Regulation of nitrate reductase and phosphoenolpyruvate carboxylase activities in barley leaf protoplasts

Barley leaf protoplasts were incubated in light or darkness in the presence of various inhibitors, metabolites or weak acids/bases. Nitrate reductase (NR) and phosphoenolpyruvate carboxylase (PEPCase) were rapidly extracted from the protoplasts and assayed under sub-optimal conditions, i.e. in the presence of Mg²⁺ and malate, respectively. Under these conditions changes in activities are thought to reflect changes in the phosphorylation states of the enzymes. The NR was activated by illumination to 90% of its maximal activity within 10 min. Photosynthetic electron transport appeared necessary for light activation of NR since activation was inhibited by the photosynthetic electron-transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and, additionally, an electron acceptor (HCO ₃ ^- ) was required. The PEPCase was also activated by light. However, this activation was not prevented by DCMU or lack of HCO ₃ ^- . Loading of protoplasts in the dark with a weak acid resulted in activation of both NR and PEPCase. For NR, full activation was completed within 5 min, whereas for PEPCase a slower, modest activation continued for at least 40 min. Incubation of protoplasts with a weak base also gave activation of PEPCase, but not of NR. On the contrary, base loading counteracted light activation of NR. Since several treatments tested resulted in the modulation of either NR or PEPCase activity, but not both, signal transduction cascades leading to changes in activities appear to be very different for the two enzymes.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) - DMO 5,5-dimethyl-2,4 oxazolidinedione - NR nitrate reductase - PEPCase Phosphoenolpyruvate carboxylase This work was supported by the Norwegian Research Council by a Grant to C.L: L.H.S. was supported by the Biotechnology and Biological Sciences Research Council.     

Effect of enhanced UV-B radiation on pollen quantity,quality, and seed yield in Brassica rapa (Brassicaceae)

We conducted three experiments to examine the influence of ultraviolet-B radiation (UV-B; 280–320 nm) exposure on reproduction in Brassica rapa (Brassicaceae). Plants were grown in a greenhouse under three biologically effective UV-B levels that simulated either an ambient stratospheric ozone level (control), 16% (“low enhanced”), or 32% (“high enhanced”) ozone depletion levels at Morgantown, WV, USA in mid-March. In the first experiment, we examined whether UV-B level during plant growth influenced in vivo pollen production and viability, and flower production. Pollen production and viability per flower were reduced by ≈50% under both enhanced UV-B levels relative to ambient controls. While plants under high-enhanced UV-B produced over 40% more flowers than plants under the two lower UV-B treatments, whole-plant production of viable pollen was reduced under high-enhanced UV-B to 17% of that of ambient controls. Whole-plant production of viable pollen was reduced under low-enhanced UV-B to 34% of ambient controls. In the second experiment, we collected pollen from plants under the three UV-B levels and examined whether source-plant UV-B exposure influenced in vitro pollen germination and viability. Pollen from plants under both enhanced-UV-B treatments had initially lower germination and viability than pollen from the ambient level. After in vitro exposure to the high-enhanced UV-B levels for 6 h, viability of the pollen from plants grown under ambient UV-B was reduced from 65 to 18%. In contrast, viability of the pollen from plants grown under both enhanced UV-B treatments was reduced to a much lesser extent: only from ≈43 to 22%. Thus, ambient source-plant pollen was more sensitive to enhanced UV-B exposure. In the third experiment, we used pollen collected from source plants under the three UV-B levels to fertilize plants growing under ambient-UV-B levels, and assessed subsequent seed production and germination. Seed abortion rates were higher in plants pollinated with pollen from the enhanced UV-B treatments, than from ambient UV-B. Despite this, seed yield (number and mass) per plant was similar, regardless of the UV-B exposure of their pollen source. Our findings demonstrate that enhanced UV-B levels associated with springtime ozone depletion events have the capacity to substantially reduce viable pollen production, and could ultimately reduce reproductive success of B. rapa.     

