resulted in a significant shortening of cdt, particularly in the PRM where cdt was reduced 4-8- and 6-1-fold at 10 and 20 °C, respectively, but only 2-fold at 30 °C. In the Swedish population at ambient [CO₂], there were no consistent temperature-dependent alterations to cdt and this population was less responsive to elevated [CO₂] than the Portuguese population. Nevertheless, elevated [CO₂] resulted in a significant shortening of the cdt for some of the zones; the maximum reduction occurred in the PRM at 30 °C. We concluded that in the shoot apex of the Portuguese population, and most notably in the PRM, 10 and 20 °C were non-optimal temperatures for cell division, whilst the Swedish population was relatively buffered against temperature change. Elevated [CO₂] resulted in substantially greater reductions in cdts in the shoot meristem of the Portuguese population than in that of the Swedish population.     

The shoot apical meristem restores its symplasmic organization during chilling-induced release from dormancy

The shoot apex of overwintering perennials ceases its morphogenetic activity at the end of the growing season and transforms into a bud which is dormant and freezing-tolerant. In birch (Betula pubescens) these events are triggered by short photoperiod, and involve the production of 1,3-beta-D-glucan containing sphincters on the plasmodesmata. As a result, all symplasmic pathways shut down. Here we show that breakage of bud dormancy by chilling involves restoration of the symplasmic organization of the meristem. This restoration is likely to be mediated by 1,3-beta-D-glucanase, which was present in small spherosome-like vacuoles that arose de novo during dormancy induction. During chilling these vacuoles were displaced from the bulk cytoplasm to the cortical cytoplasm where they became aligned with the plasma membrane, often associated with plasmodesmata. At this stage the enzyme also appeared outside the vacuoles. During chilling, 1,3-beta-D-glucan disappeared from the plasmodesmal channels and wall sleeves, and the plasmodesmata regained the capacity for cell-cell transport, as demonstrated by microinjection of Lucifer Yellow CH and Fluorescein-tagged gibberellic acid. Collectively, the present experiments demonstrate that restoration of the symplasmic organization of the meristem is indispensable for the release of buds from dormancy and the assumption of a proliferation-competent state, and implicate 1,3-beta-D-glucanase action at the plasmodesmata. Based on these findings we propose a model for 'dormancy cycling' which depicts the meristem as passing through three sequential states of cellular communication with characteristic sensitivities to distinct environmental cues.