Carrier-Mediated Uptake of Abscisic Acid by Suspension-Cultured Amaranthus tricolor Cells

Abscisic acid (ABA) uptake by Amaranthus tricolor cell suspensions was found to include both a nonsaturable component and a saturable part with K_m of 3.74 ± 0.43 micromolar and an apparent V_max of 1.5 ± 0.12 nanomoles per gram per minute. These kinetic parameters as well as the uptake by intact cells at 0°C or by frozen and thawed cells, are consistent with operation of a saturable carrier. This carrier-mediated ABA uptake was partially energized by ΔpH: it increased as the external pH was lowered to pH 4.0; it decreased after the lowering of the ΔpH by the proton ionophore carbonylcyanide-m-chlorophenylhydrazone or after the altering of metabolically maintained pH gradient by metabolic inhibitors (KCN, oligomycin). The carrier is specific for ABA among the plant growth regulators tested, is unaffected by (RS)-trans-ABA and was inhibited by (S)-ABA, (R)-ABA, and also by the ABA analog LAB 173711.     

Involvement of Endogenous Abscisic Acid in Onset and Release of Helianthus annuus Embryo Dormancy

Mature seeds of Helianthus annuus L. exhibit dormancy that is eliminated during storage in dry conditions. In vitro culture of immature embryos isolated at different times after anthesis showed that the youngest embryos are able to germinate, but within the third week after pollination, dormancy progressively affected most of the embryos. A radioimmunoassay showed that the endogenous abscisic acid (ABA) level, which increased sharply in the first half of the development period, fell at precisely the moment when embryo dormancy became established. An application of fluridone, before the increase of ABA level, prevented both ABA synthesis and development of embryo dormancy. Applied later, after the rise of the ABA level, fluridone could not prevent embryo dormancy development. Dormancy thus appears to be dependent on ABA synthesis but not concomitant with its accumulation; it must therefore be induced by ABA during maturation. Furthermore, a preincubation in water allowed dormant embryos to germinate. This acquisition of germinability could not be directly related to a leaching of free ABA. Possible effects of this treatment are discussed.     

Regulation of ABA levels in senescing petals of rose flowers

During the vase life of a rose flower, changes in the levels of abscisic acid (ABA) were observed: a decrease during the first 3 days, followed by a steady state at a low level, and finally a sharp increase in late senescence. Feeding [2-¹⁴C]ABA to isolated petals showed that metabolism was very active despite the age of the flower, oxidation processes increased with age, whereas conjugation decreased but the level of nonmetabolized ABA remained stable. When the isolated petal was subjected to water stress, whatever its age, the ABA level increased. Hydrolysis of ABA-GE was not involved in this phenomenon. Thus, ABA synthesis occurred in the isolated petal; it could be directly correlated to the decrease in water potential. However, the ABA increase in isolated petals was limited. Moreover, on the rose tree, increases in ABA levels were not correlated to water potential changes. ABA levels seemed, therefore, mainly regulated by changes in import from leaves and other parts of the flower.     

Regulation of ABA levels in senescing petals of rose flowers

During the vase life of a rose flower, changes in the levels of abscisic acid (ABA) were observed: a decrease during the first 3 days, followed by a steady state at a low level, and finally a sharp increase in late senescence. Feeding [2-¹⁴C]ABA to isolated petals showed that metabolism was very active despite the age of the flower, oxidation processes increased with age, whereas conjugation decreased but the level of nonmetabolized ABA remained stable. When the isolated petal was subjected to water stress, whatever its age, the ABA level increased. Hydrolysis of ABA-GE was not involved in this phenomenon. Thus, ABA synthesis occurred in the isolated petal; it could be directly correlated to the decrease in water potential. However, the ABA increase in isolated petals was limited. Moreover, on the rose tree, increases in ABA levels were not correlated to water potential changes. ABA levels seemed, therefore, mainly regulated by changes in import from leaves and other parts of the flower.     