Heterospecific pollen transfer between sympatric species in a midsuccessional old-field community

The cumulative (season-long) incidence of heterospecific pollen transfer (HPT) was examined using nine sympatric species in a midsuccessional old field. Inflorescences were collected weekly during the flowering season, and the proportion of foreign pollen/stigma was recorded. Flowering phenologies of sympatric species and ovule and seed counts of study species were also recorded. Heterospecific pollen was detected on some stigmas of each species. Medicago sativa (Fabaceae) received the most foreign pollen; in some cases, all of the grains on a stigma were heterospecific. Lotus corniculatus (Fabaceae) received the least amount of foreign pollen; the incidence of heterospecific pollen was near zero in most cases. The mean and range of foreign pollen received varied by as much as an order of magnitude between species. The six species with zygomorphic flowers, all Fabaceae, received more heterospecific pollen than the three species with actinomorphic flowers, Potentilla recta and P. simplex (Rosaceae) and Ranunculus acris (Ranunculaceae). This probably reflects a bias because our data were analyzed on a cumulative basis and the Fabaceae had longer flowering phenologies. HPT was not correlated with the species' relative abundance within the community. Proportion of foreign pollen received varied temporally within species, and this variation generally was not related to phenology of any sympatric taxa or the species' own phenology. Pollen grain diameter was positively related to levels of foreign pollen received by species. This might be caused by poor adhesion of large pollen grains to small stigmatic papillae or if generalist pollinators carrying large amounts of heterospecific pollen visit the large-grained species and specialists with little foreign pollen visit the small-grained species. The large proportions of heterospecific pollen on stigmas of many species indicate that HPT occurs frequently in the community we studied and the implications may include reduced seed set because of occlusion by foreign grains. As yet, however, it is unclear how important a factor HPT is in mediating pollen limitation of reproductive success.     

Fruit set in a deceptive orchid: The effect of flowering phenology,display size,and local floral abundance

We investigated the effect of flowering time, display size, and local floral density on fruit set in Tolumnia variegata, a pollination-limited orchid that offers no reward to its pollinator(s). During 1990, natural variation in flowering time, display size, and fruit set were monitored in 508 plants at one locality in Puerto Rico. The following season, orchid floral abundance per host tree (Randia aculeata) was manipulated to investigate its effect on fruit set. Four floral abundance treatments were established (700, 500, 300, and 100), each replicated four times. Flowering time was the most important trait affecting fruit set. The proportion of plants setting at least one fruit was significantly high early and late in the season, but low during the flowering peak. Thus, strong disruptive selection differential on flowering phenology was found. Display size had little effect on fruit set. A weak, but significant disruptive selection differential on display size was found. Orchid floral abundance per host tree had a significant effect on fruit set. Early in the season, T. variegata flowers with intermediate number of conspecific flowers exhibited a greater probability of setting fruit than those in host trees with fewer or more flowers. Our results show that flowering phenology may be evolutionarily unstable, possibly a consequence of the deception pollination system. Furthermore, a deception strategy would be relatively unsuccessful in populations where plants are found in either very dense or sparse patches.     

FT Ⅰ, a novel positive myeloid-lineage-specific transcription regulatory element within the mouse myeloperoxidase gene enhancer, En 1

INTanDUCTI0NMyeloidcelldifferentiati0n,inwhichmultip0tentialprogenitorcellsarec0nvertedint00neofthesixmaturedifferentiatedcells,i.e.,erythr0cytes,platelets(megakary-ocytes),macr0phages,neutr0phils,e0sinophilsandbas0phils,involvestemporalre-gulati0nofexpression0fanumberoflineage-anddifferentiationstage-specificgenes.Understandingthedevel0pmentalspecificationoflineageaJswellasmaturationstageassociatedpatterns0fgeneexpressioninmyel0idcelldifferentiationrequiresanin-sightintothecontrol0findivid…     

P48-Triggered transmembrane signaling transduction of human monocytes:mobilization of calcium ion and activation of protein kinase C(PKC)

INTRODUCTI0NThedifferentiati0nofcelIsalongthemonocyte-macr0phagepathwayandthesig-nalsinvo1vedinthesecel1sacquiringtheabilitytokilltum0rcellsarenotfllllyundersto0d.Wehavebeenstudingamoleculewhichappearst0beanimportantmemberofthecytokinenetworkinvo1vedintheregulati0nmonocyteactivation.ThiscytokinetermedP48wasisolatedfr0mthehllmannullcellleukemiacell1ineReh.IthasbeenpurifiedtohomogeneityandfOundtobedistinctfrominterferongamma,col0nystimulatingfactors(CSFs)andTNFalphaalldbeta[1,2].Func-ti…     

Release of mouse eggs from metaphase arrest by protein synthesis inhibition in the absence of a calcium signal or microtubule assembly

Mouse egg activation, which includes release from meiotic metaphase II arrest, results from fertilization-induced increase in intracellular calcium concentration ([Ca²⁺]_i). However, during egg activation caused by exposure to the protein synthesis inhibitor, cycloheximide, [Ca²⁺]_i did not change. Although eggs fertilized in the presence of microtubule inhibitors remain arrested at metaphase, eggs treated for 32 hr with cycloheximide and the microtubule inhibitor, colcemid, formed nuclei. In untreated eggs aged in culture for 24 hr, the microtubule spindles became deformed. These eggs formed nuclei after exposure to cycloheximide, but not the calcium ionophore A23187. Our results indicate that eggs in which protein synthesis is inhibited are released from metaphase without an increase in [Ca²⁺]_i, and despite disruption of the Spindle. © 1995 Wiley-Liss, Inc.     

