Effects of abscisic acid on somatic embryo maturation of hybrid larch

Somatic embryos of hybrid larch (Larixleptoeuropaea) whichhad been matured for 4 weeks on maturation medium, developednormally on medium supplemented with 60 µM ABA, but abnormallyon medium with no ABA. A comparative structural and histochemicalinvestigation was carried out on these two types of mature embryos.At the light microscope level, differences between both treatmentswere visible only after 2–3 weeks of maturation. At aroundthis time, abnormal development becomes evident macroscopically:ABA-minus embryos remain rather stubby as opposed to the morecylindrically shaped ABA-plus embryos. Whereas somatic embryosmatured with ABA consist of densely cytoplasmic cells showinga high rate of cell division, ABA-minus embryos are largelymade up of expanded and highly vacuolate cells, indicating thatgrowth in the latter is mainly due to cell expansion and notdivision. After 4 weeks of maturation, ABA-minus embryos beginto elongate in the hypocotyl region, and precocious germinationwas observed frequently. Again, these morphogenetic events werelargely due to abnormal timing of cell expansion. Histochemically,storage proteins were found only in somatic embryos maturedfor 4 weeks with ABA. This observation is in line with resultsobtained by total protein analysis, yielding significantly lowertotal protein contents in ABA-minus embryos both on a freshweight and a per embryo basis after 4–5 weeks of maturation.Deposition of starch grains mainly in the cortex tissue of thehypocotyl region was observed within 2 weeks of maturation invarying amounts regardless of ABA supply. Polyphenols, in particularcatechins and proanthocyanidins, were present in all embryosfrom the very onset of development. They were localized preferentiallyin the proximal suspensor cells and the basal region of theembryo. However, accumulation of polyphenols was generally muchmore pronounced in embryos matured without ABA, indicating alack of biochemical regulatory competence in those embryos. Key words: Abscisic acid, embryonal development, somatic embryo, storage protein, polyphenols     

EFFECTS ON FITNESS COMPONENTS OF P-ELEMENT INSERTS IN DROSOPHILA MELANOGASTER: ANALYSIS OF TRADE-OFFS

We analyzed the trade-offs between fitness components detected in four experiments in which traits were manipulated by inserting small (control) and large (treatment) P-elements into the Drosophila melanogaster genome. Treatment effects and the interactions of treatment with temperature, experiment, and line were caused by the greater length and different positions of the treatment insert. In inbred flies, the treatment decreased early and total fecundity. Whether it increased the lifespan of mated females depended upon adult density. Analysis of line-by-treatment-by-temperature interactions revealed hidden trade-offs that would have been missed by other methods. They included a significant trade-off between lifespan and early fecundity. At 25°C high early fecundity was associated with decreased reproductive rates and increased mortality rates 10–15 days later and persisting throughout life, but not at 29.5°C. Correlations with Gompertz coefficients suggested that flies that were heavier at eclosion also aged more slowly and that flies that aged more slowly had higher fecundity late in life at 25°C. The results support the view that lifespan trades off with fecundity and that late fecundity trades off with rate of aging in fruitflies. Genetic engineering is an independent method for the analysis of trade-offs that complements selection experiments.     

Acquisition and assimilation of gaseous ammonia as revealed by intracellular pH changes in leaves of higher plants

Atmospheric ammonia (NH₃) from various anthropogenic sources has become a serious problem for natural vegetation. Ammonia not only causes changes in plant nitrogen metabolism, but also affects the acid-base balance of plants. Using the pH-sensitive fluorescent dyes pyranine and esculin, cytosolic and vacuolar pH changes were measured in leaves of C₃ and C₄ plants exposed for brief periods to concentrations of NH₃ in air ranging from 1.33 to 8.29 mol NH₃ · mol^-1 gas (0.94–5.86 mg · m^-3). After a lag phase, uptake of NH₃ from air at a rate of 200 nmol NH₃ · m ^{- 2} leaf area · s^{- 1} into leaves of Zea mays L. increased pyranine fluorescence indicating cytosolic alkalinisation. The increase was much larger in the dark than in the light. In illuminated leaves of the C₃ plant Pelargonium zonale L. and the C₄ plants Z. mays and Amaranthus caudatus L., NH₃-dependent cytosolic alkalinisation was particularly pronounced when CO₂ was supplied at very low levels (16 or 20 mol CO₂ · mol^{- 1} gas, containing 210 mmol O₂ · mol^{- 1} gas). An increase in esculin fluorescence, which was smaller than that of pyranine, was indicative of trapping of some of the NH₃ in the vacuoles of leaves of Spinacia oleracea L. and Z. mays. Photosynthesis and transpiration remained unchanged during exposure of illuminated leaves to NH₃, yielding an influx of 200 nmol NH₃ · m^-2 leaf area · s^-1 for up to 30 min, the longest exposure time used. Both CO₂ and O₂ influenced the extent of cytosolic alkalinisation. At 500 mol CO₂ · mol^-1 gas the cytosolic alkalinisation was suppressed more than at 16 or 20 mol CO₂ · mol^-1 gas. The suppressing effect of CO₂ on the NH₃induced alkalinisation was larger in illuminated leaves of the C₄ plants Z. mays and A. caudatus than in leaves of the C₃ plant P. zonale. A reduction of the O₂ concentration from 210 to 10 mmol O₂ · mol ^-1 gas, which inhibits photorespiration, increased the NH₃induced cytosolic alkalinisation in C₃ plants. Suppression by CO₂ or O₂ of the alkaline pH shift caused by the dissolution and protonation of NH₃ in queous leaf compartments, and possibly by the production of organic compounds synthesised from atmospheric NH₃, indicates that NH₃ which enters leaves is rapidly assimilated if photosynthesis or photorespiration provide nitrogen acceptor molecules.This work was supported by the Biotechnology and Biological Sciences Research Council and the Deutsche Forschungsgemein-schaft within the framework of the research of Sonderforschun-gsbreich 251 of the University of Würzburg. We are grateful to Dr. B. Wollenweber (The Royal Veterinary and Agricultural University, Denmark) for discussions.     