Endogenous cytokinins in rose petals and the effect of exogenously applied cytokinins on flower senescence

In extracts from rose petals cytokinin activity was detected by Amaranthus bioassay in HPLC eluates corresponding to the standards: Z, ZR, 2iP and 2iPA; subsequently, the presence of two groups of endogenous cytokinins was confirmed by ELISA.Measurements of senesence indicators (cell sap osmolarity and conductivity) and observations of flower vase-life indicated that when the above cytokinins were applied as holding solutions they delayed flower senescence by 34–56% and prolonged rose longevity.Abbreviations B.H.T. 2.6-di-t-buytl-4-methyl phenol - ELISA Enzyme linked Immunosorbent Assay - HPLC High Performance Liquid Chromatography - 2iP isopentenyladenine - 2iPA isopentenyladenosine - Z trans-zeatin - ZR trans-zeatin riboside     

An analysis of the role of abscisic acid on the differential expansion and chlorophyll synthesis of the two cotyledons of dormant apple embryo in culture

In dormant embryos of Pyrus malus L. cv. Golden delicious laid flat on water agar and cultured under light, inhibition of growth, chlorophyll synthesis and pro-plastid development in the upper cotyledon was complete, whereas they occurred to a notable degree in the cotyledon in contact with the medium particularly between days 5 and 9 of culture. Radioimmunoassay of endogenous ABA showed a marked difference between the two cotyledons during the first days of culture: on day 2 the level in the upper cotyledon could be 8 times higher than in the lower cotyledon. Experiments using isolated cotyledons showed that this early difference may be responsible for the subsequent divergence in development of the two cotyledons. The effects of exogenous ABA differed depending on the physiological state of the material: when applied at the start of culture ABA inhibited subsequent development, but if applied after 5 days of incubation, when plastid differentiation was already established, ABA no longer prevented growth and only partially inhibited chlorophyll production. The results suggest a direct causal relationship between the levels of endogenous ABA and cotyledon development.     

The influence of light quality on rose flower senescence: involvement of abscisic acid

The aim of this work was to study the impact of light applied during preharvest culture on the subsequent senescence of cut rose flower and to analyse the possible involvement of abscisic acid (ABA). The longevity of cut rose flowers was longer when rose plants were previously grown under high pressure sodium lamps than under metal halide lamps. A change in light source did not lead to a change in leaf ABA content but significantly affected the petal ABA content. The relationship between ABA level and flower longevity, previously reported for differences of genetic origin, was again observed for culture-induced differences: the higher the ABA level at harvest, the shorter the vase-life observed.     

Oxygen availability and ABA metabolism in Fagus sylvatica seeds

At harvest, beechnuts have a very deep dormancy whichlies both in the structures surrounding the embryo andwithin the embryo itself. Covering structures preventembryo germination by interfering with water uptakeand gaseous exchange. To understand the role of thecovering structures and oxygen availability on ABAcatabolism, (+)-[³H] ABA metabolism was studied in isolated embryos as well as in intact seeds. ABAdegradation resulted essentially in oxidative products(PA, DPA). These products were more abundant inisolated embryos than in intact seeds. Theyaccumulated mainly as alkali-nonhydrolyzableconjugates of DPA. A small amount of free andesterified forms were oftenobserved. In isolated embryos a decrease in oxidativeproducts was observed either by lowering the oxygenavailability or by feeding embryos with tetcyclasis(an inhibitor of monooxygenase). In the presence of the covering structures, these oxidative products werereduced in the same manner, indicating that coveringstructures were probably responsible for limiting theoxygen supply to the embryo and for the lowgermination percentage observed in the case of intactseeds.     

ABA synthesis in protoplasts of different origin in response to osmotic stress

All the protoplasts analysed in this study whatever the original plant material were able to accumulate ABA under osmotic stress. The time course of ABA accumulation strongly differed according to the plant material. In both rose petal or Amaranthus leaf protoplasts, the increase in ABA level was significant but transient. Protoplasts prepared from Amaranthus cell suspensions behaved differently, showing a late and durable accumulation of ABA. Similar patterns of changes in ABA accumulation were observed in the original plant material under osmotic stress. A pretreatment of plant material by fluridone induced a strong inhibition of ABA accumulation whatever the origin of protoplasts was. This result suggests that ABA could be synthesised via the carotenoid pathway in the absence of the cell wall.