Na_2SO_3和NaHCO_3对叶绿体CF_1－ATPase活力作用的机制

Ｎａ２ＳＯ３对热－ＤＴＴ活化的游离ＣＦ１及类囊体膜上ＣＦ１－ＡＴＰａｓｅ活力均有显著的促进作用,ＮａＨＣＯ３亦有明显的促进作用。Ｎａ２ＳＯ３和ＮａＨＣＯ３的促进作用与它们解除Ｍｇ２＋的抑制作用有关。从ＮａＨＣＯ３和Ｎａ２ＳＯ３及它们与Ｍｇ２＋之间的竞争性关系,表明三者是结合在酶的同一部位上。Ｎａ２ＳＯ３可明显降低热－ＤＴＴ活化的游禹ＣＦ１－ＡＴＰａｓｅ催化反应的活化能,这可能与促进产物ＡＤＰ的释放有关。     

Floral evolution and male reproductive success: Optimal dispensing schedules for pollen dispersal by animal-pollinated plants

Summary Selection favouring an outcrossing plant's ability to sire seeds generally promotes floral characters that increase (1) the frequency of pollinator visits, (2) the number of pollen grains dispersed to other plants by each pollinator and (3) the probability of a pollen grain successfully fertilizing an ovule after reaching a stigma. Flowers influence pollen dispersal and fertilization probabilities by determining the pattern of pollen removal during a series of visits (dispensing schedule). We model male reproductive success to identify optimal dispensing schedules, which characteristically involve monotonic increases in the proportion of remaining pollen removed during successive visits. These schedules balance the benefits of restricted removal, which counteracts the diminishing returns associated with animal pollination (e.g. pollinator grooming, local mate competition), with the advantages of increased removal to avoid time-dependent losses in fertilization ability (e.g. pollen precedence, declining viability). Because pollinator availability mediates this balance, the most effective dispensing schedule allows dynamic adjustment of removal to the prevailing frequency of visits experienced by individual plants. As an example of such dynamic removal we demonstrate that the dispensing mechanism ofLupinus sericeus flowers allows facultative adjustment of removal to the interval between visits. Because optimal control of pollen removal can increase a plant's mating opportunities by an order of magnitude, dispensing mechanisms should be a common component of floral design.     

POLLEN DISCOUNTING AND THE SPREAD OF A SELFING VARIANT IN TRISTYLOUS EICHHORNIA PANICULATA: EVIDENCE FROM EXPERIMENTAL POPULATIONS

Floral traits that increase self-fertilization are expected to spread unless countered by the effects of inbreeding depression, pollen discounting (reduced outcross pollen success by individuals with increased rates of self-fertilization), or both. Few studies have attempted to measure pollen discounting because to do so requires estimating the male outcrossing success of plants that differ in selfing rate. In natural populations of tristylous Eichhornia paniculata, selfing variants of the mid-styled morph are usually absent from populations containing all three style morphs but often predominate in nontrimorphic populations. We used experimental garden populations of genetically marked plants to investigate whether the effects of population morph structure on relative gamete transmission by unmodified (M) and selfing variants (M‘) of the mid-styled morph could explain their observed distribution. Transmission through ovules and self and outcross pollen by plants of the M and M’ morphs were compared under trimorphic, dimorphic (S morph absent), and monomorphic (L and S morphs absent) population structures. Neither population structure nor floral morphology affected female reproductive success, but both had strong effects on the relative transmission of male gametes. The frequency of self-fertilization in the M' morph was consistently higher than that of the M morph under all morph structures, and the frequency of self-fertilization by both morphs increased as morph diversity of experimental populations declined. In trimorphic populations, total transmission by the M and M' morphs did not differ. The small, nonsignificant increase in selfing by the M' relative to the M morph was balanced by decreased outcross siring success, particularly on the S morph. In populations lacking the S morph, male gamete transmission by the M' morph was approximately 1.5 times greater than that by the M morph because of both increased selfing and increased success through outcross pollen donation. Therefore, gamete transmission strongly favored the M' morph only in the absence of the S morph, a result consistent with the distribution of the M' morph in nature. This study indicates that floral traits that alter the selfing rate can have large and context-dependent influences on outcross pollen donation.