Suppression of fungal β-glucan-induced plant defence in soybean (Glycine max L.) by cyclic 1,3-1,6-β-glucans from the symbiont Bradyrhizobium japonicum

The production of antimicrobial phytoalexins is one of the best-known inducible defence responses following microbial infection of plants or treatment with elicitors. In the legume soybean (Glycine max L.), 1,3-1,6--glucans derived from the fungal pathogen Phytophthora sojae have been identified as potent elicitors of the synthesis of the phytoalexin, glyceollin. Recently it has been reported that during symbiotic interaction between soybean and the nitrogen-fixing bacterium Bradyrhizobium japonicum USDA 110 the bacteria synthesize cyclic 1,3-1,6--glucans. Here we demonstrate that both the fungal and the bacterial -glucans are ligands of -glucan-binding sites which are putative receptors for the elicitor signal compounds in soybean roots. Whereas the fungal -glucans stimulate phytoalexin synthesis at low concentrations, the bacterial cyclic 1,3-1,6--glucans appear to be inactive even at relatively high concentrations. Competition studies indicate that increasing concentrations of the bacterial 1,3-1,6--glucans progressively inhibit stimulation of phytoalexin synthesis in a bioassay induced by the fungal 1,3-1,6--glucans. Another type of cyclic -glucan, a 1,2--glucan from Rhizobium meliloti, that does not nodulate on soybean, seems to be inactive as elicitor and as ligand of the -glucan-binding sites. These results may indicate a novel mechanism for a successful plant-symbiont interaction by suppressing the plant's defence response.Abbreviations HG-APEA 1-[2-(4-aminophenyl)ethyl]amino-l-[hexaglucosyl]deoxyglucitol - HG-AzPEA l-[2-(4-azidophenyl)-ethyl]amino-l-[hexaglucosyl]deoxyglucitol - IC₅₀ concentration for half-maximal displacement We thank Ines Arlt for excellent technical assistance. This work was supported by the Deutsche Forschungsgemeinschaft (SFB 369), the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, Fonds der Chemischen Industrie (J.E.), and USDA CSRS NRI Competitive Research grant 93373059233 (A.A.B.).     

Purification of the bacteriocin bavaricin MN and characterization of its mode of action against Listeria monocytogenes Scott A cells and lipid vesicles.

Bavaricin MN was purified from Lactobacillus sake culture supernatant 135-fold with a final yield of 11%. Sequence analysis revealed bavaricin MN to be a 42-amino-acid peptide having a molecular weight of 4,769 and a calculated pI of 10.0. Computer analysis indicated that the C-terminal region may form an alpha-helical structure with an amphipathic nature deemed important in the interaction of bacteriocins with biological membranes. Bavaricin MN rapidly depleted the membrane potential (delta p) of energized Listeria monocytogenes cells in a concentration-dependent fashion. At a bavaricin MN concentration of 9.0 micrograms/ml, the delta p decreased by 85%. Both the electrical potential (delta psi) and Z delta pH components of the delta p were depleted, and this depletion was not dependent on a threshold level of proton motive force. In addition to studying the effect of bavaricin MN on the delta p of vegetative cells, bavaricin MN-induced carboxyfluorescein (CF) efflux from L. monocytogenes-derived lipid vesicles was also characterized. Bavaricin MN-induced CF leakage was also concentration dependent with an optimum of pH 6.0. The rate of CF efflux was 63% greater in lipid vesicles in which a delta psi was generated compared with that in lipid vesicles in the absence of a delta psi.     

The differential genetic and environmental canalization of fitness components in Drosophila melanogaster

Canalization describes the process by which phenotypic variation is reduced by developmental mechanisms. A trait can be canalized against environmental or genetic perturbations. Stabilizing selelction should favor improved canalization, and the degree of a trait's canalization should be positively correlated with its impact on fitness. Here we report, for Drosophila melanogaster, measurements of environmental canalization for five fitness components. We compare them with measurements of genetic canalization, and we discuss the impact of inbreeding on both. In three experiments we measured the variation of fitness components within lines nested within temperature, treatment, and experiment. Lines differed in the position of a P element insert or in genetic background. Within lines flies were genetically nearly identical. We designated trait variation within lines as environmental canalization. The canalization of the traits increased with their impact on fitness, and the pattern was similar to that found for the canalization of fitness components against genetic differences, measured as the variation among lines nested within temperature, treatment, and experiment. This suggests that developmental mechanisms buffer the phenotype against both genetic and environmental disturbance. The results also suggest, less strongly, that inbreeding weakens canalization.